17-7PH OR (9ru10) 1

[NickE]

First the process flow as designed.

condition C -> Condition A -> blank/form -> Harden to RH950

(The reason we had to start with Cond. C is cause thats all that our supplier could deliver right now. so we Vacuum annealed the material to make prototypes. Production will use actual mill annealed coil.)

The dimensional test pieces came back from the heat treaters (VacMet, Warren, MI) today and the rockwell is roughly 10pts (HRc) under the expected (35 vs 44). I've checked the material cert from Sandvik and the chemistry is correct for 9RU10 (which has the same nominals as 17-7PH). Haven't mounted and polished a specimen yet (since we dont have a met lab at my location. I reviewed the furnace charts and all them temps and times were correct. No scale or other staining of the steel so I'm sure that there was no de-carb or carb going on.

What happened?

As an extra note: I did an air furnace anneal and treat of the same steel (from the same dang piece) last year and for condiiton TH1050 got HRc or ~42. I am completely lost as to why this happened.

nick

 

[NickE]

Furthur note: Allegheny Ludlum and AK Steel disagree as to what time to use for the 950F aging treatment. AL says 90min AK says 60min. (THis is form their data sheets. the links are:

AK-Steel (pg 4)

http://www.aksteel.com/pdf/markets_products/stainless/precipitation/17-7_PH_Data_Bulletin.pdf

AL (pg 4)

http://www.alleghenytechnologies.com/Ludlum/Documents/al17-7.pdf

)

hmmm Underaged?

nick

 

[EdStainless]

Sounds like underaging to me.
Re-age one of already aged pieces and see if it gets harder.

You know that without an anneal after forming your finished hardens will vary based on the amount of local cold worked. Heavily strained areas could end up much harder than the nominal.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[NickE]

Ed- thanks, yeah I had figured that (the hardness issue), I'm not sure that the extra hardness will be too detrimental cause its in a very high stress location.

I'm going to re-age one of the test pieces as soon as my little prototype furnace gets back up and running, Right now it just got moved badk here form the tool shop and needs some wiring to make it work.

nick

 

[israelkk]

From my experience and from MIL-HDBK-5H and 5J precipitation hardening alloys can not be guarantied for hardness. If you will look there you will not find hardness data but only tensile, yield etc. Old versions of MIL-HDBK-5 (I think it was version 5C) used to have a hardness table where it was specifically said that hardness could have a range of approximately 10RC variations at the same heat treatment. Manufacturer data are usually typical values. Only when you purchase ander a specific spec you can have a guaranty for the alloy minimum properties.

If you require to have a strict hardness (and not strength) than you will have to change/adjust the heat treatment temperature to get the hardness but with every new batch of material you will have to do tests to find the correct temperatures. However, be carefull not to loose toughness and elongation properties.

 

[NickE]

israelkk- ouch, I was hoping youd post....

What's a good way to get an assured strength, The hardness is not very importat this part operates against Al. But a strength and EL of a hardened SS is important. (quickly >200ksi UTS) We cant really use cond C (1t 90degree bent tab). I figure that the bend is going to be worked just enough to give a higher strength and prevent the tab from deforming.

I really think that the parts will work, just that I need to be able to have the assurance of a known strength level.

nick

Nick
I love materials science!

 

[israelkk]

The problem of using condition C can not be the 90 degree bent. Only the bending radius may be a probelem. However, if the bending radius (the radius to mid thickness of the material) to material thickness ratio is not smaller than 3 then you should not have any problem. Mechanical springs made of 17-7PH condition C and have the bending ration of 3 are common. I assume you are concerned with cracking of the metal due to sharp bend? Can you provide more info about the metal thickness and bending radius?

 

[Kenneth]

For what it's worth, AMS 2759/3 (Heat Treatment Precipitation-Hardening Corrosion-Resistant and Maraging Steel Parts) shows a required HRC range of 42 to 49 for the RH950 temper of 17-7PH in all product forms.

It lists the aging time as 1 hr (-0 minutes; +30 minutes) and includes a note that "An additional 1 to 1 1/2 hours at the specified temperature or an additional 10 to 20 F (6 to 11 C) degrees for an additional 1 to 1-1/2 hours after aging may be used to lower the hardness or other engineering property."

It also lists minimum tensile strengths (in "Tensile Strength Conversion to Condition" tables for this heat treat condition for various 17-7PH product forms as follows:

Sheet and Strip: 210 ksi
Plate: 200 ksi
Bar: 180 ksi

 

[NickE]

Israelkk- The thickness is ~1mm the internal callout for the bent tab is R1mm. We tried forming this radius on the same material in Condition C and had very fine cracks on the outside of the bend. (1t) We have tried to get a larger radius on the tab but to no avail.

nick

 

[NickE]

Well just an update.. I double aged (whatever you know what I mean) two of the test pieces and the hardness dropped slightly.

(addtl. ageing : 1hr at load temp of 950+/-10F)

I think that the slight dropping is due to coarsening of the Ni3Al.

Any other ideas.. (Im gonna go over to the met lab next week and mount an polish an etch and see if Ive got RA, bunches of ferrite, or other things, (any hints), I may try to look at the precipitate if I can resolve it, and I can find out some info about th size to expect for them. Unless of course the size is only resolvable in the TEM, If a slightly used decent older JEOL (6300) can get an image i will. (I can also check chemistry, however since TH1050 worked I dont think it will be too far off.)

nick

 

[israelkk]

NickE

I am confused, I understand that you want high strength of >200 ksi. Therefore, why do you keep with the hardness? Isn't it better to do a tensile test of the same material with the different heat treatments if you suspect something is wrong with the material or the heat treatment?

At TH1050 the minimum ultimate tensile strength is only 177Ksi according to MIL-HDBK-5J.

What is the calculated stress at the part?

When you sayed before "...prevent the tab from deforming.." did you mean permanent deformation or an elastic deformation?

If it is a permanent deformation, then your stresses are too high and the difference between H950 and H1050 may not be enough to give you a reasonable factor of safety.

If it is an elastic deformation then no matter what strength the metal will have it will not change the deformation. The elastic deformation is only the function of the material modulus of elasticity and the dimensions i.e. width and thickness of the tab, especially the 1 mm thickness.

 

[NickE]

I'm using HRc as an indicator of strength. I would love to spend the next month prepping tensile bars and getting data. There are however two problems: I dont have the quantity of material to test several tensile bars. Or even one. Eventually I still have to make prototypes and send those to my customer.

You're right about TH1050. The FEA work Ive done on this design gives stresses ~60-80ksi (Tensile -the other side of the beam is in approximately the same magnitude just compressive) After some number of cycles (<10^7) the last run of prototypes failed. (Air furnace TH1050) We were unsure as to wether the Use of the air furnace and the fine scale that cracked. or if there were small cracks because the air solution anneal did not produce a soft enough material to take the bend.

So we went to vacuum/inert gas processing to rule out scale. and used a reputable accredited heat treater to rule out the effect of my 1965 Lucifer Over/Under heat treating furnace. (with furnace controllers that are almost as old.)

Then I looked over AK's data book and they give In Table 3 (pg5) the following as properties acceptable for Materials Specificationall values in appropriate american units)

RH950
UTS : 210min
.2%YS: 190min
Elon.: 5%min
HRc: 44min

I figured that the extra strength wouldnt hurt so I told the project Eng. to get quotes for RH950 as opposed to any of the T treatments.

There are two needs of High UTS. 1) high endurance Limit (EL)
2) prevent plastic deformation (IE higher UTS means Higher yield)

 

[NickE]

The answer is in israelkk's first post. This alloy cannot be guarenteed to harden. There is variable instability in the austenite that can only be guarenteed to X-Form when cold working. Heat treatement will only work for each temp/time combination on certain heats of steel.