Effect of Laves Phase Precipitation on Fatigue Properties of Niobium-containing Austenitic Stainless Steel Type 347 in Laboratory Air and in 3%NaCl Solution

The effect of Laves phase precipitation on the fatigue properties of niobium-containing austenitic stainless steel, type 347, was investigated. When this material is exposed to the temperature range of 700 ∼ 900 °C, the precipitation of Laves phase (Fe2Nb) occurs, resulting in the embrittlement of the material which is known as the Laves phase embrittlement.In the present study, rotary bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using type 347 aged at 700, 800 or 900 °C. In laboratory air, the fatigue strength was improved by the aging at 700 °C due to the fine Laves phase precipitation in γ-phase. Whereas the aging at 900 °C decreased fatigue strength, which could be attributed to the coarsening of Laves phase. In 3%NaCl solution, the fatigue strength of 700 °C-aged specimen became lower than that in laboratory air. It is considered that the sensitization occurred due to the formation of chromium-depleted zone along grain boundaries during the heat treatment.