During the investigation under report, a programme for RS measurement and analysis was undertaken to quantify RS in Al-Zn-Mg alloy rings in various heat treatment conditions. A portable XRD equipment was used along with an in-situ electropolishing unit to carry out RS measurements in as-rolled, T6, T651 and T652 heat treated rings. A comparison was drawn out to evaluate the effectiveness of deploying stress-relieving operations in T651 and T652 heat treatments over T6 heat treatment. Experiments were also carried out to find out in rings, the RS generated during machining. The average values measured on rings in as-rolled, T651 and T652 treated conditions were 6.09, 4.44 and 6.40 kg/mm² respectively (all compressive), whereas average RS in T6 condition was found to be 10.76 kg/mm² (compressive).

The repeated failure of a welded toroidal tank of 2.8 meter diameter was attributed to the high values of RS measured in the welds and heat affected zones (HAZ). This observation was supported by detailed metallographic investigation on the T652 heat treated rings showing high aspect ratio in the grains representing incomplete solution treatment. Improper welding process was also found to be responsible for high RS.

A T6 heat treatment involving air cooling instead of water quenching after solution treatment followed by full ageing was successfully given to this hardware and it was experimentally found that the T6 heat treatment changed the RS from 4.8 kg/mm² tensile to 1.3 kg/mm² compressive at welds and from 5.6 kg/mm² tensile to 3.4 kg/mm² tensile at HAZ, and thus minimising the risk of RS induced cracks.