Effect Of Heattreatment To Mechanical Property And Stess Corrosion Behaviors Of 1Cr15Ni4Mo3N Steel

1Cr15Ni4Mo3N steel is semi austenitic precipitation hardening s tainless steel, with high plasticity and good processing performance, the steel is widely used in aviation, mainly used as a bolt, gear joint force sites. 1Cr15Ni4Mo3 N steel in the actual application process because of unreasonable, cause in the process of storage failure behavior of stress corrosion of serious, nearly causing major economic losses.The phases at various temperature, the dissolution-precipitation curves of 1Cr15Ni4Mo3 N were gotten by thermodynics calculations. And the calculated values of Ms,Mf are in a good agreement to the actual results. The heat treatment technologies were designed using the results above. The main conclusions are as follows:1Cr15Ni4Mo3N steel after four grade pre-annealing remaines martensite and austenite, some martensite shows recovery phenomenon. The micarostructures of 1Cr15Ni4Mo3 N steel after solid solution treatment shows martensite plus 8.69% austenite, most of the second phases dissolve into the matrix in the process, typical dislocations and dislocation net sub-structures can be obsevrd. 1Cr15Ni4Mo3 N steel after deep-cooling treatment shows martensite microstructures, only austenite below 5% content can be observed. At the same time there are some twin substructurse in martensite lath and a large number of dislocations. 1Cr15Ni4Mo3 N steel after tempering at 250 ℃ remained martensite lath morphology, the dislocation density decreases, precipitates with clubbed or elliptic shape are dispersed in the internal or lath boundary. When tempered at 350 ℃,the specimens exhibit many laths, but lath boundaries are not obvious; a lot of tprecipitated phase with round or irregular shapes are distributed in the laths, or shows chainlike distributed in the plate. The specimens tempered at 450 ℃ show lath martensites,but thelath boundaries are fuzzy. The heat treatments of 350 ℃ tempering for 2h shows a ggod combination of high strength, plastic with.The relationships bertween teh cracks propagation rate and stress intensity factor can be diveided into two stages. The crack propagation rate increses with increaing stress intensity factors in the fitst stage, durign which crack propagation are determined by stress field intensity factors; in the second stage, the crack growth rate has no relationships with stress intensity factor, during which electrochemical factor controls crack extension. For the specimens treated by deep-cooling,there are two platforms,and there are a area which shows a decreased crack propagation rates between the two platforms.Different heat treatment conditions shows different stress corrosion resistance. Stress corrosion threshold intensity factor of 87.05 MPa.m1/2 at the forging state and the deep-cooling state exibits a lowest value of 40.05 MPa.m1/2, solid solution state gives a middile value of 80.75 MPa.m1/2. Increaing tempering temperature enhaces stress corrosion resistance.The stress corrosion fracture shows intergranular and transgranular brittle fracture characteristics; Some cracks botween grains can be observed in intergranular fracture,and many small corrosion pits are found. Besides these,some small local dimples are also observed. All these phonominies shows the characteristics of stress corrosion resulted from hydrogen brittlement.The results in this paper can provid the theory base and method to solve the question above.