Stressing, post-weld

The local heating, and consequent expansion, that occurs during welding can leave the joint in a state of stress. These stresses are relieved by post-welding heat treatment. Not all vessels will be stress relieved. Guidance on the need for post-welding heat treatment is given in the codes and standards, and will depend on the service and conditions, materials of construction, and plate thickness. 

Equipment found to suffer from cracking included tanks, absorbers, carbon treater drums, skimming drums, and piping. All welds of deaerator vessels in carbon steel should be post-weld stress relieved to minimize cracking and pitting. (Bond)5... 

Figure 8. Corrosion rate is a function of the 21 Cr-1 Mo heat treatment. Key for no post weld H.T. O, unstressed coupon %, 18 ksi applied stress for post weld H.T. 0, 10 hr, 760° Z, 10 hr, 710°C Nb-stabilized parent metal A, no post weld H.T.

Stress corrosion cracking can be avoided by selecting materials that are not susceptible in the specific corrosion environment or, less certainly, by stress relieving by post-weld heat treatment. 

In fact, circumferential weld shrinkage alone, particularly in heavy-wall pipe, may create complex bending stresses at the joint. Although post-weld heat treatments should relieve many of these stresses, the subsequent cooling can reintroduce harmful stresses if there is much restraint. The fact that the much higher thermal expansion and contraction of the austenitic stainless steels may introduce unexpected restraint stresses, as well as being troublesome in piping layouts, should not be overlooked. 

Stress relief heat treatment (see Post-weld heat treatment)... 

Austenitic or duplex stanless steels undergo a solution annealing and water quenching heat-treatment. Additionnal heat-treatments such as for stress relief or post weld treatment are also used to eliminate the internal stresses of the material. 

The report of the inquiry [111] criticised the design and fabrication of the alterations made to the original pontoon. The actual cause of the accident was the failure of some tie bars in the detail around the jacking points. The failure was due to brittle fracture which initiated from severe notches such as a small radius curve at the fillet between the spade end and the shank of the tie bar. Weld defects and fatigue cracks were also present in tie bars subsequently recovered from the sea bed. The tie bars had been flame cut to shape and had weld repairs visible to the eye. There had been no post welding heat treatment of the steel. The steel complied with the original specification but tests showed low Charpy V notch impact values. Photo elastic tests indicated a stress concentration factor of 7 at the fillet between the spade end and the shank. The fracture was initiated in the opinion of the inquiry tribunal by the low ambient temperature of around 3°C. 

Quality Assurance Pian to be given by vendors (Fabrication codes and standards, stress relieving, post-weld heat treatment in case of pressure vessels). 

Records of post-weld heat treatment done for stress relieving. 

Item 2 of the report documented HTHA can accumulate in high-stress areas in carbon steel, such as non-post-weld heat-treated welds. The welds of the B and E... 

A total of 37 leaks was experienced in PFR SG units in the period 1974 to 1984 with 33 of these occurring in evaporators, 3 in superheaters and 1 in a reheater. All the leaks originated at the welds between the tubes and the tube plates associated with cracking of the tube-to-tube plate welds. These were hard and had high residual stresses beeause there was no post-weld heal treatment. The type of direct tube-to-tube plate weld (the butt/fillef weld adopted initially at PFR, which could not be heat treated after manufacture, are being avoided in future fast reactor SGs. 

Weld residual stresses can be reduced by a post-weld heat treatment (PWHT) during fabrication. To avoid sensitization, PWHT parameters have to be well controlled and adjusted to the materials and construction. 

IAEA) Nuclear Safety Standards normal water chemistry orificed fuel support post weld heat treatment reactor pressure vessel reactor pressure vessel internals reactor water clean up (system) stress corrosion cracking stand-by liquid control (system) shot peening safety relief valve... 

Stress relief Coded vessels tjrpically have a metal stamp attached that states, "Do not weld, stress relieved." That means the vessel has been post-weld heat treated to remove stresses in the vessel wall created by welding during fabrication. 

If the steel is hard, because it has lost its ductility, the vessel wall will crack. The crack will propagate along the hardened zone. The hardened area is created when the heat-affected zone of a weld cools too rapidly without being post-welded (i.e., stress relieved) heat treated. 

Still, the vessel would not have failed if the vessel welds had been stress relieved by post-weld heat treatment. But this had not been done either. 

Stress relief This occurs when the alloys after welding are subjected to stress relief by appropriate heat treatment in order to reduce the residual stresses. This is also called post-weld heat treatment. ... 

The boiler metal must be under high stress such as the areas where the boiler tubes are rolled into the drum. The welded joints become highly susceptible to caustic embrittlement. The attack proceeds on the grain boundaries, hence, it is an inter-crystalline phenomena. Post-weld heat treatment or stress relieving heat treatment can preclude cracking problems. 

The consideration of wind loads in a plant or operating facility normally happens when a section of a tower is to be stress-relieved by post weld heat treatment and an entire shell portion of the vessel is to be heated to the stress-relieving temperature. In this application, you must be cognizant of the imposing wind loads on the vessel. Another application would be the installation of guy cables, as discussed in Chapter 3, and the loads on these cables. These simations are routine in plants, hi the case of relieving stress on a process column, only the wind shear and bending moments at the location under consideration are of interest. 

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