Abstract: Tempering characteristics of carburized needle valve body for diesel engine made of G20CrNi2MoA carburizing bearing steel was studied. The chemical composition, mechanical properties and microstructure were tested in detail. The preferable heat treatment process was optimized. The needle valve body specimens were tempered at different temperatures between 180 ℃ and 250 ℃. The microstructure of the pyramidal face and core was analyzed by metallographic microscope, the residual austenite content of the end surface was detected by XRD, and the hardness of the pyramidal face and core was analyzed by means of microhardness tester. The results show that the pyramidal face hardness of G20CrNi2MoA needle nozzle can reach 740 HV(61.8 HRC) when tempered at 230 ℃, and the volume fraction of residual austenite in end surface is 0.25%, it can ensure the needle nozzle has a good wear resistance and stability of structure and size at the operating temperature of 230 ℃. The needle nozzle has a long service life and can meet the requirements of the diesel engine.

Key words: diesel engine, G20CrNi2MoA steel, needle nozzle, tempering characteristics