, Research Paper
Principles of Heat Treating of Steels
A steel is usually defined as an alloy of iron and carbon with the content between a few hundreds of a percent up to about 2 wt%. Other alloying elements can amount in total to about 5 wt% in low-alloy steels and higher in more highly alloyed steels such as tool steels and stainless steels. Steels can exhibit a wide variety of properties depending on composition as well as the phases and microconstituents present, which in turn depend on the heat treatment.
The Fe-C Phase Diagram
The basis for the understanding of the heat treatment of steels is the Fe-C phase diagram. Because it is well explained in earlier volumes of Metals Handbook and in many elementary textbooks, the stable iron-graphite diagram and the metastable Fe-Fe3 C diagram. The stable condition usually takes a very long time to develop, especially in the low-temperature and low-carbon range, and therefore the metastable diagram is of more interest. The Fe-C diagram shows which phases are to be expected at equilibrium for different combinations of carbon concentration and temperature. We distinguish at the low-carbon and ferrite, which can at most dissolve 0.028 wt% C at 727 oC and austenite which can dissolve 2.11 wt% C at 1148 oC. At the carbon-rich side we find cementite. Of less interest, except for highly alloyed steels, is the d-ferrite existing at the highest temperatures. Between the single-phase fields are found regions with mixtures of two phases, such as ferrite + cementite, austenite + cementite, and ferrite + austenite. At the highest temperatures, the liquid phase field can be found and below this are the two phase fields liquid + austenite, liquid + cementite, and liquid + d-ferrite. In heat treating of steels the liquid phase is always avoided. Some important boundaries at single-phase fields have been given special names. These include: the carbon content at which the minimum austenite temperature is attained is called the eutectoid carbon content. The ferrite-cementite phase mixture of this composition formed during cooling has a characteristic appearance and is called pearlite and can be treated as a microstructural entity or microconstituent. It is an aggregate of alternating ferrite and cementite particles dispersed with a ferrite matrix after extended holding close to A1. The Fe-C diagram is of experimental origin. The knowledge of the thermodynamic principles and modern thermodynamic data now permits very accurate calculations of this diagram. This is particularly useful when phase boundaries must be extrapolated and at low temperatures where the experimental equilibria are extremely slow to develop. If alloying elements are added to the carbon-alloy, the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed. Classical diagrams show the variation of A1 and the eutectiod carbon content with increasing amount of a selected number of alloying elements. If suffices here to mention that all important alloying elements decrease the eutectoid carbon content, the austenite-stabilizing elements manganese and nickel decrease A1, and the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A1. Modern thermodynamic calculations allow accurate determinations of these shifts that affect the driving force for phase transformation.
The kinetic aspects of phase transformations are as important as the equilibrium diagrams for the heat treatment of steels. The metastable phase martensite and the morphologically metastable microconstituent bainite, which are of extreme importance to the properties of steels, can generally form with comparatively rapid cooling to ambient temperature, when the diffusion of carbon and alloying elements is suppressed or limited to a very short range. Bainite is a eutectoid decomposition that is a mixture of ferrite and cementite. Martensite, the hardest constituent forms during severe quenches from supersaturated austenite by a shear transformation. Its hardness increases monotonically with carbon content up to about 0.7 wt%. If these unstable metastable products are subsequently heated to a moderately elevated temperature, they decompose to more stable distributions of ferrite and carbide. The reheating process is sometimes known as tempering or annealing. The transformation of an ambient temperature structure like ferrite-pearlite or tempered martensite to the elevated-temperature structure of austenite or austenite + carbide is also of importance in the heat treatment of steel.
The goal of heat treatment of steel is very often to attain a satisfactory hardness. The important microstructural phase is then normally martensite, which is the hardest constituent in low-alloy steels. The hardness of martensite is primarily dependent on its carbon content. If the microstructure is not fully martensitic, its hardness is lower. In practical heat treatment it is important to achieve full hardness to a certain minimum depth after cooling, that is, to obtain a fully martensitic microstructure to a certain minimum depth, which also represents a critical cooling rate. If a given steel does not permit a martensitic structure to be formed to this depth, one has to choose another steel with a higher hardenability.
Principles of Tempering of Steels
Martensitic is a very hard phase in steel. It owes its high hardness to a strong supersaturation of carbon in the iron lattice and to a high density of crystal defects, especially dislocations, and high – and – low angle boundaries. However, except at low carbon contents, martensitic steels have insufficient toughness for many applications. Tempering of martensitic steels, by heating for a certain time at temperatures below the A1 , is therefore introduced to exchange some of the strength for greater ductility through reduction of the carbon supersaturation initially present and replacing it with more stable structures. Additionally, the retained austenite associated with martensite in steels containing more than about 0.7 wt% C can be decomposed during the tempering process. In carbon steels containing small percentages of the common alloying elements, one distinguishes the following stages during tempering. In steels alloyed with chromium, molybdenum, vanadium, or tungsten, formation of alloy carbides occurs in the temperature range 500 to 700 oC. During stage 1, the hardness increases slightly while during stage 2, 3, and 4 the hardness decrease.
Cooling Media and Quench Intensity
The depth of hardness at a given workpiece dimension is determined by the chemical composition of the steel, the austenite grain size as established during the austenitizing treatment, and the cooling rate. The steel is normally chosen on the basis of hardenability. The choice of cooling medium, on the other hand, is less exact and crude rules are normally applied (unalloyed steel is quenched in water, alloy steels in oil, and high-alloy steels in air). Molten salt is often used for bainitic hardening of medium-carbon steels and martempering of carburized parts. Judicious selection of cooling medium is critical for obtaining optimum mechanical properties, avoiding quench cracks, minimizing distortion and improving reproducibility in hardening. Of most interest are the liquid quenching media, and they also show the most complicated cooling process.
Thermal Stressed during and Residual Stresses after Heat Treatment
Heat treatment of steel, especially martensitic hardening, is usually accompanied by the evolution of large residual stresses, that exist without any external load on the part considered. Residual stresses can be divided into three categories. A macroresidual stress is the average of the residual stress in many adjacent grains of the material. If the workpiece is cut or material is removed, the presence of macroresidual stresses into a workpiece by heat treatment or plastic deformation may also cause a distortion of the part. The pseudo-macroresidual stress is the average of the residual stress in many grains of one phase in a multiphase material minus the macroresidual stress in a part is the total residual stress minus the macroresidual and the pseudo-macroresidual stress. The thermal stress is approximately proportional to the temperature difference and is tensile in the surface and compressive in the core. Large thermal stresses are favored by low thermal conductivity, high heat capacity, and high thermal expansion coefficient. Other factors increasing the temperature difference and thermal stresses are large thickness dimensions and high-cooling intensity of the cooling medium. A large yield stress at elevated temperatures will decrease the degree of plastic flow and thus the residual stress, while the yield stress at the ambient temperature puts an upper limit on the residual stress.
Cracking and Distortion due to Hardening
There is a risk for cracking of a workpiece if large tensile stresses, transient or residual, are combined with the presence of a brittle microstructure. Thermal stresses during cooling generally increase with the size of a workpiece. For phase transformation-induced stresses, geometric dimension, hardenability of the steel, and quench intensity interact in complicated manner. However, as a general rule it holds that the use of a more efficient cooling medium, for example, water as compared to oil, will lead to larger stresses. The presence of geometric stress raisers increases the risk of cracking. Large tensile stresses in the core at lower temperatures may lead to center cracks even if the microstructure is not martensitic. Such a situation exists for larger diameter cylinders with a martensitic surface and a ferritic-pearlitic core. Case hardening may also lead to core cracking. Hardening is usually accompanied by distortion of a workpiece. The degree of distortion depends on the magnitude of the residual stresses. Hardening procedures that minimize transient and residual stresses are beneficial as well as the use of fixtures. Distortion can also occur during tempering or annealing due to release of residual stresses or phase transformations during tempering.