15-15Ti(Si) austenitic steel: creep behaviour in hostile environment

Abstract

This work aims at studying the creep behaviour of 15-15Ti(Si) austenitic steel, under uniaxial stress (range of 300-560 MPa), and its interaction with liquid lead. The steel was tested to verify its sensitivity to Liquid Metal Embrittlement (LME) and to simulate its behaviour in operating thermal and mechanical stress conditions of the IV generation Lead-cooled fast reactor. The experimental results permitted to plot the time-strain creep curve and the characteristic Norton-based curve, simulating the creep behaviour at all stress values. The comparison between the creep curves in air and in lead showed that the LME produces a decrease of creep-rupture time, a reduction of creep strain and then the loss of steel ductility. Moreover, the raw material and fracture surfaces were analyzed by Optical Microscope and Scanning Electron Microscope (SEM). SEM micrographs highlighted that lead changes both the mode and the type of specimen fracture. In addition, it was analyzed the lead action time, as the time after which the corrosion appears with macroscopic effects. Although some tests are still ongoing, it can be assumed that at high stresses, LME takes place after a long time of steel/lead contact while at low stresses, LME tends to prevail on creep effect.

Application of Small Specimen Test Technique to Evaluate Creep Behavior of Austenitic Stainless Steel

“…The n value provides important information regarding the deformation mechanism. The literatures [19,25,26] report that a value of n = 1 happens in creep diffusion, n = 2 belongs to the grain boundary sliding controlled creep, and a value of n = 3–5 reveals the main creep mechanisms in pure metals and simple solid solution alloys are the dislocation creep. The n value is highly related to the microstructures.…”

Mechanical behavior in liquid lead of Al2O3 coated 15-15Ti steel and an Alumina-Forming Austenitic steel designed to mitigate their corrosion

“…However, no LME by LBE (with low oxygen content up to 10 -12 wt%) was detected at 300 °C for the solution-annealed 15-15Ti steel [27]. An effect of the liquid lead was observed on creep resistance at 550 °C with a decrease of creep-rupture time, a reduction of creep strain and then the loss of steel ductility [50]. In LBE, the degradation of the creep performance of the solution-annealed 15-15Ti steel at 600 °C and 550 °C was attributed to uniform dissolution corrosion by LBE and by localized intergranular cracking [51].…”

A Review of the Surface Modifications for Corrosion Mitigation of Steels in Lead and LBE

“…The mechanical response of martensitic steel is much more sensitive to the liquid metal environment. T91 steel may exhibit liquid metal embrittlement (LME) e.g., [28,33,37,38], have fatigue lives e.g., [35,[39][40][41], and creep rupture times [42][43][44] are shorter in liquid metal than in air. It is important to note that LME and fatigue cracking initiate in liquid metals at the surface of the materials, as does corrosion.…”