Austenitic Stainless Alloy Composition for Creep and Crack Resistance
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Solution Overview
Problem
Austenitic stainless alloy materials used in boilers face issues with creep strength and stress relaxation cracking resistance, particularly when subjected to high temperatures and welding or bending, leading to potential grain boundary cracking.
Innovation Solution
An austenitic stainless alloy material with a specific chemical composition and controlled number density of fine precipitates, including elements like C, Si, Mn, Ni, Cr, Ti, Nb, V, and N, with a number density of precipitates having an equivalent circular diameter of 0.5 to 2.0 μm at 5000 pieces/mm² or more, to enhance both creep strength and stress relaxation cracking resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If precipitates are formed during use in high-temperature environment to increase creep strength, then creep strength is improved, but stress relaxation cracking resistance deteriorates
Solution Approach 1:
The patent applies preliminary action by forming precipitates before the material is put into service through controlled addition of Ti, Nb, and V elements. These precipitates are pre-formed to pin grain boundaries and prevent stress relaxation cracking during welding and bending operations, while also providing creep strength enhancement during high-temperature service. The key is that the precipitates are formed in advance rather than forming during service, which would cause hardening and cracking.
2Stress or pressure
If residual stress relaxation occurs in weld zone or bent portion, then internal stress is relieved, but precipitates form within grains causing grain boundary cracking
Solution Approach 1:
The patent applies preliminary anti-action by introducing Ti, Nb, and V elements that form precipitates to counteract the harmful effects of stress relaxation. These precipitates pin grain boundaries before stress relaxation can occur, preventing the formation of stress relaxation cracks. The precipitates act as a pre-established defense mechanism against the cracking that would otherwise result from residual stress relief during welding or bending.
3Strength
If alloy elements Ti, Nb, V are added to form precipitates, then creep strength increases, but material composition complexity increases
Solution Approach 1:
The patent applies merging by combining Ti, Nb, and V elements in a synergistic manner. These three elements work together to form precipitates that provide both creep strength enhancement and stress relaxation cracking resistance. By merging the functions of multiple elements into a coordinated system, the patent achieves superior performance while managing composition complexity through defined ratio relationships (P1 and P2 parameters).
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 alloy material achieves excellent creep strength and stress relaxation cracking resistance by maintaining fine grains through the pinning effect of pre-existing precipitates, ensuring durability under high-temperature conditions.
Implementation Method 1
by making P2 that is an index of Nb, V, and Ti 0.2 or more, precipitates are formed during use in a high-temperature environment and the creep strength is increased
Implementation Method 2
by containing Ti, Nb, and V, precipitates are formed during use in a high-temperature environment and the creep strength is increased
Implementation Method 3
At such time, relaxation of residual stress occurs at a weld zone of the austenitic stainless alloy material or at a portion subjected to bending. Due to the relaxation of residual stress, precipitates form within grains and the interior of the grains hardens.
Data Source
AI summary
An austenitic stainless alloy material that has excellent creep strength and excellent stress relaxation cracking resistance is provided. An austenitic stainless alloy material according to the present disclosure contains, in mass %, C: 0.03 to 0.12%, Si: 0.05 to 2.00%, Mn: 0.05 to 3.00%, P: 0.03% or less, S: 0.010% or less, Ni: 18.0 to less than 25.0%, Cr: 22.0 to less than 30.0%, Co: 0.04 to 0.80%, Ti: 0.002 to 0.010%, Nb: 0.1 to 1.0%, V: 0.01 to 1.00%, Al: 0.001 to less than 0.030%, and N: 0.10 to 0.35%. The number density of precipitates having an equivalent circular diameter of 0.5 to 2.0 μm is 5000 pieces/mm2 or more.