International Conference on Engineering and Built Environment (ICEBE) 2012
CARBIDE PRECIPITATIONS IN SS440C STAINLESS STEEL DURING TEMPERING TREATMENT
S.H.Salleh, M.Z.Omar, J.Syarif, A.G.Jaharah, S.Abdullah and M.J.Ghazali
Department of Mechanical and Material Engineering
National University of Malaysia, Malaysia.
SS440C steel is a high-carbon martensitic stainless steel, that is capable of attaining the best mechanical properties such as high strength and hardness among the others martensitic grades. The mechanical properties of the steel strongly depend on heat treatment process which can control the precipitation of carbides in SS440C steel. In this study, the microstructural change and carbide precipitation in the steel had been investigated and also the influence of them on hardness had been studied. As-quenched steel reveals a typical martensitic structure, with undissolved M7C3 carbide. Since carbon atoms have precipitated as M7C3 during solution treatment, the hardness of the as-quenched steel is lower than that of full martensitic 1%C steel. Optical observation reveals M23C6 carbide precipitates along the grain boundary of initial stage on tempering. Then, the M23C6 carbides grow as needle shape within the grain. The hardness of the steel increases owing to secondary hardening by Molydenum carbide on tempering.
Keyword: SS440C steel; heat treatment; carbides; tempering; martensite.
Martensitic stainless steel is widely used in engineering applications such as ball bearings, races, gage blocks, valve parts and many other manufacturing essential parts. It is particularly used because of its favorable overall properties such as high mechanical strength and hardness. Martensitic stainless steel is essentially Fe-Cr alloys containing 12 to 17% Cr with sufficient carbon (0.15 to 1.0% C) so that a martensitic structure can be produced by quenching from austenitic phase region (William 2002).
The mechanical properties of the steel depend on heat treatment. The conventional heat treatments of the steel are solution treatment, quenching and tempering. These heat treatment processes may cause formation of ferrite and spheroidol carbide and cementite. Because of the high content of carbon, this carbon had a mobility of high sensitivity on heating. It is thought that the carbon can change the shape and the chemistry of the carbide. That kind of change can affect the mechanical properties of this steel.
In this study, microstructural change of SS440C stainless steel on heat treatment and the precipitation of carbide during tempering process and its effect on hardness of the steel had been investigated.
SS440C martensitic stainless steel was used as a specimen in this study. It was annealed at 1040oC followed by air cooling. Table 1 shows the chemical composition of the steel. The samples were cut using the abrasive cutter and then austenised at 1150oC for 3600 seconds, followed by oil quenching. Futhermore, samples were tempered at 660oC for two heating times (600 and 3600s). Figure 1 shows the equilibrium phase diagram of Fe-18wt%Cr-0.75wt%Mo-1.0wt%C alloy which calculated by Thermo_calc. The steel has (austenite+M7C3 carbide) phase and (ferrite+M23C6 carbide) phase at austenising temperature of 1150oC and tempering of 660oC, recpectively. The samples were ground, polished and etched with Vilella’s reagent. Microstructures and hardness were investigated by Rockwell Hardness Tester and optical microscope (OM), respectively.
TABLE 1 The chemical composition of SS440C steel. (all in wt.%)
FIGURE 1 The equilibrium phase diagram of SS440C
RESULT AND DISCUSSION
Microstructural and hardness of as-received and as-quenched steel
Figure 2 shows the microstructure of SS440C steel in (a) as-received and (b) as-quenched steel. The as-received sample had been treated through the annealing process at 1040oC. This treatment results in the formation of ferrite matrix and carbide particles.
As-quenched sample exhibit a typical martensitic structure, which forms by a sudden shear process in the austenite lattice. The martensite reaction in steel is the best known of a large group of transformation in alloys in which the transformation occurs without changing the chemical composition (Honeycombe 1981). Beside martensite, the M7C3 carbides are also observed within the martensitic structure. The microstructures shown adhered well with the calculated SS440C phase diagram.
FIGURE 2 The optical micrograph of the specimens: (a) in as-received condition and (b) in as-quenched condition.
The hardness values of as-received and as-quenched steels are shown in Figure 3. Llewellyn (1992) has reported that, hardness of martensitic steel increases mainly owing to solid solution strengthening of C and the hardness of steel containing solute C of 1.0wt% will reach value of 65HRC. On the other hand, the hardness of the as-quenched sample is around 45HRC and clearly lower than that of full martensitic steel. The decrease of the as-quenched sample’s hardness is thought to be due to the decrease of solute C content owing to formation of undissolved M7C3 carbide. Furthermore, formation of retained austenite is also thought as a reason why the hardness decreases because the steel contains high concentration of carbon.
FIGURE 3 Hardness of as-received and as-quenched steel
Tempering behavior of SS440C steel
Figure 4 shows the microstructures of tempered samples. The samples were tempered at 660oC for (a) 600s and (b) 3600s were still have undissolved M7C3 carbides in this microstructure. The 600s-tempered sample shows the undissolved exist within the structure. M7C3 type carbides form along the grain boundary during this tempering process, at initial stage (600s).
After holding for 3600s, the grain boundaries completely covered by carbide. Futhermore, M23C6 carbide grows as needle-like shape within ferrite (previously martensite grain).
FIGURE 4 The optical micrograph of the tempered samples at 660oC for (a) 600s and (b) 3600s.
Figure 5 shows the change in hardness steels as a function of holding time of tempering for 600s, the hardness of the sample slightly decreased to 36HRC. It is well-known that the tempering is to relieve the stresses that are set up during the transformation of austenite to martensite, to toughen the steel and some times to reduce the hardness (Thelning 1984). Recovery of dislocation and the decrease in contribution of solid solution strengthening of carbon due to precipitation of M23C6 on tempering may cause the decrease in hardness.
The hardness of the tempered-sample for 3600s increases to 42HRC. Besides increasing the toughness of the sample through tempering, the hardness also can be increased same as the as-quenched sample. It is thought that the formation of Mo-carbide influences the increase in hardness.
FIGURE 5 The hardness value of treated and tempered steels
The change in microstructure of SS440C steel and carbide precipitation during tempering had been observed with their effect on hardness. The microstructure of as-quenched sample consist of martensite, undissolved M7C3 carbide and retained austenite while the as-tempered microstructure consist of undissolved M7C3 carbide and new precipitate M23C6 carbide. It is thought that the hardness of as-tempered sample increase as same as the as-quenched sample because of the formation of Mo-carbide during tempering.
The authors would like to thank Kyushu University, Japan for preparing the equilibrium phase diagram of SS440C steel. And also to Ministry of Higher Education (MOHE) for sponsoring this work.
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