Workability of 1045 Forgin Steel with Residual Copper

WORKABILITY OF 1045 FORGING STEEL WITH RESIDUAL COPPER

by Luis Gonzalo Garza-Martínez.

ABSTRACT

Workability of 1045 steels with residual copper was investigated. Eight 1045 steels with differing copper contents, from 0.09% to 0.39% (by weight), were tested. High strain rate compression tests of pre-bulge samples were used to simulate forging conditions. The testing was divided into two stages: Stage one, in which the steels were oxidized at different temperatures from 1100 to 1200 °C for 10 and 30 minutes and deformed to determine the critical temperature where the surface cracking becomes severe. Stage two, in which the steels were oxidized for 1, 3, 5, and 7 minutes at their critical temperature and deformed. In the stage one testing, steels oxidized for 10 minutes and deformed exhibited a critical temperature. The critical temperature decreased with decreasing copper content, from 1160 °C for the steel with the highest copper content to 1110 °C for the steel with lowest copper content. Steels oxidized for 30 minutes and deformed did not exhibit severe cracking at any temperature, but the steel with the highest copper content deformed at 1140 °C exhibited some cracking. In stage two testing, the results were less consistent. The steels with high copper content (0.39 to 0.32%) exhibited maximum cracking at shorter times, while for the steels with medium copper content (0.30 to 0.21%) the maximum cracking occurred at longer times. No steel exhibited cracking when oxidized at 1200 °C and deformed. Hot-shortness is the brittleness in metal in the hot forging range. It was found that oxidation time and temperature are critical parameters for the control of hot-shortness. Steels have a critical temperature at which cracking becomes severe; this critical temperature is dependent on copper content for oxidation time of 10 minutes. Steels oxidized and deformed above their critical temperature did not present hot-shortness cracking; therefore, a forging practice that deforms the steel above its critical temperature ensures that defects like hot-shortness could be eliminated or reduced.

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TABLE OF CONTENTS

ABSTRACT...... iii LIST OF FIGURES ...... vii LIST OF TABLES...... xi ACKNOWLEDGMENTS ...... xiii

1.0 INTRODUCTION ...... 1

1.1 Copper in Iron ...... 1 1.1.1 Copper in Iron, Historical Background...... 1 1.1.2 Residual Elements in Steel...... 2 1.1.3 Problems of Copper in Iron...... 4 1.1.4 Benefits of Copper in Iron ...... 5 1.2 Hot Shortness ...... 6 1.2.1 The Copper-Iron System...... 6 1.2.2 Hot Shortness Mechanisms...... 7 1.2.3 Elements That Prevent Hot Shortness...... 9 1.2.4 Elements That Promote Hot Shortness ...... 11 1.3 Brief Summary of Previous Studies ...... 11 1.4 Opportunities for the Present Study ...... 14

2.0 INDUSTRIAL RELEVANCE AND PROJECT OBJECTIVES...... 15

3.0 EXPERIMENTAL PROCEDURES...... 17

3.1 Experimental Steels ...... 17 3.2 Testing Methods ...... 17 3.2.1 The Gleeble...... 17 3.2.2 Testing Specimens ...... 19 3.3 The Test Matrix ...... 22 3.3.1 Stage One Testing...... 23

3.3.2 Stage Two Testing ...... 24 3.3.3 Other Tests...... 25 3.4 Surface Crack Measuring Techniques and Analysis ...... 26 3.4.1 Hot-Shortness Cracking...... 26 3.4.2 Sample Mounting and Preparation...... 28 3.4.3 Crack Measurement Techniques...... 32

4.0 RESULTS ...... 35

4.1 Stage One Results ...... 36 4.1.1 Oxidation for 10 Minutes ...... 36 4.1.2 Oxidation for 30 Minutes ...... 48 4.1.3 Determination of Critical Temperature...... 49 4.2 Stage Two Results ...... 61 4.3 Special Tests ...... 72 4.4 Summary of the Results...... 74

5.0 DISCUSSION...... 75

5.1 Stage One Testing...... 75 5.2 Stage Two Testing ...... 81 5.3 Summary of Ideas ...... 86 5.4 Industrial Insights ...... 87

6.0 SUMMARY...... 89

7.0 FUTURE WORK...... 93

REFERENCES ...... 95

APPENDIX A ...... 99 APPENDIX B ...... 101

LIST OF FIGURES

Figure 1.1 Time trends in electric furnace production, from AISI statistics [2]...... 2

Figure 1.2 Scrap requirement for the US steel industry and supply of home scrap, obsolete scrap, and prompt, low residual scrap to the industry from 1967 to 1999 [3].3

Figure 1.3 Production of raw steel and hot metal (liquid pig iron) in the Unites State and net consumption of steel mill products from both domestic and imported supply from 1967 to 1996 [3]...... 4

Figure 1.4 The copper-iron phase diagram [4]...... 7

Figure 1.5 1045 steel oxidized for 30 minutes at 1135 ºC...... 8

Figure 1.6 Effect of ternary additions of solubility of copper in austenite at 1250 °C [4]...... 10

Figure 3.1 Gleeble testing equipment...... 19

Figure 3.2 SICO test specimens, non oxidized at the top oxidized at 1000 °C for 24 hrs, both deformed at 1135 °C...... 20

Figure 3.3 Diagram of the flanged specimen (dimensions in [mm])...... 21

Figure 3.4 Pre-bulged flanged test sample...... 21

Figure 3.5 Diagram of the test matrix...... 23

Figure 3.6 Thermal cycle used during stage one testing...... 24

Figure 3.7 Thermal cycle used during stage two testing...... 25

Figure 3.8 Hot-shortness surface cracks of a steel with 0.35% copper, oxidized at 1160 °C for 10 minutes. The picture on top shows the crack groups, while the picture on bottom is a close up of the cracking...... 27

Figure 3.9 (a) micrograph of steel with 0.35% copper, oxidized at 1160 °C for 10 minutes, showing large hot-shortness cracks (steel sample is on top, epoxy mount is below), (b) SEM image of the crack surface of steel 0.39% copper, oxidized at 1135 °C for 10 minutes, showing cracks as large as 300 µm...... 29

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Figure 3.10 Diagram of mounting and sectioning procedure. (The mounts are standard 1.25” diameter mounts)...... 30

Figure 3.11 Contrast picture used in measuring and analyzing surface cracks...... 31

Figure 3.12 Crack identification and measuring for obtaining the crack index. (Steel with 0.35% Cu, oxidized at 1160 °C for 10 minutes)...... 33

Figure 4.1 Steel 39 (0.39% Cu) oxidized for 10 minutes at different temperatures and deformed...... 38

Figure 4.2 Steel 35 (0.35% Cu) oxidized for 10 minutes at different temperatures and deformed...... 39

Figure 4.3 Steel 32 (0.32% Cu) oxidized for 10 minutes at different temperatures and deformed...... 40

Figure 4.4 Steel 30 (0.30% Cu) oxidized for 10 minutes at different temperatures and deformed...... 41

Figure 4.5 Steel 25 (0.25% Cu) oxidized for 10 minutes at different temperatures and deformed...... 42

Figure 4.6 Steel 21 (0.21% Cu) oxidized for 10 minutes at different temperatures and deformed...... 43

Figure 4.7 Steel 20 (0.20% Cu) oxidized for 10 minutes at different temperatures and deformed...... 44

Figure 4.8 Steel 09 (0.09% Cu) oxidized for 10 minutes at different temperatures and deformed...... 45

Figure 4.9 Crack Index measurements of steels a) 39, b) 35, c) 32, and d) 30, oxidized for 10 minutes and deformed...... 46

Figure 4.9 Crack Index measurements of steels e) 25, f) 21, g) 20, and h) 09, oxidized for 10 minutes and deformed...... 47

Figure 4.10 Steel 39 (0.39% Cu) oxidized for 30 minutes at different temperatures and deformed...... 51

Figure 4.11 Steel 35 (0.35% Cu) oxidized for 30 minutes at different temperatures and deformed...... 52

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Figure 4.12 Steel 32 (0.32% Cu) oxidized for 30 minutes at different temperatures and deformed...... 53

Figure 4.13 Steel 30 (0.30% Cu) oxidized for 30 minutes at different temperatures and deformed...... 54

Figure 4.14 Steel 25 (0.25% Cu) oxidized for 30 minutes at different temperatures and deformed...... 55

Figure 4.15 Steel 21 (0.21% Cu) oxidized for 30 minutes at different temperatures and deformed...... 56

Figure 4.16 Steel 20 (0.20% Cu) oxidized for 30 minutes at different temperatures and deformed...... 57

Figure 4.17 Steel 09 (0.09% Cu) oxidized for 30 minutes at different temperatures and deformed...... 58

Figure 4.18 Crack Index measurements of steels a) 39, b) 35, c) 32, and d) 30, oxidized for 30 minutes and deformed...... 59

Figure 4.18 Crack Index measurements of steels e) 25, f) 21, g) 20, and h) 09, oxidized for 30 minutes and deformed...... 60

Figure 4.19 Steel 39 (0.39% Cu) oxidized at 1150 °C for different times and deformed...... 63

Figure 4.20 Steel 35 (0.35% Cu) oxidized at 1145 °C for different times and deformed...... 64

Figure 4.21 Steel 32 (0.32% Cu) oxidized at 1135 °C for different times and deformed...... 65

Figure 4.22 Steel 30 (0.30% Cu) oxidized at 1130 °C for different times and deformed...... 66

Figure 4.23 Steel 25 (0.25% Cu) oxidized at 1115 °C for different times and deformed...... 67

Figure 4.24 Steel 21 (0.21% Cu) oxidized at 1115 °C for different times and deformed...... 68

Figure 4.25 Steel 20 (0.20% Cu) oxidized at 1110 °C for different times and deformed...... 69

Figure 4.26 Crack Index measurements of steels a) 39, b) 35, c) 32, and d) 30, oxidized at their critical temperatures for 1, 3, 5, and 7 minutes and deformed...... 70

Figure 4.26 Crack Index measurements of steels e) 25, f) 21, and g) 20, oxidized at their critical temperatures for 1, 3, 5, and 7 minutes and deformed...... 71

Figure 4.27 Steel 09 (0.09% Cu) oxidized at 1110°C for 1 minute and deformed. Notice the absence of cracking...... 72

Figure 4.28 Steel 39 and 35 (0.39 and 0.35% Cu) oxidized at 1200 °C for 1 minute and deformed, showing no cracks...... 73

Figure 5.1 Critical temperature for cracking as a function of copper content...... 76

Figure 5.2 Maximum Crack Index as function of copper content...... 76

Figure 5.3 Schematic illustrating the competition between enrichment and dispersion of copper in grain boundaries near the surface of the specimen...... 78

Figure 5.4 Critical temperature for samples oxidized for 10 minutes and the solidus line of the Cu-Ni binary system...... 80

Figure 5.5 Extrapolation of the crack index measurements for Steels (a) 32, (b) 30 and (c) 25...... 83

Figure 5.6 Extrapolation of crack index measurements plot for Steels (a) 39 and (b) 35...... 84

Figure 5.7 Crack index as a function of copper content for various oxidation times.....85

LIST OF TABLES

Table 3.1 Chemical composition of the experimental steels...... 18

Table 4.1 Critical temperatures chosen for stage two testing...... 50

Table 4.2 Crack index values of Steels 39 and 35 compare with Steel 09 ...... 73

Table 4.3 Critical temperatures and crack index values ...... 74

AKNOWLEDGMENTS

Quiero expresar mi gran agradecimiento al Consejo Nacional de Ciencia y Tecnología (CONACYT) por la Beca-Crédito que hizo posible la realización de este grado de maestría.

I would like to thank the Advanced Steel Processing and Products Research Center (ASPPRC) for allowing me to participate; this project was founded by the Center. A special mention to the industrial mentors, Amy Bailey form Chaparral Steel and David Seppala form North Star Steel, for their valuable contributions. Chaparral Steel and North Star Steel provided the experimental material for this project. I thank Forging Industry Educational and Research Foundation (FIERF), for giving me the honor to be the FIERF-Fellow, and the support that this represents. I would like to thank the members of my committee, Dr. David K Matlock, Dr. Martin Mataya for their wise contributions, support and comprehension. The third member and my advisor Dr. Chester J. VanTyne, to whom I am special thankful, for his great contributions, his guidance and his friendship, all this invaluable factors made possible to achieve the quality of this project and the greatness of my experience through this process. I would like to thank all the professors that I had as teachers, all contributed to the knowledge that allowed me to realize this project. Dr. K. Miller for his help and advice with the image digital processing. I am especially thankful to all my friends and colleagues that provided the right stimulation to keep trying harder. I would like to thank Kay Eccleston who helped me making this document clearer and easy to read.

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I thank the love and company of Maximus, my little Boston Terrier who might have saved my life.

Agradezco infinitamente a mi familia, quien nunca ha dejado de estar presente en todo lo que hago. Su apoyo es invaluable y único.

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To my parents Jesus M. Garza and Ma. Olivia Martínez, who have always been with me in the easy and difficult times, and who’s education I own what I am.

Monterrey my city where I grew up breathing steel.

1.0 INTRODUCTION AND BACKGROUND

This chapter presents a summary of the relationships between copper and iron alloys, especially steels. It begins with a brief history of the two elements and an overview of the importance of copper as an alloying element in steels. Following the historical description, the circumstances of copper as a residual element in the steels are presented. The chapter also describes the problems that copper as a residual and/or alloy element in steel can cause, as well as the benefits that can be obtained. The last section of the chapter discusses the main problem that copper residuals can cause in steels during deformation at high working temperatures—hot shortness.

1.1 Copper in Iron

1.1.1 Copper in Iron, Historical Background

Copper has played a major role in the ferrous industry. Copper has been considered a good option as an alloying element in steels because of its relatively low cost and ease of handling. In the early 1900s, the mechanical properties of cast steel were considerably increased by the addition of copper. Copper improved the tensile strength of steel due to precipitation hardening. Copper increased the fluidity of molten steels during casting. Copper improved the fatigue properties of steel. As an example, in the first cast crankshafts put into production in the 1930s no less than 2% copper was used as an alloying element [1]. The best-known characteristic of copper-containing steel alloys is the increased corrosion resistance. A variety of copper-containing steels have been developed that exhibit enhanced corrosion resistance due the build up of an adherent scale layer on the

2 surface of the steel component. Corrosion resistant steels that contain copper have been used in numerous construction projects, such as bridges and buildings.

1.1.2 Residual Elements in Steel

A residual element is the one that is not removed from the liquid metal during the steelmaking process. Residual elements often continue increasing to relatively high levels with continued recycling. The usage of scrap steel for steelmaking has become more important due to the great number of electrical furnace operations (see Figure 1.1).

Figure 1.1 Time trends in electric furnace production, from AISI statistics [2].

By utilizing scrap steel instead of iron ore for steelmaking in the electrical furnace, energy consumption is reduced to 1/6 of that consumed by steelmaking from pig iron. This represents a considerable energy reduction that translates into economic savings and an environmental friendly operation. There is a major concern regarding the amount of residuals in steel. The trend of the residuals to continue increasing, the environmental concerns, and the economic issues have put the steel industry on alert. There should be an adjustment in the processing ideologies and techniques to address the anticipated problems mentioned above. Figure 1.2 and Figure 1.3 illustrate the tendencies of the scrap steel consumption and steel production in the United States [3]. It can be seen that the amount of obsolete scrap increases along with the production of steel; meanwhile, the amount of hot metal, home scrap decreases, and the low residual scrap remains low.

Figure 1.2 Scrap requirement for the United States steel industry and supply of home scrap, obsolete scrap, and prompt, low residual scrap to the industry from 1967 to 1999 [3].

Figure 1.3 Production of raw steel and hot metal (liquid pig iron) in the Unites State and net consumption of steel mill products from both domestic and imported supply from 1967 to 1996 [3].

1.1.3 Problems of Copper in Iron

Copper as a residual element, is not eliminated in the steelmaking process. Additionally, the characteristics of the Cu-Fe system produce some issues for copper-iron alloys such as the lower melting point of copper (1084.5 ºC) and the low solubility of copper in iron at low temperatures. Another issue that needs to be addressed is that copper is not soluble in iron oxide. At high temperatures, where the oxidation rate of iron is quite high, there is rejection of the copper into the metal creating a copper-enrichment zone at the metal oxide interface. At high temperatures, the copper is liquid and can penetrate along grain

5 boundaries with ease. This penetration debilitates the grain boundaries, which in the presence of a tensile stress causes them to break. The scenario is a defect that is known as hot shortness. Hot shortness is not a new problem; it has been known since the early 1900s, when it was called red-shortness. The topic arose again in the late 1950s and 1960s, when the amount of copper residuals increased in steels, and the steel industry encountered production problems. In the late 1990s, the issue has once again taken on importance, mainly for economic and environmental reasons. The hot shortness phenomenon will be discussed in more detail in a later section of this chapter.

1.1.4 Benefits of Copper in Iron

One of the primary benefits of copper in steels is the increased corrosion resistance that is imparted to the steel. Weathering steels have been commercially available for a number of years. Copper addition changes the oxide morphology, preventing it from flaking off. A protective film is formed, and the film slows the corrosion process. Because of low maintenance costs, these kinds of steels are widely used in bridges, structural steel for offshore applications, and pipelines [4]. Copper in steels is also used as a strengthener. Because of its decreasing solubility with temperature, copper can provide precipitation strengthening. The precipitates formed are ε-phase, essentially pure copper, which increases the yield point of the steel. Copper as an intentional alloying element is very intriguing considering its low cost [4-10]. Additional benefits of copper in steels have been reported in the literature, such as improved weldability and fatigue life [4, 9-14].

1.2 Hot Shortness

Volume 5 of the Metals Handbook (ASM, Metals Handbook, 8th Edition, Vol. 5, Forging and Casting) gives the following definition of hot shortness: "Brittleness in metal in the hot forging range", particularly in some types of steel containing low melting point elements, and especially copper. This phenomenon occurs commonly at the surface of these steels because, during the reheating before or during forming, the content of non- oxidizing elements such as copper increases. For this reason the term "surface hot shortness" is sometimes used. The phenomenon of hot shortness in copper-containing steels is strongly influenced by other residual elements such as antimony, tin, and arsenic. These residuals are more soluble in copper than in iron. Nickel also has an influence on the hot shortness of copper-containing steels, since it forms complete liquid and solid solutions with copper [4].

1.2.1 The Copper-Iron System

Figure 1.4 shows the copper-iron phase diagram. Some important characteristics should be noted. The melting point of pure copper is 1084.5 ºC, which is well below the normal hot working temperatures of steel. At temperatures above 1094 ºC, a liquid phase is present for concentration of copper around 8% and higher. The solubility of copper in α-Fe is 2.1% at 850 ºC and decreases with decreasing temperature, providing the opportunity for subsequent precipitation for strengthening processes. The main characteristic that should be noted is the solubility of copper at hot working temperatures of steel. The solubility increases with temperature in the hot working regime, but if the copper solubility is exceeded in this region then a liquid phase can form.

Figure 1.4 The copper-iron phase diagram [4].

1.2.2 Hot Shortness Mechanisms

The oxidation of iron at temperatures in the range of 700-1250 ºC is due to diffusion of ions through different oxide layers: Hematite Fe2O3 (1%), magnetite Fe3O4 (4%), and wüstite FeO (95%) are all present in the percentages given in parentheses [15]. Oxidation of steel is not too different from the iron, where normally only two oxide forms

are present: Fe3O4 and FeO. Nevertheless, noble elements such as copper do not oxidize. The solubility of copper in FeO is very low, so the copper is rejected towards the steel/oxide interface. A formation of a copper-enriched zone can occur. At the working temperatures of steel (above 1100 ºC), the enrichment zone is essentially pure copper and can be present as a liquid phase. Copper diffuses into the steel, and the predominant mode is penetration into the grain boundaries. Figure 1.5 is a optical micrograph of 1045 steel with 0.39% residual copper oxidized at 1135 ºC for 30 minutes. The increased copper concentration along the grain boundaries can be readily observed.

Figure 1.5 1045 steel oxidized for 30 minutes at 1135 ºC.

When copper wets the grain boundaries, they become weakened. And, in the presence of tensile stresses, the grain boundaries can separate. This is the defect know as hot shortness. In summary, hot shortness is a defect related to the rejection of copper by the oxide during high-temperature oxidation of the steels and is controlled by the penetration of the copper enrichment zone into the steel. There are some elements that help prevent hot shortness, and other elements that enhance it. The next few sections of this chapter briefly describe the effects some elements have on the problem.

1.2.3 Elements That Prevent Hot Shortness

To date, nickel has been the only viable alloying element known to effectively prevent hot shortness. Nickel prevents the copper-containing steels from experiencing hot shortness in two ways. First, it increases the solubility of copper in austenite, reducing the enrichment zone. Secondly, the concentration at the enrichment zone along the grain boundaries is composed of a Cu-Ni-Fe solution, which has a higher melting point as compared to pure copper [4,16,17]. Figure 1.6 presents the effect of ternary additions on solubility of copper in austenite at 1250 °C.

Figure 1.6 Effect of ternary additions of solubility of copper in austenite at 1250 °C [4].

Although nickel has been shown to correct the hot shortness problem, it is not the best solution because of its relatively high price. Higher additions of nickel increase the price of the steel, making it less suitable for common applications. An economical solution for hot shortness needs to come from the process area. Silicon and cobalt additions also decrease the hot shortness problem. Silicon increases the solubility of copper in austenite, but loses its efficiency at high temperatures

(1150 ºC and above). Cobalt is not as efficient as nickel, and is an even more expensive alternative [18].

1.2.4 Elements That Promote Hot Shortness

Other residual elements have detrimental effects and promote hot shortness in copper-containing steels. Antimony, arsenic, and especially tin are elements that should be controlled to very low levels if copper is present in the steel. The presence of these elements reduces the solubility of copper in austenite, increasing the amount of copper in the enrichment zone (see Figure 1.6) and making the steel more prone to hot shortness [17-21].

1.3 Brief Summary of Previous Studies

Hot shortness in steels containing residual copper is not a new phenomenon. The problem was initially identified in the early 1900s. Numerous studies and investigations have occurred since then to better understand the problem and to minimize its deleterious effects. There was a significant amount of research performed from the 1950s through the 1970s, in which the phenomenon was identified and studied in detail. Electron probe techniques were used to characterize the various elements involved and their relationship. These studies basically confirmed what had been assumed previously. Gertsman and Tardif in 1952 [19] catalogued various situations in which copper present in steels resulted in surface cracking. The cracking was worse if tin of any significant amount was present in the steel. They were the first to identify that the oxidizing atmosphere is a major influence on the problem. Later, Melford in 1962 [20] confirmed that specific residual elements caused hot shortness. Tin was found to make the situation worse, and nickel proved to be beneficial.

He stated that residual elements are catastrophic and developed an empirical equation for the amounts of those residual elements that prevent hot shortness. Nicholson and Murray in 1965 [21] investigated the influence of soaking time and

atmosphere (varying oxygen content, water vapor, and SO2 gas) by means of bend testing. They found for the first time a critical temperature at which the surface cracking was most severe. Salter in 1966 [22] determined the ternary phase diagrams of mild steel with Cu- Ni, Cu-Sn, and Cu-Sb. They demonstrated the existence of the liquid and solid phases in these ternary type systems. Other research studies performed in the late 1960s and in the 1970s focused on the diffusion of copper in steels. These studies attempted to provide a more precise description of the predominant mechanism for hot shortness via copper diffusion. These studies make clear that the diffusion of copper in steel at high temperatures is principally along grain boundaries. The studies also provide a more precise estimate of the solubility of copper in steel at high temperatures. An important conclusion resulting from these studies is that the diffusivity of copper increases considerably with increasing temperature [23-27]. In the late 1990s, there was a renaissance in the research on hot shortness in copper-containing steels. The Journal of the Iron and Steel Institute of Japan International (ISIJ International, Vol. 37, (1997), No.3) dedicated a complete issue of their publication to the copper-iron system. The studies presented in this issue of the journal show a new direction in the attempt to solve the problem via processing strategies rather than alloy modification. Imai, Komatsubara, and Kunishige [16, 17] provided a detailed description of the metallic compounds found in the scale, scale/metal interface and in the metal. They measured the contents of the elements in those compounds and identified the phases from the appropriate ternary phase diagrams. They state that the amount of nickel necessary to prevent hot shortness is half of the copper content. This contrasts with what was

13 published 20 years previously, which indicated the amount of nickel should be the same as that of copper to prevent hot shortness. In addition, they found no surface cracking in the temperature range of 1200-1300 ºC, with or without nickel. They also indicated that the surface produced by oxidation at high temperatures was very uneven. The reason attributed for the absence of cracks at these high temperatures is the fast diffusion of copper in austenite. The diffusivity of copper in iron is five times greater at 1200 ºC than at 1100 ºC. In the same issue of Journal of the Iron and Steel Institute of Japan International, Seo, et al. [18], investigated the surface hot shortness in 0.1% C - 0.5% Mn steels containing 0.5% Cu, by means of tensile testing at high temperature. They drew the following conclusions: • At 1100 ºC, addition of 0.4% Si or 0.02% P was effective in decreasing the susceptibility to surface hot shortness, although the oxidation rate was increased as a result. • At 1100 ºC, addition of 0.4% Si decreased the amount of copper-enriched liquid phase at steel/scale interface. Internal oxidation of silicon is a contributing factor to this decrease in the amount of the copper-enriched phase. The addition of 0.02% P seemed to increase the amount of copper- enriched phase slightly. • At 1200 ºC, the susceptibility to surface hot shortness in all steels deceased compared with 1100 ºC. • At 1200 ºC, in all steels, the oxidation rate was much higher than at 1100 ºC, but the amount of copper-enriched phase at steel/scale interface was reduced compared with that at 1100 ºC. Other research studies focused on the strain rate dependence of copper embrittlement in steels (Suzuki [28]) and the influence of copper and tin on the hot ductility in steels (Matsuoka, et al. [29])

The remainder of the journal issue addresses subjects related to copper in iron such as, scale properties, surface science and technology, phase transformation and microstructure, and mechanical properties of copper-containing steels.

1.4 Opportunities for the Present Study

It is clear that the motive for these recent research studies is the need to find an economically viable solution for deforming steels with residual elements at high temperatures without cracking. Because of the economic factors, it would desirable to provide a solution focused on processing changes rather than alloy modification. From the previous studies, it seems that shorter heating times, controlled atmospheres in the furnaces, or higher working temperatures are some of the options worthy of further investigation. With the increased usage of scrap steel, a more environmentally friendly operation, copper residuals in forging steels are increasing. With the new forging processing techniques that utilize induction heating, the heating times are reduced by one order of magnitude as compared to heating of the billet in a gas-fired furnace. Presently, there is an opportunity to study the effect of this reduction in heating time for forging steels that contain larger amounts of copper residuals. If it is found that with the new processing techniques, forging steels can handle higher amounts of copper, without hot shortness occurring during hot forging, it would represent substantial economic savings for both the steel suppliers and the steel forging industry.

2.0 INDUSTRIAL RELEVANCE AND PROJECT OBJECTIVES

In the last few years, the use of electrical furnaces to produce steel has increased considerably. Feeding the electrical furnaces with scrap steel results in significant savings. This method is also considered a conscientious practice that promotes the recycling of existent materials. The use of scrap steel in the steelmaking consumes only 1/6 of the energy consumed by steelmaking from iron ore. Use of electrical furnace steelmaking is the primary reason for the increased amount of residuals in the past 30 years, and these residuals are predicted to continue increasing. Significant amount of scrap steel comes from automobiles and machines in general; these scrap sources often contain copper wiring. So as the recycling of steel objects continues, the amount of residual elements, such as copper, will continue to increase. The forging industry has established rigorous restrictions governing residual elements, and in particular to residual copper. Presently 0.25% Cu is a maximum for some companies, and sometimes the restriction is as low as 0.20% residual copper. If copper is present, there is a minimum nickel content of at least half of the amount of copper required to ensure hot shortness will not occur. These restrictions were established during the 1960s and 1970s, when the process techniques used in the forging industry differed from those presently employed. During that period, the use of open-air furnaces for reheating billets to the forging temperature was common. This type of heating resulted in extended soaking times, and a significant amount of surface oxidation occurred. Hot shortness is directly related to oxidation. Because the heating method was fixed, there was little opportunity to approach the problem by modifying processing conditions.

At present, forge shops are changing their process techniques. They often use induction-heating equipment to heat the billets. In addition to being a more efficient process, heating times are also reduced considerably. As reheating times are reduced, there is less opportunity for large amounts of surface oxidation. Since steels do not experience long periods of time at high temperatures, oxidation is minimized and hot shortness could be controlled or even prevented. At the actual working temperatures (around 1260 ºC) it is reported that no cracking as a result of hot shortness occurs. Because of the induction heating process and improved temperature-control schemes in many forge shops, specification and attainment of the specified temperature is much more consistent than in previous years. These two changes (short heating times and better temperature control) in the present day process techniques could be sufficient to allow changes in the residual copper restrictions. These changes might allow a higher level of copper residual in the forging steel or less reliance on nickel additions to prevent hot shortness. It should not be overlooked that decreasing the restrictions for residual copper would represent economic savings, as well as being an environmentally friendly practice. The objective of the present thesis project is to investigate the hot shortness phenomenon caused by copper residuals in 1045 forging steel. The most important parameters for hot shortness are oxidizing temperature and time. By testing at different temperatures and times, it is intended to find relationships between these parameters and how they affect the phenomenon. With shorter heating and oxidation times, the actual forging conditions are simulated, so the steels can be observed under conditions similar to present day forging practice. Finally, if the steels with high copper content do not exhibit hot shortness behavior at conditions similar to the ones seen in actual industrial practice, the forging industry should be able to modify copper residual restrictions in forging steels without the surface cracking caused by hot shortness.

3.0 EXPERIMENTAL PROCEDURES

3.1 Experimental Steels

The steel selected for this project is 1045, because of its common chemistry and common utilization. Eight different steels were provided with different quantities of residual copper. Five steel heats with residual copper contents that ranged from 0.21% to 0.39% were provided by Chaparral Steel in the form of round bars 0.91 m (3 ft) long in several diameters ranging from 25.4 mm to 42.9 mm (1.0 in to 1.688 in). Three additional steels were provided by North Star Steel Minnesota with residual copper contents ranging from 0.09% to 0.32% in round bars 0.91 m (3 ft) long, with diameters of 25.4 mm (1in) and 31.8 mm (1.25 in). Table 3.1 presents the chemical compositions of the steels investigated in the present study. All the steels possessed a typical air-cooled, ferrite-pearlite microstructure. The average hardness of all the steels was 57 HRA.

3.2 Testing Methods

3.2.1 The Gleeble

The Gleeble 1500 is a thermo-mechanical servo-hydraulic testing device, designed to simulate metalworking processes as well as other metal processing operations. Electrical resistance heats the Gleeble sample, with a current passing directly though the specimen. To maintain cold grips, cooling liquid flows through them. The heating capacity of the Gleeble can reach 10,000 °C/s in a 6 mm diameter specimen. It has a chamber that allows testing in vacuum or in other atmospheres (Ar, He, etc.). With

18 the use of helium it is also possible to simulate quenching. Figure 3.1 presents a picture of the Gleeble equipment.

Table 3.1 Chemical composition of the experimental steels

Steel Element 09 21 20 25 30 32 35 39 C 0.43 0.44 0.44 0.44 0.44 0.43 0.46 0.45 Mn 0.77 0.74 0.74 0.71 0.72 0.69 0.81 0.80 P 0.011 0.014 0.004 0.009 0.008 0.01 0.013 0.014 S 0.017 0.024 0.032 0.0193 0.0237 0.027 0.027 0.0153 Si 0.25 0.17 0.24 0.28 0.23 0.18 0.26 0.24 Cu 0.09 0.21 0.20 0.25 0.30 0.32 0.35 0.39 Ni 0.07 0.08 0.08 0.08 0.09 0.11 0.11 0.10 Cr 0.14 0.14 0.10 0.10 0.11 0.14 0.16 0.16 Mo 0.03 0.03 0.023 0.023 0.02 0.03 0.026 0.024 Sn n/a 0.014 0.007 0.009 0.009 0.014 0.01 0.011 V 0.027 0.026 0.023 0.024 0.027 0.025 0.027 0.028 Nd 0.01 n/a n/a n/a n/a n/a n/a n/a Al 0.01 n/a 0.003 0.004 0.003 n/a 0.003 0.003 Ti 0.01 n/a 0.002 0.002 0.002 n/a 0.002 0.002 N n/a n/a 0.007 0.01 0.009 n/a 0.009 0.008 As n/a n/a 0.005 0.006 0.004 n/a 0.006 0.006 B n/a n/a 0.0005 0.0009 0.0002 n/a 0.0003 0.0002 Ca n/a n/a 0.001 0.0012 0.0015 n/a 0.0016 0.0012 Sb n/a n/a n/a 0.0003 0.002 n/a n/a n/a Zn n/a n/a n/a 0.001 0.001 n/a 0.001 0.001

The servo-hydraulic system has a capacity of 10,000 kgf, and can go as fast as 1 m/s. Cross speeds of 0.6 m/s were achieved with cylindrical samples 6 mm in diameter and 14 mm long. This speed represents an axial engineering strain rate of about 40 s-1, approaching the strain rates imparted during forging. The Gleeble programming language (GPL) software allows control of the grip movement, applied force, circumferential strain, and heating and cooling rates. All these characteristics make the Gleeble testing device an excellent tool to simulate many metalworking processes.

Figure 3.1 Gleeble testing equipment.

3.2.2 Testing Specimens

To simulate forging a compression test is the most appropriate type. Three different specimen geometries were evaluated: cylindrical, SICO (stress induced crack opening), and pre-bulged flanged sample. The cylindrical specimen was eliminated as an option because of its limited deformation and reduced circumferential strain compared to the other two. The SICO test, a standard test technique used with the Gleeble to simulate deformation at high temperatures, uses cold grips to create a temperature profile with a hot zone of 10 mm at the desired temperature. Compression of the sample deforms only

20 the heated zone. The results for these types of tests were satisfactory but the geometry of the deformed specimen is more complex to analyze and to handle after deformation. Figure 3.2 shows two SICO samples after deformation.

10 mm

10 mm Figure 3.2 SICO test specimens, non oxidized at the top and oxidized at 1000 °C for 24 hrs at the bottom, both deformed at 1135 °C.

The flanged specimen is a pre-bulged specimen that has a larger diameter in the center to enhance the circumferential strain. Hot shortness occurs when tensile stresses are present at the surface. The flanged specimen is designed to augment the tensile strain on the surface, making it the most adequate specimen geometry for the present study. The size of the specimen was designed within the capabilities of the Gleeble testing device, to obtain a minimum of 0.5 true circumferential strain and to obtain the appropriate dimensions in the deformed specimen for subsequent analysis. Figure 3.3 presents the diagram of the flanged specimen, and Figure 3.4 shows an actual specimen utilized during the analysis.

Figure 3.3 Diagram of the flanged specimen (dimensions in [mm]).

Figure 3.4 Pre-bulged flanged test sample.

3.3 The Test Matrix

The main variables affecting hot shortness are deformation temperature and time of oxidation. The test matrix was designed taking these two parameters into consideration. First, varying the temperature of oxidation and deformation to find the critical temperature at which most cracking occurs. Then varying the oxidation time at that critical temperature to observe how the cracking is affected by oxidation. The ranges of temperatures are from 1100 to 1200 ºC the range at which hot shortness is reported to occur and to become critical. The lower range (1100 ºC) is just above the melting point of copper. Although hot shortness has been reported to occur at lower temperatures, it is irrelevant to test at lower temperatures since it is further from the actual working temperatures of 1045 steels. The higher portion of the testing range is close to the limit of the thermocouple used in the Gleeble and is also above the reported critical hot shortness temperature [16-18]. The oxidation times are 10 and 30 minutes were used in determining the critical temperatures. Oxidation times of 1, 3, 5, and 7 minutes were used for the induction heating simulations. Figure 3.5 presents a diagram of the times and temperatures of the test matrix.

1220

1200

1180 ] C °

e [ 1160 ur

t by temperature for 10 minutes by times at critical temperature era 1140 Temp 1120

1100

1080 024681012 Time [minutes]

Figure 3.5 Diagram of the test matrix.

From this test matrix setup, the testing is separated into two stages: stage one and stage two.

3.3.1 Stage One Testing

In the first stage of the testing, the steels were oxidized at different temperatures for 10 and 30 minutes, temperatures were 1100, 1120, 1140, 1160, 1180, and 1200 ºC. The steels were oxidized inside a furnace in air. After the oxidation treatment, the samples were air-cooled. Before reheating, the excess scale was removed using a wire brush. The wire brushing was performed to help identify the cracking after testing. The samples were then heated in the Gleeble at a rate of 10 ºC/s, held at the testing

temperature for 30 seconds to stabilize, and deformed at the highest grip speed that resulted in a circumferential strain rate of 15 s-1. The main objective of these tests was to determine the critical temperature at which the cracking is most severe. The critical temperature is defined as the temperature that exhibits the largest amount of surface cracking. Figure 3.6 illustrates the thermal cycle for stage one testing.

1400

at the Gleeble frunace heated test temperature 1200 deformed

1000 ) C

e ( 800 ur at er

p 600 air cooled

m slow quench Te

400

200

wire brush cleaned

0 0 2 4 6 8 101214161820 Time (minutes)

Figure 3.6 Thermal cycle used during stage one testing.

3.3.2 Stage Two Testing

In stage two testing, the steels were tested at the critical temperature found in stage one, varying oxidizing times. The focus of this stage of testing is to study and understand how oxidation time affects cracking. In this stage, the samples were oxidized directly in the Gleeble in the as-machined condition. There was no intermediate cooling

25 step. Samples were heated at a rate of 10 º C/s, then held at the testing temperature for 1, 3, 5, and 7 minutes, and then deformed at a circumferential strain rate of 15 s-1. This stage simulated the conditions found in a forging practice that uses induction heating. For example, a steels oxidized at 1150 ºC for 1 minute, are exposed to 3 minutes of heat approximately, a time similar to what a 38.1 mm (1.5 in) diameter bar would experience in standard forging practices. Figure 3.7 illustrates the thermal cycle of the specimens tested in stage two.

1400

test temperature deformed 1200

1000 C]

e [° 800 r

eratu air cooled

p 600 m e T 400

200

0 02468101214161820 Time [minutes]

Figure 3.7 Thermal cycle used during stage two testing.

3.3.3 Other Tests

Finally, two steels with maximum amount of residual copper (0.39 and 0.35 % copper content) were oxidized for one minute at 1200 ºC, (with heating rate of 10 ºC/s). 1200 ºC is the maximum temperature of the test matrix. This test was performed to

compare the results of the steels with low residual copper content deformed at their critical temperatures to steels with high residual copper content deformed at a high temperature. It should be noted that 1200 ºC is closer to the actual working temperatures in forging operations.

3.4 Surface Crack Measuring Techniques and Analysis

3.4.1 Hot-Shortness Cracking

Hot-shortness cracks develop because of surface tensile stresses. As the pre- bulged sample is compressed the central section with the larger diameter expands and imparts circumferential strain. This strain causes the surface tensile stress and cracking occurs. In most of the samples the cracking is distributed in groups, instead of being uniformly distributed around the whole circumference. This non-uniform distribution of the cracks could be attributed to a localized concentration of liquid copper, which when the sample is deformed causes localized cracks and the development of more strain in the local regions. The other factor contributing to the non-uniform distribution could be a slight asymmetry of the sample, resulting in non-uniform strain distribution. Figure 3.8 shows an example of the crack distribution of a common sample. Although cracks developed all along the height of the sample, a cut through the center of the sample provides a good representation of the cracking of the sample. One concern was that decarburization during the oxidation step could affect the results, because this is also known to produce cracking. All steels had very similar composition except for the copper content. The observed cracking was consistent with the increase in copper rather than correlating with oxidation time. This correlation indicates that if there is a decarburization effect, it is not significant for the focus of the project.

Group of Cracks 4 mm

1 mm

In this study extensive characterization and description of the cracking it self was beyond the scope of the work. The experimental efforts were focused on the

28 measurement and quantification of the cracks. It was felt that quantification of the cracking would help in establishing clear relationships between the cracking and the oxidation time, cracking and oxidation temperature, and cracking and the copper content of the steel. Figure 3.9 shows a micrograph of the cracks from a section through the middle of the sample (to illustrate how the cracks were measured to get the Crack Index), and a SEM image of the cracking surface.

3.4.2 Sample Mounting and Preparation.

As seen in the figures the cracks develop perpendicular to the circumference. One way to measure the amount of cracking was to section the sample along the circumference, perpendicular to the cracks. Examination of this sectioned surface allowed counting and measuring of the width and depth of each crack that falls on the cut. Making the assumption that the cracks are uniformly distributed across the center of the circumference and making a careful cut right at the center of the sample, the measurement provides a good sampling of the cracking condition.

200 µm (a)

(b) Figures 3.9 (a) micrograph of steel with 0.35% copper, oxidized at 1160 °C for 10 minutes, showing large hot-shortness cracks (steel sample is on top, epoxy mount is below), (b) SEM image of the crack surface of steel 0.39% copper, oxidized at 1135 °C for 10 minutes showing cracks as large as 300 µm.

Each sample was mounted in clear epoxy (32mm in diameter) and then sectioned in the center. This allowed analysis of two sides for each test specimen. Figure 3.10 provides an illustration of the mounting and sectioning process.

Epoxy mounting

Top view

Figure 3.10 Diagram of mounting and sectioning procedure. (The mounts are standard 1.25” diameter mounts).

The crack measuring was taken automatically using image processing techniques. To accomplish this, an image of the specimen was captured in a digital photograph. Since the only relevant information of the test specimen was the surface morphology, taking advantage of the clear epoxy, a contrast image via digital image processing was obtained. The contrast image revealed only the circumference with the cracking information. Figure 3.11 shows a sample of one of the contrast images.

hot-shortness cracks

5 mm

Figure 3.11 Contrast image used in measuring and analyzing surface cracks.

Each image contains more than 3 million pixels. The resolution of each pixel is around 20 µm, this puts a limit to the minimum crack size that can be identified and measured. To eliminate noise on the measurements, the depth of an identified crack had to be larger than a threshold value of 40 µm (or 2 pixels). As a result, the minimum crack size measured was 40 µm deep and 20 µm wide.

3.4.3 Crack Measurement Techniques

All digital processing was performed using Image J software developed by the National Institute of Health (NIH). Once the contrast image was produced from the digital picture, the brightness and the contrast was adjusted to obtain a clear circumferential silhouette. Every point on the circumference was stored in a data file. These data were then processed to produce a radius profile. The radius profile contains information about the cracking. A measuring routine was performed on the radius profile. The routine counted each crack and provided measurements for each crack. From the measurements statistical results were obtained. Appendixes A and B explain the details of the measuring routines step by step. Appendix A is a block diagram and Appendix B provides details with actual examples for each step. The computerized routine is able identify any crack with a depth larger than an established threshold value, in the case of the present measurements the threshold is 40µm. Once a crack is identified, length and its maximum depth is registered. For comparison of the measurements a unit called crack index was define. Crack index is the sum of the product of the length by the depth of each identified crack. The crack index calculation assumes that the crack can be represented by a rectangular shape. Figure 3.12 shows that this is a reasonable assumption. The crack index has units of mm2 and provides a good representation of the surface cracking conditions. The crack index accounts for both the size and the number of cracks on the surface. Figure 3.12 shows an example of how cracks are identified, measured, and counted.

1 2 3 4

200 µm

Figure 3.12 Crack identification and measuring for obtaining the crack index. (Steel with 0.35% Cu, oxidized at 1160 °C for 10 minutes).

4.0 RESULTS

The results of the present work are separated in two parts corresponding to the two testing stages. For each stage, the results are organized by steel and are presented in order of decreasing copper content beginning with Steel 39 (0.39% Cu), with most copper, and ending with Steel 09 (0.09% Cu), with least copper content. For each of the steels, a group of photographs is presented to show the surface cracking. Following the photographs of the steels, a compilation of the crack index measurement is shown. Although for most conditions in the test matrix two samples were tested, only one image from the two is shown. A representative and clear image was chosen in each case. The photographs give the reader a clearer illustration of the cracking involved in hot shortness. The crack index is a numerical measurement that helps to compare the cracking for the different steels and different processing conditions. It is important to mention that the number presented as crack index takes into account the total number of cracks, and the depth and length of each crack. The crack index is the sum of the product of the depth by the length of each crack, as determined by the automatic measuring process. It provides a quantitative measure of the amount of surface cracking that occurs in the sample. On occasion, crack measurements could appear to deviate from the actual cracking present in the sample. The deviation resulted from the measuring technique when the sectioning of the sample was not precise, or the cracking was not uniformly distributed around the middle section of the circumference, or the sample presented a non-symmetrical deformation causing the sectioning of the sample to be off center. Nevertheless, in the majority of cases, the result of the counting is consistent with that observed in the photographic images.

4.1 Stage One Results

4.1.1 Oxidation for 10 Minutes

Figures 4.1 to 4.9 present the results of the stage one testing, where steels were oxidized for 10 minutes and deformed (Refer to Chapter 3 experimental procedure to review the thermal cycle). Figures from 4.1 to 4.8 show the surface photographs of the samples, and the crack measurements are shown in Figure 4.9. Figure 4.1 presents the photographs for Steel 39 (0.39% Cu). The photographs illustrate that the cracking is present at almost all the temperatures, but it is minimal for 1100 and 1200 °C, and it is more pronounce between 1140 and 1160 °C. In the crack index plot (Figure 4.9 (a)) the maximum point is very clear at 1160 °C. The curve is lower at 1100 and 1180 °C. The crack index measurement at 1200 °C was not a representation of the actual cracking shown in the image, resulting in a high crack index value. For this sample, the surface oxide caused the image to have more shadowing, which affected the quality of the image and resulted in an increase of the crack index value. Steel 35 (0.35% Cu) is presented in Figure 4.2. Similar to Steel 39, a minimal cracking is seen at 1100 °C, a maximum around 1140 to 1160 °C and the cracking is reduced again at 1200 °C. Also, the size of the cracking, as determined by the crack index value, is comparable to the cracking found in Steel 39. The crack index plot shows the highest point at 1160 °C, and the minimum at 1200 °C (Figure 4.9 (b)). Figure 4.3 shows Steel 32 (0.32% Cu). From the photographs it is evident that cracking is present from 1100 to 1160 °C, with maximum amount of cracking at 1140 °C. The crack index plot supports these observations, also showing a maximum at 1140 °C and a minimum at 1200 °C (see Figure 4.9 (c)). Figure 4.4 presents Steel 30 (0.30% Cu). The photographs show cracking only at temperatures from 1100 to 1160 °C. The maximum amount of cracking is found between

1120 and 1140 °C. The crack index plot shows a maximum at 1140 °C, with lowest values at 1100, 1180 and 1200 °C (Figure 4.9 (d)). Figure 4.5 shows the photographs for Steel 25 (0.25% Cu). Noticeable cracking is seen at 1100, 1120 and 1140 °C. The critical cracking temperature seems to be present at 1100 °C. The crack index plot shows a maximum at 1120 °C (Figure 4.9 (e)). The minimum has shifted to the high temperatures, 1180 and 1200 °C Steel 21 (0.21% Cu) is shown in Figure 4.6. Noticeable cracking is present only at 1100, 1120 and 1140 °C, similar to Steel 25. The crack index plot shows a maximum at 1120 °C (Figure 4.9 (f)), but the peak is much smaller than in Steel 25. For other temperatures the crack index value is small. Figure 4.7 presents the results for Steel 20 (0.20% Cu). For this steel the maximum cracking occurs at 1100 °C, and the cracking diminish as with increasing temperature. The crack index plot also illustrates the same trend (Figure 4.9 (g)). Figure 4.8 presents the photographs for Steel 09 (0.09% Cu), the lowest copper content steel in the present investigation. Only very small cracks are visible at 1100 °C. The crack index plot does not show any consistent trend, but only reflects the roughness produced by the oxidation (Figure 4.9 (h)).

38 m 2 m d C C es an r 1140° 1200° temperatu t fferen i d C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 39 (0.39% Cu 1100° 1160° Figure 4.1

39 m 2 m C C d es an r 1140° 1200° temperatu t fferen i d C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 35 (0.35% Cu 1100° 1160°

Figure 4.2

40 m 2 m C C d n a s e ur t 1140° 1200° a r mpe e t nt e r e f f di C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 32 (0.32% Cu 1100° 1160°

Figure 4.3

41 m 2 m C C d es an r 1140° 1200° temperatu t fferen i d C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 30 (0.30% Cu 1100° 1160°

Figure 4.4

42 m 2 m C C d es an r 1140° 1200° temperatu t fferen i d C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 25 (0.25% Cu 1100° 1160°

Figure 4.5

43 m 2 m C C d es an r 1140° 1200° temperatu t fferen i d C C 1120° 1180° oxidized for 10 minutes at C C Steel 21 (0.21 %Cu) deformed. 1100° 1160°

Figure 4.6

44 m 2 m C C d es an r 1140° 1200° temperatu t fferen i d C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 20 (0.20% Cu 1100° 1160°

Figure 4.7

45 m 2 m C C d n a s e ur t 1140° 1200° a r mpe e t nt e r e f f di C C 1120° 1180° ) oxidized for 10 minutes at C C deformed. Steel 09 (0.09% Cu 1100° 1160°

Figure 4.8

Figure 4.9 Crack Index measurements of steels a) 39, b) 35, c) 32, and d) 30, oxidized for 10 minutes and deformed.

Figure 4.9 Crack Index measurements of steels e) 25, f) 21, g) 20, and h) 09, oxidized for 10 minutes and deformed.

4.1.2 Oxidation for 30 Minutes

Figures 4.10 to 4.18 present the results of the steels oxidized for 30 minutes at different temperatures and deformed. Figures 4.10 to 4.17 show the surface cracking of each steel, and Figure 4.18 present the crack index measurements.. Again the order of the figures follows the copper content, beginning with Steel 39 (0.39%), the highest, and ending with Steel 09 (0.09%), the lowest. Steel 39 (0.39% Cu) is presented in Figure 4.10. The photographs show cracking at all temperatures, maximum at 1140 °C and minimal at 1200 °C. The crack index measurements show a prominent maximum at 1140 °C, with high values at 1120, 1160, and 1180 °C, and a very small value for 1200 °C (see Figure 4.18 (a)). Figure 4.11 presents the photographs for Steel 35 (0.35% Cu). The photographs do not show much cracking, however there are a few clear cracks at 1120 and 1140 °C. The crack index plot shows a clear maximum at 1140 °C (Figure 4.18 (b)), and lower values at the other temperatures. It is important to note that the crack index values for all temperatures are very low compared to the same steel oxidized for 10 minutes. Steel 32 (0.32% Cu) is presented in Figure 4.12. Cracking seems to be present at all temperatures from 1100 to 1180 °C, with a maximum at 1160 °C. The crack index plot does not show a clear trend, with some high points at 1140, 1160, and 1180 °C, and even at 1200 °C. Nevertheless, the crack index values remain low at all temperatures (Figure 4.18 (c)). Figure 4.13 shows Steel 30 (0.30% Cu). The photographs indicate some cracking only at 1100 and 1140 °C. The crack index plot shows no clear trend, the high point at 1180 °C could be the consequence of a poor quality picture, causing extra counts (Figure 4.18 (d)). Ignoring that point, the maximum would be found at 1140 °C. Figure 4.14 presents the photographs of Steel 25 (0.25% Cu). Cracking is noticeable between 1100 and 1160 °C only. The crack index shows the same features

49 with a maximum at 1100 °C, and a minimum at 1200 °C, but no clear trend can be drawn from this (Figure 4.18 (e)). The crack index levels are slightly higher than the rest of the steels. Steel 21 (0.21% Cu) is presented in Figure 4.15. There is clear cracking at 1100 and 1120 °C shown in the images. The crack index only shows a maximum at 1140 °C (Figure 4.18 (f)), and low values at every other point. This maximum point at 1140 °C does not significantly differ from any other values. Steel 20 (0.20% Cu) is shown in Figure 4.16. There is hardly any cracking produced by hot shortness shown in this image. The large cracks at 1140 and 1160 °C are likely to be oxide cracks and not metal cracks. This feature correlates with the crack index presented in the plot (Figure 4.18 (g)). The high value of the sample oxidized at 1200 °C is a consequence of roughness from the oxide formed and not due to cracking by hot shortness. Finally, Steel 09 (0.09% Cu) is presented in Figures 4.17. No cracks appeared at any temperature, however, one can observe from the image the difference between the oxides that form at the different temperatures. A rougher and more granular oxide is seen at 1200 °C, and a fine, film-like oxide is found at lower temperatures. The low temperature oxide also flakes off easily. The crack index plot only represents the value of the surface roughness of the samples (Figure 4.18 (h)).

4.1.3 Determination of Critical Temperature.

Critical temperature is defined as the temperature at which the most cracking occurs. For the samples oxidized for 30 minutes and deformed, there is no variation of the critical temperature, which seems to occur around 1140 °C. The samples oxidized for 10 minutes and deformed have different critical temperatures for the different copper contents. The maximum critical temperature resulted from Steel 39, the highest copper

50 content (0.39% Cu), between 1140 and 1160 °C, and the critical temperature decreased with the copper content. The critical temperatures for stage two testing were selected from visual observation of the surface photographs of the deformed samples. Some temperatures were selected between the actual test temperatures, due to cracking trends. Table 4.1 presents the critical temperatures selected as test temperature for stage two testing.

Table 4.1 Critical temperatures selected for stage two testing.

Cu content Steel (% by weight) Critical temperature (°C) 39 0.39 1150 35 0.35 1145 32 0.33 1135 30 0.3 1130 25 0.25 1115 21 0.21 1115 20 0.19 1110 9 0.09 1100

51 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 39 (0.39% Cu 1100° 1160°

Figure 4.10

52 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 35 (0.35% Cu 1100° 1160°

Figure 4.11

53 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 32 (0.32% Cu 1100° 1160°

Figure 4.12

54 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 30 (0.30% Cu 1100° 1160°

Figure 4.13

55 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 25 (0.25% Cu 1100° 1160°

Figure 4.14

56 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 21 (0.21% Cu 1100° 1160°

Figure 4.15

57 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at t e l C C e no deformed. Steel 20 (0.20% Cu ailab 1100° 1160° av Imag

Figure 4.16

58 m 2 m C C 1140° 1200° different temperatures and C C 1120° 1180° ) oxidized for 30 minutes at C C deformed. Steel 09 (0.09% Cu 1100° 1160°

Figure 4.17

Figure 4.18 Crack index measurements of steels a) 39, b) 35, c) 32, and d) 30, oxidized for 30 minutes and deformed.

Figure 4.18 Crack Index measurements of steels e) 25, f) 21, g) 20, and h) 09, oxidized for 30 minutes and deformed.

4.2 Stage Two Results

The results of the stage two testing are presented in Figures 4.19 though 4.26. In this testing stage, the samples were oxidized and deformed at the critical cracking temperature. The critical temperature was selected from the results of the stage one tests, for the samples oxidized for 10 minutes. Figures 4.19 to 4.25 present the photographs of the surface cracking of the samples for each steel, and Figure 4.26 present the crack index measurement plots, as a function of time Figure 4.19 presents the results of Steel 39 (0.39% Cu) deformed at 1150 °C. In the photographs one can see that the cracking is extensive for oxidation time of 1 minute. The cracking decreases and then appears to slightly increase again at 7 minutes of oxidation. The crack index plot shows a high point at 1 minute and then levels at lower values for longer times (Figure 4.26 (a)). Steel 35 (0.35% Cu) deformed at 1145 °C is shown in Figure 4.20. Cracking is noticeable for 1 minute oxidation time, and then diminishes for the rest of the times. The crack index plot for this steel does not follow any trend (Figure 4.26 (b)). The two samples tested at each time produced very broad results. Figure 4.21 presents Steel 32 (0.32% Cu) deformed at 1135 °C. The photographs clearly show that the cracking is prominent at oxidizing times for 1 minute, then cracking disappears at 3, 5, and 7 minutes. The crack index plot reflects the same behavior with oxidation time of 7 minutes starting to show an increase (Figure 4.26 (c)). This observation is significant because the same trend was observed in the next two steels. Steel 30 (0.30% Cu) deformed at 1130 °C is presented in Figure 4.22. Again the cracking is clearly present for the oxidizing time of 1 minute, absent for 3 and 5 minutes, and appears again at 7 minutes. The crack index plot shows maxima at 1 and 7 minutes, and a minimum at 3 minutes, very similar to that seen in the images (Figure 4.26 (d)).

Figure 4.23 shows the results for Steel 25 (0.25% Cu) deformed at 1115 °C. In the photographs, it appears that cracking occurs at all times, slightly larger at 5 minutes oxidation time. The crack index plot shows a very clear minimum for 1 minute, and then increases for times of 5 and 7 minutes (Figure 4.26 (e)). Steel 21 (0.21 % Cu) deformed at 1115 °C is shown in Figure 4.24. There is clear cracking for oxidation times of 1 and 3 minutes, the cracks are very small and do not extend for the entire height of the sample. The crack index plot does not indicate any trend (Figure 4.26 (f)). All of the values are low. Figure 4.25 shows the results of Steel 20 (0.20% Cu) deformed at 1110 °C. No major cracking can be seen in the images. The small cracks for 1 and 5 minutes can be assumed to be oxide cracks, rather than metal cracks. The crack index plot only shows very low values for all the oxidation times (Figure 4.26 (g)). For Steel 09 (0.09% Cu) deformed at 1110 °C shown in Figures 4.27 presents only one sample oxidized for 1 minute, with no cracking at all. The value of the crack index is the lowest of all of the steels: 0.01mm2.

63 m 2 m n n i i 3 m 7 m erent times and deformed. ) oxidized at 1150°C for diff n n i i 1 m 5 m Steel 39 (0.39% Cu

Figure 4.19

64 m 2 m n n i i 3 m 7 m erent times and deformed ) oxidized at 1145°C for diff n n i i 1 m 5 m Steel 35 (0.35% Cu

Figure 4.20

65 m 2 m n n i i 3 m 7 m erent times and deformed ) oxidized at 1135°C for diff n n i i 1 m 5 m Steel 32 (0.32% Cu

Figure 4.21

66 m 2 m n n i i 3 m 7 m erent times and deformed ) oxidized at 1130°C for diff n n i i 1 m 5 m Steel 30 (0.30% Cu

Figure 4.22

67 m 2 m n n i i 3 m 7 m erent times and deformed ) oxidized at 1115°C for diff n n i i 1 m 5 m Steel 25 (0.25% Cu

Figure 4.23

68 m 2 m n n i i 3 m 7 m erent times and deformed ) oxidized at 1115°C for diff n n i i 1 m 5 m Steel 21 (0.21% Cu

Figure 4.24

69 m 2 m n n i i 3 m 7 m erent times and deformed ) oxidized at 1110°C for diff n n i i 1 m 5 m Steel 20 (0.20% Cu

Figure 4.25

Figure 4.26 Crack Index measurements of steels a) 39, b) 35, c) 32, and d) 30, oxidized at their critical temperatures for 1, 3, 5, and 7 minutes and deformed.

Figure 4.26 Crack Index measurements of steels e) 25, f) 21, and g) 20, oxidized at their critical temperatures for 1, 3, 5, and 7 minutes and deformed.

2 mm

Figure 4.27 Steel 09 (0.09% Cu) oxidized at 1110 °C for 1 minute and deformed. Notice the absence of cracking.

4.3 Special Tests

Steels 39 and 35, with the highest copper contents (0.39% and 0.35%, respectively), were tested at the highest temperature of the test matrix, 1200 °C. Samples of these steels were oxidized for 1 minute and deformed. The 1200 °C temperature is closer to the actual temperatures that the steel would see in typical forging industry conditions. The oxidation time for the sample size is also fairly realistic. Figure 4.28 shows the photographs of the surfaces of these samples, in which no cracks can be observed. The samples were analyzed to obtain the crack index, and the values are presented in Table 4.2. This crack index values are as low as the crack index values presented by steel 09 (0.09% Cu) with the lowest copper content.

Table 4.2 Crack index values of Steels 39 and 35 compare with Steel 09 Steel Cu content (wt%) Crack Index [mm²] Critical Temperatures 1140°C for 10 minutes at 1140°C for 30 minutes at 1110°C for 1 minute 9 0.09 0.28 0.25 0.01

at 1200°C for 1 minute 39 0.39 0.65 1.60 0.015 35 0.35 0.75 0.24 0.025

Steel 39 Oxidized at 1200°C For one minute and deformed 2 mm

Steel 35 Oxidized at 1200°C For one minute and deformed 2 mm

Figure 4.28 Steel 39 and 35 (0.39 and 0.35% Cu) oxidized at 1200 °C for 1 minute and deformed, showing no cracks.

4.4 Summary of the Results

In summary, the samples in stage one, oxidized for 10 minutes, showed a critical temperature for cracking. The critical temperature decreases with copper content. Also the maximum value of the crack index decreases with copper content. For the samples oxidized for 30 minutes, the critical temperature shown by the crack index measurement it is consistently found near 1140 °C. The values of the crack index are high only for Steel 39 and Steel 35 (0.39 and 0.35 % Cu). For the remaining steels, severe cracking does not occur. For the stage two testing, the trends are more complex. For the samples with high copper content, such as Steel 39, 35, 32, and 30 (0.39, 0.35, 0.32 and 0.30 % Cu, respectively) there is a maximum amount of cracking at 1 minute. As the copper content decreases, the critical temperature decreases as well, and there is an increase in cracking at longer times. This trend is seen in Steels 32, 30, and 25 (0.32, 0.30 and 0.25% Cu, respectively). For Steels 21 and 20 with the lowest copper content (0.21 and 0.20% Cu, respectively), there is minimal cracking variation for the various oxidation times. Also, the crack index values for stage two tests are significantly lower, as compared to the values determined during stage one tests. Table 4.3 presents a summary of the principal values obtained from the different test, in stage one and stage two testing.

Table 4.3 Critical Temperatures and Crack Index Values Stage One Stage Two Oxidized for 10 minutes Oxidized for 30 minutes Steel Cu content Critical Temperature max Crack Index Critical Temperature max Crack Index Critical Time wt. % [°C] [mm²] [°C] [mm²] [minutes] 39 0.39 1160 1 1140 1.8 1 35 0.35 1160 1.2 1140 0.2 1 32 0.33 1140 0.85 1140 0.2 1 30 0.3 1140 0.75 n/a 0.2 7 25 0.25 1120 0.75 n/a 0.3 7 21 0.21 1120 0.4 n/a 0.25 5 20 0.19 1100 0.5 n/a 0.25 5 9 0.09 1100 0.2 n/a 0.2

5.0 DISCUSSION

In this chapter, the notable features observed in the results section are described. Some insights and interpretations of the results are also presented. Some of these ideas are suppositions based on observation and past experience taken from the literature on copper residuals in steels. These interpretations are presented with sufficient discussion to support the analysis made herein. First, the findings of the stage one testing will be presented. Tests during this stage of the project generated the most interesting results. Secondly, the results from the stage two testing are presented. The results from this second stage are less straightforward; therefore, more interpretation is provided. Lastly, the chapter concludes by summarizing the results and interpretations tying this thesis project together. These summary ideas attempt to establish the fundamental principles that governed and control the phenomenon of hot shortness. Some industrial insights are also presented. They offer ideas of how the results from this work can be advantageously used by industry.

5.1 Stage One Testing

The samples that were oxidized for 10 minutes and then deformed showed the most interesting results. They produced a variety of crack sizes and distributions for various steels with different copper content and various test temperatures. The temperature at which cracking is most significant is called the critical temperature. The most important feature of the critical temperature is that it decreases with decreasing copper content. Figure 5.1 shows a graph of the critical temperature as a function of copper content. The crack index also decreases with copper content; Steels 39 and 35 (with 0.39 and 0.35 % Cu) presented the highest values. Figure 5.2 presents the values of

76 the maximum Crack Index value (found at the critical temperature) as a function of copper content.

Figure 5.1 Critical temperature for cracking as a function of copper content.

Figure 5.2 Maximum crack index as function of copper content.

The decrease in critical temperature with decreasing copper content is explained by the balance between enrichment and dispersion of the copper along the surface grain boundaries. Copper enrichment at grain boundaries occurs via the oxidation of the steel surface. During oxidation, copper is rejected by the oxide and moves into the steel. The oxidation process is a parabolic function of time. Hence, enrichment of copper in the steel will be a parabolic function of time. The enrichment process also depends on the amount of copper residual in the steel as well as the temperature. The greater the amount of copper in the steel, the higher the enrichment curve, since more copper is being rejected from the region being oxidized. For higher temperatures, there is a greater oxidation rate at a given time, and more enrichment occurs for a given amount of time. Figure 5.3 shows a schematic of the enrichment process as a function of two different steels, one with a low amount of residual copper and one with a high amount of residual copper. The process of enrichment is balanced by the dispersion of copper from the grain boundary region into the bulk steel. The dispersion process occurs via solid-state diffusion of copper into the primarily iron matrix. The dispersion process is a function of temperature, in the same manner that the diffusion coefficient of copper in iron depends upon temperature. The temperature dependence of the diffusion coefficient is expressed as an Arrhenius equation. As temperature increases, the diffusion of copper into iron increases in an exponential fashion. The dispersion of copper away from surface grain boundaries will have the same type of temperature variation as the diffusion coefficient. A schematic of the dispersion process as a function of temperature is shown in Figure 5.3. Figure 5.3 provides a schematic illustration of the two competing phenomena (enrichment and dispersion) as a function of temperature. There is a maximum difference between these two curves. It can be inferred that the difference between the enrichment process and the dispersion process is a measure of the copper available to accumulate along the surface grain boundaries. The dispersion process is essentially constant for the

78 steels of differing copper contents; whereas, the enrichment rate changes significantly with copper content. For lower copper steels, the maximum difference between the two curves shifts to a lower temperature. The temperature of the maximum difference corresponds to the temperature at which the maximum amount of hot shortness cracking occurs. This temperature is the critical temperature. Figure 5.3 indicates that the critical temperature decreases as the amount of residual copper in the steel decreases.

dispersion

enrichment of steel with 0.39% copper tt ] e

m enrichment of steel with 0.30% copper content ea*ti ar / ass m [

r e ansf r t

ss critical temperatures a

m shift to the left, to lower temperatures

1000 1050 1100 1150 1200 1250 1300 temperature (°C)

Figure 5.3 Schematic illustrating the competition between enrichment and dispersion of copper in grain boundaries near the surface of the specimen.

Copper enrichment of the surface grain boundaries is a direct function of the amount of copper in the steel. The oxidation rate is the same for all the steel at the same

79 temperatures, so the amount of copper is the only parameter controlling the amount of accumulation. At constant temperature the dispersion of the copper should not vary with copper content. Although the concentration gradients at the initial moment the steel begins building the oxide layer are very drastic, this produces different cracking behavior as demonstrated in the second testing stage. The size of the cracks observed in photographs and the crack index measurements for the samples oxidized for 10 minutes are much larger that at any other times, except for Steel 39, in which the maximum cracking occurred after oxidation for 30 minutes at 1140 °C. A 10-minute oxidation was able to produce enrichment zones sufficient to cause a cracking in most of the steels at different temperatures of the test matrix. The samples oxidized for 30 minutes did not exhibit a variation in the critical temperature. Steels 39, 35, and 32, with the highest copper content, showed a maximum value at 1140 °C. The critical temperature of 1140 °C agrees with the temperatures published in the literature [16, 17, 19-21]. The other five steels did not have significant cracking, and therefore did not have a critical temperature. These results indicate that all of the steels at this longer time have achieved a steady state. The amount of copper rejected by the oxide and the dispersion of the copper into the metal is stable and in equilibrium; this contributes to the fact that the crack index values are not high. For Steels 39, 35, and 32, when steady state between enrichment and dispersion is reached, some accumulation of copper at the grain boundaries occurs. This is not the case for the other steels that exhibited cracking when oxidized for 10 minutes, but not when oxidized for 30 minutes. For these steels, the steady state condition has less accumulation at the grain surface grain boundaries, which decreases the potential hot shortness cracking. For Steel 39, the maximum crack index value was found at oxidizing time of 30 minutes; whereas, all the other steels had greater crack index values after oxidizing at 10 minutes. One of the differences of Steel 39 as compared to the other steels in the test matrix is the Cu/Ni ratio, which almost reaches a value of 4. For this Cu/Ni ratio,

observing the Cu-Ni binary phase diagram, the solidus line is approximately 1160 °C. The critical temperature for this steel in the samples oxidized at 10 minutes is closer to the solidus line of the system. This observation is an indication that Steel 39 may behave differently than the others. Figure 5.4 presents the critical temperature of the steel oxidized for 10 minutes and the solidus line.

solidus line

Figure 5.4 Critical temperature for samples oxidized for 10 minutes and the solidus line of the Cu-Ni binary system.

Figure 5.4 does not explain why there is a shift in critical temperature when oxidization for Steel 39 increases from 10 minutes to 30 minutes. However, it does explain why Steel 39 presents more severe cracking no matter the oxidation time as compared to the other seven steels in the study.

5.2 Stage Two Testing

During stage two testing, the primary parameter investigated was oxidation time before deformation. The deformation temperature for each steel was chosen to be the critical temperature as determined by stage one testing (see Table 4.1). The specimens were heated in the Gleeble in air to the deformation temperature, held for various times, and then deformed. The results from stage two testing are more difficult to interpret as compared to the results from stage one. None of the eight steels exhibited severe cracking after deformation. The crack index maximums during the stage two testing series did not exceed a value of 0.5 mm2, which represents a moderate amount cracking. Not only is the measured crack index low, but photographs of the post deformation surfaces also show equivalent results. The surfaces exhibit very small cracks as compared with the specimens from stage one. One of the more peculiar results during stage two testing is the critical oxidation time and the variation of this critical time with copper content in the steel. For Steels 39, 35, and 32, (although Steel 35 does not exhibit this very well in the measurements, it is indicated in the photographs) the maximum cracking occurs with an oxidation time of 1 minute. Steels 30 and 25 show the maximum cracking at an oxidation time of 7 minutes, rather than 1 minute. There is a significant shift in critical time for steels that have a lower copper content and were tested at a lower temperature. Steel 32 and Steel 30 show a distinctive feature. Steel 32 does have maximum in the cracking index at 1 minute; this value falls to a minimum at 5 minutes and rises again at 7 minutes. Steel 30 starts with high cracking at 1 minute, falls to a minimum at 3 minutes, and rises at five and 7 minutes. In contrast, Steel 25 starts at a minimum crack index value at 1 minute, which continues rising with time. All three of these steels (35, 30 and 25) exhibit a crack index variation with time, which if extrapolated to 10 minutes of oxidation yields a crack index value that is equivalent to one found during stage one testing in which the oxidation time was 10 minutes. For steels 39 and 35, extrapolation

82 of the stage two data to 10 minutes is more difficult to rationalize, since they do not reach the same values as was found during the stage one testing at 10 minutes. Figure 5.5 shows the extrapolation of the crack index measurement for steels 35, 30, and 25. The extrapolations for steels 39 and 35 are shown in Figure 5.6, the extrapolation line for the two steels is extended to a much great amount in order to provide at consistency between the two testing stage. It is clear from Figure 5.6 that matching of the two testing stages is less consistent for Steels 39 and 35. It is difficult to provide a rational explanation for all of these behaviors. It should be noted that there are several parameters changing when stage one data are compared with stage two data. The copper content varies among the steels, which leads to variations in the enrichment rates. The testing temperature is not the same for all the steels during stage two testing but is at the temperature associate with the maximum amount of cracking found during stage one. The variation in temperature affects the diffusivity of copper in steel and hence affects the dispersion rates. The experimental oxidation method is somewhat different between the two stages of testing. Keeping in mind that for steels with lower amounts of residual copper, the critical temperature is lower and the testing temperature during stage two is less, it is understandable that there is a delay in the penetration and dispersion phenomena. It is not possible to explain precisely why this delay promotes less cracking. In order to answer that question, the test matrix would need to be expanded. It is also possible that the extra steps used during stage one as compared to stage two testing (i.e., the extra cooling and heating as well as the brush cleaning) may have influenced the test results.

Figure 5.5 Extrapolation of the crack index measurements for Steels (a) 32 , (b) 30, and (c) 25.

Figure 5.6 Extrapolation of crack index measurements plot for Steels (a) 39 and (b) 35.

Nevertheless, there are two possible explanations or interpretations of the observed data. One is that for steels with significant copper residual content, there are two critical oxidation times. The first is soon after the oxide layer forms, due to the abrupt enrichment from oxidation and rapid penetration into the grain boundaries. Subsequently the dispersion process accommodates the excess of copper along the grain boundaries evolving into a steady state situation. This would explain the behavior seen in Steels 39, 35, and 32. Steels 30 and 25 do not receive sufficient enrichment to wet the boundaries during the early oxidation times. Even when the oxide layer is growing, and enrichment is occurring, the temperature at which the test is being conducted is lower, causing the diffusion to be reduced. The reduction in diffusion may delay penetration

85 into the grain boundaries. These two effects, a slow enrichment and slow dispersion (due to slow diffusion), could explain the maximum cracking being delayed to longer oxidation times. The second explanation is that there is some other process, such as internal oxidation, occurring at the same time. More experiments and analysis would be needed to prove the occurrence of a competing process. To summarize, the samples in stage two testing did not develop large cracks. The maximum in cracking shifted from 1-minute oxidation time to 7-minutes of oxidation with decreasing copper content. Figure 5.7 is a plot of the crack index versus copper content for the different oxidation times, in which the shift of the maximum cracking can be observed.

Figure 5.7 Crack index as a function of copper content for various oxidation times.

5.3 Summary of Ideas

Hot-shortness is the result of two competing processes—copper enrichment of the grain boundaries due to oxidation of the surface layer and copper dispersion from the grain boundaries into the steel due to diffusion. The difference in these two mechanisms can lead to copper accumulation along the grain boundaries near the surface of the steel. The accumulation of copper along the grain boundaries can lead to surface cracking during deformation, which is the phenomenon described as copper hot shortness. The enrichment process is primarily controlled by oxidation of the steel and the copper residual content present within the steel. It has been implicitly assumed in the discussion section that the oxidation rate does not vary much in the range of testing temperatures as compared to the diffusivity of copper in steel. For example, a steel with 0.40% Cu is expected to produce double the copper enrichment, as compared to a steel with 0.20% Cu. There is a clear difference in the type of oxide developed at 1100 °C, as compared to the one developed at 1200 °C. The oxide formed at low temperature does not stick as tenaciously to the surface and it is more uniform and smooth. The oxide formed at 1200 °C is rougher and adheres better to the surface. The oxide at 1200 °C appears to have more penetration into the metal. The dispersion process is controlled by diffusion, and therefore is highly dependent on temperature. The concentration gradients should also play a role, but the differences in concentration between the enriched grain boundary and the bulk steel do not differ greatly for the copper contents of the steels. Therefore, the effect of the concentration gradient on the dispersion process has been assumed to be negligible. One recommendation to prevent or delay hot shortness, which has been advocated over the years, is the addition of nickel into the steel to "cover" for the copper residual content. It has not been the focus of extensive discussion in the present study because nickel contents of the steels evaluated in the present study are relatively the same (see Table 3.1). It should not be inferred from the present work that nickel content is

87 unimportant. Steel 39 (0.39% Cu) is a prime example, since it has the highest Cu/Ni ratio of the steels studied; the effects of the nickel as a crack suppressor would be the least in this steel. Steel 39 exhibited the greatest amount of cracking at its critical temperature, which is consistent with the beneficial effects of nickel on copper residual cracking.

5.4 Industrial Insights

One of the main industrial objectives of the present project was to evaluate the newer processing conditions used in modern forge shops that might allow the use of higher copper residual contents to be used for forging without the fear of major surface cracking. As such, special tests were run using Steels 39 and 35 (with 0.39 and 0.35% Cu, respectively). The steels oxidized for 1 minute and deformed at 1200 °C. They did not exhibit any cracking. The roughness of the surface of these samples were comparable to the roughness found in the steels with very low copper content, such as Steel 09 (0.09% Cu). It is believed that at such high temperatures the dispersion rate is much higher than the enrichment rate, providing less chance for copper to accumulate and wet the grain boundaries. These two results are very encouraging, since the real working temperatures for forging of 1045 steel are around 1260 °C. At industrial forging temperatures the dispersion should be even higher. If the working temperature drops below 1200 °C, and the copper content is as high as Steel 39, hot shortness could occur. It has been proven that for steels with high copper residual content there is a critical temperature at which surface cracking is a maximum. If the deformation temperature is below the critical temperature or above the critical temperature, the cracking due to hot shortness will be less.

6.0 SUMMARY

By deforming steel specimens at high temperature after varying oxidation times, forging was simulated with the intention of investigating hot shortness due to residual copper. The results from the testing performed throughout the project showed that hot shortness is the result of two competing processes: oxidation rate causing copper enrichment along the grain boundaries and diffusion causing dispersion of copper from the grain boundaries. If one understands these two processes, an appropriate production operation can be developed that minimizes or eliminates the copper residual hot shortness in steels. The two parameters controlling the accumulation of copper along the surface grain boundaries are time and temperature. By determining the correct combination of time and temperature, hot shortness can be controlled, diminished, or eliminated. An understanding of the time temperature relationships would allow the use of steels with higher copper contents to be forged successfully without surface cracking. Several plain carbon 1045 steels with copper residual contents from 0.09 to 0.39 % were tested under different conditions. First, the steels were oxidized and deformed at different temperatures, receiving an oxidation time of 10 and 30 minutes, with the intention of finding a critical temperature at which the cracking was at a maximum. Secondly, the steels were oxidized for different periods of time at their critical temperature, in an attempt to understand the kinetics of the phenomena. The steels oxidized for 10 minutes and deformed, exhibited a critical temperature at which the cracking was most severe. The experimentally determined critical temperature depended on the copper content. As the residual copper content decreased, the critical temperature decreased (see Figure 5.1). The amount of cracking was found to be a function of the copper content (see Figure 5.2). With decreasing copper content, the amount of cracking in the steel decreased.

The steels oxidized for 30 minutes and deformed did not show a significant variation in cracking with temperature. The temperature at which the cracking became critical was 1140 °C for all the steels that exhibited a maximum point; these were Steels 39, 35, and 32. The constant critical temperature suggests that the oxidation treatment of 30 minutes was sufficiently long to create a steady-state situation for the rates of enrichment and dispersion of copper along the surface grain boundaries irrespective of copper content. Steel 39 was the only sample to exhibit severe cracking when oxidized for 30 minutes. For all the other steels the cracking during deformation after 30 minutes of oxidation was well below that shown when oxidized for only 10 minutes. Results from the time variation testing (stage two) are more complicated and more difficult to explain. During these sets of tests, the steels were oxidized at their differing critical temperatures for various times. The steels with higher copper content showed a maximum amount of cracking at short oxidation times; whereas, for steels with medium copper content, the point of maximum cracking occurred at longer oxidation times. The steels with lower copper content did not exhibit severe cracking for any of the oxidation times tested. It should be noted that the critical temperature decreases with copper content, and the enrichment rate is directly related to the copper content of the steel. At the lower temperatures, the dispersion process is slow due to slower diffusion of copper into the steel. Testing of the specimens at different temperatures makes the results more complicated to interpret. The steels with higher copper content (0.39 and 0.35 % Cu) were oxidized for 1 minute and deformed at 1200 °C, the highest testing temperature in the present project. The two steels did not exhibit any cracking when oxidized and deformed at this higher temperature. These results are consistent with what would be predicted from the other tests conducted during the study in which the cracking decreased above the critical temperature. The main inference that can be drawn from this last experiment is that above the critical temperature, hot shortness defects are diminished or eliminated. The working temperatures for 1045 steels are around 1260 °C, well above the critical

91 temperature. The results from this project suggest that cracking caused by hot shortness should not occur at those temperatures for short oxidation times, for copper contents as high as 0.39%. This should be a very encouraging result for both the steelmaking and forging industries.

7.0 RECOMMENDATIONS FOR FUTURE WORK

There are several directions that could be taken in order to extend the results of the present study. Redesigning the test matrix to include the test of time variation for several temperatures (rather than critical temperature only) for oxidation times of 10 minutes or less would provide a better means to understand the short time kinetics of the hot shortness phenomenon. Tests should be taken at intervals up to 10 minutes, to provide comparison with the stage two results in the present study. Such a testing study would require numerous tests; therefore, the examination of a fewer number of steels would be wise. For example, one steel with high copper (e.g., steel 39 with 0.39% Cu), one steel with medium copper (e.g., Steel 32 with 0.32% Cu), and one steel with low copper (e.g., Steel 25 with 0.25% Cu) should be sufficient to determine the trends and behavior. Measuring the oxidation rate of the steel at several temperatures would provide a basis for the calculation of copper enrichment rates. The feature that is more complex is the measurement of copper accumulation along the steel grain boundaries. Diffusivity itself is hard to assess because it can occur along different paths within the steel. Nevertheless, it should be measured to correlate the copper dispersion process described and proposed in the present thesis. The oxidation rate of steel and diffusivity of copper in steel should be measured independently. From these measurements a quantitative model could be developed to predict the penetration and wetting of the surface grain boundaries by copper. This method may be able to predict hot shortness as a function of the processing time and deformation temperatures. With the quantitative model and the proper values for the critical parameters, a computer model could be written to attempt to predict the presence of the copper along the grain boundaries adjacent to the surface. The computer model could predict the

94 concentration of copper at the interface of the metal and scale. Such a model could also be used to examine the concentration of other elements. With the help of thermodynamic software, such as TermoCalc, the equilibrium phases for the local alloy concentrations could be predicted. Some mechanical testing of the post-forged steels with various levels of copper residual should also be performed. These tests would provide insight into the effect of copper on the post-deformation mechanical properties. It is important to assess the effect of copper residuals not just on the deformation behavior of the steel but also on the performance of the steel in situations that are closer to the actual service conditions that they would encounter as forged components.

REFERENCES

[1] Copper in Cast Steel and Iron. Copper Development Association, Thames House, Millbank, London, 1937.

[2] E.T. Stephenson, “Effect of Recycling on residuals Processing, and Properties of carbon and Low-Alloy Steels”, Metallurgical Transaction, Vol. 14A, 1983 pp. 343-353.

[3] G. H. Geiger, “Steel at the Millennium”, Advanced Materials and Processes, Vol. 159, No. 3, March 2001 pp. 63-67.

[4] I. Le May and L.M. Schetky, Copper in Iron and Steel, John Wiley & Sons, New York, NY, 1982.

[5] G.B. Worral, J.T. Buswell, C.A. English, M.G. Hetherington and G.D.W. Smith, “A Study of the Precipitation of Copper Particles in a Ferrite Matrix”, Journal of Nuclear Materials, Vol. 148, 1987, pp. 107-114.

[6] K. Osamura, H. Okuda, M. Takashima, K. Asano, M. Furusaka, K. Kishida and F. Kurosawa, “Precipitation Hardening in Fe-Cu Binary and Quaternary Alloys”. ISIJ International, Vol. 34, 1994, pp. 359-365.

[7] A. Fujii, M. Nemoto, H. Suto and K. Monma, “Precipitation Hardening of iron- Copper Alloys”, Trans. JIM, Vol. 9, Supplement, 1968, pp. 374-381.

[8] A. Youle and B. Ralph, “A Study of the Precipitation of Copper from α-Iron in the Pre-Peak to Peak Hardness Range of Ageing”, Metal Science Journal, Vol. 6, 1972 pp. 149-152.

[9] I. Le May, L.McD. Schetky and M.R. Krishnadev, “The Role of Copper in HSLA Steels: A Review and Update”.

[10] T.L. Anderson, J.A. Hyatt and J.C. West, “The Benefits of New High – Strength Low – Alloy (HSLA) Steels”, The Welding Journal, Sept. 1987, pp. 21-27.

[11] A.D. Wilson, “High Strength, Weldable Precipitation Aged Steels”, Journal of Metals, March 1987, pp. 36-38.

[12] P.F., Wiesser, “The Effect of Residual Elements on Hot Tearing, Weldability and Mechanical Properties of Cast Steels”, ASF Transaction, Vol. 62, 1983, pp. 647- 656.

[13] M.R. Krishnadev and I. Le May, “Microstructure and Mechanical Properties of a Commercial Low – Carbon Copper – Bearing Steel”, Journal of the Iron and Steel Institute , May 1970, pp. 458-462.

[14] S. Ikeda, T. Sakai and M.E. Fine, “Fatigue Behavior of Precipitation – Hardening Medium C Steels containing Cu”, Journal of Materials Science, Vol. 12, 1977, pp. 675-683.

[15] Pfeil, JISI, Vol. 119, 1929, pp. 501-547.

[16] N. Imai, N. Komatsubara and N. Kunishige, “Effect of Cu and Ni on Hot Workability of Hot-rolled Mild Steel”. ISIJ International, Vol. 37, 1997, pp. 224- 231.

[17] N. Imai, N. Komatsubara and N. Kunishige “Effect of Cu, Sn and Ni on Hot Workability of Hot.rolled Mild Steel”, ISIJ International, Vol. 37, 1997, pp. 523- 530.

[18] S.J. Seo, K. Asakura and K. Shibata “Effects of 0.4% Si and 0.02% P Additions on Surface Hot Shortness in 0.1%C-0.5%Mn Steels containing 0.5% Cu”, ISIJ International, Vol. 37, 1997, pp. 240-249.

[19] S.L. Gertsman and H.P. Tardif, “Tin and Copper in Steel: both are bad together they’re worse”, The Iron Age, Vol. 169, 1952, pp. 136-140.

[20] D.A. Melford “Surface Hot Shortness in Mild Steel”. JISI, Vol. 200. 1962, pp. 290-299.

[21] A. Nicholson and J.D. Murray, “Surface Hot Shortness in Low-Carbon Steel”. JISI, Vol. 203, 1965, pp. 1007-1018.

[22] W.J.M. Salter, JISI, Vol. 204, 1966. pp. 478-488.

[23] S.J. Rothman, N.L. Peterson, C.M. Alter and L.J. Nowicki, “The Diffusion of Cupper in Iron”, Journal of Applied Physics, Vol. 39, 1968, pp. 5041-5045.

[24] R.R. Hough and R. Rolls, “Copper Diffusion in Iron During High-Temperature Tensile Creep”. Metallurgical Transactions, Vol. 2, 1971, pp. 2471-2475.

[25] G. Salje and M. Feller-Kniepmeier, “The Diffusion and Solubility of Copper in Iron”. Journal of Applied Physics, Vol. 48, 1977, pp. 1833-1839.

[26] H. Fredriksson, K. Hansson and A. Olsson, “On the Mechanism of Liquid Copper Penetration into Iron Grain Boundaries”, Scandinavian Journal of Metallurgy,; Vol. 30, 2001: pp. 41-50.

[27] H. Ohtani, H. Suda and K. Ishida, “Solid/Liquid Equilibria in Fe-Cu Based Ternary Systems”, ISIJ International, Vol. 37, 1997, pp. 207-216.

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International, Vol. 37, 1997, pp. 250-254.

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APPENDIX A

BLOCK DIAGRAM FOR THE CRACK MEASURING ROUTINE

Picture of the polished sample Digital picture

Contrast the picture to obtain the silhouette of the circumference Using Image-J utilities

Get every X-Y point of the circumference in a file UIT the help of an Image-J subprogram

From the X-Y points, get the radius profile with angle or the r-θ

And ovality correction is applied

With the used of a spread sheet in Excel The cracks are identified and classified by length and width, with the used of a threshold of 2 pixels (?40µm)

Cracks are count and the crack index (CI) is measured

101

APPENDIX B

CRACK MEASURING THECHINQUE, STEP BY STEP

WITH EXAMPLES

Once the sample was mounted and cut, the sample is polished and a digital image is taken. Taking advantage of the clear epoxy mount, the image is a contrasted picture that enhances the surface information.

Using Image-J, an image processing software developed by NIH, we obtain the silhouette of the circumference with the cracking information.

A plugged-in program in Image-J developed by Wayne Rasband from NIH, give us the X-Y coordinates of the circle. Manipulating mathematically this coordinates we

102 can find the center of the circle and change the system of coordinates to radial coordinates. Now with the radius and angle points, we get a radius profile that contains the cracking information. The samples after being deformed they loose their symmetry, for which the radius profile has to be corrected for ovallity in order to continue with the automatic measuring process. The ovallity correction is done by subtracting the local radius average value to the total radius average value. The result is a uniform radius profile with the cracking variances information.

radius profile, corrected

548 546 544 ovally corrected 542 original raius profile 540 local average 538 536 534 532

s 530 528 xle

pi 526 524 522 520 518 516 514 512 510 508 0 50 100 150 200 250 300 350 angle [°] With the radius profile corrected, the cracks are detected and quantify. A crack is detected when its depth is lower that an average value plus threshold value. In our case the minimum depth of a crack to be quantified is approximately 40 µm.

103

537 536 535 534 533 532 531 530 529 s 528

xle 527 pi 526 525 524 523 522 521 520 519 518 150 155 160 165 170 175 180 185 190 195 200 angle [°] The identified cracks are now measured. The length in angle is registered and the converted to length units. And the maximum deepness is measure and registered. These two parameters are multiply and give a numerical value of the dimension of the crack, in units of length square [mm2]. By adding all the products of the length multiplied by the depth of each crack identified, we get a characteristic for each sample that represents the cracking situation, this is the crack index and it has units of mm2. This crack index contains information of both the number of cracks and the dimension of the cracks.

1 2 3 4