Nitrided Steel Member Strain Control via Composition Optimization
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Solution Overview
Problem
Steel members strengthened by gas nitrocarburizing treatments exhibit inferior pitting resistance and bending fatigue strength compared to those treated by carburizing or carbonitriding, with increased strain and strain variation due to hardening processes.
Innovation Solution
A nitrided steel member with an iron nitride compound layer formed on its surface, comprising specific elemental compositions and a predetermined structure, is treated in a controlled gas atmosphere to achieve high pitting resistance and bending fatigue strength with reduced strain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If gas nitrocarburizing treatment is applied to increase strength, then strain amount and strain variation are reduced, but pitting resistance and bending fatigue strength become inferior compared to carburizing or carbonitriding
Solution Approach 1:
The invention changes the chemical composition parameters of the steel member by precisely controlling the content of specific alloying elements (C, Si, Mn, Cr, Mo, V, Al, S) within defined ranges. This parameter optimization enables the steel to achieve high pitting resistance and bending fatigue strength through nitriding treatment while maintaining low strain characteristics, resolving the contradiction between strength and strain stability.
2Strength
If carbonitriding treatment is used to improve pitting resistance, then pitting resistance increases compared to carburizing, but bending fatigue strength decreases and strain amount increases due to heat treatment in austenite transformation temperature range
Solution Approach 1:
The invention optimizes the chemical composition parameters, particularly controlling carbon content at 0.10% or less and precisely adjusting alloying element contents. This parameter control enables the steel to achieve high pitting resistance through nitriding while avoiding the strain increase associated with carbonitriding heat treatment, as the optimized composition allows nitriding to be performed without austenite transformation.
3Strength
If carburizing or carbonitriding treatment is applied to increase strength, then pitting resistance and bending fatigue strength are improved, but strain amount and strain variation increase due to hardening process
Solution Approach 1:
The invention changes the approach by optimizing the base steel composition parameters (particularly C: 0.05 to 0.14%, Si: 0.10 to 0.30%, Mn: 0.40 to 1.40%, Cr: 0.90 to 1.90%) to enable nitriding treatment to achieve high strength without requiring a subsequent hardening process. This eliminates the strain variation caused by hardening while maintaining excellent pitting resistance and bending fatigue strength.
4Stability of the object's composition
If gas nitrocarburizing treatment is used to reduce strain, then strain amount and strain variation are low, but pitting resistance becomes inferior compared to carburized members
Solution Approach 1:
The invention optimizes the chemical composition parameters, particularly increasing Cr content to 0.90-1.90% and adjusting C, Si, and Mn contents within specific ranges. This composition optimization enables the formation of a high-quality nitride layer with excellent adhesion and high hardness during nitriding treatment, achieving pitting resistance superior to conventional gas nitrocarburized members while maintaining low strain characteristics.
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 nitrided steel member demonstrates pitting resistance and bending fatigue strength equivalent to or exceeding carburizing treatments, while maintaining low strain and strain variation, thus addressing the limitations of gas nitrocarburizing treatments.
Implementation Method 1
a nitrided steel member whose surface is nitrided by a nitriding treatment in a gas atmosphere
Data Source
AI summary
A nitrided steel member including an iron nitride compound layer formed on a surface of a steel member having predetermined components, wherein: in X-ray diffraction peak intensity IFe4N (111) of a (111) crystal plane of Fe4N and X-ray diffraction peak intensity IFe3N (111) of a (111) crystal plane of Fe3N, which are measured on a surface of the nitrided steel member by X-ray diffraction, an intensity ratio expressed by IFe4N (111)/{IFe4N (111)+IFe3N (111)} is 0.5 or more; Vickers hardness of the iron nitride compound layer is 900 or less, Vickers hardness of a base metal immediately under the iron nitride compound layer is 700 or more, and a difference between the Vickers hardness of the iron nitride compound layer and the Vickers hardness of the base metal is 150 or less; and a thickness of the iron nitride compound layer is 2 to 17 μm.