In situ observation of the effect of Al N particles on bainitic transformation in a carbide-free medium carbon steel

The bainitic transformation of the steels with different mass fractions of N, ~0.002% and 0.021%, was observed in situ by using high-temperature metalloscope. Micrometer-and nanometer-sized aluminum nitride(AlN) particles were found in the steel with 0.021% N.Grain boundaries, the interior of the grains, and Al N particles were used as initial nucleation sites of bainitic ferrite, and bainitic ferrite subunits served as new nucleation sites to induce secondary nucleation. The lengthening rate of bainitic ferrite varied at different nucleation sites, which was controlled by the repeated nucleation and growth of bainitic subunits. The Al N particles not only provided several nucleation sites, but also increased the autocatalytic effect on the transformation, further shortening the incubation period, promoting the bainitic transformation, and refining the bainitic microstructure.

Microstructural characteristics of as-forged and β-air-cooled Zr-2.5Nb alloy

Multiple characterization and analysis techniques including electron backscatter diffraction（EBSD）, electron channeling contrast（ECC） imaging, transmission electron microscopy（TEM） and microhardness test were jointly employed to investigate microstructural characteristics such as local composition, morphology, grain boundary characteristics and interphase orientation relationship of a forged Zr-2.5Nb alloy before and after β-air-cooling. Results show that the as-forged specimen is composed of equiaxed and lamellar α grains and continuous net-like β-Zr films. After the β-air-cooling, the microstructure of the specimen is featured by basket-weave Widmanst？tten structure, in which the inter-α-plate second phases are nanoscale β-Zr. Analyses for crystallographic orientations reveal that the Burgers relationship has been strictly followed during the β→α cooling. Compared to the as-forged specimen, the hardness of the β-air-cooled specimen is higher, which could be attributed to the decreased structural sizes of both α and β phases, and the increased fraction of high angle boundaries as well.

Notably Accelerated Nano-Bainite Transformation via Increasing Undissolved Carbides Content on GCr15Si1Mo Bearing Steel

In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 ℃, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.

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.