Study On Microstructure Of PH13-8Mo Precipitation Hardening Stainless Steel

The influence of different heat treatment temperature on mechanical properties ofPH13-8Mo steel was studied, and the evolution of various mechanical properties withquenching and tempering temperature were shown. Using the transmission electronmicroscopy (TEM) and X-ray diffraction (XRD), the microstructure of PH13-8Mo steelat different heat treatment temperature was researched, and then the morphology anddistribution state of austenitic in different heat treatment temperature for PH13-8Mosteel was analyzed. The relationship between variation of tensile strength curve and theNiAl precipitation phase was curtained. Based on the above research, the appropriateheat treatment process parameters were determined and the microstructure evolutionmechanisms in the heat treatment process were deeply analyzed.The results show that the quenching process of PH13-8Mo steel can strongly affectthe impact toughness αkuvalue, with plays little effect on other mechanical properties.The impact toughness αkufirstly reach the peak when quenching at900℃, while will bereduced with increasing quenching temperature. Quenched lath martensite contains ahigh density of dislocations. Within the quenching temperature range, with the increaseof quenching temperature, the grain size was significantly increased and the grain sizerating dropped from9to5.The tempering process of PH13-8Mo steel can strongly affect strength, hardness,and impact toughness, while has little effect on elongation δ5and the section shrinkageΨ. When tempering temperature range is between20℃-750℃, the tensile strength σb,yield strength σp0.2and hardness increased with tempering temperature firstly and thendecrease gradually. The mechanical parameters reached the peak value at about500℃(σb1580MPa, σp0.21470MPa, the hardness47.3HRC). The variation of the impacttoughness shows opposite trend. When tempering temperature is480℃, the impacttoughness αkureached the lowest value (αku19.1J/cm2).As a precipitation hardening steel, NiAl precipitation phase is main strengtheningmechanisms in PH13-8Mo steel. No NiAl precipitated phase can not be found withtempering below400℃. NiAl ordered phase with similar substrate lattice constant B2structure can be observed with tempering at450℃. When the tempering temperature isbelow650℃, NiAl precipitation grew up with increasing of tempering temperature.However, the growth of NiAl particle directly leads to the losing of coherent relationship with the matrix and cause a rapid decrease of strength. When tempering atabout500℃, strength reached its peak value. This is because that the NiAl precipitatephase size (3nm to5nm) is located near the critical dimension and produces a strongprecipitation strengthening effect. When the tempering temperature is higher than700℃,NiAl phase dissolved.For the variation of the austenite and carbides, its microstructure contains a smallamount of residual austenite as well as the large-sized undissolved M6C carbideswithout before tempering treatment. After tempering, the alloy gradually appearsreverted austenite. With the tempering temperature increasing, reverted austeniteappeared firstly between the laths when tempering between510℃and650℃. Whenthe tempering temperature reaches570℃, the reverted austenite gradually appears inlath. When tempering at650℃, reverted austenite content reaches maximum and thesize become coarse. When tempering higher than700℃, austenite content wassignificantly reduced. However, the evolution of M6C large particles in PH13-8Mo steelis substantially independent with the influence of tempering.