Fatigue ratcheting studies on TP304 LN stainless steel straight pipes

Abstract

Fatigue Ratcheting' is a phenomenon which leads to reduction in fatigue life of a structural component by loss of ductility due to cycle-by-cycle accumulation of plastic strain. One real-life example of a situation with fatigue ratcheting possibility is piping of power plants subjected to internal pressure (primary load) and cyclic bending (secondary load). Fatigue ratcheting studies were carried out on TP304 LN stainless steel straight pipes of 168 mm outer diameter subjected to steady internal pressure and four point cyclic bending. The length and average thickness of the pipes were 2800 mm and 15 mm respectively. The thickness was reduced to 12 mm in the gauge length portion of 200 mm at the centre of the pipe. Post-yield two element rosette strain gauges were mounted at various locations within the gauge length to measure the longitudinal and circumferential strains. The pipes were filled with water and pressurized; the pressure was maintained at 35 MPa till the first through-thickness crack was observed. Cyclic bending load was applied on the pipes by subjecting them to different levels of load-line displacements. Number of cycles corresponding to through-thickness crack/s and final failure of the component were recorded. During the tests, load, load-line displacement and deflections at three locations were continuously monitored. Local bulging, ovalization and consequent thinning of the pipe cross-section were observed due to fatigue ratcheting. These ratcheting studies on pressurized piping components of power plants give valuable inputs necessary for designing the components and assuring the integrity of pressure boundary under design basis loads such as loads arising during an earthquake event.

Ratcheting in pressurized pipes and equipment: A review on affecting parameters, modelling, safety codes, and challenges

“…14,120-122 To control primary loads and evaluate the impact of static and dynamic loads, applied stress limits in the pressurized pipes were altered. 121 Figure 11A presents a straight pipe with steady internal pressure of 15 MPa and bending loads of 23, 28, 33, and 28 kN, respectively, with interval of 30 cycles in each step. Figure 11 presents ratcheting response of pressurized tubes subjected to multistep bending loads for low-carbon steel pipe 14 and Austenitic steel pipe.…”

“…Local ratcheting strains versus loading cycles for (A) various notch shapes/sizes and (B) distances from a circular notch root 95 increased through a number of loading steps. 14,[120][121][122] To control primary loads and evaluate the impact of static and dynamic loads, applied stress limits in the pressurized pipes were altered. 123 Ratcheting growth was then found highly dependent on the sequence of stress limits and number of cycles applied on steel pressurized pipes.…”

“…Figure 11 presents ratcheting response of pressurized tubes subjected to multistep bending loads for low-carbon steel pipe 14 and Austenitic steel pipe. 121 Figure 11A presents a straight pipe with steady internal pressure of 15 MPa and bending loads of 23, 28, 33, and 28 kN, respectively, with interval of 30 cycles in each step. Figure 11B presents an austenitic pressurized pipe tested with changing values of internal pressure and external bending loads.…”

Study on Fatigue Performance of Pulsed Tungsten Inert Gas Welding Joint of Duplex Stainless Steel Thin Tube

“…The shear lip zone could be seen on both sides of the pipe wall because the duplex stainless steel had good ductility [ 49 , 50 , 51 ]. There was an obvious ratchet pattern on the fracture at the stress level of 360 MPa [ 52 , 53 ] because the fracture form of the weld joint was a multi-source fatigue fracture. Under the stress levels of 225 MPa and 270 MPa, fatigue stress concentration did not easily occur at the weld joint, and the source of fatigue crack initiation was singular.…”

Microstructure Characteristics of TP304 Steel after Long-time High-temperature Service

“…The lack of adequate studies on the localization and application performance of high-temperature and high-strength materials used in ultra-supercritical units is one of the major factors limiting the development of these systems [3][4][5]. The higher temperature range that TP304 austenitic stainless steel is used in allows it to partially address the furnace flue gas temperature differential brought on by an overheated explosion tube, greatly enhancing boiler safety [6]. TP304 austenitic stainless steel is susceptible to intergranular rust and microstructure change, however, after prolonged use at high temperatures and high pressure [7].…”