Effect of Microstructure on Sliding Wear Behaviour of Modified 9Cr-1Mo steel

Abstract

Modified 9Cr.1Mo ferritic steel finds wide application in petroleum industries, chemical processing and thermal power plants because of its excellent mechanical properties and corrosion resistance. This material has become a favoured tubing material in the reheater and super heater portions of fast reaction steam generators. Excellent mechanical properties, oxidation and corrosion resistance combined with low creep rate and low thermal expansion makes it a wonderful material for high temperature applications. The present investigation aims to study the effect of microstructural variations in wear performance of modified 9Cr-1Mo steel using dry sliding wear test. Total twelve samples of the material were subjected to three different heat treatments i.e. annealing, normalizing and water quenching (to develop lower bainite) heated to a temperature of 1050¢ªC. One from each heat treated types was prepared for microstructural and hardness studies. Optical and Scanning electron microscopy along with Energy Dispersive Spectrum (EDS) analysis have been used to characterize the microstructures. The shape and percentage of phases present in the microstructure were obtained from optical microscope with image analyzer which plays a vital role in determining the wear behaviour of the material. The hardness of the three different heat treated samples was measured by Vickers hardness testing machine. Wear behaviour of this steel was investigated on a disc-on-roller multiple wear tester of model TR-25 under dry sliding condition, rubbing against EN-31 steel. Three samples of the each heat treated types were subjected to three different load variations of 30N, 50N and 100N at a constant sliding speed of 250 RPM and the results obtained were compared. The comparison among three heat treated types has been done for a constant load of 100N. SEM micrographs of worn surfaces have been used to correlate the results obtained.
Following are the conclusions based on the present investigations:
. The results obtained for the annealed samples exposed to three different load variations of 30N, 50N and 100N shows that with increase in load, the wear rate increases as there is increase in friction at the contact surface.
. For normalized and water quenched samples exposed to 100N load, deviation occurs due to formation of oxide scales on the material surface leading to oxidative wear pattern. The formation of oxide scale is due to the generation of high temperature when high load
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is applied on the material. These oxide scales protects the material surface inhibiting the further wear of the material.
. A comparison graph which has been drawn to show the variations in wear of three heat treated types at a constant load of 100N gives information that the water quenched sample experiences less wear in comparison to normalized and annealed ones. This is due to the formation of fully bainitic structure in case of quenched sample which results in higher hardness of the material and least wear among the three.
. The wear rate with respect to time or sliding distance for all the three types of samples in different loading conditions follows the decreasing trend.