17-4PH / AISI 630 machining 6

[metalguesser]

Hi All!
We made our first flange in the 17-4PH / AISI 630 grade. After forging we performed post forge thermal treatment and later aged it to achieve a hardness of 330/335 BHN. So far so good.

In the next step, we intend to machine the part and then subject it to a sub-zero quench at -60 deg or below, per customer specification. Given its martensitic structure The question that comes to the mind is;
1. Will the part fall out of its specified machining tolerance after the cryogenic quench on account of movement when martensite is formed in the metal matrix?? How much excess material should be provided to compensate for the post cryogenic-quench machining operation (which will be carried out by the customer)??
2. Will there be warpage, specially in the flange section warranting some additional treatment(s). and if yes what treatment??

Appreciate any or all help in this matter.

Thanks in advance,

Regards

 

[unclesyd]

The sub-zero treatment should have very little effect, if any on the 17/4 flange.
At times we have subjected 17/4 SS (H1125) parts to LN2 with no measurable effect.

May I ask what is the reasoning for the sub-zero treatment?

 

[tbuelna]

metalguesser,

17-4ph cres at 335 BHN is approx. equivalent to an H1025 condition, which is solution heat treated and aged. The MIL-H-6875 solution heat treatment process for 17-4ph forgings describes an austenite conditioning and sub-zero stabilization (-100degF immediately after the conditioning quench). Thus, if you are performing machining operations after the conditioning quench but before the sub-zero treatment, your sub-zero treatment will have no effect due to the time lag.

The benefit of sub-zero stabilization is to minimize retained austenite. You haven't said what your flange is used for, but this heat treat step may not be necessary, unless you really need to control the retained austenite.

The austenite-to-martensite conversion process itself usually only produces a small amount of volume reduction (ie. .0001 inch/inch?), so it would only affect very close tolerance features. But on the other hand, any quenching processes would likely produce distortions. The degree of which is dependent upon the part's shape and section thicknesses.

Finally, 17-4ph is not normally used at tensile strengths above that provided by an H1000 condition, due to stress corrosion issues. So your specified H1025 condition (335 BHN or 160KSI) is right at the very limit of what 17-4ph is normally acceptable for.

Hope that helps.
Terry

 

[mfgenggear]

Just to add my comments

precipitation heat treating prior to machining is right choice, I fail to recognize why the cryo is processed after it is machined. normally it is done right after heat treat.

secondly normally low allow carbon steels are processed with cryo treatment after quenching, snap tempered, cryo ,then temper . I have not ever processed precipitation steels that required cryo treatment.

if there is any close tolerance diameters like .0003 tolerance or tighter I would be hesitant.
if the the requirement is required then I would recommend test samples. Inspect before & after machining.

 

[mfgenggear]

the last statement should read

Inspect test sample
after machining
then after cryo

as been said there may be no change or very little
but I like to stack it on sure way.

 

[metalguesser]

Thanks Unclesyd, Terry and mfgenggear for your valuable posts. That was really helpful.

To answer all your questions,
we have no clue why the customer wants us to do the the sub-zero quench. My guess is that since they are a manufacturing set-up supplying to their customer, the sub-zero quench request must have come from their customer. We are so far down the line we have no clear picture on what the actual application is other than the part being identified as 'AC compressor flange'. As Terry has said it must be for minimizing retained austenite, I guess.

For now we are required to supply the part per ASTM A705 solution annealed condition with subzero quench. I learnt that this alloy is seldom used in the solution annealed condition and so I suspect the customer might be carrying out some age hardening at their end before doing the final machining. Since, the customer is going to do the final machining and NDE our concern is; what if the customer finds all dimensions after cryo falling out of spec. This is our first ever part in this steel grade and have no idea how much volumetric contraction occurs. Will it suffice if we maintain 5% excess stock and let the customer know of the problem?? As mfgenggear has suggested we shall check that out on test bars as a study. We have provided excess material for testing purpose though the customer has not asked for any other than the hardness test. Time constraints may make the study academic, nonetheless it would be valuable data for future use.

That brings another question; Terry has mentioned a conditioning quench before the sub-zero stabilization. I spoke to our heat-treat vendors(we have no in house facility). Opinion is divided, one school suggests a water quench with ice slush in it and then move the part in to the cryogenic enclosure. While the other group say they have never quenched 17-4 PH in water and have always used oil maintained below 90degF for a max 15 minutes before moving the part to the cryogenic enclosure. How much time should be given before moving the part to the cryogenic enclosure? We are at cross roads again. which route to take how much time in the conditioning quench before moving to cryo?? Appreciate your input and help here.

Thanks for your patience reading through this long winding post.

Regards

 

[unclesyd]

I think it will very helpful if you checkout this bulletin on 17/4 PH SS from AK Steel. I think it will clear up a lot of your questions like growth and in this case contraction.

After reading this bulletin and if you have any more questions please post,

http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-4_PH_Data_Bulletin.pdf

 

[mfgenggear]

if the parts are machined after precipitation harden then no worries about contraction.

if parts are machined prior to the harden operation then dimensions must be compensated for dimensional change.

 

[tbuelna]

metalguesser,

Take a look at MIL-H-6875, table 1D. It gives guidance on heat treatment of 17-4ph:

http://docimages.assistdocs.com/watermarker/transient/4779DC725AAA498E88B05BC93528E40D.pdf

Good luck.
Terry

 

[mfgenggear]

precipitation age hardening is the way to go.
I would not ever water quench & any intricate parts.
distortion city

 

[remetaper]

Just to have an academic discussion.
Grade 17.4PH HAS NOT retained Austenite when condition A ( ASTM A564). In this condition, this grade shows a fully hardened Martensite ( supersatured by Cu-rich phase) with +/- Ferrite. Therefore,the only reason why a cryo-heta treatment is required , depends on to be sure that whole Austenite must be transformed to Martensite ( Mf< 90 F).
This Martensite ( free of not completely transormed Austenite )is ready to be aged from H900 to H1150. When aging temperature increases from H900 to H1150 F, a "NEW" Austenite appears together with an aged and tempered Martensite whose Cu-rich phase is losting its coerence with matrix.The amount of this stable Austenite depends on chemical balance ( Cr/Niequivalent) and aging temperature. Just to have an idea, the lower Cr/Ni equivalent ratio, the higher amount of stable Austenite. Moreover, the higher ( H) aging temperature, the higher amount of stable Austenite
This means that a 15.5PH has more stable austenite at low aging temeperature compared to 17.4PH because of its Cr/Ni eq lowest.
If overcoming 1150 Ftemperature, it will appear unstable Austenite that, when cooled, will transform to a fully hardened "fresh " Martensite . This situation could prime tension and delayed cracks if not rightly evaluated.
Talking about the subject of METALQUESSER,I 'd like to remember that load cell Makers ( where a highest Rp0,2/Rm ratio is very important)prefer to be sure that whole Austenite is transformed to Martensite by a Cryo HT after quenching ( cond. A). This situation allows a good performance after H900 aging with poorest or no influence referring the dimensional tollerance.
On the contrary, the boat shaft process required a condition A (no Cryo HT) +straightening + peeling + reeling +H1150 +grindining and polishing. Only in this case ,it is mandatory to evaluate the "shrinkage" of diameter of shaft when heat treated H1150 according, for instance, AK broshure or shop experience in order to increase the stock removing before grindining and polishing.

 

[tbuelna]

remetaper,

"Just to have an academic discussion......"

That's what this forum is for. I gave you a star for your technical contribution.

Regards,
Terry