kevlar49;
I have a publication from the International Nickel Company titled "Cast Heat-Resistant Alloys". I believe I had obtained this publication from the NiDI (Nickel Development Institute) web site.

Anyway, the publication contains good technical information on the various grades of cast, heat-resistant stainless steels; HH, HF, HK...

For Grade HF (19Cr-9Ni), from what I reviewed, this material is suitable for service temperatures up to 1600 deg F. You have as cast, and aged at 1400 deg F, furnace cooled, conditions. The publication contains tensile, elevated temperature tensile, creep and stress rupture data for this material. There is no reported drop in mechanical properties to limit application at 920 deg F.

Sorry, meant HF modified.  Where can I find code allowable stresses?

kevlar49;
What is an HF modified? Is it given a separate heat treatment, 1400 deg F, held for 24 hours and furnace cooled or modified alloy composition?

I don't know of any specific allowable stress values for this material because ASTM A 297 is not endorsed by ASME B&PV Code. You will probably need to determine the allowable stress values on your own using ASME Section II.

Why don't you review ASME SA-451, which is Specification for Centrifugally Cast, Austenitic Stainless Steel Pipe for Elevated Temperature Service. The material grades are endorsed by ASME B&PV Code and have established allowable stress values.

I thought HF modified is a cast version of 347 stainless steel (http://www.hghouston.com/Technical%20FAQs/TFAQ180.html), but there is a conflicting documentation about it(http://www.metallurgical.com/Publications/Publication%2027.pdf. PAGE 6).  Does anyone know which one is right?

kevlar49;
According to the information I have from another publication, the UNS designation for ASTM A 297 Grade HF is J92603, the chemical composition is listed below;
C  0.2-0.4%
Cr 18-23%
Mn  2% max
Mo 0.5% max
Ni 8-12%
P,S  0.04% max
Si 2% max

There is another version of HF, Grade HF30 UNS J92803


As I stated, if you want cast 347 pipe material, you need to reference SA-451

C  0.25-0.35%
Cr 19-23%
Mn 1.50% max
Mo 0.50% max
Ni 9-12%
P,S 0.040% max
Si 0.5-2.0%

In my further review of the 'Handbook of Stainless Steels' by Peckner and Bernstein, the reported wrought match to Grade HF is Type 302B stainless steel, with higher carbon content. You can take this to the bank. There is no mention of a modified HF. I also checked ASTM A 297, there is no modified HF.

As I stated, you are better off referring to ASME SA 451, not ASTM A 297.