4130 cracking at LOW temperatures 3

[awol]

We have machined parts made from 4130, austenized and quenched and tempered to Rc 28-32.
They were put in service in 1977 in Alaska. Although we tested them at -50 F , these parts have developed cracks.

Would treating these parts with cryognics (liquid nitrogen) have helped avoid these cracks?

These parts could have been exposed to a shock load but doubtful.

Parts are 1.25" thick and used in shear.

Thanks,
awol

 

[TVP]

It is doubtful that treating the parts in liquid nitrogen would have prevented the cracking, but you need to provide some additional information in order to ascertain the problem. What is the function of the parts? What were the forces/moments exerted on them? Temperature fluctuations? What type of corrosive environment is/was present? Chlorine? H₂S? Are they plated or coated? Were they welded? What was the fabrication method (forging, machined from bar, tubing, etc.)? If you can provide some additional details, we can probably help you.

 

[awol]

Here's what I have:
function: The 4130 parts hold a large metal plug in a pipeline. Pipeline pressure (crude oil) is trying to push the plug out.
forces/moments : No moments. Pure (as can be) shear. We estimate 40,000 psi shear if only half of these items take the load. This is conservative.
Temperature fluctuations: In Alaska; Frozen tundra, some above ground.
corrosive environment: Chlorine? no H2S? no
plated or coated? no, oil dipped and lubricated
Were they welded? no
fabrication method: machined from bar

These didn't crack all the way thru. Just cracks were noticed during disassembly and cleaning. Disassembly and cleaning is done only when they are being cut from a pipeline for reuse elsewhere. Cleaning and disassembly not a normal process. Now you have me wandering what they used to clean these with. Maybe something harsh. I will ask.
Thanks, Awol

 

[mcguire]

Like TVP I would be very interested in knowing what environment and mechanical load his tory these components have seen. Delayed btittle failure in high strength steel always leads one to suspect hydrogen embrittlement, stress corrosion cracking or fatigue. If you can rule out the fatigue, then HE and SCC are all that's left. They share the same mechanism. It's just a question of whether the hydrogen came from corrosion or was in from the beginning.

 

[mcguire]

Delayed failure under static loads of high strength steel in the absence of corrosion at ambient temperatures can only be hydrogen embrittlement, arising from hydrogen in the material from melting. Baking the parts could have expelled hydrogen. Cryogenic treatment would have done no good.

 

[KO]

What was your soaking temperature, quenching medium, and tempering temperature?

 

[awol]

Ans. to KO's questions:
Material is bought hot rolled plate.
we have a vendor heat treat it to Rc 28/32 (we don't specify the temper or quench media; is that important?)
We do impact testing on test coupon at low temp.
No coating. Just a dry film spray (aerisol).
No welding. Just milling and drilling. No tapped threads.
Thanks for the help!
-awol

 

[TVP]

So, are we to assume that this component is exposed to atmospheric conditions, with no corrosion protection? If so, then oxidation of the surface could produce conditions leading to stress corrosion cracking or hydrogen embrittlement. Otherwise, I agree with mcguire that something from initial processing (cleaning, heat treating, etc.) led to hydrogen being charged into the steel surface, which would lead to hydrogen embrittlement if not properly relived. I strongly suggest you review the processing of these components to evaluate the following:

1. Is corrosion protection (zinc plating, organic coating, etc.) necessary?

2. Is hydrogen embrittlement causing the cracking? Failure analysis using a Scanning Electron Microscope can detect whether or not this was a brittle failure (intergranular/cleavage fracture instead of ductile failure by microvoid coalescence).

3. What processes contributed to the hydrogen environment? Or is the hydrogen from an external source once the component is in use. If it is the latter, see point number 1.

 

[gdodd]

If part has been hardened and tempered then any hydrogen introduced during manufacture should have been removed during heat treatment.
If part was pickled to remove heat treatment scale then back comes the hydrogen although i'm not sure how significant hydrogen will be in a part with hardness 28 to 32 and service life approaching 25 years.

Off hand I do not know the Mf temperature for 4130 but if its well below room temperature and your part working conditions are well below that temperature, I would have insisted on a cryogenic heat treatment operation to ensure all austenite has transformed to martensite during quenching. Then tempering can proceed without the risk of untempered martensite forming from any retained austenite remaining in the microstructure after quenching.
As part was installed in 1977 it is difficult to check heat treatment records, however if you are replacing the part, seriously consider your heat treatment tecnique.

 

[mcguire]

Whoa! Cryogenic treatment will not help prevent delayed failure on parts that are going to be used above the threshold hardness for hydrogen embrittlemment/stress corrosion cracking. One can argue whether the source of hydrogen was environmental or due to residuals from processing, but the first step must be failure analysis.
If it was brittle delayed failure was caused environmentally, then corrosion must be present for SCC to ahve occurred. In which case, coatings, etc., will help.
Absent corrosion, brittle delayed failure comes only from hydrogen embrittlment due to initial processing sources.
If failure analysis can show fatigue or shock loading or some other failure mode due to external forces, then toughness enhancement from cryogenic treatment may have helped delay or prevent failure.