SLURRY EROSION OF 13/4 MARTENSITIC STAINLESS STEEL

Abstract

In the present study, investigations have been carried out on the effect of various heat treatments on the structure, mechanical properties and erosion behavior of 13/4 martensitic stainless steel (CA-6 NM) of nominal composition (weight %): C- 0.020 Mn- 0.18 Si- 0.14 Cr- 13.75 Ni- 3.45 P- 0.018 S-0.009 This steel is most widely used in the fabrication of underwater parts in the hydroelectric power projects. The present work aims at studies to develop high erosion resistance in the cast steel by giving various heat treatments. The As received bars of the 13/4 martensitic stainless steel were given some standard heat treatments. These heat treatments involve the austenitization of cast steel at temperatures of 950°C, 1000°C and 1050°C and the holding time for each temperature is 2 hr, 4 hr and 6 hr. This is followed by oil quenching and then tempering for 1 hr at temperature of 600° C. The heat treated steel is tested for hardness, ductility (%El), toughness (Impact Strength) and tensile strength (UTS) and their effect on erosion behavior at different impingement angles i.e. 0°, 30°, 60°and 90°. The As received and the heat treated steel. which shows minimum weight loss were studied by using optical microscope to identify the different micro constituents such as lath martensite, carbides etc. which precipitates along the grain boundaries during the heat treatment. The mechanisms of -material failure in impact test, tensile test and also erosion test have been investigated using Scanning Electron Microscope (SEM). It is observed that in the first stage of erosion, plastic deformation occurs at the target surface by impact of silt particles. This results in the formation of ploughs and material lips. The lips are produced by shear and later become brittle leading to their removal from the material surface. Further impact of silt particles results in removal of material from the surface. Consequently the material is eroded gradually and causes a rough pattern on target surface. It is observed that cutting mechanism is also playing its role during erosion. This eroded surface was scanned under SEM to identify the mechanism of erosion. The microstructural aspects are also observed to affect the erosion behavior significantly. Tempered martensite is best suited for high erosion resistance. Coarsening of carbides leads to decrease in erosion resistance. SEM studies show that the erosion occurs by cutting and ploughing followed by lip formation. It is observed that the erosion behavior is affected by various mechanical properties as well as microstructure. It is also greatly affected by the inclination of the material with respect to the silt particles present in the slurry. Increase in ductility and toughness improves the erosion resistance. The erosion behavior is also dependent on UTS and hardness, as the UTS increases the weight loss also increases, same is the case with hardness. The minimum weight loss (erosion rate) is corresponds to 1050°C-6hr-OQ-600°C-Ihr. Approximately 36% reduction in weight loss is observed as compared to As received material. (