304不锈钢应力腐蚀促进马氏体相变

STRESS CORROSION CRACKING ENHANCING MARTENSITE TRANSFORMATION OF TYPE 304 STAINLESS STEEL

304不锈钢在143℃硅油中恒载荷预蠕变足够长时间后,再在143℃ MgCl2溶液中相同恒载荷下进行应力腐蚀.结果表明,在开路条件下应力腐蚀能使表层和体内α＇马氏体分别升高14％和1.5％.但如放在不发生应力腐蚀（VSCE=-600mV阴极极化）的沸腾MgCl2溶液中,则马氏体含量基本不变.测量了在不同恒电位的143℃ MgCl2溶液中形成钝化膜后产生的膜致内应力结果表明,当阴极电位VSCE<-550mV后,钝化膜应力为零或是压应力,同时亦未发生应力腐蚀;如VSCE>-550 mV,则随电位升高,钝化膜引起的拉应力也升高,与此同时,应力腐蚀敏感性也升高.因此,304不锈钢应力腐蚀促进马氏体相变和腐蚀钝化膜引起的附加拉应力有关.

After a specimen of type 304 stainless steel was pre-crept at a constant load in silicon oil at 143℃ for 24 h, and underwent stress corrosion cracking (SCC) at the same constant load in a MgCl2 solution at 143℃ at open-circuit potential for 2 h, α' martensite on the surface and over the entire specimen increased 14% and 1.5% (volume fraction), respectively. If the specimen pre-crept in the silicon oil was immersed in the 143℃ MgCl2 solution at the cathodic potential VSCE=-600 mV, in which no SCC occurs, α' martensite increased only 0.6%. The critical hydrogen concentration for formation of hydrogen-induced α' martensite was 50.9x10-4% (mass fraction), which was larger than that entered into the sample during SCC. The difference between the flowing stress of the 304 steel before unloading and the yield stress of the same specimen extended in air after unloading and immersing in the 143℃ MgCl2 solution to form a passive film is defined as passive film-induced stress. The passive film forming at, the open-circule potential generated a large tensile stress, and that at the cathodic potential VSCE=-600 mV generated a small compressive stress. Therefore, SCC enhancing martensite transformation should attribute to the passive film-induced tensile stress.