×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Doubts on 4140 steel microstructure

Doubts on 4140 steel microstructure

Doubts on 4140 steel microstructure

(OP)
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

RE: Doubts on 4140 steel microstructure

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?

RE: Doubts on 4140 steel microstructure

(OP)
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.

RE: Doubts on 4140 steel microstructure

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.

RE: Doubts on 4140 steel microstructure

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]

RE: Doubts on 4140 steel microstructure

(OP)
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.

RE: Doubts on 4140 steel microstructure

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]

RE: Doubts on 4140 steel microstructure

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?

RE: Doubts on 4140 steel microstructure

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources