Why do the needle valve parts of diesel engine fuel injectors get stuck after being heat-treated by intermediate frequency induction heating equipment? How to improve?

The needle valve coupling is a key part of the diesel injector, usually made of GCr15 steel. The heat treatment process of the intermediate frequency induction heating equipment for the needle valve coupler is heating at 840-860°C. After heating in stages with nitrate salt at 150-190°C, cold treatment is carried out within 1 hour (below -60°C), and then tempered at 160°C, and then tempered at about 130°C. Aged twice. After heat treatment, the structure is tempered martensite and retained austenite. During production, it was found that the needle valve coupling is stuck and poor atomization is a common defect, especially the needle valve coupling of high-speed supercharger, the working temperature is 250-300 ℃, and the failure of the above defects is more serious. Most of the stuck failures are caused by self-locking stuck due to workpiece deformation, and most of them are early failures that occur within 100 hours of operation.

According to the analysis, the phenomenon of stuck needle valve and poor atomization is closely related to its working environment. In the high-speed supercharged diesel engine cylinder, the working environment is worse than that of ordinary cylinders, and its temperature has reached 250-300°C during service, which exceeds the tempering temperature of GCr15 steel needle valves, so the workpiece undergoes tempering transition when heated by intermediate frequency induction heating equipment And produce micro-deformation; at the same time, the residual austenite in the workpiece transforms at this temperature to form tempered martensite, causing micro-deformation; and the needle valve is near the oil injection hole, under high temperature and stress deformation, it is easy A higher stress concentration zone is formed, and the needle valve is prone to deformation due to stress relaxation at this temperature. The needle valve is a high-precision part. The needle valve body and the whole valve are in clearance fit, and the fit gap is 0.0025-0.0035mm. Deformation occurs, thereby exceeding the fit gap of the workpiece, causing the needle valve body and needle valve to have a self-locking stuck failure accident. In addition, the inspection found that the scratched surface is mostly located at the lower part of the side near the pressure chamber, and an annular friction belt appears at the opposite position of the scratched surface of the needle valve body hole; when the needle valve bends, it is found that there are massive scratches on the needle valve. From the above analysis, it can be seen that the GCr15 steel needle valve has defects of insufficient performance and excessive deformation after adopting the heat treatment process of the usual intermediate frequency induction heating equipment.

According to the above analysis, the process improvement measures to prevent the needle valve from being stuck and bitten are proposed as follows:
(1) Appropriately reduce the diameter of the oil injection hole of the GCr15 steel needle valve body, or reduce the diameter of the middle hole, so as to increase the thickness of the annular wall in the area of the oil injection hole. It helps to reduce the deformation of this part; however, this method is limited by the structural design of the workpiece and the processing method. If the improvement is not done properly, the performance and economic benefits of the diesel engine will be reduced, which needs to be considered comprehensively.
(2) Before the heat treatment of the intermediate frequency induction heating equipment, a tight fit insert is set in the middle hole of the needle valve body, so that the annular wall is divided into two layers, and the insert acts as a buffer force and reduces the deformation of the middle hole. Although this method is theoretically desirable, there is still the problem of the first measure in the implementation, and the implementation needs to be considered comprehensively.
(3) The needle valve is made of high-alloy steel (such as 18Cr2Ni4WA steel, W18Cr4V steel, etc.), which can reduce the phenomenon of workpiece jamming and poor atomization, but the material is expensive, cold and hot processing is difficult, and the processing cost is high. In addition, high-alloy steel needle valve parts are used in high-speed supercharged diesel engines, and there are still stuck and damaged phenomena, so high-alloy steel still has deficiencies.
(4) Improve the heat treatment process of the intermediate frequency induction heating equipment for the needle valve couple, so that the workpiece can obtain a sorbite structure with higher thermal stability than the tempered martensite, which can better solve the problem of GCr15 steel needle valve couple stuck and seized Dead defect failure problem. Improvement measures are adopted in production
(5) GCr15 steel needle valve is processed, the production service life reaches 3000h, and various performance indicators and technical and economic indicators reach the high-quality product level of 18Cr2Ni4WA steel and W18C Hong V steel needle valve couples.