Yongjoon Kang

Influence of Ti on Non-Metallic Inclusion Formation and Acicular Ferrite Nucleation in High-Strength Low-Alloy Steel Weld Metals

The phase transition behaviors of non-metallic inclusions as a function of Ti content were investigated by monitoring changes in the microstructure and mechanical properties of high-strength low-alloy steel multipass weld metals. Weld metals with Ti contents ranging from 0.007 to 0.17 wt% were prepared using a gas metal arc welding process. The inclusion analysis was performed based on thermodynamic calculations and transmission electron microscopy, accompanied by energy-dispersive spectrometry and selected area electron diffraction. With increase in the Ti content of weld metals, the chaotic arrangement of ferrite laths in the columnar zone was transited to a well aligned arrangement and the impact toughness of the weld metals drastically deteriorated in response to the decrease in the Mn content of the inclusion. The effective inclusion phase for intragranular nucleation contained considerable amounts of Mn and a Mn depleted zone was observed around the effective nucleant.

Factors Affecting the Inclusion Potency for Acicular Ferrite Nucleation in High-Strength Steel Welds

Factors affecting the inclusion potency for acicular ferrite nucleation in high-strength weld metals were investigated and the contribution of each factor was qualitatively evaluated. Two kinds of weld metals with different hardenabilities were prepared, in both, MnTi2O4-rich spinel formed as the predominant inclusion phase. To evaluate the factors determining the inclusion potency, the inclusion characteristics of size, phase distribution in the multiphase inclusion, orientation relationship with ferrite, and Mn distribution near the inclusion were analyzed. Three factors affecting the ferrite nucleation potency of inclusions were evaluated: the Baker–Nutting (B–N) orientation relationship between ferrite and the inclusion; the formation of an Mn-depleted zone (MDZ) near the inclusion; and the strain energy around the inclusion. Among these, the first two factors were found to be the most important. In addition, it was concluded that the increased chemical driving force brought about by the formation of an MDZ contributed more to the formation of acicular ferrite in higher-strength weld metals, because the B–N orientation relationship between ferrite and the inclusion was less likely to form as the transformation temperature decreased.

Mn-Depleted Zone Formation in Rapidly Cooled High-Strength Low-Alloy Steel Welds

The mechanism of formation of an Mn-depleted zone (MDZ) near the inclusion in a steel weld was elucidated based on quantification of MDZ depth and thermodynamic calculations. The effective inclusion phase for intragranular nucleation, which increased as a function of increasing chemical driving force, satisfied the requirements for presence of a considerable quantity of Mn in the phase, and a lower precipitation temperature compared with the solidus temperature of the matrix.

Variation in the Chemical Driving Force for Intragranular Nucleation in the Multi-pass Weld Metal of Ti-Containing High-Strength Low-Alloy Steel

The variation of the Mn-depleted zone (MDZ) around the inclusion during multi-pass welding of Ti-containing high-strength low-alloy (HSLA) steel was investigated by taking the changes in the impact toughness and microstructure into account. As-deposited weld metal specimens were prepared by single-pass, bead-in-groove welding, and reheated weld metal specimens were obtained by a thermal simulation technique. Two types of chemical compositions were prepared, mainly by controlling the Ti content in order to form two types of phases at inclusion/matrix interface: spinel and ilmenite. When the reheating thermal cycle is applied to the as-deposited weld metal, the MDZ depth varied depending on the inclusion surface phase; this could be explained by the competition of the homogenization effect and the dissolution effect, which occurred near the inclusion/matrix interface. In order to enhance the chemical driving force for intragranular nucleation in both as-deposited weld metal and reheated weld metal, the formation of ilmenite phase is recommended.

Influence of Ni on the Microstructure and Mechanical Properties of HSLA Steel Welds

The microstructure and mechanical properties of the high-strength low-alloy steel weld metals with a variation of nickel content were investigated. The weld metals with a variation of nickel content from 2.3 to 3.3 wt% were prepared using Gas Metal Arc Welding process. The amount of acicular ferrite decreased with increasing nickel content; this is accompanied with an increase in the region of bainite and martensite, hence the hardness and tensile strengths were increased with the increase in nickel content, whereas the impact energy was deteriorated.

Correlation Between Microstructure and Low-Temperature Impact Toughness of Simulated Reheated Zones in the Multi-pass Weld Metal of High-Strength Steel

A large fraction of reheated weld metal is formed during multi-pass welding, which significantly affects the mechanical properties (especially toughness) of welded structures. In this study, the low-temperature toughness of the simulated reheated zone in multi-pass weld metal was evaluated and compared to that of the as-deposited zone using microstructural analyses. Two kinds of high-strength steel welds with different hardenabilities were produced by single-pass, bead-in-groove welding, and both welds were thermally cycled to peak temperatures above Ac3 using a Gleeble simulator. When the weld metals were reheated, their toughness deteriorated in response to the increase in the fraction of detrimental microstructural components, i.e., grain boundary ferrite and coalesced bainite in the weld metals with low and high hardenabilities, respectively. In addition, toughness deterioration occurred in conjunction with an increase in the effective grain size, which was attributed to the decrease in nucleation probability of acicular ferrite; the main cause for this decrease changed depending on the hardenability of the weld metal.

Microstructure and Mechanical Properties of Reheated Zonesin the Multi-pass Weld Metal of High-Strength Steel

A large fraction of reheated weld metal is formed during multi-pass welding, which significantly affects the reliability and stability of the welded structures. In this study, the effect of reheating on the mechanical properties and microstructure of high-strength steel welds during multi-pass welding was investigated. Two kinds of high-strength steel welds with different hardenabilities, i.e., welds L (low hardenability) and H (high hardenability), were produced by single-pass, bead-in-groove welding, and both welds were thermally cycled to various peak temperatures to simulate the reheated welds using a Gleeble simulator. In as-welded weld L, acicular ferrite developed extensively in the grain interior, while grain boundary ferrite and Widmanstätten ferrite formed along the prior austenite grain boundaries. The microstructure of as-welded weld H consisted mainly of bainite, with some acicular ferrite and coalesced bainite. The microstructural changes due to thermal cycling were observed by scanning electron microscopy and correlated with the mechanical properties.

Effect of Restraint Stress on the Precipitation Behavior and Thermal Fatigue Properties of Simulated Weld Heat Affected Zone in Ferritic Stainless Steel

Abstract Thermal fatigue life of the automobile exhaust manifold is directly affected by the restraint force according to the structure of exhaust system and bead shape of the welded joints. In the present study, the microstructural changes and precipitation behavior during thermal fatigue cycle of the 18wt% Cr ferritic stainless steel weld heat affected zone (HAZ) considering restraint stress were investigated. The simulation of weld HAZ and thermal fatigue test were carried out using a metal thermal cycle simulator under complete constraint force in the static jig. The change of the restraint stress on the weld HAZ was simulated by changing the shape of notch in the specimen considering the stress concentration factor. Thermal fatigue properties of the weld HAZ were deteriorated during cyclic heating and cooling in the temperature range of 200to 900due to the decrease of Nb content in solid solution and coarsening of MX type precipitates, laves phase, M 6 C with coarsening of grain and softening of the matrix. As the restraint stress on the specimen increased, the thermal fatigue life was decreased by dynamic precipitation and rapid coarsening of the precipitates.Key Words : Thermal fatigue, Ferritic stainless steel, Heat affected zone, Precipitation