4140 Cold Drawn Bar Fractures
4140 Cold Drawn Bar Fractures
(OP)
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.
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.





RE: 4140 Cold Drawn Bar Fractures
Regards,
Cory
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RE: 4140 Cold Drawn Bar Fractures
RE: 4140 Cold Drawn Bar Fractures
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RE: 4140 Cold Drawn Bar Fractures
Maui
RE: 4140 Cold Drawn Bar Fractures
RE: 4140 Cold Drawn Bar Fractures
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RE: 4140 Cold Drawn Bar Fractures
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RE: 4140 Cold Drawn Bar Fractures
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.
RE: 4140 Cold Drawn Bar Fractures
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.
RE: 4140 Cold Drawn Bar Fractures
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: 4140 Cold Drawn Bar Fractures