Ramaswamy Viswanathan

Weld repair of steam turbine casings and piping: An industry survey

As the U.S. fleet of fossil power plants age, utilities are forced to perform more and more repairs on such components as turbine casings, main and reheat piping, headers, and other components that have experienced high-temperature degradation. This paper presents information from two surveys on the weld repair technologies currently used by utilities and repair organizations to extend the life of high-temperature, high-pressure components. The initial survey included responses from 28 EPRI member utilities on various repair issues ranging from condition assessment to preheat/postweld heat treat to filler metals employed. The second survey was forwarded to repair vendors and OEMs to gain their perspective on utility industry repairs.

Performance of Repair Welds on Service-Aged 2-1/4Cr-1Mo Girth Weldments

This paper discusses the results of evaluations performed on service-aged piping using both conventional postweld heat treatments and temperbead repair techniques. The two repair weldments were accomplished on two 2-1/4Cr-1Mo pipe girth weldments which were removed from a utility hot reheat piping system in the fall of 1992 after 161,000 h of operation at 1,000 F (538 C). Each repair was performed around one-half of the diameter of a pipe girth weldment, while the remaining half of the girth weldment was left in the service-aged condition. Post-repair metallurgical and mechanical test results indicated that both weld repairs produced improved remaining lives when compared to the service-aged girth weldments. Since the two ex-service weldments that were utilized in weld repairs exhibited different stress rupture strengths to start with, the performance of temper bead and postweld heat-treated (PWHT) repair could not be compared directly. It was clear, however, that life extension periods exceeding 30 yr could be achieved by temperbead repairs, with improved toughness and with no loss of stress rupture ductility, tensile strength, or yield strength. The temperbead repair improved the toughness of the service-aged weldment, while the postweld heat-treated repair lowered the HAZ toughness.

Weld repair of 2-1/4Cr-1Mo service-aged header welds

The objective of this investigation was to evaluate the efficacy of different weld repair techniques as applied to service-aged 2-1/4Cr-1Mo steel weldments. A header which had been in service for 244,000 h at 1,050 F (565 C) was utilized for the study. Three girth welds were partially excavated and subjected to repairs using gas tungsten arc welding (GTAW), shielded metal arc welding (SMAW) with postweld heat treatment (PWHT), and without postweld heat treatment using a temperbead technique. Results show that all the weld repairs improved the creep rupture lives of the ex-service weldments and that remaining lives of several decades could be achieved in the repaired condition. The SMAW-temperbead repairs resulted in increase of future life, tensile strength, and impact toughness compared to the SMAW-PWHT repairs. The GTAW-PWHT repairs also produced a superior combination of mechanical properties. Remaining creep rupture lives were a function of the extrapolation procedure and specimen size. These results are described here and discussed in comparison with results previously reported for a less severely degraded condition of the steel in order to delineate the effect of prior degradation on weld repair performance.

Historical Overview of Improving Cleanliness of Rotor Steels for Electric Utility Applications

Improved cleanliness is perhaps the only approach that results in simultaneous improvement in strength and ductility at elevated temperatures as well as toughness at low temperatures of steels. In addition, superclean steels in which manganese and silicon have also been reduced, provide greater resistance to stress corrosion cracking than conventional steels. Major projects are underway worldwide to promote the use of clean/superclean steel rotor and disk forgings, both for low temperature and for high temperature applications in steam and combustion turbines. An international workshop sponsored by EPRI was held in 1995 in London, at which turbine manufacturers and steelmakers discussed ongoing activities with respect to clean steels. This paper summarises EPRI sponsored research on rotor steel chemistries and provide an overview of developments over the last 15 years.