×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
This is the new problem I am working on, on one of the mounting bolts. This time I will try my level best to give you guys as much of info as possible with pictures. This is our major issue (Customer satisfaction)

Problem where find: In the field, somewhere between 5hr to 440hr of engine run in water (no problem in assembly). Problem is seen only on high horsepower engines.

Joint Description: the whole engine will be supported by a structure, the engine is mounted on to the structure with 2 upper mount bolts and 4 lower mount bolts. So in service the whole load is carried by those 6 bolts and the problem is with the 2 upper mount bolts which are breaking underhead. Warranty issue. Some of them breaking at underhead of bolt and some of them come loose.

Fasteners used: ½-13 UNRC-2A,17.4 strength bolt (grade 8 high strength stainless steel) cadmium coated, fastener driven into blind hole (no nut), split lock washer under head and loctite 272 on the threads.
Depending on the heat treatement of the bolt, the proof load of the bolt can vary from 14,687lbs min to 17500lbs max. our supplier can get a mixed fasteners so we really want to keep the 14,687lbfs as proof load for margin of safety.But our tensile lab did some tests on 4 production bolts and they found the proofload to be 18000lbs.

Clampload we are shooting to: 9000 lbs average on each bolt.
 
Service loads on the joint : don’t know, design engineer dosent know either,but he said he gonna try to get the number.

Engineering dept thought: our eng dept thinks that , the failure/fracture of bolt head is due to hydrogen embrittlement or stress corrosion failure. But we see some of the bolts failed in 5 hrs of the engine run, so how can it be a corrosion failure if u have the engine in water for just 5 hrs? some of them failed in 440 hr (this I can agree it may be of corrosion failure)

What my guess is 9000 lbs of clamp load is like 50-60% of the bolt proof load. I think the bolts are failing because of the low clampload as cyclic loading/vibrations.services loads acting on the joint.

What do u guys think?

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Now I am struck, its like a dual edged sword. Without knowing the root cause I cant take further steps because,

1)If I feel the problem is low clamp load, and bump up the torque and get the clamp load to 13000 lbs which is close to min proofload, and the problem really is hydrogen embrittlement or stress failure, then I am even making the situation worst because I am over stressing the bolt.
2)If I think the problem is stress failure, then I don’t over stress the bolt (keep the clampload what I am right now) and if the problem is really low clampload, then I  end up continuing breaking more bolts in the field

I can get you guys the pictures of bolts that being failed, with very good resolution (material lab pictures) and also I can take the pictures of the actual joint.our material lab looked at the pictures and saw some beachmarks and the failure area and decide its hydrogen embrittlement or stress corrosion factor.As I told you earler, I some how feel its low clampload problem. But I have to believe the materials lab (more scientific).

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

1. What is the exact process used to manufacture the bolts starting from raw material to the point at which preload installs them in his engine mount?  What are the times and temperatures used for heat treating?

2. Have you performed any dimensional inspection on these suspect bolts, specifically on the transition area from head to shank?  How is this specified on the drawing?  Do the parts meet the drawing requirements?

3. Your materials lab should be performing a more thorough examination.  What is the microstructure for the failed bolts?  Did they use a Scanning Electron Microscope to evaluate the fracture surfaces at higher magnification?  Hydrogen embrittlement (more properly termed delayed fracture) will produce a characteristic fracture pattern that is different from fatigue.  Any cracks, surface defects, etc. that cannot be explained by corrosion?  What about the coating?  Is it uniform?  Any defects, poorly adhering spots, etc.?

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
TVP,

1)I dont have an answer for the first question. I have to ask my fastener supplier for thier process and will do that.
2)Material lab did dimensional inspection, please see pics in the 2nd post of the thread.Regarding proof load and tensile strength, parts dint meet the drawing spec.Drawing spec says 14500lbs and the tensile test says 18000 lbs for the proof load.fasteners are over designed.
3)yes we did SEM, please see the two pics attached down.the inspection confirms that the microstructure is consistent with the material and heat treatment specified

http://img106.imageshack.us/img106/457/umfb6ak0.png
http://img164.imageshack.us/img164/1241/umfb7ce0.png

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Are you saying these parts are made with 17-4 precipitation hardening stainless steel?  If so, please stop using the term Grade 8, it does not apply and is confusing to those of us trying to provide tips.

Proof loads are minimums.  Therefore, your parts with 18000 lbf proof load meet the requirement of 14500 lbf minimum.  They are not overdesigned.

Intergranular fracture typically is an indicator of delayed fracture due to hydrogen assisted cracking.

Regards,

Cory

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

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

The damage to the fastener proper prior to failure has a strange look, not unlike electrolysis.  

Can you give the heat treatment for the 17/4 PH SS?

What is the material that the bearing surface of the fastener head lands on?

I've never seen 17/4 fasteners Cd plated. Can you post the plating process used?

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)

first pic is the tapped hole. engine is mounted on the black adapter u see in the picture
http://img163.imageshack.us/img163/175/holeef2.jpg

the part in this pic surrounded by a rubber is fastened to the  tapped hole part above with the fastener.
http://img355.imageshack.us/img355/7770/fastenermatingsurfacerj8.jpg

this pic is the assembly pic (it just shows one fastener)
http://img181.imageshack.us/img181/2219/assembly1yb9.jpg

this pic is the whole view of assembly (both fasteners)
http://img181.imageshack.us/img181/4294/assy2pr8.jpg

this pictures shows the list of failure modes with hours of run.please enlarge the pic
http://img337.imageshack.us/img337/5465/hrscc9.png

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Corypad,

I am really sorry for the misleading information. yes it is 17-4 ph stanless steel. So regarding the proofload for any calculatios, which number should I consider? 18000 or 14500?

Unclesyd,
At this point I dont know the plating and heat treatment process. I will definetly call my supplier tomorrow and will get some documents and post them here. Reg the bolt head mating surface, I will post that early tomorrow.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

The presence of beach marks on the fracture surface that your materials lab pointed out could imply fatigue crack propagation OR stress corrosion cracking. I am leaning toward stress corrosion cracking as the culprit base don the macrophotographs you posted.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Can you check if your installation isn't resonating at some speed (if it is varaible speed conditions), or for instance during misfiring conditions (providing it is a combustion engine). You may get exceissive movements and related bolt loads during these conditions.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

For calculation, you normally use the minimum allowed by specification since you are likely to have one part near the lower limit if you have a large population of parts.  If you are trying to back calculate forces/stresses/fracture toughness, then use the measured number not the specification limit number.

After reviewing this issue more, I have to agree with Syd and question the Cd plating.  Why use it?  Low friction?  The electroplating process can generate hydrogen which can lead to delayed fracture (usually for medium carbon steels).  The intergranular fracture pattern would support this.  But the beach marks suggest failure due to cyclic loading.  Perhaps there are multiple failure mechanisms involved.

Regards,

Cory

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

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Cory,

we using the Cd plating for lubrication.As the bolt is stainless steel we dont ned more corrosive protectant coating, its simply for lube.

so you saying that the issue here is with both h2 embrittlement (intergranular fracture) and low preload (cyclic loading)?

is 1/2-13 17-4 phss a medium carbon steel?

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
\\\Can you check if your installation isn't resonating at some speed (if it is varaible speed conditions), or for instance during misfiring conditions (providing it is a combustion engine). You may get exceissive movements and related bolt loads during these conditions. \\\

Rob,

Could you please explain it more.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

No, 17-4 PH stainless steel is not a medium carbon steel.  The maximum carbon content is 0.07% by mass.  

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
TVP,

Thanks.

Tool we are using is clutch tool on this critical application and loctite 271 also.we got some alignment issues too.

where can I get K values for loctite 271

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Thanks Cory,

Regarding the same joint, I just now did ultrasonic clampload study on 6 bolts.
Assy has 2 bolts and my study is on 3 assy's.

I got some weird results

In 1 assy I used ,

Locitite 242, splitlock washer, torque - 66 ft-lbs,1/2-13 17-4 phss cadmium plated bolt.
My clamp load figures are 16200 and 13200 lbs

In 2 other assys I used,
Locitite 271, splitlock washer, torque - 66 ft-lbs,1/2-13 17-4 phss bolt
My clamp load figures are (11,900 and 14000 lbs) and (12,200 and 14,000lbs)

Th only diference is loctite. Why do u guys think I have that much of a clamp load difference?

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Hello,

Is your application in fresh water or salt water?

Matt Evans

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

That sure looks like SCC on the SEM photo.  The question is one of what is causing it.  You wouldn't expect to see pitting like that on 17-4 parts and not that agressive of attack.  I was wondering if there may be some sort of an electrical problem that may be inducing a stray current through the bolts which in effect accelerate the creation of hydrogen.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Screwman,

Yes our material lab confirms the same thing. But corrosion is something which u expect to see after the engine runs for some hrs in the water. But if u see the pic below,shows the no of hours the engine run in service and the failure mode.
http://img337.imageshack.us/img337/5465/hrscc9.png
u can see some of them broke, or fractured in 0 hr or 5 hrs. In this case how can we attribute the failure to corrosion?

Volpe,
Depends on the customer use, some of them will be in salt water (florida) and some of them will be in clear water (in lakes)

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

It is possible that the difference is due only to the different Loctite products.  242 is a medium viscosity product, while 271 is a low viscosity product.  Perhaps 271 is squeezed out of the joint more than 242, allowing more metal-to-metal contact, and hence, more friction.

Regards,

Cory

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

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

One thing that you should do right away is to eliminate the helical "lock washer".  It is contributing nothing to keeping the joint intact and may be creating a stress riser on the underhead surface that is contributing to the failures.  242 Locktite should be more than enough to keep that bolt from loosening.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

I completely agree with Screwman-- get rid of the lockwasher.  The Loctite plus preload is sufficient for resistance to loosening.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Unclesyd,

Here is the info you asked for,

Fastener material - Stainless Steel: Group 7 per ASTM F593 Except to age condition to H-1100 (HRC 30-38 REF) with cadmium plating. I have the the treatement process for the fastener with me. Please let me know what you want.It is a 16 page process , so I dint upload it. I can do that if u want.

Washer: split lock stainless steel washer. Fastener under head contact  surface.

Tapped hole material: Investment Cast SS CF-8 per ASTM A743-81A. Fastener thread contact surface.

Please let me know if u need more info

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
TVP & Screwman,

Yes, we are planning to get rid of that lock washer for the next year model.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

You can test your inventory for hydrogen embrittlement.  See the ASTM F519-06e1 Standard Test Method for Mechanical Hydrogen Embrittlement Evaluation of Plating/Coating Processes and Service Environments for details.  It is a fairly quick (4 days max, often times 2-3 hours) and dirty way to check your inventory.    

Essentially you load the bolts up to the proof load on a fixture and wait 96 hours.  Remove them and if they haven't completely failed check for cracks.  

If you have lot control you can sample a few from each lot.  If not, take a larger sample from your inventory.  

It is a destructive test, so even if they pass do not use the test specimens.
  
Also, stainless steel can be effected by localized galvanic corosion if exposed to salt water.  The Cad plating provides a sacrifical layer, so you may want to keep it or something similiar (zinc).

good luck

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Odd.  Loctite 271 is normally used for sealing porosity in castings, or for application _after_ wrenching a threaded connection.  It's plenty strong, essentially permanent, but it wicks into every available crevice, which may not be a desired behavior in all circumstances.  Its low viscosity suggests that it may not be a particularly good thread lubricant.

The 242 is a better choice if you ever want to get the joint apart.





Mike Halloran
Pembroke Pines, FL, USA

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
I just did a comparison study using skid more and ultrasonics in the lab. I see a real big difference in the clampload in production line joint and in the lab

In production line,  I gave the machined bolts to the operator and asked him to run them at 66 ft-lb torque and used ultrasonics to see the clampload and I got an average of 14500 lbs .

I replicated the same test in the lab using skidmore and also using ultrasonics (used both to see if there is any big difference between ultrasncs and skidmore).
On skidmore I always got less than 10000lbs for the same 66 ft-lb torque. Please see the attached results.

http://img255.imageshack.us/img255/3080/skidmoreil4.png

What is the reason of getting higher clamploads ( around 5000 lbs) for the same torque on production line than in the lab.

FYI: clutch tool we are using on line is doing really good. I cannot attribute the reason to the gun.

And also could you please help me understand why the torques are different to achieve the same load in different bolts?

For example, if you see the link, to achieve 10,000 lbs
Bolt 1 took 94ft-lbs
Bolt 2 took 77ft-lbs
Bolt 3 took 83ft-lbs

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
For those who dont know,
minimax is the ultrasonic instrument.

Initially I thought if skidmore is doing something wrong, but I get the same value of load when I cross checked with ultrasonics minimax. so skidmore proved that the results are good.

For some reason, the same minimax is hsowing very hight loads,with the same torque on production line. I dont understand why?

Thanks in advance for your ideas

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Friction variation and geometry variation can explain differences of 10-15%, which is what you are seeing.

Difference between lab and line results can be due to tool speed variation, which affects friction.

Did you know that the most highly engineered fastener coatings have friction variation of ± 25%?  For example, Magni 565 with μ = 0.13 ± 0.03.  

Regards,

Cory

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

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Cory,

Does friction due to tool speed and goemetry variation has such a big effect (around 5000 lbs difference)?

friction variation gives me the answer to clamp load variation, but how can I attribute the friction variation to consistent low tension in skidmore and consistent high tensin on line?

Thanks a lot.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Usually not a 34% difference (5000 out of 14,500), more like 5% to 10%.  Do you know the rotation speed for each test?  If the Skidmore test was 10 rpm and the power tool was 10,000 rpm, then maybe it can account for this.  Your materials and coatings are not ideal to minimize variation.

Regards,

Cory

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

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Cory,

If I have a grip length change from lab to production joint,will there be a difference in clampload for the same torque and same environment?

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

When doing the comparative tests make sure that you duplicate both the production line nut member and bearing surface.  Each of those will contribute nearly 45% of the total friction in the joint.  Also use all new (virgin) components for each test.  Once the parts have been used you change the friction characteristics significantly.
One thing that mey be contributing some amount of difference between the production line and the lab could be the comparative differences in joint stiffness between the two systems.  This will only account for maybe 10% difference though.  But with the "lockwasher" in the joint you may be getting quite a bit of emmbedment when power tool tightening.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Screwman,

If there is an embedmenet due to lockwasher and power tool on the production line, then I should see less clampload right? But I am seeing more clampload on the production line.

The differences in the test are

On the skidmore, our technician used hand wrench , in production line its clutch tool (will find out the speed)

Tech dint use any loctite on lab joint  (on production line we use loc 272)

Tech used washer plates to simulate the joint and grip length(geometry variation),In production its actual production joints

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Did the technician use any lubricant at all?  If not, then this is the only answer you need.  As mentioned previously, Loctite 272 will act as a lubricant during tightening.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

The locktite and also the use of washer plates if they were what the lock washer bore up against, along with the power tool vs hand tool all added to the variation.  
You can not believe how critical lubrication consistency has to be on some joints.  We had a case where head bolts were breaking at random and it turned out that a leaky cylinder was occaisionally dripping oil into the tapped hole and that was enough to cause the bolts to tensile during installation.  Process control is critical on really fussy joints; everything has to be kept spot on for the units to turn out right.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Make sure the washers that you are using on the test stand are ground and better yet hardened. Stacking washers can act like a short spring.

The mention of speed of tightening can play an big part in the actual applied torque. We had an incident where the mechanics were running in a 3/8-16 H11 SHCS with a speed handle and then walking around the fixture with a torque wrench.  The torque wrench clicked at the 45 ft lb set point but the actual torque was above 55 ft lb. We lost fasteners when we went to dismantle the fixture.



Here is a nice bolt stress calculator that I ran across while looking for an article on the values of K and thread friction factors. Click on the high high level of detail buttons to get all the results.

http://www.turula.com/bolt.html


RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Unclesyd,

Thanks for the ideas.

In the link u provided, how can I get the value of
"Bolt Torque Factor = q = ratio of torque in the stressed bolt cross-section to the torque applied to the bolt"

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

I take it to be the ration of torque that is strictly applied to the bolt. The problem is that that this is called out to be 10% to 30% depending on a bushel of other variables. Your two problems are prime examples in that in one you are turning just the nut and the other your are engaging the threads.

I'm still after an article that I ran across that developed a relationship between the two, K and mu.


  

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Unclesyd,

are u looking for  formula that relates k and mu? If that is what u r looking, I have the formula and some values of mu to the corresponding K

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Thanks for the reply. Here is a excerpt from a book that has some of the information that I spoke about. There was an article that also covered the topic to the point of determining which value to use. Though not a participant in the testing aspect we did a lot of work in this area when we started using H11 SHCS and Allen Nuts. As I recall there was a lot conflict when it came to the torque values and the thread deformation. We also had to go to hardened washers to achieve some of bolt stresses we required to meet our process requirements. I was looking at the values of mu versus the K that might govern the bolt stress in these two cases.
Start on page 156

http://books.google.com/books?id=TAWznUs54uwC&pg=PA157&ots=qTRI6pInDI&dq=nut+factor&sig=SyQ2VQ1U4gTierWyBpgT3TllqAQ#PPA22,M1

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Preload,

sorry I was away for a couple days and haven't checked forums. What i meant was that there may be resonance conditions during which excessive bolts stresses occurr. Apart from broken bolts, you also mention bolts running loose and in my book that means vibrations.
And don't know anything on the installation or oprating conditions, but if it is a diesel, misfiring is a condition whhich is often overlooked when designing an installation.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

(OP)
Rob768,

Thanks for the explanation.

Thanks a lot guys for your ideas on this issue. Our engineering dept has identified the root cause and they are pretty confident solving this problem. So for now I dont need to focus on this issue.

RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help

Preload,

  What were the final conclusions from the engineering department?  Looks like a very interesting problem with multiple potential causal issues.

bob

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources