Ferritic Stainless Steel Nitrogen-Enriched Surface Brazing
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Solution Overview
Problem
Ferritic stainless steels face challenges with brazing properties when using Ni-containing brazing metals at high temperatures due to oxide film formation, leading to inadequate joint strength and infiltration, and existing solutions either compromise corrosion resistance or increase production costs.
Innovation Solution
A ferritic stainless steel with a nitrogen-enriched surface layer, created through controlled atmosphere heat treatment before brazing, inhibits the formation of Ti and Al oxide films, enhancing brazing properties while maintaining corrosion resistance and allowing for efficient production.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If ferritic stainless steel is used for heat exchanger parts requiring high-temperature brazing with Ni-containing brazing metal, then cost is reduced and heat fatigue properties are improved, but oxide film formation occurs on the surface leading to poor brazing properties
Solution Approach 1:
The patent applies preliminary action by forming a nitrogen-enriched layer on the steel surface before brazing through controlled atmosphere heat treatment. This pre-treatment prevents oxide film formation during subsequent high-temperature brazing with Ni-containing brazing metal, thereby improving brazing properties without compromising the use of cost-effective ferritic stainless steel
Solution Approach 2:
The patent utilizes an inert atmosphere principle by conducting heat treatment in a nitrogen-containing atmosphere (with dew point of -40°C or lower) to create a protective nitrogen-enriched layer on the steel surface. This nitrogen-rich environment prevents oxidation during heating, eliminating the harmful oxide films that would otherwise form and interfere with brazing
2Reliability
If Ti or Nb is added to prevent sensitization, then corrosion resistance is improved, but oxide film formation is promoted leading to poor brazing properties
Solution Approach 1:
The patent applies parameter changes by precisely controlling the nitrogen content (0.005-0.030 mass%) and the combined Ti+Nb content (0.05-0.50 mass%), along with the carbon content (0.003-0.020 mass%). By adjusting these parameters and conducting heat treatment in a nitrogen-rich atmosphere, the nitrogen-enriched layer forms preferentially, preventing Ti and Nb from forming oxides while maintaining their corrosion-resistant carbide-forming capabilities
Solution Approach 2:
The patent uses an inert nitrogen atmosphere during heat treatment to prevent oxidation of Ti and Nb elements. The nitrogen-rich environment (dew point -40°C or lower) ensures that nitrogen diffuses into the steel surface, creating a protective layer that prevents these active elements from reacting with oxygen and forming harmful oxide films
3Ease of manufacture
If Si content is reduced to improve brazing properties, then oxide film formation is suppressed, but corrosion resistance deteriorates
Solution Approach 1:
The patent applies parameter changes by optimizing the Si content within a specific range (0.03-1.00 mass%) and compensating with controlled nitrogen content (0.005-0.030 mass%). The nitrogen-enriched layer formed through heat treatment provides oxidation protection, allowing Si to maintain its optimal balance between brazing properties and corrosion resistance without needing to be excessively reduced
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 nitrogen-enriched surface layer effectively prevents oxide film formation, ensuring good brazing properties and corrosion resistance, and enables a cost-effective, efficient production process for high-temperature brazing with Ni-containing brazing metals.
Implementation Method 1
a nitrogen-enriched surface layer, created through controlled atmosphere heat treatment before brazing, inhibits the formation of Ti and Al oxide films
Implementation Method 2
a nitrogen-enriched surface layer, created through controlled atmosphere heat treatment before brazing
Data Source
Figure 1
Figure 2A~2B
AI summary
Provided is a ferritic stainless steel that has excellent corrosion resistance and displays good brazing properties when brazing is carried out at high temperature using a Ni-containing brazing metal. These effects are obtained as a result of the steel having a chemical composition containing, in mass%: 0.003% to 0.020% of C; 0.05% to 1.00% of Si; 0.10% to 0.50% of Mn, 0.05% or less of P; 0.01% or less of S; 16.0% to 25.0% of Cr; 0.05% to 0.35% of Ti; 0.005% to 0.05% of Al; and 0.005% to 0.025% of N, the balance being Fe and incidental impurities, and as a result of a nitrogen-enriched layer being created that has a nitrogen concentration peak value of 0.05 mass% to 0.30 mass% at a depth of within 0.05 µm of a surface of the steel.