I would think that the use or existence of an S-N curve is questionable unless you specify the condition of the 420 SS.  
Martensitic 420SS is commonly used in the annealed form as forgings and obviously can be heat-treated to a wide range of hardnesses (i.e strengths).  Since fatigue strength and the UTS are related, you can see why there is unlikey to be a universal fatigue curve for "420SS".

Nevertheless, I'll wait to be corrected...

  Dear rustbuster, because 420SS is commonly used in various hardened conditions, I was hoping an S-N Curve existed which included various curves for various heat treat and tempering temperatures, as is the case for 420SS in the ASM reference.  But if it does matter, the condition I'm interested in is heat treated at 1875F for 15min, quenched in inert atmosphere to room temp., then tempered at 400F for 3 hours.  Resulting UTS is 230KSI and YS is 195KSI.  But really, any information would be better than none.

Recommend buy The Atlas Of Fatigue Curves by "Boyer". Several data and curves are presented for 422 SS which should be very similar to 420 SS. Bob

Thanks,
I did find a copy of Boyer's Altas at our local university library.  As you have said, 422SS is in the Atlas, but not 420SS.  But tring to convince a manager that one is can be similar to another is sometimes not easy.

Does anyone out there have any advice on welding the following combination of metals to each other???

1. SA 335-GR P91 pipe this is a high chrome alloy with a 9Cr,1Mo,1V

to

2. Din #17240 UNS#42200 material 1.4923 ( I think this stuff is pretty similar to a 420 SS)

Any advice would be greatly appreciated....

Did you ever find any SN curves for 420 Stainless Steel.  Where?  Do you have a copy you could send me.  We are currently using it at a Brinell hardness of 331.  I could really use a curve.  Thanks for any help.

Sorry JAB, I do not believe any published S-N Curves exist for 420SS.  I've looked in Boyer's Atlas and the ASM handbooks.  I feel if they existed, they'd be in one of those references.  The closest thing are curves for 422SS, which is a modification of 420SS with additions of nickel, moly, tungsten, and vanadium.