3Cr-W, 3Cr-W-Co, 12Cr-W-Co型熱間工具鋼の焼戻挙動について

抄録

The changes of hardness, Cahrpy impact value, lattice parameter of ferrite, internal stress in matrix, carbide reaction, elements in carbide, and electron microstructure occuring during isothermal tempering of hot-work tool steels of 3Cr-5W-0.3V(DSS), 3Cr-9W-0.3V/(DSH), 3Cr-5W-0.3V-3Co(CoSS), 3Cr-9W-0.3V -3Co(CoSH), 12Cr-7W-0.5V-5Co(DSE), and 12Cr-7W-0.5V-10Co(DSF) types for up to 1000 hr in the range 400-700°C have been studied.
The results obtained were plotted against the tempering parameter P=T(20+logt) x 10^-8, and discussed in detail. Peak of secondary hardening appeares in P=17-18 for 3Cr-W-(Co) type, and at about P=16 for 12Cr-W-Co type. Charpy impact values show a broad valley near the parameter of peak hardness for 3Cr type, and show it in higher parameter than that of peak hardness for 12Cr type. Lattice parame-meters of matrix in DSS and DSE decrease with increasing parameter, and a maximum internal stressof about 100 kg/mm² is obtained at about P=16 and 15.5 for DSS and DSE, respectively. On the above isothermal tempering, the following sequence of changes in the carbides takes place in each type: Fe₈C→Fe₈C+W₂C→W₂C→W₂C+M₆C→M₆C→ for 3Cr-5W-(Co) type, Fe₈C→resolution→W₂C→W₂C+M₆C→M₈C for 3Cr-9W-(Co)type, Fe₃C→resplution→M₇C₃→M₇C₃+M₂₃C₆→M₂₃C₆+M₇C₃+Fe₂W→Fe²W+M₂₃C₆ for 12Cr-W-Co type. Cobalt decreases the rate of growth of carbide, and the increase of resistance to softening during tempering is attributed to this effect.