UNS S32750超级双相不锈钢激光焊接头微观组织与耐蚀性能

Microstructure and corrosion resistance of UNS S32750 super duplex stainless steel laser welded joint

以UNS S32750超级双相不锈钢为研究对象,采用光学显微镜、扫描电子显微镜、透射电子显微镜、电子探针显微分析仪对激光焊接头微观组织和元素分布进行表征,采用临界点蚀温度测试方法研究激光功率对激光焊接头点蚀行为的影响规律.结果表明,提高激光功率可显著增加焊缝中奥氏体含量,并且在一定程度上抑制Cr_(2)N析出.此外,激光焊焊缝具有比母材更低的耐点蚀性能.但随着激光功率的增加,焊缝中耐蚀的奥氏体含量增加,同时降低了Cr_(2)N析出倾向,因此其耐点蚀性能也逐渐增强.与双相钢母材相比,激光焊焊缝中Cr,Ni,Mo等合金元素在铁素体与奥氏体中的分配差异性显著减小,而N原子的分配差异性增加,因此焊缝中铁素体具有比奥氏体更低的耐点蚀指数,进而优先被选择性腐蚀.对于双相钢母材,点蚀主要发生在δ/γ相界和夹杂处,而激光焊焊缝的点蚀主要以铁素体内大量析出的Cr_(2)N作为点蚀萌生位置,并向弱相铁素体内快速发展.

Holding UNS S32750 SDSS as the object of study,this paper studied the influence of laser power on the microstructure and pitting behaviour of laser welded joints using optical microscopy,scanning electron microscopy,transmission electron microscopy,electron probe microanalyzer and critical pitting temperature.The results show that increasing the laser power can significantly increase the austenite content while inhibiting the precipitation of Cr_(2)N in the laser welded seam.In addition,the laser welded seam has a lower pitting resistance than that of the base metal.However,with the rising of the laser power,the corrosion-resistant austenite content of the weld grows and the tendency for Cr_(2)N precipitation is weakened,resulting in a gradual increase in the pitting resistance.Compared with the base metal,the difference in the distribution of alloying elements such as Cr,Ni and Mo between ferrite and austenite in laser welded seam is significantly reduced,while the difference in the distribution of N atoms grows,so that the ferrite in the weld has a lower pitting resistance index than austenite.Therefore,it is thus preferentially selectively corroded.For base metal,pitting occurs mainly at theδ/γphase boundary and inclusions,while pitting in laser welded seam is mainly caused by the large amount of Cr_(2)N precipitated in the ferrite where pitting happend and the rapid development of the weak phase ferrite.