Microstructure and property of bearing steel with and without nitrogen addition were investigated by microstructural observation and hardness measurement after different heat treatment processing. Based on the microstructural observation of both 9Cr18 steel and X90N steel, it was found that nitrogen addition could effectively reduce the amount and size of coarse carbides and also refine the original austenite grain size. Due to addition of nitrogen, more austenite phase was found in X90N steel than in 9Cr18 steel. The retained austenite of X90N steel after quenching at 1050℃ could be reduced from about 60% to about 7 9% by cold treatment at -73℃ and subsequent tempering, and thus finally increased the hardness up to 60 HRC after low temperature tempering and to 63 HRC after high temperature tempering. Furthermore, both the wear and corrosion resistance of X90N steel were found much more superior than those of 9Cr18 steel, which was attributed to the addition of nitrogen. It was proposed at last that nitrogen alloying into the high chromium bearing steel was a promising way not only to refine the size of both carbides and austenite, but also to achieve high hardness, high wear property and improved corrosion resistance of the stainless bearing steel.
Hai-feng XuFeng YuChang WangWan-li ZhangJian LiWen-quan Cao
A hot-rolled medium Mn(0.2C5Mn)steel is annealed at 650℃ to produce an ultrafine-grained duplex microstructure with different austenite volume fractions by austenite reverted transformation(ART)annealing,and the orientation relationship strictly obeys K-S orientation relationship before deformation.Tensile tests are carried out in a temperature range from-196 to 400℃ to examine the effects of the austenite volume fraction and the deformation temperature on the tensile properties and the austenite stability.Microstructural observations reveal that the metastable austenite gradually transformed into a-martensite,which is controlled by the deformation strain,the temperature and the austenite volume fraction.Both strain hardening behavior and ductility of the studied steel are dependent on austenite volume fraction and deformation temperature significantly.The stress-strain curves of ART-annealed 0.2C5Mn steel assume an S shape and a very large work hardening rate of about 10 GPa is obtained at liquid nitrogen deformation temperature.Based on the experimental data,a quantitative relation is proposed to describe the ductility dependence on both the austenite volume fraction and its mechanical stability.
Si-lian CheZhao-xi CaoChang WangChong-xiang HuangDirk PongeWen-quan Cao
A medium-Mn steel （0.2C5Mn） was processed by intercritical annealing at different temperatures （625 ℃ and 650 ℃ ）. An ultrafine-grained micro-duplex structure consisting of alternating austenite and ferrite laths was de- veloped by austenite reverse transformation （ART） during intercritical annealing after forging and hot rolling. Ultra- high ductility with a total elongation higher than 30% was achieved in the temperature range from -196 ℃ to 200 ℃, and high impact toughness no less than 200 J at -40 ℃ was obtained. Based on the analysis of microstructure and mechanical properties, it was found that the enhanced ductility was determined by the phase transformation effect of austenite （TRIP effect）, while the delayed ductile to brittle transition was controlled by austenite stability.
Si-lian CHENJun HUXiao-dan ZHANGHan DONGWen-quan CAO
A duplex ultrafine microstructure in a medium manganese steel （0.2C-5Mn） was produced by austenite re- verted transformation annealing （ART-annealing）. The microstructural evolution during annealing was examined by scanning electron microscopy （SEM）, transmission electron microscopy （TEM） and X-ray diffraction （XRD）. Based on the microstructure examination, it was found that some M3C type carbides appeared in the martensitic matrix at the beginning of the ART-annealing. But with further increasing annealing time, these carbides would be dissolved and finally disappeared. Meanwhile, the austenite lath was developed in the ART-annealing process and the volume fraction of austenite increosed with the increase of the annealing time, which resulted in a duplex microstructure con- sisting of ultrafine-grained ferrite and large fraction of reverted austenite after long time annealing. The mechanical property examinations by uniaxial tensile tests showed that ART-annealing （6 h, 650 ℃） resulted in a superhigh product of strength to elongation up to 42 GPa ·%.
