Microstructure and mechanical property relationship for different heat treatment and hydrogen level in multi-pass welded P91 steel joint

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.

Section: Introductionmentioning

confidence: 99%

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

Silva

Pinho

Péreira

et al. 2020

“…Due to additional precipitation of carbides and/or their coarsening during thermal exposure, the number of irreversible hydrogen traps is increased and by this way the volume of free mobile hydrogen is decreased [17]. Because only diffusible hydrogen plays a role in the hydrogen embrittlement [52,53], the performed thermal aging has led to decreased measure of hydrogen embrittlement and the dominance of thermal embrittlement [26,27]. The details on the fracture behavior of the studied weldments will be discussed in the following section.…”

Section: Mechanical Propertiesmentioning

confidence: 99%

The Effects of Electrochemical Hydrogen Charging on Room-Temperature Tensile Properties of T92/TP316H Dissimilar Weldments in Quenched-and-Tempered and Thermally-Aged Conditions

Ševc

Falat

Čiripová

et al. 2019

The influence of isothermal aging at 620 °C in combination with subsequent electrochemical hydrogen charging at room-temperature was studied on quenched-and-tempered T92/TP316H martensitic/austenitic weldments in terms of their room-temperature tensile properties and fracture behavior. Hydrogen charging of the weldments did not significantly affect their strength properties; however, it resulted in considerable deterioration of their plastic properties along with significant impact on their fracture characteristics and failure localization. The hydrogen embrittlement plays a dominant role in degradation of the plastic properties of the weldments already in their initial material state, i.e., before thermal aging. After thermal aging and subsequent hydrogen charging, mutual superposition of thermal and hydrogen embrittlement phenomena had led to clearly observable effects on the welds deformation and fracture processes. The measure of hydrogen embrittlement was clearly lowered for thermally aged material state, since the contribution of thermal embrittlement to overall degradation of the weldments has dominated. The majority of failures of the weldments after hydrogen charging occurred in the vicinity of T92 BM/Ni weld metal (WM) fusion zone; mostly along the Type-II boundary in Ni-based weld metal. Thus, regardless of aging exposure, the most critical failure regions of the investigated weldments after hydrogen charging and tensile straining at room temperature are the T92 BM/Ni WM fusion boundary and Type-II boundary acting like preferential microstructural sites for hydrogen embrittling effects accumulation.

“…It was also found that when the TIG process is accompanied by the addition of filler metal, the same temperature can be used for PWHT (760 • C), but the duration of the treatment must be much shorter, i.e., only 2 h, while welding TIG autogenous require treatment at the same temperature, but for 6 h, in order to reach the toughness level usually required, i.e., 47 J. Another study carried out by the same team [23], but this time using shielded metal arc welding (SMAW), allowed to quantify the maximum level of diffusible H that the P91 joints may contain without hydrogen embrittlement. This value was established at 6.21 mL/100 g. Higher values result in increased porosity in the joints and a progressive loss of toughness presented by the joints.…”

Section: Introductionmentioning

confidence: 99%

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

Sousa

Silva

Pinho

et al. 2021

Grade P91 is a relatively new class of steel, which has received special attention from designers because it presents extremely interesting characteristics for specific applications. This steel exhibits ideal properties for demanding applications, especially involving high temperature and pressure, being employed in facilities such as power plants and other equipment, such as heat exchangers. P91 welds usually need heat treatments, which are already parameterized in the codes. However, standardized treatments are time-consuming and harmful to the environment, as they massively consume energy. Some attempts have been made in the past to reduce the time and energy spent on these treatments. This work aims to extend this study, now presenting better solutions than those obtained previously. This work presents four new conditions for the heat treatment of joints carried out on P91 steel, with a view to reducing processing time, reducing energy consumption, and an even better balance between mechanical strength and elongation after failure. Heat treatment conditions were established in which there was a loss of about 14% in Ultimate Tensile Strength (UTS), but in which a gain of about 50% in elongation was obtained, compared to welding without any treatment, but also with 10% losses in the UTS and 30% gains in elongation when compared to the solution recommended as more correct in the codes, saving a lot of time and energy in the treatment process. Thus, these solutions may be adopted in the future with gains in terms of productivity and economic and environmental sustainability.