Evaluation of Welded Joints in P91 Steel under Different Heat-Treatment Conditions

Abstract

P91 steel has been of interest to many researchers over the past two decades. This interest is because this steel has very interesting characteristics for application in power plants, where it is common to have pipes that need to support steam at temperatures between 570 and 600 °C, and at pressures in the range of 170 to 230 bar. These working conditions are quite severe for most common steels, requiring increased high-temperature mechanical strength as well as high creep resistance. The manufacture of these pipes normally includes welding operations, which must preserve the main characteristics of this type of steel. This justifies the concern of the researchers to ensure the best welding conditions so that the preservation of the properties of these steels becomes possible. The present work intends to depict the best results obtained varying the heat-treatment conditions applied to weldments made on heat-resistant steel P91. This steel usually takes the designation SA 213 T91 (seamless tube) or SA 335 P91 (seamless pipe), according to ASME II, as well as the designation X10CrMOVNb9-1 according to EN 10216-2. The purpose of this study is to compare the behavior of pipe welding under different post-welding heat-treatment (PWHT) conditions. One of them is performed with thermal cycles (preheating, post-heating, and the post-weld heat treatment) in agreement with most construction codes and standard rules. The second one is performed without any thermal cycle before and after welding. Both welds were made by the same process, TIG (Tungsten Inert Gas, or GTAW—Gas Tungsten Arc Welding) in the horizontal position (2G according to ASME IX) and the same welding parameters. In order to evaluate the results obtained in the welds, microstructure analyses, hardness measurements, bending tests, and tensile tests at room and high temperature (600 °C) have been performed. Other tests were also carried out according to the quality procedures, such as visual, penetrant dye, and X-ray tests. Regarding the different strategies used in the heat treatments, the best results have been obtained using a strategy similar to the one currently in use and recommended by construction codes and steel manufacturers but excluding the phases’ transformation time, and it was possible to observe that the tensile strength is impaired by about 2% to 9% at room and elevated temperatures, respectively; the elongation is reduced by 39% at room temperature but keeps a good performance at elevated temperature; the hardness profile is very similar at both temperatures; the microstructure presented is compatible with the requirements; and no cracking trend has been reported. Thus, a new strategy for the welding heat treatment of grade 91 steels was drawn, saving energy and processing time.

Enhancing Heat Treatment Conditions of Joints in Grade P91 Steel: Looking for More Sustainable Solutions

“…In this paper, different thermal cycles were applied to the various welded samples in order to evaluate their influence in the joint's mechanical properties. These different thermal cycles can be observed in Figure 1 (the reference coding of these cycles is also displayed in this figure, being the same as the one presented in [27]). Table 5 provides information on the temperature that was used in the different thermal cycles.…”

“…As in the previously performed work [27], the experimental work was conducted on a SA 355 P911 steel pipe, with 48.3 mm in diameter and 5 mm thickness, which was provided by Vallourec Brazil (Belo Horizonte, Brazil). The material's mechanical properties and chemical composition are presented in Tables 1 and 2, respectively.…”

“…The main objective of this paper remains the same as the one presented in the paper by Silva et al [27], being the evaluation of the various heat treatment cycles on the grade P91 steel welded joints, looking for obtaining a more favorable thermal cycle and expanding the knowledge about this process with this base metal. The pipes were first cut in a circular sawing machine Kaltenbach KKS 450 S (Kaltenbach GmbH, Lorrach, Germany), and the chamfer was made using a conventional lathe, Knuth V-Turn Pro (Knuth Werkzeugmaschinen GmbH, Wasbek, Germany).…”

“…Prior to the weld, the samples were properly aligned, furthermore, thermocouples were attached to the sample's surface, close to the weld joint. Regarding the chosen parameters, these were the same as the ones used in the previously presented paper [27], with a heat input of 1.33 kJ/mm being selected and a current intensity of 95 A (this was selected taking the thickness of the filler-metal wire into account). Table 4 shows the welding parameters used for the conducted experimental work.…”

“…Different temperatures and times have been repositioned for the PWHT of P91 steels, varying between 760 • C and 770 • C, and lasting around 2 h [25,26]. Recently, a study was carried out in order to optimize the cycle of heat treatments to be applied to welds made of P91 steel [27], given that the treatments recommended by the metal building codes are complex and unfriendly to the environment, consuming immense energy and being a time-consuming task. Several strategies have been established and alternatives were pointed out to the set of complex treatments recommended in the building codes.…”