Effect of manganese content and microstructure on the susceptibility of X70 pipeline steel to hydrogen cracking

Abstract

The influence of composition and microstructure on susceptibility to hydrogen induced cracking (HIC) was investigated in high strength pipeline steels, with Mn contents of 1.2% (standard, X70), and 0.5% (medium, MX70). The HIC resistance of the simulated coarse grained heat affected zone microstructures and normalized X70 transfer bar was also investigated. Notched and fatigue pre-cracked samples were charged with hydrogen prior to three point bend tests. The conditional fracture toughness JQ was determined. The results are discussed in relation to grain size, microstructure, composition and the type and distribution of non-metallic inclusions and precipitates. Published as "Effect of manganese content and microstructure on the susceptibility of X70 pipeline steel to hydrogen cracking" AbstractHydrogen, even in very low concentrations, can diffuse to regions of high stress concentration resulting in a degradation of mechanical properties. However, the transport of hydrogen depends on the interaction between hydrogen atoms and microstructural traps which, in turn, is related to microstructure. The influence of composition and microstructure on susceptibility to hydrogen embrittlement was investigated by selecting pipeline steels with different Mn contents, i.e., standard Mn (X70) and medium Mn (MX70) strips. The HAZ (heat affected zone) microstructures of these strips as well as a normlised transfer bar (TB) were also investigated. HAZ simulations were conducted using Gleeble thermo-mechanical machine to simulate a thermal cycle typical of inservice repairs. Notched and fatigue pre-cracked samples were subjected to electrochemical hydrogen charging using a solution of H 2 SO 4 and NaAsO 2 to achieve 2 and 4 ppm hydrogen content. Three point bend tests were conducted on as-received and hydrogen-charged samples. The X70 strip exhibited a higher J Q than the MX70 strip before and after charging, whereas the TB displayed the lowest J Q . The fracture performance of both steels deteriorated almost linearly with an increase in the hydrogen content. Because of their coarser prior austenite grain sizes and the presence of relatively hard bainitic-ferrite structure with martensite/austenite islands, the simulated HAZ microstructures of the two steels demonstrated significantly lower fracture resistance both with and without hydrogen. The results obtained were compared and discussed in relation to the grain size, microstructure, composition and the type and distribution of precipitates in the samples.

Effect of gaseous hydrogen charging on the tensile properties of standard and medium Mn X70 pipeline steels

“…Due to the sample geometry of the tensile test pieces, the volume containing centreline segregation was removed, whereas it was present in the TPB samples. Unlike S-X70, the centreline region of M-X70 strip contained elongated MnS stringers with some blocky inclusions among these stringers, as well as a higher number density of oxides [27]. These microstructural features have been reported by Domizzi et al [37] to promote HICC and they could, therefore, have contributed to the higher incidence of HICC in the TPB tests of M-X70.…”

“…In all cases of hydrogen atmosphere testing, partial or complete brittle fracture by quasi-cleavage was observed. HICC associated with inclusions and other stress-concentrating defects would be required to initiate and extend cracking by quasi-cleavage, which dominated the fracture mode of the hydrogen-charged samples that were tensile tested at 25 o C. Electrolytic charging of the same steels [27] clearly established the presence of hydrogen cracking throughout the strip cross-sections and it is therefore reasonable to expect the presence of cracking in regions surrounding the final crack surface in the present case. However, the presence of sub-critical cracks was not confirmed because it was not considered to have a significant bearing on the main findings of the work.…”

“…Although the S-X70 steel was found to be more susceptible to HE than M-X70 for tensile testing under 10 MPa hydrogen pressure, it was found to be less susceptible to HE when electrolytically hydrogen-charged samples were subjected to TPB tests [27]. In the TPB tests the surface subjected to bending was parallel to the rolling plane, so that the microstructure ahead of the advancing notch-induced crack was banded, particularly for the S-X70 steel.…”

Effect of microstructure and composition on hydrogen permeation in X70 pipeline steels

“…Recent studies involving precipitate analysis of the two steels using scanning electron microscopy [51] have shown that the medium Mn steel exhibits a significantly larger number density of precipitates and inclusions. Most of these precipitates were found to be complex carbonitrides of Ti and Nb having an average diameter of 0.8  0.8 m.…”

Segregação Em Aços Alta-Resistência Baixa Liga (Arbl) Para Aplicações Em Serviço Com H2s- Avaliação Por Termodinâmica Computacional

“…O efeito simulado da segregação é apresentado na Figura 8. É evidente que, quando a segregação é levada em conta, o enriquecimento interdendrítico em Mn e S leva a precipitação de MnS no final da solidificação, criando regiões concentradas neste sulfeto, muito desfavoráveis a resistência a HIC [41]. A adição de Ca, além de garantir a lingotabilidade, leva a formação de CaS, considerado o melhor sulfeto, nestes aços [2,6].…”