Delayed Cracking (Hydrogen Embrittlement)

Testing and Characterization

Delayed cracking caused by hydrogen embrittlement results in decreased toughness, ductility, and load-bearing capacity of a material. It is a concern with some Advanced High-Strength steel (AHSS) grades.

Three triggers must be present for there to be an embrittlement risk:

  1. Use of a susceptible material, such as steel with tensile strength above approximately 1000 MPa and/or the presence of retained austenite in the microstructure,
  2. An environment or processing step where hydrogen is available to diffuse into the steel, and
  3. Presence of a high stress condition, either from forming or during product usage and life cycle.

The interaction between hydrogen and the steel crystallographic structure in the presence of these conditions limits the ability of the steel to deform under load. The steel part will fracture at a lower stress than is anticipated under other conditions. Of particular concern is that fracture may not occur at the time of stamping, since hydrogen can diffuse out of the lattice over time.

High strain areas on parts formed from AHSS are at increased risk for delayed cracking, since the distorted crystallographic lattice allows for an increased hydrogen solubility.  Cut edges are especially troublesome, since the shearing operation work hardens the steel (increasing strength) and damages the microstructure (increasing strain).

AHSS grades that contain retained austenite are also at increased risk for delayed cracking from hydrogen embrittlement. Stamping strains provide the driving force for the TRIP effect, where austenite coverts to martensite. The austenite crystallographic lattice allows for increased hydrogen solubility, so the TRIP effect creates a hydrogen-rich high strength martensite – conditions that promote a greater risk of embrittlement.

Sources of hydrogen include:

  • Breakdown of mill or press applied lubricants,
  • Annealing or heat treating atmospheres, and
  • Electrochemical operations in steel mills or vehicle production such as pickling, electrogalvanizing, welding, phosphate coating, and paint pretreatment, such as e-coating.

Actions to minimize the risk of hydrogen embrittlement includes not over-pickling the surface prior to coating (since pickling uses an acid), heat treating in dew-point controlled atmospheres (below -10 °C or even lower for higher strength grades), and using the e-coating operation as a means to degas the steel.

Global steelmakers take alloying and processing approaches that are specific to their particular infrastructure and equipment. The same grade from two different steelmakers will likely use different alloying approaches. This may lead to different hydrogen embrittlement risks, as well as different tensile property ranges. Use caution when switching suppliers for all AHSS grades.

The procedures to characterize delayed fracture resistance typically involve using a machined sample, pre-strained and placed in an acidic solution for a defined time. SAE J3215S-94 is one such procedure.

Note that cracking in simulative tests does not necessarily indicate that parts made from that material would crack in an automotive environment due to the specific conditions that need to be present for hydrogen embrittlement to occur. When selecting any AHSS, results from simulative tests should be considered in conjunction with the forming and processing history of the steel, the strain state of the formed and processed part, and the operating environment of the part.

Liquid Metal Embrittlement: Results of a 3-Year Study

Liquid Metal Embrittlement: Results of a 3-Year Study

Blog, RSW Joint Performance Testing

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Results of a Three-Year LME Study

WorldAutoSteel releases today the results of a three-year study on Liquid Metal Embrittlement (LME), a type of cracking that is reported to occur in the welding of Advanced High-Strength Steels (AHSS).The study results add important knowledge and data to understanding the mechanisms behind LME and thereby finding methods to control and establish parameters for preventing its occurrence. As well, the study investigated possible consequences of residual LME on part performance, as well as non-destructive methods for detecting and characterizing LME cracking, both in the laboratory and on the manufacturing line (Figure 1).

Figure 1: LME Study Scope

Figure 1: LME Study Scope

The study encompassed three different research fields, with an expert institute engaged for each:

A portfolio containing 13 anonymized AHSS grades, including dual phase (DP), martensitic (MS) and retained austenite (RA) with an ultimate tensile strength (UTS) of 800 MPa and higher, was used to set up a testing matrix, which enabled the replication of the most relevant and critical material thickness combinations (MTC). All considered MTCs show a sufficient weldability under use of standard parameters according to SEP1220-2. Additional MTCs included the joining of various strengths and thicknesses of mild steels to select AHSS in the portfolio. Figure 2 provides the welding parameters used throughout the study.

Figure 2: Study Welding Parameters

Figure 2: Study Welding Parameters

In parallel, a 3D electro-thermomechanical simulation model was set up to study LME. The model is based on temperature-dependent material data for dual phase AHSS as well as electrical and thermal contact resistance measurements and calculates local heating due to current flow as well as mechanical stresses and strains. It proved particularly useful in providing additional means to mathematically study the dynamics observed in the experimental tests. This model development was documented in two previous AHSS Insights blogs (see AHSS Insights Related Articles below).

 

Understanding LME

The study began by analyzing different influence factors (Figure 3) which resembled typical process deviations that might occur during car body production. The impact of the influences was analyzed by the degree of cracking observed for each factor. A select number of welding set-ups from these investigations were rebuilt digitally in the simulation model to replicate the process and study its dynamics mathematically. This further enabled the clarification of important cause-effect relationships.

Figure 3: Overview of All Applied Influence Factors (those outlined in yellow resulted in most frequent cracking.)

Figure 3: Overview of All Applied Influence Factors (those outlined in yellow resulted in most frequent cracking.)

Generally, the most frequent cracking was observed for sharp electrode geometries, increased weld times and application of external loads during welding. All three factors were closely analyzed by combining the experimental approach with the numerical approach using the simulation model.

Destructive Testing – LME Effects on Mechanical Joint Strength

A destructive testing program also was conducted for an evaluation of LME impact on mechanical joint strength and load bearing capacity in multiple conditions, including quasi-static loading, cyclic loading, crash tests and corrosion. In summary of all load cases, it can be concluded that LME cracks, which might be caused by typical process deviations (e.g. bad part fit up, worn electrodes) have a low intensity impact and do not affect the mechanical strength of the spot weld. And as previously mentioned, the study analyses showed that a complete avoidance of LME during resistance spot welding is possible by the application of measures for reducing the critical conditions from local strains and exposure to liquid zinc.

 

Controlling LME

In welding under external load experiments, the locations of the experimental crack occurrence showed close correlation with the strains and remaining plastic deformations computed by the simulation model. It was observed that the cracks form at the location of the highest plastic strains, and material-specific threshold values for critical strains were derived. The threshold values then were used to judge the crack formation at elongated weld times.

At the same time, the simulation model pointed out a significant difference in liquid zinc diffusion during elongated weld times. Therefore, it is concluded that liquid zinc exposure time is a second highly relevant factor for LME formation.

The results for the remaining influence factors depended on the investigated MTCs and were generally less significant. In more susceptible MTCs (AHSS welded with thick Mild steel), no significant cracking occurred when welded using standard process parameters. Light cracking was observed for most of the investigated influences, such as low electrode cooling rate, worn electrode caps, electrode positioning deviations or for gap afflicted spot welds. More intense cracking (higher penetration depth cracking) was only observed when welding under extremely high external loads (0.8 Re) or, even more, as a consequence of highly increased weld times.

For the non-susceptible MTCs, even extreme situations and weld set-ups (such as the described elongated weld times) did not result in significant LME cracks within the investigated AHSS grades.

Methods for avoidance of LME also were investigated. Changing the electrode tip geometry to larger working plane diameters and elongating the hold time proved to eliminate LME cracks. In the experiments, a change of electrode tip geometry from a 5.5 mm to an 8.0 mm (Figure 4) enabled LME-free welds even when doubling the weld times above 600 ms. Using a flat-headed cap (with small edge radii or beveled), even the most extreme welding schedules (weld times greater than 1000 ms) did not produce cracks. The in-depth analysis revealed that larger electrode tip geometries clearly reduce the local plastic deformation around the indentation. This plastic strain reduction is particularly important, as longer weld times contribute to a higher liquid zinc exposure interval, leading to a higher potential for LME cracks.

Figure 4: Electrode Geometries Used in Study Experiments

Figure 4: Electrode Geometries Used in Study Experiments

It was also seen that as more energy flows into a spot weld, it becomes more critical to parameterize an appropriate hold time. Depending on the scenario, the selection of the correct hold time alone can make the difference between cracked and crack-free welds. Insufficient hold times allow liquid zinc to remain on the steel surface and increased thermal stresses that form after the lift-off of the electrode caps. Elongated hold times reduce surface temperatures, minimizing surface stresses and thus LME potential.

Non-Destructive Testing: Laboratory and Production Capabilities

A third element of the study, and an aid in the control of LME, is the detection and characterization of LME cracks in resistance spot welds, either in laboratory or in production conditions. This work was done by the Institute of Soudure in close cooperation with LWF, IPK and WorldAutoSteel members’ and other manufacturing facilities. Ten different non-destructive techniques and systems were investigated. These techniques can be complementary, with various levels of costs, with some solutions more technically mature than others. Several techniques proved to be successful in crack detection. In order to aid the production source, techniques must not only detect but also characterize cracks to determine intensity and the effect on joint strength. Further work is required to achieve production-level characterization.

The study report provides detailed technical information concerning the experimental findings and performances of each technique/system and the possible application cost of each. Table 1 shows a summary of results:

Table 1: Summary of NDT: LME Detection and Characterization Methods

Table 1: Summary of NDT: LME Detection and Characterization Methods

 

Preventing LME

Suitable measures should always be adapted to the specific use case. Generally, the most effective measures for LME prevention or mitigation are:

  • Avoidance of excessive heat input (e.g. excess welding time, current).
  • Avoidance of sharp edges on spot welding electrodes; instead use electrodes with larger working plane diameter, while not increasing nugget-size.
  • Employing extended hold times to allow for sufficient heat dissipation and lower surface temperatures.
  • Avoidance of improper welding equipment (e.g. misalignments of the welding gun, highly worn electrodes, insufficient electrode cooling)

In conclusion, a key finding of this study is that LME cracks only occurred in the study experiments when there were deviations from proper welding parameters and set-up. Ensuring these preventive measures are diligently adhered to will greatly reduce or eliminate LME from the manufacturing line. For an in-depth review of the study and its findings, you can download a copy of the full report at worldautosteel.org.

 

 

LME Study Authors

LME Study Authors

The LME study authors were supported by a committed team of WorldAutoSteel member companies’ Joining experts, who provided valuable guidance and feedback.

 

 

Journal Publications:

 

 

Auto/Steel Partnership LME Testing and Procedures

 

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Embrittlement Phenomenon

Embrittlement Phenomenon

RSW Joint Performance Testing

It has been a long-standing challenge to extend the usage of the very high-strength steels in hydrogen-rich environments, given that these steels are prone to Hydrogen Cracking, due to their increased mechanical strength. Hydrogen embrittlement (HE) is known to be a premature fracture caused by a small amount of hydrogen atoms concentrated at highly stressed regions inside susceptible high tensile strength materials. (See examples in Figure 1.)

Figure 1: Examples of hydrogen embrittlement (HE) failure in RSW.

Figure 1: Examples of hydrogen embrittlement (HE) failure in RSW.

The factors controlling the occurrence of HE, susceptible microstructure, stress, and the presence of hydrogen, are well known and have been sufficiently quantified to develop procedures that minimize its occurrence mostly in arc welding thicker gauge steels. When advanced high strength steel (AHSS) with tensile strength over 980 MPa are applied in automotive applications, there is a small risk that hydrogen embrittlement fracture (sometime called delayed fracture) may occur after welding, while a vehicle is in use. Although there have been no reports that automotive parts made of AHSS have fractured due to hydrogen embrittlement, a risk assessment of delayed fracture for AHSS is considered necessary to ensure the safety of the automotive body and encourage wider use of Advanced High-Strength Steel (AHSS) sheets.

Another common embrittlement phenomenon involves the zinc coating discussed previously. Resistance spot welding is dependent on the interfacial contact resistance between the electrodes and the material. During welding, a metal with a lower melting point, such as zinc can penetrate in a liquid state into the grain boundaries of the material. By the end of the welding process, liquid metal embrittlement (LME) can become a problem due to the ductility of the grain boundary being reduced by the impeding tensile stress. (Example can be seen in Figure 2.) Also, brittle intermetallic compounds, such as Cu5Zn8, are created by the reaction with the Cu electrode and the material at the high temperature, which promotes LME or surface cracking.

Figure 2: Description of (top image) and actual example (bottom) of the LME phenomenon in zinc coated AHSS.

Figure 2: Description of (top image) and actual example (bottom) of the LME phenomenon in zinc coated AHSS.

 

There are many research papers investigating and analyzing Liquid Metal Embrittlement in mild steel and AHSS. LME is not unique to automotive AHSS steels or RSW, but is discovered in other ferrous materials, heat treatment and other welding processes. In spot welding AHSS, the complex microstructure and the greater spring back behavior of AHSS eventually lead to weld discontinuities such as LME.

A detailed multi-year, multi-organization study investigated the root causes of LME and how to test for, control, and prevent LME. The full report can be downloaded here, and is summarized in an AHSS Insights Blog.

The Joining Team of the Auto/Steel Partnership (A/SP) also recognizes the concerns surrounding LME, and conducted several rounds of testing, summarized here. 
A/SP developed two separate procedures to test for Liquid Metal Embrittlement:

In summary, LME cracking may occur when a combination of tensile stress, liquid metal and susceptible microstructure exist. Studies are being performed to evaluate whether LME is mitigated by today’s automotive RSW processes, where the volume of welds significantly exceeds engineering requirements, or whether the occurrence of LME actually affects in-use properties at all.