EFFECT OF PLASMA ASSISTED NITRIDING IN SULPHIDE INCLUSION OF AUSTENITIC STAINLESS STEEL

L. ESCALADA; S. SIMISON; E. DALIBON; M. AVALOS; S. BRUHL

Cancún

Congreso; XXV International Materials Research Congress; 2016

IMRS

The austenitic stainless steels are used in several industrial applications due to their excellent corrosion resistance. They are typically identified as alloys with low hardness and wear resistance. In order to improve these weaknesses, several surface treatments have been developed during last decades. An example of that are the plasma assisted treatments like nitriding or carburizing that may produce a modified surface layer with five times the hardness of the base material and an increase in the wear resistance without compromising the original corrosion resistance.Under certain conditions the diffusion of nitrogen in the austenitic stainless steel lattice produced a layer that is commonly named expanded austenite. The nitrogen fill the interstitial sites producing and expansion of the metallic matrix, introducing internal stresses that change the surface properties. This is observable with XRD as a shift in the peaks of the austenite to lower diffraction angles.When in contact with chloride media, the untreated steels show crevice corrosion as the main attack morphology with low amount of isolated pits. The attack is originated in second phase particles like inclusions. After  nitriding treatment and in the same corrosion condition, the modified surface layer change its corrosion morphology to semispherical pits below the nitride layer. The inclusions still participate in the corrosion attack but this behavior is not observed for all the second phase particles.In this work, two main kinds of inclusions have been identified. We characterize their microtextural relationships with the expanded austenite layer around them by using EBSD, SEM, TEM and EDS, showing the effect of the nitriding process.