The Effect Of Heat Treatment On The Impact Toughness Of High-Chromium Heat Resistant Steels

Microstructure and properties of X12CrMoWVNbN10-1-1（X12）, X22CrMoV12-1（X22） and 4Cr9Si2 have been studied in this paper by the scanning electron microscopy, transmission electron microscopy equipped with energy dispersive spectrum analysis, X-Ray diffraction,hardness and impact test. The conclusions are made as follows:As-cast 4Cr9Si2 steel has a typical lath martensitic microstructure. After annealing at 860℃ for 2h, coarse M₂₃C₆ carbide with particular and stripy shapes precipitates in the ferrite matrix, and the hardness of the steel decreases obviously but its impact absorbed energy is still very low. When the as-cast 4Cr9Si2 steel is normalized at 1020℃ for 1h followed by air cooling then tempered at 720℃ for 2h, the cooling rate after the tempering treatment has much influence on the microstructure and impact absorbed energy of the steel and high temperature tempering embrittlement is found. Furnace cooling following the tempering treatment causes a lower impact absorbed energy and a higher hardness than the water cooling does because the fine dispersive carbide particles are densely distributed in the matrix and secondary hardening is obtained in the furnace-cooling condition.After quenching at 1070℃ for 1h followed by oil cooling, both X12 and X22 steels have lath martensitic microstructure, and there are some large sized MX-type carbide particles existing in matrix of X12 steel. Both steels maintain lath martensitic microstructure after tempering at different temperature. Tempering below 600 ℃, needle-liked M3C-type carbide precipitates from martensitic lathes, and with increasing of the tempering temperature,the M3C-type carbide gradually disappears,instead of the M₂₃C₆-type carbide particles are formed in the prior austenite grain boundaries and lath boundaries. With the increase of tempering temperature, the hardness decreases firstly, then increases, and it reaches a peak value at 500℃,after that, the hardness decreases with increasing tempering temperature. The changing tendency of impact absorbed energy is contrary of its hardness for X12 steel, however, the changing tendency for X22 steel is more complicated and shows increasing at first followed by a decrease, then rising and dropping off again, and finally increasing with the increase of tempering temperature. In this process, there are two low valleys at 500℃ and 650℃, respectively.After high-temperature tempering treatment of X22 steel, the number of carbides distributed in the martensitic lath boundaries and prior austenite grain boundaries gradually decrease with the increase of cooling rate, and the size of the particles increases with decreasing the cooling rate. The impact toughness increases with the increase of cooling rate following the high-temperature tempering treatment, but the hardness change is not obviously. The impact fracture morphology is characterized by ductile rupture tempered at 750℃ followed by different cooling rate. The tempering brittleness tempered at 650℃ can be eliminated by secondary tempering at 750℃.