INVESTIGATION OF THE THERMAL FATIGUE BEHAVIOUR OF T-23 STEEL TUBE COMPARED WITH PREDICTED NUMBER OF CYCLES TO FAILURE USING COFFIN-MANSON EQUATION

摘要

Steels for high temperature service play a vital role in construction of power stations. For a long time, the ASTM A 213 Gr T-22 steel tube has been in use world wide in high capacity power stations with excellent operating behaviour. The increase of operating steam parameters like pressure, temperature etc, and set a greater demand for the development of high strength steels. ASTM A 213 Gr T-23 material was evolved as an alternative to T-22 steel tube. It is a low alloyed steel (2.25% Cr) derived from grade T22 substituting part of the Mo content with W and adding Nb, V, B. These super heater tubes subjected to alternate heating and cooling in power plants. Thermal shock loading of operating pressure equipment is a common occurrence, particularly in thermal power stations. These tubes are subjected to the temperature of 600℃ and producing steam at 540℃ and a pressure of 160 bar. In this paper, a laboratory simulation for reproducing thermal fatigue phenomenon is developed to determine the number of cycles to failure occurring in super heater tubes. The experimentally obtained cycles are compared with predicted number of cycles to failure using Coffin-Manson equation. The base tube is subjected to thermal cycles from 800℃ (accelerated temperature) to room temperature. Oxy-acetylene flame is utilised as a heating source, whereas an improvised facility, water bath is utilised for quenching purpose. The test is carried out until open cracks are identified Surface cracks are observed in the base tube after 120 cycles. In base tube lot of longitudinal cracks are observed on the heating zone. The length of the longitudinal crack is about 20 mm and small transverse cracks also seen. The tube is then sectioned and subjected to optical microscopy, SEM and EDAX analyses. Cracks are observed in the failed base tube. A comparison of predicted thermal fatigue lives with experimental results shows the difference of 8 cycles.