Martensitic Stainless Steel Heat Treatment Without Surface Ferrite
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Solution Overview
Problem
Highly corrosion-resistant martensitic stainless steel components face challenges in achieving both high corrosion resistance and hardness due to ferrite formation at the surface layer, which compromises their performance in severe corrosive environments.
Innovation Solution
A highly corrosion-resistant martensitic stainless steel component is developed with a two-phase mixed structure of retained austenite and martensite at the surface layer, achieved by heating the steel to 1050-1120°C under a nitrogen atmosphere with a partial pressure of 1000-10000 Pa, preventing ferrite formation and ensuring a hardness of HRC 57 or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If nitrogen is added to suppress ferrite formation and improve corrosion resistance, then corrosion resistance is improved, but nitrogen escapes during vacuum hardening causing ferrite formation and reduced hardness
Solution Approach 1:
The invention changes the nitrogen partial pressure parameter during hardening from conventional vacuum conditions to a controlled nitrogen atmosphere with partial pressure of 1000-10000 Pa. This parameter change prevents nitrogen escape, suppresses ferrite formation, and maintains both high corrosion resistance and hardness of HRC 57 or more
Solution Approach 2:
The invention applies periodic nitrogen supply during the hardening process to maintain stable nitrogen partial pressure. This periodic nitrogen replenishment prevents nitrogen concentration decrease at the surface layer, thereby preventing ferrite formation while maintaining the desired hardness and corrosion resistance
2Strength
If conventional vacuum hardening is used, then hardness is achieved, but nitrogen escapes from the surface layer causing ferrite formation and reduced corrosion resistance
Solution Approach 1:
The invention replaces conventional vacuum atmosphere with a nitrogen-rich atmosphere having controlled nitrogen partial pressure (1000-10000 Pa). This inert nitrogen environment prevents nitrogen escape from the steel, suppresses ferrite formation, and maintains high corrosion resistance while achieving the required hardness
3Stability of the object's composition
If ferrite is formed at the surface layer, then nitrogen escape is accommodated, but carbon concentration increases around ferrite forming chromium carbide and creating chromium-deficient layers
Solution Approach 1:
The invention applies preliminary anti-action by maintaining high nitrogen partial pressure during hardening to prevent nitrogen escape before ferrite formation can occur. This preventive measure stops the chain reaction that would lead to carbon concentration, chromium carbide formation, and chromium-deficient layers, thereby preserving corrosion 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 provides a stainless steel component with enhanced corrosion resistance and high hardness without ferrite at the surface layer, meeting the requirements for rolling bearings and other mechanical components in severe corrosive environments.
Implementation Method 1
heating the steel to a temperature in the range from 1050 to 1120° C. under a nitrogen atmosphere having a nitrogen partial pressure of 1000 Pa or more and less than 10000 Pa and hardened
Implementation Method 2
under a nitrogen atmosphere having a nitrogen partial pressure of 1000 Pa or more and less than 10000 Pa
Implementation Method 3
the nitrogen as a solid solution escaping from the surface layer portion is suppressed, and no ferrite structure is formed at the surface layer portion
Implementation Method 4
a matrix structure of a surface layer portion of an entire outer surface is a two-phase mixed structure containing retained austenite and martensite
Data Source
AI summary
To provide a highly corrosion-resistant stainless steel component made of martensitic stainless steel achieving both high corrosion resistance and high hardness without containing ferrite at a surface layer portion. The highly corrosion-resistant stainless steel component is made of martensitic stainless steel containing, by weight, from 0.35 to 0.43% of C, 0.5% or less of Si, 0.5% or less of Mn, 0.04% or less of P, 0.04% or less of S, from 15 to 17% of Cr, from 0.1 to 0.3% of W, from 1.5 to 3.0% of Mo, from 0.001 to 0.005% of B, and from 0.12 to 0.18% of N, with the balance being Fe and an inevitable impurity. The matrix structure of the surface layer portion of the entire outer surface is a two-phase mixed structure containing retained austenite and martensite, and the surface hardness is HRC 57 or more.