Duplex Stainless Steel Welded Joint Sigma Phase Suppression
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Solution Overview
Problem
Duplex stainless steel welded joints experience a decrease in corrosion resistance and embrittlement due to sigma phase precipitation during high heat input welding, particularly under high-temperature chloride environments, leading to stress corrosion cracking and sulfide stress cracking.
Innovation Solution
A line pipe with a welded joint of duplex stainless steel comprising a base metal and a weld metal, where the chemical composition is optimized to suppress sigma phase precipitation, with specific element ratios ensuring improved SCC resistance, including C: ≤0.030%, Si: 0.20-1.00%, Mn: ≤8.00%, P: ≤0.040%, S: ≤0.010%, Cu: 2.00-4.00%, Ni: 4.00-8.00%, Cr: 20.0-30.0%, Mo: 0.50-2.00%, N: 0.100-0.350%, and optionally V, Ca, Mg, B, or rare earth metals, satisfying specific expression criteria to enhance passive film strength and inhibit sigma phase nucleation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If high heat input welding is applied to duplex stainless steel, then welding efficiency and productivity are improved, but sigma phase precipitation occurs causing embrittlement and decreased corrosion resistance
Solution Approach 1:
The invention changes the chemical composition parameters of the duplex stainless steel, specifically controlling Cr (23-30%), Mo (1.50-3.00%), and Cu (2.00-4.00%) content to suppress sigma phase precipitation during high heat input welding, enabling both high productivity and maintained reliability
Solution Approach 2:
The invention creates a composite material system with optimized multi-element composition (Cr, Mo, Cu, Ni, Mn) that synergistically prevents sigma phase formation while maintaining the duplex microstructure, allowing high heat input welding without embrittlement
2Reliability
If Cu content is increased to improve corrosion resistance under chloride and sulfide environments, then SCC resistance improves, but sigma phase precipitation is promoted during welding
Solution Approach 1:
The invention optimizes the Cu content parameter to a specific range (2.00-4.00%) and combines it with controlled Cr and Mo content, changing the overall compositional parameters to achieve both improved SCC resistance and suppressed sigma phase precipitation
Solution Approach 2:
The invention creates local quality differentiation by having specific element distributions and interactions (Cu with Cr and Mo) that provide corrosion resistance in the matrix while preventing sigma phase formation at grain boundaries during welding
3Reliability
If Cr and Mo content are increased to enhance passive film strength and corrosion resistance, then SCC resistance improves, but the risk of sigma phase precipitation increases
Solution Approach 1:
The invention changes the compositional parameters by setting specific ranges for Cr (23-30%) and Mo (1.50-3.00%) and combines them with Cu content control, achieving both enhanced corrosion resistance and suppressed sigma phase precipitation through optimized parameter combinations
Solution Approach 2:
The invention introduces Cu as an intermediary element that mediates between Cr/Mo and the sigma phase formation process, where Cu suppresses sigma phase precipitation while allowing Cr and Mo to maintain high corrosion resistance
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The optimized composition effectively suppresses sigma phase precipitation and enhances SCC resistance under high-temperature chloride environments, maintaining excellent corrosion resistance and toughness in both the base metal and weld metal.
Implementation Method 1
the area fraction of a ferrite phase in the duplex stainless steel to 40 to 70%
Data Source
Figure 1
Figure 2
Figure 3A~3B
AI summary
Provided is a welded joint of duplex stainless steel that can suppress precipitation of a σ phase at the time of high heat input welding and is excellent in SCC resistance under high-temperature chloride environments. A weld metal of the welded joint of duplex stainless steel according to the present invention contains, in mass percent, C: at most 0.030%, Si: 0.20 to 1.00%, Mn: at most 8.00%, P: at most 0.040%, S: at most 0.0100%, Cu: at most 2.00%, Ni: 7.00 to 12.00%, Cr: 20.0 to 30.0%, Mo: 1 to 4%, N: 0.100 to 0.350%, sol. Al: at most 0.040%, and O: at most 0.035%, the balance being Fe and impurities, and the weld metal satisfies Expression (1) and Expression (3): 2.2Cr+7Mo+3Cu>66 Cr+11Mo+10Ni-12Cu+30N<100 where a content (mass percent) of each element in one of the base metal and the weld metal is substituted into a symbol of each element in Expression (1) and Expression (3).