Shot peening is a typical but very important process for the improvement of product operating life in an automotive industry. Especially, among such products, coil springs must have two or more shot peening processes for extremely high fatigue life expectation. As a tendency of lightweight design, the coil spring, being a relatively light component in a car, also becomes lighter and therefore, the higher design stress would threaten its fatigue life reduction and even lead to failure rather quickly.
The conventional coil spring used for automotive suspension is 30~40 cm as height and 10~15cm as outer-diameter, with the wire diameter being 9~17mm. If the coil is developed, the length of the used wire is almost 2 m. The coil spring individually passes through the shot peening tunnel, and this intensive shot peening care is commonly checked by the Almen Strip, where its holder is attached to the centre of the spring. (Fig. 1)
From the manufacturing facility control manager’s viewpoint, the Almen strip is a good measuring tool. But for the designer, test engineer, quality engineer and even the customer who all wish to know the peening effect on the coil spring, the Almen strip may reveal its limitations. A number of the tiny shot balls, under 1mm as diameter, will be accelerated by impeller wheels and finally impinge on the surface of the coil. The wheel type impeller has its arbitrary shot direction and is usually perpendicular to the axis of the spring. The upper and the lower part of the coil spring will be weakened owing to its direction, and these weaknesses can be checked when more Almen strip holders are welded and attached there, but it is a problem that the three holders are heavy compared to the coil spring and tend to hinder the shot balls. Furthermore, the real motion of the coil spring under shot peening will become different because of the increased weight; more technically, the shape of the coil spring will be changed after shot peening and might affect the final shape of the coil spring. This means the specific load and the spring stiffness can be different from the desired specification.
Suggestion of the Peening Ring and its Results
To measure the local peening intensities on the coil spring, a new method must satisfy some requirements such as having light weight and small size compared to the spring and being easier-to-use relative to the conventional Almen Strip and its holders. The inner diameter of the ring is a little bigger than the wire diameter of the coil spring (1~3mm bigger). The ring(s) will be temporally fixed in the measured area(s) by epoxy glue. The rubber ring, or conventional oil ring, will be placed on both sides of the ring. The material of the ring can be applied by general steel such as S45C or SAE9254 for an automotive coil spring. In this test, SAE9254 is used.
The coil spring attached with four Peening Rings is inserted in the peening facility. The residual stress can be measured by XRD and compared with the values of the ring before and after shot peening. (Fig. 3) The measuring points on the ring and their closely-near points on the coil spring are selected to compare the values of the ring and the spring. Generally, the residual stress is a deliberate indicator of the peening performance and must be controlled by automotive makers. The values in the red rectangle, which show the bigger difference than the others, are ignored since its values are similar with the values before shot peening, and therefore, the residual stresses measuring both of them are similar and the peening ring jig has potential to be used in the coil spring peening process.
This is similar to the Almen strip method, where some areas in the ring jig are deleted and the arc height can be measured. The deleted area has a gap distance. A number of shot balls will hit on the outer surface of the ring and tend to increase the length and conversely, the inner surface remains as the original length, elastic deformation occurs and the distance of the original gap, 2.0mm, becomes 1.7mm. The peening intensity can be directly estimated by measuring the gap distance.
Conclusions and Discussions
The ring-type jig is suggested to locally measure the peening performance. It is validated by the residual stress measurement and the proposed jig concept reveals potential to apply precise measurement of the peened surface of the coil spring. Since it is small and light, several rings can be simultaneously attached on every point on the coil spring. The ring-type jig can be modified by eliminating some part of the ring, and this modified version jig can show the peening intensity by simply measuring the gap distance; this is similar to the arc height of the Almen Strip.
The shot peening process plays a role in high fatigue life of the spring product and therefore, the conventional Almen Strip must be used to check the performance and quality of the peening process. As a high durability and lightweight trend, the shot peening process is more demanding for the current and the future coil spring and therefore, the shape change of the coil spring must become a problem and be controlled properly. To measure the peening intensities, for every point on the coil spring, the ring-type jig can be an alternative solution rather than the Almen Strip method.
Our company, YOUNGWIRE, always wish to cooperate with any other company and any suggestions are always welcomed.
This research was supported by the Ministry of Trade, Industry and Energy and Korea Evaluation Institute of Industrial Technology (KEIT) in 2022. (20017248)
The author thanks Solution Lab (located at Daejeon, Republic of Korea) for the computer simulation using DEFORM.
Republic of Korea