Effect of P and Si on creep induced precipitation in 20% CW Ti-modified 14Cr-15Ni stainless steel fast reactor clad

Abstract

The effect of phosphorus and silicon on the creep properties and the resultant creep induced precipitation are investigated on ‘20% cold worked Titanium-modified 14Cr-15Ni stainless steel’ with varying contents of phosphorus (P: 0.025 and 0.04 wt%) and silicon (Si: 0.75 and 0.95 wt%). The creep tests were conducted at 973 K and at a stress level of 150 MPa. The microstructures of creep exposed specimens revealed needle shaped phosphides, titanium carbides, M6C carbides and intermetallic Laves phase (Fe2Mo) in the transmission electron microscopy investigations (TEM). The relative fraction of the precipitates is observed to be governed by the rupture life and the P and Si contents. The aforementioned precipitates are quantified for the four combinations of P and Si, and it must be mentioned that phosphide and titanium carbide precipitates played beneficial role in enhancing the creep strength of the alloy. The latter is markedly perceived in the alloy containing 0.025 wt % phosphorus and 0.75 wt % silicon that exhibited high creep rupture life as compared to the other three combinations of P and Si. The higher the amount of P and Si, the excessive is the precipitation of M6C carbides and/or Laves phase that are coarser in size, and consequently the material has shown considerably lower creep lives.