Stability of fatigue cracks at 350 °C in air and in liquid metal in T91 martensitic steel

Abstract

The low cycle fatigue behaviour of a 9Cr1MoNbV martensitic steel has been investigated at 350°C in air and in lead-bismuth eutectic (LBE) with low and high oxygen content. Total strain controlled tests were performed from t = 0.40% to 1.2%. The material exhibited a pronounced cyclic softening in both environments. LBE reduced the fatigue resistance. LBE accelerated the formation of the long crack by promoting the growth of the first short cracks. A clear change in propagation mode was observed. In air, ductile fatigue striations were observed while in LBE a brittle fracture decorated by voluminous and largely spaced striations were visible. From EBSD analysis, it was concluded that in LBE, the long crack advanced quickly by repeated and discontinuous cleavage.

Liquid metal embrittlement sensitivity of the T91 steel in lead, in bismuth and in lead-bismuth eutectic

“…a decrease in the surface energy of the solid alloy caused by adsorption of Pb and Bi atoms. The occurrence of LME by LBE in T91 has been reported later on by other researchers for other sets of conditions and the fractography more deeply studied [11][12][13][14][15][16][17][18][19][20][21][22][23]. The brittle fracture discussed at the mesoscopic scale was either at prior austenitic grain boundaries [11,13,15,21] or transgranular.…”

“…In their study, the embrittling criterion was built on a LME model based on the reduction of the surface energy by liquid metals adsorption. They also ignored the presence of oxygen which has an influence on the sensitivity of the T91 steel by the LBE [21][22][23].…”

“…The influence of oxygen content in LBE on the corrosion resistance and on the LME sensitivity of the T91 steel has been studied [21][22][23]46]. A low oxygen content in LBE increases the sensitivity to corrosion and LME of T91 steel in LBE.…”

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.…”

Investigation into creep‐to‐rupture of SIMP steel in stagnant LBE at 300–450°C

“…[15] It is well known that LME can deteriorate the mechanical properties of steels, such as the reduction of fracture toughness, [16] and a drop of the total elongation. [17][18][19] Therefore, it is worth investigating the mechanism and underlying physics of LME and how to predict the LME phenomenon of steels in LBE. Generally, there are four main explanations of LME occurrence: (i) the brittle fracture theory-based model proposed by Stoloff and Johnson, [20] (ii) the ductile failure based models of Lynch, [21] (iii) the liquid metal atoms penetration based model of Gordon and An, [22] and (iv) the dissolutioncondensation based models of Robertson and Glickman.…”