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

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.

Characteristics and Evolution of Inclusions in Ti‐Bearing Peritectic Steel with Mg Treatment

“…According to the SEM–EDS mapping results, the inclusion is identified to be MgO·Al 2 O 3 ·TiN·MnS inclusion with the diameter of about 2 μm. It is therefore demonstrated that the Mg containing complex inclusions with the size above 1 μm can effectively promote the nucleation and growth of intragranular ferrite, which is consistent with the results of our previous researches, [ 29,32,55,56 ] so as to prevent the generation of transverse corner cracks. In addition, the formed complex inclusions of the Mg containing oxides+(Ca, Mn)S, (Mg, Mn)S, or (Mg, Ca, Mn)S can also reduce the harmfulness of the extended MnS [ 57 ] to improve the transverse impact toughness of steel plate.…”

“…[ 27,28 ] With the development of injection metallurgy technology, Mg can be smoothly fed into molten steel by using Mg cored wire technology. At present, Mg treatment has been used in thick plate for the high heat input welding, [ 29–32 ] sulfur free‐cutting steel, [ 33,34 ] and ferritic stainless steel, [ 35,36 ] but the application of Mg treatment in peritectic steel is rarely reported.…”

In Situ Observation of Microstructural and Inclusions Evolution in High-Strength Steel Deposited Metals with Various Rare Earth Pr Contents

“…The existence of Si and Mn in the inclusions is caused by the absorption of Si and Mn in the base metal and welding wire during the growth of inclusions. Mn in the inclusions is also an important factor affecting AF nucleation [ 28 , 29 , 30 , 31 ].…”

Oxide Metallurgy Technology in High Strength Steel: A Review

“…A range of ship plate steel products for high heat input welding were developed [43]. Furthermore, Yang's team from Shanghai University had long studied the effects of Mg treatment on inclusions in steel and the properties of welding HAZ, and they concluded that Mg treatment could refine inclusions and improve the toughness of HAZ [44][45][46]. Scholars from Northeastern University found that the low temperature impact toughness of HAZ were optimum when the Zr content in low carbon steel was 0.003%.…”