Effects of tempering on corrosion properties of high nitrogen alloyed tooling steels in pyrolysis oil

Abstract: Nowdays biofuels are becoming a good alternative for petroleum fuels due to environmental issues like high carbon dioxide emission and increasing vehicles population, together with the high price and fast depletion of petroleum oils. This project aims to investigate the corrosive effects of wood Pyrolysis oil on a special grade of nitrogen alloyed tooling steels to be used for injector nozzles in Diesel engines, where high stress and strain encounter high acidity and corrosivity of the Pyrolysis oil and cause breakdown over short periods. Vanax 35 and Vanax 75 manufactured in Uddeholm are two types of powder metallurgy high nitrogen alloyed martensitic stainless steel with a high combination of hardness (over 56HRC), low friction properties, wear resistance, anti-galling and corrosion properties. In this work, the newly developed Vanax material together with the tool steels Elmax and AISI O1 were tempered at various temperatures from 200°Cto 500°C. The tempered steels were then exposed in pyrolysis oil at 4 different temperatures, 20°C, 70°C, 95°Cand 130°C. The materials were investigated by means of corrosion rate measurements, microscopy (LOM, SEM, confocal) and Thermo-Calc calculations. The corrosion rate measurement proved that Vanax tempered at lower ranges (200°C, 400°Cand 450°C) showed the best corrosion resistance while higher tempering temperatures such as 500°C, Elmax and AISI O1 tempered at 200°Csuffered a great deal of general corrosion attack. Thermo-Calc calculations showed the formation of a hard phase, VN as primary nitrides instead of primary chromium carbides at austenizing temperature for the Vanax group. Higher amount of chromium is dissolved in solid solution in Vanax at austenizing temperature hence the martensite matrix has, after quenching, a higher chromium content that helps passivation. The loss in corrosion properties at higher tempering temperatures was due to the formation of CrN secondary phase at around 400˚C which reduces the chromium content of the martensite matrix. The results of light optical and confocal microscopy showed the presence of pits when tempering at 400˚C and 450˚C. No pits were observed at 200˚C. Elmax was not passivated at all which resulted in general corrosion attacks, due to a high chromium loss from the austenite solid solution at the austenizing procedure temperature and also the tempering temperatures. The chromium depletion from the austenite can be explained by a high carbon and a low nitrogen content in the composition which resulted in formation of a high amount of Cr7C3.