Stress Corrosion/Hydrogen Embrittlement/low Clampload help

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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?

 

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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).

 

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Pics of the failed bolts uderhead from field and material lab inspections pics

http://img108.imageshack.us/img108/6087/umfbzw4.png

http://img112.imageshack.us/img112/881/umfb1fl7.png

http://img108.imageshack.us/img108/1852/umfb2zc1.png

http://img102.imageshack.us/img102/1807/umfb3ar9.png

http://img453.imageshack.us/img453/4927/umfb4ba8.png

http://img128.imageshack.us/img128/4250/umfbqu7.png

 

[TVP]

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.?

 

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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

 

[CoryPad]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

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?

 

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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

 

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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.

 

[metengr]

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.

 

[rob768]

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.

 

[CoryPad]

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 for tips on how to make the best use of Eng-Tips Fora.

 

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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?

 

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\\\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.

 

[TVP]

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

 

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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

 

[CoryPad]

You can contact Loctite directly:

http://www.loctite.us/int_henkel/loctite_us/meta/index.cfm?&pageid=16&page=Product_Information

Regards,

Cory

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

 

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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?

 

[volpe2]

Hello,

Is your application in fresh water or salt water?

Matt Evans

 

[Screwman]

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.