Tempering behavior of Fe-X-C steels (X: W, Mo, Ni, Si) has been studied. Especially the existence of retained austenite and the formation of boundary carbides following the decomposition of the retained austenite have been examined by the analytical transmission electron microscopy. And the relationship between the formation of boundary carbides and tempered martensite embrittlement (TME) has been studied. The effect of impact test temperature on the existence of TME has been also investigated.
Fe-X-C steels were austenitized at $1200^\circ{C}$ and then quenched in ice water or oil and then tempered at $200^\circ{C}$-600^\circ{C}$. The mechanical test was performed at room temperature. The impact test was done in the range of $-196^\circ{C}$-230^\circ{C}$ to investigate the relation between test temperature and TME.
In the present Fe-X-C steels the retained austenite was observed at lath and/or grain boundaries. The effect of alloying element on the amount of retained austenite was little. From this observation the mechanical stabilization may play a primary role in stabilizing austenite in fast quenched condition. The retained austenite decomposed and the resultant carbides formed at lath and/or grain boundaries at $300^\circ{C}$ tempering temperature in W-, Mo-, and Ni-steel and at $400^\circ{C}$ tempering temperature in Si-steel.
The hardness and tensile properties were closely related with the alloying elements. W and Mo retarded the growth of cementite and exhibited the high tempering resistance in all tempering temperature range studied. A clear secondary hardening occurred in Mo-steel, compared to W-steel. Ni little affected the growth of cementite and little exhibited tempering resistance. Si inhibited the cementite formation and shifted the tempering behavior to higher temperature. However, the impact property was different from the hardness and tensile properties. TME occurred at $300^\circ{C}$ tempering temperature in W-, Mo-, and Ni-steel. TME phenomenon was associated with the formation of carbides at lath and/or grain boundaries resulting from the decomposition of retained austenite. In Si-steel, however, TME did not occur since the boundary carbides were not coarse and elongated probably due to the inhibition of cementite formation by the alloying element, Si.
In W-and Mo-steel the transgranular TME was observed in a limited test temperature range where matrix toughness was adequate for lath boundary carbide to act as an embrittler. A limited test temperature range was similar to a test temperature range where the specimens tempered at $200^\circ{C}$ and $300^\circ{C}$ showed the ductilebrittle transition. In test temperature ranges where the specimens tempered at $200^\circ{C}$ and $300^\circ{C}$ exhibited almost entirely brittle or ductile fracture behavior, the embrittling role of the lath boundary carbides would be weak and the impact toughness would be controlled primarily by the matrix toughness.
On the other hand Ni-steel exhibited the intergranular TME below the specific test temperature ($\sim-40^\circ{C}$) where both the impurities and the carbides at grain boundary were able to contribute to the embrittlement. Below the specific test temperature, the grain boundary carbides could act as the crack initiators and the impurity weakened grain boundary could act as the easy fracture path. In the specimen tempered at $200^\circ{C}$, however, the amount of the intergranular fracture decreased with decreasing the test temperature below the specific test temperature where TME was observed. This result indicated that the grain boundary carbides were essential to the intergranular TME. Above the specific test temperature, however, the embrittling role of the impurities and carbides at the grain boundary would be weak and thus the impact toughness would be controlled by the matrix toughness.
From the above results, in both the transgranular and the intergranular TME, the embrittling role of the lath and/or the grain boundary carbides was essential and the matrix toughness which enabled the boundary carbides to act as the effective embrittler was necessary.
