Nitrided Plate Part Fatigue Strength via Pre-Straining
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Solution Overview
Problem
Nitrided plate parts, particularly those used in torque converters, face challenges in achieving fatigue strength equivalent to carburized members due to stress concentration and crack occurrence patterns, which existing techniques fail to adequately address.
Innovation Solution
Controlled nitrogen concentration and shearing strain history, combined with specific chemical compositions and production conditions, are used to enhance fatigue strength by optimizing the nitrogen content and metal structure in the nitrided plate parts, particularly in the sheared end face and central regions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If gas softnitriding treatment is performed to improve wear resistance and reduce heat treatment distortion, then surface hardening depth and dimension precision are improved, but fatigue strength is insufficient due to lack of martensitic transformation and compression residual stress
Solution Approach 1:
The invention applies preliminary plastic deformation (pre-straining) to the steel sheet before nitriding treatment. This pre-straining introduces dislocations and compression residual stress in advance, creating a metal structure that will generate sufficient compression residual stress after nitriding without requiring martensitic transformation, thereby solving the fatigue strength problem while maintaining the dimensional precision benefits of gas softnitriding
Solution Approach 2:
The invention changes the metal structure parameters by controlling the ferrite fraction (50-80%) and introducing dislocations through pre-straining. By adjusting these parameters, the material achieves adequate compression residual stress after nitriding treatment, resolving the contradiction between maintaining dimensional precision and improving fatigue strength
2Reliability
If the corner between plate and pawl is made gentle and thickened to reduce stress concentration, then durability is improved, but spatial constraints and power transmission efficiency are compromised
Solution Approach 1:
The invention extracts the stress concentration problem from the geometric design and transfers it to the material structure level. By introducing dislocations and compression residual stress through pre-straining, the material itself resists stress concentration at sharp corners, eliminating the need for geometric modifications that would compromise spatial constraints and power transmission efficiency
Solution Approach 2:
The invention replaces the mechanical solution (geometric modification) with a material science solution (dislocation control and residual stress engineering). The pre-strained metal structure provides inherent resistance to stress concentration, allowing maintenance of sharp corner geometry for efficient power transmission while achieving improved durability
3Productivity
If shearing process is used to obtain plate part shape, then manufacturing efficiency is improved, but fatigue strength is reduced due to high roughness and microscopic stress concentration at end face
Solution Approach 1:
The invention converts the harmful effect of shearing (microscopic stress concentration) into a beneficial one by pre-straining the material before shearing. The dislocations introduced during pre-straining alter the stress distribution pattern, and when combined with subsequent nitriding, transform the stress concentration sites into regions of adequate compression residual stress that actually improve fatigue strength
Solution Approach 2:
The invention changes the metal structure parameters (dislocation density, ferrite fraction) through pre-straining before shearing. This parameter change ensures that even though shearing creates surface roughness, the underlying metal structure has adequate compression residual stress that compensates for the surface defects, maintaining both manufacturing efficiency and fatigue strength
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 approach results in nitrided plate parts with fatigue strength comparable to or exceeding that of carburized members, improving durability and resistance to stress concentrations.
Implementation Method 1
gas softnitriding treatment performed in a bath or an atmosphere including carbon together with nitrogen
Implementation Method 2
nitrogen average content in a range in which a distance from the sheared end face in a sheared end face normal direction is equal to or greater than 0.05 mm and equal to or less than 0.10 mm is equal to or greater than 0.4000% and equal to or less than 1.2000% in mass %
Implementation Method 3
martensitic transformation as in carburizing treatment and induction hardening treatment does not occur
Implementation Method 4
a slab is subjected to hot rolling at hot finish rolling exit-side temperature in a range of equal to or greater than 850° C. and less than 960° C.
Data Source
AI summary
[Object] To provide a nitrided plate part that exhibits fatigue strength equivalent to or better than that of a carburized member, and a method for producing the same.[Solution] Provided is a nitrided plate part having predetermined components and structure. Nitrogen average content in a range in which a distance from a sheared end face of the part toward an interior of the nitrided plate part in a sheared end face normal direction is equal to or greater than 0.05 mm and equal to or less than 0.10 mm is equal to or greater than 0.4000% and equal to or less than 1.2000% in mass %, and minimum nitrogen content in a range in which the distance is equal to or greater than 0.015 mm and equal to or less than 0.200 mm is 0.0600% or more. After uncoiling a steel sheet coil, stretch and compressive deformation in a range of equal to or greater than 0.03% and equal to or less than 3.00% in amount of plastic strain are alternately applied to a surface layer of a steel sheet. Then, shearing and press-forming are performed to make the steel sheet into a plate part shape, without recoiling the steel sheet again. Then, nitriding is performed under predetermined conditions.