1. Introduction
Hydrogen embrittlement is a degradation phenomenon that occurs, particularly in high-strength and alloyed steels, in which hydrogen entrapment occurs, and the microstructure becomes brittle, leading to a pre-mature failure under stress. It can occur through several mechanisms, such as hydrogen enhanced decohesion (HEDE), hydrogen localized plasticity (HELP), hydrogen formation, stress-driven diffusion, etc. In HEDE, hydrogen reduces the cohesive strength of metallic bonds at grain boundaries or phase interfaces, promoting cracking, whereas, in the HELP mechanism, hydrogen enhances dislocation mobility, leading to localized plastic deformation and microcrack formation in regions of high-stress concentration1). In certain steels, hydrogen can combine with specific alloying elements to form brittle hydrides, leading to cracking under applied stress. Hydrogen migrates to high-stress concentrations like crack tips or inclusions, accelerating crack propagation. Manufacturing processes such as casting, welding, or galvanization, externally corrosive environments, cathodic protection systems, and hydrogen storage systems are major sources of hydrogen. Atomic hydrogen enters the metal lattice, converts into diffusible hydrogen soon after, and results in a decrease in the toughness and ductility of the microstructure. The sources of the hydrogen, the microstructural susceptibility, the presence of tensile stress, and the operating environment are important variables that affect hydrogen embrittlement1,2). Modifying the susceptible microstructure, lowering the hydrogen content, or reducing the tensile residual stress levels for a particular steel and joint effectively reduces the risk of hydrogen embrittlement3,4).
High-strength steel (HSS) is a type of micro-alloyed steel that offers greater strength and corrosion resistance than traditional carbon steel. With a minimum tensile strength of 440 MPa, HSS is commonly used to construct structures in shipbuilding, aerospace, and automotive5-17). Automobile weight reduction is necessary in the manufacturing sector to increase vehicle fuel efficiency. Automobile body parts are increasingly being made using high-strength and ultra-high-strength steel sheets with thin thicknesses to minimize weight. Despite their extreme strength, Advanced/ultra-high- strength steel sheets (AHSS/UHSS) with tensile strengths over 1000 MPa are frequently used in the aerospace and automotive industries. However, issues arise in cold stamping ultra-high strength steel sheets due to excessive forming stress, limited formability, tool malfunction, and hydrogen-induced delayed fracture6,18,19).
Since ultra-high-strength steels (UHSS) possessing martensitic or bainitic microstructure have high strengths and high hardness levels, they are more vulnerable to hydrogen-induced cracking. Hydrogen embrittlement is a major concern in the context of UHSS, considering their great mechanical characteristics like high strength- to-weight ratios and exceptional resistance to deformation. However, these steels’ vulnerability to HE could compromise their reliability and efficiency20-22).
A prominent joining method for UHSS in automotive industries is resistance spot welding (RSW). The localized heating and cooling cycles connected to RSW may create an environment favourable to hydrogen penetration and accumulation, particularly in the weld nugget and heat-affected zone. Enhancing the durability and safety of welded UHSS structures requires understanding the interplay between hydrogen entry and the microstructural alterations RSW brings. Laser welding of advanced high-strength strength is also considered one of the commonly used fabrication methods in automobiles, energy sectors, etc, where the chances of hydrogen embrittlement is a major concern considering the microstructure of the steel. High Entropy Alloys (HEAs) have attracted significant interest owing to their distinctive properties, notably the lattice distortion characteristic, which is advantageous for hydrogen storage. J chen et al synthesized TiZrFeMnCrV high- entropy alloys with varying vanadium concentrations and analyzed the influence of vanadium on hydrogen storage characteristics23). To better understand the mechanisms of hydrogen embrittlement in RSW and laser joints of AHSS, this research review will analyze the different variables that affect the microstructural susceptibility, methods used to estimate hydrogen entrapment and associated embrittlement behaviour and understand the mechanism involved in the fracture, etc.
Considering the important aspects of hydrogen embrittlement associated with the Laser weld joints and Resistance Spot Weld (RSW) joints of HSS going to discuss in this article is categorized and major discussion topics are shown in Table 1.
Table 1
Major and important techniques used to identify and/or to estimate the impact of Hydrogen embrittlement of high strength steel weld (Laser and RSW) joints
To understand the diffusion of hydrogen inside a microstructure, it needs to be charged in a hydrogen-free microstructure, where it can get entrapped in different interfaces, grain boundaries, or imperfections of the microstructure. Hydrogen charging methods used for the process are discussed below.
2. Hydrogen Charging Methods
2.1 Testing for Hydrogen Embrittlement in Ultra High Strength Steels and Stampings by Acid Immersion
Any steel that possesses a tensile strength of ~1000 MPa or more is susceptible to hydrogen embrittlement. This standard test procedure determines how susceptible, uncoated, cold-rolled, and hot-rolled Ultra High Strength Steels (UHSS) are to hydrogen embrittlement. When choosing any UHSS, the test results should be considered along with the material’s strain state and the part’s operating environment.
There are a few factors that need to be considered before hydrogen charging. Samples were tested without zinc-based or other anodic coatings that will react with the acid in the bath, i.e., the evolution of hydrogen from zinc and other metallic anodic coatings at low pH will generate hydrogen in a manner that is not representative of service environmental conditions, artificial charging the samples with hydrogen and thereby invalidating the test. Testing is performed at room temperature using 0.1 N hydrochloric acid as the test solution. It is prepared from concentrated acid using deionized or distilled water, and the solution must be in sufficient quantity to cover the samples in the testing container completely. An initial tensile test was performed to estimate the yield strength of the material, a strain gauge was attached to the specimen, and the fixture was to bend the specimen to achieve a pre-determined stress level, represented as a percentage of the baseline yield strength as determined in the tensile test performed on the unstrained material. After pre-straining, the samples are set aside for 24 to 48 hours in ambient environmental conditions, and strains are re-measured. Hydrogen embrittlement tests can be performed by removing the strain gauge from the pre-strained specimen and immersing it in 0.1N hydrochloric acid. Test specimens must be visually inspected every 2 hours for the first eight hours and then every 24 hours. If possible, set up a camera system to take photographs at regular intervals, ideally no less. Visual observation of the cracks ends the test; if no cracks are visually apparent after 120 hours, remove the specimen from the solution and confirm that no cracks are apparent on any of the three samples (SAE Intrernational-J3215)24). The experimental set up and the material cracked after testing are shown in Fig. 1.
2.2 Electrochemical Hydrogen Charging
An aqueous solution of 3% NaCl and 0.3% NH4SCN, subjected to a current density of 0.5 A/m² at ambient temperature, can function as the charging solution, with a platinum wire utilized as the counter electrode. A Schematic of hydrogen charging in a laser weld joint is shown (Fig. 2(a-c)). Hydrogen was electrochemically injected into the sub-tensile and tensile shear test (TST) specimens of resistance spot welded tensile specimens (Fig. 2(d)) utilizing a galvanostatic mode on a Gamry Instruments Interface 1010 device25). Charging durations ranged from 0 hours (no charging) to 24 hours. Cleaning and hydrogen charging techniques were used for the samples, similar to those used for hydrogen measuring and mechanical testing. Before beginning the mechanical tests, the samples were electro-galvanized immediately following cathodic charging and left in the open for a full day.
Fig. 2
(a)-(b) Electrochemical hydrogen charging of laser weld joints. Geometry and dimension of tensile test specimen used for slow strain rate test (SSRT) of (c) laser weld joints and (d) Resistance spot weld joints25)
The hydrogen charging region has been defined as the aggregate surface area of both sides of specimen (20 mm×20 mm×2mm) and the lateral area (20 mm×1.4 mm×4), subsequent to the masking of the extraneous area designated for hydrogen charging in the specimen.
2.3 Galvano static charging and electro galvanizing
Galvanostatic charging was applied to alter the materials’ hydrogen concentration. The steel sheets were galvanized as they were delivered. First, the samples were de-coated for two minutes using an acid solution containing 1.1 g/l HCl and 3.5 g/l hexamethylenetetramine. Depending on the objective of the study, several sample preparations were made. The samples used in the forming and slow strain rate tests (Hydrogen embrittlement susceptibility test) were polished to a surface roughness of 0.4 μm. The original edge morphology was not maintained by grinding the samples for U-bending. Before charging, the samples were cleaned for 10 seconds using an acid solution of 1.1 g/l HCl with 3.5 g/l Hexamethylene Tetramine. The contaminants were then removed using distilled water and acetone26).
3. Quantifying hydrogen entrapped in different traps of automotive steel weld joints
Hydrogen absorption can happen in steel in various processes, including steelmaking, press hardening, welding, and electrochemical processes like e-coating27). Hydrogen embrittlement occurs at remarkably low average concentrations. Since 1875, it has been established that a hazardous one is diffusible hydrogen. Conventional wisdom suggests that diffusion is essential for hydrogen accumulating at stress concentrations, such as sharp cracks. As a result, the consequences of hydrogen are more significant than what a low average concentration would imply28). Even in the complicated process of producing automobile body parts, hydrogen can readily pass through the crystal lattice of steel because of its small mass and atomic size. Different traps where hydrogen can be accumulated are schematically shown (Fig. 3), “Diffusible hydrogen” refers to hydrogen atoms or molecules that can pass through steel materials at different points during the manufacturing process. It is crucial to study the methods and theories used to identify the hydrogen traps in materials and to quantify the hydrogen content in respective traps, especially in AHSS, which is widely used in automobile body parts.
Fig. 3
Schematic representation of hydrogen traps in steels, (a) Interstitial sites, (b) Surface traps, (c) Sub-surface traps, (d) grain boundary traps, (e) dislocation traps and (f) Vacancy traps30)
Welding is an inevitable and important process during manufacturing automobile bodies; depending upon the welding technique used for joining, the hydrogen content can also vary. That can be attributed to the welding atmosphere, such as the cleanness of the material and the baking condition of consumables in the case of arc welding. Shielding gas, coolant, and many other factors can also contribute to hydrogen penetration. Hence, most care is necessary in the case of welding steels with a susceptible microstructure. Several methods are used to quantify the hydrogen entrapped in steel weld joints1,18,28,29). Extraction of hydrogen and determination of hydrogen embrittlement susceptibility or delayed cracking susceptibility are critical processes that indicate the chances of premature failure of high- strength steel weld joints that require longer life. Since most of the automotive parts are made of AHSS, our discussion focuses on RSW, laser welding, and the associated hydrogen embrittlement mechanism of AHSS.
Among the different methods used to extract hydrogen entrapped in the weld joints applies the theory of activation energy (Ea, m2/s), the energy required to release the diffusible hydrogen atom entrapped in the material. It can be termed as the energy required for interstitial displacement of hydrogen within the metallic lattice, which indicates the material or microstructural capability to bind hydrogen. It can subsequently, after a while, escape from the workpiece; this process is known as desorption. On the other hand, hydrogen may also gather in certain areas known as “hydrogen traps,” creating internal pressures that risk the integrity of the material by causing cracks to form27). The diffusion coefficient D(m2/s) represents the time hydrogen takes to move through the metal. The activation energy required for hydrogen release also changes depending on the trap. Considering this concept, hydrogen extraction from materials is carried out, and thermal desorption mass spectroscopy (TDMS) is a common technique used to measure diffusible hydrogen content18,30,31). Major factors influencing the activation energy of hydrogen are expressed in the Kissinger equation (Eq. (1), 32)), where Φ is the heating rate, Tp corresponds to a peak temperature, and R is the gas constant. In the TDMS Method, a standard specimen is heated at a defined heating rate, and hydrogen starts to release with increasing temperature. It can be identified from the desorption rate vs temperature plot in Fig. 4(a). The hydrogen desorption rate was integrated over time and divided by the specimen’s weight to get the hydrogen concentration. How the extracted hydrogen from the material can be quantified is shown in Fig. 3 using thermal desorption analysis (TDA). Here, the time required to effuse 50% of the hydrogen in the sample at a temperature is expressed as t50%. That can be used to measure the diffusion coefficient using Eq. (1).
Fig. 4
Martensitic steel specimens underwent 24-hour hydrogen charge, followed by three analyses, (a) change in hydrogen desorption rate as heating rate increases, (b) Thermal desorption analysis employing peak deconvolution and (c) Activation energy estimates for various hydrogen trapping locations estimated using the Kissinger equation35)
The hydrogen content was examined in most of the studies since it influences hydrogen embrittlement. Fig. 4(a) shows the hydrogen desorption rates at various quantities of entrapped hydrogen. It can be seen that all the major peaks are below 300 ºC. Earlier studies reported the reversible hydrogen33) is trapped at activation energies of 30kJ/mol or lower34,35). At the same time irreversible hydrogen33) is trapped with activation energy of 60kJ/mol or higher require temperature over 300 ºC for desorption34,35).
3.1 Thermal desorption spectroscopy method
Thermal Desorption Spectroscopy (TDS) was used to measure the concentration of hydrogen. Fig. 3 shows a schematic of the TDS equipment. The samples were put inside a glass conduit inside the furnace, and the thermocouple was put outside in a different glass chamber in a comparable spot. Before measurement, the system was vacuumed to get the internal pressure from 10-8 to 10-2 Pa. Under extreme vacuum, the sample’s and the thermocouple’s surrounding circumstances were quite similar34). The thermocouple was used to measure the radiation furnace’s temperature during heating. As the temperature rises, the absorbed atomic hydrogen (Hab) desorbs from the substrate and recombines at the sample surfaces. Consequently, quadruple spectrometers were used to measure the amount of desorbed hydrogen molecules, and a computer was used to record the amount. The TDS equipment in this work was calibrated using standard TiH2. It heated at a rate of 0.1°C/s.
3.2 Estimating different Hydrogen traps
Measuring the activation energy required to release the hydrogen from different trapping sites can be helpful in understanding the characteristics of those regions that are basically decided by the microstructure. Typically, grain boundaries are regarded as weak trapping sites with an activation energy (Ea) of less than 60 kJ/mol. The GB angle can affect the Ea range for GBs, i.e. LAGBs, 17.2< Ea<18.6 kJ/mol, for HAGB, in between 53 to 59 kJ/mol. For cementite, the activation energy (Ea) ranges from 8.4 to 18 kJ/mol, indicating it is a weak trapping site; nonetheless, the Ea for the F-martensite phase contact remains unreported. In dual-phase steels, the ferrite/martensite contact serves as an efficient hydrogen trap. F-Mi/f, TDS peak temperature observed for DP steel was 150ºC at 100ºC/h, for the same interphase, the TDS peak temperature observed for AHSS equal to 210ºC at 100 ºC/h. Dislocations are classified as weak trapping sites with activation energy required to reach 26-29 kJ/mol. TiC precipitates serve as strong trapping. sites, with activation. energies ranging from 68 to 137 kJ/mol, depending. on the interface coherency. The addition of solute Mo, to form (Ti, Mo) C, needs higher activation energy32). Few scientists demonstrated that hydrogen is pinned to different interphases using cryo-transfer atom probe tomography. Developing materials that are more resistant to hydrogen embrittlement should benefit from this direct examination of hydrogen trapping36).
Fig. 2(a) shows the hydrogen desorption peaks generated at different heating rates used to quantify the hydrogen trapped in martensitic steels2). Different peaks marked in Fig. 2(b) corresponding to different heating rates and temperatures represents the possibility of different locations; using that, the activation energy is calculated and confirmed the possible traps exist in the microstructure.
4. Hydrogen absorption and embrittlement of different AHSS
Various types of AHSS have been developed to meet the demanding specifications of present-day vehicle architectures in the ongoing development of AHSS for automotive applications. Since the steels possess a balanced strength and ductility, first-generation AHSS, such as dual-phase and TRIP steels, laid the foundation for subsequent developments. Formability was further improved by introducing twinning-induced plasticity (TWIP) steels in the 2nd generation. Medium manganese steels, Q&P steels, and tempered bainitic ferrite- TBF, are coming under the 3rd-gen AHSS. Different alloying and heat-treating techniques are required for each of these steel varieties to have the requisite microstructural properties. The following discussion will explore these various approaches, looking at how Hydrogen embrittlement affects AHSS performance and the significant efforts to use them more commonly in automobile design37). High-strength automobile parts are frequently made by hot stamping of Al-Si-coated 22MnB5 steel, although this process has disadvantages such as high energy costs, low die durability, and reduced productivity due to die quenching. Warm stamping, which uses air cooling and lower austenitizing temperatures, can help to reduce these problems. It can be done with medium-Mn steels (3-7 wt% Mn, 0.1-0.2 wt% C). However, 22MnB5 steel is unsuited for this operation due to its high austenite. formation (Ac1) temperature and low hardenability2,19,38). The high tensile characteristics (Y.S=1180 MPa, U.T.S=1850 MPa, T.E=9.3%) of the warm-stamped specimen were superior to those of the hot-stamped 22MnB5 steel (Y.S= ~1000 MPa, U.T.S=~1500 MPa, T.E=~8%) due to its fine-grained (FG) martensitic microstructure containing Nano-NbC particles19,35).
Hydrogen embrittlement susceptibility of such AHSS was difficult to predict since the microstructure is very complex. Lyu et al. discussed different medium (3-7 wt%) Mn martensitic steels used in automotive applications and were examined for hydrogen absorption and embrittlement resistance39). The 7wt% Mn specimens had the most hydrogen content when electrochemically hydrogen-charged since it possesses largest reversible hydrogen26) trap density. A higher Mn concentration was associated with a lower embrittlement resistance with the same hydrogen content. This is probably because of hydrogen segregation and grain boundary decohesion. Nonetheless, this situation was ameliorated by the addition of B, which promoted grain boundary cohesion and inhibited H segregation into the grain boundaries39). The strength-ductility of DP steels was diminished due to their susceptibility to hydrogen absorption. Hydrogen absorption weakens the interfaces between ferrite and other constituents, leading to a considerable degradation in tensile characteristics, as revealed by fractographic studies and tensile tests37).
Effects of laser beam welding (LBW) and RSW on the microstructure evolution, load endurance capabilities, corrosion resistance, and HAZ softening of dual phase 1000 grade UHSS joints welded in lap joint design examined by Rajalingam et.al. where the softening due to martensitic tempering and grain coarsening in HAZ was one among the major reason for the failure of both RSW and LBW joints6). The following section, in detail, discusses the hydrogen embrittlement phenomenon of AHSS associated with the LBW and RSW operations.
4.1 Laser welding and influence of hydrogen embrittlement in HSS
Fu et al. investigated the hydrogen-induced (HI) stress corrosion cracking behaviour of SUS301L-MT laser arc hybrid welded (LAHW) joints25). This study involved specimens extracted from base metal-BM, heat-affected zone-HAZ, and weld metal-WM in SUS301L-MT SS laser arc hybrid welded joints, which underwent in situ electro-chemical hydrogen charging during slow. Strain rate tensile. testing (SSRT, discussed in section 5.1.1, widely used method in laser and RSW weld joints to estimate the cold cracking susceptibility) to investigate the HE behavior of each zone in addition to microstructures, crack propagation paths, and fracture surface morphologies analysis. The findings showed that during SSRT tests, deformation-induced martensite transformation took place in the BM and HAZ specimens, producing dimples in some places and cleavage morphologies in the fracture center. The WM specimen’s fracture surface was entirely covered in dimples. The α,/γ contact near the specimen edge is where the cracks in the BM and HAZ specimens started during in situ hydrogen charging. The δ-ferrites found at the borders of the austenitic grain served as the initial site of the cracks in the WM specimen. Following the initiation of cracks, the propagation of cracks would be induced by “hydrogen enhanced local plastic deformation” and “hydrogen enhanced decohesion” mechanisms, which would be combined with the hydrogen concentration distribution and the stress field at the front of the fracture tips. Consequently, the fracture surface developed primarily fracture characterization of intergranular cracking, transgranular cleavage, and secondary fractures25). Recently, Hopf et al., Xue et al., and Rajalingam et al. discussed the possible chances of hydrogen embrittlement UHSS, and a few suggested SSRT as one of the efficient methods among the different methods developed to estimate the embrittlement behavior28,30,31). Recently, Hopf et al. suggested self-restraining tests of HSS thin sheets welded without any consumables and external loading to understand the cold crack susceptibility.
Anton Hopf and co-authors studied three different cold-rolled AHSS samples with varying conditions of the surface, such as coatings lubrication, to assess their susceptibility to cold cracking. They quantified the diffusible hydrogen entrapped in the weldments and how it promotes the hydrogen cracking tendency. A greater understanding of the critical hydrogen levels leading to hydrogen-assisted cracking (HAC) is obtained through combining the assessment of hydrogen concentration and cracking susceptibility. The findings of this investigation show that adding lubricant, coating, or both enhances the likelihood of cold cracking, particularly in AHSS samples. Furthermore, it is discovered that adding hydrogenous layers to any material increases the amount of diffusible hydrogen present in the welds of that material. While a critical hydrogen level is found, no discernible relationship is found between the hydrogen content and cracking susceptibility. These results have important relevance for cold-formed AHSS welding, especially in the automobile sector, where lightweight design and safety are critical considerations30,31).
In a recent study on laser weld joints of QP980 steels, it is unknown how complicated microstructures experience hydrogen embrittlement (HE). During a slow strain rate tensile test, the joint subjected to hydrogen pre-charging fractured at the inter-critical heat-affected zone (ICHAZ), exhibiting a relative elongation loss of 84.8%. About 60% of hydrogen-induced (HI) microcracks started inside martensite. Transgranular martensite propagation and intergranular ferrite/martensite interface propagation were the joint’s fracture propagation pathways influenced by hydrogen. The joint’s hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) actions combined to form the HE mechanism29).
4.2 Estimating the hydrogen delayed cracking susceptibility in Laser weld joints.
4.2.1 Bead on Plate Test
This test, unlike traditional cold crack testing methods, utilizes self-restraint testing of specimens fabricated from thin sheet materials without requiring external force or welding consumables. A proposed test procedure is established, and the system contains a specific sample geometry (Fig. 5), a clamping device, and fracture documentation. It is demonstrated that the weld’s location on the specimen is an important factor and that the sensitivity to cold cracking rises as edge distance does. Two distinct steels’ susceptibilities to delayed cracking are effectively ranked using the test methodology and the most critical seam position28). The study also proved that lubrication significantly affects a laser-welded component’s vulnerability to cold cracking. This testing approach is suitable irrespective of the material’s coating; it permits the utilization of various specimens and stack thicknesses or combinations for both analogous and heterogeneous joints. It is also feasible to utilize materials or high-strength alloys of increased thickness with identical geometry in the form of weld beads on plates. This specimen geometry and testing procedure can be utilized to assess the vulnerability to cold cracking in high-strength structural materials, particularly fine-grained structural steels that are susceptible to delayed cracking.
Fig. 5
Sample geometry for estimating cold cracking (Hydrogen delayed cracking) susceptibility of laser weld joints30)
The test procedure is simple: once the specimen is prepared as per the dimension in the figure, pre-treatments such as cleaning, oiling, pre-straining, H-charging, heat-treatment, etc., depending upon the purpose. This is followed by welding preparation, such as gap set-up and clamping of the samples in fully overlapped conditions, and welding needs to be performed. Visual inspection using a magnifying glass or stereo microscope within the time intervals directly after 1h, after 24h, and after 7 days30).
Cold crack testing at a specific condition can determine the probability of hydrogen cracking of steel weld joints, and the TDMS method used at respective conditions also measures the hydrogen. The test was conducted for a number of conditions by Anton Hopf et al. and the team; when shielded by argon, welding enables the release and conversion of bound hydrogen into diffusible hydrogen, making it resistant to the surrounding atmosphere. Conversely, uncovered welding permits the absorption of more than 0.50 ppm hydrogen from ambient moisture. The presence of hydrogen- containing layers, including zinc coatings or lubrication on the uncoated substrate, increases the hydrogen concentration in the weld seam by approximately 0.14 ppm and 0.22 ppm, respectively. Introducing a lubricant or water onto the coated steel surface drastically raises the hydrogen content. The incorporation of zinc with forming oil results in an increase of approximately 0.8 ppm in hydrogen content relative to uncoated welds. Moreover, welding coated material in the presence of condensed water significantly increases hydrogen content, augmenting it by 1.69 ppm.
4.2.2 Slow strain rate test (SSRT)
The tensile samples were subjected to a steady, slow strain rate (0.005 mm/min) in the SSRT until fracture occurred. Different amounts of hydrogen are charged into the laser-welded specimen using any charging method, for example, in Fig. 2, using the electrochemical method. After testing it can be correlated with H-free specimens and the impact can be measured (Fig. 6). It is visible that, with increasing H-charging time, the tensile properties such as UTS, YS and total elongation (TEL) of the specimen after the test were decreasing (Fig. 6(b)). The fracture site of weld joint without hydrogen pre-charging was situated in BM, whereas the fracture site with hydrogen pre-charging was placed in ICHAZ. Microcracks caused by hydrogen primarily originate within the martensite. and along the ferrite/martensite interface. A significant quantity of quasi-cleavages was noted in the brittle fracture region.
Fig. 6
Mechanical properties of joints under varying hydrogen charging conditions (a) Engineering stress-engineering strain curves and (b) tensile characteristics of the laser-welded joints25)
The solidification enhances the occurrence of cold cracks at the end crater, which may be interpreted as a geometric notch or welding flaw (Fig. 7). The occurrence of hydrogen cracking and the identification of brittle fracture surfaces exhibiting transgranular and intergranular features serve as definitive evidence of hydrogen-assisted cracking (HAC).
Fig. 7
Assessment of cold cracking susceptibility in laser weld joints, (a) cross-section, (b) top view, (c) bottom view. SEM pictures illustrating (d) a crater, (e) a mixed crack surface comprising hot and cold crack sections, and (f) a close view of the dendritic hot crack area, and (g) examination of transgranular and inter-granular fractures in the cold crack region of coated and lubricated DH10030)
5. Resistance spot welding and influence of hydrogen embrittlement in HSS
Nagasaka et al. recently studied the influence of hydrogen in ultra-high-strength TRIP-aided martensitic steel (TM steel) during the RSW process. The steel is widely used for automobile applications and possesses a base metal strength of 1532MPa. Same steel specimens tested after hydrogen charging reduced the tensile strength to 1126MPa. It shows the negative influence of hydrogen on tensile strength. At the same time, hot- stamped steel (HS) possessing a BM strength of 1438 MPa, drastically reduced to 725MPa after hydrogen charging, shows much more severe failure chances in the presence of hydrogen40). TM steel possesses a martensitic microstructure with retained austenite in its PAGBs, while HS steel is completely martensitic. Many previous studies stated that the presence of retained austenite can suppress the hydrogen embrittlement, i.e., RA suppresses the deterioration of strength in TM steel. Si stabilizes the retained austenite in TRIP steels26,41). In addition to that, the martensitic microstructure of HS steel contains a higher fraction of dislocation density that can act as hydrogen trapping sites; hence, more hydrogen volume in the steel leads to severe hydrogen embrittlement compared to TM steel. The comparison of maximum stress after hydrogen charging and maximum stress in spot welded specimens of TM steel and HS steel were similar. It is probably due to the stress concentration generated in the heat-affected zones (HAZ) and its softening during RSW operation. In most welding operations, HAZ is considered as the region that possesses the least strength compared to nugget and BM due to the non- uniform microstructures formed in different HAZs such as coarse-grained (CGHAZ), Fine-grained (FGHAZ), inter-critical (ICHAZ) and Sub-critical (SCHAZ) can adversely influence the strength. Microstructural characterization and mechanical testing of different resistance spot weld joints of AHSS shows that the microstructure of the typical ICHAZ was composed of scattered martensite islands and undissolved ferrite along with some retained austenite, whereas martensite in CGHAZ and FZ41). Nevertheless, melted regions (nuggets) in spot-welded advanced high-strength steel sheets experience quick cooling and solidification. These elements alter the crystalline structure and hardness of the nugget, which raises concerns about greater sensitivity to hydrogen embrittlement.
5.1 Estimating the hydrogen delayed cracking susceptibility in RSW joints
Researchers use different methods to understand hydrogen embrittlement and associated delayed cracking susceptibility of RSW joints. To meet design criteria, the right welding conditions must be chosen when spot welding different material combinations. This is required to guarantee that hydrogen penetration during painting or application will not result in embrittlement. Consequently, a rapid and quantitative evaluation approach to quantify the influence of hydrogen on the final strength of welded joints is necessary to establish the proper welding settings to minimize hydrogen embrittlement in spot welds. Microstructural c-s fo RSW eld joints and associated HAZ’s shown in Fig. 8 and the corresponding phase diagram as well2). The most commonly used mechanical tests to investigate HE behavior are discussed below.
Fig. 8
Cross-section of resistance spot weld joint showing different regions and corresponding Fe-C diagram showing the different possible phases in HAZ’s with varying peak temperature2)
5.1.1 Slow Strain Rate Test (SSRT)
A Slow Strain Rate Test (SSRT) was conducted using pre-charged and Zn-coated samples in the ambient environment. The tensile samples were subjected to a steady slow strain rate (0.005 mm/min) in the SSRT until fracture occurred (Fig. 9). SSRT experiments were conducted using a Zwick Z100 universal extension machine26). To mitigate the effects associated with the onset of cracks, the sample edges were subjected to milling. Subsequently, the surfaces of the sample were polished. The samples were initially infused with varying quantities of hydrogen and subsequently subjected to electro-galvanization in the laboratory (Hydrogen charging methods explained in section. 2.1. A consistent rate of deformation of 10-5s-1 was imposed. The strain in the axial loading direction was measured throughout the test using a laser extension sensor called laserXtens, manufactured by Zwick Company. The engineering stress-strain curves were computed and documented automatically. Each test was carried out for three different specimens to confirm the observed result 18,27,28,42,43). Hydrogen delayed cracking susceptibility studied in hot stamped steels by conducting SSRT on resistance spot welded specimens before and after H charging, is shown in Fig. 5. It is clearly visible from Fig. 5(d) that the H-charged specimen fractured perpendicular to the tensile direction in a brittle manner at the same time, Fig. 5(c) shows the 45-inclined fracture mode40). This clearly establishes the H embrittlement behavior in hot-stamped steels. Another study by Gatu Kitahara et al also reported the SSRT method to understand the delayed cracking susceptibility due to hydrogen18).
5.1.2 Incremental Loading test (ILT)
The specimens were subjected to progressive loading while being stretched in a NaCl solution. The modified perforated samples, by the SEP1970 standard, were subjected to testing, as well as the samples with welded joints. The sample geometries depicted in Fig. 10 consist of holed samples with punched edges and welded samples with welding points applied using spot welding26). The edges of the welded samples were then machined to match the test geometry. The ILT was conducted using a NaCl solution with a concentration of 3.5% and an initial pH value of 4, which changed using HCl. A quasi-static tensile test determined each material’s maximum force until fracture (Fmax). ILT began at 75% of Fmax and maintained at a constant level based on Fmax. After 48 hours, the load was incrementally increased by 5% of the maximum load (Fmax) every 2 hours until either fracture occurred or 100% of Fmax was reached. Fig. 3.6 depicts the loading curve. The duration of the test and the time it took for a fracture to occur were recorded. The test was conducted five times consecutively26).
5.1.3 Constant Loading test (CLT)
The Constant Loading Test (CLT) was conducted in a solution containing 3.5% NaCl with a pH of 4 (adjusted using HCl). The sample geometry for CLT conforms to the specifications outlined in the SEP1970 standard, as depicted in Fig. 7. The samples were severed using punching and thereafter adhered using the plastic container. The samples were securely fastened using bolts in the loading rings to conduct the testing, as depicted in Fig. 11. The load was then applied by adjusting the bolts to create a displacement, which was maintained at a constant level. Previous research indicates that the notch effect on a sample is minimized when the yield point (Rp0.2) is reached, particularly for materials with a high yield ratio (Rp0.2/Rm, where Rm=UTS). Thus, for materials with a yield ratio greater than 0.9, a stress of 80% of the tensile stress (Rm) was used for testing. The material’s Rm (ultimate tensile strength) and Rp0.2 (yield strength) were established using tensile tests conducted at a strain rate of 10-3s-1. In this study, the applied load remained constant while the stress at the notch reached Rp0.2, or 80% of the tensile strength (Rm). The specimens underwent a 96-hour loading process, and the time at which the fracture occurred (tf) was recorded. Three samples were evaluated for each substance.
In order to ascertain the threshold stress of the SEP1970 samples in the presence of NaCl, the stress applied in the CLT was modified. Initially, it was set at 90% of the ultimate tensile strength (Rm), and then gradually reduced by 5% of Rm. This adjustment was made until fracture did not occur over the 96-hour testing period across three repeated samples. The recorded value represents the maximum stress at which fracture occurs (σmax)26).
Similarly, other methods, such as U Bending test four point bend test, can also be utilized to estimate the hydrogen embrittlement behavior26).
5.2 Hydrogen Delayed cracking and Fracture mechanism in RSW weld joints
Along with the microstructural features, crack initiation, crack propagation, and mode of failure, such changes need to be addressed with H content changes. To assess the correlation between diffusible hydrogen (HD) content and tensile shear strength (TSS), spot- welded specimens fabricated from AHSS (1.5GPa) sheets were subjected to tensile shear tests at varying tensile rates during hydrogen charging (hydrogen charging methods are discussed in session 3.2). Consequently, as the amount of hydrogen increases, the tensile shear strength of spot-welding drops. The intergranular fracture surface and the quasi-cleavage fracture surface were noticed at the nugget and within the crack formed at the interface of the nugget heat-affected zone. Moreover, the findings on growth of cracks and hydrogen TDA analysis suggest that hydrogen embrittlement occurs in spot welds. is associated with stress-induced hydrogen diffusion as well as hydrogen trapped in dislocation and vacancy clusters at crack tips18). Fig. 12 shows the importance of the slow strain rate tensile testing (SSRT) method compared to conventional tensile testing (CTST) in understanding the influence of hydrogen content in steel. Schematics showing a cross-section of weld nuggets at different amounts of hydrogen content undergone SSRT, CTST, and corresponding fracture surfaces demonstrates the correlation between the tensile-shear strength and the amount of diffusible hydrogen present in C-TST and S-SRT specimens. Within the context of C-TST, the strength of the specimen remained constant until the hydrogen content reached 0.5 wt. ppm. However, once the hydrogen content is beyond this threshold, the strength of the specimen declines. In S-SRT, the tensile-shear strength showed a progressive drop until the HD content reached 0.003 wt. ppm. However, above this value, the strength decreased significantly. The tensile strength of S-TST specimens, which were subjected to a lower rate of tension, was markedly inferior to that of C-TST specimens. S-TST specimens exhibited a higher susceptibility to hydrogen embrittlement than C-TST specimens, even though the difference in hydrogen content was very small and in the ppm range. The corresponding difference in higher brittle surface fraction observed in SSRT-tested specimens reported confirms the explained facts18).
Fig. 12
Correlation between diffusible hydrogen concentration, tensile shear strength, and fracture morphology18)
Yagita et al. investigated the impact of sheared-edge and overlap-length on the diminution of the tensile fatigue limit in RSW’d UHSS sheets, before to and subsequent to hydrogen embrittlement. Static tensile shear and fatigue-tests were conducted on ultra-high-strength steel sheets that had undergone shearing and laser cutting. The steel sheets were then resistance spot welded, and hydrogen embrittled using cathodic hydrogen charging. Adjusting The overlap length was adjusted to evaluate the impact of the weld position, altering the distance from the resistance spot weld to the sheared and laser-cut edges. The maximum load in the tensile shear test dropped with decreasing overlap length and hydrogen embrittlement, whereas the fatigue test showed a greater effect from hydrogen embrittlement. As the repetitive load intensified during the fatigue test, the fatigue mode transitioned from the width to the sheared edge orientation. The fracture shifted to the sheared edge even when the overlap length decreased. The effect of hydrogen embrittlement-induced fatigue limit dropping was higher in the specimens with sheared. edges than in the specimens with laser surfaces44).
Recently, Park and co-researchers investigated the embrittlement mechanism of advanced HS martensitic steels during RSW operation2). The percentage reduction in elongation and fractur displacement, the displacement or strain of the specimen at which fracture occurs before and after H charging, was used to calculate the HE resistance. The study revealed that the tensile-shear strength,TSS and fracture displacement of spot-welded steels decrease with higher quantities of diffusible hydrogen. Fracture displacement experiences a rapid drop within the initial 3 hours of hydrogen charging. However, the pace of this decrease dramatically slows down afterward. The change in the slope is caused by a shift in the path by which the fracture spreads. At first, cracks spread along the fusion zone line, exhibiting transgranular failure. However, once the hydrogen concentration reaches a certain critical level, the cracks start spreading along the boundaries between the grains of the preceding austenite in the higher critical heat-affected zone. This type of failure is known as intergranular failure. When the hydrogen content exceeds a certain critical threshold, the PAGB in UCHAZ becomes weaker because of hydrogen- enhanced decohesion (HEDE) and hydrogen-enhanced localized plasticity (HELP). This same phenomenon was explained in the case of the laser weld joint of QP980 steels earlier by Xue et al.44). Cracks subsequently spread along the prior-austenite grain boundary (PAGB) in the direction of the highest stress during the tensile-shear test (TST), resulting in a modification of the course of crack propagation2).
Macrographs elucidate the crack propagation behavior of spot welds following cessation at various stress levels on the stress-strain curve, where Fig. 13(a) specimen stopped after crack initiation stress shows the initiation of crack from the endpoint of nugget and prolonged 45º direction (shear direction), further, the main crack propagate along sheet thickness (Fig. 9(b)) at maximum TSS, and Fig. 9(c) indicates local deformation, characterized by a fracture extending from nugget-interface to the heat-affected zone, a branching crack originating from the primary crack, additionally cracks arising from the blow hole at the nugget’s center. The emergence of the principal crack on the specimen’s surface under maximum TSS, post-hydrogen charging indicates that diffusible hydrogen promotes the development and propagation of primary fractures, together with the reduction of TSS. It is possible to explain the origins and propagation of the main fractures (Fig. 9(a)) along a direction inclined at a 45° angle from the nugget end, taking into account the plastic yield that occurs within the stress accumulation zone illustrated in Fig. 13(d). A schematic illustration of the initial cracks brought on by stress accumulation at nugget end can be found in Fig. 13(d). As seen in Fig. 13(e), slip occurs and deformation happens in the 45° direction when stress concentration happens at the end of the nugget, forming a plastic area. When absorbed under load, a hydrogen concentration region forms next to the nugget end. The grain boundary becomes extremely brittle due to accumulating several dislocations at the junction of the slip line and GB, potentially leading to grain-boundary fracture. Fracture happens at a lower stress level when samples are subjected to an environment with a continuous diffusible hydrogen supply in the stress- concentrated area. The phenomenon is caused by hydrogen embrittlement occurring in the stress-concentration region and fracture tip at grain boundaries. Hydrogen embrittlement characteristics of RSW and LW of AHSS steels are summarized in Table 2.
Fig. 13
(a-c) Macrographs showing cross section of spot weld nuggets of SSRT specimens indicating the crack initiation and propagation corresponding to different stress levels. (d) schematics of initial crack generated during tensile shear test, and (e) hydrogen affected zone near edge of nugget in enlarged view18)
Table 2
Hydrogen embrittlement characteristics of RSW and LW on AHSS
6. Other common Hydrogen cracking susceptibility tests and diffusible hydrogen measurements from weld joints.
6.1 Implant test
The implant test is an externally restrained test where the load applied on the weld is known, and susceptibility to HAC is checked for the corresponding load. The implant test is an externally restrained to evaluate hydrogen assisted cracking (HAC). A rod of the steel base metal to be tested is machined, and the end of the rod having a helical groove notched (Fig. 14) proved to be more sensitive to the cracking susceptibility. When a rod is inserted into a hole in the middle of a plate, its top flushes with the plate’s top. A weld bead is applied to the upper surface of the plate, directly encompassing the rod. The rod’s groove is consequently situated in the coarse-grained HAZ since the weld was made over the rod. The cracking occurs depending on factors such as diffusible hydrogen remaining, the microstructure, and the applied constant load45,46).
6.2 Y-Groove Tests
Specimens for Y-groove tests are fabricated with two distinct groove geometry. The straight Y-groove test is typically conducted to evaluate the cracking susceptibility of weld metal. In contrast, the inclined Y-groove test determines the cracking susceptibility of a weld joint’s heat-affected zone (HAZ). The samples were subjected to preheating and post-heating using oxy-acetylene flame heating. The samples were retained for 24 hours post-welding and underwent dye penetrant testing. Microscopic analyses to identify cracks were conducted following the sectioning of each sample at three distinct sites48).
6.3 Hot extraction Gas chromatography Thermalconductivity detector.
To quantify the diffusible hydrogen content of a shield metal arc weld joint specimen, it were produced per ISO 3690 specifications. The specimen measuring (30×15×10)mm3 was secured in a copper jig, accompanied by run-on and run-off components, each measuring (40×15×10)mm3 (Fig. 15). Bead-on-plate welding can be performed using a suitable electrode, preheated at 300 °C for 2 hours before welding. Immediately following the conclusion of welding process, the specimens submerged in ice-cold water for 5 seconds and subsequently placed in liquid nitrogen to achieve subzero temperatures until they were obtained for hydrogen extraction and quantification. Before measurement, the sample was removed from liquid nitrogen, and the central length was separated from the continuous run-on and run-off sections47).
Fig. 15
(a) Heating elements for performing pre-heating and post-heating of BM and weldment, and the Cu-Jig with weldment constitutes run-off and run-on pieces, (b) Hot extraction chamber (where the extraction of diffusible hydrogen is performed), a part of (c) Gas chromatographic thermal conductivity detector. Different units are mentioned in each figure47)
The specimen (30×15×10mm3) was immersed in acetone for de-icing and cleansing, air-dried, and placed into the sealed extraction chamber. The entire procedure of transferring the sample was executed swiftly to minimize hydrogen discharge. Hot extraction, gas chromatography, thermal conductivity detector (HE_GC_ TCD) apparatus employed for hydrogen measurement comprises a hydrogen collection chamber, a heater for chamber heating, and a gas chromatograph (GC)52-55). The specimen was preserved in a container at 400 °C for a minimum of 30 minutes to separate HD from the test piece. The hydrogen collected in the chamber was transferred to the gas chromatograph, which was fitted with a thermal conductivity detector (TCD) and employed argon as carrier gas. Prior to measurement, the gas chromatograph (GC) was calibrated using known volumes of hydrogen. The volume of hydrogen generated from the weld sample was determined via a software interface. The volume of hydrogen was quantified in ml/g based on the mass of the deposited weld metal.
Discussed experimental methods widely used for HE susceptibility of HSS, principle, advantages, disadvantages etc. are summarized in Table 3.
Table 3
A summary of various experimental methods used to estimate the hydrogen embrittlement susceptibility of AHSS materials
| Test method | Principle | Test conditions | Measured parameters | Advantages | Limitations |
|---|---|---|---|---|---|
| Slow Strain Rate Test (SSRT)25,40) | Tensile test at low strain rates in hydrogen environment | Strain rate: 10-6 to 10-4 s-1 in air or hydrogen | Elongation loss, reduction in area, UTS degradation | Simple setup, quantitative HE index | Requires careful strain rate control, slow testing |
| Incremental Load Test (ILT)26) | Stepwise increasing load in hydrogen environment until fracture | Load steps applied at specific intervals | Threshold stress for HE cracking | Identifies HE threshold stress | Time-consuming, stress concentration effects |
| Constant Load Test (CLT)26) | Specimen held at constant load in hydrogen environment | Fixed stress level, environmental exposure | Time to failure under sustained load | Simulates real-world conditions | Long test durations |
| Implant test45,47) | Small cylindrical specimen with notch under tensile load in hydrogen | Load applied in controlled environment | Threshold stress for crack initiation | High sensitivity to HE | Complex setup, needs specialized fixtures |
| Y-Groove test48) | Welded specimen with groove, subject to external stress | Simulated welding conditions | Crack length and depth | Useful for weldability assessment | Requires large samples, limited standardization |
| Gap Bead on Plate (GBOP) test30) | Simulated welding test with a gap between plates to study HE cracking in weld metal and HAZ | Welding performed with controlled gap, preheated/non-preheated conditions | Crack formation, crack length, susceptibility ranking | Directly evaluates HE cracking in welds | Affected by welding parameters, requires metallographic analysis |
6.4 Summary
After Analyzing the recent research publications discussing the hydrogen embrittlement behavior of AHSS used in automotive applications, the following points have been summarized.
AHSS and UHSS are prone to hydrogen-delayed cracking, where the martensitic microstructure containing a higher fraction of dislocation density (much higher after die quenching), interfaces, grain boundaries, etc. can entrap hydrogen and lead to a pre-mature failure to RSW and laser welding joints.
Crack initiation, propagation, fracture mechanism, and associated microstructural phenomenon during hydrogen embrittlement were discussed.
Different methods used to measure diffusible hydrogen content in the microstructure and cold cracking susceptibility of steels are also discussed in detail.
Hydrogen traps and recent techniques to estimate the hydrogen-entrapped locations in a microstructure and cold cracking susceptibility tests were also discussed.
Mechanisms, Susceptibility, and Evaluation methods of Hydrogen Embrittlement in Resistance Spot and Laser Welded Advanced High-Strength Steels reviewed and reported the most important and widely used, current methods for better understanding of the phenomenon and how the drawbacks can be avoided further for safe operations.
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