Analysis of application of shot blasting and shot peening technology

The application of shot blasting technology can effectively improve and enhance the fatigue life and corrosion resistance of key automotive components. At present, many well-known automobile manufacturers and component manufacturers in the world have included strengthening in the standard production process. At the same time, strengthening equipment has also formed a complete modern manufacturing line like other manufacturing equipment. With the continuous development of shot blasting technology, its use in the automotive manufacturing field to improve and increase the fatigue life of key automotive components has become more and more the focus of people’s attention, and it is used in the design of cars, trucks, motorcycles, etc. It has been fully considered and valued. At present, spray/shot blasting strengthening technology and process applications will be applied in the design of most engine parts, including: crankshaft (descaling and strengthening), connecting rod (strengthening), transmission gears and other shaft parts, ring gear, Pistons, sun gears and planetary gears, leaf springs and round springs, etc. A large number of auto parts, whether it is casting/forging parts, die casting parts, or machine cutting parts, welding parts, need to use different types of spraying/polishing equipment for surface treatment, such as removing oxide scale, deburring, sanding and cleaning other surfaces Impurities.

There are conclusive data prove that through shot peening, the fatigue life of the leaf spring can be extended by 600%, the fatigue life of the transmission gear can be extended by 1500%, and the fatigue life of the crankshaft can be extended by 900%. Effectively improve the fatigue resistance and corrosion resistance of parts, which is of great significance to its service life and safety in use. Relying on shot peening technology, parts can be designed to be lighter and lighter. Some parts that had to use expensive materials due to process specifications can now also be replaced with low-priced materials. The shot peening process can achieve the same or even better performance standards.

1.Shot blasting in the crankshaft manufacturing process As a part of the manufacturing process, the heat-treated crankshaft needs to be shot blasted to remove the thermal scale on the surface. The crankshaft is placed on the rotating rollers. When rolling, all the surfaces of the crankshaft are fully exposed to the stream of shots projected by multiple blasting heads. The impact of the multi-angle shots makes the outer surface of the crankshaft thoroughly cleaned. The size of the crankshaft determines the type of shot blasting machine used. For large engines, the size of the crankshaft may be as large as φ762mm and 6096mm long. The crankshaft is placed between a set of rollers installed on the trolley. There are several ways of working. Customers can choose a fixed throwing head according to the actual situation of their own workshops. They can either move the trolley under the throwing head, or fix the trolley to move the top throwing head. No matter which method is chosen, the crankshaft placed between the rollers is constantly rotating, so that all surfaces are fully shot blasted.

Smaller crankshafts, such as φ152～203mm and length 914mm, are usually shot blasted by a rotary crane shot blasting machine. The crankshaft is hung on a hook, and then sent into the shot blasting chamber with multiple blasting heads through the rotation of the catenary, for shot blasting. The hook rotates in the shot blasting chamber, so that the workpiece is fully exposed to the high-speed shot flow, and passes while rotating. The cleaning speed can reach 250 pieces/h, and the cleaning effect is very good.

Although the requirements for process control are not as stringent as strengthening, modern crankshaft shot blasting equipment also monitors process parameters to ensure cleaning quality.

2. Strengthening of the crankshaft

Since the crankshaft works under the action of alternating stress, the risk of stress fatigue and strain failure at the change of the journal surface and the fillet is extremely high. At present, the use of shot peening to change the fatigue resistance of crankshafts has been widely used in a wide range, and the effect is satisfactory.

The defect of the traditional rolling process is that due to the limitation of the crankshaft processing technology, the rounded corners of each journal are difficult to match with the rollers, which often results in the phenomenon of gnawing and cutting of the rounded corners, and the crankshaft after rolling deforms greatly, and the effect is not good. The mechanism of shot peening is to use pellets with a strictly controlled diameter and certain strength to form a stream of projectiles under the action of high-speed airflow and continuously spray them on the metal surface of the crankshaft, just like hammering with countless small hammers to produce the surface of the crankshaft. Extremely strong plastic deformation, forming a cold work hardened layer. In short, because the crankshaft is subjected to various mechanical cutting forces during processing, the stress distribution on its surface, especially the cross-section transition of the crankshaft, is extremely uneven, and it is subject to alternating stress during work, which is easy to produce Stress corrosion reduces the fatigue life of the crankshaft. The shot peening process is to introduce a pre-compression stress to offset the tensile stress that the parts will receive in the future working cycle, thereby improving the fatigue resistance and safe service life of the workpiece.

There are two most critical parameters for the shot peening process. One is the stress intensity, which usually uses "Almen test strips" for strength testing. Multiple test pieces are fixed on different surfaces of the crankshaft, especially the corners of the cross section of the crankshaft where the stress is the most concentrated, and shot peened together. The compressive stress generated on the test piece causes the test piece to bow. The expansion change of the curvature is proportional to the energy of the shot impact. Another main parameter to determine the quality of shot peening is the coverage rate, which mainly refers to the ratio of the area occupied by the surface craters after strengthening to the total strengthened surface. This parameter is defined by the crankshaft design engineer, and usually requires 100% to 200% , Some crankshaft applications may require coverage greater than 200%.

According to the hardness of the crankshaft and the ideal compressive stress intensity, usually the shot peening used for crankshaft shot peening has a hardness of 50～55HRC and a size of S280～S330 (0.7mm～0.84mm). In this way, the intensity range of the "Almen test piece" is about 0.008～0.010C (0.025ontheAscale). Compared with shot blasting, the process parameter monitoring of shot peening is more stringent. For crankshaft strengthening applications, the parameters that need to be monitored include: shot peening speed, shot peening intensity, shot diameter, shot peening distance, strengthening time and coverage. Changes in any of these parameters will affect the effect of crankshaft surface strengthening to varying degrees.

The correct application of controllable shot peening technology can greatly increase the fatigue strength of crankshafts and other parts that work under high load conditions, thereby greatly extending the fatigue life of the parts. Advanced and precise shot peening equipment with computer program control technology can closely monitor the shot peening process to ensure the constancy and repeatability of shot peening quality. At present, many well-known automobile manufacturers and component manufacturers in the world have included strengthening in the standard production process. The strengthening equipment has also formed a complete modern manufacturing pipeline like other manufacturing equipment.