cracking in 420 ss at 48 RC

[dlangt]

Hello All,

Looking for some advice. We make injection molds and have an issue where 420 hardened to 48-50RC is cracking in our material delivery systems.
Looking for a material with increased tensile and elongation at 120C with similar corrosion resistance.
Was thinking about using a precipitation hardening grade like 17-4
As always cost is a concern so this eliminates the maraging grades

Would appreciate any comments or recommendations.

thanks,

 

[EdStainless]

Is this cracking from mechanical fatigue to thermal cycling?
If mechanical you may need thicker material or better support.
If thermal then slower heating/cooling might be in order.
Why not move to 425 or 440A.
Similar or slightly higher Cr content, and you could raise the temper temperature to 900-950F and still have similar hardness.
This would improve the toughness.

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P.E. Metallurgy, consulting work welcomed

 

[dlangt]

Sorry i should have added more details.

It is from mechanical failure. It is a nozzle that can see upwards of 30,000 psi in an orifice of 0.5" x 4" long
From a real estate perspective, it is not easy for us to change the design. We looking at this to increase the wall thickness by approx 15%

I think i really need a material with similar hardeness but increased ductility/elongation to avoid the stress cracking

 

[EdStainless]

It isn't stress cracking is it? Far more likely fatigue.
My alloy suggestions still stand.
17-4PH would not help you at all.
In order to get decent toughness you need to age at at least 1025F.
And then your hardness would be <40HRC.
You need to figure out how to add material to the structure.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[dlangt]

Hard to say if it is stress or fatigue. Pressures can be variable. Probably more related to cyclic stress which would fall under fatigue.
With regards to 17-4 we would age to condition H900
At H900 we would get an increase in tensile and elongation over 420. Will look at 440C to compare as recommended.

My experience with 440 C is that it cracks easier than 420 but all my experience is with it at 58 RC
We also use 440C for a lot of mold cavities.

Do you know of anywhere to get good tensile/elongation data at varying degrees of hardness for 420 / 440 C. I have this info for 17-4 at all conditions.
We do plan to change the o-ring gland where we believe the cracking to be originating at. Other than that it is not possible to change the design as whatever we do must be backwards compatible with existing tooling in the field.

We can do a field test with 440C fairly easily.

Thanks for your help, i appreciate the feedback. Always nice to bounce ideas around.

 

[MagBen]

17-4 will not get you >48RC by any means.

if still having tolerance of cost, check other PH grades. custom 465, then a little cheaper 455, then cheaper 13-8. These all can get you to 48rc or above with some roughness, all are more expensive than 17-4 though.

 

[EdStainless]

Go to the ATI web site.

https://www.atimaterials.com/Produc.../ati_410_420_425_mod_440a_440c_tds_en2_v2.pdf

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

You may want to look at the geometry of this area. The notch cuts through the radius on the step and the notch appears to have unfinished edges. These may contribute as stress raisers.

 

[mrfailure]

I still think it is worthwhile to perform a metallurgical analysis to better understand. Break open the crack and look at morphology under the SEM to confirm it isn't fatigue, identify where the crack first began, and establish the type of fracture (i.e. transgranular vs. intergranular?). Also cross-section transversely through the fracture origin to establish whether secondary cracking is present and confirm microstructure consistent with specified heat treatment.

Given the hardness, is it possible that hydrogen embrittlement occurred through hydrogen pickup either during processing or subsequent service?

 

[mfgenggear]

I agree with Mr Failure, Send the part out to a met lab to process a failure analysis.
now my educated guess we are looking at a heat treat crack.
do a fish bone analysis and write down what the possible causes.
then eliminate each one to prove what it is not to narrow down what it may possibly be.
I would suggest may changing material AISI 9310 that is surface carburized, 4340 Mod that is
induction harden , why was 420 SS chosen.

 

[mfgenggear]

o ya the way figure out if it is a heat treat crack when they section the pc if there is corrosion in the crack area, then yes it was initiated at heat treat. be sure to have every part before heat treat and after final inspection , a NDT of the part to verify it was free from cracks. then after so many hours of service yank it out and have it NDT .
if cracks present then could be due to incorrect heat treat, stress risers, wrong material for fit form or function.