Low temperature impact testing - 13/4 stainless

[bruv]

I am having a big problem trying to meet low temp impact properties on ASTM A352 CA6NM. The spec call for 27J at -73 deg C - I am getting less than 20J average. I have tried replicating a previous occasion when I managed to get around 80J, but still failed. Does anybody have any ideas?

 

[Hush]

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.

 

[Grelling]

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

 

[bruv]

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.

 

[metman]

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?

 

[bruv]

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

http://www.key-to-steel.com.

 

[Hush]

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.

 

[arunmrao]

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.

 

[bruv]

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%.

 

[bruv]

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...

 

[bruv]

Yippee!!!

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