Effect Of Heat Treatment On Strength,Toughness And Hydrogen Embrittlement Of USS122 Ultra-high Strength Stainless Steel

To be an important branch of ultra-high strength stainless steel, martensitic precipitation hardening stainless steel is universally applied as structural components into aerospace industry, marine engineering and energy industry due to its excellent combination of high strength, ductility and corrosion resistance. However, its alloy system is complex and a variety of precipitates precipitated through heat treatment, which considerably limits its further development and application. Meanwhile, the sensitivity to hydrogen embrittlement increased drastically along with the enhancement of strength. Therefore, there will be a significant theoretical and practical value to investigate the effect of heat treatment on the strengthening, toughening mechanism and hydrogen embrittlement of the steel.In this article, the microstructures were investigated by means of OP(Optical Microscopy), SEM(Scanning Electron Microscopy), TEM(Transmission Electron Microscopy), EBSD(Electron Back-scattered Diffraction) and Thermal-calc software. The results show that a large amount of coarse M6 C phase and χ phase distributing on the original boundries completely dissolved after solid solution treated at 1080 ℃. Thus the excellent combination of strength and toughness was achieved. Moreover, there precipitates would not completely dissolve when solid solution treated at 1000℃ which caused the deterioration of toughness. On the other hand, the coarsening of microstructures happened when treated at1100℃ leading to the decline of yield strength.The optimized comprehensive mechanical property was obtained after 1050℃×1h OQ,-73℃×8h AC,540℃×4h AC( aging peak) heat treatment. The ultimate tensile strength reached above 1900 MPa with 45 J impact energy. The dispersive precipitation of a large amount of fine Laves phase with size ranging from 5~11nm caused tremendous precipitation hardening effect which significantly strengthened the steel. Before the aging peak(i.e. the aging time or temperature were less or lower), the amount of precipitates was insufficient to generate precipitation hardening effect. After aging peak, the coarsening and over aging of precipitates had a negative effect on strength of steel. Besides, the thin films of reverted austenite located in seams of matrix played an important role to promote the toughness of steel.The notch tensile strength and hydrogen contents of hydrogen charging and uncharging specimens were measured through slow strain tensile test and thermal desorption spectrometry.The results show that hydrogen embrittlement happened in the charging specimens. The root of fracture exihibited typical fractograph of intergranular fracture. And the notch tensile strength of charging specimen decreased dramatically than uncharging specimens. The sensitivity of hydrogen embrittlement firstly declined then rised with the rising of aging temperatures which had a similar variation with the effect of aging temperature on strength.The peak of hydrogen embrittlement occuered when aged at the temperatures ranging from520~560℃.