Hydrogen Assisted Cold Cracking Research Articles

Investigations on hydrogen-assisted cold cracking of laser welded AHSS

AbstractThis study aims to investigate the impact of various surface conditions prior to welding on the susceptibility of materials to cold cracking, including an analysis of fracture surfaces. Additionally, a novel method is introduced for quantifying the presence of diffusible hydrogen using thermal desorption analysis (TDA). This method allows for the determination of diffusible hydrogen concentration in thin sheet welded joints without use of welding consumables. Three different cold-rolled Advanced high-strength steel (AHSS) samples with different surface conditions, such as coatings, lubrication, or water, are examined to assess their susceptibility to cold cracking. In addition to measuring the diffusible hydrogen content in both the base material and the coating, the overall hydrogen content of the base material is also measured using the melt extraction (ME) method. The new method for quantifying diffusible hydrogen in weld metal is applied to investigate different welding variations, intentionally introducing hydrogen through coatings and hydrogenous fluids on the sheet surface. By combining the assessment of cracking susceptibility and hydrogen content, a better understanding of critical hydrogen levels leading to hydrogen-assisted cracking (HAC) is achieved. The results of this study demonstrate that the occurrence of cold cracking in specific AHSS samples increases when either lubrication or both coating and water or lubricant are added. Additionally, the presence of diffusible hydrogen in the welds of all materials is found to increase with the introduction of hydrogenous layers to the material. Although a critical hydrogen content is identified, no clear correlation between the amount of hydrogen and cracking susceptibility can be determined. These findings have significant implications for the welding of cold-formed AHSS, particularly in the automotive industry where safety and lightweight design are of paramount importance.

Quenched and Tempered Steels Welded Structures: Modified Gas Metal Arc Welding-Pulse vs. Shielded Metal Arc Welding

Quench and tempered (Q&T) steels are widely used for a diverse range of applications, particularly in the mining and defence industry, where wear and unconventional loading are common. Furthermore, they are particularly prone to hydrogen assisted cold cracking (HACC), imposing a more careful selection of consumables and requiring a comparably higher welder skill level to fabricate defect-free structures. Therefore, the cost of fabrication of welded structures is higher when the more preferred welding technique of shielded metal arc welding, SMAW, is employed. The introduction of the modified pulsed arc mode of depositions, a variation to pulsed arc deposition, has improved the productivity rates and can be utilised by welders with a greater skill variations. In this study, full-strength butt welds of Q&T steel (AS/NZS 3597 Grade 700), with the thickness of 20 mm, are fabricated under a high level of restraint using both conventional SMAW and modified pulse gas metal welding (GMAW-P). The study investigated the economic feasibility of the two deposition modes and the propensity to cracking for the welded joints under high restraint conditions. Utilising the modified GMAW-P resulted in 63% and 88% reduction in the ‘Arc-On’ time and the total normalised fabrication time, respectively. However, strict controls must be implemented, due to the increased propensity to lack of fusion-type defects, to optimise the welding procedure and mediate for such defects if GMAW-P is to provide a techno-economically beneficial alternative to conventional SMAW when welding Q&T steels.

Investigation of Diffusible Hydrogen Concentration in Gas Metal Arc Brazing by Carrier Gas Hot Extraction Method Referring to ISO 3690

Arc brazing is an alternative joining technology well-suited for processing thermally sensitive materials and to produce mixed material connections. Due to the technological similarities of gas metal arc brazing to gas metal arc welding, it can be assumed that the process-related hydrogen input is of similar magnitude for both joining technologies. Since diffusible hydrogen is known to cause embrittlement in metallic materials, it is necessary to know the amount of diffusible hydrogen introduced by different manufacturing processes. Regarding the qualification of welding procedures, hydrogen ingress is an important factor to evaluate the risk of hydrogen-assisted cold cracking, especially when processing high-strength steels. For arc brazing, there is a lack of knowledge about the process-related hydrogen input. Hence, to study the influence of different brazing filler materials and varying levels of heat input on the diffusible hydrogen concentration in arc braze metal, a methodology to determine hydrogen content in arc weld metal in accordance with international standard ISO 3690 based on carrier gas hot extraction was applied to arc brazed specimens. Very low diffusible hydrogen concentrations of about HD = 0.1 to 0.3 mL/100 g were found for GMAB without significant influence of arc energy or filler metal used.

Hydrogen effect on mechanical properties and cracking of creep-resistant 9% Cr P92 steel and P91 weld metal

Martensitic 9% Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen-assisted cracking. The focus of this study was the microstructure and heat treatment effect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition. Tensile tests with hydrogen-free reference samples and electrochemically hydrogen charged samples were carried out; the mechanical properties were assessed and supported by detailed fractographic analysis. Finally, a hydrogen and microstructure-dependent fracture criterion is established. All investigated microstructures showed a hydrogen-influenced degradation of the mechanical properties compared to the hydrogen-free reference samples. The as-welded martensitic P91 weld metal had the highest degree of degradation in the presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a significantly increased risk for hydrogen-assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.

Diffusible Hydrogen Concentration in Drawn Arc Stud Weldments

AbstractThe mechanized drawn arc stud welding is a well established process in steel construction for joining technique for pin shaped elements, especially in the field of composite constructions. Therefore, the process is commonly applied on construction sites under different climatic conditions. The process is characterized by its relatively high heat input due to high current and a weld pool protection that is realized by a ceramic ferrule holding the metal vapor as a shielding gas. However, turbulences inside the ferrule during welding cause contaminations with atmospheric humidity. Furthermore, a high cooling rate results in a strongly hardened heat‐affected zone and a considerable residual stress state in the parent material. Thus, there is a risk of hydrogen assisted cold cracking, especially for stud welding on high strength steels or under adverse welding conditions. The knowledge about the expectable hydrogen concentration is fundamental for evaluating the cold cracking susceptibility. In this study, the effect storage conditions of the consumables and atmospheric conditions are investigated and evaluated regarding their effect on the resulting hydrogen concentration in weld metal. For determination of diffusible hydrogen concentration in stud weld metal a method based on the carrier gas hot extraction is used.

Waiting time before NDT of welded offshore steel grades under consideration of delayed hydrogen-assisted cracking

Offshore wind turbines (OWT) are a major goal of the energy strategy of Germany encompassing the increase of the installed wind power. OWT components are manufactured from welded steel plates with thicknesses up to 200 mm. The underlying standards and technical recommendations for construction of OWTs encompass specifications of so-called minimum waiting time (MWT) before non-destructive testing of the weld joints is allowed. Reason is the increased risk of time-delayed hydrogen-assisted cold cracking as hydrogen diffusion is very slow due to the very thick plates. The strict consideration of those long MWT up to 48 h during the construction of OWTs leads to significant financial burden (like disproportionately high costs for installer ships as well as storage problems (onshore)). In this study, weld joints made of S355 ML were examined in comparison with the offshore steel grade S460 G2+M. The aim was to optimize, i.e., reduce, the MWT before NDT considering varied heat input, hydrogen concentration and using self-restraint weld tests. This would significantly reduce the manufacturing time and costs of OWT construction. To quantify the necessary delay time until hydrogen-assisted cold cracks appear, acoustic emission analysis was applied directly after welding for at least 48 h.

Self-Gathering Effect of the Hydrogen Diffusion in Welding Induced by the Solid-State Phase Transformation.

The hydrogen diffusion in welding was investigated by using thermal-mechanical-hydrogen diffusion sequential coupled procedures based on finite element method. A self-gathering effect induced by the solid-state phase transformation was discovered. Because of the self-gathering effect, the hydrogen concentration in weld metal was accumulated to a peak value which can be larger than the initial hydrogen concentration in molten pool, and subsequently the hydrogen concentration in heat affect zone was redistributed. In multi-pass welding, the gathered effect not only happened inside a weld pass, but also in the inter-pass, which further increased the sensitivity of the hydrogen-assisted cold cracking. Controlling should be adopted to restrain the hydrogen accumulation. Welding stress evolution during the solid-state phase transformation process had limited effect on the hydrogen diffusion.

Techno-economic Feasibility of Modified Pulse Arc Deposition on Thick Section of Quenched and Tempered Steel

Quenched and Tempered (Q&T) steels welded structures that have numerous applications, particularly in the defence industry. However these steels are particularly prone to Hydrogen Assisted Cold Cracking (HACC) and require a highly-skilled welder to fabricate defect-free structures. This is due to the selection of the manual metal arc welding process of shielded metal arc welding (SMAW). The introduction of Modified Pulsed arc mode of depositions; a variation to Pulsed Arc deposition, has advanced deposition rates and can be employed by welders with a greater variation in skill. In this body of work, full strength butt welds are fabricated on 20mm, sections of Q&T AS/NZS 3597 Grade 700 steel under a high level of restraint using Modified Pulse Gas Metal Welding (GMAW-P) and conventional Shielded Metal Arc Welding (SMAW). The study investigates the economic feasibly of the two modes of deposition and the propensity for cracking when welded under high restraint. The study concluded that modified GMAW-P achieved reduction of 63% in the ‘Arc-On' time and an 88% reduction in the total normalised fabrication time. However, due to the increased propensity to lack of fusion type defects, strict controls must be employed in optimising the welding procedure to mediate for such defects if GMAW-P is to provide a techno-economically beneficial alternative to conventional SMAW when welding Q&T steels.

Hydrogen diffusion mechanism of the single-pass welded joint in welding considering the phase transformation effects

To date, the hydrogen diffusion mechanism in the welded joint of high strength steels is poorly understood. A thermal-mechanical-hydrogen diffusion sequentially coupled procedure was built to investigate the hydrogen diffusion behaviors during the whole welding process with considering the nonlinearity of the material and the effects of the phase transformations. By using this modelling, the dehydrogenations of the welding conditions were comparably studied. A first-time numerically observed self-gathering effect occurred during the transformation of undercooled austenite in the cooling process, which caused a hydrogen concentration jump even larger than the initial concentration of the molten pool at the bottom of the weld metal. That significantly influenced the subsequent hydrogen diffusion of the whole welded joint. The retained hydrogen was dominantly driven by the normalized concentration difference. Surprisingly, the stress-assisted diffusion was limited in the joint unless the hydrogen concentration was at a very low level. The hydrogen concentration in both weld metal and heat affect zone should be treated with caution for preventing the hydrogen assisted cold cracking. The results also indicate that there is a more economical and energy saving treatment in contrast with the over conservative ones commonly used in engineering.

Failure Analysis of Weld Metal Cracking in Weld Joint of API 5L X 75 Pipelines Steel

The development of steel alloy which result thin walled, high strength API 5L X75 grade, a restricting parameter controlling widespread use of X75 is the susceptibility to weld metal tracks. The excellent weld ability of this grade of the pipe steel has enhanced the potential for the use of high strength cellulose consumable like E6010 in root pass welding, but the risk of hydrogen assisted cold cracking (HACC) is also increased because of the high strength weld metal. This investigation outlines , the use of grade (E6010) of commercial cellulose consumable to assess conditions leading to hydrogen assisted cold cracking in the diluted weld metal. The research contained clarification of the link among microstructure; preheat temperature and hardness amounts for the weld consumable and its effect on cracking susceptibility. The cracking morphology studies indicated that there are many ways in which the crack can propagate in the weld metal and HAZ region. The mode of cracking observed was microvoide coalescence.

Susceptibility of Acicular Ferrite and Upper Bainite Microstructures to Hydrogen Assisted Cold Cracking Propagation

Acicular ferrite (AF) and upper bainite (UB) are microstructural constituents commonly found in ferritic weld metals. Both microstructures are formed within a similar temperature range and by the same type of transformation mechanisms. They have however, substantially different morphologies and microstructural features that govern both their mechanical properties and hydrogen embrittlement susceptibility. This work shows that despite substantial microstructural differences, the mechanical properties of both microstructural constituents were quite similar. However, the microstructural differences were found to significantly affect the hydrogen crack propagation resistance. Hydrogen assisted cold cracking (HACC) propagates along a path of least resistance through the surrounding microstructure. The unit crack path was significantly shorter for AF than for UB, which implied more frequent changes in direction and thus increased dissipation of energy from the crack driving force. These results suggest that AF, possessing fine interlocking grains and high angle grain boundaries (HAGB), increases the localised resistance to HACC propagation more than UB due to the impediment of brittle, cleavage-like crack propagation at HAGB’s.

Prediction of welding stresses in WIC test and its application in pipelines

In the present study, the Welding Institute of Canada (WIC) restraint test was used to simulate the restraint conditions of full-scale girth welds on energy pipelines to ascertain the influence of welding process parameters on welding stresses. Finite element models are developed, and validated with neutron diffraction measurements, to evaluate the welding stresses for under-matched, matched and over-matched welds. The effects of heat input, wall thickness and variable restraint lengths of WIC sample are systematically investigated. As a practical outcome, this work can help in selection of the appropriate restraint length for WIC tests to simulate the specified stress conditions in the pipeline, and, ultimately, reduce the risk of Hydrogen Assisted Cold Cracking (HACC) in high strength low alloy.This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.

Hydrogen-assisted cold cracking in welded joints of press-hardened 22MnB5

Over the last decade, press hardening has become all pervasive for automotive body-in-white design. Press-hardened boron micro-alloyed steels are widely used in modern body structure for high safety standard and lightweight achievement. Considering the welding of press-hardened components, some special considerations have to be taken into account compared to conventional deep-drawing steel grades. The mechanical properties of the welded joints are influenced by the hydrogen content in the base metal as well as the weld metal. Hydrogen absorption may result from the press-hardening process, as well as the welding process. Due to the martensitic microstructure and the high strength level, a critical hydrogen content may cause hydrogen embrittlement or hydrogen-assisted cold cracking (HACC). This paper focuses on the determination of the diffusible hydrogen content in the base metal after the press-hardening process, as well as in gas metal arc (GMA) welded joints of 22MnB5. Hydrogen source during press hardening is the humidity at the austenitizing temperature in the furnace atmosphere. During welding, for this study hydrogen was deliberately introduced by hydrogenous fluids on the sheet surface. The diffusible hydrogen content in the base metal and the weld metal was quantified by thermal desorption mass spectroscopy (TDMS) technique. The influence of the diffusible hydrogen on the mechanical properties of the welds was determined by four-point-bend testing. The time to fracture was identified using acoustic emission technique.

Micromechanical characterisation of weld metal susceptibility to hydrogen-assisted cold cracking using instrumented indentation

Hydrogen-assisted cold cracking is generally accepted to be the consequence of a critical concentration of hydrogen trapped within a susceptible microstructure and subjected to a threshold level of stress. Traditionally, hardness has been used as a proxy for establishing the critical limits above which the risk of a hydrogen crack propagating is considered significant. However, developments in the steel-making process, in particular thermomechanically controlled processing, has brought into question the suitability of empirical hardness limits developed using older generation steels. In this paper, a safe welding boundary was established for single-pass root runs for API 5 L X70 steel welded with E6010 electrodes. Across this boundary, it was shown that hydrogen cracks were present in welds with hardness’s well below the traditionally accepted threshold of 350 HV. This paper explores the use of nanoindentation as means of quantifying the susceptibility of welds deposited on high-strength low-alloy steels, using shielded metal arc welding, to hydrogen-assisted cold cracking. It is suggested that the use of the hardness/elastic modulus (H/E) ratio, which is directly related to the yield strength of a material, is a more suitable parameter to predict weld metal hydrogen-assisted cold cracking (HACC) susceptibility than is the hardness alone.

Towards the establishment of weldability test standards for hydrogen-assisted cold cracking

Industry and research have long desired the establishment of standards for weldability testing in regards to hydrogen-assisted cold cracking formation. This would have the obvious advantage of allowing data to be reliably compared between different research labs. But making decisions regarding standards requires some careful thought and agreement on i) how test parameters affect test results, ii) what exactly needs to be measured, and iii) how test results should be interpreted and reported. Our depth of understanding on these points has matured significantly over time and, while there is not always universal agreement, it is at least possible to start highlighting factors important to standards. This paper examines these factors, including the welding parameters, restraint, hydrogen, and cracking index. When comparing different alloys having different thermal characteristics, the use of constant welding parameters (common practice) will result in variable weld penetration and weld pool shape, which can influence grain shape and microstructural features, which can result in inequitable weldability comparisons. Welding on test coupons having different dimensions can affect restraint, which will influence the residual stresses around the weldment. High restraint usually results in higher crack susceptibility. Also, hydrogen content present in a weldment depends on the thermal history, welding parameters, and surrounding atmosphere humidity, with high hydrogen contents associated to great cracking susceptibility. Finally, the selection of an appropriate cracking index is required for data analysis. Quantifications of crack length and minimum preheat temperature are common indexes used for comparison. Critical stress and hydrogen content are other indexes. But how well these indexes actually represent weldability are contentious issues. This paper will examine and quantify these issues in detail, thus providing the reader with an appreciation of all things that must be considered when preparing a standardized procedure for weldability testing.

Determination of hydrogen input in welded joints of press-hardened 22MnB5 steel

Over the last decades, press hardening or hot forming has become all pervasive for automotive body-in-white design. Press-hardened boron-microalloyed steels are widely used in modern body structure for high safety standard and lightweight achievement. Considering the welding of press-hardened components, some special considerations have to be taken into account compared to conventional deep-drawing steel grades. The welding process results in a significant hardness drop in the heat-affected zone (HAZ) which may lead to a change in failure mode or premature failure. Furthermore, the mechanical properties of the welded joints are influenced by the weld metal hydrogen content. Hydrogen pickup of press-hardened steel may result from the heat treatment or the welding process. Due to the martensitic microstructure and the high strength level, a critical hydrogen content may cause hydrogen embrittlement or hydrogen-assisted cold cracking (HACC). This paper contributes to the determination of the diffusible hydrogen content in resistance spot-welded and gas metal arc-welded joints of press-hardened boron-microalloyed steel. It also promotes the understanding of the effect of diffusible hydrogen on the mechanical properties of those joints. Hydrogen was deliberately introduced during welding by wetting of the sheet surface with hydrogenated fluids. The hydrogen content in the weld metal was quantified by thermal desorption mass spectrometry (TDMS) technique. Finally, the influence on the mechanical properties of the welded joints was determined based on a simple component test. A GMA-welded component was set under a constant static load and evaluated for delayed hydrogen-assisted cracking.

Development of safe optimized welding procedures for high strength Q&T steel welded with austenitic consumables

High strength quenched and tempered (Q&T) steels offer obvious economic benefits originating from their advantageous strength to price and weight ratios. These steels are usually welded using ferritic consumables and for this combination the risk of hydrogen assisted cold cracking (HACC) is high. The use of austenitic stainless steel (ASS) consumables has great potential to significantly improve this issue. Yet, there are no guidelines for determination of safe level of preheat for welding ferritic steels with ASS consumables. For this reason manufacturers adopt this parameter from procedures developed for conventional ferritic consumables thus significantly limiting the benefits ASS consumables are capable to deliver. Productivity could be further enhanced by identifying the upper interpass temperature threshold, thus reducing the stand-off times. Aim of this work is to develop safe highly optimised procedures for welding of high strength Q&T steel with ASS consumable.

Numerical investigations on cold cracking avoidance in fillet welds of high-strength steels

Industry faces a growing demand for high-strength structural steels with yield strengths of up to 1,300 MPa in order to cope with increasingly higher strength requirements in engineering. Higher strength levels are achieved by a special coordinated production process and an adapted chemical composition. Nevertheless, disastrous damage cases with high-strength steels have occurred in the past. The sensitivity to mechanical property degradation by hydrogen increases dramatically with strength. This phenomenon leads to hydrogen-assisted cold cracking. T-joints with fillet welds made from one side with an included angle of 60° were examined for their cold cracking behavior. Based on the T-joint, a modified heat input, even interpass temperature, plate thickness, and length ones were examined. The diffusion behavior and the effectiveness of different post-weld heat treatments in joints were simulated. The results of post-weld heat treatments are illustrated in practical hydrogen removal heat treatment diagrams. It is noticed that the T-joint is subject to a very high risk of hydrogen-assisted cold cracking (HACC). Contrary to other joints, its most critical area for cracking is not the weld metal but the heat-affected zone surrounding area of the root pass. The simulation shows that HACC in the T-joint can only be avoided by applying a sufficient post-weld heat treatment.

Structural and Hardness Gradients in the Heat Affected Zone of Welded Low Carbon Martensitic Steels

Welding of low carbon martensitic steels with yield strengths above 690 MPa requires careful attention to the welding procedure to avoid hydrogen assisted cold cracking (HACC) and to minimise degradation of the mechanical properties of the weldment. Investigations of the microstructural and hardness gradients in the heat affected zone (HAZ) of these types of steels revealed that the peak hardness does not occur in the grain coarsened heat affected zone (GCHAZ) adjacent to the fusion boundary, as normally observed for ferritic steels, but is displaced towards the grain refined region (GRHAZ). This phenomenon, referred to as the displaced hardness peak (DHP) effect, is considered to arise when the hardenability of the steel is sufficiently high to produce the same microstructure in the both the GC and GR heat affected zones, but the enhanced structural refinement of the GRHAZ increases the hardness and strength above that of the GCHAZ. Implications relative to the susceptibility of the weldments to HACC are discussed.

Diffusible Hydrogen Content Depending on Welding and Cooling Parameters

Safety of welded components made from high-strength materials is assured above all by the avoidance of hydrogen-assisted cold cracking which occurs during fabrication and may have effects during service. In this context, continuous advancement of the procedures and standards for hydrogen analysis is required. Exact determination of the weld metal diffusible hydrogen according to ISO DIS 3690 is therefore highly relevant as regards the welding and cooling parameters to be maintained. In the investigations presented in this paper, the influence of the welding process parameters and of the cooling parameters on the diffusible hydrogen content was examined and the procedures and parameters specified in ISO DIS 3690 for the weld specimen preparation were checked. The influence of cooling conditions, arc length and contact tube distance (CTD) on the weld metal hydrogen concentration was studied for a high-strength flux cored wire electrode and a high-strength solid wire electrode. Measurement of the diffusible hydrogen was carried out by means of carrier gas hot extraction using a sensitive and long-term stable thermal conductivity detector (TCD).