Influence of purge quality and surface preparation on weld microstructure and degradation of austenitic stainless steel pressure piping

316或316L级不锈钢焊接管线钢广泛应用于石油和天然气工业，主要用于输送腐蚀性很强的物质。焊接这些钢的主要挑战之一是焊接区发生变色，也称为热着色，这通常会导致零件报废。变色现象的产生与根部焊道及随后的填充和盖层焊接过程中铬氧化物的形成有关。最近的研究表明，熔合区的氧捕获也可能是变色现象发生的一个可能来源。然而，焊接过程中氧气水平对变色程度和对显微组织和腐蚀性能的破坏程度的影响尚未量化。除了氧气水平外，表面处理已被确定为一个非常重要的工艺参数，它可能会影响接头的质量及其在使用条件下的耐用性。因此，本研究的主要目的是通过确定控制机制来研究起始基材、焊接工艺参数和表面准备对变色的程度和严重程度的影响及其与接头组织和性能的关系。工业伙伴的项目成果将是根据所选材料改进表面准备和焊接程序，以达到可接受的变色程度。

Welded pipeline steels made of 316 or 316L grade stainless steels are widely used in oil and gas industry mainly for the transportation of very corrosive substances. One of the main challenges in welding of these steels is the occurrence of discoloration, also called heat tinting, of the weld region which results often in the rejection of the part. The origin of the discoloration phenomenon has been related to the formation of chromium oxides during the welding of the root pass and subsequent fill and cap passes. Recent research has indicated that oxygen trapping in the fusion zone could also be a probable source for the occurrence of the discoloration phenomenon. However, the influence of the oxygen level present during welding on the extent of discoloration and level of damage to the microstructure and corrosion properties has not been quantified yet. In addition to the oxygen level, surface preparation has been identified as a very important process parameter that could affect the quality of the joint and its durability under service conditions. The main objective of the present research is therefore to investigate the influence of starting base material, weld process parameters, and surface preparation on the extent and severity of the discoloration and its correlation with joint structure and properties through the identification of the governing mechanisms. The outcome of the project for the industrial partner will be to implement improved surface preparations and welding procedures as a function of the selected material in order to achieve acceptable discoloration levels.