Development of Guidelines for Warm Forging of Steel Parts

Home , Boriding, Cold sizing, Forging, Forging temperature, Forming (metalworking), Hot working

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Niranjan Rajagopal, B.Tech

Graduate Program in Industrial and Systems Engineering

The Ohio State University

2014

Master's Examination Committee:

Dr.Taylan Altan, Advisor

Dr.Jerald Brevick

Niranjan Rajagopal

2014

ABSTRACT

Warm forging of steel is an alternative to the conventional hot forging technology and cold forging technology. It offers several advantages like no flash, reduced decarburization, no scale, better surface finish, tight tolerances and reduced energy when compared to hot forging and better formability, lower forming pressures and higher deformation ratios when compared to cold forging.

A system approach to warm forging has been considered. Various aspects of warm forging process such as billet, tooling, billet/die interface, deformation zone/forging mechanics, presses for warm forging, warm forged products, economics of warm forging and environment & ecology have been presented in detail.

A case study of forging of a hollow shaft has been discussed. A comparison of forging loads and energy required to forge the hollow shaft using cold, warm and hot forging process has been presented.

DEDICATION

This document is dedicated to my family.

iii

ACKNOWLEDGEMENTS

I am grateful to my advisor, Prof. Taylan Altan for accepting me in his research group, Engineering Research Center for Net Shape Manufacturing (ERC/NSM) and allowing me to do thesis under his supervision. The support of Dr. Jerald

Brevick along with other professors at The Ohio State University was also very important in my academic and professional development.

I would also like to thank the Forging Industry Association (FIA) and the Forging

Industry Educational and Research Foundation (FIERF) for funding this research.

Sincere thanks are extended to all the students and visiting scholars of ERC for their help and suggestions in different parts of this research work. Special thanks go to Siddharth Kishore, Adam Groseclose, Xi Yang, Ganapathy Srinivasan,

Tingting Mao, Soumya Subramonian, Eren Billur, Varun Nandakumar, Akshay

Jain, Neeraj Joshi and Linda Anastasi as well as many others for their help and encouragement.

Lastly, I would like to thank my family for their support and encouragement during my graduate studies.

VITA

March 28 th 1989 ...... Born, Coimbatore, India

2010 ...... B.Tech. Mechanical, Amrita University

Coimbatore, India

2010-2011 ...... Programmer Analyst Trainee, Cognizant

Technology Solutions, India.

2012-2013 ...... Graduate Research Associate,

Engineering Research Center for Net

Shape Manufacturing, The Ohio State

University

FIELD OF STUDY

Major Field: Industrial and Systems Engineering

TABLE OF CONTENTS

ABSTRACT ...... ii

DEDICATION ...... iii

ACKNOWLEDGEMENTS ...... iv

VITA ...... v

LIST OF TABLES ...... ix

LIST OF FIGURES ...... xi

CHAPTER 1 INTRODUCTION ...... 1

1.1 Forging ...... 2

1.2 Warm forging Operation as a System ...... 9

CHAPTER 2 BILLET ...... 11

2.1 Materials ...... 11

2.2 Forging Temperature Range ...... 13

2.3 Determination of Material Properties (Flow stress) ...... 15

2.3.1 Plain Carbon Steel, 0.15% carbon ...... 19

2.3.2 16MnCr5(SAE5115) ...... 20

2.3.3 AISI ( E52100,4140,1053,5117) ...... 21

2.3.4 42CrMo4 (AISI 4140), 100Cr6 (AISI 52100), 15CrNi6 (AISI 3115),

20MnCr5 (SAE 5120) and Ck15 (AISI 1015) ...... 23

2.4 Billet preparation ...... 23

2.4.1 Physical Aspects...... 25

2.4.2 Heating ...... 25

2.4.3 Pre-coating ...... 26

CHAPTER 3 TOOLING ...... 27

3.1 Tool Design ...... 27

3.2 Tool Materials ...... 29

3.3 Tool Wear ...... 40

3.4 Tool Coatings and Surface Treatments ...... 41

CHAPTER 4 BILLET-DIE INTERFACE ...... 43

4.1 Lubrication system ...... 43

4.2 Types of lubricants ...... 44

4.3 Application ...... 47

4.3.1 Lubricant volume and die temperature ...... 49

4.3.2 Lubrication film thickness and die life ...... 51

CHAPTER 5 DEFORMATION ZONE/ FORGING MECHANICS ...... 53

5.1 Case study - Hollow Shaft Forging ...... 53

vii

CHAPTER 6 PRESSES FOR WARM FORGING ...... 62

CHAPTER 7 WARM FORGED PRODUCTS ...... 65

7.1 Physical Properties...... 65

7.2 Mechanical Properties ...... 67

CHAPTER 8 ECONOMICS OF WARM FORGING ...... 68

CHAPTER 9 ENVIRONMENT AND ECOLOGY ...... 69

REFERENCES ...... 72

APPENDIX A: FLOW STRESS OF STEELS ...... 77

APPENDIX B: TOOL STEELS...... 85

APPENDIX C: SUPPLIERS ...... 92

viii

LIST OF TABLES

Table 1-1: A Comparison of Typical Forging Process Characteristics [ICFG 2001]

...... 4

Table 1-2: Advantages and Disadvantages of cold, warm and hot forging processes ...... 7

Table 2-1: Examples of Warm Forgeable Steels. [Shichun, 1982], [Sheljaskov,

1994] ...... 12

Table 2-2: Chemical composition of different steels [Neugebauer, 2003] ...... 12

Table 2-3 : Chemical composition of stainless steels [Shichun, 1982] ...... 13

Table 2-4: Warm Forging temperature ranges of stainless steels. [Shichun, 1982]

...... 14

Table 2-5: Deforming force in warm upsetting Cr12Mn5Ni4Mo3Al. [Shichun,

1982] ...... 14

Table 2-6: Rapid oxidation temperatures of stainless steels [Shichun, 1982] ...... 15

Table 2-7: Symbols of flow stress, strain, strain rate and ductility...... 17

Table 3-1: Composition of materials considered suitable for extrusion tooling

[Altan, 2011] ...... 31

Table 3-2: Tool Steels used in Warm Forging. [Sheljaskov, 1994] ...... 35

Table 4-1: Mean tool service life. [Sheljaskov, 2001] ...... 45 ix

Table 4-2: Die lubricants & billet coatings developed for warm forging. Deltaforge

– Henkel, LUBRODAL - Fuchs [www.henkelna.com/metals] [www.fuchs- lubritech.com] ...... 46

Table 5-1: Simulation results of Isothermal Hot forging at 1100° C (2012 F)...... 60

Table 5-2: Simulation results of Isothermal Warm Forging at 700°C (1292 F) .... 60

Table 5-3: Simulation results of Isothermal Cold Forging at 20°C (70F) ...... 61

Table 7-1: A Comparison of Forged Properties [ICFG, 2001]...... 67

Table 7-2: Mechanical properties of warm forged parts. [ICFG, 2001] ...... 67

Table 8-1: Comparative cost of different forging processes [Hawkins, 1985] ...... 68

Table A-1: Mechanical Properties of Cr17Ni2 at different temperatures [Shichun,

1982] ...... 81

Table A-2: Deforming force in warm upsetting Cr12Mn5Ni4Mo3Al. [Shichun,

1982] ...... 82

LIST OF FIGURES

Figure 1-1 : Approximate values of dimensional accuracies achievable in various processes. [Lange et al., 1985]...... 2

Figure 1-2: Comparison of temperature, shapes and materials between cold, warm and hot forging [Sheljaskov, 1994]...... 3

Figure 1-3: Warm Forged Components [Sheljaskov, 1994] ...... 9

Figure 1-4: Warm Forging of a valve considered as a system...... 10

Figure 2-1: Compression test specimen. (a) View of specimen, showing lubricated shallow grooves on the ends. (b) Shape of the specimen before and after the test [Altan, 2005]...... 16

Figure 2-2: Compression test tooling [Altan, 2005]...... 17

Figure 2-3: press setup and fixture used in heating and compression of cylinders and rings. [Altan, 2005] ...... 19

Figure 2-4: Effect of Temperature on Flow Stress, Ductility & Scale formation

(Carbon Steel, 0.15%C) [Hirschvogel, 1979]...... 20

Figure 2-5: Stress-strain curves of steel 16 MnCr5 (SAE 5115) at 0.5 strain &

10s -1strain rate. [Neugebauer, 2003] ...... 21

Figure 2-6 : Flow stress of steels as a function of temperature at a strain of 0.8.

[Sheljaskov, 1994] ...... 22 xi

Figure 2-7 : Mean flow stress on stress strain curve [ICFG, 2011] ...... 22

Figure 2-8: Temperature Dependence of Mean Flow Stress of steels [ICFG,

2001] ...... 23

Figure 2-9: Customary setup of a warm forging plant. [Sheljaskov, 1994] ...... 24

Figure 3-1: Toolset Assembly for Backward extrusion of a CV Joint Component.

[ICFG, 2001] ...... 28

Figure 3-2: Heat Treatment cycle of hot work tool steels [Roberts, 1998] ...... 32

Figure 3-3: Comparison of tempering curves [Altan, 2011] ...... 33

Figure 3-4: Comparison of hot hardness curves [Altan, 2011]...... 34

Figure 3-5: Comparison of hot hardness of ceramic die material (Sialon) and advanced hot working tool steel grades. [Altan, 2011] ...... 37

Figure 3-6 : Forging tests with super alloy and cermet materials. a) Influence of warm forging temperature on the strength of die material, b) Die seizure when supper alloy dies were used. [Mitamura, 1999] ...... 38

Figure 3-7: Forward extrusion of outer race part using MoB dies [Mitamura, 1999]

...... 38

Figure 3-8: Schematic representation of the warm forging (upsetting) die assembly [Manas, 2008]...... 39

Figure 3-9: Die surface temperature at point P2 during upsetting on die insert made of different materials [Manas, 2008][Point P2 is shown in Figure 3-8]...... 40

Figure 3-10: Results of thermal softening experiments for various billet heating temperatures: (a) 550, (b) 600, and (c) 650°C [Jeon g, 2001]...... 42

xii

Figure 4-1: (a) Extrusion punch shape and lubrication method (b) Measurement of punch temperature [Iwama, 1997] ...... 48

Figure 4-2: lubricant volume and maximum die temperature [Iwama, 1997]...... 49

Figure 4-3: The sticking of graphite at different billet temperatures: (a) Oil-based graphite and (b) Water-based graphite [Jeong, 2001]...... 50

Figure 4-4: Lubrication film model [Iwama, 1997] ...... 51

Figure 4-5: Adhesion layer thickness and die life. [Iwama, 1997] ...... 51

Figure 4-6: The time to form a lubrication adhesion layer thickness of 10 m

[Iwama, 1997] ...... 52

Figure 4-7: Change of lubrication adhesion layer thickness by different spray conditions [Iwama, 1997]...... 52

Figure 5-1: Hollow Forged Shaft (Dimensions from [Kim, 2005], [Kang, 2005] and modified)...... 53

Figure 5-2: Forging process steps ...... 54

Figure 5-3: Approximate billet dimension by operation sequence (in inches). .... 54

Figure 5-4: Schematic of upsetting process with dimensions & volume calculations...... 55

Figure 5-5: Schematic of backward extrusion process with dimensions...... 55

Figure 5-6: Schematic of punching operation...... 56

Figure 5-7: Schematic of forward extrusion process...... 56

Figure 5-8: Simulation setup for cold, warm & hot forging ...... 57

Figure 5-9: Flow stress of AISI 1043 at Cold (70F) [Deform Database] ...... 57

xiii

Figure 5-10: Flow stress of AISI 1043 at Elevated temperatures (1292F-2498F)

[Deform Database] ...... 58

Figure 5-11: Estimated Load Stroke curve of Isothermal upsetting process at different strain rates (0.01, 2 & 3.93 in/s) and temperature (2012 F) (1100 C) ... 58

Figure 5-12: Estimated Load Stroke curves of Isothermal Backward extrusion process at different strain rates (0.01, 2 & 3.93 in/s) and temperature (2012 F)

(1100 C)...... 59

Figure 5-13: Estimated Load-Stroke curves of Isothermal Forward extrusion process at different strain rates (0.01, 2 & 3.93 in/s) and temperature (2012 F)

(1100 C)...... 59

Figure 5-14: Comparison of Estimated forging load & Energy in Cold Vs Warm

Vs Hot Forging (Isothermal FE simulations) ...... 61

Figure 6-1: Relationship between process and machine variables in hot forming process conducted in presses [Altan, 2005]...... 62

Figure 6-2: Comparison of Presses [Aida, 2013] ...... 63

Figure 7-1: Achievable Tolerances [Schuler, 2013] ...... 65

Figure 7-2: Explanation of IT specification [Schuler 2013]...... 66

Figure 7-3: Achievable Surface Quality [Schuler, 2013] ...... 66

Figure A-1: Comparison of TTT- diagram and the flow stress for steel quality

Ck45. (Plain Carbon Steel, 0.45%C) [Hirschvogel, 1979]...... 77

Figure A-2: Effect of heating temperature on flow stress curves of 42CrMo4.

[Behrens, 2008] ...... 78

xiv

Figure A-3: Stress-strain curves of steel 16 MnCr5 at various forging temperatures at strain rate of 10s -1[Neugebauer, 2003] ...... 79

Figure A-4: a) Tensile properties of 1Cr18Ni9Ti at different temperatures. b)

Equilibrium diagram for austenitic stainless steel [Shichun, 1982]...... 80

Figure A-5: Strength-temperature curve for 2Cr13 (tempered) [Shichun, 1982]. 81

Figure A-6: The flow stress curves of AISI 1006 at different strain rates. [Xinbo,

2002] ...... 83

Figure A-7: The flow stress curves of AISI 5140 at different strain rates. [Xinbo,

2002] ...... 84

Figure B-1: Comparison of thermal properties of carbides and ceramic to steels

(ThCond: Thermal conductivity; ThExp: Coefficient of Thermal Expansion)

[Manas, 2008]...... 85

Figure B-2: Ranges of hardness levels of various materials and surface treatments. EB, electron beam; high-strength, low alloy [Davis, 2002]...... 86

Figure B-3: Approximate relative costs of various surface treatments. [Davis,

2002] ...... 87

Figure B-4: Slug coating and die lubricants developed for warm forging.

[Sheljaskov, 2001] ...... 89

Figure B-5 : Calibration curve of ring compression tests [Jeong, 2001] ...... 90

Figure B-6 : Force & Energy calculation for punching operation...... 91

CHAPTER 1 INTRODUCTION

The manufacture of metal parts and assemblies can be broadly classified, in a simplified manner, into five general areas: primary shaping processes (casting, powder metallurgy), metal forming processes (forging, stamping, bending), metal cutting processes (turning, machining, milling), metal treatment processes (heat treatment, anodizing, surface hardening) and joining processes (welding, riveting, shrink fitting). Among all manufacturing processes, metal forming plays an important role because it helps to produce parts of superior mechanical properties with minimum waste of material. [Altan, 2005].Forging is a very important process since it is used to produce parts of higher strength with reliable mechanical properties for automotive, aerospace, nuclear, and various other industries. Figure 1-1 shows the dimensional accuracy that is achievable by different processes. The values given in the figure must be considered as guidance values only.

Figure 1-1 : Approximate values of dimensional accuracies achievable in various processes. [Altan, 2005] 1.1 Forging

In forging, an initially simple part- a billet, for example – is plastically deformed at room or elevated temperature between two tools (or dies) to obtain the desired final configuration. Thus simple part geometry is transformed into a complex one, whereby the tools “store” the desired geometry and impart pressure on the deforming material through the tool/material interface. Depending upon the temperature of the slug, forging can be classified into cold, warm and hot forging. Figure 1-2, shows the general temperatures ranges for cold, warm and hot forging of steels.

Figure 1-2: Comparison of temperature, shapes and materials between cold, warm and hot forging [Sheljaskov, 1994]. The world- wide growth in the demand for forgings by the automobile industry since the 1950s resulted in the application of cold forging to serial mass production of a wide range of component types. Cold forging techniques have been developed and refined to enable near -net and in some cases net-shape components to be produced in large batch quantities. Limitations to the economic production of complex parts, large size and difficult to forge alloys revealed the potential for commercial warm forging between the market sectors of hot and cold forging. [ICFG 2001]

Table 1-1: A Comparison of Typical Forging Process Characteristics [ICFG 2001]

Characteristic Hot Forging Warm Forging Cold Forging (Die (Extrusion) Forging)

Shape spectrum Arbitrary Rotationally Mainly symmetrical if rotationally possible symmetrical Use steel quality Arbitrary Arbitrary Low alloyed steels C<0.45% Normally achievable IT 12- IT 16 IT 9 – IT 12 IT 7 – IT 11 accuracy Economic lot size >500 parts >10,000 parts >30,000 parts

Initial treatment of Generally Generally none or Annealing/phos billets and slugs none a graphite layer phat-ing Intermediate None Generally none Annealing/phos treatment phat-ing Tool materials Hot work tool Hot work tool Cold work tool steels steels, high speed steels, high steels, hard speed steels, metals hard metals Typical tool life 5,000 – 10,000 – 20,000 20,000 – 10,000 parts parts 50,000 parts Material utilization 60-80% Approximately 85-90% 85% Energy required per 460-490J 400-420J 400-420J Kg gross of forging.

In Table 1.1, the main principles, requirements and results of the forging methods hot, cold and warm are put together. It must be noted that all values are only guiding principles showing the various tendencies and are not to be taken as an absolute statement. Under consideration of these technical and economic aspects, the aims of these three methods can be summarized as follows:

1. A great variety of shapes nearly out of any steel grade can be manufactured by hot forging. Tolerances and surface quality are rough. Smaller volumes can be manufactured economically.

2. Cold forgings are limited to simpler and more compact shapes and to lower alloyed steel grades. Accuracy and surface quality are excellent; development costs are normally higher and, therefore, there has to be a sufficient total life-time production volume per part to amortize the tool costs.

3. The most complicated process is warm forging; but as it combines the advantages of hot forging (i.e., good formability) with those of cold extrusion (i.e., close tolerances and good surface qualities), it is a very promising process. But, it has to be pointed out, that development costs and costs for a die-set are normally considerably higher than that for a cold or hot forging. Therefore, the aim of warm forging can be expressed as follows: producing precision parts in high volumes made of steel grades, that cannot be cold formed.

Cold forgings are limited to a certain deformation ratio per press stage. In order to achieve higher ratios, an intermediate annealing together with a subsequent new surface treatment may be necessary, before starting the next press operation. On the other hand, higher deformation ratios can be achieved with warm forging due to the fact that recrystallization occurs in parallel with the forging process. This means that a part can be manufactured in one press cycle with 4 or 5 press stations – it is evident that this process will be cheaper instead of having two press cycles with an intermediate heat and surface treatment.

However, if cold forming a part with three, four or five stages without intermediate annealing is possible, it will be cheaper than the corresponding warm forging. Depending on the weight of the component, a modern cold forging process can produce between 30 and 120 parts per minute, a warm-forging process normally

5 produces 20 pieces/min (for a part weighing approximately 1 kg). The production is limited to this rate due to the necessary die cooling and lubrication during the process [Hirschvogel, 1992].

None of the three technologies is generally superior to the others but depending on the requirements regarding part geometry, material, production quantity and accuracy, the best suited technology can be chosen. Warm forging technology is often combined with cold forging operations. A warm forged component can initially be pre-formed or finally be calibrated by cold forming to obtain improved dimensional tolerances. [Behrens, 2008]

Table 1-2: Advantages and Disadvantages of cold, warm and hot forging processes

COLD WARM HOT

Advantages Advantages Advantages

Precision Process Combines Advantages of Can forge complex (Tight Tolerances) cold & hot forging shapes Improved part Better formability Good formability strength Better surface Lower forming pressures Lowest forming finish pressures Material Higher deformation ratio Can forge parts of conservation higher weight & volume No annealing required Disadvantages Disadvantages Disadvantages

High forming High tooling costs Formation of scale pressures Several pre-forming Tooling must withstand Decreased accuracy steps needed forming pressures as well (Larger tolerances) as high temperatures Annealing steps may be required during process Low formability In warm forging process, the tool design is similar to that of cold forging due to fact that warm forged products do not have flash. Warm forging technology is continuously further developed, particularly in aspects of surface treatment, lubrication and tooling. If it is feasible to forge a particular component by hot, warm, or cold, due to lack of quantitative data and therefore uncertainty about the

7 time required to setup a warm forging line, engineers tend to avoid warm forging and tend to choose hot or cold forging to produce a given part.

Warm forging is particularly applicable in the following situations:

• For non-circular shapes of large quantities.

• When the flow stress of the work piece material is too high for cold forging.

• The number of process steps in a cold forging route is too large for economic production.

• To provide a highly accurate preform for a cold forming operation.

• When the component is too large for the capacity of a cold forging press.

• When the accuracy, surface finish, material yield or subsequent machining associated with the alternative hot forging route is unacceptable.

• When the cost of annealing and re-lubricating in a cold forging process is too high.

Figure 1-3: Warm Forged Components [Sheljaskov, 1994] 1.2 Warm forging Operation as a System

A forging system comprises all the input variables such as the billet or blank (geometry and material), the tooling (geometry and material), the conditions at the tool/material interface, the mechanics of plastic deformation, the equipment used, the characteristics of the final product, and finally the plant environment where the process is being conducted.

The “systems approach” in forging allows study of the input/output relationships and the effect of the process variables on product quality and process economics. Figure 1-4, shows the different components of the forging system. The key to a successful forging operation, i.e. to obtain the desired shape and properties, is the understanding and control of the metal flow. The direction of metal flow, the magnitude of deformation, and the temperatures involved greatly influence the properties of the formed components. Metal flow determines both the mechanical properties related to local deformation and the formation of 9 defects such as cracks and folds at or below the surface. The local metal flow is in turn influenced by the process variables as shown in Figure 1-4 [Altan, 2005].

Figure 1-4: Warm Forging of a valve considered as a system.

CHAPTER 2 BILLET

In this chapter, various aspects of billet are discussed in detail namely: billet materials, forging and annealing temperature range, material properties and billet preparation.

2.1 Materials

Most engineering steels may be warm forged but there are limitations to deformation, depending on chemical composition and forging temperature. Formability of cold forgeable steels is increased under warm forging conditions.

Warm forging is used for manufacturing components from unalloyed or low alloyed cementation steels, heat treatable steels, steels for superficial hardening, ball bearing steels and stainless steels. Some examples are given below

Table 2-1: Examples of Warm Forgeable Steels. [Shichun, 1982], [Sheljaskov, 1994]

Cementation Steels Ck10(AISI1010),Ck15(AISI1015),15CrNi6(AISI43 20), 16MnCr5(AISI 5117), 20MnCr5(AISI 5120

Heat-treatable Steels Ck35(AISI 1035), Ck45(AISI 1045), Ck60(AISI1060), 34Cr4(AISI 5132), 34CrMo4(AISI 4137), 42CrMo4 (AISI 4140), 50CrV4(AISI6150).

Superficial hardening Cf53 (AISI 1053) steels

Ball-bearing steels 100Cr6 (AISI E52100).

Stainless steels (General) X5CrNi189(AISI 304L),

X10CrNiMo 17 12 (AISI 316) a)Austenitic Cr18Ni9Ti b)Martensitic 2Cr13, 4Cr13 and Cr17Ni2 c)Precipitation Hardening Cr12Mn5Ni4Mo3Al

Table 2-2: Chemical composition of different steels [Neugebauer, 2003]

Table 2-3 : Chemical composition of stainless steels [Shichun, 1982]

2.2 Forging Temperature Range

In cold forging, the billet is at room temperature, while in warm forging, the billet is heated to a temperature below the recrystallization temperature of the material. Different classes of steel have characteristics which influence the choice of working temperature.

• Carbon Steels: They should be forged at temperatures above 600°C to obtain a significant reduction in flow stress. At temperatures between 200°C and 550°C these steels may be brittle.

• Alloy Steels: Generally the flow stress decreases with increasing temperature. Any temperature in the warm forging range may be used, depending on circumstances.

• Austenitic stainless steels: Flow stress reduces greatly with small increases in temperature although they may strain harden significantly. Temperatures between 200°C and 300°C are commonly u sed. They are not susceptible to the intergranular corrosion when they are warm forged at 500°C- 600°C.

Table 2-4: Warm Forging temperature ranges of stainless steels. [Shichun, 1982]

STAINLESS STEEL TYPE PREHEAT TEMPERATURE Cr18Ni9Ti (Austenitic Stainless Steel ) Lower – 200°-3 00°C Higher - 500°-800°C 2Cr13, 4Cr13 and Cr17Ni2 (Martensitic 650°- 800°C Stainless Steel) Cr12Mn5Ni4Mo3Al (Precipitation 650°-800°C Hardening Stainless Steel )

In general, the temperature interval of warm forging (650-950°C) is considerably wider than the temperature interval of hot forging (1000-1200°C). [Sheljaskov, 1994]

From Table 2-5, it can be seen that the deforming force decreases significantly when the preheat temperature is 900°C; when the red uction is 53-54%, the warm upsetting force at 650°C increases by 41% in compar ison with that at 900°C, when the reduction is 65-65.5%, the force at 650°C increases by 31% in comparison with that at 900°C; when the reduction i s 34-34.5%, the force at 350°C increases by 187% in comparison with that at 900°C. It is seen from these results that a suitable temperature for warm forging Cr12Mn5Ni4Mo3Al is above 650°C at the least.

Table 2-5: Deforming force in warm upsetting Cr12Mn5Ni4Mo3Al. [Shichun, 1982]

In order to avoid scale formation, the warm forging process should be carried out at a temperature lower than the temperature at which rapid oxidation of the metal begins as shown in Table 2-6.

Table 2-6: Rapid oxidation temperatures of stainless steels [Shichun, 1982]

Material Temperature at which rapid oxidation begins

2Cr13 750°C

4cr13 650°C

Cr17Ni2 750-800°C

1Cr18Ni9Ti 800-850°C

2.3 Determination of Material Properties (Flow stress)

Flow stress is very important to understand the forces and stresses involved in metal forming processes. The metal starts flowing or deforming plastically when the applied stress (in uniaxial tension without necking and in uniaxial compression without bulging) reaches the value of the yield stress or flow stress. The flow stress is very important because in metal forming processes the loads and stresses are dependent on (a) the part geometry (b) friction, and (c) the flow stress of the deforming material [Altan, 2005].

The flow stress ( σ) can be expressed as a function of the temperature, strain, strain rate and the microstructure S.

In hot forming of metals at temperatures above the recrystallization temperature the effect of strain on flow stress is insignificant and the influence of strain rate becomes increasingly important. The flow stresses of metals should be determined experimentally for the strain, strain rate, and temperature conditions

15 that exist during the forming processes. The most commonly used methods for determining flow stress are the tensile, uniform compression and torsion tests [Altan, 2005].

Figure 2-1: Compression test specimen. (a) View of specimen, showing lubricated shallow grooves on the ends. (b) Shape of the specimen before and after the test [Altan, 2005]. The compression test is used to determine the flow stress data (true stress/true- strain) relationships for metals at various temperatures and strain rates. In this test, the flat platens and the cylindrical sample are maintained at the same temperature so that die chilling, with its influence on metal flow, is prevented. To be applicable without corrections or errors, the cylindrical sample must be upset without barreling; i.e., the state of uniform stress in the sample must be maintained as shown in Figure 2-1. Barreling is prevented by using adequate lubrication, e.g., at room temperature, Teflon or machine oil and at hot and warm forging temperatures, graphite in oil for aluminum alloys, and glass for steel, titanium and high temperature alloys. The load and displacement, or sample height, are measured during the test. From this information the flow stress is calculated at each stage of deformation, or for increasing strain [Altan, 2005].

Figure 2-2, shows the tooling used for compression tests conducted at the Engineering Research Center for Net Shape Manufacturing (ERC/NSM) of the Ohio State University.

The symbols of flow stress, strain, strain rate and ductility used in this report are shown in Table 2-7.

Table 2-7: Symbols of flow stress, strain, strain rate and ductility.

Figure 2-2: Compression test tooling [Altan, 2005].

The following relationships are valid for the uniform compression test:

Where V is the instantaneous deformation velocity; h 0 and h are initial and instantaneous heights, respectively, and A 0 and A are initial and instantaneous cross-sectional surface areas, respectively.

At, hot working temperatures, i.e. above the recrystallization temperature, the flow stresses of nearly all metals are very much strain-rate dependent. Therefore, whenever possible, hot compression tests are conducted on a machine that provides a velocity–displacement profile such that the condition = velocity/sample height can be maintained throughout the test. Mechanical cam- activated presses called plastometer or hydraulic programmable testing machines (MTS, for example) are used for this purpose. In order to maintain nearly isothermal; and uniform compression conditions, the test is conducted in a furnace or a fixture such as that shown in Figure 2-3.The specimens are lubricated with appropriate lubricants – for example, oil graphite for temperatures up to 800°F (425 °C) and glass for temperatures up to 2300°F (1260°C ). The fixture and the specimens are heated to the test temperature and then the test is initiated [Altan, 2005].

Figure 2-3: press setup and fixture used in heating and compression of cylinders and rings. [Altan, 2005]

2.3.1 Plain Carbon Steel, 0.15% carbon Figure 2-4, shows the “decision making facts” for the forming temperature: in this diagram flow stress, formability and scale formation are plotted versus temperature. The following conclusions are possible:

• Blue brittleness – The one at lower temperature. This range is totally unsuitable for warm forging because the ductility is even lower than for pure cold forming.

• Red brittleness – The one at around 1023 K (750°C) . This zone goes in parallel with the ferrite- austenite transition and therefore depends on the rate at which the steel billet has been heated. As opposed to the blue brittleness, this red brittleness temperature range can be chosen for warm working. Figure 2-4, shows that the ductility is two and half times larger than at room temperature, whereas the flows stress are relatively constant 19

in this zone. The constant flow stress causes a constant forging pressure, and therefore, always the same elastic deformation of the press. Due to this, close thickness tolerances can be achieved, even when there are small changes in the forging temperature.

• Scale formation: Figure 2-4, also shows the scale thickness plotted versus temperature at various set times. It can be seen that there is nearly no loss of steel due to scale below 1073 K (800°C), st rong oxidation begins at about 1173K (900°C). It can therefore be concluded that warm forging temperatures for steel should not exceed 1073 K (800°C) in order to prevent scale formation. [Hirschvogel, 1979]

Figure 2-4: Effect of Temperature on Flow Stress, Ductility & Scale formation (Carbon Steel, 0.15%C) [Hirschvogel, 1979].

2.3.2 16MnCr5(SAE5115) From Figure 2-5, for 16MnCr5, the first local maximum at about 400°C is defined as the blue brittleness where cementite dispersions occur. The second local 20 maximum at about 850°C shows the region of red brit tleness. Both phenomena cause an increase of the flow stress and a reduction of the formability.

Temperatures higher than 800°C enable thermo- mecha nical processing causing structural changes. The temperature range between 580°C and 880°C was selected for the lateral extrusion process. [Neugebauer, 2003]

Figure 2-5: Stress-strain curves of steel 16 MnCr5 (SAE 5115) at 0.5 strain & 10s - 1strain rate. [Neugebauer, 2003]

2.3.3 AISI ( E52100,4140,1053,5117) Figure 2-6, shows the flow stress of different steels at different temperatures. The flow stress of individual steels clearly differs in the sector of cold forging, the stress curves rather coincide in warm forging.

Figure 2-6 : Flow stress of steels as a function of temperature at a strain of 0.8. [Sheljaskov, 1994]

Figure 2-7 : Mean flow stress on stress strain curve [ICFG, 2011]

In Figure 2-7, the mean flow stress K fm is defined by (k f 0.1 +k f max)/2.

2.3.4 42CrMo4 (AISI 4140), 100Cr6 (AISI 52100), 15CrNi6 (AISI 3115), 20MnCr5 (SAE 5120) and Ck15 (AISI 1015)

Figure 2-8: Temperature Dependence of Mean Flow Stress of steels [ICFG, 2001] The mean flow stress for various case hardening and heat treatable steels is plotted against temperature in Figure 2-8 for strain rates 38 s -1and 20 s -1 respectively.

2.4 Billet preparation

The fundamental structure of a warm forging plant as shown in Figure 2-9, normally includes a rod hopper, shear, an inductor for preheating slugs, a dipping for coating slugs, an inductor for heating slugs to forging temperature and a press with a tool lubrication system and a system for rapid tool change.[Sheljaskov, 1994]

In some cases, during the last step, the component is finish formed by a cold sizing operation, so that parts are endowed with the highest possible formed 23 dimensions on one or more of its features. In such cases the warm forged parts must be cleaned and re-lubricated before cold sizing. If severe deformation is necessary and high quality finish is required, the parts should be lubricated as for conventional cold forging. If little metal movement occurs in the cold sizing an oil spray onto the tools might be sufficient. [ICFG, 2001]

Particular attention should be paid to the hardness of warm forged parts to ensure they are soft enough for the cold sizing operation. The cheapest way of achieving the correct hardness is by controlled cooling from the warm forging process but when this is not possible (due to material specification or work piece temperature after warm forging) an intermediate annealing heat treatment will be necessary.[ ICFG, 2001]

Figure 2-9: Customary setup of a warm forging plant. [Sheljaskov, 1994]

2.4.1 Physical Aspects Typically the workpiece families that are presently manufactured by warm forging are strongly related to the ones of cold forging i.e. they are to a very large extent rotationally symmetrical or axially symmetrical. Billets may be graded by weight and the heavier ones used later in the process as tool wear results in cavity growth. Shearing may be used if billets are flattened and sized (formed into a slug) in the initial stages of a multi-stage forging process. The normally achievable weight tolerances when shearing to length are close to 1-1.5%. If even more exacting tolerances are required, the slugs must be weighted and sorted into different weight groups that can be processed with different settings of the press. [Sheljaskov, 1994]

To more readily attain billet weight accuracy and eliminate surface defects, peeled bar is used as raw material. After shearing and sawing to length billets may be tumbled or shot blasted to remove sharp edges which otherwise might cause surface defects or reduce lubricant performance. [ICFG, 2001]

De-scaled hot rolled bar may be used provided surface defects are small. Monitoring of hot rolled bar diameter is important to enable cut length to be varied to obtain weight accuracy. [ICFG, 2001]

2.4.2 Heating Induction heating is the most suitable method for billet heating as it takes only short heating times and also accurate temperature control are guaranteed

Heating of billet is done by induction and is in two stages:

• A short heater is used to bring the billet temperature to a value suitable for lubrication. This is usually between 120°C and 150° C, depending on lubricant temperature, billet weight and billet geometry.[ICFG, 2001]

• The main heater brings billet to a temperature required for forging. It usually consists of a horizontal coil through which the billets are passed.

The coil should increase temperature fairly evenly throughout a billets volume. If the surface reaches a high temperature too soon the lubricant will degrade to a large extent allowing oxidation and decarburization top occur and become ineffective before forging starts. Relatively long coils are used to ensure temperature uniformity. Typically temperatures are controlled to within ± 15°C of nominal. The size of induction heater may be estimated as follows: 760 to 860 kJ/kg of billet. [ICFG, 2001]

2.4.3 Pre-coating Pre-coating on a billet helps to reduce oxidation and decarburization, enhance metal flow and to reduce tool wear.

Often colloidal graphite in a water carrier with organic binders is used as billet coating. Preheated billets are immersed in or sprayed with lubricant which coats them with a solid layer as the water carrier evaporates. The binders in the lubricant increase the physical robustness of the lubricant coating and increase the resistance of the graphite to oxidation at high temperatures [ICFG, 2001]. The coating layer produced by this procedure on the slugs support the direct tool lubrication that is additionally taking place during the forging process [Sheljaskov, 1994].

A lot of parameters such as bath concentration, bath temperature, slug surface quality and the dipping time play an important part for a good quality of the slug coating. The formation of a continuous graphite layer after dipping and the stability of the coating during heating to forging temperature are criteria for the coating quality [Sheljaskov, 1994].

Specific information on pre-coating and lubrication are discussed further in Chapter 4.

CHAPTER 3 TOOLING

In this chapter, various aspects of the tooling are discussed in detail namely: tool design, tool materials, tool wear and tool coatings and surface treatments.

3.1 Tool Design

The tooling for warm forging is similar to that for cold forging, with some modifications made in the die to allow increased temperatures, internal die cooling, and venting of coolants [Altan, 2005].

The construction of a tool for the backward extrusion stage of a CV joint component is shown in Figure 3-1.The following guidelines are generally applicable [ICFG, 2001]:

1. Can extrusion punches

• Land length between 3 mm and 5 mm.

• Relief behind a land between 0.3 mm and 0.6 mm depending on temperature.

• Nose radius not less than 1 mm and preferably between 2 mm and 3 mm.

2. Forward extrusion dies

• Cone/land transition radius between 1mm and 4mm depending on temperature. 27

• Land length between 3 mm and 5 mm.

• Relief behind a land between0.1 mm and 0.2 mm

3. Clearances

• These must be greater than for cold forging to allow for thermal expansions which may not be uniform in the toolset.

Figure 3-1: Toolset Assembly for Backward extrusion of a CV Joint Component. [ICFG, 2001]

3.2 Tool Materials

The selection of the die materials is a very significant decision in the production of precise components by forging. Appropriate selection of die materials is imperative to get acceptable die life at a reasonable cost. The most important criteria for selection of die steel material for forging are its resistance to wear, plastic deformation and fatigue (mechanical and thermal). Selection of proper die materials is very important for reducing the production costs and setting narrow tolerance for the forged part [Tulsyan, 1993].

In choosing a suitable material for use in warm forging tools consideration must be given to the following [ICFG, 2001]:

• Surface transient temperatures of up to about 800°C and bulk temperatures of about 400°C can be encountered in m ass production.

• Depending on preheat temperature warm forging pressures are typically 25% to 50% the value of those arising in cold forging.

• Thermal shock can arise when water based lubricants are used to cool tool surfaces.

• Surface abrasion conditions, although at a lower level than in hot forging are of greater importance because of the precision nature of warm forging.

Thus the essential characteristics of the warm forging tools are:

• High hot strength/hardness and wear resistance.

• Good thermal shock resistance and thermal fatigue strength.

A number of special steel compositions have been formulated for warm forging applications and the user should refer to tool steel manufacturers for advice on appropriate steel specifications, when designing tools [ICFG, 2001].The tool materials used can be broadly classified as:

1. High Speed Steel :

Longest tool life is achieved with high speed steel such as M2 or its

equivalent in powder metallurgy form. High speed steels have low

temperature shock resistance. They must be preheated and should be

cooled during production, only by air. The use of water cooling or water

based lubricants will cause rapid fracture. This characteristic of high speed

steels can limit productivity if a high rate of forging would result in

significant heat buildup in the tools [ICFG, 2001].

2. Hot Work Steels :

Steels of the chromium, molybdenum, vanadium family such as,

X40CrMoV51 and H13 do not possess the strength or hardness of high

speed steels but are tougher with greater resistance to thermal shock.

Therefore they can be rapidly cooled in service. A variety of these steels is

available enabling hardness between HRC 50 and 60 to be achieved

appropriate to various process conditions. Tungsten alloy steels are not

resistant to thermal shock and must not be cooled intermittently with water

[Altan, 1983].

The chromium hot work steels are the most commonly used for forging

applications. In general, chromium die steels retain their hardness upto

425°C, tungsten hot work steels retain much of thei r hardness up to

620°C. The properties of molybdenum based hot work steels is in between

that of chromium based and tungsten based hot work die steels [Altan,

2005].

Table 3-1: Composition of materials considered suitable for extrusion tooling [Altan, 2011]

Table 3-1, lists some commercially available hot work die steels. The

compositions and the hardness ranges recommended by the

manufacturer are also shown in the table. Except DRM1 and DURO F1

die materials, all other materials were regular chromium and molybdenum

based hot working die steels. These materials are suitable for the

extrusion die inserts and other surrounding tooling such as the extrusion

bushing and die holder. DRM1 and DURO F1 are matrix high speed steels

(MHSS) which are suitable for extrusion punches, were also tested in dies.

MHSS usually contain higher percentage of tungsten or molybdenum

which provides high hardness to the dies. [Altan, 2011]

Hot work die steels are usually subjected to a heat treatment cycle prior to

use, see Figure 3-2. After hardening (by quenching) they are usually

tempered (once or multiple times) to provide enough toughness.

Figure 3-2: Heat Treatment cycle of hot work tool steels [Roberts, 1998]

The tempering curves for the nine die materials are given in Figure 3-3.

Tempering temperature also defines the working temperature range for the dies. During forging, if the temperature on the die exceeds the tempering temperature, the dies soften and lose their hardness quickly

[Altan, 2011].

Figure 3-3: Comparison of tempering curves [Altan, 2011]

Die materials are considered to be better if they are able to retain their hardness at elevated temperature i.e. they should have better hot hardness. As the temperature in the warm forging tooling is about 400°C, it is imperative for the die materials to retain their hardness prior to dropping down. Moreover, materials which have steep decrease in hardness are considered to be unsuitable compared to the dies which have moderate hardness and gently reducing hot hardness curves. It can 33 be observed from the curves that DURO F1 and DRM1 (both MHSS) can be tempered to a higher hardness compared to other materials.

Figure 3-4: Comparison of hot hardness curves [Altan, 2011].

It can be observed from Figure 3-4, that W360 and DRM1 are more likely to retain their hardness (at temperatures above 550°C) compared to other materials.

Tool steels produced under protective atmosphere such as Vacuum Arc

Re-melting (VAR) produce cleaner steels with better structural homogeneity compared to Electro Slag Re-melting (ESR).

The various tool steel materials for warm forging used in Japan and

Germany are listed in Table 3-2.

Table 3-2: Tool Steels used in Warm Forging. [Sheljaskov, 1994]

3. Specialty alloys (High Nickel & Cobalt Alloys) :

Nickel alloys can be used to achieve excellent service lives under extreme

conditions. These alloys have: high fatigue strength, high thermal shock

resistance and high toughness over a range of temperatures. Alloys, such

as Inconel 718 and 18% nickel maraging steel were successfully used in

tooling. The use of these alloys is usually prohibited by their high cost

which can be an order of magnitude greater than that of conventional

steels. [ICFG, 2001]

Cobalt based alloys, available under the trade name Stellite, may be used

in cases where some areas of tools deform and erode due to overheating.

These alloys are usually welded to the surface of the tools. E.g.: A ring

made of Stellite welded around the periphery of a hot work steel

counterpunch which could not be adequately cooled. [ICFG, 2001] 35

4. Ceramics and Carbides :

Ceramic inserts have been used in the machining industry for reducing

tool wear and improving the tool performance. Some ceramic materials

are found to have significantly better properties over the traditional hot

work die materials (Cr-Mo-W based steels) used in hot forging

[Deshpande, 2010].

The ceramic forging dies should be designed in such a way that dies are

not subjected to stresses which can lead to failure due to cracking.

Ceramics are used as small inserts which are fitted onto large hot work

steel dies/container rings as they have low tensile strength and high cost.

The ceramic dies are usually pre-stressed (compressive stress) in order to

prevent brittle fracture due to internal pressures during forging. At different

temperatures, the compressive state of stress between the ceramic inserts

and the container rings should be maintained [Deshpande, 2010].

Some of the potential ceramic materials that can be used for hot/warm

forging applications are Silicon Nitride, Sialon and Silicon carbide. Silicon

Nitride is a ceramic that can be used in warm forging applications as it has

high hardness, high toughness, good wear resistance and adequate -

thermal shock resistance, hot hardness and resistance to oxidation.

Silicon Aluminum Oxynitride (Sialon) retains their hardness more

efficiently at elevated temperatures when compared to hot working tool

steels [Altan, 2007]. Carbides can be used as inserts for warm forging 36

tools. However it has been reported that failure of carbide tools can occur

when forging ceases and cooling takes place [ICFG, 2001].

Figure 3-5: Comparison of hot hardness of ceramic die material (Sialon) and advanced hot working tool steel grades. [Altan, 2011]

Ceramic dies have been tested in Japan for warm forward extrusion

process. Nissan motor company has tested cermet dies made of MoB

(ceramic). The material is powder formed and sintered. MoB cermet dies

and nickel based super alloy dies were tested on forward extrusion of

outer race part under warm forging conditions. It was observed that the

MoB cermet dies could withstand high temperatures (800°C) even better

than nickel based super alloy.[Mitamura, 1999].

Figure 3-6 a, shows the high temperature strength of the two die materials.

Even though the axes are not labeled, it can be clearly seen that MoB

cermet die can withstand higher temperature (800°C/ 1470°F) than the

super alloy. Figure 3-6 b, shows the die seizure observed with the super

alloy material

Figure 3-7: Forward extrusion of outer race part using MoB dies [Mitamura, 1999]

[Manas, 2008], conducted FE simulations of warm upsetting of automotive

transmission shaft as shown in Figure 3-8. Four different die insert

materials namely carbide, ceramic and hot working die materials (H21 and

MHSS) were tested). 38

Figure 3-8: Schematic representation of the warm forging (upsetting) die assembly [Manas, 2008].

Figure 3-9, shows that the die surface temperature of ceramic material was observed to be higher, due to low thermal conductivity of ceramic material.

Figure B-1, in Appendix B shows the comparison of thermal properties of carbides and ceramics to steels. [Manas, 2008]

Figure 3-9: Die surface temperature at point P2 during upsetting on die insert made of different materials [Manas, 2008][Point P2 is shown in Figure 3-8].

3.3 Tool Wear

The factors influencing die life are thermal fatigue, plastic deformation, wear etc.

Amongst these, wear is the predominating factor in warm forging process.[Lange

1993] reported that wear is the dominating failure mechanism for forging dies, being responsible for approximately 70% of die failures. [Kang, 1999].Abrasive wear occurs due to the removal of material of the die surface caused by sliding of a rough, hard surface against a softer material i.e. forging [Choi, 2012].

A large number of researchers have tried to use finite element simulation to predict die wear in metal forming and machining operations. The basic concept is to determine process variables (such as temperatures, stress, and sliding distances using finite element simulation. Then a wear prediction equation or a

40 set of equations is derived in terms of these process parameters. [Altan, 2011]

[Archard, 1953] [Painter, 1996] [Kang, 1999] [Behrens, 2005] [Kim, 2005]

[Deshpande, 2010] [Choi, 2012].

3.4 Tool Coatings and Surface Treatments

The properties of die materials can be locally influenced by the surface engineering techniques such as heat treatment and surface coating techniques.

The die life can be increased dramatically by proper selection of surface engineering techniques. [Deshpande, 2010]

Die surface treatments such as nitriding, weld overlays (or hardfacing) and chemical and physical vapor deposition of heat resistant ceramic materials can substantially increase the life of the hot work dies. Nitriding is the most common surface treatment for hot forging dies. Boriding and surface welding are also used in many cases. Surface welding is also used to rebuild worn dies. The vapour deposition techniques such as PVD, CVD are more commonly used for cold forging applications but also have some success in hot forging applications

[Deshpande, 2010] [Jeong, 2001].

From Figure 3-10 , it can be seen that the carbon-nitrided die has the highest hardness regardless of the heating temperature and time. Ion-nitride is next and no-treatment is the last. The higher is the heating temperature, the lower is the hardness. Especially, the hardness drop with no-treatment is very rapid. Surface treatments make the dies harder than the no-treatment case. [Jeong, 2001]

[Jeong, 2001] also found that among carbon-nitrided, ion-nitrided and no- treatment dies, the heat transfer coefficient of carbon–nitride is lowest and that of no-treatment is the highest. Therefore the carbon nitride die is less affected by the heat than by the other treatments.

More information on hardness & costs of surface coatings are given in Appendix

B-2.

Figure 3-10: Results of thermal softening experiments for various billet heating temperatures: (a) 550, (b) 600, and (c) 650°C [Jeon g, 2001].

CHAPTER 4 BILLET-DIE INTERFACE

In this chapter, various aspects of billet-die interface are discussed in detail namely: Lubrication system, function, types of lubricants and application.

4.1 Lubrication system

The factors such as spray pressure, lubricant flow rate, spray angle, spray distance, and spray pattern are of great importance for a successful warm forging operation. [Altan, 2005]

The four most common lubrication systems are MoS 2, graphite, synthetics, and glass; MoS 2 is useful at warm forging temperatures (up to 750° F, or 400°C).

MoS 2 and graphite are solid lubricants. Because of their layered molecular structure, they demonstrate low frictional stresses. They are usually mixed into an aqueous solution and sprayed onto the dies. This serves two purposes. First, the aqueous solution evaporates upon contact with the dies, thus acting to cool the dies and protect them against increased wear due to thermal softening.

Second, a MoS 2 or graphite layer remains on the dies following evaporation of the aqueous solution. This layer not only acts as a lubricant, but also as insulation against excessive die heating. [Schey, 1983] [Manji, 1994].Today, concerns over the environmental friendliness of graphite as well as the accumulation of graphite within the dies have led to the development of water- based synthetic lubricants. [Manji, 1994]

See Apendix B, section 3 for function of lubricants and slug coatings.

4.2 Types of lubricants

Lubricants may be grouped into the following types:

a) Colloidal Graphite-Oil

These lubricant types have a lower latent heat of evaporation than the water based ones and are suitable for use with high-speed steel tools for which rapid cooling is detrimental. In many cases flooding of the tools is employed (the tools are designed with drains and vents to allow its escape) and if extra cooling is required an air blast is also used. A disadvantage of this type of lubricant is the smoke and fumes which are formed on contact with a heated work piece as well as the high cost of disposal in conformation with environmental control. [ICFG, 2001].The heat transfer coefficients of oil-based graphite are higher than those of water based graphite at various experimental temperatures [Jeong, 2001].

b) Colloidal Graphite-Water

These lubricants are essentially pollution free and non-toxic. Also small amounts of organic compounds added to improve adhesive properties and increase temperature stability, decompose on contact with the work piece to form traces of noxious gas but with no known significant health hazard. They are corrosive due to the presence of ammonia as a bactericide and fungicide, so the pipes and fittings should be of stainless steel.

Water based lubricants used under the correct conditions form a solid coating on the tools. This is an advantage in situations when an oil based lubricant would run off the tool surface or be squeezed out of the work piece/tool interface. To perform properly, a water based lubricant must adhere to the tools as carrier evaporates. The durability of graphite coatings is enhanced by the presence of binding agents which also raise the wetting temperature of the lubricants. Current products are commonly used to coat tool surfaces at temperatures of 400°C and for temperatures above 400°C the coating is done us ing special spraying techniques. Because of their good coating and friction reducing properties, water 44 based graphite lubricants are probably the most popular at present. However, although they are environmentally, virtually harmless the fact that they cause plant and equipment to be covered in a black film has brought them into disfavor and attempts are underway to develop white or colorless lubricants. Another disadvantage of colloidal graphite is that unless continuously agitated it tends to flocculate, forming sediment in the holding tanks and clogging up pipes and spray nozzles. [ICFG, 2001]

[Sheljaskov, 2001] investigated tool life in the warm forging of constant velocity joints as shown in Table 4-1.The table shows the tool life of the die in the first forging station (forward extrusion) and of the punch in the fourth forging station (backward extrusion). High tool life was achieved during the production tests compared with tool life monitored with lubricants used before the project was started.

Table 4-1: Mean tool service life. [Sheljaskov, 2001]

Table 4-2, shows the various die lubricants and slug coatings developed for warm forging by Henkel & Fuchs. [www.henkelna.com/metals] [www.fuchs- lubritech.com]

Table 4-2: Die lubricants & billet coatings developed for warm forging. Deltaforge – Henkel, LUBRODAL - Fuchs [www.henkelna.com/metals] [www.fuchs- lubritech.com]

Product Features & Benefits Suitable for GRAPHITE Deltaforge GP -190 Excellent high temperature die wetting up to 900°F Billet coating for warm forging (482°C) Deltaforge F-31 The industry benchmark for performance. Contains Hot & warm forging of steel the finest particle size graphite. Provides the best forging performance, productivity & die life. Has widest range of application & is the most forgiving in operation. Deltaforge F-42 Versatile & reliable product, containing mid-particle Hot & warm forging on mechanical size graphite for a wide range of forging applications. presses. Deltaforge F-65C Best graphite in oil emulsion. Oil gives carrying Circulation systems; hot forging and properties on hot des, helping to give total die warm forging of steels; mechanical coverage on difficult to spray configurations. Capable presses of recycling due to re-emulsification properties of the formulation. Deltaforge F-87 New generation of high performance graphite Hot & warm forging of steel ; dispersion where enhanced die wetting temperature & extrusions. die adhesion capabilities are desired; high dilutability. LUBRODAL F 24 W Contains finely ground graphite in a water based Warm forging steel operations emulsion which contains oil. It is a homogenous, low sediment, concentrated lubricant which can be diluted with tap water to any strength required. NON GRAPHITE Deltaforge F-60A High diluting range , general forging applications Hot and warm forging; hammer where graphite is not required. Excellent wash-off forging properties to keep machinery and dies clean. BILLET COATINGS Deltaforge 144 Oxidation resistant graphite film with effective Protective coating of steel billets for lubricity; water dilutable. warm forging.

See Appendix B, section 3 for friction factor values of oil based graphite and water based graphite.

c) White mineral in Water

The compositions of these lubricants are largely known only to the manufacturers. The substances used have a molecular structure which allows the relative sliding of lamellae. Boron nitride and aluminum silicate are minerals known to have been used and although they form robust interfacial boundaries to elevate pick-up they are not very effective at reducing friction [ICFG, 2001].

d) Colorless Solutions in Water

Solutions of up to 18% salts or soaps in water have wetting temperatures of about 400°C. They can be used as the sole lubricant in a process but with pre- graphited billets the lubricity of the system is improved significantly.

The development of white/colorless lubricants has been underway for several years and noticeable performance improvements have been achieved. The question of tool life vis- a- vis graphite lubricants has yet to be answered.

One distinct advantage of colorless solution in water lubricants is that being water soluble the plant and workshop environment is readily washed clean. Also as the lubricating substances remain in solution, pipework does not become clogged [ICFG, 2001]

4.3 Application

Some lubrication systems sequentially blast air into the cavities to clear debris, blow air/water mist mixture for cooling and then spray a lubricant. Depending on the tool materials and the lubricant one or all three of these operations can be used in a sequence of forging operations.

If the lubricant has sufficient cooling properties additional water or air will be unnecessary. However in general large amounts of heat can be extracted more economically with a water/air spray than by applying more lubricant than is needed for establishing a satisfactory tool/work piece interface alone. In addition to the inefficiency of using an expensive lubricant for a task better accomplished by cheaper substances, the over application of a lubricant leads to problems of die clogging and environmental pollution [ICFG, 2001]

An important rule to observe is that the quantity of lubricant to be applied should be sufficient only to maintain an interfacial barrier between work piece and tools

47 during forging. If this is inadequate to cool the tools sufficiently two possibilities exist to remedy the situation:

• If the lubricant is water-based dilute it and increase the spray time. By this means a greater quantity of water will be applied to the tools for a given quantity of lubricant.

• Apply an air, water or air/water spray before applying the lubricant. Such a measure depends on sufficient time being available in a forging stroke, for two different spraying operations.

Figure 4-1: (a) Extrusion punch shape and lubrication method (b) Measurement of punch temperature [Iwama, 1997] Figure 4-1, shows the experimental setup used by [Iwama, 1997] to study the effect of lubricants.

4.3.1 Lubricant volume and die temperature Figure 4-2, shows the result of an investigation on the relationship between the lubricant volume and the maximum die temperature based on a punch equipped with the thermocouple. Decrease in lubricant volume can lead to a rise in the die temperature. It was found that the lubricant dries up faster when using an appropriate lubricant volume and remains wet if the lubricant volume is high. The water-soluble graphite lubricant forms a lubrication film which binds to the die after the water has dried. This lubricant appears to prevent the die temperature from rising, due to the lubrication effect of the graphite [Iwama, 1997]

Figure 4-2: lubricant volume and maximum die temperature [Iwama, 1997]. [Jeong, 2001] conducted experiments to observe the sticking of graphite onto the metal surface, when using oil based graphite and water based graphite lubricants. Oil based lubricant is a mixture of soybean oil and graphite powder with 1:1 mixture rate. Deltaforge31 (Acheson), which is a mixture of water and graphite powder in the ratio 3:1, was used as the water based graphite lubricant. The lubricants were sprayed onto the surfaces of heated billets at various temperatures, which were 150,250, 350,450, 500,550,650,750 and 850°C.

Figure 4-3: The sticking of graphite at different billet temperatures: (a) Oil-based graphite and (b) Water-based graphite [Jeong, 2001]. From Figure 4-3, it can be seen that oil-based graphite is well stuck at over 450°C and the water-based graphite is stuck well in the range of 150 to 350°C.

4.3.2 Lubrication film thickness and die life Figure 4-4, shows that the dried lubrication film can be classified into two layers: adhesion layer (one which is firmly adhered to the die) and accumulation layer (one which has a lower level of adhesion power).

Figure 4-4: Lubrication film model [Iwama, 1997] Figure 4-5, shows the relationship between the thickness of the lubrication adhesion layer and the die life as well as the type of die life, for the conditions used by [Iwama, 1997] this study showed that the die life can be improved by ensuring the appropriate lubrication adhesion layer.

Figure 4-5: Adhesion layer thickness and die life. [Iwama, 1997] The conditions that are required for an appropriate lubrication adhesion layer to exist after the lubricant has dried are: die temperature and spray granularity. It

51 was also found from experiments that the temperature range in which the lubrication adhesion layer will form is between 200 and 300°C. Figure 4-7, shows the relationship between the initial temperature of test piece and the time it takes to achieve a lubrication adhesion layer thickness of 10 m. If spraying of the lubricant is started when the initial temperature of the test piece is 300°C, a lubrication adhesion layer thickness of 10 m can be achieved in the shortest time possible.

Figure 4-6: The time to form a lubrication adhesion layer thickness of 10 µm [Iwama, 1997] Figure 4-7, shows the relationship between the spray time according to the size of the spray granules and the lubrication adhesion layer thickness.

Figure 4-7: Change of lubrication adhesion layer thickness by different spray conditions [Iwama, 1997] 52

CHAPTER 5 DEFORMATION ZONE/ FORGING MECHANICS

The use of FEM in forging die and process design is well known. A case study further illustrates this point.

5.1 Case study - Hollow Shaft Forging

Overall dimensions of a hollow forged shaft are shown in Figure 5-1. Detailed dimensions of the part cannot be provided due to confidentiality issues. The objective of the study was to determine the force and energy required to form the part.

Figure 5-1: Hollow Forged Shaft (Dimensions from [Kim, 2005], [Kang, 2005] and modified). 53

Figure 5-2 & Figure 5-3, show the forging process steps & billet dimensions. This part is very similar to that studied by [Kim, 2005] [Ssemakula, 2013] [Kang, 2005].First operation is upsetting which is followed by a backward extrusion. The third operation is punching and finally the hollow billet is forward extruded to final dimensions.

1 • Upsetting

• Backward extrusion 2 • Punching 3

4 • Forward Extrusion

Figure 5-2: Forging process steps

Figure 5-3: Approximate billet dimension by operation sequence (in inches). Figure 5-4, shows the schematic of upsetting process with volume calculations. Volume constancy is maintained throughout all the stages. Figure 5-5, shows the schematic of backward extrusion process. Figure 5-6 & Figure 5-7, shows the

54 schematic of the punching and extrusion operation. The forward extrusion operation could be done in 2 steps in order to reduce high thermal and mechanical stresses in tool and dies.

Figure 5-4: Schematic of upsetting process with dimensions & volume calculations.

Figure 5-5: Schematic of backward extrusion process with dimensions.

Figure 5-6: Schematic of punching operation.

Figure 5-7: Schematic of forward extrusion process.

All three stages of the hollow shaft forging process were simulated in 2D using FE software DEFORM, except punching operation as the punching load would be very low compared to other operations. Punching load was estimated using equations shown in Appendix B section 4.

FE Model Axisymmetric, Isothermal

Billet Material AISI1043, (700-1370C) (DEFORM Database)

Initial Billet Temperature Hot - 2012 F (1100C) at each stage Warm – 1292 F (700 C) at each stage Cold – 70F (20C) at each stage Flow Stress From DEFORM Database

Die Material AISI H13 (Rigid)

COF 0.3 (hot forging – lubricated) from DEFORM 0.25 ( warm forging) 0.12 (cold forging) Punch Velocity 0.01in/sec 2 in/sec 3.93 in/sec

Figure 5-8: Simulation setup for cold, warm & hot forging

Flow stress at 68 F & strain rate 1.6S-1 100s-1. 160 140 120 100 80 1.6/s 60 Stress (ksi) Stress 40 100/s 20 0 0 0.5 1 1.5 2 2.5 Strain

Figure 5-9: Flow stress of AISI 1043 at Cold (70F) [Deform Database]

Flow Stress at Strain rate 1.6s-1 Flow Stress at Temperature 2012F 60 30 50 25 1292 F 40 20 1472 F 30 15 1652 F 1.6/s 20 Stress(Ksi) 10 Stress (Ksi) Stress 1832 F 40/s 10 5 2012 F 0 0 2498 F 0 0.2 0.4 0.6 0.8 1 0 0.2 0.4 0.6 0.8 1 Strain Strain

Figure 5-10: Flow stress of AISI 1043 at Elevated temperatures (1292F-2498F) [Deform Database]

300 3.93 in/sec 250

200

150 2 in/sec

Force (Klb) Force 100

50 0.01 in/sec

0 0 0.5 1 1.5 2 2.5 3 3.5 Stroke (in)

Figure 5-11: Estimated Load Stroke curve of Isothermal upsetting process at different strain rates (0.01, 2 & 3.93 in/s) and temperature (2012 F) (1100 C) From Figure 5-11, it can be seen that with increase in strain rate the load increases. Thus strain rate plays an important effect in forging at elevated temperature.

120 3.93 in/s 100

80 2 in/s

60 Force (Klb) Force 40 0.01 in/s 20

0 0 1 2 3 4 5 Stroke (in)

Figure 5-12: Estimated Load Stroke curves of Isothermal Backward extrusion process at different strain rates (0.01, 2 & 3.93 in/s) and temperature (2012 F) (1100 C). From Figure 5-12, it can be seen that forging load increases with strain rate, the backward extrusion was done till the load starts shooting up.

1400 3.93 in/s 1200

1000

800 2 in/s 600 Load (Klb) 0.01 in/s 400

200

0 0 1 2 3 4 5 6 Stroke (in)

Figure 5-13: Estimated Load-Stroke curves of Isothermal Forward extrusion process at different strain rates (0.01, 2 & 3.93 in/s) and temperature (2012 F) (1100 C).

Table 5-1: Simulation results of Isothermal Hot forging at 1100° C (2012 F).

Process Step Punch velocity : Punch velocity : 2 in/sec Punch velocity 3.93 0.01in/sec in/sec Maximum Energy Maximum Load Energy Maximum Energy (in- Load (ton) (in-ton) (ton) (in-ton) Load (ton) ton)

Upsetting 19.25 35 111 244 249 571

Backward 22 86 49 188.5 54 209 extrusion

Punching 9 2.5 15 4 15 4

Forward 194 586 505 1420 572 1586 extrusion Total 244.25 ton 709.5 680 ton 1856.5 890 ton 2370

The total load and energy (Estimated by FEM for Isothermal conditions) required to hot (2012 F) forge the hollow shaft at different punch velocities are shown in Table 5-1.

Table 5-2: Simulation results of Isothermal Warm Forging at 700°C (1292 F)

Process Step Punch velocity 3.93 in/sec

Maximum Load (ton) Energy (in-ton)

Upsetting 471.5 1044

Backward extrusion 190 722

Punching 41 11

Forward extrusion 1793 4705

Total 2495.5 ton 6482 in-ton

The total load and energy (Estimated by FEM for Isothermal conditions) required to warm (1292 F) forge the hollow shaft at a punch velocity of 3.93 in/s are shown in Table 5-2.

Table 5-3: Simulation results of Isothermal Cold Forging at 20°C (70F)

Process Step Punch velocity 3.93 in/sec

Maximum Load (ton) Energy (in-ton)

Upsetting 1452 3074

Backward extrusion 487 1706

Punching 87.5 23.3

Forward extrusion 3847 10412

Total 5873 ton 15215 in-ton

The total load and energy (Estimated by FEM for Isothermal conditions) required to cold (70F) forge the hollow shaft at a punch velocity of 3.93 in/s are shown in Table5-3.

Max Force (ton) Energy (in-ton)

7000 16000

6000 14000

12000 5000 10000 4000 8000 3000 FORCE FORCE (ton)

6000 (in-ton) Energy 2000 4000

1000 2000

0 0 Cold (70F) Warm (1290 F) Hot (2012 F)

Figure 5-14: Comparison of Estimated forging load & Energy in Cold Vs Warm Vs Hot Forging (Isothermal FE simulations) From Figure 5-14, it can be seen that the load and energy requirement for warm forging is only 43% of that required for cold forging and that of hot forging is only 40% of that of warm forging. 61

CHAPTER 6 PRESSES FOR WARM FORGING

The choice of forging machine will depend upon a number of factors such as batch quantities, required productivity, rate at which the energy is applied to the work piece & the capability to control the energy [Altan, 2005].

Figure 6-1: Relationship between process and machine variables in hot forming process conducted in presses [Altan, 2005]. It is useful to consider forming load and energy as related to forming equipment. For a given material, a specific forming operation (such as hot closed-die forging with flash, warm -forward or backward extrusion, upset forging etc) requires a certain variation of the forming load over the slide displacement (or stroke). For a 62 given part geometry, the absolute load values will vary with the flow stress of the given material as well as with frictional conditions. The interaction between the principal machine and process variables is illustrated in Figure 6-1[Altan, 2005].

The ideal warm forging machine should have a rapid stroking rate to reduce dwell times and avoid overheating of tools. However the higher stroking rates can increase the flow stress of the work piece material and hence the forging load. Press stiffness and accuracy of ram guidance should be similar to those for cold forging. Slide geometry should be temperature compensated so that small clearances can be maintained. Ejection should be built into the press operating system and should be rapid acting to minimize work piece die contact time [ICFG, 2001].

Figure 6-2: Comparison of Presses [Aida, 2013] Figure 6-2, shows a comparison between the features of mechanical, hydraulic and servo drive presses. Mechanical Crank or eccentric presses with their rapid stroking rates are the ideal type of machine for warm forging. They can accommodate multi-stations with mechanical ejection and transfer systems, and

63 integral lubrication/cooling facilities. Production rates of typically 40 parts a minute are achievable depending on part size. [ICFG, 2001]

Knuckle joint presses may be also used with due recognition of their greater dwell times and the probable requirement for increased cooling. Hydraulic presses are used in some companies for relatively small batch quantities and large (more than 5Kg) parts.

Lack of automation results in less process control (transport and dwell time in particular) and therefore a tendency for lower product consistency than for completely automated systems [ICFG, 2001].

CHAPTER 7 WARM FORGED PRODUCTS

In this chapter, various aspects of warm forged products are discussed in detail namely: physical and mechanical properties.

7.1 Physical Properties

The accuracy and surface finish of warm forged parts depend significantly on process control, tool design and precision [ICFG, 2001]. Figure 7-1, shows the range of tolerances achievable by different forging processes in terms of IT specifications as per DIN ISO quality.

Figure 7-2, shows the explanation of various IT specifications. Figure 7-3, shows the surface quality achievable by different forging processes.

Figure 7-1: Achievable Tolerances [Schuler, 2013]

Figure 7-2: Explanation of IT specification [Schuler 2013].

Figure 7-3: Achievable Surface Quality [Schuler, 2013]

The following values as shown in Table 7-1 may be expected to be achieved if good practice is employed.

Table 7-1: A Comparison of Forged Properties [ICFG, 2001].

Comparison item Hot Forging Warm Forging Cold Forging

Temperature range 1000 -1250°C Over Ac 1 Below Ac 1 Room temp Decarbonized layer 0.4-0.4 0.10-0.25 0.1 0 (mm)

Roughness (R A) >100 m <50 m <20 m <10 m Draft <7° <1° <1° 0° Dimension (mm) ± 0.5 – ±1.0 ± 0.05 – ± 0.20 ± 0.05 – ±0.15 ± 0.005 – ±0.1

Thickness (mm) ± 0.5 – ±1.5 ± 0.20 – ±0.40 ± 0.10 – ±0.25 ± 0.10 – ±0.20

Eccentricity (mm) 0.5-1.5 0.10-0.70 0.10-0.40 0.05-0.25

7.2 Mechanical Properties

As the mechanical properties of warm forged parts depend on deformation, final forming temperature, strain rate and post forged cooling arte, precise values cannot be given. The following are guidelines: [ICFG, 2001]

Table 7-2: Mechanical properties of warm forged parts. [ICFG, 2001]

Finish Forging Temperature Range °C Mechanical Prope rties

200°C - 400°C Similar to cold forged parts with equiv alent deformation. Above 400°C Tensile yield strength 1.1 to 1.5 times that of normalized material.

Note that due to heat loss in transport and to tool surfaces and heat generation due to deformation, the finish forming temperature will be different to the pre-heat temperature.

CHAPTER 8 ECONOMICS OF WARM FORGING

The total cost of warm forging, compared with both hot and cold for the production of a socket spanner and a rear axle has been estimated as shown in Table 8-1.

Table 8-1: Comparative cost of different forging processes [Hawkins, 1985]

CHAPTER 9 ENVIRONMENT AND ECOLOGY

The influence of warm forging practice on the environment has to be considered with the same care as for any forging or other industrial activity. Health, safety and environmental aspects of the following areas of the manufacturing and supply system have to be in accord with local, national and international laws, rules and standards which include; processes, machines and equipment for raw material handling, billet preparation, forging, pre-processing and post processing [ICFG, 2001]. Specific operations are;

• Storage

• Transportation

• Cutting (sawing and cropping)

Both processes have associated noise problems and adequate sound proofing should be ensured. Swarf is produced by sawing and this is often covered in an oil emulsion. The metal and the lubricant must and the lubricant must be separated and disposed of as legislation demands [ICFG, 2001].

• Cleaning (pickling, blasting, degreasing)

Any of these methods of cleaning, results in residues which have to be dealt with in the appropriate manner. Pickling leads to acidic sludge, blasting to abrasive particles contaminated with metal and mill scale, degreasing to hydrocarbon contaminated solvents [ICFG, 2001].

• Heating and cooling (gas and/or induction heating, air and/or liquid cooling)

Heating by gas and particularly induction is efficient but ultimately on cooling the forgings thermally load the environment [ICFG, 2001].

• Coating and lubrication

Graphite and the bases of the ‘white’ lubricants are generally nontoxic. Graphite however is considered to aesthetically degrade the environment. Airborne lubricant droplets, vapours emanating from heated oil-based lubricants and the stabilizers existing in water-based colloidal graphite lubricants should not be inhaled. All presses should be fitted with ducting and fans to draw the fumes and floating particles away to suitable filters [ICFG, 2001].

The lubricant waste from presses consists of graphite, salts, organic substances, metal oxides, water, ammonia and oil. If phosphate billets have been used the detritus will probably contain zinc also. Disposal of this effluent must be in accordance with the national laws [ICFG, 2001].

The best results concerning environmental impact and tool life are achieved by the application of tool lubricating systems consisting of a graphite-based slug coating and an oil-graphite- free die lubricant [Sheljaskov, 2001].

• Forging and piercing

Both involve high forces and are accompanied by noise and vibration. Adequate protection both to hearing and physical wellbeing of operators in case of breakage or misplaced work piece should be ensured.

Each process is associated with a degree of noise, vibration, dust, vapor and waste products which have attendant health, safety and ecological considerations. These lead to the need for personnel protection, exhausting and ventilating gases, particle separation form: exhaust gases, air and waste liquids, process automation, shielding of machines and other measures to separate people and animals from inimical conditions. Protection relates to

• Health and safety of people and animals.

• Safeguarding of buildings and equipment.

• Protection and conservation of natural environment and resources.

[ICFG, 2001]

REFERENCES

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[Altan, 2005] Altan, T., Ngaile, G., & Shen, G. (2005)."Cold and Hot Forging Fundamentals and Applications". ASM International. [M]

[Altan, 2011] Altan, T., & Deshpande, M. (2011). "Selection of die materials and surface treatments for increasing die life in hot and warm forging". Paper no 644-FIA Tech Conference, April 2011. [T]

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[Bariani, 1996] Bariani, P. F., Berti, G. A., D'Angelo, L., & Guggia, R. (1996). “Wear in hot and warm forging: Design and validation of a new laboratory test”. CIRP Annals - Manufacturing Technology, 45(1), 249-253. [T]

[Behrens, 2002] Behrens, A., & Just, H. (2002). “Extension of the forming limits in cold and warm forging by the FE based fracture analysis with the integrated damage model of effective stresses”. Journal of Materials Processing Technology, 125–126(0), 235-241. [M]

[Behrens, 2005] Behrens, B.A., Barnert, L., & Huskic,A. (2005)."Alternative techniques to reduce die wear - hard coating or ceramic?". Annals of the German academic Society for Production Engineering (WGP), 2005. [T]

[Davis, 2002] Davis, J.R. (2002).”Surface hardening of Steels - Understanding the basics". ASM International.2002. ISBN: 0-87170-764-0.

[Deshpande, Deshpande, M., & Altan, T. (2010)."Die materials and 2010] coatings for hot forging of steel in mechanical presses". ERC for Net Shape Manufacturing, Report No. ERC/NSM- 10-R-06, 2010, The Ohio State University. [T]

[Doege, 1978] Doege, E., Melching, R., & Kowallick, G. (1978). “Investigations into the behaviour of lubricants and the wear resistance of die materials in hot and warm forging”. Journal of Mechanical Working Technology, 2(2), 129-143. [L]

[Fujikawa, 1992] Fujikawa, S., Yoshioka, H., & Shimamura, S. (1992). “Cold- and warm-forging applications in the automotive industry”. Journal of Materials Processing Technology, 35(3–4), 317- 342. [M], [T] & [L]

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[Hirschvogel, Hirschvogel, M. (1979). “Recent developments in industrial 1979] practice of warm working”. Journal of Mechanical Working Technology, 2(4), 317-332. [M]

[Hirschvogel, Hirschvogel, M., & Dommelen, H. v. (1992). “Some 1992] applications of cold and warm forging”. Journal of Materials Processing Technology, 35(3–4), 343-356. [M]

[ICFGG, 2001] ICFG (2001).”Warm Forging of Steels”. International Cold Forging Group (ICFG) Document No.12/01 May 2001. [M]

[Iwama, 1997] Iwama, T., & Morimoto, Y. (1997). “Die life and lubrication in warm forging”. Journal of Materials Processing Technology, 71(1), 43-48. [L]

[Jeong, 2001] Jeong, D. J., Kim, D. J., Kim, J. H., Kim, B. M., & Dean, T. A. (2001). “Effects of surface treatments and lubricants for warm forging die life”. Journal of Materials Processing Technology, 113(1–3), 544-550. [L] 73

[Kang, 2007] Kang, J. H., Lee, K. O., & Kang, S. S. (2007). “Characterization of cooling heat transfer for various coolant conditions in warm forging process”. Journal of Materials Processing Technology, 184(1–3), 338-344. [L]

[Kang, 2005] Kim, D.H., Kim, B.M., & Kang, C.G. (2005). “Die life considering the deviation of the preheating billet temperature in hot forging process". Finite Elements in Analysis and Design.41(2005) 1255-1269

[Kang, 1999] Kang, J. H., Park, I. W., Jae, J. S., & Kang, S. S. (1999). “A study on die wear model considering thermal softening (II): Application of the suggested wear model”. Journal of Materials Processing Technology, 94(2–3), 183-188. [T]

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[Ssemakula, Ssemakula, H. (2013). “Minimization of stock weight during 2013] close-die forging of a spindle”. Material Sciences and Applications. 217-224

[Switzner, 2010] Switzner, N. T., Van Tyne, C. J., & Mataya, M. C. (2010). “Effect of forging strain rate and deformation temperature on the mechanical properties of warm-worked 304L stainless steel”. Journal of Materials Processing Technology, 210(8), 998-1007. [M]

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[Xinbo, 2002] Xinbo, L., Fubao, Z., Jianhua, F., & Zhiliang, Z. (2002). “Research on the flow stress characteristics of AISI 1006 and AISI 5140 in the temperature range of warm forging by means of thermo-mechanical experiments”. Journal of Materials Processing Technology, 122(1), 38-44. [M]

[Xinbo, 2003] Xinbo, L., Hongsheng, X., & Zhiliang, Z. (2003). “Flow stress of carbon steel AISI 1006 in temperature range of warm-forging”. Journal of Materials Processing Technology, 139(1–3), 543-546. [M]

APPENDIX A: FLOW STRESS OF STEELS

1. Plain carbon steel (Ck45) - flow stress &TTT Diagram

Figure A-1: Comparison of TTT- diagram and the flow stress for steel quality Ck45. (Plain Carbon Steel, 0.45%C) [Hirschvogel, 1979]. Figure A-1, shows the flow stress plotted in parallel with the TTT diagram. If we look at heating rates between 1°C/s and 10°C/s – fi ve different regions for forming operations become apparent. The region below 500°C is not suitable due to the blue brittleness, whereas the region of the inhomogeneous austenite is not suitable due to starting of scale formation. There are therefore three regions left for warm forging, and the properties of the forged part depend on which of these three regions is selected for forming, as a different microstructure will occur in each region. The recrystallization starts at temperatures as low as approximately 400°C; at 500°C it takes place simultaneously with the deformation process, and at temperatures higher than 670°C total recrystalli zation occurs. In the first region, during forging, the part will be work-hardened, because the deformed 77 microstructure was not totally recrystallized. In the second region, the tensile strength of the forging will be even lower due to deformation and recrystallization occurring simultaneously, the whole microstructure renews itself. In the third region; the microstructure partially changes into austenite during the heating process. [Hirschvogel, 1979]

2. 42CrMo4 (AISI 4140) – Flow Stress In Figure A-2, the stress strain curves of 42CrMo4 are presented. The flow stress is decreasing with increasing temperature .The temperature range 850-900°C has been selected for warm forging of 42CrMo4.The graph at 20°C shows a strong increase of the flow stress for low strains due to work hardening. This effect is reducing with increasing temperature. [Behrens, 2008]

Figure A-2: Effect of heating temperature on flow stress curves of 42CrMo4. [Behrens, 2008]

3. (16MnCr5) (SAE5115) - Flow Stress In Figure A-3, the stress strain curves of the case hardening steel 16MnCr5 at various forming temperatures are shown. It is obvious that the yield stress increases very rapidly at forming temperatures up to 750°C, whereas the increase is much lower at higher temperatures

Figure A-3: Stress-strain curves of steel 16 MnCr5 at various forging temperatures at strain rate of 10s -1[Neugebauer, 2003]

4. (1Cr18Ni9Ti) - Tensile strength & Equilibrium Diagram Figure A-4, shows the effect of temperature on the resistance to deformation of 1Cr18Ni9Ti as follows: the tensile strength decreases rapidly with increase of temperature from ambient to 200°C; it changes only slightly in the temperature range from 200°C to 500°C; it again decreases rapid ly when the temperature is above 500°C.The warm forging temperature for 1Cr18N i9Ti should therefore be either in the range 200-300°C or above 500°C. Accor ding to the equilibrium diagram for austenitic stainless steel, the structure of 18-8 stainless steel is austenitic above120°C, and austenite plus ferrite a t room temperature.

Figure A-4: a) Tensile properties of 1Cr18Ni9Ti at different temperatures. b) Equilibrium diagram for austenitic stainless steel [Shichun, 1982].

5. (2Cr13) As shown in Figure A-5, the strength decreases significantly at temperatures above 500°C; warm forging should therefore be carri ed out above 600°C in order to reduce the resistance of the steel to deformation.

Figure A-5: Strength-temperature curve for 2Cr13 (tempered) [Shichun, 1982].

6. (Cr17Ni2) The mechanical properties of tempered Cr17Ni2 at different temperatures are presented in Table A-1. Results show that the preheat temperature for the warm forging of Cr17Ni2 should be higher than its tempering temperature (550°C), with the aim of lowering resistance to deformation.

Table A-1: Mechanical Properties of Cr17Ni2 at different temperatures [Shichun, 1982]

7. (Cr12Mn5Ni4Mo3Al) From Table A-2, it can be seen that the deforming force decreases significantly when the preheat temperature is 900°C; when the red uction is 53-54%, the warm upsetting force at 650°C increases by 41% in compar ison with that at 900°C, when the reduction is 65-65.5%, the force at 650°C increases by 31% in comparison with that at 900°C; when the reduction i s 34-34.5%, the force at 350°C increases by 187% in comparison with that at 900°C. It is seen from these results that a suitable temperature for warm forging Cr12Mn5Ni4Mo3Al is above 650°C at the least.

Table A-2: Deforming force in warm upsetting Cr12Mn5Ni4Mo3Al. [Shichun, 1982]

8. (AISI 1006) From Figure A-6, it can be seen that the forming temperature is the primary factor affecting the flow stress of AISI 1006 in the temperature range of warm forging. The effect caused by temperature is more obvious than that caused by strain arte. The choice of warm forging temperature for AISI 1006 should be 850°C. This will be helpful in decreasing the forgi ng pressure and promoting metal flow.

Figure A-6: The flow stress curves of AISI 1006 at different strain rates. [Xinbo, 2002]

9. (AISI 5140) From Figure A-7, it can be seen that the effect of temperature on the flow stress of AISI 5140 is very obvious at the early period of deformation and after that, its effect on the flow stress begins to decrease gradually.

Figure A-7: The flow stress curves of AISI 5140 at different strain rates. [Xinbo, 2002]

APPENDIX B: TOOL STEELS

1. Thermal properties of carbides, ceramics and steels

Figure B-1: Comparison of thermal properties of carbides and ceramic to steels (ThCond: Thermal conductivity; ThExp: Coefficient of Thermal Expansion) [Manas, 2008]. Figure B-1,shows that Carbide had approximately 125% greater thermal conductivity compared to steels, which in turn is 200% greater than that of ceramic.. Also elastic modulus of carbides was 200% greater than that in steel and 80-90% greater than that of ceramic. The thermal fatigue in the die surface is influenced by the interaction between the thermal conductivities and thermal expansion [Manas, 2008]. 85

2. Hardness & costs of surface treatments Figure B-2, shows the typical ranges in hardness for many of the surface- engineering processes used to control wear. The diffusion treatments that produce harder surfaces are nitriding, boronizing (boriding) and chromizing. The hardest metal coating is chromium plate, although hardened electro-less nickel plate can attain values just under that of chromium. The surfaces that exceed the hardness of chromium are the cermets or ceramics. The popular solid ceramics used for wear applications – aluminum oxide, silicon carbide, and silicon nitride- generally have hardness in the range of 2000 to 3000 Kg/mm2.

Figure B-2: Ranges of hardness levels of various materials and surface treatments. EB, electron beam; high-strength, low alloy [Davis, 2002].

Figure B-3, provides some general guidelines for cost comparisons of different surface treatments [Davis, 2002]. 86

Figure B-3: Approximate relative costs of various surface treatments. [Davis, 2002]

3. Function of lubricant and slug coatings Lubrication and methods of application are under continuous development to achieve good lubricity, long tool life, reduced environmental impact at low cost.

A lubricant should fulfill the following requirements [ICFG, 2001] [Sheljaskov, 2001] [Doege, 1978]:

• Maintain a barrier between tool and work piece thereby eliminating pick-up and eliminating wear.

• Reduce tool/work piece friction thus reducing forging loads and tool stresses.

• Cool the tools to maintain their hardness and avoid plastic deformation.

• Good film formation on the dies (the die temperatures attain normally 150- 250°C)

• Low impact on the environment.

• Good propulsive effect. In order to avoid the sticking of the work piece in the die, it may be necessary for the lubricant to have a propulsive effect. A propulsive effect is indispensable for deep die cavities.

• Thermal stability has to be guaranteed during the time of deformation. For

eg. MoS 2 can be used successfully in cold forging; it is unstable at temperatures which occur during warm forging. Lubricants containing graphite are superior at this range of temperatures.

• Wet the die surface, even at elevated temperatures, and must also have good adherence.

A slug coating should fulfill the following requirements [Sheljaskov, 1994]

• Easy application.

• Uniform wetting of the slug.

• Good adherence to the slug.

• Good temperature resistance.

• Good lubricating properties during the forging operation.

Figure B-4, shows the various slug coatings and die lubricants developed from the study conducted in co-operation with two lubricant suppliers (Acheson 88

Colloiden B.V and Carl Bechem GmbH), a press manufacturer (Schuler GmbH), an environmental company (Fisia S.p.A, Italy) and three forging companies (Teksid S.p.A, Hirschvogel Unformtechnik GmbH and SNR Roulements, France) [Manas, 2008]

Figure B-4: Slug coating and die lubricants developed for warm forging. [Sheljaskov, 2001] Figure B-5, shows the friction factor values of oil- based graphite and water- based graphite lubricants, determined by ring compression tests. It was found that the friction factor of oil-based graphite is 0.3 and that of water-based graphite is 0.35 [Jeong, 2001].

Figure B-5 : Calibration curve of ring compression tests [Jeong, 2001] 4. Punching Force & Energy Estimation

Figure B-6, shows the punching force and energy calculation for two different punching velocities 0.01 in/s and 2 in/s. The calculations for punching velocity 3.93 in/s are assumed to be same as that of 2 in/s.

Figure B-6 : Force & Energy calculation for punching operation

APPENDIX C: SUPPLIERS

Suppliers of billet materials, die steels, lubricants and forging equipment are listed below.

1. Billet materials 1. Alton Steel, Inc 2. ArcelorMittal 3. ATI Allvac 4. Bohler-Uddeholm Corporation 5. Eaton Steel Bar Company 6. Electralloy 7. Ellwood Quality Steels Co. 8. Gerdau (GLN) 9. Gerdau (GSN) 10. Hamilton Specialty Bar Inc. 11. Kreher Steel Company, LLC 12. Latrobe Specialty Metals a Carpenter Company 13. Lehigh Specialty Melting Inc. 14. Nucor Corp. 15. Nucor Steel Memphis, Inc. 16. Nucor Steel-Darlington, SC 17. Nucor Steel-Norfolk, NE 18. Ovako North America, Inc. 19. Perlow Steel Corporation 20. Remelt Sources, Inc. 21. Republic Special Metals Inc. 22. Republic Steel Corporate Headquarters 23. Saarstahl dba Saarsteel USA, Inc. 24. Schmolz + Bickenbach USA, Inc. 25. Steel Dynamics - Engineered Bar Products Div. 26. Tata Steel 27. The Timken Company Steel Division 92

28. Turret Steel Industries, Inc. 29. Universal Stainless & Alloy Products, Inc. 30. Valbruna Slater Stainless Inc. 31. Vulcan Steel Products. 32. Warren Steel Holdings LLC. 33. Weld Mold Company.

2. Die Materials/Die Blocks 1. A. Finkl & Sons Co. 2. ATI Allvac. 3. Bohler-Uddeholm Corporation. 4. Composite Forgings, Ltd. 5. Cor-Met, Inc. 6. Ellwood Specialty Steel Co. 7. Schmolz + Bickenbach USA, Inc.

3. Lubricants 1. Henkel 2. ZWEZ – Chemie GmbH 3. Quaker Chemical Corporation 4. Budenheim USA, Inc. 5. Condat Corporation 6. Fuchs Lubricants Co. Lubrodal Division 7. Gerdau-Lansing Mt. Hope Facil

4. Forging Equipment 1. Schuler Group 2. Kurimoto 3. Elwood 4. Advanced Machine Design Company, Inc. 5. Aiken Engineering Company 6. Ajax-CECO Park-Ohio Capital Equipment Group. 7. Dango & Dienenthal Incorporated 8. Danieli Corporation 9. Erie Press SystemsErie Press Systems 10. Fontijne Grotnes, Inc. 11. Forge Enterprises International

12. Forging Developments (Int'l), Inc. Ajax-CECO Park-Ohio Capital Equip. 13. Forging Equipment Solutions. 14. GERB Vibration Control Systems, Inc. 15. Hariton Machinery Co., Inc. 16. Heating Induction Services, Inc. 17. Henkel 18. Hoffman Machinery Corp. 19. Interpower Induction USA 20. LASCO Engineering Services L.L.C. 21. Macrodyne Technologies Inc. 22. National Machinery LLC 23. Pillar Induction 24. Siempelkamp L.P. 25. Siempelkamp Pressen Syteme GmbH & Co 26. SMS Elotherm Induction Systems 27. SMS Meer Inc. 28. TrueForge Global Machinery Corp. 29. Wepuko Pahnke Engineering, LP.