Study On The Weldability Of The Third Generation Pipeline Steels

Welding is a necessary and inevitable process in pipeline industry, e.g. pipe-making and field work in pipeline construction. Thus, the quality of weld joint is very important for the reliability of the whole pipeline project. Low C high Nb X80was proved to have very good weldability. But, with the increasing of pipeline steel strength level, more problems caused by welding are noted, for example, local embrittlement/softening in the heat affected zone (HAZ); strength/toughness mismatch between baseplate and weld metal; hydrogen induced crack, cold crack and so on so forth. Therefore, the research on the weldability of higher level pipeline steel (e.g. X90/X100) is of significance.In this paper, the microstructure and mechanical properties of the HAZ of X100pipeline steel were characterized and examined. The resuls showed that the microstructure and properties varied a lot in different zones in the HAZ, due to different thermal history they went through. The mechanical properties of X100pipeline steel weld joint are very good except Charpy impact toughness. The absorbed energy in the HAZ varied a lot. The CVN value in the’equivalent’ fusion line was only51J, because the notch encountered ICCGHAZ. While without the exsistence of ICCGHAZ in the notch of’equivalent’HAZ, the CVN value went up to206J. Therefore, ICCGHAZ, which is consisted of coarse prior austenite grains and necklacing M-A (Martensite-Austenite) constituent along grain boundaries, is the root cause for the low toughness. The presence of ICCGHAZ caused unexpected brittle fracture before the fracture stress was reached, and there was no’stable propagation’stage during the impact process of ’equivalent’fusion line sample. Results on the characterization of fracture surfaces showed that ICCGHAZ was the crack initiation site of the whole fracture surface. Meanwhile, the crack of M-A constituent or debonding of M-A from matrix can cause microcrack and hence can also be the initiation site of big cleavage facets. Therefore, how to enhance the toughness of ICCGHAZ is the key to improve the toughness of the whole HAZ.The microstructure of different regions in the HAZ was simulated using Gleeble thermal simulation machine, and the Charpy impact toughness of each region was also tested. The results showed that the toughness of CGHAZ, FGHAZ and ICHAZ were very good, with CVN value higher than200J. While, the toughness of ICCGHAZ was the lowest, with the average CVN value of lower than50J. It’s also because of the existence of necklacing M-A constituent. The way how to improve the toughness of ICCGHAZ was explored by changing the thermal simulation parameters. The results showed that with the increasing of the second peak temperature, the distribution of M-A constituent was scattered and the size of M-A was also decreased. Hence the toughness was improved. When the grain size of ICCGHAZ was refined, the nucleation sites for M-A constituent increased. Therefore, the distribution of M-A became less continuous, the size of M-A was refined, and also the toughness was notebaly improved. The matrix microstructure of ICCGHAZ can also influence the toughness of ICCGHAZ. When the matrix microstructure was consisted of lath-like bainite which has good toughness, the toughness of ICCGHAZ was higher. In contrast, when the matrix was granular bainite or lath bainite, the toughness was lower. Based on the above results, only if the necklacing M-A constituent can be changed into non-continuous or dispersive state, the size of M-A would be smaller and the Charpy impact toughness could be evidently improved.Detailed structure and chemical composition of M-A constituent in ICCGHAZ were characterized and examined. The layer-by-layer structure was proved to be martensite-austenite lath structure, with the fraction of47%and43%respectively. The atom probe results showed that the carbon content in M-A was only0.45%, which is not enough to stabilize43%austenite. But due to the3-dimensional compressive stress and the size effect, the retained austenite was much more stable. Therefore,43%retained austenite in M-A is reasonable. The formation mechanism of M-A constituent in ICCGHAZ was summaried as:(1) CGHAZ was formed in the first pass thermal cycle due to high peak temperature.(2) In the second thermal cycle, reverted austenite nucleated along grain boundaries. Carbon in the nearby area diffused into the reverted structure, which is called1st carbon enrichment.(3) Durig cooling stage, part of the reverted austenite transformed into bainitic ferrite. Carbon in the bainitic was expelled into un-transformed autenite, which made the austenite more stable. This is called2nd carbon enrichment.(4) When cooled to Ms temperature, martensite transformation happened. But at room temperature, martensite transformation was not completed, hence, martensite-austenite layer-by-layer structure was formed. The results on the characterization of secondary microcracks beneath the fracture surface showed that fracture mechanism changed from nucleation-controlled in CGHAZ into propagation-controlled in ICCGHAZ, due to the exsistence of necklacing M-A constituent. And also, necklacing M-A constituent can obviously reduce the surface energy during the crack propagation stage. Study on the structure-property relationship of X70/X80girth weld metal showed that microstructure of the weld metal, instead of inclusions, is the key factor which influences Charpy impact energy. Toughness of weld metal would notably decrease if necklacing M-A constituent showed up in the weld metal or the wires do not have enough hardenability. But until now, self-protected flux-cored semi-auto arc welding was primarily used in China. So many factors can cause the scattering problem of Charpy impact toughness of girth weld metal, for example, various wires, different welding parameters, environment and climate during welding, complicated microstructures in multi-pass caused by overlapped thermal history and so on. Lot of intensive work needs to be done in the future.