Cast 17-4PH Heat Treat - Hardness vs Yeild Strength? 1

[AndrewM7]

Anyone with good insight on heat treating investment cast 17-4. Most recipes I’ve looked at, of course, start with the material at Condition A (annealed) from the mill. I’ve also seen the anneal processed outlined, 1950 ± 25°F (1066 ± 14°C) for ½ hr, but is it necessary for cast parts, and how critical is the temp? The part requires a tight balance of yield strength, impact strength, fatigue, and resistance to fracture witch we had originally. It’s small part with a service temp potential dipping to -50°C, so I need to avoid brittle fracture. In a switch from Oil quench to Air quench I’m able to sustain hardness values, but they are NOT correlating to good yield strength, i.e. failing in torsion.

Currently using this, basically what the heat treat facility pulled out and like I said hardness values are within H925 range (desired…), but failing yield strength.
1. Homogenize ~1150° C +0/-10 - 120min (Higher temp and longer than what I’ve seen suggested, Could this be my greatest issue?)
2. Cool - Temp below 30° C
3. Solution Heat treat 1040° C - 60min (This is also much longer than I’ve seen suggested…)
4. Air Quench below 30° C
5. Precipitation hardening 500+/-5° C*90min (Shooting for H925 condition, this should be adjusted to 4 hours, but they adjusted to reach desired hardness)
6. Cool to room temp

I guess in outlining this I’ve realized some obvious process changes I need to correct, and will start with the times and temps of process 1 and 3, but if there are suggestions on the following I could use them…

A. Is the Homogenize (to Condition A, step 1) critical for cast 17-4?
B. Is excessive time and temp at Homogenize promoting an undesired ‘grain growth’ issue?

Thanks

 

[kenvlach]

Agree with 1)-2). Solidification segregation may be the problem.
Homogenize at a minimum of 1900 ^oF (1040 ^oC)* for a minimum of 90 minutes, then cool to below 90 ^oF (32 ^oC), as per para. 5.2.1 of ASTM A747/A747M, 'Standard Specification for Steel Castings, Stainless, Precipitation Hardening.'

http://www.astm.org/cgi-bin/SoftCar...EDLINE_PAGES/A747A747M.htm?L+mystore+fwcr1683

*Homognize as necessary as high as 2100 ^oF (1150 ^oC), followed by air cooling to below 90 ^oF (32 ^oC).

Solution anneal and age by the book, e.g., ASTM A747 paragraph 5.2.2 and Table 2. Messing with the ageing time may adversely affect impact properties.

What specification are you following, and what is the maximum material thickness?

 

[AndrewM7]

Thanks,
I’m not exactly sure where those numbers came from; I believe they were pulled from a previously supplied part of the same material. I pulled the print and it just said “Precipitation harden to achieve Rc 38-42”, but like I said there was a switch from Oil to Air quench before my time, and the process was an outside service for us…

The part has complex geometry with it’s thickest cross section around .27” and as thin as .06”. Some deformation is expected, manual straightening is acceptable.

I’ll switch to the ASTM spec
Thanks again

 

[AndrewM7]

Another question, I haven’t seen an acceptable rate of cooling listed for the Homogenize/Anneal step, is it possible to cool to fast? I'd assume so...

 

[EdStainless]

I have never seen cooling too fast being an issue metallurgically. I have seen it cause excessive distortion. I don't like quenching other than forced air unless there is no other way to get the part cool in 30 min.
Have you talked with the casting house? They may be able to put the hot castings into a holding funace and carry out the homogenize step right then.

Some points.
- The higher temp homogenize is important. Your casting will have significant segregation.
- Atmospher control will be critical. You can't do the high temp steps in air. Oxidation will kill you. Surface damage, alloy depleation and such.
- If low temp toughness is your goal then you are going to need a higher aging temp (lower strength). The 925F condition is not known for beign forgiving.
- The aging should be 4 hours. You adjust the temp, not the time. Playing with time can give you unstable or unrepeatable results.

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[israelkk]

Here is a quote from MIL-HDBK-5J regarding 17-4PH castings "Alloy 17-4PH castings are produced in sand molds, investment molds, and by centrifugal casting.
While 17-4PH has good castability, it is subject to hot-tearing, so heavy X or T sections, sharp corners, and
abrupt changes in section size should be avoided. Alloy 17-4PH castings are susceptible to microshrinkage
which will decrease the ductility but have no effect on the yield or ultimate strength. During heat treatment,
care must be exercised to avoid carbon or nitrogen contamination from furnace atmospheres. Combusted
hydrocarbon and dissociated ammonia atmospheres have been sources of contamination. Air is commonly used
and both vacuum and dry argon are effective for minimizing scaling. Oxides formed during solution treating
in air may be removed by grit blasting or abrasive tumbling. Alloy 17-4PH can be heat treated to develop a wide range of properties. Heat treatment procedures
are specified in applicable material specifications and MIL-H-6875."

As to hardness correlation to yield/tensile strength, there is no direct and exact correlation. For the same yield/tensile strength there may be up to 10Rc hardness variations. Therefore, if hardness is required, for each batch you will need to test heat treatment parameters which is not practical and you may end up with non standard heat treatments.

If this part is for aerospace/military use you probably do not have the freedom to use non standard heat treatments but only those allowed by MIL-H-6875. Therefore, hardness can not be defined but only yield and tensile strength.