INVESTIGATIONS ON PWHT EFFECTIVENESS OF SA213 T23 TUBULAR BUTT WELDS USING FLEXIBLE CERAMIC RESISTANCE HEATING ELEMENTS AND FINGER ELEMENT HEATING COILS - A COMPARATIVE CASE STUDY

摘要

This paper presents a comparative analysis between finger element resistance heating coil (FEC) and flexible ceramic pads (FCP) methods of Post Weld Heat treatment (PWHT) of butt welds in tubular panels made of SA 213 T23 tubes. The SA 213 T23 material (2.25Cr-1Mo-1.6W) is a candidate material for membrane water wall panels of supercritical and ultra-supercritical boilers due to the high creep strength compared to the conventionally used T22 steel. But this material has been found to be very sensitive to the PWHT after welding. In order to ascertain the most effective method of PWHT for T23 tubular panels, the present study was conducted. A Tubular panel was fabricated having 8 tubes and 7 fins with a length of 1.2 m, simulating field conditions. Spool pieces of length 200 mm were introduced in two of the eight tubes thereby resulting in 4 weld joints. The butt welds in this panel were subjected to PWHT using "flexible ceramic" resistance heating pads. Similarly, butt welds in tubular panels in a thermal power plant site were heat treated using "finger element" (FEC) resistance heating coils. The PWHT was done maintaining the parameters as per provisions of ASME code case 2199-5. After PWHT, the weld joints were subjected to detailed mechanical and metallurgical investigation. It is observed that although the effectiveness of PWHT of both the methods were comparable as measured in terms of reduction of hardness after PWHT, flexible ceramic resistance heating pad method is found to give better spread of heat and was easier for use. The consistent performance of flexible ceramic resistance heating pads easily overweighs the finger element type of resistance heating coils, in several ways.