Effects of annealing and quenching on fatigue behaviour in type 444 ferritic stainless steel

Abstract

A B S T R A C T In order to understand the effects of annealing and quenching on fatigue behaviour in type 444 stainless steel, fully reversed axial fatigue tests have been performed using smooth specimens of heat-treated materials in laboratory air and 3%NaCl aqueous solution.Three materials subjected to different heat treatments, annealing at 960 and 1000 • C, and water-cooling at 960 • C, were prepared. In laboratory air, the fatigue limit of the annealed specimens was lower than that of the as-received specimen and decreased with increasing annealing temperature. The subsequent grain coarsening from the heat treatments was primarily responsible for the lower fatigue strength in the annealed specimens. The fatigue strength of the water-cooled specimen was lower than that of the corresponding annealed specimen. In the annealed specimens, cracks were generated within ferritic grains, while in the water-cooled specimen, at or near grain boundaries. In 3%NaCl solution, the fatigue strengths of all specimens decreased compared with those in laboratory air. Only in the water-cooled specimens, crack initiation at grain boundary and intergranular crack growth were observed, indicating the most sensitive to corrosion environment.Because ferritic stainless steels with extremely reduced interstitial content possess excellent resistance to stress corrosion cracking, good toughness, ductility and weldability, as compared with conventional austenitic stainless steels, 1,2 their applications to structural components in machines and structures are increasing, particularly in elevated temperature applications such as exhaust system products of automobiles. 3 In such components, it is expected that grain growth and sensitization would take place due to frequent heating and cooling at the temperature range above recrystallization temperature. Thus evaluation on the fatigue properties of materials subjected to annealing and quenching becomes extremely important in order to ensure their safety and reliability. For several years, we have studied on type 444 ferritic stainless steel, 4-8 while in newly developed high purity ferritic stainless steels, the studies on the fatigue behaviour of the steels heated above the sensitizing temperature, usually Correspondence: K. Tokaji.above 900 • C, have been very limited. 9 For increasing practical use, such a condition is unfavourable from the viewpoint of the safety and reliability. The present paper gives the result of fatigue testing on the ferritic stainless steel, which was heated above the sensitizing temperature, i.e. fully reversed axial fatigue tests have been performed on materials annealed at 960 and 1000 • C and water-quenched at 960 • C of type 444 ferritic stainless steel in laboratory air and 3%NaCl solution. The effects of heat treatment on fatigue behaviour were discussed on the basis of crack initiation, small crack growth and fractographic analysis. E X P E R I M E N T A L D E T A I L S MaterialThe material used is a high purity ferritic stainless steel, type 444 (18Cr-2M...

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