HAZ microstructure and properties of pipeline steels

The weld heat affected zone (HAZ) in steels differs appreciably in both microstructure and properties from the parent steel as a consequence of the thermal cycles involved in either pipe manufacture or during the laying of transmission pipelines. In the former case, two pass welding is often used in producing the individual lengths of pipe which constitute the pipeline; firstly a pass is made filling the inside joint, then a final pass sealing the outer pipe surface. The thermal cycles surrounding such welds are determined by the welding heat input used and will vary with plate thickness. Examples of the range of microstructures encountered for typical pipe plate compositions are reviewed and their influence on the mechanical properties of the seam weld assessed. There are several regions of the HAZ where significant changes in properties may be encountered. The first of these is the coarse grained HAZ (CGHAZ), the focus of this paper, secondly in the intercritical (IC) or grain refined (GR) regions of the HAZ and, thirdly, those regions where the CGHAZ microstructure of the first welding pass is modified by the subsequent weld run on the outside surface. The latter causes tempering of the original CGHAZ structure, whilst there may be additional embrittlement from precipitation or other microstructural changes. The factors leading to the evolution of the CGHAZ microstructure during cooling after welding are, primarily, the austenite grain size resulting from the weld heating cycle and the steel composition. The major microstructural changes are the formation of martensite or mixtures of martensite/bainite and these depend, critically, on the transformation behaviour of the steel. As carbon contents have been reduced over the past 3 decades, increasingly, ‘carbide free’ bainitic microstructures have become a feature of the CGHAZ and some consideration is given to the prospects for developing acicular ferrite HAZ microstructures. The poorer properties associated with the IC and GRHAZ remain and may be exaggerated by higher alloy content used to depress transformation temperature in more recent pipe steel compositions. The role of microalloying in controlling the grain coarsening response is assessed in the light of the size and distribution of the microalloy precipitates in the parent plate. These considerations lead to the conclusion that a significant part of the microstructural changes, including changes in precipitate response to the thermal cycle, in various regions of the HAZ are indirectly linked to the process history of the parent steel. Examples are described for some simple steels. (AU) Copyright © 2013 Companhia Brasileira de Metalurgia e Mineração (CBMM) All Rights Reserved