Cold-formed springs are also called precision springs and are one of the mechanical elements that require high spring accuracy shape and spring constant. In order to improve the accuracy of these springs, it is indispensable to improve the accuracy of each factor related to the spring constant. In this paper, we will discuss the variations in elastic modulus and coil diameter after low-temperature annealing, which are considered to play an important role. Especially, we reviewed a difference in elastic modulus depending on the measurement method, and the variations in coil diameter after low-temperature annealing, and then studied the cause of these issues suggesting the problems and remaining issues.

Since the metal texture of springs is related to fatigue, creep, and especially strength, we investigated the microstructure in order to clarify the changes of the texture during the manufacturing process of the springs, and to contribute to further strengthening of springs. In this study, the crystal structure of the austenitized and oil-tempered wires was observed by electron backscattering diffraction (EBSD) and confirmed that axial direction of the specimen was 〈110〉. We simulated the texture of a wire produced by drawing in the spring manufacturing process according to the predictive theory of crystal rotation presented by Katoh, which is based on the theorem of minimizing resultant of shear strain and maximizing plastic work and the result shows that the crystal orientation of axial direction rotates toward 〈110〉. Comparing the simulation results with the experimental results, it was confirmed that the metal structure of spring formed 〈110〉//RD texture, which is produced during drawing in the manufacturing process. Furthermore, austenitizing and oil-tempering have almost no effect on the texture.

The risk of delayed fracture tends to increase by increasing hardness of coil spring, and to establishing suitable and safe evaluation method test method is essential. Once the evaluation method is standardized, automotive maker and spring maker can make fair judgement based on common criteria. It is expected that spring maker can reduce evaluation cost. Previous report was to establish hydrogen analysis method to separate hydrogen occurred in the rust remained on material surface. And was to estimate maximum hydrogen content entered from automotive environment. What are the dominant factor of spring to promote hydrogen penetration caused by grinding was also discussed. This study reports experiment results to confirm an evaluation method of test piece to monitor delayed fracture by holding coil spring simulating fracture pattern in the market. This paper proposes an evaluation method using test piece to reproduce fracture pattern in the market.

The guideline as kinetic equation of decarburizing to prevent excess decrease in fatigue strength, which is expressed as temperature the quenching starts, has been proposed from the systematic investigation on decarburizing spring steels;

T_Q = 910 + 44.7^%Si - 30^%Mn - 11^%Cr - 15.2^%Ni + 31.5^%Mo - 203√^%C - (^%C - C_S) {1 - exp (-(3.29 × 10⁸t) / 2d exp (-(3.61 × 10⁴) / (T_h' + 273)))}

where ^%M[mass%] indicates content of element M in steel, d[m] initial particle size of austenite. Referring the guideline, quenching from above the evaluated temperature, T_Q can prevent excess decrease in fatigue strength the spring steel exposed to high temperature decarburizing atmosphere, whose temperature is T_h'[°C] and decarburizing time is t[s], because the quenching leads to form the quench microstructure without surface ferrite layer. In the case that the exposed atmosphere is excess CO₂ content, as same as the atmosphere of gas combustion furnace, the value of surface carbon content, C_S[mass%] can be employed to be 0.2. Moreover, if the cooling rate is grater than 80 °C/s corresponding to the cooling rate of oil quenching, the guideline can be used to every spring steels. Similarly if temperature rise is slow, the guideline can be acceptable with modification of the term of decarburizing time to t + 0.03t_r, where t_r[s] is the time period of temperature rise.

Though there are many activities to define delayed fracture method by professors, professional engineer and any specialists, there is still no standardized delayed fracture test method, and Automotive OEMs have their own ideas and test methods for reasons of the above. Considering background of the recent high hardness spring design, we need to prepare standardized test method to prevent delayed fracture risk in the future. Therefore, the research committee have been in action to standardize delayed fracture test method simulated automotive environment. The issues to be solved are as bellows, First issue is to establish hydrogen analysis method to separate hydrogen occurred in the rust remained on material surface.

Second issue is to estimate maximum hydrogen content entered from automotive environment.

Third issue is to confirm an evaluation method to occur delayed fracture by grasping the coil spring simulated fracture pattern in the market.

4th issue is to establish an evaluation method used test piece to be reproduced fracture pattern in the market.

This study reports experiment results for the first and second issues as in September 2019.