Influence of Welding Thermal Cycle on Grain Growth, Microstructural Transformations and MA Formation on CGHAZ of TMCP Steel

Abstract

High heat input welding is one of the alternatives adopted by shipyards and wind tower manufacturers for increasing productivity. However, the thermal cycles generated during welding may cause microstructural transformations that are detrimental to the toughness in the heat-affected zone, especially in the coarse-grained region (CGHAZ). In this work, the grain growth and the microstructural changes in the CGHAZ of 355 MPa class steel, produced by controlled rolling followed by accelerated cooling - TMCP (Thermo-Mechanical-Control-Process) were compared to a conventional steel of the same grade. High heat inputs welding conditions were simulated in the dilatometer and Gleeble®. It was observed that previous austenite grain size in the CGHAZ follows a log-normal distribution and the TMCP steel showed significantly lower values of grain size. This result was associated with pinning effect of TiN precipitates homogeneously distributed and coherent with the matrix in regions where the maximum temperature is not enough for its dissolution. The results showed that it is possible to obtain excellent toughness at low temperature in high heat input welded joints when using TMCP steel.

Key-words:
CGHAZ; TMCP Steels; Dilatometry; Gleeble®; High heat input welding