Ferritic Stainless Steel Brazing via Nitrogen-Enriched Surface Layer
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Solution Overview
Problem
Ferritic stainless steels face challenges with high production costs due to expensive materials like Mo and inadequate brazing properties when using Ni-containing brazing metals, leading to potential brazing failures and insufficient bond strength, while Cu-containing steels lack sufficient corrosion resistance and brazing efficiency.
Innovation Solution
A Cu-containing ferritic stainless steel with a specific chemical composition and a nitrogen-enriched layer formed through controlled heat treatment, optimizing the steel's surface for improved brazing properties and corrosion resistance during high-temperature brazing with Ni-containing brazing metals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Mo is added to ferritic stainless steel to improve corrosion resistance, then corrosion resistance is improved, but production cost increases
Solution Approach 1:
The patent replaces expensive Mo (molybdenum) with cheaper Cu (copper) as the primary alloying element to achieve corrosion resistance. The chemical composition specifies Cu content of 1.5-3.0 mass% while limiting Mo to 0.05-0.20 mass%, significantly reducing material cost while maintaining protective functionality through Cu-rich oxide layer formation.
Solution Approach 2:
The patent optimizes the chemical composition parameters by precisely controlling Cu content (1.5-3.0 mass%), Cr content (17.0-23.0 mass%), and C content (0.005-0.030 mass%) to achieve the desired balance between corrosion resistance, brazing properties, and cost. This parameter optimization eliminates the need for high Mo content while maintaining performance.
2Strength
If Ni-containing brazing metal is used for high-temperature brazing, then brazing temperature is sufficient for strong bonding, but Cr oxide layer forms on the steel surface preventing effective brazing
Solution Approach 1:
The patent performs preliminary action by controlling the steel's chemical composition to pre-form a Cu-rich oxide layer (CuO or Cu2O) on the steel surface before brazing. This is achieved by optimizing Cu content (1.5-3.0 mass%) and Cr content (17.0-23.0 mass%), which ensures that during high-temperature brazing, the Cu-rich oxide layer forms first and prevents Cr oxidation, creating a brazing-friendly surface.
Solution Approach 2:
The patent introduces Cu (copper) as an intermediary element that mediates between the steel substrate and the brazing metal. The Cu-rich oxide layer acts as an intermediate layer that prevents direct reaction between Cr and oxygen, and also prevents Cr depletion at grain boundaries, thereby enabling effective brazing with Ni-containing brazing metals while maintaining corrosion resistance.
3Ease of manufacture
If Cu is added to ferritic stainless steel to reduce production cost, then production cost decreases, but corrosion resistance and brazing efficiency become insufficient
Solution Approach 1:
The patent creates a composite material system by combining Cu (1.5-3.0 mass%), Cr (17.0-23.0 mass%), and C (0.005-0.030 mass%) in specific proportions. This composite composition works synergistically: Cu provides cost reduction and forms protective oxide layers, Cr provides corrosion resistance through Cr2O3 layer formation, and C controls carbide precipitation. The combination achieves both cost reduction and maintained corrosion resistance.
Solution Approach 2:
The patent optimizes the Cu content parameter to a specific range (1.5-3.0 mass%) that is sufficient to form protective Cu-rich oxide layers and reduce cost, but not excessive to compromise corrosion resistance. This precise parameter control, along with optimizing Cr (17.0-23.0 mass%) and C (0.005-0.030 mass%), transforms Cu from a potentially harmful impurity into a beneficial alloying element.
4Strength
If high-temperature brazing is performed to ensure strong bonding, then bond strength is improved, but Cr depletion layer forms at grain boundaries reducing corrosion resistance
Solution Approach 1:
The patent uses Cu as an intermediary element that prevents Cr depletion at grain boundaries during high-temperature brazing. The optimized Cu content (1.5-3.0 mass%) ensures that Cu-rich oxide layers form on the surface and Cu carbides form at grain boundaries, acting as barriers that prevent Cr from reacting with C and N at elevated temperatures, thereby preventing sensitization and maintaining corrosion resistance.
Solution Approach 2:
The patent extracts Cr from the grain boundary region by controlling the chemical composition to prevent Cr carbide precipitation. By optimizing C content (0.005-0.030 mass%) and Cr content (17.0-23.0 mass%), and adding Cu (1.5-3.0 mass%), the patent ensures that Cr remains in solid solution in the matrix rather than depleting at grain boundaries, thereby maintaining corrosion resistance even after high-temperature brazing.
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 excellent corrosion resistance and effective brazing properties at high temperatures, reducing production costs and preventing Cr oxide layer formation, thus enhancing the steel's performance in heat exchanger components.
Implementation Method 1
a nitrogen-enriched layer is present that has a nitrogen concentration peak value of 0.03 mass % to 0.30 mass % at a depth of within 0.05 μm of a surface of the steel
Implementation Method 2
formed through controlled heat treatment, optimizing the steel's surface for improved brazing properties
Implementation Method 3
optimized the steel's surface for improved brazing properties and corrosion resistance during high-temperature brazing with Ni-containing brazing metals
Data Source
AI summary
Provided is a ferritic stainless steel having a chemical composition containing, in mass %: 0.003% to 0.025% of C; 0.05% to 1.00% of Si; 0.05% to 1.00% of Mn; 0.04% or less of P; 0.01% or less of S; 16.0% to 23.0% of Cr; 0.20% to 0.80% of Cu; 0.05% to 0.60% of Ni; 0.20% to 0.70% of Nb; 0.005% to 0.020% of N; and the balance being Fe and incidental impurities, in which a nitrogen-enriched layer is present that has a nitrogen concentration peak value of 0.03 mass % to 0.30 mass % at a depth of within 0.05 μm of a surface of the steel.