The Method And Key Technique For Preparation Of Reference Material For Quantification Of Retained Ausenite In GCr15Bearing Steel

As a typical bearing steel, GCr15has good performance and wideapplication, with a certain amount of retained austenite in the microstructureafter quenching and tempering. Since the content of retained austenite has animportant influence on materials’ mechanical properties, service behavior,dimensional stability, etc., it is significant to measure and control the content ofretained austenite for improving the manufacturing technique of bearing parts inadvanced manufacturing industry.Firstly, an accurate approach of quantifying retained austenite wasinvestigated. X-ray stress analyzer can be used to measure the content of retainedaustenite accurately when the carbide’s influence is eliminated. But in themeasurement process of X-ray diffraction, considerable error exists because ofoverlapped diffraction peaks, peak separation, carbide’s influence, and so on.Thus, Electron backscatter diffraction (EBSD) can be used as a reference methodfor measuring the content of carbide for GCr15bearing steel.The state of sample surface and affected layer, formed during mechanicalprocess or heat treatment, were studied by X-ray diffraction method on samplesurface. The results showed that the affected layer and the state of surface hadgreat influence on the measured results of retained austenite. Therefore, it isnecessary to eliminate the affected layer and polish the surface of sample beforeexperiment.The orthogonal experiment method was introduced to optimize theexperimental parameters by X-ray diffraction method for quantifying retainedaustenite. The results showed that samples with different content of austeniteshould select different parameters. When the content of retained austenite was5%in the sample, five parameters, including diameter of collimator, voltage andcurrent of X-ray tube, counting time of martensite, counting time of austenite and2θ scanning range of austenite, all have significant effects. When the content ofretained austenite was10%in the sample, the effects of diameter of collimatorand2θ scanning range of austenite were all highly significant, but fortunately theothers had no significant effects.Metallographic method, EBSD method, thermodynamic calculation method and electrolytic extraction method were used to measure the content of carbidefor GCr15bearing steel, and the effects of carbide on measurement of residualaustenite content were discussed. The results showed that carbide had a certaininfluence on the measured results of retained austenite, including the calculationprocess of austenite content, diffraction intensity of martensite and austenite.Electrolytic extraction is an accurate method for quantifying the carbide inGCr15bearing steel. And it is feasible to estimate the content of carbide by othermethods for low requirement.An approach of controlling austenite and carbide was introduced toinvestigate the correlation between austenite, carbide and heat treatmenttechnique. At the same time, a correction curve for revising measured results ofretained austenite was established, with which the measured results of retainedaustenite can be adjusted according to the heat treatment technology even thoughthe content of carbide was unknown in GCr15bearing steel.The determination results of X-ray diffraction method, electrolyticextraction method, etc. had been validated by comparing results with thosemeasured by external laboratories. The verification of certification method wascompleted preliminarily for retained austenite reference material.The results of homogeneity testing, stability testing and characterizationanalysis showed that retained austenite and carbide were homogeneous. And thestability of austenite was acceptable not only in the temperature range of-40℃~40℃, but during14weeks. These data were statistically processed throughoutlier testing, normality testing, and accuracy testing. The certification analysisresult of retained austenite was12.0%±0.2%(volume fraction). The standarddeviation was0.14%. And the content of carbide was7.5%. Therefore, thepreparation technique of retained austenite reference material for GCr15bearingsteel was produced preliminarily.