Selecting high strength stainless based on machinablility? 1

[BryanS]

We are wanting to fabricate rotation shafts out of high strength stainless. Due to lead times of our product, it would be best to machine the shafts in the high strength condition rather than heat treating afterwards. Does anyone have a recommendation on an alloy that would meet the following criterion:
- a minimum yield strength of 150ksi or greater
- minimal corrosion resistance is necessary
- machinable (need to turn the exterior on a lathe and bore a hole in it preferably in the high strength condition)
- no adverse mechanical characteristics between approx -40° F and 140° F (such as a drastic decrease in notch resistance)
- single 12' bar length quantities available for purchase (in other words relatively common)
- ability to get a relatively fine surface finish after machining in order to run (very slow rpm) in a bushing
- characteristics that don't make much difference
- magnetic properties
- thermal conductivity
- electrical resistivity/conductivity
- ability to retain a cutting edge
- young's modulus

Thank you.

 

[Jimbo51]

15/5 or 17/4 in commonly heat treated to 180 - 200 ksi
and would most likely work well for what you want. It's common to
Jim

 

[BryanS]

Thanks Jimbo51 for the reply. I've been looking at the 15-5 (H900), the 17-4 (H900) and the 440 series stainless. One ASM chart shows the 440 series to be easier to machine than the precipitation hardened options, however this is in the non-heat treated state for all of them. I do not know the results after being tempered.

The other factor I don't know is at what temperature each of these steels exhibit a significant decrease in resistance to impact thus exhibiting more brittle characteristics. This had played a limited role in a failure of similar application of a different non-stainless alloy.

Lastly, the 15-5 appears to be somewhat available in a non-heat treated state. I don't know about the others....Any idea?

 

[Jimbo51]

H900 is simply heatying to 900 deg for one hour and getting about 38 or 40 on the C scale....send it out to the heat treater and have him precipitation harden either 17/4 or 15/5 to 180/200ksi and it is still quite machinable...I do this all the time with turned parts to avoit the warpage
Hope this helps
Jim

 

[Jimbo51]

I also think that either 15/5 or 17/4 will give better surface finish's than 440 or 440c. 440 will go higher on the C scale if you need it to though. How hard do you need to be?
Jim

 

[BryanS]

For the application, the hardness is not a characteristic that I care much about. In general it comes with the territory of the high strengths and consequently often makes the machining a slower process.

Do these alloys result in a "case hardened" form after heat treating? If this is true, machining after tempering would remove the hardened/strengthened surface. I'm questioning this because the precipitation hardened alloys (15/5 & 17/4) start in the austenitic face centered cubic (fcc) structure and transform to the martensite (bcc) through the heat treatments. I realize they also precipitate out to increase strength (which should be a relatively uniform result), but I'm not well versed in how uniform the crystaline structure will be over the cross-section after heat treating.

Sorry about the string of questions. I haven't found good resources on alloy specifics and I'm trying to cover my bases so I don't have a bunch of product recalls on my hands. Thanks.

 

[TVP]

BryanS,

No, the heat treatment of these alloys does not result in surface case that is harder than the core material. I could have sworn that I replied to this post earlier with some websites for more information, but here they are again:

http://www.cartech.com

(click on Technical Information then Alloy Category then Stainless Steel then Precipitation Hardenable Stainless Steels)

http://www.timken.com/products/specialtysteel/engineering/tech_info/stainlesssteels.asp

http://www.aksteel.com/markets_products/stainless_precipitation.asp

http://www.allvac.com/allvac/pages/Steel/UNSS15500.htm

http://www.allvac.com/allvac/pages/Steel/UNSS17400.htm

http://www.ssina.com/SSINA2002/SSINAsite/publications/index.htm

http://www.nidi.org/index.cfm/ci_id/9/la_id/1.htm

http://www.slater.com/stainless/literature/index.htm

 

[BryanS]

Thanks for the reply TVP! I found the carpenterdirect.com (cartech.com) about the same time as I got the e-mail. To this point, this website has been the most comprehensive source that I've found. I'll look into the other web sites as well.

Thanks for the help, I think I've found enough info to select a suitable alloy for the application.

 

[bama1]

Have you considered the Inconel family? Possibly Inconel 718 (Carpenter's Pyromet 718). Hardness should come in at 220-230 HB and tensile should be 185 ksi min (per AMS 5662L). Try National Specialty Alloy for a source.