JSC Russian Research Institute of Pipe Industry, Chelyabinsk, Russia¹ ; South Ural State University, Chelyabinsk, Russia²:

I. I. Suleymanova, Engineer, Laboratory for Pipe Strength and Welding¹, Postgraduate Student, Dept. of Equipment and Technology of Welding Production², e-mail: suleimanova@rosniti.ru

D. I. Derbenev, Junior Researcher, Laboratory of Pipe Strength and Welding¹, Postgraduate Student, Department of Equipment and Welding Technology²

JSC Russian Research Institute of Pipe Industry, Chelyabinsk, Russia:
V. D. Kvashnin, Head of Welding Technology Section, Laboratory of Strength and Pipe Welding
D.S. Yakovlev, Cand. Eng., Head of Laboratory, Laboratory of Pipe Strength and Welding

N. V. Elistratova, Engineer of the laboratory of Metallurgy and Heat Treatment took part in the work.

The features of the formation of the structure of coarse-grained heat-affected zone (CGHAZ) of a welded joint of pipe steel grade X100 are investigated depending on the cooling rate by simulating welding thermal cycles on thermo-mechanical Simulator – Gleeble 3800. As a result, a continuous cooling thermokinetic (CCT) diagram of austenite decomposition in the CGHAZ metal of pipe steel grade X100 was constructed, which makes it possible to study the structure and kinetics of phase transformations when the steel is exposed to the welding thermal cycle. It was found that the austenite decomposition is accompanied by bainitic and martensitic transformations in the range of cooling rates of 0.1–150 ºC/s. With an increase in the cooling rate, the bainite structure changes its morphology from a globular and coarse acicular to a dispersed acicular with a pronounced packet organization and a lath morphology, islands of the martensite-austenitic (MA)-constituents, which have an acute-angled shape and are distributed at the boundaries between laths, packets and inside them, are gradually transformed into thin layers. The martensite structure appears at a cooling rate of more than 15 ºC/s. Large blocks (crystals) of bainite with MA components can be observed in the structure despite an increase in the cooling rate. Hardness values change from 245–285 to 307–353 HV with the appearance and increase in the fraction of the martensite structure.

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