超声冲击改善P355NL1钢焊接接头腐蚀疲劳性能研究

Improvement of Corrosion Fatigue Performance of P355NL1 Steel Welded Joint by Ultrasonic Impact

对P355NL1钢焊接接头进行超声冲击处理,研究超声冲击对腐蚀疲劳性能的影响。利用光学显微镜和扫描电镜对超声冲击前后的表层显微结构以及腐蚀疲劳失效断口进行分析,利用电化学工作站对超声冲击前后试样电化学腐蚀速率进行测定。结果表明,当试样处于6%(质量分数) NaCl腐蚀溶液中时,焊态试样疲劳强度下降12.5%;在6%NaCl腐蚀液和水介质中,冲击态试样相较于焊态试样疲劳强度分别提高了75%和53%,S-N曲线斜率分别改变75.4%和60.4%。经超声冲击处理后试样表层产生明显的塑性变形层,最大变形层深度约350μm;冲击之后试样的疲劳寿命有很明显提高,疲劳断裂位置也由焊趾处转移到焊缝或者母材区,腐蚀坑数目明显减少。表明超声冲击可以细化晶粒,同时降低应力集中现象,消除有害残余拉应力,引入有益的残余压应力,降低电化学腐蚀速率,提高焊接接头的腐蚀疲劳性能。

The effect of ultrasonic impact on corrosion fatigue properties of P355 NL1 steel welded joints was studied, while the surface morphology and the fractured surface were characterized by optical microscope and scanning electron microscope. Meanwhile, the electrochemical corrosion rate of the weld joints before and after ultrasonic impact was comparatively examined by electrochemical workstation. The results show that the fatigue strength of the welded joint in the as welded state decreases by 12.5% in 6%(mass fraction) Na Cl corrosion solution in comparison with that of the substrate. In 6%Na Cl solution and water, the fatigue strength of the ultrasonic impacted welded joints is increased by 75% and 53%, and the slope of S-N curve is changed by 75.4% and 60.4%, respectively, in comparison with that in the welded state. The maximum depth of the plastic deformation layer is about 350 μm. The fatigue life of the welded joints is significantly improved by the ultrasonic impact, correspondingly, the fatigue fracture location is also transferred from the weld toe to the weld or base metal area, and the number of corrosion pits is significantly reduced. The results clearly show that the ultrasonic impact can refine grain, reduce stress concentration,eliminate harmful residual tensile stress, introduce beneficial residual compressive stress, reduce electrochemical corrosion rate, and improve the corrosion fatigue performance of welded joints of P355 NL1 steel.