Abstract
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.
