Characteristics and Transformation Mechanism of Nonmetallic Inclusions in 304 Stainless Steel during Heat Treatment at 1250 °C.

Evolutions of two typical types of nonmetallic inclusions, i.e., inclusions based on CaO-SiO2-Al2O3 and MnO-SiO2-Al2O3 of 304 stainless steel were investigated in laboratory-scale experiments under isothermal heat treatment at 1250 °C for 0, 30, 60 and 120 min. Results show inclusion population density increases at the first stage and then decreases while their average size decreases and then increases. Moreover, almost no Cr2O3 content within the inclusion before the heat treatment, but Cr2O3 content increases gradually along with increasing heat treatment time. Furthermore, the increasing of Cr2O3 content in the inclusions would increase their melting points and reduce their plasticities. The experimental results and thermodynamic analysis indicate that there are three steps for inclusion evolution during the heat treatment process, in which Ostwald ripening plays an important role in inclusion evolution, i.e., inclusions grow by absorbing the newly formed small-size MnO-Cr2O3 inclusions.

Influence of the Nitrogen Content on the Carbide Transformation of AISI M42 High-Speed Steels during Annealing.

Attempts were made to elucidate the effect of nitrogen on primary eutectic carbides in as-cast and annealed AISI M42 high-speed steel. Particular emphasis was placed on the transformation of carbides during forging and annealing in steels with different nitrogen concentrations and the influence of final carbides on the impact toughness of the steel. Microstructural observation, electrolytic extraction method, X-ray diffraction analysis, automated inclusion analysis (INCASteel), and impact toughness measurement combined with fractographic observation were conducted on the specimens. Primary M2C carbides were found to be dominant precipitates in the as-cast ingot, with a certain amount of V(C,N). Nitrogen addition promoted the formation of fibrous M2C, whereas lamellar M2C predominated in M42 steel with a low nitrogen concentration (w[N]% = 0.006). Fibrous carbides M2C tend to decompose into more stable carbides M6C and MC during forging and annealing compared to lamellar M2C. Nitrogen alloying only affected the morphologies and dimensions of carbides, but did not change the types of carbides. These improvements in the dimensions and fractions of carbides naturally increased the impact toughness of annealed steel. Hence, it was suggested that the addition of nitrogen to AISI M42 high-speed steel was required to achieve homogeneous distribution of carbides and sufficient impact toughness.