Relationship between microstructure and impact toughness of weld metals in pipe high-strength low-alloy steels (research review)

Introduction. The modern pipeline industry requires the development of materials of high strength and toughness for the production of steels for oil and gas pipelines. Changes in steel production and rolling technologies have become a challenge for welding consumables and joining technologies. This is more critical for strength levels above 830 MPa, where there are no specific regulations for the approval of welding consumables. Research methods. The failure of stainless steel pipeline welds is becoming a serious problem in the pipeline industry. Multiphase microstructures containing acicular ferrite or an acicular ferrite-dominated phase exhibit good complex properties in HSLA steels. This paper focuses on the results obtained using modern methods of scanning electron microscopy for microstructural analysis, backscattered electrons (BSE) for electron channel contrast imaging (ECCI) and orientation microscopy based on electron backscatter diffraction (ORM), as well as characteristic X-rays for compositional analysis using energy-dispersive X-ray spectroscopy (XEDS) and secondary electrons (SE) for observing surface morphology. Results and discussion. This paper analyzes the characteristics of the microstructure of the weld and its relationship with impact toughness. It is shown that predicting impact toughness based on the microstructural characteristics of steel weld metals is complicated due to the large number of parameters involved. This requires an optimal microstructure of the steel. Satisfactory microstructure depends on several factors, such as chemical composition, hot work processing, and accelerated cooling. Alloying elements have a complex effect on the properties of steel, and alloying additives commonly added to the steel composition include Mn, Mo, Ti, Nb and V. From a metallurgical point of view, the choice of alloying elements and the metallurgical process can greatly influence the resulting microstructure. A longer cooling time tend to improve the toughness and reduce the mechanical strength of weld deposits on high-strength steels. Welding thermal cycles cause significant changes in the mechanical properties of the base material. The analysis showed that impact toughness strongly depends on the microstructure of the multi-pass weld of the material under study, which contains several sources of heterogeneity, such as interdendritic segregation, and the effective grain size can also be a significant factor explaining large deviations in local impact toughness values. Acicular ferrite nucleated in intragranular inclusions has been shown to produce a fine-grained interlocking arrangement of ferrite plates providing high tensile strength and excellent toughness, and is therefore a desirable microstructural constituent in C-Mn steel weld metals. At the same time, discussion regarding the relationship between acicular ferrite and toughness is very complex and still open at present. Relating impact toughness to acicular ferrite, taking into account the top bead, is not a reliable procedure, even for single-pass deposit welding. Impact strength depends on several factors, and the strong effect of acicular ferrite is generally recognized due to its fine-grained interlocking structure, which prevents the propagation of brittle cracks by cleavage. The large-angle boundaries and high dislocation density of acicular ferrite provide high strength and toughness. However, for the same amount of acicular ferrite, different viscosity values may be observed depending on the content of microalloying elements in the steel. An analysis of the results of various studies showed that other factors also affect the impact strength. For example, microphases present along the Charpy-V notch are critical for the toughness of weld metals. The combination of OM, SEM and EBSD techniques provides an interesting method for metallographic investigation of the refined metal microstructure of stainless steel pipeline welds. Conclusion. This review reports the most representative study regarding the microstructural factor in the weld of pipe steels. It includes a summary of the most important process variables, material properties, regulatory guidelines, and microstructure characteristics and mechanical properties of the joints. This review is intended to benefit readers from a variety of backgrounds, from non-welding or materials scientists to various industrial application specialists and researchers.