High Strength Steel Welds Research Articles

Metallurgical Design Rules for High-Strength Steel Weld Metals

A review of a dilatometric analysis of selected Fe-C-Mn high-strength steel shielded metal arc weld metals showed that balanced Ti, B, Al, O, and N additions reduced the austenite-to-ferrite transformation-start (TS) temperature. These microalloy additions must match the following aim levels for composition control: Ti at 400 ppm (0.04 wt-%), B at 40 ppm (0.004 wt-%), Al at 200 ppm (0.020 wt-%), O at 400 ppm (0.04 wt-%), and N preferably below 80 ppm (0.008 wt-%) to ensure effective deoxidation, form complex inclusions, and distribute them to enable development of highly fracture-resistant refined weld metal microstructures. It may be wiser to avoid the rich and lean ends for these microalloy additions, except N, which should be held at the lean end, preferably much below 80 ppm (0.008 wt-%). The balanced Ti, B, Al, O, and N additions offered nearly a 100°C shift in lowering the Charpy V-notch (CVN) test temperature for either 28 or 100 J absorbed energy. Dilatometric evaluations of reheated weld metals showed that 1) the balanced Ti, B, Al, O, and N additions lowered the actual TS temperature by about 60°C compared to the calculated austenite-to-ferrite transformation temperature obtained from the constitutional equation; 2) N with more than 100 ppm (0.010 wt-%) effectively nullified the beneficial effects of Ti, B, and Al additions in lowering the transformation temperature; and 3) at N content much below 80 ppm (0.008 wt-%), both a lower TS temperature and a narrow start-to-finish (TS–Tf) temperature range helped in achieving exceptional weld metal CVN impact toughness.

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.

Eliminating the Brittleness Constituent to Enhance Toughness of the High-Strength Steel Weld Heat-Affected Zone Using Electropulsing.

The evolution of the martensite–austenite (MA) constituent in the heat-affected zone (HAZ) of high-strength steel FH690 welds when subjected to electropulsing (EP) treatment was investigated herein, with the aim of eliminating brittle MA to enhance toughness. The features induced by EPT were correlated with the microstructure and fractography through scanning electron microscopy and electron backscatter diffraction analyses, together constituting an impact property evaluation. The Charpy V-notch impact results showed EPT could improve toughness of the HAZ from 34.1 J to 51.8 J (the calibrated value was 46 J). Examinations of EP-treated microstructure showed a preferred Joule heating: at the site of the MA constituent, the cleavage fractography introduced by the MA constituent was substituted with ductile dimples with various sizes. Decreases in grain size of 40% and 47% for the matrix and the retained austenite, respectively, were achieved; while for regions without the MA constituent, microstructural modification was negligible. The temperature rise at sample surface was less than 60 °C. The mechanism behind this favorable Joule heating for the MA constituent was correlated with the electrical properties of the MA constituent in contrast with martensite matrix. The toughness enhancement of the HAZ was thus attributed to the elimination of the coarse MA constituent. The present investigation suggested that electropulsing, characterized as a narrow-duration current, is a promising method for preferred elimination of brittle factors and thus improving the toughness of HAZ of high-strength steel within a limited region with a width less than 2 mm.

High-temperature phase evolution of the ZnAlMg coating and its effect on mitigating liquid-metal-embrittlement cracking

Ternary zinc-aluminum-magnesium (ZnAlMg) alloy coatings are the focus of significant attention in the automotive and steel industries due to several advantages over traditional Zn-based coatings. Currently, the literature on this type of coating is limited and focuses mainly on their corrosion resistance and room temperature tensile properties. To assess the relevance of ZnAlMg coatings in current manufacturing processes such as hot stamping and welding of advanced high strength steels (AHSS), it is essential to understand their high-temperature performance, particularly their resistance to liquid metal embrittlement (LME) cracking. This study showed that the ZnAlMg coating had complete resistance to LME cracking at a temperature of about 900 °C, which is traditionally recognized as the temperature at which the highest levels of LME susceptibility are observed in the different families of AHSS. Elemental distribution analysis confirmed that due to an increase in the testing temperature, the lamellar eutectic microstructure of the coating dissolved into the Zn-matrix, with the constituent elements, Al and Mg, segregating towards the steel substrate and the coating surface, respectively. This led to the in-situ formation of a uniform α-Fe(Zn, Al) layer at the steel/coating interface which prevented the direct contact of liquid metal with the steel substrate, resulting in complete suppression of LME at high temperature. Numerical calculations of interdiffusion flux were used to investigate the diffusion behavior of the elements of interest at the interface which indicated that the α-Fe(Zn, Al) layer formed due to the high diffusion rate of Al towards the Fe substrate at 900 °C. The effectiveness of the α-Fe(Zn, Al) layer in mitigating LME was evaluated by calculating the work of adhesion, which showed that this layer preserved its integrity under an applied tensile load, successfully mitigating the initiation of LME. The findings of this study offer valuable insights into developing new avenues for advancing LME resistant coatings by utilizing ternary Zn-based alloy systems.

Model description of hydrogen-induced damage evolution in high-strength steel weld metal based on results of a novel test method

In this paper, the development of a novel method for testing welded tensile specimens under two-stage external tensile loading is presented. The test method is intended to investigate the susceptibility of submerged arc welded high-strength steel specimens of type S690QL-S3Ni2.5CrMo to hydrogen-induced damage. Multiple cracking events within one specimen prove that the locally prevailing boundary conditions decisively determine the hydrogen-induced damage sequences.By means of SEM and EBSD analyses on specimens after test completion, interrelationships about the microstructural damage behavior are being examined. The crack tip environments are of particular importance here, since complex stress and strain conditions tend to cause hydrogen accumulation and further propagation of inter- and transgranular cracks here. In the vicinity of crack tips, retained austenite is of special importance. This metastable phase might undergo a deformation-induced transformation into crack-sensitive martensite and simultaneously release high local hydrogen contents as a former hydrogen trap. Finally, a model description including various sections of the stress- and hydrogen-induced damage evolution is derived from the empirical-analytical results.

New insights into the fracture behavior of advanced high strength steel resistance spot welds

While the automotive industry is striving for the reduction of car body weight to increase the fuel efficiency and to reduce CO2 emission without compromising the safety and crashworthiness of vehicles, a new generation of advanced high strength steels (AHSS) have emerged as excellent candidates to meet these requirements. However, their integration into the car body structure is associated with welding-related problems. This research utilizes a novel approach to establish a fundamental correlation between welding parameters, microstructure and mechanical performance of AHSS resistance spot welds. In-situ micro-cantilever bending experiments are executed and analyzed in a quantitative manner to evaluate the effect of texture and post-welding heat treatment on the local fracture toughness of spot welds. A striking finding is that, through a switch from single to double pulse weld scheme, the texture of martensite formed in the fusion zone becomes responsible for a significantly higher fracture toughness of the area in front of the pre-crack. In addition, it is found that paint baking heat treatment also results in a much enhanced fracture toughness through tempering of the martensitic microstructure. A quantitative correlation is made between the micro-scale fracture toughness and macro-scale mechanical performance of advanced high strength steel welds.

Microstructure analysis and hardness prediction based on microstructure evolution theory in multi-pass welded joint

During multi-pass butt welding of high-strength steel (HSS) thick plate, it is difficult to estimate the hardness performance of thick weld since multithermal cycles can drive complex microstructure evolution. In this study, butt-welded joint of 75 mm-thick Q690 HSS was prepared by multi-pass shielded metal arc welding (SMAW) in advance; meanwhile, experiments were carried out to characterize microstructure and measure hardness distribution in butt-welded joint. Then, the welding temperature history of butt-welded joint was numerically examined. Based on the irreversibility of Martensite and Bainite, the microstructure evolution model was modified to predict the complicated microstructure evolution in multi-pass welded joint. Compared with experimental data, the validity of modified microstructure evolution model was verified. With the application of simulated thermal cycles and material chemical compositions as input parameters, volume fraction distribution of each phase was obtained by modified microstructure evolution model and hardness value was calculated by hardenability algorithm. Both experimental and predicted results exhibited that microstructure in weld metal (WM) is mainly composed of granular Bainite with a small amount of Ferrite and Martensite, and heat-affected zone (HAZ) consists of predominantly lath Martensite. Hardness distribution in butt-welded joint obtained by the prediction method is in accordance with the measurement. Average hardness value in WM and BM is 290 VPN and 250 VPN. A sharp decrease in HAZ hardness can be found from the fusion line to base metal (BM). The maximal value in the HAZ close to fusion line and the corresponding prediction deviation is 6%.

Resistance spot welding of high-strength DP steel and nano/ultrafine-grained IF steel sheets

In the current study, microstructural characteristics, mechanical behavior, and failure analysis of resistance spot welded joints for high-strength dual-phase (DP) steel and nano/ultrafine-grained interstitial free (IF) steel was investigated. The DP steel side indicated three different heat-affected zones including intercritical, fine-grained, and coarse-grained. The volume fraction of martensite in the coarse-grained heat-affected zone was higher than in other regions due to more growth of γ grains. The IF steel side exhibited three different subregions in the heat-affected zone, namely, ultrafine-grained, fine-grained, and coarse-grained heat-affected zone. In the coarse-grained heat-affected zone, the iron oxide was effective on the microstructure by suppressing the grain growth. The uniformity of microhardness in the fusion zone of 10 kA welding current was more than other samples due to faster homogenization of melt. All welded samples indicate hardening at fine-grained heat-affected zone (on both sides) and ultrafine-grained heat-affected zone (on the interstitial free steel side). With increasing the current of welding from 6 kA to 10 kA, the peak load increased from 7.38 kN to 11.23 kN and the energy absorption improved from 6.01 kJ to 11.72 kJ due to increasing the diameter of the fusion zone. All samples failed through partial thickness-partial pullout mode. Cleavage facets and shear dimples were observed in all samples, which indicated a mixture of brittle and ductile fractures.

Resistance spot welding of advanced high strength steel for fabrication of thin-walled automotive structural frames

This investigation presents the process parameter optimization of resistance spot welded (RSW) advanced high strength steel (AHSS) joints using response surface methodology (RSM) for fabrication of thin-walled automotive structural frames. Resistance spot welding (RSW) is used to minimize solidification cracking, heat affected zone (HAZ) softening and distortion related problems in joining of thin advanced high strength steel (AHSS) sheets. The three factor – five level central composite design (3 × 5 - CCD) matrix pertaining to the minimum number of experiments was chosen for designing the experimental matrix. The polynomial regression analysis was employed to develop the empirical relationships between RSW parameters and performance characteristics of AHSS joints. The 3-dimensional (3D) response surfaces were developed using RSM which shows the optimal process window to enhance the strength of spot joints. The RSW joints made using welding current of 5.0 kA, electrode pressure of 4.0 MPa, and welding time of 1.50 s displayed maximum lap tensile shear fracture load (LAP-TSFL) of 21.7 kN, cross tensile shear fracture load (CROSS-TSFL) of 17.65 kN, and nugget zone hardness (NZH) of 589 HV0.5 respectively. The results of ANOVA indicates that welding current, electrode pressure and welding time significantly influences the LAP-TSFL, CROSS-TSFL and NZH of joints respectively. The superior TSFL and NZH of joints made using optimized RSW parameters is attributed to the evolution of needle/lath martensite in nugget zone .

Induction Heating in Underwater Wet Welding-Thermal Input, Microstructure and Diffusible Hydrogen Content.

Hydrogen-assisted cracking is a major challenge in underwater wet welding of high-strength steels with a carbon equivalent larger than 0.4 wt%. In dry welding processes, post-weld heat treatment can reduce the hardness in the heat-affected zone while simultaneously lowering the diffusible hydrogen concentration in the weldment. However, common heat treatments known from atmospheric welding under dry conditions are non-applicable in the wet environment. Induction heating could make a difference since the heat is generated directly in the workpiece. In the present study, the thermal input by using a commercial induction heating system under water was characterized first. Then, the effect of an additional induction heating was examined with respect to the resulting microstructure of weldments on structural steels with different strength and composition. Moreover, the diffusible hydrogen content in weld metal was analyzed by the carrier gas hot extraction method. Post-weld induction heating could reduce the diffusible hydrogen content by −34% in 30 m simulated water depth.

Oxide Metallurgy Technology in High Strength Steel: A Review.

Oxide metallurgy technology plays an important role in inclusion control and is also applied to improve the weldability of high strength steel. Based on the requirements of the weldability in high strength steel, the influencing factors of weld heat affected zone (HAZ) as well as the development and application status of oxide metallurgy technology are summarized in this review. Moreover, the advantages and difficulties in the application of rare earth (RE) oxide metallurgy technology are analyzed, combined with the performance mechanism of RE and its formation characteristics of fine and high melting point RE inclusions with distribution dispersed in liquid steel. With the weldability diversities of different high strength steels, the research status of weldability of high strength steel with high carbon equivalent and the effects of RE on the microstructure and properties of HAZ are discussed, and some suggestions about further research in the future are proposed.

The Toughness of High-Strength Steel Weld Metals

Low-temperature phase transformation (LTPT) welding consumables are a new class of welding wires developed to mitigate hydrogen-induced cracking in the welding of high-strength steels without preheating or postweld heat treatment. LTPT weld metals have a high strength, but their toughness needs further investigation. LTPT weld metals predominately contain a martensite microstructure, which is necessary to achieve high strength; however, martensitic weld metals containing oxide inclusions have relatively poor toughness. Three welding processes — gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), and hot wire GTAW — were investigated. Optical microscopy, scanning electron microscopes, and transmission electron microscopes were employed for characterization. The role of the shielding gas in the formation of oxide inclusions in LTPT weld metals was investigated. The formation of oxide inclusions in the weld metals was related to the CO2 in the shielding gas. When 100% Ar or a pure inert shielding gas mixture was used for all three welding processes, oxide inclusions were greatly reduced, and the weld metal toughness improved considerably, matching the base metal toughness. The mechanism by which inclusions promote fracture propagation in the weld metal was proposed.

Investigation of Weld Root Defects in High-Power Full-Penetration Laser Welding of High-Strength Steel

The currently available high-power laser shows promising opportunities for the welding of thick plates in a single pass. However, weld-root defect frequently occurs when a high-power laser is used to join thick plates in a full-penetration mode, which has a significantly adverse effect on the serviceability of the weld joint. The purpose of this work is to understand the defect formation mechanism and reduce these defects through controlling welding parameters. In this study, the characteristics of weld root defects were investigated using a 10 kW fiber laser using a program of experiment and theoretical analysis. The corresponding defect formation mechanisms were discussed based on the bottom molten pool behaviors observed by the high-speed camera. The results showed that there were four types of weld-root appearances as follows with an increase of linear heat input from 300 J/mm to 1000 J/mm: weld-root humping (30 mm/s), sound weld (25 mm/s), weld sagging (20 mm/s) and excessive weld sagging. The remedies for reducing weld-root defects were also presented to obtain sound weld bead by optimizing welding parameters. Weld-root humping was formed due to the quasi-full-penetration keyhole. Weld sagging resulted from the imbalance of the hydrostatic pressure and surface tension in the condition of a through keyhole. It was also found that the sound weld was formed when a through keyhole and a proper molten pool size were obtained. Thus, the state of the keyhole and molten pool geometry were the major factors that affect weld-root defects.

Multiple-wire submerged arc welding of high-strength fine-grained steels

The Paton Welding Journal, 2022, №01. International Scientific-Technical and Production Journal «The Paton Welding Journal» «The Paton Welding Journal» has been published monthly since 2000 in English, ISSN 0957-798X. «The Paton Welding Journal» is a cover-to-cover English translation of the «Avtomaticheskaya Svarka» (Automatic Welding) journal. The «Avtomaticheskaya Svarka» journal has been published monthly since 1948 in Russian, ISSN 005-111X.

Analysis of welding solidification crack in narrow gap laser welding of high-strength steel

Analysis of welding solidification crack in narrow gap laser welding of high-strength steel

Post-Treatment of Welding Joints of High Strength Steels II.:Improving Residual Stress Condition – Overview

High strength steel welding joints fatigue strength could be improved by changing the as weld condition with post-weld treatment. These post-weld treatments are classified to two categories which are the weld geometry and residual stress condition improvement methods. Post-weld treatment methods of high strength steel welding joints by improving weld geometry were overviewed in the first part of our article. The post-weld treatment methods for improving the welding joint residual stress condition are also applicable in high strength steels, but some consideration have to take in place when applying these techniques. The aim of this paper to review the welding joint residual stress condition improvement techniques considers application of these techniques in high strength steels.

Model Description of Hydrogen-Induced Damage Evolution in High-Strength Steel Weld Metal Based on Results of a Novel Test Method

Model Description of Hydrogen-Induced Damage Evolution in High-Strength Steel Weld Metal Based on Results of a Novel Test Method

Pulse spray gas metal arc welding of advanced high strength S650MC automotive steel

With an increasing demand for safer and greener vehicles, mild steel and high strength steel are being replaced by much stronger advanced high strength steels of thinner gauges. However, the welding process of advanced high strength steels is not developed at the same pace. The performance of these welded automotive structural components depends largely on the external and internal quality of weldment. Gas metal arc welding (GMAW) is one of the most common methods used in the automotive industry to join car body parts of dissimilar high strength steels. It is also recognized for its versatility and speed. In this work, after a review of GMAW process and issues in welding of advanced high strength steels, a welding experiment is carried out with varying heat input by using spray and pulse-spray transfer GMAW method with filler wires of three different strength levels. The experiment results, including macro-microstructure, mechanical properties, and microhardness of weld samples, are investigated in detail. Very good weldability of S650MC is demonstrated through the weld joint efficiency > 90%; no crack in bending of weld joints, or fracture of tensile test sample within weld joint or heat affected zone (HAZ), or softening of the HAZ. Pulse spray is superior because of thinner HAZ width and finer microstructure on account of lower heat input. The impact of filler wire strength on weldability is insignificant. However, high strength filler wire (ER100SG) may be chosen as per standard welding practice of matching strength.

Microstructure, Toughness and Hardness of the Simulated HAZ Area of Steel S1300QL

The introduction contains information (found in available reference publications) concerning the weldability of steel S1300QL. The introduction also presents general information concerning the effect of the microstructure and the chemical composition of filler metals used in the welding of high-strength steels on the mechanical properties of weld deposit. The subject of simulation tests (discussed in the article) was structural steel S1300QL having a yield point of more than 900 MPa S1300QL. Simulations involved both single (Tmax = 1250°C) and double welding thermal cycles (Tmax = 1250°C + 600°C, Tmax = 1250°C + 760°C and Tmax = 1250°C + 900°C) as well as cooling times t8/5 = 3 s, 5 s and 10 s. Specimens with simulated HAZ areas were subjected to impact strength tests performed at a temperature of -40°C and +20°C, Vickers hardness tests (HV10) and microscopic metallographic tests involving the use of light microscopy. The test results are presented in related diagrams and photographs. The final part of the article contains a discussion concerning test results and concluding remarks. The tests revealed that, in terms of the test steel, the number of repetitions of thermal cycles having pre-set parameters did not explicitly translated into changes of impact energy values concerning the simulated HAZ areas. The tests also revealed that recommended thermal cycles making it possible to obtain the required combination of the high toughness and hardness of the simulated HAZ area of steel S1300QL (similar to that of the base material) were double thermal cycles of maximum temperature Tmax1 = 1250°C + Tmax2 = 600°C and Tmax1 = 1250°C + Tmax2 = 900°C combined with cooling times t8/5 = 5 s and 10 s and all numbers of repeated thermal cycles.

Analysis of a High-Strength Steel SMAW Database

Recently, Dr. Glyn M. Evans posted a large shielded metal arc (SMA) weld metal (WM) database on the ResearchGate website (researchgate.net). This database contains more than 950 WM compositions, along with their respective WM tensile and Charpy V-notch (CVN) impact properties. In particular, the CVN impact properties list the test temperatures that achieved 28 and 100 J impact energy for each WM composition. While the availability of this SMA WM database is a valuable and rare gift to the welding community, how could the welding community analyze this database to gain valuable insights? This paper utilizes a constraints-based model (CBM) as a simple and effective framework to organize and analyze this very large Fe-C-Mn SMA WM database. A CBM is built on the metallurgical principle that one needs to lower relevant solid-state phase transformation (i.e., austenite decomposition) temperatures to improve WM strength and fracture toughness while simultaneously reducing carbon content and Yurioka’s carbon equivalent number (CEN) to improve the weldability of high-strength steels. To this end, a CBM identifies and simultaneously solves several statistical (regression) equations that relate the chemical composition of high-strength steel WM with Yurioka’s CEN and selected solid-state phase transformation temperatures related to austenite decomposition. The results of the current effort demonstrate that the analysis of Evans’s shielded metal arc welding database using a CBM as a framework reaffirms that controlling carbon content, the value of the CEN, and calculated solid-state phase transformation temperatures, particularly the difference between the calculated Bs (bainite-start) and Ms (martensite-start) temperatures, is critical to developing and identifying high-performance, high-strength steel welding electrodes. A dual approach that manipulates the contents of principal alloy elements such as C, Mn, Ni, Cr, Mo, and Cu, and adds controlled amounts of Ti, B, Al, O, and N, appears to offer the best means to lower relevant solid-state phase transformation temperatures to produce high-strength and high-toughness WMs.