Fracture behavior of steam-grown oxide scales formed on 2–12%Cr steels

Abstract

A room temperature three point bent-beam test was conducted for preoxidized coupon specimens of ferritic 2%Cr (T22), 9%Cr (T91), and 12%Cr (T122) steels in order to examine the fracture/spalling behavior of steam-grown oxide scale. Test specimens were reacted with atmospheric 100% steam at 550–750°C for 1000-4800 h. Oxide scale thickness of the tested steels was 15–1150 μm for the 2Cr steel, 30–450 μm for the 9Cr steel, and 25–60 μm for the 12Cr steel. External tensile strain of up to 1.86% was loaded to each preoxidized specimen surface and the fracture/spalling behavior of steam-grown oxide scale was investigated visually and by microscopic observation. For the 2Cr steel, scale greater than 380 μm exfoliated without applying any external strain. For thin-scaled specimens of 15–20 μm thick, a tensile strain of 0.25% and more caused through-scale cracking perpendicular to the scale/metal interface. For these specimens, cracks along the scale/metal interface also resulted, and the oxide scale became separated from the base metal. For the 9Cr steel, scale exfoliation due to cooling was not prominent even for specimens with 450 μm thick scales. External tensile strain of 0.91% and 1.86% caused through-scale cracking to the oxide scale, but the scale/metal interface remained intact and scale exfoliation did not take place. This was the same for the 12Cr steel. Clearly, spalling resistance of the steam-grown oxide scale was significantly higher for the 9Cr and 12Cr steels than for the 2Cr steel.