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?
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
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
h
h
ht
ht
ht
h
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
h
ht
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
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://i
this pic is the assembly pic (it just shows one fastener)
http:/
this pic is the whole view of assembly (both fasteners)
ht
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
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
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
Rob,
Could you please explain it more.
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
ht
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
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
Is your application in fresh water or salt water?
Matt Evans
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
Yes, we are planning to get rid of that lock washer for the next year model.
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
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:
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
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
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
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
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
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
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
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
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
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'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
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
Start on page 156
http://boo
RE: Stress Corrosion/Hydrogen Embrittlement/low Clampload help
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
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
What were the final conclusions from the engineering department? Looks like a very interesting problem with multiple potential causal issues.
bob