论文总字数：19691字

摘 要

在奥氏体不锈钢和马氏体不锈钢的焊接过程中使用双层保护气体，在外层保护气体中添加适量活性气体，而内层保护气体保持纯惰性气体的方法，能够在保护电极不被氧化的同时，增加焊缝的熔深，提高焊接效率，这种方法被称为AA-TIG焊。

本研究对这一先进焊接方法是否适用于双相不锈钢进行了探讨。研究了三种不同氧气添加量情况下2205双相不锈钢焊件焊缝中的金相组织、焊缝形状、氧化物的形貌及分布，并对硬度、屈服强度及冲击韧性进行了测试。研究结果表明：随着保护气中氧气添加量的增加，焊缝从宽而浅变得窄而深，焊缝的深宽比增加，焊接效率得到提高。焊缝凝固后的室温组织包括母相铁素体和新相奥氏体两个相，而新相奥氏体分为晶界奥氏体（包括魏氏体型奥氏体）和晶内奥氏体。外层保护气体中氧气添加量的增加会抑制铁素体到奥氏体的相变；保护气体中的氧含量会影响焊缝形状和焊缝中氧化物的平均直径和数量密度；检测焊件硬度、屈服强度和冲击韧性，实验发现保护气体中氧含量对焊缝硬度几乎没有影响，但是会略降低焊缝的屈服强度和冲击韧性。随后对冲击韧性试样断面进行观察，发现裂纹产生于奥氏体相和铁素体相的相界面。

关键词：双层保护电弧焊；双相不锈钢；氧气添加量；机械性能；焊缝深宽比

ABSTRACT

The double-shielded arc welding has been used in the welding process of the austenitic stainless steel and the martensitic stainless steel by adding appropriate amount of active gas to the outer protective gas, and maintaining the inert gas in the inner protective layer. This method could protect the electrode from being oxidized while could increase the weld penetration and thus improve the welding efficiency. This method is known as AA-TIG welding.
This study discussed whether this advanced welding method is suitable for the duplex stainless steels. The microstructure, the weld shape, the morphology and the distribution of oxides in the welds of the 2205 duplex stainless steel welds were studied under three different oxygen addition conditions. The hardness, the yield strength and the impact toughness were tested. The results showed that as the amount of oxygen added in the protective gas increased, the weld became narrow and deep, and the D/W ratio of the weld increased and thus the welding efficiency was improved. The room temperature microstructure after solidification of the weld were consisted of two phases: the parent ferrite phase and the new austenite phase, and the new phase austenite had two morphologies: the grain boundary austenite (including the Widmanstatten-type austenite) and the intragranular austenite. The increase in the amount of the oxygen in the outer protective gas inhibited the phase change from the ferrite to the austenite; the oxygen content in the protective gas affected the weld penetration and the average diameter and number density of the oxides in the weld. It was found that the oxygen content in the protective gas had little effect on the hardness of the weld, but it would slightly reduce the yield strength and impact toughness of the weld. Then the cross section of the impact toughness specimen was observed and the crack was found at the interface between the austenite phase and the ferrite phase.

Key words: Double-shielded arc welding; duplex stainless steel; oxygen addition; mechanical properties; D/W ratio

目 录

摘要..........................................................................................................Ⅰ

ABSTRACT..............................................................................................Ⅱ

第一章 绪论 1

1.1 双相不锈钢简介及分类 1

1.1.1 简介 1

1.1.2 分类及牌号 1

1.2 双相不锈钢的焊接性 2

1.2.1 双相不锈钢与奥氏体、铁素体不锈钢的不同 2

1.2.2 耐蚀性 3

1.2.3 焊接热影响区的组织转变 5

1.2.4 焊缝金属的组织转变 9

1.2.5 双相不锈钢凝固模式 10

1.3 双层保护电弧焊发展背景 11

1.4 氧对金属机械性能影响的研究 14

1.5 本课题研究的目的及内容 16

第二章 焊缝金属的微观组织形貌 17

2.1 实验方法 17

2.2 实验结果 18

2.3 本章小结 18

第三章 焊缝形状及氧化物分布 19

3.1实验方法 19

3.2实验结果 19

3.3本章小结 21

第四章 焊缝金属的机械性能 22

4.1硬度 22

4.1.1 实验方法 22

4.1.2 实验结果 22

4.2屈服强度 23

4.2.1 实验方法 23

4.2.2 实验结果 23

4.3冲击韧性 24

4.3.1 实验方法 24

4.3.2 实验结果 25

4.4 本章小结 27

第五章 总结 28

参考文献..................................................................................................29

致谢..........................................................................................................31

绪论

1.1 双相不锈钢简介及分类

1.1.1 简介

双相不锈钢是在固溶体中铁素体相和奥氏体相各约占一半，一般较少相的含量至少也需要达到30%的不锈钢。除此以外，Mo、Cu、Si、Nb、Ti，N等合金元素也有可能在一部分钢中存在。双相不锈钢的成分决定了它将继承铁素体不锈钢和奥氏体不锈钢的特质。相较于铁素体，双相不锈钢不仅延续了其导热系数高，具有超塑性等优良特点，还继承了一部分缺点，如475℃脆性。由于有奥氏体相的加入，双向不锈钢在塑韧性，耐晶间腐蚀性能和焊接性能上体现出明显的优势。与此同时，铁素体的加入使双相不锈钢的强度和耐晶间腐蚀性能与奥氏体不锈钢相比有了显著的提升。双相不锈钢具有优良的耐孔蚀性能，也是一种节镍不锈钢^[1]。

1.1.2 分类及牌号

一般情况下，双相不锈钢按照合金元素含量划分可以分为低合金型、中合金型、高合金型和超级双相不锈钢型四种类型。常用牌号和各国牌号对照如下表：

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