Doubts on 4140 steel microstructure

[GonzaloMartinez]

Hello everyone,
I would like to hear (read) your thoughts on the image I'm posting. It belongs to a 4140, 2" diameter steel bar . Notice the coarse acicular structure + fine pearlite grains (?). I was told by a material specialist that it corresponds to the absence of heat treatment after rolling (uncontrolled cooling) but I could not find any similar image in the bibliography to support it. Do you agree with that statement?

Best Regards,

Gonzalo M.

Magnification 100X

 

[priyam07]

I would agree, looks like uncontrolled cooling (faster cooling in this case) and forming bainitic instead of pearlitic microstructure. Do you have hardness measurement done, high hardness might confirm this?

 

[GonzaloMartinez]

Yes I have! Hardness measurement resulted in values around 24-25 HRC. The component under study fractured in a brittle way, probably under a shock load. We have done charpy-V tests resulting in very low values for the absorbed impact energy, aproximately 6 Joules at room temperature.

 

[CoryPad]

This is bainite microstructure, and based on low toughness, it must be upper bainite. This structure is possible with 4140 steel when it has been subjected to intermediate cooling from the austenite phase.

 

[WKTaylor]

GonzaloMartinez...

WHAT material spec [and HT, etc] was the 4140 bar SUPPOSED to be made per: ASTM, AMS, MIL-, foreign, etc?

Did wet chemistry verify the alloy chemical constituents/ratios... or perhaps by spectroscopy?

CAUTION. Materials definition/requirements should be crystal clear and unambiguous from the beginning of procurement... thru to to the final packing of the finished parts.

An old engineer acquaintance in the fastener business wisely advised me: "You deserve to get what you ask for: but if You don’t ask for very much, don’t-expect to get very much."

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[GonzaloMartinez]

WKTaylor there wasn't any precise specification, the component was made with the material available at the shop. And well..... there are the results...

Thank you all for your help! I think I got enough information to reduce my bibliographic search.

 

[WKTaylor]

GM... I've heard this answer/excuse before...

More than once my shops have complained that raw steel stock 'wouldn't harden' like it was supposed to; or it cracked; or warped; etc.

When pressed, I found out that the shop was sure' it was a specific alloy because it was 'in the right scrap bin'... but was otherwise was unmarked.

This is precisely why aircraft/aerospace/ordnance parts mandate [anal] traceability. Detailed material certs [from a recognized lab] directly accompanying raw-stock made per a recognized spec is worth the effort and expense. Yeah, trust is still required that the 'system worked' [integrity prevailed]... but the ground-work is a solid foundation with checks/balances.

I am working a somewhat similar issue... but with a steel finish processing bath solution that 'went out-of-spec' for several months [getting more acidic]. Traceability has finally caught-up and we've been able to identify the discrepant parts in various assemblies and place them under quarantine. Imperfect as this system is, the 'system eventually worked'... better late than never.

BTW a couple of other Eng-Tip threads are also dealing with 4140 steel discrepancies... worth a look.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[priyam07]

Late reply I guess, anyways 24-25 HRC and low impact indicates Bainite. Was it supposed to be heat treated though? Again do you have the chemistry confirmed?

 

[GonzaloMartinez]

Hi sravii, chemistry was confirmed by optical emission spectroscopy; all elements of interest fall within the ranges for SAE 4140 (SAE J404). As to the material and heat treatment specification, I live in an area with lots of machine shops, few material specialist and poor steel suppliers. You can get the picture of the environment... Whenever we have the opportunity we advise our clients to be more specific in their manufacturing requirements, but sadly there is a general lack of understanding on the criticity of the issue.