Leaf Spring Localized Hardness for Hydrogen Cracking
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Solution Overview
Problem
Higher hardness leaf springs for vehicle suspensions are prone to early failures due to hydrogen environment-assisted cracking (HEAC) in high-strength steels, particularly at sections experiencing static assembly stress, which is exacerbated by the inverse relationship between steel strength and threshold stress for cracking.
Innovation Solution
Implementing a secondary tempering method to achieve higher hardness in the arms and parabolic sections of leaf springs while maintaining lower hardness in sections that experience static assembly stress, such as the eyes and seat, by applying localized heat and rapid cooling to selectively temper these areas, thereby reducing the incidence of hydrogen cracking and improving fatigue life.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If the leaf spring is through hardened to high hardness (461-514 BHN) to improve fatigue life and reduce weight, then the fatigue life and weight performance improve, but the incidence of hydrogen environment assisted cracking increases in sections experiencing static assembly stress
Solution Approach 1:
The patent applies different hardness levels to different sections of the leaf spring. Specifically, the eyes and seat sections are through hardened to a lower hardness (375-461 BHN) to resist hydrogen cracking, while the arms and parabolic sections are through hardened to a higher hardness (461-514 BHN) to improve fatigue life and reduce weight. This local differentiation of material properties resolves the contradiction between overall high hardness benefits and localized vulnerability to hydrogen cracking.
2Weight of moving object
If the steel strength is increased to reduce the number and thickness of leaf springs, then vehicle weight and unsprung suspension system weight are reduced, but the threshold stress required to trigger hydrogen assisted cracking decreases
Solution Approach 1:
The patent implements local quality by varying the hardness and strength characteristics across different sections of the leaf spring. The high-strength, high-hardness material is applied only where needed for weight reduction and fatigue resistance (arms and parabolic sections), while lower hardness material is used in sections subject to static assembly stress (eyes and seat). This resolves the contradiction by allowing high overall strength while protecting vulnerable locations from hydrogen cracking.
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 effectively reduces hydrogen cracking and enhances the fatigue life of leaf springs by maintaining high strength and hardness in critical areas while minimizing stress concentrations, leading to improved performance and reliability of suspension systems.
Implementation Method 1
Localized heat is applied to a section of the leaf spring, bringing the heated areas within the section to a temperature that is above the temperature at which the leaf spring undergoes primary tempering and below austenitic transformation temperature
Implementation Method 2
The leaf spring is then rapidly cooled from a temperature that is at least 50° F. and preferably at least 75° F. to 100° F. above the temperature at which tempered martensite embrittlement can occur down to a temperature that is less than about 150° F., by quenching the leaf spring with an aqueous solution
Data Source
AI summary
Leaf springs, and methods of manufacturing thereof, having first and second sections, spaced apart along the length of said leaf spring, said sections are through hardened and tempered to achieve, respectively different levels of finished through hardness, are disclosed.