Study On Martensitic And Ageing Transformation Of C250and15-5PH Steels

In this paper, the C250maraging steel and15-5PH stainless steel are taken as theobjects of this study. Solid-state phase transition of the solid solution process and thesubsequent martensitic phase transition and solid-state phase transition of the ageingprocess and its aging kinetics are researched by the kinetics equation of JMA and thetheories of martensitic phase transition and diffusion and phase transition in the solidmetal; the solid-state phase transition and its influence on hardening behavior、mechanical properties and corrosion resistance of15-5PH stainless steel subjected tolong-term intermediate temperature are researched, too, by means of various ofanalytical techniques such as DIL805A dilatometer、metallographic microscope、SEM、XRD、TEM and microhardness tester.Based on the above research, the results can be obtained as follows: The Ms pointincreases with the increase of solution temperature, increases at first and then decreaseswith the increase of solution holding time; and the Mf point remains basicallyunchanged. Solution treatment at900℃, the C250recrystallized, the recrystallizationhas the priority of nucleation at the grain boundary the morphology of martensitepresents irregular bans and stringers before recrystallization, whereas presents blockythereafter. The recrystallization of C250results in the increase of Ms, the subsequentresolution of Fe2Mo leads to the decrease of Ms. The Ms and Mf of15-5PH stainlesssteel decrease with the increase of solution temperature; with the increase of solutionholding time, the Ms decreases at first and then decreases, the Mf remains unchanged.The resolution of ε-Cu results in the decrease of Ms, the separation of NbC leads to theincrease of Ms. The twin crystal and NbC exist after solution treatment, and the ferriteexists after solid solution treatment at900℃in the15-5PH stainless steel.The C250and15-5PH stainless steel were aged. With the prolong of ageing time,ageing treatment in the low temperature, the hardness increases and then remains stable;ageing in the high temperature, the hardness increases to the peak and then decreases.With the increase of ageing temperature, the decline of hardness after peak increases;the peak of hardness and the time of reaching hardness peak decreases. In the C250, theprecipitation of Ni3Ti and Fe2Mo results in the increase of hardness, because of theageing of the precipitation and the formation of reverse austenite, the hardnessdecreases. The precipitation of ε-Cu results in the decrease of hardness in the15-5PHsteel, because of the ageing of ε-Cu and the formation of reverse austenite, the hardness decreases. With the proceeding of ageing, the morphology of ε-Cu becomes fromspherical to short rod-shape. The reverse austenite forms when the ageing temperature is550℃and distributes in the martensite lath boundaries; When the ageing temperature is580℃, the reverse austenite forms in the matrix. Calculated by the ageing kinetic, theactive energy of precipitation is41kJ/mol and the index of Avrami is0.36in C250; theactive energy of precipitation of ε-Cu is256kJ/mol and the index of Avrami is0.613in15-5PH steel.The15-5PH stainless steel is subjected to long-term ageing at400℃. As theageing time, the ε-Cu precipitates and gradually grows up, the NbC gradually grows up,the quantity of reverse austenite increases, the spinodal decomposition occurs. As theageing time, the tensile strength first increases and then decreases, the elongation andreduction of area first decreases and then increases, the absorbed energy graduallydecreases. The way of fracture becomes from ductile fracture to brittle fracture. Thehardness first increases and then decreases, the corrosion resistance reduces. Theprecipitation、reverse austenite and spinodal decomposition result in the change ofmechanical properties and corrosion resistance.