Thermophysically simulated weld HAZ and CCT diagram of high strength low alloy pipeline steels

Dilatometry was used to generate continuous cooling transformation (CCT) diagrams for two different L485ME pipeline steels for hydrogen transportation. One material originated from an in-service pipeline, while the other, newer material was taken from plate material. To generate the CCT diagrams, specimens were heated up to peak temperatures of 1250°C using dilatometry to generate coarse-grained microstructure and subsequently cooled with t 8/5 cooling times varying from 2 to 500 s. In addition, heat-affected zone (HAZ) subregions were thermophysically simulated using peak temperatures between 700 and 1250°C at t 8/5 cooling times of 6 s and 15 s. The newer plate material exhibited hardening across the entire investigated HAZ region, whereas the in-service material showed significant hardening only at peak temperatures above 1100°C (15 s) and 1000°C (6 s). These findings provide a basis for understanding the influence of cooling conditions on microstructural evolution and hardness in pipeline steels relevant to hydrogen transport applications.