ABSTRACT

This study focuses on the challenges associated with welding stainless steel, specifically addressing the issue of heat tint that occurs during high-temperature welding. This tint is recognized as a consequence of the formation of an oxide layer. This layer, less dense than the passive film formed at room temperature, leads to discoloration and reduced corrosion resistance. Common practices like argon purging are not always effective, requiring additional measures such as physical removal or acid treatment of the oxide layer. Excessive argon usage and labor for re-welding are concerns. The research includes an analysis of corrosion resistance differences due to discoloration in actual operating conditions, establishing criteria for visual inspection of weld bead and heat-affected zone discoloration. Additionally, the study develops a program for quantitative assessment of discoloration, providing more objective standards for pass/fail judgments in stainless steel pipe welding.

INTRODUCTION

In welding stainless steel piping, the welder creates a back bead between the grooves of the pipe from the outside, where a gap is present. During welding, the high temperatures cause the weld and heat-affected zone exposed to the atmosphere to react with oxygen, leading to oxidation and the formation of a thick oxide layer, which results in discoloration (Sunil Kumar et al., 2013; Trigwell et al., 2005). Stainless steel, containing chromium, forms a chromium-inclusive oxide layer; however, the proportion of iron oxides is higher (Trigwell et al., 2005). The iron oxide layer grows rapidly and, due to its porous structure, easily forms corrosion pathways when exposed to corrosive environments (Trigwell et al., 2005). As stainless steel piping is often used in environments requiring corrosion resistance, these oxide layers are typically removed through physical or chemical means (Kearns et al, 1994). However, removing such layers from the inaccessible interior of the piping is challenging, which is why purging with inert argon gas is performed to eliminate internal oxygen before welding. While extended argon purging and complete oxygen removal prevent oxidation and discoloration, this process is costly and time-consuming due to the high price of argon gas. Additionally, oxygen can infiltrate through gaps created during welding, potentially leading to partial discoloration. In some instances, areas that are difficult to access may be ground down and rewelded. Currently, there is no known acceptable range of discoloration for stainless steel piping in operational environments due to these various factors. Therefore, this study explores the permissible range of discoloration for stainless steel piping used in gas carriers.

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