CN7M casting to Alloy 20 Pipe - 1-1/2" Sch 40: Welding Procedure

I'm not overly familiar with american designations of these steels (and the sometimes small variations between the different designations), but in general, no, elevated heat should be avoided.
Did you encounter hot cracking or other defects/cracks? What filler? Depending on a.o. the Ni%, either ferrite number or heat input should be carefully predicted/applied.
Should be kept - very - clean before/during welding!

I presume that cleanliness was not an issue here, though these alloys are very sensitive.
These alloys in general and a 0 FN, which makes them very susceptible to hot cracking.
A little pre-heat can help and it shouldn't be an issue with the metallurgy.
It is more common to address this with controls on the heat input, weld speed, inter-pass temp, and cooling rate.
Allowing stresses to build during welding nearly guarantees cracking.
Castings can be problematic because the microstructures can vary.

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P.E. Metallurgy

Did you gap the root of the socket joint as required by ASME?
EdS raises relevant points, to which I would add: was the casting sound to begin with?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

Thanks for the prompt replies - and apologies for not responding until now.

The customer of ours with the issue sent his GTAW WPS. It specifies ER320LR, F#45, A#N/A(non-ferrous) and .030 to 3/16 filler metal with a 60°F preheat temperature.
As I noted, I'm not a welding engineer, but in my limited experience it seemed off, starting with "non-ferrous".
Checked vendor data: F6 & A9. Confirmed with a copy of Section IX.

We had one of our shop welders visit - he observed #1 - they were welding it cold, with no preheat - January in New Jersey cold.
#2 they were laying the wire down hot and doing a single pass.
He worked with them and made some successful welds with 150° preheat and 300° max interpass in two passes.

I also did some internet searching and found good information in a publication by the Nickel Development Institute.
It notes that some cast grades "such as CN7M" are fully austentic (i.e. 0 FN as EdStainless pointed out) and goes on to state that special techniques may be needed to prevent microfissures next to the weld.
Pretty spot-on for what we saw - pinhole leaks after welding in the heat affected area.
And the end of that paragraph say "Techniques available are low interpass temperatures, low heat input and peening of the weld to relieve mechanical stresses." Our experienced welder doesn't know the metallurgy, but knew to start warm but don't get it too hot.

I'm getting someone to create a formal WPS for our shop.