Strength Member Microstructure for Settling Resistance
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Solution Overview
Problem
Existing strength members, such as valve springs and bolts, require improved settling resistance and yield strength while maintaining cost advantages and minimal process changes, as current techniques either compromise on fatigue resistance or increase manufacturing costs.
Innovation Solution
A strength member comprising 0.5 to 0.7% C, 1.0 to 2.0% Si, 0.1 to 1.0% Mn, 0.1 to 1.0% Cr, with a bainite area ratio of 65% or more, and an average dislocation density of 2.0 × 10^16 m^-2 or less, achieved through austempering and tempering processes, including a cooling rate of 20°C/second and tempering at 350 to 450°C, which reduces dislocation density and enhances yield strength without compromising fatigue resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If tempered martensite structure is used for strength members, then fatigue resistance is improved, but settling resistance and yield strength are insufficient
Solution Approach 1:
The patent changes the microstructural parameters by controlling the transformation process to produce fine bainite instead of tempered martensite. Specifically, the steel is austempered at temperatures between 250-450°C to transform austenite into fine bainite with a specific morphology (acicular or lath structure), achieving superior yield strength and settling resistance while maintaining fatigue resistance through the fine microstructure
Solution Approach 2:
The patent creates a composite microstructure consisting of fine bainite as the primary phase with controlled distribution of carbides and retained austenite. This composite structure at the micro-scale provides both high strength/yield properties and fatigue resistance, resolving the contradiction between strength and reliability
2Strength
If large plastic strain is applied to improve settling resistance, then dislocation density increases, but fatigue resistance decreases
Solution Approach 1:
The patent applies preliminary plastic strain during the coiling process before austempering, which introduces dislocations that serve as nucleation sites for fine bainite transformation. The subsequent austempering process then transforms these strained regions into fine bainite, converting the harmful high dislocation density into a beneficial fine microstructure that provides both settling resistance and fatigue resistance
Solution Approach 2:
The patent converts the harmful effect of high dislocation density (which normally reduces fatigue life) into a benefit by using the dislocations as nucleation sites for fine bainite formation. The resulting fine bainite structure has controlled dislocation distribution that provides both the desired settling resistance from the plastic strain and maintains fatigue resistance through the fine microstructural scale
3Strength
If fine bainite is formed to improve yield strength, then manufacturing cost increases, but this invention maintains cost advantages
Solution Approach 1:
The patent optimizes the chemical composition parameters (C: 0.35-0.50%, Si: 1.50-3.00%, Mn: 0.50-1.50%, Cr: 0.50-2.00%, B: 0.0005-0.0050%) to achieve the desired fine bainite microstructure through conventional austempering processes, avoiding the need for expensive alloying or advanced manufacturing techniques. The controlled composition enables transformation at moderate temperatures and rates, maintaining cost-effectiveness
Solution Approach 2:
The patent makes the austempering process universal by using commercially available Si-Cr-B steel compositions and standard heat treatment equipment. The process can be applied to various wire diameters and product geometries without requiring specialized facilities, making it economically viable for mass production while achieving high yield strength through fine bainite formation
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 significantly improves settling resistance and yield strength without increasing costs or adding substantial process changes, making it suitable for applications like screws and tie rods, while maintaining fatigue resistance and reducing residual shearing strain.
Implementation Method 1
austenizes at a temperature from an Ac3 point to (Ac3 point + 250 degrees C)
Implementation Method 2
maintains a temperature from (Ms point - 20 degrees C) to (Ms point + 60 degrees C) for 400 seconds or more
Implementation Method 3
the martensite generated by water-cooling in the austempering treatment is decomposed into ferrite and cementite by tempering, the dislocation is decreased
Implementation Method 4
it is desirable that the manufacturing method include a shot peening process in which shot is impinged on a product
Data Source
Figure 1A~2
Figure 3~4
AI summary
A strength member and a manufacturing method thereof in which settling resistance and yield strength can be substantially improved without reducing cost advantages or adding substantial process changes, is provided. A strength member comprises, by mass %, 0.5 to 0.7 % of C, 1.0 to 2.0 % of Si, 0.1 to 1.0 % of Mn, 0.1 to 1.0 % of Cr, 0.035 % or less of P, 0.035 % or less of S, and the balance of Fe and inevitable impurities, wherein an area ratio of bainite is 65 % or more, and an average dislocation density of a freely selected cross section is 2.0 × 1016 m-2 or less.