Year

2014

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Material and Mechatronic

Abstract

A wide range of ferrite-based microstructures were produced for high strength pipeline steels with standard X70 (1.2 wt.%) and lowered MX70 (0.5wt.%) Mn content. These include as-processed X70 and MX70 strips, as well as X70 transfer bar in the asprocessed and normalised conditions. In addition, simulated coarse grain heat affected zone (CGHAZ) microstructures were produced. The main objective of the research was to establish and rationalise the differences in hydrogen pick-up and susceptibility to hydrogen embrittlement (HE), both under three point bending (TPB) of notched hydrogen-charged samples and tensile testing of tubular samples pressurized with hydrogen. A particular concern was whether the susceptibility to hydrogen induced cracking (HIC) was compromised by design of a lower Mn steel meeting X70 mechanical property specifications.

The effects of electrolytic hydrogen charging on surface and internal damage, as well as fracture toughness, were studied in relation to grain size, microstructure, composition and the type and distribution of non-metallic inclusions and precipitates. The X70 steel consistently exhibited higher JQ fracture toughness values (derived from TPB tests) than the MX70 strip, both before and after hydrogen charging.

Electrolytic hydrogen charging experiments on thin strip samples (about 1 mm thick) indicated that the most rapid formation of surface blisters and HIC occurred for the banded ferrite-pearlite microstructures of the as-processed strip, followed by equiaxed TB ferritepearlite microstructure in the normalised condition, and then by the as-received TB sample with a ferrite-bainite microstructure, no blistering was observed in the heat affected zone (HAZ) samples for up to 24 hours charging.

In terms of measured diffusible hydrogen content after charging, the equiaxed (normalised) microstructure showed the lowest diffusible hydrogen content, followed by the banded ferrite-pearlite microstructures, the quasi-polygonal ferrite with bainite microstructure and finally the HAZ samples. In relation to residual hydrogen monitored after its release from stronger traps, the HAZ microstructures exhibited the lowest hydrogen content; followed by the equiaxed (normalized TB) microstructure; the quasi-polygonal ferrite and bainite microstructure of the as-received TB; and the banded ferritepearlite microstructures of the as-received strip steels.

HIC in these charged samples often initiated from oxide particles and propagated mainly along intragranular paths, and also along ferrite/pearlite interfaces. The role of ferrite grain size in HIC was also evaluated and it was confirmed that microstructures with an intermediate average ferrite grain size (46 μm) exhibit higher residual and diffusible hydrogen contents than samples with lower and higher grain sizes.

The plastic behavior of the X70, MX70 and normalized TB samples was investigated by tensile testing of tubular specimens under 10 MPa hydrogen gas at 25, 50 and 100°C, and 10 MPa argon at 25°C for reference. The results showed that X70 steel is more susceptible to hydrogen embrittlement by external gaseous hydrogen charging than MX70, an opposite finding to the JQ results obtained from TPB.

It is therefore clear that evaluation of HE can depend on the method of testing, particularly the presence/absence of a notch and the whether the hydrogen is internal or is externally supplied. The investigation overall established that there was no significant loss in resistance to HIC associated with the use of a medium Mn X70 alloy design.

Share

COinS