Tempering of Steel - Laboratory 4 | MAT E 443, Lab Reports of Materials science

Download Tempering of Steel - Laboratory 4 | MAT E 443 and more Lab Reports Materials science in PDF only on Docsity! Mat E 443 – Laboratory 4: Tempering of Steel Background To this point, we have discussed the formation of the very hard martensite phase through the diffusionless transformation of austenite. Indeed, the enhancement of hardenability is the principle motivation for the alloying of steels, illustrating the utility of the martensitic structure. In practice however, due to the low toughness of this hard but brittle phase, martensitic steels are rarely used in the as-quenched condition. Rather, these steels are heat treated so that diffusional processes can operate to relieve or “relax” the state of this metastable structure. Such a process is known as tempering, and it permits decomposition of the BCT martensite phase, where the c/a ratio decreases with increasing tempering temperature or time (see below left).1 As the structure moves closer to equilibrium, the ductility of the steel is increased dramatically with an associated decrease in strength (see below right).2 Accordingly, well controlled tempering processes are critical to the production of high strength high toughness steels with controlled properties. The property changes observed during tempering come about through a number of diffusional processes that change the microstructure of the steel. We recall that the BCT martensite phase itself is metastable and also that it is generally heavily defected with either a high density of twins or dislocation structures. Therefore, a large number of process may be set into operation when sufficient thermal activation is provided upon heating. The driving forces for these processes and associated rates are temperature dependent and different types of relaxation processes tend to dominate in different temperature regimes. For this reason, tempering has 1 Kurdjumov, J. Iron Steel Inst. 195 (1960) 26. 2 Modern Steels and their properties, Handbook 2757, 7 th ed. Bethlehem Steel Corp., Bethlehem, PA, 1972. traditionally been divided into several “stages” which simply denote these temperature ranges where different mechanisms become dominant. It should be noted, of course, that many mechanisms may be operative simultaneously. The stages only serve as a general guide as to which mechanisms may be dominant at a particular temperature. The stages of tempering are listed below. Stage 1: (100-250ºC) Precipitation of fine iron carbides ( ) with a decrease in the c/a ratio. Stage 2: (200-300 ºC) Decomposition of retained austenite to ferrite and cementite (not pearlite). Stage 3: (250-350 ºC) Dissolution of of -carbide and precipitation of cementite. Stage 4: (300-700 ºC) Coarsening and spheroidization of cementite. Recrystallization of ferrite. These stages are indicated in the figure below (left),3 on a plot of hardness vs tempering temperature for plain carbon steels of different carbon content. There is one other important microstructural change that may occur during high temperature tempering of alloy steels. We note that several alloying elements may form carbides (as well as nitrides and/or borides) which are more stable than cementite.4 Specifically, the alloying elements of Cr, Mo, V, W, and Ti are all good carbide formers and may form carbide phases during tempering treatments in the range of 500 to 600 ºC. This process, known as secondary hardening, is not generally considered to be a stage of tempering (although some may refer to is as such), and it may result in a substantial increase in strength (counter to the processes associated with all true tempering stages). The figure above (right) shows the marked effect of molybdenum content on this secondary hardening behavior in a steel with 0.1 wt% carbon.5 3 Speich, Trans. Met. Soc. AIME 245 (1969) 2553. 4 R.W.K. Honeycombe and H.K.D.H. Bhadeshia, Steels: Microstructure and Properties, Edward Arnold, London 1995, p.184. 5 Irving and Pickering, J. Iron Steel Inst. 194 (1960) 137.