| The Swagelok Company has recently developed a low-temperature (470°C) carburization technology for austenitic stainless steels, that increases the surface hardness from 200 to 1200 HV25 without sacrificing corrosion resistance. In order to investigate the microstructural changes responsible for these outstanding properties, bulk specimens, thin foils, and powder specimens of several different low-temperature carburized 316 stainless steels have been studied.; XRD studies revealed that the low-temperature carburization of 316 austenitic stainless steels lead to a colossal supersaturation of interstitial carbon in the austenite. While the equilibrium solubility of carbon is 0.03 at% at the carburization temperature of 470°C, high-precision XRD determination of the lattice parameter after carburization indicated a carbon concentration of >10at% in solid solution—a colossal supersaturation! This astonishing result was confirmed by a completely independent experimental method, X-ray photoelectron spectrometry (XPS).; Residual stress measurements indicated that low-temperature carburization caused an enormous compressive residual stress of 2 GPa at the surface. The enormous compressive residual stress and a high density of stacking faults caused broadening and shifting of the austenite peaks in X-ray diffraction scans.; Analysis of the underlying thermodynamics and kinetics indicate that the key to colossal supersaturation is to kinetically suppress the formation of M23C6. The colossal supersaturation of carbon in the austenite is the dominant feature responsible for the unusual hardness.; Only during the extended (>40h) carburization times, M5C 2 carbide (Hägg carbide), instead of M23C6, was observed to form. In addition, TEM studies indicated the presence of a small amount of a second carbide phase, M7C3. The particles of both carbides have the shape of long needles, containing a high density of planar defects normal to the long axis of the needles.; The concept of “low-temperature colossal supersaturation” (LTCSS) with interstitial solutes appears to apply more generally than only to 316 stainless steel. It should also occur, for example, in Al-, Ti-, and Co-based alloys. In fact, LTCSS with interstitial solutes will probably bestow a new generation of far-from-equilibrium alloys with outstanding hardness, wear- and corrosion resistance. |
