threaded steel pipe for a suction line

--------------------------------------------------------------------------------

I had a customer today that wanted me to make a copper to steel transition .

This is for a rig to cool natural gas so they can crack it.

Evap temp is gonna run -10f

They wanted to run the suction line in schedule 80 threaded pipe . I said not a good idea ,may hold at first but after cycling the machine I was afraid thermal expansion and contraction would cause leaks . They are now going to socket weld it . They wouldn't let me run copper as they already have pipe and fittings and say they are close to losing money on this deal.

Am I correct in telling them no threaded steel for refrigerant