Austenitic Stainless Steel Pipe Creep and SRC Resistance
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Solution Overview
Problem
Existing austenite-based stainless steel pipes with large diameter and large wall thickness face challenges in achieving high creep strength, good stress corrosion cracking resistance (SCC), and stress relaxation cracking resistance (SRC) due to non-uniform processing strain and Nb segregation, which affects grain structure.
Innovation Solution
Austenite-based stainless steel pipes with specific chemical compositions and controlled microstructures, including a recrystallization ratio X of not lower than 0.90, austenite crystal grain size of 20 to 300 µm, and two-stage heating during the push-bench process, are manufactured to ensure uniform strain relief and optimal grain size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If austenite-based stainless steel containing Nb is used for large diameter and large wall thickness pipes, then good weldability and high-temperature strength are improved, but SRC resistance deteriorates due to non-uniform processing strain and Nb segregation
Solution Approach 1:
The patent applies parameter changes by precisely controlling the Nb content within 0.20-1.00% and implementing specific heat treatment parameters (heating temperature 1140-1190°C, holding time 30-180 minutes) to achieve uniform microstructure and eliminate Nb segregation, thereby resolving the contradiction between high-temperature strength and SRC resistance
Solution Approach 2:
The patent applies local quality by ensuring uniform distribution of Nb elements throughout the pipe wall through controlled heat treatment, preventing local segregation that would cause non-uniform microstructure and poor SRC resistance in large diameter and thick wall pipes
2Reliability
If finer crystal grains are used, then SRC resistance is improved, but creep strength deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing the austenite crystal grain size to fall within 20-300 μm through controlled heat treatment parameters, achieving the optimal balance between SRC resistance (requiring finer grains) and creep strength (requiring coarser grains)
3Reliability
If a recrystallization ratio of not lower than 0.90 is achieved through heat treatment, then uniform microstructure and SRC resistance are improved, but manufacturing process complexity increases
Solution Approach 1:
The patent applies preliminary action by performing heat treatment after the push-bench process to ensure uniform microstructure and high recrystallization ratio (≥0.90) before final pipe formation, preventing non-uniform processing strain and Nb segregation that would require additional corrective operations
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 solution provides austenite-based stainless steel pipes with large diameter and wall thickness that exhibit high creep strength, good SCC resistance, and effective SRC resistance, maintaining mechanical integrity in high-temperature, high-pressure environments.
Implementation Method 1
heat treating the intermediate product at 1140 to 1190 °C
Implementation Method 2
a recrystallization ratio X being not lower than 0.90
Data Source
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AI summary
An austenite-based stainless steel pipe is provided that has large diameter and large wall thickness and that provides high creep strength, good SCC resistance and good SRC resistance. An austenite-based stainless steel pipe has a chemical composition including, in mass %: up to 0.030 % C; 0.10 to 1.00 % Si; 0.10 to 2.00 % Mn; 15.00 to 25.00 % Cr; 6.00 to 15.00 % Ni; 0.020 to 0.200 % N; up to 0.030 % sol. Al; 0.001 to 0.045 % P; up to 0.010 % S; and 0.20 to 1.00 % Nb, the steel pipe having an outer diameter not smaller than 200 mm, a wall thickness not smaller than 15 mm, an austenite crystal grain size of 20 to 300 µm, and a recrystallization ratio X not lower than 0.90.