Microstructure and Mechanical Properties of 50SiMnNiNb Steel by a Novel Quenching-Partitioning-Austempering Heat Treatment

For the purpose of reducing weight of steel parts, save raw materials and keep or even improve safety standards, the development of advanced high strength steels is increasingly demanded in the automotive industry and engineering applications. We have proposed a novel heat treatment （quenching-partitioning-austempering treatment, Q-P-A） to obtain steel parts with high strength and good ductility. The Q-P-A process is intended to produce microstructure consisted of carbon-depleted martensite, carbon-enriched retained austenite and nanostructured bainite. Quenching（Q） treatment fabricates mixed microstructure of carbon-supersaturated martensite and certain amounts of untransformed austenite. Partitioning（P） thermal treatment accomplishes fully diffusing of carbon from the supersaturated martensite phase to the untransformed austenite phase and enriching the amount of carbon in untransformed austenite. Further low-temperature austempering（A） process induces incredible thin bainite from the carbon-enriched untransformed austenite. A study of the microstructure and mechanical properties of 50SiMnNiNb steel subjected to the novel Q-P-A treatment is presented. Microstructure is assessed by optical microscope（OM）, field emission scanning electron microscope（FESEM） and transmission electron microscope（TEM）, and the corresponding mechanical properties are measured. The experimental results indicate that attractive mechanical properties of steels during the Q-P-A process are attributed to the complex multi-phase structure. Slender plates of bainite with 20-40 nm thick are generated in the medium carbon steel. Meanwhile, with increasing of the volume fraction of nanostructured bainite, yield strength of steel parts is increased with little degradation of ultimate tensile strength. In this paper, a novel quenching-partitioning-austempering heat treatment is proposed, and the attractive mechanical properties of steels are obtained during the Q-P-A process.

Comparison on wear resistance of nanostructured bainitic bearing steel with and without residual cementite

The sliding wear property of high-carbon nanostructured bainitic bearing steel with the equal initial hardness and different microstructures was investigated,and the reasons for the difference of wear resistance between the cementite-bearing(CB)and cementite-free(CF)specimens were analyzed.The results show that CF specimens have lower mass loss and surface roughness and shallower wear depth than CB specimens during wear process.Compared with CB specimen,CF specimen presents superior wear resistance.This is due to two reasons:(1)a lot of retained austenite in CF specimen is easy to produce TRIP effect and be transformed into martensite during wear process,which notably increased the surface hardness of worn specimen;(2)there is a nondestructive oxide layer in the surface of cementite-free worn specimen,which can protect the surface of worn specimen from destruction.Under the combined effect of retained austenite and oxide layer,the loss of matrix is reduced.Thus,CF specimen exhibits high wear resistance.It reveals that the wear mechanism of high-carbon nanostructured bainitic bearing steel with different microstructures can provide a reference for improving the wear resistance in high-carbon nanostructured bainitic bearing steel in future.

Microstructure and mechanical properties of nanobainitic steel subjected to multiple isothermal heat treatments

Nanostructured bainite in 62MnSiCr steel was prepared by two-stage transformation process at different temperatures for less than 2 h. Microstructures, phase distribution and mechanical properties of the obtained steel were investigated. The results showed that the thickness of bainite plate and the amount of retained austenite decreased obviously after the twostage transformation, while the carbon concentration in the retained austenite showed a small change. With increase in the second holding temperature within the bainite transformation range, all of them increased slightly. The additional formation of bainite at the second transformation stage is beneficial to refining the austenite and further enriching it with carbon, resulting in the enhancement of the mechanical stability. Bainite transformed in two-stage process showed a better comprehensive performance. Absorbed impact energy of 88 J and an ultimate tensile strength of 1818 MPa have been achieved by isothermal heat treatment at 300 ℃ followed by 260 ℃. Meanwhile, there was a slight change in mechanical properties when the second transformation temperature varied from 260 to 220 ℃.

Wear Resistance of Two Nanostructural Bainitic Steels with Different Amounts of Mn and Ni

Extremely valuable mechanical properties in combination with acceptable wear resistance can make nanostructural bainitic steels to be used extensively in different engineering and tribological applications. However, it is critical to characterize the contributed factors to investigate the wear response of these high-strength materials. This work aims to study the wear behavior of two nanostructural bainitic steels with different amount of austenite stabilizer elements Mn and Ni. For this purpose, wear resistances of the materials were evaluated using the pin-on-disk method. The results indicated that the hardness of the sample is a critical factor affecting the tribological behavior, and the volume fraction and morphology of high-carbon retained austenite are also of considerable importance. It has also been demonstrated that transformation-induced plasticity effect during the wear test and oxide formation at worn surfaces are critical factors.