Relationship between the Microstructure and Impact Toughness of the Coarse‐Grained Heat‐Affected Zone for Offshore Engineering Steels with Different Mg Contents

Abstract

The relationship between the microstructure and impact toughness of coarse‐grained heat affected zone for offshore engineering steels with 0.0002 and 0.0042 wt% Mg, termed as 2 and 42Mg, respectively, is clarified. With increasing the Mg content from 0.0002 to 0.0042 wt%, the average impact energy at −40 °C decreases from 201 to 59 J. The prior austenite grain size (PAGS) increases from ≈92 to ≈163 μm, the average thickness of lath bainites increases from ≈234 to ≈361 nm. The addition of Mg makes the particle increase in number and refine in size, but the amount of the effective pinning particles at the temperature close to 1350 °C in 42Mg is smaller than that in 2Mg, which leads to the larger austenite grain growth rate and larger PAGS in 42Mg. The larger PAGS with the higher bainite transformation temperature and smaller PAG boundary density in 42Mg makes the bainite coarser. So, in 2Mg, the effective bainitic grain size with misorientation above 45° is smaller and the density of high‐angle grain boundaries (HAGBs) is larger, resulting in its higher toughness. Besides, the coalesced bainite containing cementite colonies in 42Mg makes the bimodal bainitic lath thicknesses and heterogeneous microhardness distribution, reducing the toughness.