Coatings for PHS

Coatings for PHS

 

Overview

The initial press hardening steels of the 1970s were delivered bare, without a galvanized or aluminized layer for corrosion protection (i.e., uncoated). During the heating process, an oxide layer of FeOx forms if the furnace atmosphere is not controlled. Through the years, several coating technologies have been developed to solve the following problems of uncoated steelsF-14, F-33:

  1. Scale formation, which causes abrasive wear and requires a secondary shotblasting process before welding,
  2. Decarburization, which leads to softening close to the surface,
  3. Risk of corrosion.

The first commercially available coating on press hardening steels was patented in 1998. The coating was designed to solve the scaling problem, but it also offered some corrosion resistance.C-24 Since the coating composition is primarily aluminium, with approximately 9% silicon, it is usually referred to as AlSi, Al-Si, or AS.

Coating thickness is nominally 25 μm (75 g/m2) on each side and referenced as AS150. A more recent offering is a thinner coating of 13 μm (AS80).A-51 The AS coating requires a special heating curve and soaking time for better weldability, corrosion resistance and running health of the furnace. Most OEMs now include furnace dew point limitations to reduce/avoid hydrogen embrittlement risk.

In 2005, Volkswagen was looking for a method to manufacture deep drawn transmission tunnels and other complex-to-form underbody components using press hardened steels. Although AS coatings were available, parts could not be formed to the full draw depth using the direct process, and AS coated blanks cracked during the cold forming portion of the two-step hybrid process. Using uncoated blanks led to severe scale formation, which increased the friction coefficient in hot forming. For this particular problem, a varnish coating was developed. The coating was applied at a steel mill, and shipped to Volkswagen’s stamping plant. The parts were first cold pre-formed and then heated in a furnace, as seen in Figure 1a. Hot pre-forms were then deep drawn to tunnels. As shown in Figure 1b, scale formed on parts which did not have the coating. A varnish coated blank could be cold formed without any scale, Figure 1c.S-63, F-34 Since then, some other varnish coatings also have been developed.

Figure 1: Transmission tunnel of 2005 Volkswagen Passat: (a) hot forming of pre-form, and final parts: (a) uncoated blank would suffer from scaling, (c) scale-free parts can be formed from varnish-coated blanks [REFERENCE 7]

Figure 1: Transmission tunnel of 2005 Volkswagen Passat: (a) hot forming of pre-form, and final parts: (a) uncoated blank would suffer from scaling, (c) scale-free parts can be formed from varnish-coated blanks.F-34

 

In car bodies, components that are sealed from external moisture are referred to as dry areas. These areas have low risk of corrosion. Areas that may be exposed to moisture are wet areas. Precautions must be taken to avoid corrosion of the sheet metal, such as using galvanized or pre-coated steel. Sealants can also be applied to joints to keep out moisture. The presence of humidity in these areas increases the risk of forming a galvanic cell, leading to accelerated corrosion. These areas have higher risk of corrosion and may require additional measures. Figure 2 shows dry and wet areas. In this figure, parts colored with yellow may be classified as wet or dry, depending on the vehicle design and the OEMs requirements.G-41

Figure 2: Dry and wet areas in a car body. [REFERENCE 8]

Figure 2: Dry and wet areas in a car body.G-41

 

An estimated ~40% of press hardened components are in dry areas. Thus, high corrosion protection is desired in the 60% of all press hardened components which are employed in wet areas.B-48  Zn-based coatings are favored for their cathodic protection, but require tight process control. The first commercial use of Zn-coated PHS was in 2008, using the indirect process.P-20 Since then, direct forming of Zn-coated PHS has been studied. When direct formed, furnace soaking temperature and time must be controlled carefully to avoid deep microcracks.G-41, K-20  Recently developed are two new Zn-coated press hardening steel grades, 20MnB8 and 22MnSiB9-5, both reaching approximately 1500 MPa tensile strength after processing. Using grades requires a pre-cooling process after the furnace to solidify the Zn-based coating. 20MnB8 can be direct hot formed to final shape, whereas 22MnSiB9-5 can be formed in a transfer press in the “multi-step” process.K-21, H-27

Depending on the coating type and thickness, the process type, controls and investment requirements may change significantly. For example, some press hardening lines may be designed to form blanks with only Al-based coatings. Table 1 summarizes the advantages and disadvantages of several coating systems.

Table 1: Summary of coatings available for press hardening steels.

Table 1: Summary of coatings available for press hardening steels.

Uncoated Blanks

The earliest press hardening steels did not have any coating on them. These steels are still available and may be preferred for dry areas in automotive applications. If the steel is uncoated and the furnace atmosphere is not controlled, scale formation is unavoidable. Scale is the term for iron oxides which form due to high temperature oxidation. Scale thickness increases as the time in furnace gets longer, as seen in Figure 3. Scale has to be removed before welding, requiring a shotblasting stage. Thicker scale is more difficult and more costly to remove.M-53 Early attempts to reduce (if not avoid) scale formation saw the use of an inert-gas atmosphere inside the furnace.A-52  Today, a mixture of nitrogen (N2) and natural gas (CH4) is typically used.F-35 In China, at least one tier supplier is using a vacuum furnace to prevent scale formation.A-68

Figure 3: Oxide layer (scale) on press hardened steel after: (a) fast resistance heating (10 seconds in air), (b) furnace heating (120 seconds in air) [REFERENCE 14]

Figure 3: Oxide layer (scale) on press hardened steel after: (a) fast resistance heating (10 seconds in air), (b) furnace heating (120 seconds in air).M-53

 

While heating uncoated steel in the furnace, if the conditions are favorable for iron (Fe) oxidation, carbon (C) may also be oxidized. When the carbon is oxidized, layers close to the surface lose their carbon content as gaseous carbon monoxide (CO) and/or carbon dioxide (CO2) is produced.S-87 The depth of the “decarburization layer” increases with dwell time in the furnace, until an oxide layer (scale) formed. Scale acts as a barrier between the bare steel and atmosphere. As the carbon is depleted in the “decarburization layer”, the hardness of the layer is decreased, as seen in Figure 4. Decarburization is usually undesirable since it lowers the strength/hardness and may negatively affect fatigue life.C-26

Figure 4: Hardness distribution of an uncoated steel after 6 minutes in a 900 °C furnace, showing hardness decrease as the surface layers lose their carbon. Image recreated after REFERENCE 19.

Figure 4: Hardness distribution of an uncoated steel after 6 minutes in a 900 °C furnace, showing hardness decrease as the surface layers lose their carbon. Image recreated after C-26.

 

Several methods are available to improve the corrosion resistance of uncoated PHS parts:

  1. E-coating after welding, before painting is a typical step of car body manufacturing, for rustproofing.
  2. If descaling can be done by using chromium shots (in shotblasting), a thin film of chromium-iron may grow on the surface and improve the corrosion resistance.F-14
  3. Vapor galvanizing (also known as Sherardizing) of uncoated steel after descaling, an experimental study described in Citation G-42.
  4. Electro-galvanizing after hot stamping, as described in Citation A-68.
  5. Change the base metal chemistry to one that is more oxidation resistant.L-60  Figure 5 compares the shiny non-oxidized surface appearance of parts made from this grade with that made from a conventional uncoated press hardening grade on the same production line with the same processing conditions.W-28

 

Figure 5: Oxidation resistant PHS grades may not need descaling or coatings for sufficient corrosion resistance. Citation W-28

Figure 5: Oxidation resistant PHS grades may not need descaling or coatings for sufficient corrosion resistance.W-28

 

Aluminium-Based Coatings

The first commercially available coating on press hardening steels was patented by Sollac (now part of ArcelorMittal) in 1998. This coating was designed to address the scaling problem, but also offers some barrier corrosion resistance.C-24  The nominal coating composition is 9-10 wt.% Si, 2-4 wt.% Fe, with the balance Al.L-39 The coating may be referred to as AlSi, Al-Si, AluSi or more commonly AS. Nominal as-delivered coating thickness is 25 μm (approximately 75 g/m2) on each side, and is usually referred to as AS150, with 150 referencing the total coating weight combining both sides, expressed as g/m2. More recently, a thinner coating of 13 μm (30-40 g/m2 on each side, AS60 or AS80) is now commercially available.A-51 When AS coated blanks are “tailor rolled,” the coating thickness is also reduced in a similar percentage of the base metal thickness reduction. Corrosion protection is similarly reduced, and furnace parameters need to be adjusted accordingly.

As delivered, AS150 has a coating thickness of 20-33 μm and a hardness of approximately 60 HV. The “interdiffusion layer” (abbreviated as IDL) has a high hardness and low toughness at delivery, as seen in Figure 6a. Due to the brittle nature of the IDL, AS coated blanks cannot be cold formed unless very special precautions are taken. During heating, iron from the base metal diffuses to the coating forming very hard AlSiFe (or AlFe) layers close to surface. At the same time, Al and Si of the coating diffuse to the IDL, growing it in thickness and reducing its hardness, Figure 6b. Earlier studies have shown that heating time (and also furnace temperature) has direct effect on the final thickness of IDL, as shown in Figure 7. Once the IDL thickness surpasses approximately 16 to 17 μm, the welding current range (ΔI = Iexpulsion – Imin) may be well below 2 kA.V-15, V-21, W-34  The dwell time must be long enough to ensure proper surface roughness (see Figure 6b) for e-coatability.M-27, T-40  Figure 10 summarizes the heating process window of AS coatings. The process window may change with base metal and coating thicknesses.

Figure 5: AS coating micrographs: (a) as-delivered, (b) after hot stamping process (re-created after REFERENCES 21, REFERENCE 22, REFERENCE 23, REFERENCE 26)

Figure 6: AS coating micrographs: (a) as-delivered, (b) after hot stamping process (re-created after V-15, V-21, W-34, G-32)

 

Figure 6: IDL thickness variation with furnace dwell time (Image created by REFERENCE 43 using raw data from REFERENCE 22, REFERENCE 26, and REFERENCE 27]

Figure 7: IDL thickness variation with furnace dwell time (Image created by B-55 using raw data from V-21, G-32, K-41.)

 

Hydrogen induced cracking (HIC, also known as hydrogen embrittlement) has been a major problem for steels over 1500 MPa tensile strength. AS coated steels may have higher diffusible hydrogen, when delivered, due to the aluminizing process occurring at 680 °C. In addition, AS coated grades may have a hydrogen absorption rate up to three times higher during heating.C-27  To reduce the hydrogen diffusion, it is essential to control the heating process (both heating rate and dew point in the furnace). AS coated blanks absorb hydrogen at room temperature; however, this happens at much lower rates than uncoated or Zn-coated blanks.J-21  Diffusible hydrogen can be removed from the press hardened part by re-heating the part to around 200 °C for 20 minutes or longer, in a process called de-embrittlement.V-21, G-32, G-43, J-21

For the abovementioned reasons, AS coated higher strength grades (i.e., PHS1800 and over) are required to have precise “dew point regulations” during the heating in furnace. Their final properties, especially elongation and bending angle, may be guaranteed only after bake hardening, as shown in Figure 8.B-32  Paint baking is standardized in Europe as a treatment for 20 minutes at 170 °C, which may act like a de-embrittlement treatment.E-10  Some OEMs also require dew point control and “subsequent de-embrittlement treatment” for AS coated PHS1500.

Figure 7: Effect of diffusible hydrogen (Hdiff) on mechanical properties of: (a) uncoated PHS2000, (b) AS coated PHS2000 in an uncontrolled furnace atmosphere (REFERENCE 43 using raw data from REFERENCE 28)

Figure 8: Effect of diffusible hydrogen (Hdiff) on mechanical properties of: (a) uncoated PHS2000, (b) AS coated PHS2000 in an uncontrolled furnace atmosphere (B-55 using raw data from C-27).

 

Another method to reduce the risk of hydrogen embrittlement is to adjust the coating composition. The bath chemistry for a standard AlSi coating consists of up to 90% aluminum, about 8% to 11% silicon and a maximum of 4% iron. Adding a maximum of 0.5% alkaline earth metals, like magnesium, for example has been shown to result in 40% less hydrogen diffusion into steel.R-29, T-45

Although not common in the industry, Al-Zn and Zn-Al-Mg based coatings have also been developed for press hardening processes.F-14 Recently introduced is an aluminium-silicon coating with magnesium additions. When oxidized with water vapor, Mg releases less H2 and thus may reduce the diffusible hydrogen.S-88

AS coatings may cause costly maintenance issues in roller hearth furnaces, as the coating may contaminate the rollers.B-14 Special care has to be taken to avoid the issue or prolong the maintenance intervals.

 

Zinc-Based Coatings

AS coatings provide some corrosion protection, known as “barrier protection”, as the coating forms a barrier between the oxidizing environment and the bare steel. It is quite common in Europe for a car to have 12 years corrosion protection warranty. To achieve such corrosion resistance, a typical car may have over 85% of its components galvanized.S-89

The use of Zn-coated PHS has been relatively low, compared to AS coated and uncoated grades. In 2015, 76% of the PHS sold in EU27+Turkey was AlSi coated. In these markets, 18% of the PHS sold was uncoated and only 6% was Zn coated.D-20 This can be attributed to the susceptibility of Zn-coated PHS to Liquid Metal Embrittlement (LME, also known as Liquid Metal Assisted Cracks (LMAC) and Liquid Metal Induced Embrittlement (LMIE)).C-28, L-46

After heating and soaking in the furnace, the base metal should be in the austenitic phase. During heating, the Zn coating reacts with the base metal and forms a thin solid layer of body-centered-cubic solid solution of Zn in α-Fe, shown as α-Fe(Zn) in Figure 9. During deformation, a microcrack can be initiated in this layer at the grain boundaries of the austenite in the base metal, as indicated in Figure 9a. As the crack propagates, zinc from the α-Fe(Zn) layer diffuses along the austenite grain boundary and combines with iron from the base steel to form additional α-Fe(Zn), Figure 9b. Cracks propagate through the weak a-Fe(Zn) grain boundary layer, allowing liquid zinc (with diffused iron) to advance into the capillary crack (Figure 9c). After quenching, the base metal transforms to martensite and the liquid Zn transforms to a hard and brittle intermetallic phase, Γ-Fe3Zn10.C-28

Figure 8: Schematic illustration of microcrack formation. (re-created based on REFERENCE 37.)

Figure 9: Schematic illustration of microcrack formation. (re-created based on C-28.)

 

To avoid LME, three methods can be employedK-20:

  1. Forming in the absence of liquid Zn,
  2. Reducing stress level,
  3. Reducing material susceptibility.

There are no breakthroughs to address the last two items. Forming a part in the absence of liquid Zn involves either of two process routes: (1) Indirect press hardening (also known as form hardening), or (2) Pre-cooled direct processes.

In the direct forming of Zn-coated blanks, with or without pre-cooling, microcracks in the base metal may be observed. Microcracks less than 10 μm into the base metal does not affect the fatigue strength of the part.K-20 Microcrack depth is a function of coating thickness, furnace conditions (temperature and dwell time, see Figure 10), forming severity and forming temperature. It may be possible to direct form galvannealed (GA coated) blanks.

The boiling point of pure zinc (907 °C) is very close to the austenitization temperature of 22MnB5 (885 °C), so the heating process window of Zn-coated blanks must be controlled precisely. When the furnace dwell time is too short, deeper microcracks may be observed. When the furnace dwell time is too long, corrosion performance may be degraded. Thus, heating process window of Zn-coated blanks is significantly narrower than that of AS-coated blanks.B-14, S-90

Figure 9: Heating process window of AS and Zn coatings (representative data, may not be accurate for all sheet and coating thicknesses, re-created based on REFERENCE 34 and REFERENCE 39).

Figure 10: Heating process window of AS and Zn coatings (representative data, may not be accurate for all sheet and coating thicknesses, re-created based on B-14, S-90.

 

Zn-based coatings may result in very low diffusible hydrogen after press hardening. In one studyJ-21, no diffusible hydrogen was detected, as long as the furnace dwell times are shorter than 6 minutes. Even after 50 minutes in the furnace, diffusible hydrogen was found to be around 0.06 ppm. Zn coatings do not act as a barrier for hydrogen desorption (losing H through the surface). Even at room temperature, Zn coated blanks may lose most of the diffusible hydrogen within a few days (also referred to as aging).

Figure 10: Evolution of galvanized coating: (a) as delivered: Ferrite+Pearlite in base metal, almost pure Zn coating with Al-rich inhibition layer, (b) at high temperatures: austenite in base metal + α-Fe(Zn) and liquid Zn + surface oxides, (c) after press hardening: martensite in base metal + α-Fe(Zn) and Γ-phase coatings + surface oxides. The oxides are removed prior to welding and painting [REFERENCE 30]

Figure 11: Evolution of galvanized coating: (a) as delivered: Ferrite+Pearlite in base metal, almost pure Zn coating with Al-rich inhibition layer, (b) at high temperatures: austenite in base metal + α-Fe(Zn) and liquid Zn + surface oxides, (c) after press hardening: martensite in base metal + α-Fe(Zn) and Γ-phase coatings + surface oxides. The oxides are removed prior to welding and painting.J-21

 

Zn-based coatings may have a yellowish color after hot stamping. The surface oxides have to be removed before welding. This is typically done by shotblasting.

PHS blanks with a ZnNi coating were previously available. The ZnNi coating provided a low friction coefficient, a large process window in the furnace, the ability to be cold formed (indirect or two-step hybrid processes were also possible) and decreased susceptibility to LME.B-56  ZnNi coated PHS was used in the rear rail of the Opel Adam city carH-57 for a short period, until the coating was discontinued.C-29

 

Varnish Coatings

Another method to avoid scaling and decarburization is to apply varnish coatings. In this method, uncoated steel can be either coil coated or blanks can be manually coated with the paint-like varnish coatings.B-14  These coatings may also be known as “paint-type” or “sol-gel”.

Figure 11: Manual application of a varnish coating. [REFERENCE 7]

Figure 12: Manual application of a varnish coating.F-34

Depending on the type of coating, they may allow very fast heating – including induction and conduction heating with electric current. Since the coating does not require time to diffuse, furnace heating may be completed in less than 2 minutes.F-34 Again, depending on the type, surface conditioning may not be required before welding or e-coating.B-14

They were used in automotive industry between 2005 and 2010. By 2015 there were four different types of varnish coatings, some of which are now discontinued.B-14  These coatings may be useful for prototyping and low volume production.

 

eren billur, PhD Thanks are given to Eren Billur, Ph.D., Billur MetalForm, who contributed this article.

 

 

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Delayed Cracking (Hydrogen Embrittlement)

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.

Analyze Hydrogen Induced Cracking Susceptibility in Resistance Spot Welds

Analyze Hydrogen Induced Cracking Susceptibility in Resistance Spot Welds

This articles summarizes a paper entitled, “New Test to Analyze Hydrogen Induced Cracking Susceptibility in Resistance Spot Welds,” by M. Duffey.D-10

This study aims to develop a new weldability test to analyze the susceptibility of HIC in RSW of different steels. A total of eight different steel samples were analyzed with their carbon content, associated American Welding Society (AWS) carbon equivalencies, and gauges shown in Table 1. All materials were tested in the full-hard condition (all have been cold-rolled).

Table 1: Tested Steels, Carbon Equivalencies, and Steel Gauge.

Table 1: Tested Steels, Carbon Equivalencies, and Steel Gauge.D-10

 

The associated parameter ranges for welds made with each steel are in Table 2. The resistance spot weld was made in the middle of the sheets, as shown in the test setup in Figure 1. There was a total of 18 test samples (nine not painted and wiped, nine painted) for each material tested.

Table 2: Welding Parameters

Table 2: Welding Parameters.D-10

 

Figure 1: HIC Test for Resistance Spot Welds Schematic.

Figure 1: HIC Test for Resistance Spot Welds Schematic.D-10

 

Figure 2A shows the results of the 3- × 3-in. tests. Figure 2B shows the results of the 4- × 4-in. tests. Figure 2C shows the results of the 5- × 5-in. tests.

 

Figure 2: Results of the Three Different Test Sizes on AHSS.

Figure 2: Results of the Three Different Test Sizes on AHSS.D-10

 

Cracks consistently initiated at the periphery of the weld nugget where the two steel sheets came together. Cracks then propagated either in the weld metal or HAZ, as shown in Figures 3 and 4.

Figure 3: Cracking in the Weld Metal of Steel 8.

Figure 3: Cracking in the Weld Metal of Steel 8.D-10

 

Figure 4: Cracking in Both the Weld Nugget and HAZ in Steel 8.

Figure 4: Cracking in Both the Weld Nugget and HAZ in Steel 8.D-10

 

Figure 5 displays the results from Steels 1, 4, and 5. Steel 1 is the most resistant (of the three) to HIC. For the three steels shown in Figure 5, the crack length (at each gap spacing) was longer for the painted sample than the non-painted sample.

 

Figure 5. Test Results for Steels 1, 4, and 5.

Figure 5. Test Results for Steels 1, 4, and 5.D-10

 

The microstructure of IF Steels 1-3 (Figure 6A) was ferrite. The microstructure of the high-strength low-alloy (HSLA) Steel 4 (Figure 6B) was a mixture of grain boundary ferrite, martensite, and possibly some bainite. The microstructure of the specialty alloy Steels 5-7 and AHSS Steel 8 (Figure 6C) was entirely martensite.

 

Figure 6: Microstructures of the Different Weld Nuggets.

Figure 6: Microstructures of the Different Weld Nuggets.D-10

 

Figure 7 shows the fracture surface of a crack completely through Steel 7.

Figure 7: Fracture Surface of Cracked Weld in Steel 7.

Figure 7: Fracture Surface of Cracked Weld in Steel 7.D-10

 

The average and maximum hardness results of spot welds in each material are summarized in Figure 8.

Figure 8. The Average and Maximum Hardness of HAZ and Weld Nugget in Resistance Spot Welds of Each Steel

Figure 8. The Average and Maximum Hardness of HAZ and Weld Nugget in Resistance Spot Welds of Each Steel.D-10

 

Figure 9 is a graph that displays the carbon equivalence, number of washers where cracking first began, and average hardness of the weld nugget and HAZ in each steel.

Figure 9: Carbon Equivalency vs. Number of Washers to Initiate Cracking

Figure 9: Carbon Equivalency vs. Number of Washers to Initiate Cracking.D-10

 

Embrittlement Phenomenon

Embrittlement Phenomenon

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.