Coil Spring Fatigue Resistance via Carburized Layer
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Solution Overview
Problem
Coil springs used in automotive applications require enhanced fatigue resistance to withstand increased stress and weight reduction demands, with existing solutions falling short in achieving the desired level of durability.
Innovation Solution
A coil spring made of steel with a specific chemical composition (C: 0.40 to 0.70%, Si: 1.50 to 3.50%, Mn: 0.30 to 1.50%, Cr: 0.10 to 1.50%, V: 0.50 to 1.00%, and Al: 0.01% or less, with a carburized layer depth of 0.30 to 1.00 mm and Vickers hardness of 600 to 750 at 1/4 × diameter depth, manufactured through vacuum carburization at 1,000°C to 1,100°C followed by gas cooling or oil quenching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the coil spring is designed to have a carburized layer with predetermined composition and hardness (650 to 1,000 HV at 0.02 mm depth) to improve fatigue resistance, then the fatigue resistance is improved to a level of fifty million times, but the spring becomes heavier and larger, preventing further weight reduction
Solution Approach 1:
The patent changes the chemical composition parameters of the steel wire (specific C: 0.45-0.65%, Si: 1.50-3.00%, Mn: 0.30-1.50%, Cr: 0.10-1.50%, V: 0.05-1.00%, Al: 0.003-0.020%) and the carburized layer parameters (depth: 0.05-1.00 mm, hardness: 650-1,000 HV) to achieve the optimal balance between fatigue resistance and weight. By precisely controlling these parameters, the patent achieves fatigue resistance of over fifty million times while preventing spring sagging, thus avoiding the need for excessive material that would increase weight.
Solution Approach 2:
The patent creates a composite structure with a carburized layer on the surface of the steel wire. This composite material structure combines the high hardness and wear resistance of the carburized layer with the toughness of the underlying steel wire, achieving superior fatigue resistance without requiring uniform thickening of the entire spring structure, thereby maintaining weight efficiency.
2Weight of moving object
If the coil spring is modified to improve fatigue resistance and made more compact, then weight reduction is achieved, but the fatigue resistance may be compromised without proper control of metallographic structure and carburized layer
Solution Approach 1:
The patent precisely controls the chemical composition parameters (C, Si, Mn, Cr, V, Al) and the metallographic structure parameters (prior austenite grain size number: 10.0-14.0, carburized layer depth: 0.05-1.00 mm) to achieve the optimal balance between compactness/weight and fatigue resistance. By controlling these parameters within specific ranges, the patent ensures that the spring achieves sufficient fatigue resistance even in a compact design.
Solution Approach 2:
The patent applies vacuum carburization treatment before spring setting, and shot peening treatment before or after setting, to preliminarily enhance the surface hardness and introduce compressive residual stresses. This preliminary action ensures that the spring has sufficient fatigue resistance built-in before final assembly, allowing for optimized compact design without compromising durability.
3Strength
If conventional wire drawing and quenching tempering processes are used to produce high strength wire, then the desired strength is achieved, but the fatigue resistance is insufficient for modern automotive applications requiring over sixty million fracture-lifetime tests
Solution Approach 1:
The patent creates a composite structure with a carburized layer on the surface of the steel wire. This composite material structure combines the high hardness and wear resistance of the carburized layer with the toughness of the underlying steel wire, achieving superior fatigue resistance that exceeds conventional single-structure wires, enabling over sixty million fracture-lifetime tests.
Solution Approach 2:
The patent changes the chemical composition parameters (adding V: 0.05-1.00%, optimizing Si: 1.50-3.00%, Cr: 0.10-1.50%, Mn: 0.30-1.50%) and the metallographic structure parameters (prior austenite grain size number: 10.0-14.0) to achieve superior fatigue resistance. These parameter changes enable the wire to withstand over sixty million fracture-lifetime tests while maintaining the desired strength level.
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 provides a coil spring with significantly improved fatigue resistance, exceeding sixty million fracture-lifetime tests, by balancing strength and toughness through controlled chemical composition and metallographic structure, effectively addressing the limitations of previous technologies.
Implementation Method 1
a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface
Implementation Method 2
vacuum carburization at 1,000°C to 1,100°C followed by gas cooling or oil quenching
Implementation Method 3
gas cooling or oil quenching
Implementation Method 4
followed by gas cooling or oil quenching
Data Source
Figure 1~2
AI summary
To provide a coil spring having excellent fatigue resistance. Disclosed is a coil spring made of steel, including (in % by mass, the same shall apply for a chemical composition): C: 0.40 to 0.70%; Si: 1.50 to 3.50%; Mn: 0.30 to 1.50%; Cr: 0.10 to 1.50%; V: 0.50 to 1.00%, and Al: 0.01% or less (excluding 0%), with the balance being iron and inevitable impurities, wherein an average crystal grain size number of prior austenite crystals in a depth of 0.3 mm from a surface is 11.0 or more, while a difference in grain size number between the respective prior austenite crystals is in a range of less than 3 from a grain size number observed at the maximum frequency, and wherein a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface, while an average Vickers hardness is 600 or higher at a position in a depth of (1/4) × diameter in the depth direction from the surface.