Bearing Wire Rod Microstructure for Shorter Softening Heat Treatment
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Solution Overview
Problem
The high cost and long duration of softening heat treatment processes for bearing wire rods, which increase manufacturing costs and reduce ductility and toughness due to proeutectoid cementite precipitation during cooling, necessitate a method to shorten or omit this treatment.
Innovation Solution
A bearing wire rod composition of 0.8-1.2% C, 0.01-0.6% Si, 0.1-0.6% Mn, 1.0-2.0% Cr, 0.01-0.06% Al, and the balance Fe, with specific grain boundary characteristics and a manufacturing method involving heating, finish hot rolling, and controlled cooling to achieve desired microstructure and mechanical properties without extensive softening heat treatment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If softening heat treatment is performed at high temperature for long time to spheroidize cementite and improve cold workability, then cold forgeability is improved, but manufacturing cost increases and production time increases
Solution Approach 1:
The invention performs spheroidizing heat treatment during the initial wire rod manufacturing process, before subsequent drawing and processing operations. This preliminary spheroidization creates a microstructure that maintains good cold workability throughout subsequent processing without requiring additional softening heat treatments, thereby reducing total heat treatment time and manufacturing cost
Solution Approach 2:
The invention optimizes the spheroidizing heat treatment parameters (temperature range of 700-800°C, holding time of 30 hours or more) to achieve complete cementite spheroidization and uniform grain distribution. By precisely controlling these parameters, the treatment achieves maximum softening effect with minimal time, improving cold workability while reducing production time
2Productivity
If drawing process is introduced first to accelerate spheroidization, then spheroidization is accelerated, but breakage occurs due to internal defects in high carbon wire rod
Solution Approach 1:
The invention reverses the conventional sequence by performing spheroidizing heat treatment on the as-rolled wire rod before any drawing operations. This preliminary softening creates a more ductile microstructure with spheroidized cementite and uniform grain distribution, which can then withstand subsequent drawing operations without internal defect formation or breakage
Solution Approach 2:
The invention inverts the conventional process sequence: instead of drawing first to accelerate spheroidization (which causes breakage in high carbon wire rod), it performs spheroidizing heat treatment first to create a soft, ductile microstructure that can then be drawn without damage. This inversion eliminates the breakage problem while still achieving accelerated spheroidization
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 method reduces manufacturing costs by shortening or omitting softening heat treatment while maintaining high tensile strength and ductility, achieving a tensile strength of 1,200 MPa or more and a reduction in area of 20% or more, with a spheroidized microstructure and reduced cementite aspect ratio.
Implementation Method 1
heating a billet including, in percent by weight (wt %), 0.8 to 1.2% of C, 0.01 to 0.6% of Si, 0.1 to 0.6% of Mn, 1.0 to 2.0% of Cr, 0.01 to 0.06% of Al, 0.02% or less (exclusive of 0) of N, and the balance of Fe and inevitable impurities, in a temperature range of 950 to 1,050° C.
Implementation Method 2
cooling the wire rod to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, and cooling the wire rod at a rate of 1° C./sec or less
Implementation Method 3
proeutectoid cementite precipitated along grain boundaries of prior austenite during cooling
Implementation Method 4
A bearing wire rod includes, in percent by weight (wt %), 0.8 to 1.2% of C, 0.01 to 0.6% of Si, 0.1 to 0.6% of Mn, 1.0 to 2.0% of Cr, 0.01 to 0.06% of Al
Data Source
AI summary
Disclosed is a bearing wire rod includes, in percent by weight (wt %), 0.8 to 1.2% of C, 0.01 to 0.6% of Si, 0.1 to 0.6% of Mn, 1.0 to 2.0% of Cr, 0.01 to 0.06% of Al, 0.02% or less (exclusive of 0) of N, and the balance of Fe and inevitable impurities, wherein a prior austenite grain size of a microstructure is from 3 to 10 μm, and a sum of lengths of high angle grain boundaries having a misorientation angle of 15° or more per unit area is from 1,000 to 4,000 mm/mm2.