Technical question - U-BOLT microscopic cracks on the tooth of the rolled thread

[FRIDENS]

We found in U-BOLTS (4 from 7 of them) microscopic cracks on the tooth of the rolled thread.

Do you know what the reason for these microscopic cracks?
Is this failure critical for the function of the u-bolt in the vehicle?
How can we make a sorting?
Do microscopic cracks in thread rolling process typical?
Do you have any experience with these smaller cracks?

Appreciate your answer.​

 

[brahmaiah]

Hairline cracks are less dangerous.But everything depends on application.

An experianced metallurgist can throw some light on this if foolowing additional information is provided:
a)Material specification
b)Any heat treatment carried out before rolling
c)Hardness value
d)Exact location of crack on the thread
V.J.Brahmaiah

 

[Umang Vidyarthi]

We found in U-BOLTS (4 from 7 of them) microscopic cracks on the tooth of the rolled thread.

Do you know what the reason for these microscopic cracks?
Is this failure critical for the function of the u-bolt in the vehicle?
How can we make a sorting?
Do microscopic cracks in thread rolling process typical?
Do you have any experience with these smaller cracks?

Appreciate your answer.​

Hello Fridens, welcome to the cove

I'll try to answer in serial order:

1. The main cause of surface defects in 'Rolled threads' are misalignment of the roll working thread tracks on the roll thread surface, which causes asymetrical deformation of metal, and rolling with the 'roll working thread profiles' full of metal, which causes axial displacement of surface layers. The defect/crack caused may be superficial, id est, only skin deep or may penetrate deeper.

2. If the defect is superficial then it may not be critical, depending upon the permissible deviation/limit given by your customer if you are the manufacturer, or by your quality dept. if you are the end user.

3. You may segregate the defective components by using any of the NDI (Non Destructive Inspection) technique viz: Ultrasonic (used to detect fractures in metals), X-Ray, Eddy current(for close to surface defects) or 'Thread spooler' equipped with a thread defect sensor.

4. I manufacture components with rolled threads, and have encountered minor cracks on the thread surface. But permissibility of defects depends upon the usage. What is okay for me may not necessarily be fine for you.

Hope this helps.

Umang

 

[brahmaiah]

The suggested methods of detection of surface cracks on threads has some practical difficulties:
1.How can we probe the narrow surface of the threads for ultrasonic and eddycurrent crack detection?Should we make special probes to match the thread profile?

2.When hairline cracks are on the threads,How can we x-rax the threads?

3)But the last method suggested is special.

Could you please tell us more on above points?

Thanks,
V.J.Brahmaiah

 

[Miner]

Umang gave some good information. I ran into something similar as a tier 1 automotive supplier receiving bolts from a tier 2 supplier. They are definitely defects, but are not strictly cracks. During rolling, the metal forming the thread is deformed incorrectly such that two separate sections of metal are rolled up against each other forming a seam between the two that looks like a crack.

From a practical view, the difference is that a crack will have a sharp root that will progress over time. The rolled defect will have a rounded root that is less likely to progress over time. However, the thread itself is still weaker than a correctly rolled thread.

 

[MIREGMGR]

Are there any "vehicle" ( I assume automotive) parts for which the risk associated with the function of that part in its intended use, including warranty-related financial risk, is less than substantial?

Quality should handle such problems only up to detecting them. From that point forward, they're an engineering issue...unless in the OP's company the quality folks are credentialed and have formal responsibility in structural analysis, program management, production engineering and metallurgy.

 

[Wes Bucey]

We found in U-BOLTS (4 from 7 of them) microscopic cracks on the tooth of the rolled thread.

Do you know what the reason for these microscopic cracks?
Is this failure critical for the function of the u-bolt in the vehicle?
How can we make a sorting?
Do microscopic cracks in thread rolling process typical?
Do you have any experience with these smaller cracks?

Appreciate your answer.​

Hairline cracks are less dangerous.But everything depends on application.

An experianced metallurgist can throw some light on this if foolowing additional information is provided:
a)Material specification
b)Any heat treatment carried out before rolling
c)Hardness value
d)Exact location of crack on the thread
V.J.Brahmaiah

Hello Fridens, welcome to the cove

I'll try to answer in serial order:

1. The main cause of surface defects in 'Rolled threads' are misalignment of the roll working thread tracks on the roll thread surface, which causes asymetrical deformation of metal, and rolling with the 'roll working thread profiles' full of metal, which causes axial displacement of surface layers. The defect/crack caused may be superficial, id est, only skin deep or may penetrate deeper.

2. If the defect is superficial then it may not be critical, depending upon the permissible deviation/limit given by your customer if you are the manufacturer, or by your quality dept. if you are the end user.

3. You may segregate the defective components by using any of the NDI (Non Destructive Inspection) technique viz: Ultrasonic (used to detect fractures in metals), X-Ray, Eddy current(for close to surface defects) or 'Thread spooler' equipped with a thread defect sensor.

4. I manufacture components with rolled threads, and have encountered minor cracks on the thread surface. But permissibility of defects depends upon the usage. What is okay for me may not necessarily be fine for you.

Hope this helps.

Umang

Umang gave some good information. I ran into something similar as a tier 1 automotive supplier receiving bolts from a tier 2 supplier. They are definitely defects, but are not strictly cracks. During rolling, the metal forming the thread is deformed incorrectly such that two separate sections of metal are rolled up against each other forming a seam between the two that looks like a crack.

From a practical view, the difference is that a crack will have a sharp root that will progress over time. The rolled defect will have a rounded root that is less likely to progress over time. However, the thread itself is still weaker than a correctly rolled thread.

I'm presuming from the context of the first post that your organization is manufacturing the bolts. Is this true? or are you inspecting bolts from a supplier?

If you are manufacturing, then you should collaborate with the customer (internal or external) who can determine (from the function of the bolt in use) whether the "perceived" cracks will affect the function of the bolt.

If you are the customer and using the bolts, then your engineers make the determination of whether there is a detriment to the function of the bolt.

If the condition does not affect, then the situation is not critical and you can examine the root cause and inspection methods at leisure.
If the condition prevents use of the bolts for intended use,

1. then you need to determine EXACTLY what the condition is - real cracks or seams?
2. then root cause - caused by metal condition or by misaligned thread rollers?
3. then corrective action - machine or material?

If the situation to determine any of these steps is beyond the capability or capacity of your present staff, you may have to call upon an outside specialist (metallurgist, thread roller supplier, other) to help you sort through the actual condition, cause, corrective action.

My own experience with machining problems has always been that someone has to actually feel, see, smell, or otherwise handle product and machine to come to a final determination, especially when some of the suggested investigation activities entailed expertise or instrumentation we did not possess. I often used the services of a metallurgical lab to investigate problems using their special equipment and instrumentation. The fee always seemed reasonable versus spending interminable time, money, and effort trying to learn and gear up for an investigative technique the lab performed on nearly a daily or weekly basis.

Ultimately, every business needs to make the most effective and efficient use of its resources and not expend time and money learning and using a process that may be used only once in a lifetime compared to a hiring a professional who routinely performs the task on a regular basis.

 

[FRIDENS]

Following is answers to mr. V.J.Brahmaiah:

Material is SAE 5140 H
No heat treatment carried before rolling
Hardness value 16-18 RC
See exact location of crack on the thread in picture attached
Its U-BOLT for connecting Rear Axle of a vehicle to its leaf springs.2 U-BOLT holding each leaf spring total 4 U-BOLT per Axle.

Regards, Shlomo - Mechanical Engineer

 

[Wes Bucey]

Following is answers to mr. V.J.Brahmaiah:

Material is SAE 5140 H
No heat treatment carried before rolling
Hardness value 16-18 RC
See exact location of crack on the thread in picture attached
Its U-BOLT for connecting Rear Axle of a vehicle to its leaf springs.2 U-BOLT holding each leaf spring total 4 U-BOLT per Axle.

Regards, Shlomo - Mechanical Engineer

Three identical tears certainly leads one to suspect the machining process, not material. Thread rolling was never my thing - we cut metal, not deformed it.

I do know there are at least two ways to roll a thread

1. hold workpiece still and rotate thread rolling dies around the piece
2. rotate workpiece against thread rolling dies

Given that, I'd suspect die alignment first, then die pressure. Seems simple enough to experiment to discover whether one or both are primary cause, then to explore root cause of how the condition occurred (bad setup, bad die, bad force calibration, wrong speed, wrong feed, etc.)

 

[Miner]

This looks like an extreme example of the type of rolling defects that I remember receiving. This definitely appears to be a problem with material deformation, not a true crack.

Given your automotive suspension application, this would not be acceptable given the severity of the failure mode.