Crystallographic Texture Evolution of γ′-Fe 4 N and Its Influences on Tribological Property of Nitrided Steel

The crystallographic texture of γ′-Fe4N in compound layer and its influences on the tribological properties of nitrided steel 38CrMoAl are investigated in the study. The preferred orientation of (200)γ′ is produced by low-temperature nitriding in atmosphere with low nitrogen–hydrogen ratio and increases with the nitriding time. The preferred orientation of (220)γ′ appears after 72 h cyclic nitriding. The orientation relationships (0001)ε//(101)α′ and [110]ε//[111]α′, (111)γ′//(0001)ε and \(\left[ {011} \right]_{{\gamma^{\prime } }} //\left[ {1\overline{2} 10} \right]_{\varepsilon }\), (200)γ′//(110)α′ and [011]γ′//[111]α′, as well as \(\left( {1\overline{1} 03} \right)_{\varepsilon } //\left( {220} \right)_{{\gamma^{\prime } }}\) and \(\left[ {0100} \right]_{\varepsilon } //\left[ {1\overline{1} 0} \right]_{{\gamma^{\prime } }}\) are established by first-principles method. The misfit of interatomic distance (δ), determining the phase transition resistance, is calculated. Accordingly, two reaction pathways during nitriding, α′→γ′ and α′→ε→γ′, are assumed, which determines the preferred orientations of γ′-Fe4N. Results of wear tests demonstrate that the specimen with preferred orientation of (200)γ′ exhibits lower frictional coefficient and lower wear rate in comparison with the specimen with (220)γ′ preferred orientation. (111)γ′ texture usually relates to the lower frictional coefficient but higher wear rate due to the main slip system parallel to the sliding plane. Therefore, the (200)γ′ preferred orientation has a positive significance in improving the wear properties of steels.