Effect of cold-work on the oxidation resistance of AISI 304L in O2 atmosphere

摘要

In order to ensure a good resistance to high temperature oxidation, the oxide scales formed on stainless steels should remain thin, rich in Cr with good adhesion properties to the metallic substrate. However, the formation of the protective Cr-rich oxide scale induces a decrease of the Cr concentration in the bulk zone underneath the oxide scale. The balance between Cr consumption in the oxide scale and Cr diffusion from the bulk is a key parameter for the steel durability: below a critical level, Cr depletion leads to a loss of the scale protectiveness, and nucleation and fast growth of non-protective Fe-rich oxide nodules happens. The Cr critical value needed to ensure a good oxidation resistance varies with the operating conditions or the steel metallographic state. High values are required for severe oxidation conditions such as wet or carburizing atmospheres. On the contrary, thinner alloy grain size or cold working before oxidation (grinding or traction) are believed to improve the steel durability. Since Cr diffuses faster along the grain boundaries than in the steel bulk, lowering the grain size improves the Cr effective diffusion. This effect is more pronounced in the fcc-structure of austenitic stainless steels where the lattice diffusion of Cr is slower than in bcc-structure of ferritic stainless steels. Moreover, a higher density of grain boundaries provides more oxide nucleation sites on surface, allowing efficient formation of a continuous oxide scale. Mechanical treatments prior to oxidation may also improve the oxidation resistance. Since strain increases the concentration of defects (dislocations, grain boundaries), Cr diffusion is improved as well as the early formation of Cr-rich oxides. This paper aims at studying the effects of cold working on the oxidation behaviour at high temperature of AISI 304L austenitic stainless steel. Specimens, subjected to 38 % strain using tensile testing at room temperature, were oxidized in O_2 at either 830 °C or 850 °C from 48 to 312 h. Evolution of grain size and surface hardness of the alloy substrate with oxidation time at 850 °C, show that the strain effects are withdrawn after 48 h of oxidation. Thermogravimetric analysis performed for 312 h at 830 °C indicate that the mass gain per surface unit area of the 38 % pre-strained sample is decreased by a factor two compared to the as-received sample. After 312 h at 830 °C, SEM investigations on sample surfaces and cross-sections show that the oxide scale formed on the pre-strained sample is thin while nodules locally appear on the as-received sample. After 48 h at 850 °C, Fe-rich oxide nodules appear on pre-strained sample but remains localized while for as-received samples a protective regime is never reached. In both cases, SEM investigations show that cold work promotes the formation of a protective duplex oxide scale from the first stages of oxidation that condition the long-term oxidation resistance.