Tonight when I'm back home I'll take a look at my references and I'll be able to answer your question tomorrow.
For the moment, I can tell you that Carpenter 20Cb3 belongs to the so called 25-20 family of alloys, that contain 25% chrome and 20% nickel, along with other metals.
Giovanni S. Crisi
Sao Paulo - Brazil

Hi Strat!

Carpenter 20CB-3 (UNS# N08020) - An austenitic stainless steel with excellent resistance to hot sulphuric acid, and many aggressive environments which readily attack Type 316 stainless. Excellent resistance to Stress Corrosion Cracking (SCC). Weldable, machinable, and cold formable.

Chemical composition:

0.06 C, 2.00 Mn, 0.035 P, 0.035 S, 1.00 Si, 19.00/21.00 Cr, 32.50/35.00 Ni, 2.00/3.00 Mo, 3.00/4.00 Cu, 8xC min./1.00, max. Cb + Ta, Bal. Fe (Single figures are maximums).

http://www.cartech.com/product_literature/pdf/GuideToSelecting.pdf?leftn=cartech&lefto=empty&lefti=cartech_pl0

The information above can be found in the .pdf file above on page 16 of 83.

One can get this, and more technical information if one clicks one of the links below ;) :

http://www.cartech.com/product_literature/pdf/SelectaloyBklt2001.pdf?leftn=cartech&lefto=empty&lefti=cartech_pl0

http://www.cartech.com/servlet/ProdLitOrderFormServlet?leftn=cartech&lefto=empty&lefti=cartech_pl0

http://www.cartech.com/

Respectfully,
Henry

Strat,
you've already got the answer.
Giovanni S. Crisi