Nitrided Steel Microstructure for Fatigue Strength and Machinability
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Solution Overview
Problem
Existing roughly-shaped steel materials for nitrided parts face challenges in achieving both high fatigue strength and straightening property, particularly in portions with diameters or widths ranging from 60 to 130 mm, while also requiring improved machinability, especially deep-hole drilling capabilities.
Innovation Solution
A roughly-shaped steel material with a specific chemical composition and microstructure is developed, comprising tempered martensite and tempered bainite near the surface and ferrite and perlite internally, optimized for nitriding to achieve excellent fatigue strength, straightening property, and machinability through controlled quenching and tempering processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quenching and tempering process is performed before nitriding to improve straightening property and fatigue strength, then fatigue strength and straightening property are improved, but machinability of roughly-shaped steel materials deteriorates
Solution Approach 1:
The patent applies local quality by creating different microstructure types in different regions of the steel material. The surface region (depth 0-10mm) contains tempered martensite and tempered bainite for high strength, while the internal region (depth 10mm or more) contains ferrite and perlite for good machinability. This spatial differentiation of microstructure properties resolves the contradiction between surface strength and overall machinability.
Solution Approach 2:
The patent segments the steel material into two distinct microstructural zones based on depth from the surface. The first zone (0-10mm) is optimized for fatigue strength with tempered martensite and bainite, while the second zone (10mm or more) is optimized for machinability with ferrite and perlite. This segmentation allows each region to independently satisfy its specific functional requirements.
2Device complexity
If surface microstructure is made uniform with internal microstructure to simplify production, then manufacturing complexity is reduced, but deep-hole machinability deteriorates
Solution Approach 1:
The patent deliberately creates non-uniform microstructure where the internal region (depth 10mm or more) has ferrite and perlite for machinability, while the surface region has tempered martensite and bainite for strength. This local differentiation specifically addresses deep-hole machinability requirements without compromising surface fatigue strength.
3Strength
If higher carbon content is added to increase fatigue strength, then fatigue strength is improved, but straightening property deteriorates
Solution Approach 1:
The patent concentrates higher carbon content (0.35-0.50%) in the surface region to form tempered martensite and bainite for fatigue strength, while the internal region maintains lower effective carbon content to form ferrite and perlite for straightening property. This spatial distribution of carbon content resolves the contradiction between strength and ductility.
Solution Approach 2:
The patent creates a composite microstructure consisting of two distinct microstructure types: tempered martensite and bainite in the surface region, and ferrite and perlite in the internal region. This composite approach combines the high strength of martensite with the good ductility and machinability of ferrite-perlite, achieving both fatigue strength and straightening property simultaneously.
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 steel material provides nitrided parts with enhanced fatigue strength, straightening property, and deep-hole machinability, effectively addressing the trade-off between these properties and improving manufacturing efficiency.
Implementation Method 1
Machine parts used in vehicles, ships, industrial machines, and the like may be nitrided to improve the fatigue strength
Implementation Method 2
A pre-heating process such as quenching and tempering or normalizing of steel before nitriding process improves the straightening property and the fatigue strength after nitriding process
Data Source
AI summary
Provided are a roughly-shaped steel material for a nitrided part, and a nitrided part obtained by nitriding the roughly-shaped steel material for a nitrided part, having a determined chemical composition, in which the portion with a diameter or width ranging from 60 to 130 mm of the roughly-shaped steel material for a nitrided part has a microstructure at a depth of 14.5 mm from a surface including, in terms of area fraction: tempered martensite and tempered bainite in total: from 70 to 100%; remaining austenite: from 0 to 5%; and a balance: ferrite and perlite; and has a microstructure at a depth of 15 mm or more from the surface including, in terms of area fraction: tempered martensite and tempered bainite in total: from 0 to less than 50%; remaining austenite: from 0 to 5%; and a balance: ferrite and perlite.