Stainless steel cracks

I am trying to weld a stainless steel fitting to 5/8" stainless barstock that has been gundrilled 3/8" inside.Everything looks great after grinding and polishing the weld smooth,but after pressure testing to 3500 PSI with water in the tube,I keep getting hairline cracks allowing water seepage.I am using a TIG at 55 amps on a 115 circut with 17 CFH of argon.Too much/not enough heat?Too much/not enough argon?Work cooling too rapidly?Thanks & happy holidays to ya.

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55 amps sounds about right for the 1/16" wall

if you are not sure what grade stainless, its possible that it is 303 free machining which is not recommended for welding. People do weld it.
but it should not be used for critical applications and if you are testing at high pressures, that might be the cause of the cracking.

303 machines great, welds crappy

other grades like 304 weld great, machines crappy