Valve Body Repairs

[tc7]

I have a need to repair blind threaded holes in some valve bodies (used for studs to secure bonnets and flanges). The threads are 3/4x16(UNF)x1.5" deep. The valve body is a steel casting, ASTM A437 Gr 4 (w/nickel, chrome and moly). The stress analysts tell me I cannot drill and tap an oversize one inch thread (high pressure and fatigue concerns). The only other options I can think of are:
a. drill out the existing threads, plug weld fill the hole and retap. Can this be done in a 1.5"-1.75" deep hole? I am sure post-heat treat will be necessary.
b. Can I drill out the hole and flame spray the I.D. then redrill and retap? Will Flame sprayed material provide a high strength bond? (Nuts on studs will be torqued to 200 ft-lbs.)
c. Scrap the valve bodies and buy new ones at $50K each.

Has anybody been there/done this? All advice welcome.
Thankyou,
Tom C

 

[CRG]

Use a thread insert? HeliCoil or similar.

 

[tc7]

CRG-
We have tried helicoils and also keen-serts. They both gave us less than reliable results as far as maintaining torques.
T