Study on the mechanism of low temperature toughness of 9NI steel is good, has been the material researchers interested in the topic, after years of research, the existing three kinds of interpretation mechanism to get everyone's approval, an explanation is attributed the low temperature toughness of good appearance in rotary austenite, it has extended the organization that crack tip blunting crack effect, when the crack tip encountered reversed austenite region, the tip steering and bifurcation, in order to increase energy consumption and absorption, but was blocked from crack tip, the crack tip passivation mechanism, the internal microstructure of main crack to change as well as the crack tip bifurcation, in which a crack to move on, moreover influence on impact fracture zone in many different size holes and isolated micro crack. Not only can improve the toughness of reversed austenite, when the lath martensite could also increase the low temperature toughness of role, when the lath martensite bunch transverse to the forward direction of the crack tip, will cause the crack tip blunting and hindered, increase the load to the crack tip forward must be accompanied by a sharp crack tip deformation, at the macro performance for the impact toughness increased. Cementite also has the crack tip passivation effect, which indicates that the necessary condition for the existence of reverted austenite is not crack passivation. Another explanation is that the change of deformation induced martensite in the fracture process, namely the local transformation induced plasticity, in front of the crack in the plastic zone induced by mechanical stress, rotary austenite into martensite, absorb the additional energy, so that the plastic deformation of local stress concentration caused by the relaxation, because phase transition occurs, stop crack initiation and growth. Rotary austenite to different local transformation induced plasticity, low temperature toughness increase its contribution is different, reversed austenite transformed before metamorphosis (heat labile), reversed austenite transformation in the deformation (mechanical stability) have no effect on the impact toughness, the former is the worst. This is best, the third center. In addition, there is an explanation is reversed austenite can absorb the ferrite brittle C, N and other elements, and the martensite purification, so as to improve the low temperature toughness, namely rotary austenite purification. The concentration of interstitial atoms and mismatch atomic rotary in austenite to martensite microstructure is higher than the adjacent, C, N and other interstitial atom enrichment in rotary austenite, colleagues matrix toughness makes martensite matrix purification also increased, while NI, Mn atoms tend to segregate at rotary austenite, strengthen the rotary austenitic heat stability and mechanical stability, more favorable to improve the low temperature toughness.
There is an important role on low temperature toughness of 9NI steel rotary austenite increase, but not the determinants and the only factor, even after quenching and tempering and dual phase heat treatment of 9NI steel, in order to get the same amount of reverted austenite, but its low temperature toughness of different, even a large gap, the low temperature toughness and reversed austenite on distribution and morphology. Rotary austenite has two forms, one is the irregular mass, mainly in the original austenite grain boundary precipitation, another form of flake reversed austenite, which is precipitated in the martensite lath in between, reversed austenite between the laths on low temperature toughness plays an important role in.
Have an important influence on low temperature toughness of 9NI steel rotary austenite, but not that the number of reverted austenite, low temperature toughness is higher, started with a rotary austenite increase low temperature toughness increased, but to a certain amount, low temperature toughness reaches the maximum, and then with the reversed austenite decreases with increasing the amount of, the relevant this and the rotary Austenite Stability, formation, stability and atomic diffusion of reverted austenite is concerned, the higher the tempering temperature, holding time is long, reversed austenite more generated, of which the stability element soluble factors such as C, NI, Mn and other less, the stability of reverted austenite decreases, the increase is not the stability of reversed austenite, form mutual continuous network in the organization, the external mechanical force and temperature, produce martensite transformation, become the crack propagation channel, reduce toughness.