I've never spec'd CA6NM below -50 C. I'm impressed that your values you're getting now are as high as they are.

You should take care that the retained austenite will be still present when testing. The retained
austenite brings the toughness.

Do I need to play around with the composition (I currently run Ni at around 4.2% with a 4.5% max). If not, can anybody recommend a suitable heat treatment. At the moment I am solution treating followed by a double temper.

bruv-  I have some thoughts about your problem that might be helpfull but I am not familiar with this particular alloy.  Somewhere on this site I found a thread to a site called "About metals Guide."  When I searched using keyword of ASTM, it gave me 38000+ choices.  Cannot zero in on chemistry, properties, heat treat, etc for ASTM A352 CA6NM and when I search under that same site under 'ASM's Alloys Online', I come to a choice of 4 categories of S.S. cast alloys and then I am stuck again.  For example; what is the the Mf (Martensite finish)temperature for this alloy which is a very important factor for analyzing your problem since it bears heavily on the question of retention/transformation of Retained Austenite especially since you are testing cryogenically.

Can anyone give me a step-by-step method to find specific info for a particular ASTM designation?

Metman,

I have looked in all the available (to me) literature, and cannot find any details of Mf temps for this alloy. The current H/T cycle for this alloy is 1020C, air cool, followed by temper at 690C and a second temper at 620C

For chemistry of this (and many other) alloys, try www.key-to-steel.com.

Your HT sounds like NACE MR0175 (H2S service)which specifies a heat treat of 1010C min, air or oil quench, 648 to 690C, air cool, 593 to 620C, air cool. We had trouble with a supplier not meeting max hardness and min impacts and determined the following possible faults. Not allowing the material to cool below 100C before 2nd temper. To short a time at temper. Leaky seals on furnace doors creating cold zone. To rapid a cool after tempering. Hope some of this helps. The factor that gave the most improvement was allowing the material to cool right to ambient before second temper. Hope this helps.

My experience is restricted to room temperature properties assessment. How low is the carbon in your CA6NM alloy. Prefer to keep it at around 0.03%C. Also please ensure that between each stage of heat tretment the castings are properly cooled to near room temperatures before the next cycle begins.
I had once increased the Ni to beyond 5% to improve the impact properties,but this deviation will need to be cleared by the buyer.

The H/T is used for NACE service components. I have recently re-run the H/T with longer tempers, and am currently waiting on the results from the test lab. I will post the answers when I have them. The components in question a fairly small, and we normally leave 6-8 hours from the end of one H/T cycle to the start of the next.

Carbon is run at about 0.025%.

Apologies to one and all for the delay - results on the trials came in at 35-40J at -73(C). All I have to do now is repeat that on the production work.

Watch this space...

Yippee!!!

Average of 33J at -73C on the production testing. Thanks to one and all for the ideas given