Microstructure and corrosion resistance of laser-MIG 316L stainless steel under the nitrogen assistance
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摘要: 为了提高纯氩气下MIG焊接316L不锈钢的稳定性、改善焊缝组织以及强化耐腐蚀性能，引入1 200 W小功率激光对MIG电弧进行诱导压缩，同时在氩气中混入氮气，探索不同流量比的Ar-N2混合气体对焊缝微观组织及其耐腐蚀性能的影响. 结果表明，激光的诱导作用能够收缩并稳定MIG电弧，随着氮气流量的增加，焊缝的熔合线逐渐平缓，内部气孔缺陷明显降低；XRD测试和显微组织分析发现，渗氮后的焊缝内部γ相含量明显增多，中下部区域均为细小均匀的γ胞状晶，中上部区域为γ树枝晶，并且一次枝晶间距逐渐减小. 当氮气流量增加到5 L/min，焊缝的显微硬度可综合提升20 HV；电化学极化测试发现，渗氮之后的焊缝表现出更强的耐腐蚀性能. 试验证实，氮气辅助激光-MIG复合焊接工艺能够改善316L不锈钢焊缝的显微组织和耐腐蚀性能，当Ar∶N2气体流量比为20∶5时，γ相的强化效果最显著，综合耐腐蚀性能最好.
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关键词:
- 激光诱导电弧复合焊接 /
- 316L不锈钢 /
- 氮气 /
- 耐腐蚀性能 /
- 微观组织
Abstract: In order to enhance the MIG arc stability, improve the internal microstructure and strengthen the corrosion resistance of 316L stainless steel weldments manufactured by MIG under the pure argon gas, a 1 200 W low power laser was introduced to induce compression on the MIG arc, with N2 mixed into Ar to explore the effect of Ar-N2 mixed shielding gas with different flow rates on the microstructure and corrosion resistance of the 316L welding seam. Experimental observations display that the MIG arc became more stable under the induced effect of 1 200 W laser. With the increase of N2 gas flow rate, the fusion line of the molted pool become smoother and the internal porosity defects are significantly reduced. XRD tests and microstructure observations indicate that the content of internal γ-phase increase significantly. It can be clearly seen that most fine cellular γ phase distributed uniformly in the lower middle regions of the molted pool, and the upper middle regions were dendritic γ phase, with its primary dendrite spacing gradually decreased. As the N2 gas flow rate increase to 5 L/min, the micro-hardness of the welding seam could be enhanced by 20 HV. Electrochemical polarization tests revealed that the Laser-MIG 316L welding seam formed under the Ar-N2 mixed gas exhibit stronger corrosion resistance. Above experiments confirmed that the N2-assisted laser-MIG hybrid welding technology can improve the microstructure and corrosion resistance of 316L stainless steel weldments, and when the Ar : N2 gas flow rate is 20 : 5, the strengthening effect of γ phase is most significant and the best corrosion resistance can be achieved comprehensively. -
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图 1 激光-MIG复合焊接工艺示意图
Figure 1. Schematic diagram of laser-MIG hybrid welding technology
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图 2 用于电化学测试的纵向焊道工作电极选区示意图
Figure 2. Schematic diagram of welded zone selected as working electrode for electrochemical tests. (a) cross welding; (b) longitudinal welding
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图 3 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢焊道的表面形貌
Figure 3. Macroscopic of 316L stainless steel welding bead formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas
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图 4 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢横向焊缝组织形貌
Figure 4. Microstructure of 316L transverse welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas. (a) cross section; (b) upper middle region; (c) middle region; (d) lower middle region
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图 5 MAG焊(80%Ar-20%CO2)316L不锈钢横向焊缝组织形貌
Figure 5. Microstructure of 316L transverse welding seam formed by MAG welding under the 80%Ar-20%CO2 shielding gas. (a) lower middle region；(b) middle region；(c) bottom region；(d) fusion line
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图 6 不同Ar- N2气流量比下的激光-MIG复合焊接316L不锈钢纵向焊缝组织图
Figure 6. Microstructure of 316L longitudinal welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas. (a) top region；(b) upper middle region；(c) middle region；(d) lower middle region
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图 7 不同Ar-N2气流量比的激光-MIG复合焊接316L不锈钢纵向焊缝X射线衍射图
Figure 7. XRD patterns of 316L longitudinal welding seam formed by laser-MIG hybrid welding under the different flow rate ratios of Ar-N2 shielding gas
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图 8 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢纵向焊缝的显微硬度
Figure 8. Vickers hardness of 316L longitudinal welding seam formed by laser-MIG hybrid welding under the different flow rate ratios of Ar-N2 shielding gas
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图 9 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢纵向焊缝的开路电位曲线
Figure 9. Open circuit curves of 316L longitudinal welding seam formed by laser-MIG hybrid welding under the different flow rate ratios of Ar-N2 shielding gas
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图 10 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢的纵向焊缝电化学阻抗图
Figure 10. Electrochemical impedance spectroscopy of the 316L longitudinal welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas. (a) Nyquist; (b) Bode plot
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图 11 316L纵向焊缝在3.5% NaCl溶液中的等效电路
Figure 11. Simplified equivalent circuit of 316L longitudinal welding seam in 3.5% NaCl solution
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图 12 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢纵向焊缝的动电位极化曲线
Figure 12. Dynamic cycle polarization curves of 316L stainless steel longitudinal welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas
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图 13 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢纵向焊缝点蚀电位与自腐蚀电位
Figure 13. Epit and Ecorr of the 316L longitudinal welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas
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图 14 激光-MIG复合焊接316L钢熔池的吸氮与脱氮
Figure 14. Schematic of nitrogen absorption and desorption during the 316L laser-MIG hybrid welding process.
表 1 Bohler 316L不锈钢焊丝的化学成分(质量分数，%)
Table 1 Chemical composition of Bohler 316L stainless steel solid wire
C Si Mn Cr Ni Mo P S Cu Fe 0.015 0.45 1.6 18.5 12.0 2.6 0.017 0.007 0.04 余量 
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表 2 不同Ar-N2气体流量比的激光-MIG复合焊接316L不锈钢焊道尺寸
Table 2 Dimensions of 316L stainless steel welding bead formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas
Ar∶N2
气体流量比熔宽
W/mm熔深
H/mm余高
h/mm24∶1 7.93 3.78 2.80 22.5∶2.5 7.85 3.67 2.59 20∶5 7.61 3.70 2.66 17.3∶7.7 7.49 3.66 2.78
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表 3 不同Ar-N2气体流量比的纵向焊缝顶部区域树枝状晶的一次枝晶间距和二次枝晶间距
Table 3 Primary dendrite spacing and secondary dendrite spacing of dendritic crystals in the top region of the longitudinal welding seam formed under the different flow rate ratios of Ar-N2 shielding gas
Ar : N2
气体流量比一次枝晶间距
S1 / μm二次枝晶间距
S2/ μm24 : 1 21.38 7.46 22.5 : 2.5 18.30 7.74 20 : 5 16.62 6.96 17.3 : 7.7 12.08 7.32
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表 4 不同Ar-N2气流量比下的激光-MIG复合焊接316L纵向焊缝的电化学阻抗谱参数
Table 4 Electrochemical impedance spectroscopy parameters of 316L longitudinal welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas
Ar : N2
气体流量比溶液电阻
Rs/(Ω·cm2)电荷转移电阻
Rp/(kΩ·cm2)比例系数
Y0/10−6(Ω−1·cm−2·sn)经验系数
n双电层电容
Cdl/(μF·cm−2)24 : 1 25.23 5.07 64.81 0.84 19.08 22.5 : 2.5 18.11 7.25 45.15 0.87 15.60 20 : 5 24.26 8.44 49.10 0.89 21.36 17.3 : 7.7 25.49 6.30 58.04 0.85 18.37
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表 5 不同Ar-N2气体流量比的激光-MIG电弧复合焊接316L不锈钢纵向焊道的动电位循环极化曲线
Table 5 Dynamic cycle polarization curves of the welding seam formed by laser-MIG hybrid welding technology under the different flow rate ratios of Ar-N2 shielding gas
Ar∶N2
气流量比自腐蚀电位
Ecorr/V自腐蚀电流
Icorr/(10−7A·cm−2)点蚀电位
Epit/V再钝化电位
Erep/V电位误差
(Epit−Ecorr)/V电位误差
(Erep− Ecorr)/V24∶1 −0.057 5.31 0.516 −0.098 8 0.573 −0.041 8 22.5∶2.5 −0.076 4.55 0.575 −0.138 6 0.651 −0.062 6 20∶5 −0.159 7.43 1.203 −0.091 7 1.362 0.067 3 17.3∶7.7 −0.123 0.535 1.047 −0.1011 1.170 0.021 9
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