What is your "Target Prestress" for the bolts? What are you using for lubrication (both between the bolt and the nut AND the nut face and the washer/flange face)?
The formula is:
T=F/2*(dn*fn+d2*((f2+cos(alpha)*tan(lambda))/(cos(alpha)-f2*tan(lambda))))
where:
T is the target torque
F is the target bolt tensile load
dn is the mean diameter of the nut
fn is the coefficient of friction between the bolt nut and the flange
d2 is the pitch diameter (or mean thread contact diameter)
f2 is the coefficient of friction between the bolt and nut threads
alpha is the thread flank angle
lambda is the lead angle
Note that for UN and UNR screw threads the flank angel is 30°, the lead angle is equal to atan(L/pi*d2) and L is equal to the pitch of the threads (for 8-thread series this would be 1/8 in)
This one is simple. 4" class 1500 flange has qty 8 - 1.25" dia. bolts. You have your materials backwards. Bolt material is B7M (softer than B7) anf the nuts are 2H. Based on the tensile stress you wnat the torque for each nut is 500 to 750 ft-lbs. Of course the torque is applied in 25% increments and to bolts/nut are clean and lubricated.
