The effect of furnace atmosphere carbon potential on the development of residual stresses in 52100 bearing steel
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The Effect of Furnace Atmosphere Carbon Potential on the Development of Residual Stresses in 52100 Bearing Steel XU NAISONG, C. A. STICKELS, and C. R. PETERS It has been shown ~ that samples of SAE 52100 bearing steel, austenitized at 800 to 850 ~ in a carburizing furnace atmosphere, will be carburized at the surface, even though undissolved carbides exist in the steel at these temperatures. Carburizing produces (1)compressive residual stress at the surface to a depth approximately equal to the depth of carbon penetration from the atmosphere, (2) a higher retained austenite content at the surface, (3) a higher fraction of primary carbides at the surface, and (4) higher surface hardness. The development of residual stress and the surface-to-center variation in retained austenite are accounted for primarily by differences between the amount of carbon dissolved in austenite at the surface and in the interior of the samples. (The austenite carbon content at the surface approaches equilibrium with the furnace atmosphere while the austenite carbon content in the interior approaches equilibrium with high-chromium cementite.) The increase in surface hardness is mainly due to the higher carbide fraction near the surface. Carburized samples were found to have better rolling contact fatigue life than samples with uniform carbon content. In the previous work ~ the atmosphere carbon potential during austenitization was not well characterized nor was it automatically controlled. The present experiments were intended to remedy these deficiencies and to establish the atmosphere carbon potential needed for carburizing to occur. Circular disks of spheroidize-annealed 52100 steel, 7.67 cm in diameter and 1.78 mm in thickness, were austenitized for one hour at 815 ~ in a small sealed-quench carburizing furnace using atmospheres prepared from propane and air. Oxygen sensor voltages corresponding to carbon potentials of 0.15, 0.30, 0.50, 0.70, and 0.90 wt pct carbon in pure iron were used as control points. The atmosphere control system is described in Reference 2. The CO, CO2, and CH4 contents of the furnace atmosphere were
monitored by infrared gas analysis. After austenitizing, samples were quenched in 55 ~ oil, then tempered for two hours at 150 ~ Residual stress was measured as before' and computed using the analysis of Treuting and Read 3 (see Appendix for details). Retained austenite was measured using a vertical diffractometer with a diffracted beam monochromator and Cu K a radiation. The integrated intensities of the (200), (220), and (311) austenite lines and the (200) and (211) martensite lines were measured, corrected for overlap of carbide lines, and the austenite content was computed using Dickson's method 4 to compensate for preferred orientation in the samples. (A total of nine reflections was measured on one sample to confirm that preferred orientation could be adequately compensated for with five reflections.) Carbide fraction was estimated by measuring the integrated intensity of the (112, 021) cementite reflection.
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