Continuous cooling transformation and strengthening mechanism of G30Cr15Mo1N high nitrogen martensite steel
The CCT curve of G30Cr15Mo1N steel was measured by Formastor-F2 automatic phase transformation instrument,and the microstructure change and phase transformation of high nitrogen steel in the process of continu-ous cooling transformation were analyzed to guide the formulation of heat treatment process.In addition,the micro-structure and mechanical properties of G30Cr15Mo1N steel at different tempering temperatures were studied by XRD,SEM,EDS and TEM characterization methods,and the strength contributions of grain size,solid solution,dis-location and precipitation to the yield strength of G30Cr15Mo1N steel was analyzed.The results show that the Ac1 and Ac3 temperatures of the experimental steel are 840 and 870℃respectively,the temperature range of lamellar pearlite transformation is 600-700 ℃,and the fastest cooling rate of pearlite formation is 1℃/s;after tempering at a lower temperature(≤500℃),the microstructure of the experimental steel is composed of carbon nitride,tempered martensite,and remained austenite,and maintains high yield strength and tensile strength,exceeding 1 750 and 2 050 MPa,respectively,while the elongation is less than 3%;after tempering at 600℃,the martensite recovers and recrys-tallizes,and the strip shape of martensite plate basically disappears.A short rod like Cr2(C,N)precipitate with the size of 20-80 nm was observed in the TEM,which led to a significant decrease in the strength of high nitrogen steel and a full recovery of plasticity.When tempered at 150℃,the yield strength of G30Cr15Mo1N steel mainly comes from high solid solution and dislocation strengthening,among which solid solution strengthening plays a major role;after tempering at 500 ℃,the strength contributions of dislocation and solid solution respectively decrease 83.7 and 249.3 MPa,the main source of the significant secondary hardening effect is a higher proportion of precipitates with a size smaller than 10 nm.When the tempering temperature is as high as 600 ℃,the solid solution,dislocation,and sec-ond phase strengthening all decrease significantly,and the contribution to the yield strength is relatively balanced,which together play an important role in strengthening.
high nitrogen martensite steelcontinuous cooling transformationtemperingmicrostructurestrength-ening mechanism
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