Oxidation and wear behaviour of a work roll grade high speed steel

Abstract

The oxidation behaviour of high speed steel (1.55 % C, 7.70 % Cr, 4.90 %V, and 2.00% Mo) was studied isothermally at 550 and 615'C for three different environmental conditions, two of which consisted in a mixture of dry air and water vapour flowing at a rate of 0.45 or 1.45 cm'/min respectively, compared to dry air conditions. At 615'C and for the maximum water vapour content, the oxidation behaviour was initially logarithmic followed by linear stage. In contrast, for a water vapour flow rate of 0.45 CM3/Min, the oxidation was parabolic, which resulted in a greater mass gain of the samples after I hour oxidation, even though there was a lower water content. This was believed to be related to the partial pressure of oxygen and therefore to the amount of oxygen available for oxidation. When exposed to dry air, logarithmic kinetics were observed, with much lower mass gain compared with the other two environmental conditions. Reducing the test temperature to 550'C only reduced the mass gain, with the same oxidation kinetics for each condition, suggesting the same oxidation mechanisms at both temperatures. For the humid conditions, an iron-chromium spinel (Fe, Cr)304was formed along with magnetite Fe304 and hematite Fe203- In dry conditions the spinel and hematite were also present. In addition, a VO vanadium oxide layer was located at the top of the oxide layer, indicating oxidation of the MC vanadium rich carbides, promoted by the high partial pressure of oxygen of the environment. The steel was also exposed to cyclic temperature fluctuations, of two different frequencies, in an attempt to more closely simulate the conditions found during operation of rolls in industry. For the high frequency tests, it was difficult to establish a mathematical relationship for the oxide growth, with the kinetics being rather stochastic. The cyclic oxidation produced a very thin granular layer which appeared to be the spinel (Fe, Cr)304. Local surface regions exhibited high plastic deformation associated with cracks that facilitated the oxygen dissolution within the steel. In the low frequency cyclic tests, the oxidation kinetics were parabolic at both test temperatures. Quenching in water, resulted in the preferential spallation of the hematite by a mechanism known as "buckling", generated from compressive thermal stresses. Material was removed at a rate of -0.13mg/cycle and -0.07mg/cycle at 615 and 550'C respectively, leaving the surface only protected by a layer of iron-chromium spinel. The relationship between the wear of the high speed steel and oxidation was investigated at 600,500 and 4001C in a rolling-sliding disc on disc configuration using a 2.5 kg load for a sliding distance of III km for two different environmental conditions (water, both gaseous and liquid, and laboratory dry air). The specific wear rate of the high speed steel discs was greater for the dry tests compared to the wet ones. However, the specific wear rate of the dry tests was strongly temperature dependent, while for the wet tests, the wear rate was insensitive to temperature. The dry tests exhibited a combination of metallic and oxidational wear, while the wet tests were almost entirely oxidational, with a different oxide phase constitution to the dry tests. Surprisingly, the wet tests exhibited higher friction compared to the wet tests. The reasons for this and the difference in wear rates are discussed and compared to the static oxidation tests