Valve Spring Heat Treatment for Deep Compressive Stress Layers
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Solution Overview
Problem
Current methods for producing valve springs with high tensile strength face challenges such as increased notch sensitivity, brittle fracture, and reduced fatigue strength due to residual stresses, which are difficult to mitigate with existing surface treatments like shot peening, and often require expensive alloying elements and complex heat treatments.
Innovation Solution
A production method involving high-temperature austenitizing and rapid cooling to reduce residual stresses, followed by tempering and shot peening to create a balanced tempered martensitic structure with a thick compressive residual stress layer, improving fatigue resistance without the need for expensive materials or complicated heat treatments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of spring wire is increased to reduce valve spring weight, then the fatigue resistance and sag resistance improve, but the notch sensitivity to cracks and defects increases greatly, making the spring wire more likely to break during cold spring forming and for brittle fracture to occur while in use
Solution Approach 1:
The patent applies induction heating to change the thermal parameters of the spring wire, heating it to austenite transformation completion temperature and holding it for a specific time. This thermal parameter change allows the high-strength wire to be tempered, reducing brittleness and notch sensitivity while maintaining high tensile strength, thus resolving the contradiction between strength and fatigue resistance
Solution Approach 2:
The patent utilizes the phase transition of steel from martensite to austenite through induction heating. By heating the coiled spring wire to the austenite transformation completion temperature and holding it, the martensitic structure transforms to austenite, which then transforms to tempered martensite during cooling. This phase transition reduces the brittleness and notch sensitivity of the high-strength wire while preserving its high tensile strength
2Reliability
If shot peening is used to provide compressive residual stress on the surface layer to improve fatigue resistance, then breakage originating from the surface may be prevented, but the yield strength increases and the amount of plastic strain that can be provided decreases, making it difficult to form a thick compressive residual stress layer
Solution Approach 1:
The patent applies induction heating and tempering to the coiled spring wire before shot peening. This preliminary thermal treatment reduces the yield strength and hardness of the wire, creating a more ductile state that allows shot peening to penetrate deeper and form a thicker compressive residual stress layer, thereby improving fatigue resistance without being limited by high yield strength
3Reliability
If high compressive residual stress is provided on the surface layer from surface to deep inside to improve fatigue resistance, then the fatigue strength increases, but the production process becomes more complex and material costs increase
Solution Approach 1:
The patent merges the heat treatment process with the spring coiling process by applying induction heating directly to the coiled spring wire. This combination allows the tempering treatment to be performed in-situ, eliminating the need for separate heat treatment equipment and processes, thereby reducing production complexity while achieving deep compressive residual stress for improved fatigue resistance
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 results in a spring with superior fatigue resistance and sag resistance, achieved using an inexpensive oil tempered wire, with a simplified process that reduces material costs and production complexity, while maintaining high strength and ductility.
Implementation Method 1
a method for induction heating a coiled member
Implementation Method 2
heating it up to a temperature to complete transformation into austenite
Implementation Method 3
cooling it down to room temperature at a cooling rate of not less than 20°C/second
Implementation Method 4
tempering step for heating the spring and cooling the same to room temperature
Implementation Method 5
a first shot peening step for projecting shots to the spring
Data Source
Figure 1
AI summary
A spring with superior fatigue resistance and a production method therefor are provided by decreasing the material cost and simplifying the production process. There is provided with a spring comprising: a composition consisting of, 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, not more than 0.035 % of P, not more than 0.035 % of S, and the balance of Fe and inevitable impurities; a structure including not less than 95 % of tempered martensitic structure by area ratio in a cross section of a wire material; a compressive residual stress layer formed from a surface to a depth of 0.35 mm to D/4, in which D (mm) is a circle-equivalent diameter of a cross section, the compressive residual stress layer having maximum compressive residual stress of 800 to 2000 MPa; a center portion with Vickers hardness of 550 to 700 HV in the cross section; and a high hardness layer with greater hardness than the center portion by 50 to 500 HV from a surface to a depth of 0.05 to 0.3 mm.