等温时效对S32707不锈钢析出行为和耐蚀性的影响

Effect of isothermal aging on precipitation behavior and corrosion resistance of S32707 stainless steel

为了研究等温时效对特超级双相不锈钢S32707析出和耐腐蚀性能的影响,利用微观组织表征和电化学测试相结合的手段,研究了950℃时效不同时间下钢的析出行为、耐点腐蚀和耐晶间腐蚀性能。在950℃时效过程中,S32707可以析出σ相、γ_(2)和Cr_(2)N,结果表明,时效5~30 min后,σ相经共析反应(δ→σ+γ_(2))优先在δ/δ晶界和δ/γ相界析出,并逐渐向δ晶粒内生长。时效2 h后,δ相完全转变成σ相和γ_(2)相,时效12 h后,γ晶内析出σ相。时效初期,Cr_(2)N在σ/γ_(2)界面形核,随后在σ/γ和γ/γ界面形成。950℃时效导致S32707耐点腐蚀和晶间腐蚀性能下降,随时效时间延长,钢的点腐蚀电位(Epit)、电荷转移电阻(Rp)和双电层电容(Q)均呈3阶段变化规律,0~2 h,Epit和Rp急剧减小,Q急剧增大;2~12 h,Epit和Rp缓慢减小,Q缓慢增大;12~48 h,Epit和Rp略有增大,Q略有减小。钢的晶间腐蚀敏感性随时效时间延长也呈3段式变化规律,0~2 h急剧增大,2~12 h缓慢提高,12~48 h略有降低。σ相和Cr_(2)N析出会诱导其周围形成贫Cr区和贫Mo区,钢的耐腐蚀性能取决于Cr、Mo元素贫化和自愈的竞争结果。0~2 h,析出相快速形成诱导产生大量Cr、Mo贫化区,耐腐蚀性能显著下降;2~12 h,Cr、Mo贫化速率大于自愈速率,耐腐蚀性能缓慢下降;12~48 h,Cr、Mo贫化速率小于自愈速率,耐腐蚀性能略有提高。

In order to research the effect of isothermal aging on precipitation and corrosion resistance of hyper duplex stainless steel S32707,the precipitation behavior,pitting corrosion and intergranular corrosion resistance of the steel aged at 950℃ for different time were studied by means of microstructure characterization and electrochemical tests.The results show that,after aging for 5-30 min,σ phase preferentially precipitated at δ/δ grain boundary andδ/γphase boundary through eutectoid reaction(δ→σ+γ_(2)),and gradually grew into grains.After aging for 2 h,δ phase was completely transformed intoσandγ_(2) phases.After 12 h aging,σphase nucleated inside theγphase.At the beginning of aging,Cr_(2) N nucleated at the σ/γ_(2)interface,and then precipitated at the σ/γ and γ/γ interfaces.With the prolongation of aging time,the pitting potential(Epit),charge transfer resistance(Rp)and double-layer capacitor(Q)of steel changed in a three-stage manner.During 0-2 h,Epit and Rp sharply decreased,and Q sharply increased;during 2-12 h,Epit and Rp slowly decreased,and Q slowly increased;during 12-48 h,Epit and Rp slightly increased,and Q slightly decreased.The intergranular corrosion sensitivity also changed in a three-stage manner,sharply increased during 0-2 h,slowly increased during 2-12 h,and slightly decreased during 12-48 h.σphase and Cr_(2) N precipitation induced Cr-and Mo-depleted zones around them.The corrosion resistance of steel depended on the competition results of Cr and Mo depletion and self-healing.During 0-2 h,the rapid formation of precipitates induced extensive Cr-and Mo-depleted zones,and the corrosion resistance significantly reduced.During 2-12 h,the depletion rate of Cr and Mo was greater than their self-healing rate,so the corrosion resistance slowly decreased.During 12-48 h,the depletion rate of Cr and Mo was less than their self-healing rate,so the corrosion resistance slightly improved.