Nitrogen stainless steel bearing ring the lower hardness of root cause analysis and prevention

Nitrogen stainless steel bearing ring 2019 - the analysis of the causes of low hardness and prevention 11 - 15 Liu Hui river, Wu YongGang, once the van, zhang ling, government of luoyang bearing co. , LTD. , the national key laboratory of aerospace precision bearing Cao Di, air force military agent's room in luoyang district nitrogenous stainless steel is a new type of the corrosion resistance of bearing steel, the brand is 40 cr15mo2vn. Compared with the traditional 9 cr18 bearing stainless steel, nitrogen stainless steel under the condition of the mechanical performance fairly, not only the contact fatigue life is better than nine cr18, and salt fog corrosion resistance is far better than nine cr18. Nitrogen stainless steel after heat treatment hardness requirements & ge; 58 HRC, a type of nitrogen stainless steel bearing ring found after heat treatment of 55 ~ 56 HRC surface hardness, lower than the standard requirements. The ring processing technology for: blanking & rarr; Forging & rarr; Car processing & rarr; Heat treatment. 1. After the thermal physical and chemical inspection method selected ring and ring after car processing, horizontal line made of metallographic specimen, the section for the metallurgical structure observation of sample surface. Metallographic specimen after mechanical grinding and polishing with a saturated solution of picric acid hydrochloric acid alcohol corrosion, Zeiss M2M metallographic microscope and electronic JMS - Japan 6610 scanning electron microscope observation of microstructure and distribution of carbide form. The ONH - nitrogen element 2000 determination of oxygen and nitrogen analyzer, the rest of the element type application ARL4460 direct reading spectrometer. 2. Observation and analysis, 1) Chemical composition analysis of chemical composition analysis results are shown in table 1. The analysis results show that the ring material chemical composition in accordance with standard requirements. （ 2) Hardness test respectively test specimen surface rockwell hardness and cross section of the core area, the surface hardness of 55 ~ 56 HRC, cross section is 58 ~ 59 HRC hardness, nitrogenous stainless steel after heat treatment hardness requirement for & ge; 58 HRC, the surface hardness is lower than the technical requirements, section hardness comply with technical requirements. （ 3) Quenching tempering tissue contrast of nitrogen stainless steel after heat treatment, the microstructure should be tempered martensite + a small amount of a carbide, diffuse distribution of secondary carbides. Select the ring line made of metallographic sample after heat treatment, after saturated solution of picric acid hydrochloric acid alcohol corrosion under the metallurgical microscope and scanning electron microscopy (sem) observation, respectively. Normal regional quenching tempering organizations as shown in figure 1, globular carbide dispersion distribution on the martensitic matrix; Area near the surface quenching tempering organization as shown in figure 2, compared with normal region, the carbide distribution significantly reduced, and the martensite structure is unusual, with grain boundary. Figure 1 normal regional quenching tempering organization chart 2 near surface quenching tempering organization ( 4) Machined after annealing organization compared with batch selected ring made of metallographic sample, before heat treatment after corrosion respectively under the metallurgical microscope and scanning electron microscopy (sem) observation, normal area spheroidizing annealing organization as shown in figure 3, normal area uniformly distributed small ball and granular carbides; Near the ring surface area spheroidizing annealing organization as shown in figure 4, compared with the normal area, close to the surface area of carbide distribution decreased significantly, and are distributed in part of the chain, similar to the 9 cr18 steel annealing twin carbide. Figure 3 normal regional figure 4 near the surface field annealing annealing organization ( 5) Surface carbon content type using ARL4460 direct reading spectrometer detection after quenching tempering carbon content on the surface of the ring, the WC is 0. About 20%, lower than the standard values. 3. Process analysis to review this batch of ring processing technology and processing equipment, heat treatment and forging process parameters in accordance with relevant requirements. Quenching heat treatment using a vacuum furnace, equipment running records without exception. Forging USES the electric box furnace heating, due to the damage of a resistance band electric furnace, heating furnace heating up slowly. Forging blank in a furnace, heat preservation time is too long, lead to the poverty carbon depth beyond the normal requirements. Poverty at the surface of the carbon residue after race car processing results in the decrease of surface hardness after heat treatment. 4. Epilogue ( 1) Ring the cause of the low surface hardness is caused by poor carbon, the cause of poor carbon for forging heating furnace of failure. （ 2) Ring surface quenching tempering tissue abnormalities associated with chain carbides in annealing organization, raises its brittleness. （ 3) In forging and heat treatment process, should be strictly monitor equipment operation state is normal, keep the record complete processing parameters, and in strict accordance with the process execution, found to have abnormal condition should be double sampling observation, avoid the nonconforming into the next working procedure. Source: the metal processing ( Hot working) Magazine,