Mechanism of Improving Heat-Affected Zone Toughness of Steel Plate with Mg Deoxidation after High-Heat-Input Welding

Longyun Xu,Joohyun Park,Jian Yang,Hideki Ono

doi:10.3390/met10020162

Longyun Xu, Joohyun Park + Show 2 more

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https://doi.org/10.3390/met10020162

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Journal: Metals	Publication Date: Jan 21, 2020
Citations: 8	License type: CC BY 4.0

Affiliation: Shanghai University, Hanyang University, University of Toyama

Abstract

In the present study, the mechanism of improving HAZ toughness of steel plate with Mg deoxidation after the simulated welding with the heat input of 400 kJ/cm was investigated through in situ observation, characterization with SEM-EDS and TEM-EDS, and thermodynamic calculation. It was found that intragranular acicular ferrite (IAF) and polygonal ferrite (PF) contributed to the improvements of HAZ toughness in steels with Mg deoxidation. With the increase of Mg content in steel, the oxide in micron size inclusion was firstly changed to MgO-Ti2O3, then to MgO with the further increase of Mg content in steel. The formation of nanoscale TiN particles was promoted more obviously with the higher Mg content in the steel. The growth rates of austenite grains at the high-temperature stage (1400~1250 °C) during the HAZ thermal cycle of steels with conventional Al deoxidation and Mg deoxidation containing 0.0027 and 0.0099 wt% Mg were 10.55, 0.89, 0.01 μm/s, respectively. It was indicated that nanoscale TiN particles formed in steel with Mg deoxidation were effective to inhibit the growth of austenite grain. The excellent HAZ toughness of steel plates after welding with a heat input of 400 kJ/cm could be obtained by control of the Mg content in steel to selectively promote the formation of IAF or retard the growth of austenite grain.

Highlights

In recent years, there is an increasing demand for steel plates with excellent weldability after high-heat-input welding used in the areas of shipbuilding, architectural construction, offshore structures, multipurpose gas carriers, and pipeline fields [1]
Its microstructure becomes coarse and forms a coarse-grained heat-affected zone (CGHAZ), which is a local brittle zone resulting in the deterioration in its toughness
Low-content Mg deoxidation, and high-content Mg deoxidation termed as A, low-content Mg deoxidation (LM), and HM, respectively

Summary

Introduction

There is an increasing demand for steel plates with excellent weldability after high-heat-input welding used in the areas of shipbuilding, architectural construction, offshore structures, multipurpose gas carriers, and pipeline fields [1]. During the thermal cycle of high-heat-input welding, the base metal near the fusion line is heated to 1400 ◦ C or higher for a considerable time. It undergoes a rather low cooling rate in the subsequent temperature zone of phase transformation. Oxide metallurgy technology has been considered as an effective method to improve the heat-affected zone (HAZ) toughness of steel plates after high-heat-input welding [2,3,4]. The oxide metallurgy technology is to make use of oxide particles or other kinds of inclusions and precipitate particles as the nucleation sites of intragranular acicular ferrite (IAF), or for pinning the growth of austenite grain in HAZ of steel plate during the welding process. It is widely accepted that the single-phase inclusions such as the single oxide (Al2 O3 , MnO, and SiO2 ) and the isolated sulfide (MnS)

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