Microstructures of 27SiMnMoVA steel intended for nozzle in raw, as-brazed and as-normalized states were surveyed by using OLS4000 laser scanning confocal microscope, Axiovert-25CA optical microscope and QUENTA-400 scanning electron microscopy. It is found that the microstructures of raw 27SiMnMoVA steel are ferrite and pearlite; the microstructures of the as-brazed 27SiMnMoVA steel are ferrite, martensite, bainite and troostite; the microstructures of as-normalized 27SiMnMoVA steel are sorbite and bainite; and that the as-normalized microstructures are appreciably finer than the as-brazed microstructures. Adjusting the carburizing and hardening processes, a cryptocrystalline martensite provided with better wear-resistance is obtained in the case of 27SiMnMoVA steel specimen, and the effective hardened depth is increased from 0.48mm to 0.75mm. Retained austenite content and wear-resistance of the 27SiMnMoVA steel specimens in different heat treatment conditions were measured by means of D8 Advance X-ray diffractometer and MM200 Pin-on-Disk wear testing machine. The results show that the retained austenite contents in the specimens are six to eight percent, and the abrasion marks are as about 3mm. The volume and superficial area of the cooling chamber were increased by 13% and 23% respectively by the way of improving it's structure, which made the cooling performance of the needle nozzle enhanced significantly. The purpose of increasing the flow coefficient of the nozzle spray holes, improving the spray quality and performance of the nozzle and prolonging the nozzle assembly service life were achieved by adopting liquid extrusion grinding process and carried out the liquid extrusion grinding in spray holes of the nozzle by using KYM-2 liquid extrusion grinder.
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