锰氮配比对22%Cr节Ni型双相不锈钢焊接热影响区组织及性能的调控机制研究

The low nickel type duplex stainless steels (DSS) are to stabilize austenite phase by substitution combination of Mn and N for expensive Ni resource, which is an effective way to obtain high performance of DSS and expand their application fields. However, in the process of welding thermal cycle, the formation of precipitated phase and unbalance of two phase ratio in heat affected zone (HAZ) will degrade mechanical property and corrosion resistance of DSS. Furthermore, Mn and N have different effects on the stability of austenite phase and the formation of precipitates, thus, different Mn/N ratio will affect precipitated phase formation and two phase ratio caused by reformed austenite. For 22% Cr low nickel type DSS for engineering application, the project intends to study the influencing mechanism of Mn/N ratio on microstructure and properties of HAZ. On the condition of different welding peak temperatures, heat inputs and multi pass welding thermal cycles, the following studies will be conducted. To study the influencing rules and mechanism of Mn/N ratio on reformed austenite formation, two phase proportion and element distribution in welding HAZ. To investigate the influencing rules of Mn/N ratio on precipitated phase type, distribution and size in HAZ, and to explore thermodynamic and kinetic mechanism of nitride formation. To study the influencing mechanism of Mn/N ratio on impact toughness of HAZ under the condition of single pass welding thermal cycle. And finally, the influencing rules and mechanism of Mn/N ratio on pitting corrosion and intergranular corrosion will be investigated for welding HAZ. Therefore, the research can provide theoretical basis for microstructure optimization of low nickel type DSS during welding thermal cycles, which can improve its weldability and promote its development and application.

Mn、N配比能有效替代昂贵Ni稳定奥氏体，是获得高性能节Ni型双相不锈钢(DSS)并扩大其应用的有效途径。然而焊接热影响区(HAZ)中析出相形成和两相比例失衡会明显降低其力学和耐蚀性。因Mn、N稳定奥氏体能力和对析出相形成类型影响差异较大，不同Mn/N比将影响HAZ析出相形成和转变奥氏体形成导致的两相比例变化。对22%Cr工程用节Ni型DSS，拟对其焊接HAZ的Mn/N比影响机制进行研究。在单道次不同焊接峰值温度、热输入及多道次热循环条件下，研究Mn/N比对HAZ转变奥氏体形成、两相比例及元素分配的影响规律及机理；探讨Mn/N比对HAZ析出相类型、分布及氮化物形成热力学和动力学影响机理；研究不同Mn/N比焊接HAZ组织和两相比例变化对HAZ冲击韧性，点蚀和晶间腐蚀的影响规律和机理。为节Ni型DSS焊接HAZ组织调控优化提供理论基础，以改善其焊接性，推进其研制和应用。

Ni价格的持续攀升限制了双相不锈钢(DSS)在工程应用领域的扩展，通过Mn、N合理配比代替Ni可获得两相平衡节约型DSS，因Mn、N含量对氮化物、σ相和转变奥氏体形成影响差异较大，不合适的成分和焊接工艺参数配合会导致焊接热影响区(HAZ)性能明显下降。研究了Mn/N配比对22%Cr节Ni型DSS焊接HAZ组织、力学和耐蚀性影响规律和机理，为增强节Ni型DSS焊接HAZ综合性能提供理论基础和有效途径。固溶态组织和性能分析可知，其抗拉强度和延伸率随Mn/N比增加而降低，在3.77-52.14Mn/N比范围获得两相平衡组织，1780Mn/N比DSS固溶态相对2205DSS有更优异综合性能。低热输入时Mn/N比增加促进HAZ晶内和晶界转变奥氏体(IGA和GBA)形成，高热输入则一定程度降低了N稳定奥氏体能力。Mn/N比增加抑制Cr2N形成，并在较高热输入促进了HAZ相间位错缠结和σ相形成。动力学曲线分析可知随Mn/N比增加，Cr2N的形成鼻点温度降低，且开始形成时间延长，与TEM分析结果一致。探讨了中低热输入时Mn/N比对力学性能影响得出，因N的固溶强化和HAZ晶粒细化作用，3.77Mn/N比DSS的HAZ抗拉和屈服强度、延伸率均高于2205DSS。随Mn/N比增加，HAZ冲击断裂从韧性转变为脆性，两相硬度和弹性模量差的扩大增加了HAZ裂纹形成，明显降低了高氮DSS韧性。中低热输入HAZ耐蚀性分析得出HAZ的平均点蚀电位随Mn/N比增加先增后迅速减少。高Mn/N比DSS中的Fe(III)和Cr，NH4+含量降低和疏松MnO形成导致HAZ钝化膜稳定性和致密性降低。获得了中等热输入条件下不同焊接峰值温度和Mn/N比对HAZ转变奥氏体和析出相形成、冲击韧性及耐蚀性影响规律，发现氮含量的增加不仅能有效稳定不同峰值温度HAZ奥氏体相，还能有效增强整体的耐点腐蚀性能。获得了大热输入条件下Mn/N比对焊接HAZ组织演变、耐蚀和力学性能的影响规律。由于大热输入HAZ转变奥氏体组织明显增加，17.80Mn/N比DSS的HAZ耐点蚀性高于2205DSS，但冲击韧性仍处于低值，3.28Mn/N比高氮奥氏体钢HAZ冲击韧性则明显提高。探讨了多道次焊接条件下Mn/N比对焊接HAZ组织演变和力学性能影响，可对节Ni型双相不锈钢焊接道次工艺和成分合理配比提供理论基础。