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

The effect of Laves phase precipitation on the fatigue behavior of a 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 the Laves phase (Fe2Nb) occurs, resulting in the embrittlement of the material which is known as the Laves phase embrittlement. Rotary bending fatigue tests have been conducted in laboratory air and in 3%NaCl solution using a type 347 aged at 700, 800 or 900°C. In laboratory air, the fatigue strengths were 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 strengths, which could be attributed to the coarsening of precipitated Laves phases. In 3%NaCl solution, the fatigue strengths of 700°C aged specimens were lower than those in laboratory air. It is considered that the sensitization took place during the heat treatment, where chromium-depleted zones were formed along grain boundaries.