Year

2013

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering

Abstract

This research investigates the detailed microstructure-mechanical property relationships in the weld heat affected zone (HAZ) of commercially fabricated API 5L grade X70 and X80 line pipe steels. High strength line pipe steels are widely used for the transmission of energy such as crude oil and natural gas and provide an important service. Although the combination of high strength and good toughness can be achieved in the base metal (BM) through the adoption of clean steelmaking, advanced thermomechanical controlled processing (TMCP) technology and tailored alloy design, the welding thermal cycles can dramatically change the properties in the HAZ. Microalloying addition of Ti is employed to control austenite grain coarsening at the peak temperature in the HAZ. The formation of thermally stable TiN precipitates is effective in control of grain growth in the HAZ. This study investigated the microstructure and mechanical properties of both the actual steel welds and the critical regions produced by Gleeble weld simulation of X70 steels with different Ti/N ratios ranged from 1.88 to 4.88. Moreover, the microstructure and properties in the simulated HAZ of high Nb X80 steel produced by high temperature processing (HTP) were studied.

The mechanical properties in the weld zone of commercially fabricated line pipes are critical to the in-service operation and reliability of the pipelines. The microstructure of two-pass submerged arc welding (SAW) joints with various Ti/N ratios, including BM, HAZ and weld metal (WM), was examined by optical microscope (OM) and scanning electron microscope (SEM). The hardness profiles across the welds were also recorded. The strength and Charpy impact toughness of the BM did not appear to be influenced by Ti/N ratio, within the range evaluated. The toughness of actual welds (V-notch sectioned 50% HAZ+50% WM) tested at a series of temperatures did not show any correlation with the Ti/N ratio in the studied range.

To elucidate the influence of Ti/N ratio on the microstructure and mechanical properties of a typical commercial line pipe welds, Gleeble thermomechanical simulation was performed to critically assess the different subzones within the actual weld HAZ, with specific attention directed to the coarse grained heat affected zone (CGHAZ). The microstructure of the simulated specimens, over a range of Ti/N ratios was characterised by means of OM, SEM, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The microstructure of simulated CGHAZs mainly consisted of a relatively coarse austenite grain size containing packets of aligned bainitic ferrite laths, with a small volume fraction of martensite-austenite (M-A) constituent finely dispersed throughout the microstructure. Evaluation of the prior austenite grain size, transformed microstructure and hardness revealed subtle differences in mechanical properties as a function of Ti/N ratio. The results confirmed that stoichiometric ratio of Ti/N provided optimum austenite grain size control and therefore higher CGHAZ toughness.

Intercritically reheated coarse grained heat affected zone (IC-CGHAZ) is another critical region in the HAZ of two-pass SAW joints. The simulated IC-CGHAZ was produced for steels over a range of Ti/N ratios. The microstructure and mechanical properties of simulated IC-CGHAZ were evaluated over the entire intercritical temperature range. The IC-CGHAZ exhibited extremely low toughness, and there was no correlation with Ti/N ratio in the studied range of Ti and N concentrations. The beneficial effect of near-stoichiometric Ti/N ratio observed in CGHAZ did not translate to IC-CGHAZ due to the negative effect of blocky M-A constituent along the prior austenite grain boundaries. Moreover, the IC-CGHAZ toughness gradually improved as the intercritical temperature increased, but did not return to the values of the original CGHAZ due to the presence of isolated large M-A particles and coarse microstructure. Significance of M-A constituent to toughness is firstly related to the location of particles along prior austenite grain boundaries, followed by the size of individual M-A particles.

For the high Nb, high temperature processed (HTP) steel, the microstructure and properties of simulated CGHAZs over a range of welding cooling time, from 800°C to 500°C (Δt8/5), to represent different welding processes and production rates. The HTP steel maintained HAZ fracture toughness over a wide range of heat inputs (HIs), as evidenced by 20°C lower ductile brittle transition temperature (TB) compared to the conventional X70 steel. This was attributed to the more homogeneous microstructure, much finer prior austenite grain size and lower fraction of dispersed M-A islands. Finally, HTP X80 grade steel, conventional X70 grade steel and HSLA65 steel were selected to investigate the influence of Nb concentration on the grain coarsening behaviour in simulated HAZ. The HTP steel with high Nb content (~0.11 wt.%) showed lower degree of grain coarsening at peak temperature of 1350°C compared to the other two steels with lower Nb contents.

FoR codes (2008)

0912 MATERIALS ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.