Stress corrosion cracking? 3

[escapega]

We manufacture adjustable wrenches from 5151 steel. They handles are hardened to Rc 40-50. We have recently begun experiencing some cracking during nickel-chrome plating. (1-2% failure rate) The cracking does not occur if we stress relieve the parts prior to plating using a 600-degree stress relief operation, so we feel that it is related to internal stresses and maybe some form of hydrogen embrittlement that we have not had previously. We know that the load strength of the parts decreases about 10% after plating, but have not had the parts crack during the process.

We are thinking some new form of hydrogen embrittllement that is reacting to the internal stresses in the parts. We have checked some of the alkaline cleaner baths and found that we have about 0.5 parts per million of hydrogen sulfide as tested using the hach methylene blue method.

Any comments, suggestions, or ideas would be appreciated.

Roy Hill, cooper tools

803-481-1425

 

[metengr]

Sounds reasonable to me, as long as you have not had any failures or cracks with the added SR step.

 

[CoryPad]

I assume you don't want to continue using the stress relief operation due to cost?

When you say there is cracking during plating, do you mean the parts crack while immersed in the plating bath?

A hydrogen concentration less than 1 part per million is not the problem, but you may have more hydrogen somewhere else in your system.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[escapega]

Cory, thank you for your answer.

1)Yes- we wish to eliminate the stress relieve operation due to cost.
2) Our test indicate that they break right after nickel plating and prior to chrome plating. I was thinking that the HE in plating was the straw that broke the camel’s back. Where they break is a very thin section between two holes.

My major interest was to get some feed back on hydrogen level concentrations as I am thinking as you stated that we have an unusual build up of hydrogen in our plating system somewhere.

 

[escapega]

Thank you metengr for your reply. i am very much interested in how much hydrogen sulfide concentration in a cleaner or rinse might lead to stress related cracking or other HE problems as this is something new as opposed to a long term issue.

regards,

roy

 

[metengr]

Unfortunately, hydrogen embrittlement is a synergistic damage mechanism in steels that requires the combined effects of residual tensile stress and nascent hydrogen. You have one of two options - either eliminate the source of nascent hydrogen in your process or reduce tensile stresses to a level that is below the threshold for cracking. Stress relieving does both - increase the diffusivity of hydrogen and reduces residual tensile stresses.

 

[escapega]

metengr- thanks for your interest and help-

I am thinking that maybe we are getting hydrogen sulfide from anaerobic bacteria growing in the system so I am interested in what concentration of hydrogen sulfide in the cleaner baths and rinses should we consider treatment with something like carbon to reduce the hydrogen sulfide?

I just got the reagents to test the baths and have only sampled and tested one bath, once, just to see that the hach system worked- so I will be doing a wider range of testing in the near future.

 

[swall]

Can you described where on the wrench handle cracking is occurring and is it always in the same place? Am I correct in assuming that the wrench is forged?

 

[swall]

Oh--have you mag particle inspected parts prior to plating to make sure that they are not cracked before plating?

 

[escapega]

swall- thanks for ure reply

1) Not sure about how the posting of pictures works- so- if you look at an adjustable wrench on end so that the movable jaw is on top, you will see 2 holes and a slot which the jaw moves in. the crack is in the thin cross section between the large hole that the barrel of the jaw enters and the smaller hole that holds the pin upon which the knurl rides.

2) we have done magnaflux testing and determined with high confidence that the cracks occur after the plating cycle starts. it also does not seem to make any difference how long the parts a held prior to plating- we held a box for a month to see it was a time related thing.


3) we ran a test to check parts after each step of the plating process and believe it to be occurring after the nickel plating bath prior to chrome plate. (after soak clean, electrol clean and HCL sour dip)

As this appears to be different than our run of the mill HE situation of just a 10% loss of strenght, we believe that it may be a combination of factors such as steel related factors(who knows where the steel actually comes from these days?) as well as plating process related.

roy

 

[swall]

OK, I grabbed wrench and know the area you are talking about. How are these holes put in? I am thinking that the machining operation may be abusive to the metal surface, leaving residual tensile stresses which then set the stage for hydrogen imbrittlement. But, as this happens only 10% of the time, fixing the machining operation such that it is more consistent ( more frequent tool changes, coolant change, more consistency in depth of cut, etc)may solve most of your problem.

 

[escapega]

Thank you for ure reply- that very well may be part of the problem as we are using high speed cnc machines to drill, or rather almost punch the holes into the parts. The problem did start soon after we switched methods and we have made changes as you suggested to insure the tools are changed more frequently.

It is a 1% problem, not a 10% problem and thus is like finding a needle in the haystack.

Do you know of a test where we could economically have a few hundred parts tested to check for the level of internal stress and see if there is a difference between parts- and then see if it makes a difference in the cracking?

 

[swall]

Two NDT ways to measure surface residual stresses--X-ray diffraction and Barkhausen. The former is fairly expensive, but will give you quantitative data. The latter is less expensive and will give you qualitative data. Since you will want to examine a large number of parts, and just get an idea of extreme variations in stress levels, Barkhausen would probably be the way to go. I would contact American Stess Technologies and discuss your needs. They offer both XRD and Barhausen equipment.

 

[escapega]

i will do- it another frog to kiss- i hope it turns into a princess!

i will be out of pocket for several days, so no more replys today.

many thanks to those who have posted today and i will follow up in a few days.

hopefully i can one day give someone else some help.

 

[TVP]

escapega,

One other area to consider is the "HCl sour dip". Acid cleaning and electroplating processes are the most common sources of nascent hydrogen. Do you know the concentration and pH of the HCl bath? How long are the parts immersed in the bath? Steel parts that have been hardened and tempered to a hardness greater than ~ 32 HRC are very susceptible to hydrogen embrittlement during acid cleaning/pickling. The following is an excerpt from SAE/USCAR-5 AVOIDANCE OF HYDROGEN EMBRITTLEMENT OF STEEL:

If high hardness parts are to be acid cleaned, it is required that a procedure be established for
processing the parts that limits the time in the acid bath to a maximum of 10 minutes.

I also agree with swall that residual tensile stresses may be the result of abusive machining operations. These stresses should be minimized or eliminated by proper machining practices.

 

[kenvlach]

escapega,
First, acquaint the bean-counters with the fact that all chromium plating specifications (AMS, ASTM, Federal, MIL & international) require a post-machining, pre-plating stress relief bake for such parts. Compliance is probably required by contract. Neglecting this step may result in hidden damage to parts which will be uncovered in liability lawsuits.

To the above responses, I will add
1) restrict the use of the cathodic cycle during electrocleaning -- most operators cycle between anodic & cathodic),
2) add an inhibitor to the HCl pickling solution -- this will minimize hydrogen pick-up. It must be an inhibitor compatible with subsequent plating.
3) see ASTM B849 Standard Specification for Pre-Treatments of Iron or Steel for Reducing Risk of Hydrogen Embrittlement (equivalent to ISO/DIS 9587).
4) see ASTM F519 Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating Processes and Service Environments. This test is a quality control requirement for many plating specifications.

P.S. I doubt that 0.5 parts per million of hydrogen sulfide in an alkaline cleaner bath matters; adsorption of hydrogen sulfide sometimes even inhibits hydrogen pick-up.

 

[EdStainless]

I have learned a lot from this thread.
My only additional comment also relates to the holes. It may not just be local residual stress but also surface damage in the ID of hte holes that can accelerate the hydrogen damage. I have seen this in high strength wire. Small surface defects are the first places that HE will show up.

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[escapega]

Thank you all for your comments.

1) Based on this input, I am planning to revisit the machining and try some speed/feed experiments to see how it correlates to the problem. We had been hoping for a “gold” nugget, but I am now thinking that it is simply that just too much internal stress has been added to the parts using the new high-speed method. It is the simplest explanation.

2) I will also be getting a copy of the ASTM B849.

3) i will update this thread when i have some data from the experiment- it may take 2-3 weeks-

regards to all,

roy