4140 Cold Drawn Bar Fractures 2

[paw1448]

I am investigating a couple of failures in 4140 cold drawn round bar. One material was hot rolled 1-3/8" diameter, the other 1-3/4" diameter. Both were cold drawn to pitch diameter for subsequent thread rolling. After drawing and before threading, the bars were induction heat treated to A193-B7 condition. After threading and one size was Zn plated, the other was "black" as threaded. During installation, both failed at apparently low tensile forces resulting from torqueing the nut. Both fractures were identical. Fracture initiated at center of bar and propagated radially to the OD (somewhat starburst pattern). There was a ductile shear lip at OD with slight reduction of area, slight increase in thread pitch and elongation of thread crest.

I suspect that the bar might have ruptured internally during cold drawing and managed to stay together during processing until tensile stress applied during installation with nuts. Has anyone seen similar failures in this type of material?

I have also seen the same fracture pattern in a larger diameter 1045 cold drawn bar that was not subsequently heat treated. Upon machining the 1045 bar into a shaft, a crack became visible and upon releasing the pressure from the lathe tailstock, the material separated, showing the same fracture pattern as seen in the 4140 material.

 

[CoryPad]

Your idea is possible. This is known as a central burst defect. This has been studied extensively and an Internet search will return a lot of information.

Regards,

Cory

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

 

[swall]

I have seen this type of failure caused by thread rolling on warm forged then turned 1050 material. I would expect a failure caused by cold drawing to exhibit a "chevron" separation rather than a "starburst" fracture.

 

[EdStainless]

The original defect may have formed in the hot rolling process. Some simple NDT would detect this.

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[Maui]

I was thinking the same thing Ed. I've seen internal burst defects that resulted from either overheating or underheating during rolling, and this sounds like a defect that was not detected during hot rolling. If you could post a photograph, or provide a link where one could be viewed, that would help a great deal in determining the type of failure and the likely cause.

Maui

 

[paw1448]

Based on CoryPad's response, I did some web searching and based on the information I found, combined with the processes my client is using, I think that CoryPad hit the nail on the head. I'm trying to get in touch with who appears to be the expert in central burst defects to get his opinion. However, I would like to post a photo for all of you to see and maybe that could help you help me. Please tell me how to post a photograph and I'll do it. Thanks for your input.

 

[metengr]

paw1448;
Go to the web site below and copy your file with the pics. Once you have the link, copy this link to your post, and others will be able to view it directly.

http://www.tinypic.com/

 

[paw1448]

Here is the link to the photograph of the fracture. All of my fractures, whether drawn, heat treated and roll threaded or only cold drawn appear as this one does.

<a href="

http://tinypic.com"

target="_blank"><img src="

http://i18.tinypic.com/6czjy8y.jpg"

border="0" alt="Image and video hosting by TinyPic"></a>

 

[metengr]

paw1448;
I reviewed the pic. The fracture in your picture looks brittle with uniform, radial-oriented ridges emanating from the center, as you described. However, in my opinion, I would not hang my hat on any of the aforementioned causes until you have this failure analyzed (chemistry, SEM of the fracture surface, metallographic examination) by a metallurgical laboratory.

 

[paw1448]

Thanks, metengr for your additional input.

I have looked at the fracture surface on the SEM. Some small areas near the center are ductile (microvoid coalescence - dimpled) and all of the shear lip around the OD is the same. The majority of the surface with the radially oriented ridges are cleavage and quasi-cleavage with no sign of intergranular fracture. I have not performed chemistry, however, the mechanical properties are right in line with what has been heat treated and tested over a 10 year period with no complaints from my client with regards to chemical composition from the mill that produced the material (or on samples that we have analyzed here for them). Our met lab has been performing these tests for this client for the 10 years, so, I don't doubt that the material is 4140. Yes, I should and will run a chemistry to verify but without it, I am 100% sure of the alloy. Regarding the microstructure, I also have not examined this specific sample. However, a specimen analysed from this lot of material at the time of production was tempered martensite. Again, with the history of this product, I am sure that that is what I will find on this sample. Also, other samples of this lot of material that we tensile tested full size showed tensile strengths in line with machined specimens from this broken sample and also had significant reduction of area and elongation in the threads. The sporadic and intermittent failure occurrence along with the other observations are some of the reasons I am leaning to a burst that occurred during drawing prior to HT and threading.

 

[CoryPad]

The definitve ways to look for bursts include 1) using x-radiography of the initial bar and final bolts; and 2) longitudinally cross-sectioning the bar and bolts. Usually bursts are clustered, so you may see more than one, including on the sample shown in your picture.

Regards,

Cory

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

 

[swall]

I looked at the photo. It shows a tensile overload fracture. Happened a room temperature. I see this stuff all the time in wire drawing (room temperature) when a strand breaks. Definitely not a burst.