Effect of Heat Input on the Microstructure and Mechanical Properties of Low Alloy Ultra‐High Strength Structural Steel Welded Joint

Abstract

Microstructural evolution and mechanical properties of an ultra‐high strength structural steel welded joint at different heat inputs are investigated by metal active gas arc (MAG) welding technique. It is found that the microstructure of weld metal (WM) contained high volume fraction of acicular ferrite regardless of heat input. However, the acicular ferrite changes from bainite acicular ferrite (B‐AF) to Widmanstätten acicular ferrite (WF‐AF) with the increase in heat input. The microstructure of coarse grain heat affected zone (CGHAZ) is characterized by complete martensite, a mixture microstructure (lath martensite and bainite), and granular bainte, respectively, with increasing heat input from 7.5 to 10.5 and then to18.5 kJ cm−1. The acicular ferrite in WM nucleated at complex oxide inclusions and the number density of inclusions with the size of ≈0.6–1.2 µm significantly increases with the increase in heat input. The maximum drop of strength (113 MPa) occurs when the heat input is up to 18.5 kJ cm−1. High volume fraction of acicular ferrite in WM improves the impact toughness because of increased proportion of high angle grain boundaries and interlocking structure. The heat input should be higher than 7.5 kJ/cm to avoid the formation of welding cold crack and twinned martensite in CGHAZ.