Effect of microstructure on the indentation creep behaviour of 2.25Cr-1Mo and its comparison with modified 9Cr-1Mo ferritic steel

Abstract

2.25Cr-1Mo and modified 9Cr-1Mo are extensively used for super heater and reheater tube, header pipe in large-scale thermal and nuclear power plant. Some of the important factors to be considered for selection of such steels are resistance to creep deformation, resistance to environmental attack, corrosion resistance, oxidation resistance. Long term exposure at elevated temperature and pressure can result in microstructural changes. The effective strength, i.e. load bearing capacity of the tubes due to microstructural damages decreases. The failure occurs when it falls below a critical level determined by component geometry and loading. The objective of the investigation is to study the effect of microstructure on the indentation creep behaviour of 2.25Cr-1Mo and its comparison with modified 9Cr-1Mo.
Indentation creep tests for 2.25Cr-1Mo ferritic steel and modified 9Cr-1Mo steel were carried out at different temperature and different load by using Spranktronics indentation creep testing machine. The microstructural examinations and phase analysis were carried out with the help of an image analyser. The microstructural examinations and EDAX were taken by SEM. Hardness measurements were done using a Vickers hardness tester by applying 500g load with dwell time of 15 s. The plates of two alloy steels were first normalised at 950°C for 2hrs. Indentation creep tests for 2.25Cr-1Mo ferritic steel were carried out at different temperature (4800C, 5000C, 5200C) and different load (10 Kg, 20Kg). The indentation creep test of modified 9Cr-1Mo ferritic steel was done at 5200C with a load of 10kg.
The results highlight that creep of 2.25Cr-1Mo ferritic steel was load and temperature dependent, larger load and higher temperature resulted in a larger creep deformation. It was also found that higher load and higher temperature caused higher creep rate .Creep rate for all temperature and loads had a maximum value at the initial creep stage and it decreased dramatically after a period of holding time. The value of activation energy was found to be 95kJ/mol. From the microstructural analysis it was revealed out that with increase in temperature there is a decline in creep strength because there is a reduction in the amount of bainite with the progress of temperature. Modified 9Cr-1Mo steel shows best compressive behaviour than 2.25Cr-1Mo.Because it contains more amount of alloying element.