Abstract
In the present work, the effect of the welding heat input on the microstructure, martensite–austenite (M–A) constituents, and impact toughness of the coarse-grained heat-affected zone (CGHAZ) in offshore engineering steel with Ca deoxidation is studied. With the heat input increased from 50 to 100 kJ/cm, the HAZ toughness decreased rapidly, while the measured microhardness decreases steadily. The grain sizes are increased from 52 to 132 μm, and the width of bainite lath increased from 0.4 to 2 μm. The area fraction of lath bainite (LB) decreased, while the area fraction of granular bainite (GB) increased. The average width of M–A constituents grows from 0.3 to 0.6 μm, and the average length grows from from 0.5 to 0.9 μm. Its area fraction is increased from 5.3 to 8.6% and then decreased to 6.1%, and its number density decreased from 0.7 to 0.2 μm−2. The morphologies of M–A constituents change from dot-like to slender and blocky, which are deleterious to impact toughness. The fracture mechanism changes from ductile to quasicleavage and cleavage as the heat input is increased. As the M–A constituents are always found as the cleavage initiation, they should be responsible for the decrease in HAZ toughness when the heat input is above 100 kJ/cm.
Highlights
Offshore engineering steel is widely used in the manufacture of marine engineering equipment, such as marine platforms, marine energy equipment, submarine oil, and gas pipelines
The matrix microstructure in coarse-grained heat-affected zone (CGHAZ) is divided into lath bainite (LB) and granular bainite (GB) based on its morphology according to the SEM observation, rather than upper bainite and lower bainite as reported in the work of Bramfitt and Speer, 1990
As the M–A constituents are always found as the cleavage initiation, they should be responsible for the decrease in HAZ toughness when the heat input is above 100 kJ/cm
Summary
Offshore engineering steel is widely used in the manufacture of marine engineering equipment, such as marine platforms, marine energy equipment, submarine oil, and gas pipelines. It demands high strength, excellent toughness, fatigue, and corrosion resistance for use in cold environments (Suzuki et al, 2005). It is desired to increase the welding heat input to improve the welding efficiency and shorten the processing time. Oxide metallurgy technology is an efficient way to improve CGHAZ toughness, which is often used in manufacturing steel plates with a thickness larger than 50 mm (Kojima et al, 2004; Suzuki et al, 2005). The addition of strong deoxidizers, Ti, Zr, Mg, etc., can produce the nonmetallic
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