Effect of Solution Carbon and Nitrogen on the Microstructural Size and Crystallography of Lath Martensite in Fe–N and Fe–C Alloys

Abstract

Microstructures of lath martensite that contains bcc or bct martensite crystals in FeC alloys are known to depend on the carbon content of the alloys. The effect of nitrogen content on microstructure, however, has not yet been elucidated. This study elucidates the effects of carbon and nitrogen content on microstructures via local crystallographic analysis. We found that the packet sizes are similar when the nitrogen content in the alloys are the same as the carbon content, with the packet size decreasing with increasing carbon and nitrogen content. The block and subblock thicknesses in low nitrogen lath martensite are smaller than those in low carbon lath martensite, whereas those in medium and high carbon and nitrogen lath martensites are similar. Martensite lath thickness and dislocation density in the FeN alloy laths are lower than those in the Fe C alloys laths, and the distribution of misorientation angles between adjacent the blocks and sub-blocks in FeN alloys is similar to that in FeC alloys.

Comparison of Microstructure and Hardness between High-carbon and High-nitrogen Martensites

“…The reduction of FWHM with increasing carbon and nitrogen contents in 7.3C and 7.5N, respectively, were attributed to their lower dislocation density from lattice-invariant deformation by twinning instead of dislocation gliding during martensitic transformation. Morito et al 24) investigated the dislocation substructure in Fe-C and Fe-N alloys with martensitic structure with TEM, and reported that the dislocation density of lath martensite in Fe-C alloys (C% < 2.77 at%) was higher than that in their Fe-N alloy (N% < 2.70 at%) counterparts, which agrees with the results of this study. The difference in dislocation density between Fe-C and Fe-N steels is probably due to the difference in M s temperature shown in Fig.…”

“…The lath width of 3.9C and 3.7N steels was 250 and 360 nm, respectively, indicating that Fe-C martensite has thinner laths than Fe-N martensite. Morito et al 24) compared the lath widths of Fe-N alloy ( < 2.7 at% N) and Fe-C ( < 2.6 at% C). They reported that the lath width of Fe-C alloy was greater than that of Fe-N alloy in the low-concentration regime.…”

“…Since the development of the high-temperature gas nitriding and quenching method (N-quench) 16) as an alternative surface-hardening method to conventional carburizing and quenching, studies on nitrogen-bearing martensitic steels have gradually increased. Hitherto, the effects of nitrogen content on lattice parameter, [17][18][19][20][21][22] hardness, 16,23) microstructural size, and crystallography 24) have been reported. For example, Chiba 16) reported that the hardness of a Fe-N system martensite increases with the amount of nitrogen in the solid solution, but eventually decreases at nitrogen concentrations over 5 at% due to the formation of retained austenite.…”

“…The extent of the difference of their effects has not been adequately discussed. Morito et al 24) investigated and compared the substructures of lath martensite in Fe-C and Fe-N steels with hypoeutectoid compositions by transmission electron microscopy and reported that both carbon and nitrogen refine the size of block and packet structures and increases the dislocation density in their corresponding steels, but with a different degree of influence. Though we can obtain information on nitrogen-containing martensite from these limited studies, they lack details on steels containing more than 1 mass% nitrogen or sufficient comparison between the effects of carbon and nitrogen.…”

A physically-based high temperature yield strength model for 9Cr steels

“…Figure 9a presents the measured effect of carbon content on microstructural properties in Fe-C steels. The dislocation density increases with increasing carbon content [49] due to the decreasing block and lath width, as measured in [52]. Below a carbon content of approximately 0.12 wt.%, the microstructure is predominantly ferritic [40].…”