Hardened Washer are Necessary

Problem

Numerous studies and investigations have found that without "hardened washer" under bolted connections, ~25% of the torque is lost due to the poor interface, machining of the flange or gland follower (ie if you torque a joint to 100ft-lbs without "hardened washers" on 75 ft-lbs of torque is transmitted to the joint)

Due to concerns for full thread engagement, the use of hardened washers is not pursued. For good "leak-free" service both are necessary

Solution

Install hardened washers under all bolting