| Recently,with the rapid development of infrastructure construction,great progress have been achieved in the field such as high speed railway,precision machine tool equipments,domestic vehicle and large sized industrial equipments.The processing and manufacturing technique of steel used in advanced equipments industry is one of the key impact factor that greatly influence the national economy of our country.In this work,GCr15 bearing steel and 55 SiCrV spring steel provided by Daye Special steel and Baosteel,respectively,were adopted as research materials to understand the relationship betweern heat treatment,microstructural differences and resistance to hydrogen embrittlement by means of scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),hydrogen bubbles test,mechanical property test and hydrogen permeation test.The main achievements are concluded as following:1.Adopted the Pre-Quenching and Austempering(PQA)heat treatment to treat GCr15 bearing steel,and obtained the multiphase microstructure combined with high strength and improved plasicity.The microstructure of PQA-treated sample are consist of martensite + bainite + stable retained austenite + spherical undissolve carbides.2.According to the research result of indentation test,hydrogen bubbles test and hydrogen permeation test,properly conclusion have been drawn that,compared with traditional Quench and Tempering(QT)heat treatment processed material,PQA-treated material have the stronger capability to inhibit crack generation,lower effective diffusion coefficient and much more hydrogen trapping sites,performed a lower susceptibility to hydrogen embrittlement.3.X-Ray diffraction analysis result shows that Under the condition of 10% engineering strain,the volume fraction of retained austenite in PQA-treated material was lower than QT-treated material before compression,but after 10% engineering compression strain,it is higher than QT-treated material,which indicate that PQA-treated material have higher mechanical stability of retained austenite.4.TEM micrographs of PQA-treated sample shows that nanoscale film-like retained austenite distributed on inside of bainitic ferrite and the gap between martensite interface,which benefit to the mechanical stability of retained austenite.Meanwhile,stable film-like retained austenite can act as hydrogen trapping sites to arrest hydrogen atoms and decrease the number of diffusible hydrogen atoms.so the deleterious effect of hydrogen to stress concentration area or crack initiation was suppressed5.Introduce advanced Q-P-T heat treatment and adjust parameters to make the mechanical property of 55 SiCrV spring steel is adjustable over a wide range.#2 sample have the best combination of strength and plasitic deformation ability,the ultimate strength and elongation are 2094 MPa and 12.4% respectively.both of these two values are higher than traditional QT-treated material,and elongation of #2 sample is 74.6% more than QT-treated material.6.SEM observation shows that the microstructure of 170 ? austempering sample approximately consist of martensite,but if austempering temperature is 190? or above,the main microstructure become bainite.Especially for sample austempering at temperature slightly higher than Ms,the formation of nanoscale bainite was formed.7.Hydrogen permeation test showed that sample austempering at 190? have the lowest hydrogen diffusion coefficient and largest number of hydrogen trapping sites,which indicate the lowest hydrogen embrittlement susceptibility.With the increasing austempering temperature,the hydrogen embrittlement susceptibility increased accompanied by increased hydrogen diffusion coefficient and decreased hydrogen trapping sites. |
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