铌对65SiCrV6弹簧钢氧化增重的影响

 铌对Si-Cr-V系弹簧钢强度和脱碳层特征的影响已受到较多关注,但铌对该系弹簧钢氧化增重影响的研究还较少。以65SiCrV6弹簧钢为研究对象,在其中添加质量分数约0.017%的铌(65SiCrV6Nb)。采用SEM+EDS、XRD、TEM、FactSage化学热力学软件、反应扩散理论和数理统计相结合的方法,从研究铌加入是否会对该弹簧钢在炉氧化增重和氧化铁皮物相组成等产生明显影响的角度,对铌加入是否会对该弹簧钢高压水除鳞难易度产生影响进行评价。结果表明,铌的加入提高了锻态65SiCrV6钢中的珠光体和未溶M(C,N)的相对含量,降低了铁素体的相对含量,并细化了组织。在氧气浓度为2%~7%(体积分数)、加热速度为8~20 ℃/min、保温温度为1 050~1 150 ℃和保温时间为60~90 min等工艺条件下,铌的加入使65SiCrV6钢的氧化增重明显提高,提高幅度为2.54%~27.82%且具有统计学意义。影响试验钢在炉氧化增重的主次效应依次为保温温度>保温时间>加热速度>氧气浓度,保温温度和保温时间对试验钢在炉氧化增重的影响为正相关,氧气浓度和加热速度对试验钢在炉氧化增重的影响为负相关。65SiCrV6钢在炉氧化增重达最小值的工艺为氧气浓度为7%、加热速度为14 ℃/min、保温温度为1 050 ℃和保温时间为60 min;65SiCrV6 Nb钢在炉氧化增重达最小值的工艺为氧气浓度为7%、加热速度为8 ℃/min、保温温度为1 050 ℃和保温时间为60 min。影响铌对65SiCrV6钢在炉氧化增重提高幅度的主次效应为保温时间>保温温度>加热速度>氧气浓度。铌的加入使65SiCrV6钢在炉氧化增重提高幅度最小的工艺为氧气浓度为2%、加热速度为8 ℃/min、保温温度为1 050 ℃和保温时间为75 min。由反应扩散控制的氧化固态相变是造成保温温度、保温时间、加热速度和氧气浓度对试验钢在炉氧化增重产生不同影响的主要原因。铌的加入未改变65SiCrV6钢表面氧化铁皮的3层结构特征,氧化铁皮由外向钢基体主要由Fe₂O₃、Fe₃O₄和FeO(或FeO+Fe₂SiO₄)等构成。铌的加入降低了65SiCrV6钢氧化铁皮中Fe₂SiO₄的相对含量,对高压水除鳞有利。

Effects of Nb on oxidation weight gain of 65SiCrV6 spring steel

The influence of Nb on the strength and decarbonization characteristics of Si-Cr-V series spring steels has received more attention,but less study has been focused on effect of Nb on the oxidation weight gain of the series spring steels. One of the Si-Cr-V series spring steels being 65SiCrV6 steel was selected as the research object,in which the mass percent of about 0.017%Nb was added(65SiCrV6 Nb). Using a combination of SEM+EDS,XRD,TEM,FactSage chemical thermodynamics software,diffusion theory,and mathematical statistics,it would be evaluated whether the addition of Nb in 65SiCrV6 steel would affect the ease of high-pressure water descaling of the steel from the perspective of researching whether the addition of Nb would have an obvious influence on oxidation weight gain and phase composition of iron oxide scales. The results show that the addition of Nb increased the relative content of pearlite and undissolved M(C,N) in 65SiCrV6 steel,reduced the relative content of ferrite,refined the as-forged microstructure of the steel. Under experimental conditions such as the oxygen concentration of 2%-7%(volume percent),the heating rate of 8-20 ℃/min,the holding temperature of 1 050-1 150 ℃,and the holding time of 60-90 min,the addition of Nb increased the oxidation weight gain of 65SiCrV6 steel obviously,and the increase range was about 2.54%-27.82%,which was statistically significant. The main and second effects that affect in-furnace oxidation weight gain of the experimental steels were holding temperature>holding time>heating rate>oxygen concentration. The effects of holding temperature and holding time on in-furnace oxidation weight gain of the experimental steels were positively correlated;the effects of oxygen concentration and heating rate on in-furnace oxidation weight gain of the experimental steels were negatively correlated. The process for 65SiCrV6 steel to reach the minimum in-furnace oxidation weight gain was oxygen concentration of 7%,heating rate of 14 ℃/min,holding temperature of 1 050 ℃,and holding time of 60 min;the process for 65SiCrV6 Nb steel to reach the minimum in-furnace oxidation weight gain was oxygen concentration of 7%,heating rate of 8 ℃/min,holding temperature of 1 050 ℃,and holding time of 60 min. The main and second effects of Nb on the increasing magnitude of in-furnace oxidation weight gain of 65SiCrV6 steel were holding time>holding temperature>heating rate>oxygen concentration. The process with the smallest increase in the in-furnace oxidation weight gain of 65SiCrV6 steel by the addition of Nb was oxygen concentration of 2%,heating rate of 8 ℃/min,holding temperature of 1 050 ℃,and holding time of 75 min. The oxidation-solid phase transformation controlled by reaction-diffusion was the main reason for the different effects of the holding temperature,the holding time,the heating rate,and the oxygen concentration on in-furnace oxidation weight gain of experimental steels. The addition of Nb did not change the three-layer structure of iron oxide scale on the surface of 65SiCrV6 steel. The iron oxide scale was Fe₂O₃,Fe₃O₄ and FeO(or FeO+Fe₂SiO₄) in turn from the outward to the steel matrix. The addition of Nb reduced the relative content of Fe₂SiO₄,in the iron oxide scale of 65SiCrV6 steel,which was beneficial to high-pressure water descaling.