High-strength Low-alloy Steel Weld Research Articles

Improvements in welding continuous cooling transformation diagram of high-strength low-alloy steel weld

The welding continuous cooling transformation (CCT) diagram concerning microstructure evolution in a fine zone of the weld joint provides essential data for microstructure control in practice. In this study, a welding CCT diagram was improved to be elaborate by using the expanded lever rule. Processes for obtaining pieces of phase information are explained in detail. Improvements are: (i) phase areas are clearly presented, (ii) phase volume percentages are given, and (iii) overlapped phase areas are revealed. To meet high-strength requirement, a heat input of 21 kJ cm−1 was referenced from the welding CCT diagram, for achieving acicular ferrite and lath-like martensite. Transformed phase volume fractions are estimated and high strength and good toughness are obtained due to the elaborate phase information.

Study on microstructure evolution and mechanical properties of high-strength low-alloy steel welds realized by flash butt welding thermomechanical simulation

Defects would occur in the weld joint of the wheel rims during the post-flash butt welding (FBW) process suffering from poor plasticity, which will deteriorate the quality and lifecycle of finish products. Therefore, the FBW process of the 440CL high-strength-low-alloy (HSLA) steel was physically simulated and the influence of flash parameters on FBW joints was systematically evaluated in this study. The results showed that the width of heat affected zone increased with accumulated flash allowance (δf) while declined with accelerated flash speed (vf). The recrystallization level would be intensified with increased δf. Meanwhile, the acceleration in vf populated the WZ with a more homogeneous microstructure, higher recrystallization degree and lower dislocation density. The hardness in WZ slightly reduced (202 → 195 HV) as increased δf but obviously dropped (192 → 177 HV) as increased vf. All tensile samples were fractured at the BM location and the tensile properties of FBW joints exhibit a good match with those of BM, with a slight increase in strength (UTS: 468 ~ 493 MPa; YS: 370 ~ 403 MPa) but a mild decrease in plasticity (EL: 39 ~ 44%; RA: 74 ~ 79%). Furthermore, both the joint strength and ductility showed a downward tendency with the increment of δf. However, the strength slightly decreased while the ductility increased with the advancement of vf. These findings would be valuably referential to the real FBW of HSLA steels with optimized microstructure and mechanical performance.

Effect of the Thermal History on Macrostructure and Microstructure Development in High-Strength Steel Welds

The present work addresses the microstructural and macrostructural development in multipass welded joints. It focuses on multiple thermal cycles induced by successive deposition of welding passes. The local welding-related thermal history was related in detail to the evolution of austenite grains during the manufacturing of two high-strength low alloy steel welds. The analytical Rosenthal thermal model was used to identify the thermal cycles experienced within typical weld metal regions. Selected heat cycles were applied to laboratory specimens, taken from the same weld metal, to investigate microstructural evolution during the welding process. Heat cycle experiments, involving full austenitization, showed the persistence of columnar zones resulting from a memory effect of the prior austenite grains during the reverse transformation. Intercritical heat cycles led to white etching, softer regions with high fractions of retained austenite. They also showed that the memory of austenite grains was actually stored in elongated retained austenite particles that remained after complete welding. This memory effect vanished under high peak temperatures (typically, 130 °C higher than Ac3); this was linked to a competition between growth and merging of elongated, intragranular retained austenite particles, and growth of equiaxed, intergranular austenite particles. Finally, a low peak temperature promoted refined, harder final microstructures.

Effect of holding time on strain relaxation in high-strength low-alloy steel welds: An in-situ neutron diffraction approach

In-situ neutron diffraction was employed in the present investigation to study the effects of holding time on the relaxation of residual strains during Post-Weld Heat Treatment (PWHT) for multi-pass High-Strength Low-Alloy (HSLA) steel welds. Different holding time intervals (0.5, 1 and 3 h) were utilized to evaluate and monitor the residual strain relaxation during PWHT. It was found that the holding time has little effects on the strain relaxations as strain relaxations occurs predominantly during the reheating stage of the heat treatment (~80% of strain relaxation). A similar trend is observed for all three holding time arrangements with the high strain relaxation during the reheating stage followed by linear strain relief may confirm the hypothesis that creep (primary and secondary) is responsible for strain relaxation during PWHT. It further confirms the selected soaking temperature of 600 °C has a significant effect on the stress relaxation.This finding is in line with the microstructural characterization studies indicating the formation of sub-grains through polygonization in the Heat Affected Zone (HAZ) and Weld Metal (WM) of the heat treated joints. Ex-situ neutron diffraction which was conducted after PWHT confirmed the in-situ neutron diffraction results and showed slightly higher level of residual stresses for the heat-treated specimen with 1/2 hour holding time (~32% of yield strength of the WM). Furthermore, the tensile, hardness and Charpy impact test results support the notion that a shorter holding time is more beneficial, not only for the cost issues but also for the microstructural and mechanical properties of the welded joints. The findings of this study can be used to optimize the current PWHT codes and standards. It can also be used for the validation studies of the finite element modeling of this process.

Influence of Titanium-Containing Inoculants on the Structure of Metal in the Welds of High-Strength Low-Alloy Steels

We study the influence of titanium-containing refractory inoculants (nitrides, carbides, and intermetallic materials) on the structure of metal in the welds of high-strength low-alloy steels. The character of structural-and-phase transformations and the specific features of fine structures are determined by the method of transmission electronic microscopy with regard for the dislocation density distribution. The influence of structural components formed as a result of introduction of various types of inoculants on the mechanical properties of welded joints is investigated. The roles played by the structural factors (phase composition, subgrain structure, and the distribution of dislocations) in the behavior of local internal stresses and stress concentrators in the process of crack initiation are analyzed. For the case of welding of high-strength low-alloy steels, we determine the optimal compositions of applied inoculants guaranteeing the possibility of getting of high-quality and reliable welded joints with high mechanical characteristics and crack-growth resistance.

Effect of weld seam orientation and welding process on fatigue fracture behaviors of HSLA steel weld joints

Welded steel structures can be exposed to high dynamic forces and failure typically occurring at the weakest zone within a few (less than 102) cycles. In order to investigate low-cycle fatigue fracture in weld joints of high-strength low-alloy (HSLA) steels, we have studied the fatigue properties in different weld seam orientations and welding processes. The fatigue performance showed that with transverse welding, fatigue lives at high strains (2%) were approximately seven times higher than with diagonal welding, and, for laser welded seams, fatigue lives were approximately two times higher than those for the plasma arc welded seams.

Basic Physicochemical Concepts for Controlling the δ-Ferrite Content When Welding with Austenite-Ferrite Materials

Abstract—By the example of the metal of a seam obtained using welding materials based on 10Kh19N11M4F steel which are currently used for the welding of high-strength low-alloy steels, the influence of its chemical composition on the behavior of δ-ferrite throughout the entire temperature range of its existence is shown. On the basis of these studies, the prospects of the use of 10Kh19N11M4F steel for the welding of high-nitrogen corrosion-resistant steels with the retention of their nonmagnetization, including in the area of the welded joint, are shown. The critical parameters determining the behavior of δ-ferrite upon crystallization and subsequent cooling of solid steel are found using thermodynamic modeling. It is shown that the depth of the σ-ferritic transformation and the maximum equilibrium temperature of austenitization used for the interpretation of the experimental data obtained during hot physical modeling of welding are the most important among them. The areas of promising compositions of the metal of the seam in the case of welding of low-alloy high-strength and high-nitrogen corrosion-resistant steels without hot cracks and providing, if necessary, nonmagnetization of the seam are found and depicted on the fragment of the improved Scheffler–Speidel diagram.

In situ neutron diffraction measurement of strain relaxation in welds during heat treatment

ABSTRACTNeutron diffraction (ND) is commonly used to investigate the stress redistribution before and after post-weld heat treatment (PWHT) in welded structures. However, there is a lack of information on the evaluations of strains during PWHT. The present work employed in situ ND to measure the relaxation of residual strains during conventional PWHT in multi-pass high-strength low-alloy steel welds. It was found that strain relaxation occurs principally during the heating stage of the heat treatment. The findings have important economic bearings and can be used to characterise comparable material combinations and optimise the PWHT process for high-strength low-alloy weld joints. This unique information also provides a valuable benchmark for the finite element modelling of this complex process.

Investigating the effects of welding process on residual stresses, microstructure and mechanical properties in HSLA steel welds

One of the important steps in the design and fabrication of welded structures is the selection of the welding process and the filler consumables. This is because these two factors control the mechanics of thermal distribution and the chemistry of the welded join, which in turn affect weld integrity through the resulting microstructure and residual stresses. The present study employed neutron diffraction to investigate the effects of welding process on the residual stresses in high-strength low-alloy steel weld joints made by SMAW (shielded metal arc welding) and combined MSAW (modified short arc welding) and FCAW (flux cored arc welding) processes. A significantly higher level of residual stress was found in the MASW+FCAW combination which was shown to be in line with the microstructural and mechanical properties. Higher levels of residual stresses may be related to the formation of bainite and Widmanstätten ferrite in the weld metal and HAZ of the combined MSAW and FCAW processes.

Effect of welding processes on mechanical and metallurgical properties of high strength low ALLOY (HSLA) STEEL joints

High Strength Low Alloy (HSLA) steels were developed primarily for the automotive industry to replace low-carbon steels in order to improve the strength-to-weight ratio and meet the need for higher-strength construction grade materials. During this period, micro-alloyed or HSLA steels became an indispensable class for different applications like automobile industry, ship building, line pipe, pressure vessels, building construction, bridges, and storage tanks. Different welding techniques are used in this study to evaluate the mechanical properties of weldments of HSLA steel. Weldments are prepared using three welding processes such as shielded metal arc welding (SMAW), flux cored arc welding (FCAW) and friction stir welding (FSW). Friction stir welding (FSW) is a solid state technology that has attracted considerable interest since it was invented at TWI in 1991. Structural integrity issues in welding of HSLA steels are associated with different problems encountered in friction stir welding of these steels. The other important problem in welding HSLA steels is to prevent brittle fracture of welded joints due to increased strength of HSLA steels. The solid state characteristics of FSW eliminate the susceptibility of porosity and cracking. The objective is to determine the best welding process for HSLA steel by evaluating weld metal microstructure and mechanical properties including weld metal tensile strength and Charpy V-notch impact toughness are investigated. Charpy impact and tensile tests are performed on standard notched specimens obtained from the welded and main sections of the material. The hardness distribution measurements on the differently welded specimens are conducted in order to gain a deep insight of different welding processes.

Evaluation of the factors influencing the strength of HSLA steel weld joint with softened HAZ

Softening in the heat-affected zone (HAZ) may occur at welding of high-strength low-alloy steels and might have a negative effect on the static strength of welded joints. This study investigates the influence of HAZ softening in combination with different filler metal strength classes, weld seams, and hot rolled strips on static performance of a GMA-welded joints. The results of these welding experiments were compared and discussed with a numerical investigation on factors influencing the static strength of HSLA steel welds. At the numerical investigation, a systematical study on potential strength influencing factors and a Pareto analysis based on a multiple regression was done and the potential strength influencing factors were ranked according their degree of influence. The comparison of the results of this experimental and numerical study showed a good accordance.

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.

Role of welding flux in formation of weld metal during arc welding of high-strength low-alloy steels

Role of welding flux in formation of weld metal during arc welding of high-strength low-alloy steels

Prediction of element transfer due to flux and optimization of chemical composition and mechanical properties in high-strength low-alloy steel weld

The transfer of elements C, Si, Mn, P and S from slag into the weld metal or from weld metal into the slag and microhardness has been studied using formulated fluxes. The fluxes have been formulated using extreme vertices design with an aim to develop mathematical models for change in element content and mechanical properties versus flux constituents for submerged arc welding of high-strength low-alloy steel. It is found that CaO is the most significant flux constituent and Al2O3 is the second most significant constituent among individual mixtures. CaO·MgO and CaO·Al2O3 among binary mixtures have significant effect on element transfer and microhardness. Developed mathematical models have been checked for adequacy using t-test and analysis of variance (F-test). Flux mixtures’ composition has been provided for optimum chemical composition and mechanical properties. One of the optimum flux mixture with composition, CaO 11.61, Al2O3 12.33, CaF2 15.00 and MgO 39.06, would be providing desirable chemical composition and mechanical properties.

Microstructure-properties correlation in fiber laser welding of dual-phase and HSLA steels

Similar and dissimilar welds of dual-phase (DP) and high strength low alloy (HSLA) steels were made by fiber laser welding (FLW). The welds were characterized with respect to microstructure, micro- and nano-hardness, and tensile properties. The fusion zone (FZ) in the DP welds consisted of fully martensitic structure; whereas HSLA and dissimilar weld FZ microstructure were mixture of martensite and bainite. Analytical transmission electron microscopy (TEM) confirmed bainite structures containing bainitic ferrite laths with intralath and interlath cementite. Precipitation of single variant carbides inside the bainitic ferrite laths were confirmed by measuring the interplanar spacing. The cooling rate in the FZ, estimated using Rosenthal equation, and continuous-cooling-transformation diagrams corroborated the microstructure formed. Nanoindentation was used to verify the hardness of these individual microconstituents, since a much lower nano-hardness for bainite (4.11GPa) was observed compared to martensite (6.57GPa) phase. Tensile failure occurred in the tempered area of the heat affected zone (HAZ) in the DP steel welded, which was confirmed by typical cup-like dimple fracture; likewise failure in the HSLA base metal, which occurred in dissimilar and HSLA welds, indicated distinctive dimple and shear dimple ductile morphology.

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.

Investigation on flux design for submerged arc welding of high-strength low-alloy steel

Fluxes in submerged arc welding of high-strength low-alloy steels are not readily available, flux compositions are not clear and compositions are patented. This study aims at the design, development and optimization of flux for submerged arc welding of high-strength low-alloy steel. Extreme vertices design suggested by McLean and Anderson is used to formulate twenty-one fluxes to study the effect of flux constituents on tensile strength, percentage elongation, impact strength, diffusible hydrogen content and microstructure of the weld metal. Mathematical models for ultimate tensile strength, percentage elongation, impact strength and diffusible hydrogen content for welded specimens versus flux constituents have been developed. From the experiments, it is found that tensile strength and elongation are affected the most with Al2O3 content, whereas CaO and CaF2 contents have significant effect on impact strength. Synergic binary effect of CaO·CaF2, CaO·MgO and Al2O3·CaF2 mixtures is more than other binary mixtures on the mechanical properties. Developed regression models have been checked for adequacy using t-test for regression coefficients and analysis of variance (F-test) for whole regression equation. Finally, optimum flux composition giving optimum mechanical and microstructural characteristics is suggested.

고강도 저합금강 용접금속의 미세조직과 기계적 특성에 미치는 니켈 함량의 영향

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.

Issues in Welding of HSLA Steels

The application of High Strength Low Alloy (HSLA) steels has expanded to almost all fields viz. automobile industry, ship building, line pipe, pressure vessels, building construction, bridges, storage tanks. HSLA steels were developed primarily for the automotive industry to replace low-carbon steels in order to improve the strength-to-weight ratio and meet the need for higher-strength materials. Due to higher-strength and added excellent toughness and formability, demand for HSLA steel is increasing globally. With the increase of demand; other issues like the selection of filler grade and selection of suitable welding process for the joining of these steels have become very significant. This paper discusses the various issues regarding selection of suitable grade and selection of suitable welding process for joining of HSLA steels and issues concerning the structural integrity of HSLA steel welds.

Variation of microstructures and mechanical properties in the post-weld heat-treated HAZ of Cu containing HSLA steel welds

The present study focuses on the effect of Cu and post-weld heat treatment (PWHT) on the microstructure and mechanical properties in the coarse grained heat affected zone (CGHAZ) of Cu containing high strength low alloy (HSLA) steel welds. For this study, HSLA steels with and without Cu were prepared in laboratory. Mechanical properties were estimated by Vickers hardness and Charpy impact tests. As a result, despite PWHT, the Vickers hardness did not change noticeably; however, the impact toughness deteriorated significantly with increasing PWHT time. In case of Cu added steel, intergranular brittle fractures occurred after PWHT, due to the strengthening of the grain interior by precipitation of Cu particles. Additionally, the formation of a soft denuded grain boundary zone also played a significant role in lowering the fracture toughness during PWHT.