The use of the bainitic class of creep strength enhanced ferritic steels T/P23 and T24 has increased over the last decade in a wide range of applications including replacement headers, superheater and reheater tubing and in waterwall tubing. Many issues have been reported in one or both of these materials including hydrogen induced cracking, reheat cracking and stress corrosion cracking. To appropriately address these issues, work has been initiated that includes a literature review, development of a database of phase transformation temperatures, investigation of tempering behavior, and an analysis of the effect of phase transformation on residual stresses. Such information will be provided in the context of understanding why these two materials appear highly susceptible to these cracking mechanisms.

Xcel Energy’s Comanche Unit 3 experienced widespread cracking of T23 membrane wall tubes within the evaporator section, initially occurring during the boiler construction phase, primarily at shop and field tube butt welds. The majority of the tube cracking was attributed to stress-corrosion cracking (SCC), and a lesser number of fabrication-related hydrogen induced cracking (HIC), weld solidification cracking, and brittle cracking within tube swage sections were also experienced. Hundreds of tubes were replaced prior to Unit commissioning, due to both actual tube leaks and those replaced due to weldment cracking and other identified weld defects during radiographic testing. Elevated stress levels and material susceptibility (i.e. hardness in the as-welded condition) were considered the critical factors in the tube cracking.

In this paper, the performance of T23 and 12Cr1MoVG water wall tubes as well as their welded joints in engineering applications is reported. It was found that the T23 water wall tube may have water leak problems during its operation. In order to make sure the safe operation, leakage reasons of T23 water wall tube were analyzed and improvement measures were taken. Recommendations on the choice of water wall material of 1000MW USC tower boiler are given.

This paper explores the development and qualification of a bainitic-martensitic steel grade and its matching welding consumables for power plants operating under ultra-supercritical steam conditions (605/625°C and 300/80 bar). It provides insights into recent developments and offers practical considerations for handling this material (grade T24) from the perspective of both tubular component manufacturers and welding consumable producers. The paper is structured into three main sections: (1) Development and qualification of the T24 steel base material. (2) Development, qualification, and recommendations for welding consumables compatible with T24 steel. (3) Experiences during manufacturing and installation of components using T24 steel, concluding with key takeaways.

T24 tube material (7CrMoVTiB10-10), with its combination of high creep strength and potential to be welded without using preheat, is regarded as a candidate waterwall material for Ultra Supercritical (USC) boilers. However, its reputed sensitivity to hydrogen and potential for secondary hardening may have adverse impacts on construction of waterwall panels. Doosan Babcock Ltd have investigated the response of welds made in T24 tubing to secondary hardening via changing hardness in a series of ageing heat treatment trials. Also, the response of the material to hydrogen infusion has been investigated

The objective of this study was to determine the typical range of weld metal cooling rates and phase transformations during multipass gas-tungsten arc (GTA) welding of Grade 23 (SA-213 T23) tubing, and to correlate these to the microstructure and hardness in the weld metal and heat affected zone (HAZ). The effect of microstructure and hardness on the potential susceptibility to cracking was evaluated. Multipass GTA girth welds in Grade 23 tubes with outside diameter of 2 in. and wall thicknesses of 0.185 in. and 0.331 in. were produced using Grade 23 filler wire and welding heat input between 18.5 and 38 kJ/in. The weld metal cooling histories were acquired by plunging type C thermocouples in the weld pool. The weld metal phase transformations were determined with the technique for single sensor differential thermal analysis (SS DTA). The microstructure in the as-welded and re-heated weld passes was characterized using light optical microscopy and hardness mapping. Microstructures with hardness between 416 and 350 HV 0.1 were found in the thick wall welds, which indicated potential susceptibility to hydrogen induced cracking (HIC) caused by hydrogen absorption during welding and to stress corrosion cracking (SSC) during acid cleaning and service.