The effect of Al on the morphology of MnS in medium-carbon non-quenched and tempered steel was investigated at three different cooling rates of 0.24, 0.43, and 200°C·s^-1. The formation mechanisms of three types of MnS were elucidated based on phase diagram information combined with crystal growth models. The morphology of MnS is governed by the precipitation mode and the growth conditions. A monotectic reaction and subsequent fast solidification lead to globular Type I MnS. Type II MnS inclusions with different morphological characteristics form as a result of a eutectic reaction followed by the growth in the Fe matrix. Type III MnS presents a divorced eutectic morphology. At the cooling rate of 0.24°C·s⁻¹, the precipitation of dispersed Type III MnS is significantly enhanced by the addition of 0.044wt% acid-soluble Al (Al_s), while Type II MnS clusters prefer to form in steels with either 0.034wt% or 0.052wt% Al_s. At the relatively higher cooling rates of 200°C·s⁻¹ and 0.43°C·s⁻¹, the formation of Type I and Type II MnS inclusions is promoted, and the influence of Al is negligible. The results of this work are expected to be employed in practice to improve the mechanical properties of non-quenched and tempered steels.

The changes of microstructures, element distribution, and comprehensive properties were studied to explore their interactions with each other, induced by solution treatment of UNS S32750 super duplex stainless steel. The results showed that the ferrite content improved, while the austenite content declined as the temperature increased. From 900 to 1000 °C, the σ phase existing at α/γ grain boundaries and in ferrite grains gradually dissolved. At 1050 °C, the microstructures consisted of only ferrite and austenite. From 1050 to 1300 °C, the Cr₂N precipitated in ferrite and gradually grew and coarsened. The impact energy and pitting potential of UNS S32750 first improved and then weakened, while the hardness is the opposite, owing to the combined effects of element distribution, microstructures, and precipitates. In the presence of the σ phase, the corrosion resistance and mechanical properties of UNS S32750 correspond directly to the σ phase fraction. Subsequently, the rise in temperature promoted γ → α phase transformation, and the elements partitioning ratios of Cr and Mo declined, resulting in reduced toughness and corrosion resistance and a rise in hardness. Consequently, when the solution treatment temperature is 1050 °C, the α/γ ratio of UNS S32750 approached 1:1, with excellent overall properties.

The effect of cooling rate on martensitic structure and carbides precipitation behavior was investigated based on Cr-Mo alloy steel with different quenching media of oil, water and 10% NaCl-water, respectively. The influence mechanism of martensite structure and carbide precipitates on mechanical properties was also studied. The results showed that martensite packets and blocks were refined with water quenching, however, they were coarse with oil or 10% NaCl-water quenching. Martensite laths were refined and dislocation density increased with increasing cooling rate. The carbides in tempered steel were coarse obviously with 10% NaCl-water quenching. The impact toughness deteriorated significantly with 10% NaCl-water quenching due to coarsening of martensite structure and carbides.

The fatigue property is significantly affected by the inner inclusions in steel. Due to the inhomogeneity of inclusion distribution in the micro-scale, it is not straightforward to quantify the effect of inclusions on fatigue behavior. Various investigations have been performed to correlate the inclusion characteristics, such as inclusion fraction, size, and composition, with fatigue life. However, these studies are generally based on vast types of steels and even for a similar steel grade, the alloy concept and microstructure information can still be of non-negligible difference. For a quantitative analysis of the fatigue life improvement with respect to the inclusion engineering, a systematic and carefully designed study is still needed to explore the engineering dimensions of inclusions. Therefore, in this study, three types of bearing steels with inclusions of the same types, but different sizes and amounts, were produced with 50 kg hot state experiments. The following forging and heat treatment procedures were kept consistent to ensure that the only controlled variable is inclusion. The fatigue properties were compared and the inclusions that triggered the fatigue cracks were analyzed to deduce the critical sizes of inclusions in terms of fatigue failure. The results show that the critical sizes of different inclusion types vary in bearing steels. The critical size of the spinel is 8.5 μm and the critical size of the calcium aluminate is 13.5 μm under the fatigue stress of 1200 MPa. In addition, with the increase of the cleanliness of bearing steels, the improvement of fatigue properties will reach saturation. Under this condition, further increasing of the cleanliness of the bearing steel will not contribute to the improvement of fatigue property for the investigated alloy and process design.