December 1, 2011

Using Temperature- Measuring Indicators

By Louis A. Chismar, and Kenneth R. Stockton

Correct welding procedures are extremely important elements of the work done by the PSEG Central Maintenance Shop (serving Public Service Electric and Gas Co., PSEG, a New Jersey utility). We have, for example, a Critical Weld Inspection Program for high-temperature pressure piping whose goal is to identify cracks in high-temperature piping welds.

The stations work very closely with the PSEG Engineering department, based in Newark, N.J., to complete the inspections, and the Servco maintenance shop then makes the repairs. The shop commonly fabricates new spool pieces with dissimilar welds to replace existing ones.

There is always a high potential for failure cracking with transition welds joining dissimilar metals such as chrome and stainless steel. The weld procedure is to cut above and below the transition, do the more complex transition welds in the shop under shop conditions, and then replace the piece in the field by making welds between similar/same base metals.

How Weld Cracks Form

Weld cracking is usually caused by thermal stresses imposed on the weld metal and adjacent heat-affected zones. In welding carbon and alloy steels, cracking often occurs at the junction. The formation of hard, brittle regions within the weld occurs as the result of rapid cooling during welding. The presence of hydrogen can also cause weld problems.

Stress builds in the assembly as the weld metal shrinks as it cools and the weld metal is restrained by the surrounding colder metal. Also, heat is drawn away from the weld zone by the surrounding colder metal, accelerating stress formation. The rate of heat flow away from the weld is greater during welding of thick sections and in metals having a high thermal conductivity. In metals susceptible to quench hardening, such as high-carbon and alloy steels, the rapid movement of heat away from the weld area can result in the formation of hard, brittle regions.