Study On Heat Treatment Characteristics Of Low Carbon Martensitic Stainless Steel

In this paper,the microstructure evolution of low carbon martensitic stainless steel ZG0Cr13Ni4Mo was observed in-situ by high temperature laser confocal microscope in the temperature range of 30? to 1040?.The effect of different normalizing processes on the microstructure and properties of ZG0Cr13Ni4Mo was investigated.Furthermore,the heat treatment process of ZG0Cr13Ni4Mo was optimized to reach high strength,good toughness and suitable yield ratio.In situ observation of heating process shows that austenite begins to form at 570?and martensite disappears at 815?.In the temperature range of 570?815?,austenite needle-like austenite appears at the beginning with the same orientation as original martensite.Then needle-like austenite gradually grows to form massive austenite with increasing temperature.Over 900? the austenite recrystallization occurs,the bulge phenomenon and martensite-like orientation gradually disappear.The recrystallization effect is most significant at 950?,and the newly formed austenite with smaller size appears in the original massive austenite.With the increase of temperature,the grain boundary between fine austenite gradually disappears and becomes massive austenite,and the refinement effect of recrystallization disappears.The normalizing process adjustment is to change the normalizing times,normalizing holding time and normalizing temperature on the basis of the conventional normalizing and tempering heat treatment process(1040?+620?+600?)of ZG0Cr1 3Ni4Mo hydraulic turbine castings.For different normalizing times,The results show that the plastic and toughness of ZG0Cr1 3Ni4Mo decreases slightly,but the strength increases obviously.Among them.the secondary normalizing structure is the finest and the highest yield strength(787MPa).With more normalizing times,the maximum length of martensitic lath slowly increases,and the strength slightly decreases,but both of them are higher than the corresponding strength of primary normalizing.For different holding time,the results show that the properties of ZG0Cr13Ni4Mo are not deteriorated obviously,but the plasticity decreases slightly.The highest strength(836 MPa)was obtained at 4 h,and the strength decreased slowly with the extension of holding time.The reason is that the prolongation of holding time is beneficial to solution of alloying elements and then the solution strengthening effect.But at the same time,it promotes the growth of grains and weakens the effect of fine grain strengthening.Finally,the sampless were normalized at 1040?,950? and 900?,and the yield strength of the sampless at 950? was the highest(703 MPa),which was the best temperature for recrystallization,as the effect of fine grain strengthening was the most significant.Considering the service safety,the yield ratio of hydraulic turbine castings is often limited under 0.90.However,the experimental results show that the yield ratio is higher than 0.90 after multiple normalizing and prolonging the holding time,while the normalizing strength at 950? is much lower,and the temperature cannot guarantee the sufficient solid solution of alloy elements in large-size castings.According to the actual production requirements,this work optimizes the heat treatment process and puts forward two kinds of new heat treatment processes:1040?+950?+620?+600? and 1040?+950?+650?+620?.The former reduces the secondary normalizing temperature to maintain the plastic toughness and improve the strength while reducing the yield ratio to 0.89;the latter reduces the yield ratio to 0.80 by increasing the tempering temperature.The above two processes can realize the flexible adjustment of yield ratio and strength of ZG0Cr13Ni4Mo material.