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Hardenability of Steel

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Hardenability of Steel HARDENABILITY OF STEEL Lauralice C.F. Canale1, L. Albano1, George E. Totten2, Lemmy Meekisho2 1. Universidade de São Paulo, Escola de Engenharia de São Carlos, São Carlos, SP. Brazil. 2. Portland State University, Department of Mechanical and Materials Engineering, Portland, OR, USA ABSTRACT Hardening and depth of hardening of steel is a critically important material and process design parameter. Hardenability is the property of steel that describes the ability to harden steel and it is related to the composition of steel. A selective overview of experimental and predictive procedures to determine steel hardenability is presented here. This discussion covers the breadth of steel hardenabilities ranging from shallow, very difficult to harden to air-hardening steels. In addition sufficient detail and references are provided to enable the reader to adequately understand and practice those methods discussed. INTRODUCTION Plain-carbon and low- and medium-alloy steels are typically hardened by heating to the austenization temperature and then cooled sufficiently fast to form the desired structure, typically martensite in order to maximize the attainable strength. Martensite is characteristically hard and brittle. When the cross-section size is sufficiently large, the hardness varies across the section with the greatest hardness at the surface and with lower hardness at the center which is due to increasing amounts of softer pearlitic structure. The variation of the steel structure across the section is due to a gradation of cooling rates ranging from fastest at the surface to slowest at the center and the hardness distribution across the section is dependent on the transformation characteristics and composition of the steel. Krauss has defined the relative ability of a ferrous alloy to form martensite when quenched from a temperature above the upper critical temperature ( AC3 ), known as the austenitizing temperature, as hardenability [G. Krauss, Steels: Heat Treatment and Processing Principles, 1990, ASM International, Materials Park, OH, USA, p. 460.] Therefore, steel hardenability refers to the potential to be hardened by a thermal treatment. The greater the hardenability of a steel, the greater will be the depth of hardening when quenched from the AC3 temperature. Hardenability, should not be confused with hardness which is a material property and refers to the resistance to penetration which is proportional to strength. It is important to note that hardness is dependent only on carbon content in the steel whereas hardenability, which refers to the ability to achieve a certain hardness at a certain depth and which depends on both the carbon content as shown in Table 1 [1] and on the presence and amount of alloying elements. The actual depth of hardness depends on: Size and shape of the cross-section, Hardenability of the material, Quenching conditions. The effect of quenching conditions on the depth of hardening is not only dependent on the quenchant being used and its physical and chemical properties but also on process parameters such as bath temperature and agitation. These effects will be discussed subsequently. The focus of this chapter will be on plain-carbon and low-alloy carbon steels. Plain carbon steels typically contain no additional major concentrations of alloying elements other than carbon. Low-alloy carbon steels are a category of steels that exhibit mechanical properties superior to plain carbon steels due to the addition of alloying elements including: nickel, chromium, molybdenum, vanadium, manganese, and copper. Total alloy content can range from 2.07% up 10% chromium, a level just below that of stainless steels. The primary function of the alloying elements in low-alloy steels is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. This chapter describes factors that affect steel hardenability and methods to quantify it. DISCUSSION Time-Temperature-Transformation (TTT) diagrams, which are also called Isothermal- Transformation (IT) diagrams [2,3], are developed by heating small samples of steel to the temperature where the austenite structure is completely formed, i.e. austenitizing temperature, and then rapidly cooling to a temperature intermediate between the austenitizing and MS temperature, and then holding for a fixed period of time until the transformation is complete, at which point the transformation products are determined. This is done repeatedly until a TTT diagram is constructed such as those shown in Figure 1 for AISI 1045 and AISI 4140 steel. TTT diagrams can only be read along the isotherms shown and these diagrams strictly apply only to those transformations conducted at constant temperature. By comparing the position of the nose of the pearlite transformation, it is evident from transformation times that the AISI 1045 plain carbon steel is considerable less hardenable than AISI 4140. However, relatively few commercial heat treatments are conducted isothermally. Instead, steel is heated to the austenitizing temperature and then continuously cooled in a quenching bath at a temperature and at a cooling rate suitable for the alloy and section size being treated. To better model this process, steel samples can be cooled at different specified rates and the proportion of transformation products formed after cooling to various temperatures intermediate between the austenitizing and MS temperatures are determined and a Continuous-Cooling-Transformation (CCT) diagram is constructed like those shown for AISI 1045 and AISI 4140 steel in Figure 2. CCT curves provide data on the temperatures for each phase transformation, the amount of phase transformation product obtained for each cooling rate with respect to time, and the cooling rate necessary to obtain martensite. CCT diagrams can only be read along the curves for different cooling rates. The CCT diagrams show from the displacement of the AISI 4140 transformation curve to the right relative to the curve for AISI 1045 plain carbon steel that it is the more hardenable steel. Table 1 Effect of Carbon Concentration and Martensite Content on the As-Quenched Hardness of Steel Carbon Hardness (HRC) % 99% 95% 90% 80% 50% Martensite Martensite Martensite Martensite Martensite 0.10 38.5 32.9 30.7 27.8 26.2 0.12 39.5 34.5 32.3 29.3 27.3 0.14 40.6 36.1 33.9 30.8 28.4 0.16 41.8 37.6 35.3 32.3 29.5 0.18 42.9 39.1 36.8 33.7 30.7 0.20 44.2 40.5 38.2 35.0 31.8 0.22 45.4 41.9 39.6 36.3 33.0 0.24 46.6 43.2 40.9 37.6 34.2 0.26 47.9 44.5 42.2 38.8 35.3 0.28 49.1 44.8 43.4 40.0 36.4 0.30 50.3 47.0 44.6 41.2 37.5 0.32 51.5 48.2 45.8 42.3 38.5 0.34 52.7 49.3 46.9 43.4 39.5 0.36 53.9 50.4 47.9 44.4 40.5 0.38 55.0 51.4 49.0 45.4 41.5 0.40 56.1 52.4 50.0 46.4 42.4 0.42 57.1 53.4 50.9 47.3 43.4 0.44 58.1 54.3 51.8 48.2 44.3 0.46 59.1 55.2 52.7 49.0 45.1 0.48 60.0 56.0 53.5 49.8 46.0 0.50 60.9 56.8 54.3 50.6 46.8 0.52 61.7 57.5 55.0 51.3 47.7 0.54 62.5 58.2 55.7 52.0 48.5 0.56 63.2 58.9 56.3 52.6 49.3 0.58 63.8 59.5 57.0 53.2 50.0 0.60 64.3 60.0 57.5 53.8 50.7 Figure 1 – Representations of TTT diagrams for AISI 1045 (on left) and AISI 4140 (on right). Figure 2 - CCT diagrams for AISI 1045 (on left) and AISI 4140 (right). Critical Cooling Rate The oldest concept of assessing hardenability based on transformation diagrams is the “critical cooling rate” [4]. Krauss has defined the critical cooling rate of a steel as: “The rate of continuous cooling required to prevent undesirable transformation. For Steel, it is the minimum rate at which austenite must be continuously cooled to suppress transformations above the MS temperature” [ 5] or the slowest cooling rate that will produce 100% martensite. Since the cooling curve from which the critical cooling rate is derived is not linear, there are various methods for its determination. Using the TTT curve for the alloy of interest, examples include: the time to cool half way between the austenitizing temperature and the temperature of the quench bath; average cooling rate calculated from the (austenitizing temperature – quenchant bath temperature)/elapsed time to cool; or the slope of the tangent to the transformation curve denoting the onset of austenite precipitation at a given temperature (“nose method” [6]) such as 700 ºC – although the selection of this temperature is not obvious [7]. Sverdlin and Ness have reported that the so-called “nose method” provides an estimate that is approximately 1.5 times the true critical cooling rate [8]. Furthermore, the critical cooling rates have been obtained from either the TTT or CCT diagrams which are clearly different. Therefore, the method by which the critical cooling rate is calculated must be provided. A CCT diagram for a eutectoid steel is shown in Figure 3 . Cooling curves for different cooling processes including a water and an oil quench and a normalizing and a full-annealing cooling process. Cooling after normalizing is typically performed by removing the load and cooling in air at ambient temperature. Typically, the full-annealing process is conducting by heating the steel in the furnace and then shutting off the heat and cooling the load in the furnace (furnace cooling).
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