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Hydrogen embrittlement of fasteners

  • 1.  Hydrogen embrittlement of fasteners

    Posted 07-16-2020 15:10

    Hydrogen embrittlement of fasteners is a problem caused by improper baking that can lead to failure under static tensile loading.  There is a difference of materials professional opinions on whether or not hydrogen embrittlement can be reversed.  Some professionals say that hydrogen embrittled fasteners can be reclaimed by rebaking them.  Others say that once the hydrogen embrittlement is in the fasteners it cannot be reversed.  Which do you support, and what is your technical reasoning/data for your position?



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    Robert Gaster
    Retired Senior Staff Engineer
    John Deere Moline Technology Innovation Center
    (563) 340-8862
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  • 2.  RE: Hydrogen embrittlement of fasteners

    Posted 07-17-2020 08:25
    if there are hydrogen cracks (or "flakes") in the fasteners, welds, ingots, or acid cleaned steels from the hydroegn and residual stresses the damage is done and only in the case of ingots that can be forged over ~40 % reduction in area can the hydrogen cracks be "healed" really fused closed.

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    Edward Vojcak
    Senior Metallurgical Engineer
    SGS North America
    Blue Island IL
    (708) 595-8734
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  • 3.  RE: Hydrogen embrittlement of fasteners

    Posted 07-17-2020 09:27
    As per my experience, It happens more in diameters greater than 28 mm round bars, I have also seen embrittlement in thinner wall thickness stabilizer bars which were pickled in acid. So I do agree that pickling introduces hydrogen during the process.
    The suggestion would be to get degassed material from steel suppliers for higher diameter applications. All the big mass steel components like gears, shafts are heat treated from degassed steel. As a mater of fact most OEM's are requiring hydrogen, and oxygen regulated to a certain PPM amount to avoid embrittlement.  Degassing also regulates incisions in steel which is another advantage of degassing except the cost goes up.

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    Sanjay Kulkarni
    Materials Engineer
    MSSC
    Troy, MI
    248-840-1056
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  • 4.  RE: Hydrogen embrittlement of fasteners

    Posted 07-21-2020 15:53
    In the vehicle braking industry we have to ask ourselves as design engineers, " Can the system withstand one failed fastener IF the supplier 1.) misses the bake out for a lot of fasteners, 2.) delays the bake out, or 3.) the oven does not reach the prescribed temperature?"  The fastener industry has a little more secret weapon with zinc or zinc-aluminum flake mechanical coatings (ie. Geomet / Dacromet). 
    We can either use lower hardness fasteners that do not require full zinc plating, go to phos & oil, or use the flake coatings.
    We also tend to use stainless steel springs in valves if corrosion is anticipated. Steel cleanliness and surface issues can cause low cycle fatigue cracking, so we tend to avoid  zinc plated steel springs for the 3 reasons stated above. Under ASTM recommendations, there is B850 that spells out hydrogen bake out relief.  It is not always 4 hours at 400F within 4 (or some rules of thumb say12) hours of plating.

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    Patrick Mizik
    Principal Metallurgical Engineer
    Haldex
    Leawood KS
    (816) 510-2458
    pat.mizik@haldex.com
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  • 5.  RE: Hydrogen embrittlement of fasteners

    Posted 07-17-2020 08:26
    I don't know why anyone would want to re-bake hydrogen embrittled fasteners. In my experience I didn't know they were brittle until at least some bolts in a joint failed. At this time all the fasteners are considered suspect and I'd throw them away. All bolts would then be replaced. To me it is more important to pinpoint the source of the hydrogen. Was it a result of poor quality control of processing during manufacture or the result of hydrogen diffusion from corrosion in service. The effectiveness of re-baking might depend n the surface and the ease with which the hydrogen can diffuse through any surface coating.

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    David Schonfeld
    Principal Consultant
    UQ Materials Performance
    Graceville QLD
    61407894727
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  • 6.  RE: Hydrogen embrittlement of fasteners

    Posted 07-17-2020 09:54
    Yes. The best is to save the fasteners from embrittlment by quickly baking them on exposure to acids.

    if the fasteners is high strength and already embrittled they are likely to suddenly fail in service.

    My experience is in such cases not to take risk and avoid using them.

    C. RENGANATHAN
    Director
    Chennai Metco - India

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    Renganathan Chellamraja
    Chennai Metco Pvt Ltd
    Tamil Nadu
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  • 7.  RE: Hydrogen embrittlement of fasteners

    Posted 07-17-2020 16:03
    Some recent papers indicate that higher strength alloy steels, such as SAE 4340 after Cd plating, still had lower impact toughness even after baking.  Baking at very high strengths didn't improve toughness.  In my estimation, alloy steels have more sites, such as martensite, grain boundaries, carbides and other discontinuities to have the hydrogen to diffuse out at 400-500F.  I have asked one of the authors to continue further work on this matter because so many high strength fasteners are electroplated or cleaned in acids prior to galvanizing.  Considering that bolting is torqued to 60-75% of yield strength in many applications, the fracture toughness of bolting, combined with threads, can result in bolt cracking in acidic or hydrogen-embrittled fasteners.

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    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023
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  • 8.  RE: Hydrogen embrittlement of fasteners

    Posted 07-18-2020 09:36
    Re: the original question,  

    The specifications that require baking to remove hydrogen are based on realizing that it will be there after pickling and are mostly likely based on experience of the spec authors that if the hydrogen is baked out quickly enough, damage will be unimportant.  Work to quantify how quickly it must be removed would be interesting. Cracking would probably have to occur in areas of stress - I have seen a cold-bent, then pickled, part that H cracked just until it relieved the cold bending residual stresses, then stopped.

    Regarding the other answers:
    If the hydrogen problem occurred prior to final product form and pickling, then it seems unlikely that the material can be healed by that much forging - if cracking was found then there are already breaks to the surface.

    Degassing and such affect impurity levels but existing hydrogen isn't, in my opinion, why VIM/VAR are done.  Hydrogen introduced during part processing is what is being discussed here I think, not from the initial melt.

    If H cracking occurred from corrosion in service, then that is indeed a different animal, and the fastener is unsuitable for that service.  Some analysis of the SF-Oakland Bay Bridge fastener failures suggests that the fasteners were heat treated to provide an adequate level of strength, but in such a manner that the surface hardness exceeded spec to make up for sub-spec core properties, and SCC failures occurred in the too-hard case.

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    Paul Tibbals
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  • 9.  RE: Hydrogen embrittlement of fasteners

    Posted 07-19-2020 11:28
    Hydrogen embrittlement that's a real bag of worms many opinions. Here is my outlook from mostly aerospace outlook and I'm far from an expert. Hydrogen  embrittlement is basically a diffusion problem. Aerospace standards are very strict for any process that might general hydrogen at the surface. If the steel is above a certain strength you have to bake at a certain temp within a certain time. 24 hours in most cases if memory serves me. Or 12 hours if above a higher strength. Generally no rebake allowed. The thinking as I understand it is that because a hydrogen atom can diffusion in any direction you have a better chance of getting rid of it if it's close to the surface. Think of dropping a ping pong ball in a room with many door and windows. If I drop it (bake) when it's near a door or window it has a better chance of leaving the room then if I wait to drop it until I'm reach the middle of the room. Any explanation of hydrogen embrittlement like so many things has a lot of ifs, and, and buts. And many books and articles written on it.

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    Joe Epperson FASM
    Senior Metallurgist retired
    National Transportation Safety Board Retired
    Waldorf MD
    (301) 885-9418
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  • 10.  RE: Hydrogen embrittlement of fasteners

    Posted 07-20-2020 08:20
    AMS2759/9, Hydrogen Embrittlement Relief (Baking) of Steel Parts, covers the requirements for embrittlement relief (baking) of heat treated steel parts to remove hydrogen infused during plating and certain other chemical processing such as stripping, chemical milling, pickling, and etching. The AMS specifications are published by the Aerospace Materials Council of SAE International.  

    Steel parts of high hardness are generally given a thermal stress relief some time prior to electroplating. The reason is that the acids used to remove oxides and activate the surfaces can discharge hydrogen into the metal and cause spontaneous cracking before the plating process. (Without this acid treatment, the parts would have remained sound.)  After electroplating, the same parts need to be baked shortly afterwards to expel the hydrogen introduced during that process (which can cause cracking either immediately or over time during service).  Presumably the immediacy of the baking is due to much more hydrogen charged into the metal by the plating process than would come from just pickling.

    Some plating solutions and processes are more likely to discharge a large amount of hydrogen into the part and so have stricter baking requirements than others.  I believe there are tables in AMS2759/9 that indicate the baking requirement,  Individual AMS and government plating specs (maybe ASTM as well) will have similar plate-specific requirements.

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    [Dave] Himmelblau
    [Retired Materials & Processes Engineer]
    Mountain View CA
    (650) 968-1121
    CharlesRetired
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  • 11.  RE: Hydrogen embrittlement of fasteners

    Posted 07-20-2020 12:00
    But you don't need to eliminate the hydrogen, just reduce it below a critical concentration. Hydrogen diffuses poorly through nickel, so it is difficult to remove after nickel plating. It is more likely the hydrogen is diffusing away from the surface towards the core. as the hydrogen spreads out and becomes less concentrated near the surface, hydrogen embrittlement is reduced or eliminated.

    I always felt that limiting the time before bake was to avoid hydrogen cracking due to residual stress. Hydrogen embrittlement requires sustained stress for a period of time before any damage takes place.

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    Ken Kirby
    Snap-on, Inc.
    Kenosha WI
    (262) 748-3836
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  • 12.  RE: Hydrogen embrittlement of fasteners

    Posted 07-20-2020 12:26
    If you read the submittal I made late on July 19, you’ll see that the same substrate hardness criteria for hydrogen baking after plating apply to stress relief baking before plating. Now where and how the hydrogen is dispersed after post-plating baking may be another matter. People with for more research or elemental testing than me will have to respond to your comment.


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  • 13.  RE: Hydrogen embrittlement of fasteners

    Posted 07-19-2020 11:28
      |   view attached
    One of my students worked on a similar question for his MS thesis and found that higher-strength fasteners (RC hardness > 37, achieved by more extensive case hardening through to the tapping screw center) were more susceptible to hydrogen embrittlement, (due to a zinc acid plating process), as expected, (and is geometry dependent). 
    Subsequent baking for 8-12 hours at 200C showed no positive effects, and in some cases worsened embrittlement (as evidenced by lower fracture loads and notch toughness). 
    So, this experimental work suggests that baking may not be a good idea (at least for the conditions investigated in this limited study). 
    If you are interested in further detail, please check the paper (attached).
    Medcalf, John S., Thomas, Brian G., and Brahimi, Salim V., "Hydrogen Embrittlement Susceptibility of Case Hardened Steel Fasteners," SAE Internat., Technical Paper 2018-01-1240, WCX Conf., Detroit, MI, Apr. 10-12, 2018, pp. 1-17, 2018, DOI:10.4271/2018-01-1240.
    --Brian.

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    Brian Thomas FASM
    Professor of Mechanical Engineering
    Colorado School of Mines
    Golden CO
    (217) 493-9633
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  • 14.  RE: Hydrogen embrittlement of fasteners

    Posted 07-20-2020 11:59
    I would agree with some of the other posters that hydrogen uptake recovery can be a mixed bag.  My senior project in college many years ago was to investigate the cause of failures of a percentage of bolts that were embedded in concrete.  The fasteners were torqued to specification then the next day half had failed.  These were specified at 32HRc maximum and were heavily zinc plated.  Turned out the steel supplier falsified the records and provided two lots of material under one certificate but were heat treated as a single lot resulting in one heat meeting specification (~30HRc) and the other not (~38 HRc).  Quality sampling did not catch the issue.  We performed some hydrogen relief thermal cycles on the failed bolts.  Some bolts exhibited recovery of impact energy some not. 

    Most recently I worked on a project regarding eliminating the 48hr hold after temper bead welding for performance of the NDE.  The intention in to allow any diffusible hydrogen to cause any damage before performing the examination.  What we found in the literature was that the microstructure and the hydrogen trapping capability of the material influences the cracking susceptibility.

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    David Segletes
    Materials and Process Engineer
    Blue Origin
    Taft TN
    (803) 487-2368
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  • 15.  RE: Hydrogen embrittlement of fasteners

    Posted 07-22-2020 08:27
    David Segletes said, in part:...investigate the cause of failures of a percentage of bolts that were embedded in concrete. The fasteners were torqued to specification then the next day half had failed. These were specified at 32HRc maximum and were heavily zinc plated. Turned out the steel supplier falsified the records and provided two lots of material under one certificate but were heat treated as a single lot resulting in one heat meeting specification (~30HRc) and the other not (~38 HRc). Quality sampling did not catch the issue. We performed some hydrogen relief thermal cycles on the failed bolts. >
    Thank you for sharing that!  I found a part made of 17-4PH (S17400) that had failed from SCC under static loading in an oceanside environment.  It turned out to have been treated to H900 condition, which is susceptible to SCC under some conditions.  The "martensitic or ferritic and >HRC37" rule trumps the "stainless" rule. 

    That flashed me back to an assignment very early in my career - an embedded, galvanized stud in concrete for a tank foundation at a remote construction site broke off when being "persuaded" with a hammer to line up with the tank flange.  Initial analysis showed that the broken fastener was not bolt material, but rather O-1 tool steel. While the galvanizing didn't have anything to do with the failure (now I'm going off-topic) we still needed to see how many of the studs were of improper material. We did not have any sort of portable elemental analyzer at that time.  But Cal Poly was still home to old-school metallurgists who had instructed us in spark testing.  After testing to make sure that I could reliably differentiate O-1 vs. medium carbon grades, I hauled out an air grinder, nitrogen bottle to power it, and a black backdrop for visibility, and worked my way through the site.  Only 1 or 2 more studs were found to have been from a maverick round that had "snuck into" the bar stock being threaded for this job, so the foundation did not need to be removed and replaced.  Zinc paint was used to cover the ground areas.

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    Paul Tibbals
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  • 16.  RE: Hydrogen embrittlement of fasteners

    Posted 07-23-2020 09:49
    Paul, What's interesting about hydrogen embrittlement is that it manifests itself when the Rockwell hardness is sufficiently high and there is exposure to acid environments, whether they be plating baths or scale removal prior to galvanizing.  When a part or component is quenched, the cooling rate at the surface is always greater than at the center, resulting in a greater amount of martensite formation.  If the outer surface of the part is under bending or torsional forces, this makes it most susceptible to fracture.  So many of our tools and bolting are sporting protective finishes, we are literally inviting hydrogen embrittlement.  There is a process called "cold galvanizing" where the parts or bolts are circulated in a slurry of ceramic pellets, zinc particles and a carrier solution.  Whether that solution is acidic or neutral or alkaline could provide some relief from hydrogen entry.  The only problem with bolts treated by cold galvanizing is that threads are roughened and torque values must be increased compared to zinc or cadmium plated fasteners.

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    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023
    ------------------------------

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  • 17.  RE: Hydrogen embrittlement of fasteners

    Posted 07-23-2020 10:24

    From my recollection there are 3 ways to reduce hydrogen intake during zinc electroplating:

    1. There is a big difference between acid or alkaline baths ( I am thinking alkaline baths are lower).
    2. It is all about current density, and time.  For platers, the time in the zinc bath is the bottle neck.  They may have 2 bathes for each set of wash and rinse tanks.  So if the they reduce current density, they increase time .  Time is money for electroplaters.
    3. If the parts are slightly rusty or mildly oxidized (even from machining friction at the surfaces, so stamped cold rolled steel parts would be preferred) then a reverse polarization is added before plating to electrolytically remove the oxide all while evolving hydrogen.  This is not a very long cycle.

     

    Perhaps some one from the plating industry can better explain these.

     

     

    Patrick Mizik, P.E.

    Principal Metallurgical Engineer
    Haldex Brake Systems

     

    Direct:  816-801-2304

    Fax:     816-891-9432

    E-mail: pat.mizik@haldex.com

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  • 18.  RE: Hydrogen embrittlement of fasteners

    Posted 07-24-2020 13:41
    I have seen hydrogen embrittlement on galvanized fasteners that arose from in-service corrosion. While corrosion of zinc in cold, fresh water liberates little or no hydrogen, in salt water it does liberate hydrogen. Also, fresh water can become saline through evaporative concentration. 

    --
    John Grubb



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  • 19.  RE: Hydrogen embrittlement of fasteners

    Posted 07-21-2020 08:57
      |   view attached

    One of my students worked on a similar question for his MS thesis and found that higher-strength fasteners (RC hardness > 37, achieved by more extensive case hardening through to the tapping screw center) were more susceptible to hydrogen embrittlement, (due to a zinc acid plating process), as expected, (and is geometry dependent). 

    Subsequent baking for 8-12 hours at 200C showed no positive effects, and in some cases worsened embrittlement (as evidenced by lower fracture loads and notch toughness). 

    So, this experimental work suggests that baking may not be a good idea (at least for the conditions investigated in this limited study). 
    If you are interested in further detail, please check the paper (attached).

    Medcalf, John S., Thomas, Brian G., and Brahimi, Salim V., "Hydrogen Embrittlement Susceptibility of Case Hardened Steel Fasteners," SAE Internat., Technical Paper 2018-01-1240, WCX Conf., Detroit, MI, Apr. 10-12, 2018, pp. 1-17, 2018, DOI:10.4271/2018-01-1240.
    --Brian.



    ------------------------------
    Brian Thomas FASM
    Professor of Mechanical Engineering
    Colorado School of Mines
    Golden CO
    (217) 493-9633
    ------------------------------

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  • 20.  RE: Hydrogen embrittlement of fasteners

    Posted 07-27-2020 10:18
    "Hydrogen embrittlement of fasteners is a problem caused by improper baking that can lead to failure under static tensile loading.  There is a difference of materials professional opinions on whether or not hydrogen embrittlement can be reversed.  Some professionals say that hydrogen embrittled fasteners can be reclaimed by rebaking them.  Others say that once the hydrogen embrittlement is in the fasteners it cannot be reversed.  Which do you support, and what is your technical reasoning/data for your position?"

    There are two aspects of hydrogen embrittlement, the first step is to have hydrogen infiltrate the base metal. It is at this step that the material can be baked and the hydrogen removed. There can be constraints in time and base material size as to how long one can wait before baking to remove the hydrogen. I have found in small diameter fasteners that this time can be rather long, as in weeks. So as long as the hydrogen embrittlement has only progresse to this point, then re-baking can be successful. When you get to larger diameters there may be more time constraints, I don't have direct experience.

    The second step of hydrogen embrittlement is when the hydrogen protons have migrated through the material, come out at the grain boundaries and have then pushed the grains apart in the typical grain boundary separation that is the classic appearance of hydrogen embrittlement. This step typically happens after the part has been loaded. Once the part has progressed to this stage of hydrogen embrittlement baking does nothing. The parts are not recoverable. 

    I recommend you read AMS 2759-9 "Hydrogen Embrittlement Relief (Baking) of Steel Parts"

    dj

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    David Jones
    Senior Principal Materials Engineer
    Stryker Orthopeadics
    Mahwah NJ
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