Influence of Nitrogen Content on the Evolution of Creep Damage in 316 LN Stainless Steel

The influence of nitrogen content on the high temperature mechanical properties of 316LN stainless steel has been studied at nitrogen levels of 0.07, 0.11, 0.14 and 0.22 wt. %. These studies have shown that 316LN SS containing 0.14 wt. % nitrogen has optimum mechanical properties. Detailed creep studies were carried out on 316LN SS at 823 K, 873 K and 923 K at stress levels ranging from 140 to 300 MPa. The longest rupture life in this study was 32,500 hours. Creep strength was found to increase substantially with increase in nitrogen content. Metallographic studies were performed on creep tested specimens to understand the influence of nitrogen content on the evolution of creep damage. The extent of internal and surface creep damage decreased drastically with increase in nitrogen content. Area fraction of internal creep damage was measured and it was observed to decrease significantly with increase in nitrogen content. The steel containing the highest nitrogen content showed a negligible amount of creep damage irrespective of the stress level. Hence, increasing the nitrogen content increases the resistance to creep damage formation leading to improved rupture life. The extent of internal and surface creep damage decreased with increasing temperature and decreasing stress. Fracture mode was found to predominantly intergranular in the steel containing the higher nitrogen content.