Austenitic Stainless Steel Tube Steam Oxidation Resistance
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Solution Overview
Problem
Existing steel pipes for high-efficiency boilers face challenges in maintaining steam oxidation resistance at elevated temperatures due to issues with uniform Cr oxidation scale formation and stability, leading to potential stress corrosion cracking and reduced longevity.
Innovation Solution
An austenitic stainless steel pipe with a controlled metal structure featuring subgrains, formed by blasting shots or sands onto the inner surface, ensuring a volume ratio of 0.3 or higher of small-angle and large-angle grain boundaries, and a crystal grain size of 50 µm or smaller, which enhances uniform Cr oxidation scale production and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional cold working or peening is performed to form a worked layer, then hardness and initial oxidation resistance are improved, but uniform Cr oxidation scale formation and long-term stability at high temperatures deteriorate
Solution Approach 1:
The invention changes the fundamental parameter of the metal structure by introducing subgrains with specific orientation differences (5-50 degrees) through controlled hard working. This creates a unique microstructure where adjacent crystals have specific orientation relationships that promote uniform Cr diffusion and oxidation scale formation, resolving the contradiction between initial hardness and long-term oxidation uniformity.
Solution Approach 2:
The invention creates a composite microstructure consisting of subgrains embedded within larger crystal grains. The subgrain structure (with orientation differences of 5-50 degrees) acts as a diffusion pathway network for Cr atoms, while the base metal matrix provides structural integrity. This composite microstructure enables both uniform oxidation scale formation and maintained strength at high temperatures.
2Strength
If high working ratio cold working is performed to increase hardness, then initial protectability is improved, but uniform Cr oxidation scale formation deteriorates
Solution Approach 1:
The invention optimizes the working ratio parameter to a specific range (20-80%) that is sufficient to create subgrains with the required orientation differences but not so high as to cause excessive grain refinement or heterogeneity. This controlled parameter change achieves the balance between hardness and uniform oxidation scale formation.
3Strength
If solution heat treatment is performed to uniformize metal structure, then high-temperature strength is improved, but fine grain structure and oxidation resistance deteriorate
Solution Approach 1:
The invention performs hard working to create the subgrain structure before solution heat treatment. This preliminary action ensures that the beneficial subgrain microstructure is established in advance, and the subsequent heat treatment uniformly distributes these subgrains throughout the metal structure without eliminating them, maintaining both strength and oxidation 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 solution significantly improves steam oxidation resistance by forming a protective Cr oxidation scale uniformly, inhibiting abnormal oxidation even at temperatures near 750°C, thereby extending the pipe's service life and maintaining effective protection.
Implementation Method 1
a region satisfying Formula (1) exists in a metal structure at a depth of 10 to 20 μm from the inner surface of steel pipe, and the region is formed by blasting shots or sands
Implementation Method 2
where g is a value calculated from Formula (2)... the volume ratio of a region in which the orientation difference of adjacent crystals is 5 to 50 degrees
Implementation Method 3
scale excellent in protectability can be formed uniformly on the surface of the steel pipe... forming a protective Cr oxidation scale uniformly, inhibiting abnormal oxidation
Data Source
AI summary
There is provided an austenitic stainless steel pipe excellent in steam oxidation resistance. The austenitic stainless steel pipe excellent in steam oxidation resistance contains, by mass percent, 14 to 28% of Cr and 6 to 30% of Ni, and is configured so that a region satisfying the following Formula exists in a metal structure at a depth of 5 to 20 µm from the inner surface of the steel pipe: α/β×δ/ε×100≥0.3 where the meanings of symbols in the above Formula are as follows: α: sum total of the number of pixels of digital image in region in which orientation difference of adjacent crystals detected by electron backscattering pattern is 5 to 50 degrees β: the number of total pixels of digital image in region of measurement using electron backscattering pattern ε: analysis pitch width of electron backscattering pattern (µm) δ: grain boundary width (µm).