In spite of the formation of a high fraction of deformation-induced a¢ martensite, the tensile elongation of a cast high-nitrogen austenitic stainless steel was found to enhance at lower temperatures, a behavior deviating from that exhibited by wrought and homogenized austenitic stainless steels. The observed behavior was explained by the presence of microstructural regions with different stabilities with respect to deformationinduced a¢ martensite formation caused by the segregation of alloying elements. Tensile elongation near room temperature of low stacking fault energy (SFE) austenitic steels including stainless, [1][2][3][4][5][6][7][8][9] high Mn, [10][11][12][13] and high Ni [14] steels varies in three temperature regimes as follows. At the highest temperature range (regime I), tensile elongation remains more or less constant or exhibits a weak temperature dependence. At intermediate temperatures (regime II), elongation increases as the temperature decreases. The enhancement of ductility at reduced temperatures in regime II is commonly attributed to such deformationinduced microstructural changes as e martensite formation, the e-TRIP effect, [15] and deformation twinning, the TWIP effect.[2] These are common microstructural features of deformed austenitic stainless steels [16,17] and high Mn [12,18,19] steels and are thought to be consequences of high glide planarity. Enhanced glide planarity caused by reduced cross slip of screw dislocations at lower temperatures has been correlated with an underlying temperature dependence of SFE.[20] The latter dependence is available for many fcc metals and alloys including austenitic steels. [20][21][22][23][24] The enhancement of tensile elongation at lower temperatures in regime II of austenitic stainless steels is interrupted at the temperature below which the deformation-induced formation of a¢ martensite is enabled, namely below the M d cfia¢ temperature. The loss of ductility caused by the deformation-induced formation of a¢ martensite initiates the regime III of elongation, characterized by reduced elongations at lower temperatures. The large number of investigations in support of the detrimental effect of a¢ martensite formation on the tensile elongation of metastable austenitic stainless steels [1][2][3][4][5][6][7][10][11][12] indicates that any possible contribution to the ductility of a TRIP effect would not be large enough to prevent the loss of ductility below the M d cfia¢ temperature. This has been also suggested by the modeling of the a¢ TRIP effect contribution to the ductility.[25] The present study demonstrates how compositional inhomogeneities in a cast high-nitrogen austenitic stainless steel can cause deviation from the behavior described above.
The reliability of rolling bearings is affected by white etching crack (WEC) or white structure flaking (WSF) failures, causing tremendous commercial burdens for bearing manufacturers and operators. The research for the underlying failure mechanism has attracted interest from a large scientific community over decades. Despite the significant amount of efforts, a root cause of white etching cracking is still missing. Amongst other factors, lubricant chemistry is considered to be essential in WEC formation. The authors aim to elucidate this key parameter by provoking white etching crack formation on a FE8 bearing test rig using a well-described set of chemicals in high- and low-reference lubricants. Scanning electron microscopy and energy dispersive X-ray analysis prove the presence of a patchy tribofilm on the surface of bearing washers, leading most likely to a higher frictional torque at the early stages of operation when the low reference oil is used. Secondary neutral mass spectrometry (SNMS) shows a hydrogen containing tribofilm in the shallow subsurface of about 30 nm depth, suggesting that hydrogen proliferating into bearing material may subsequently facilitate crack propagation via dislocation pileups, leading to premature bearing failure.
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