DEVELOPMENT OF EROSION RESISTANT NITRONIC STEEL

Abstract

Erosive wear is caused by the impact of abrasive particles against a solid surface. Erosion is rapid and severe forms of wear and can results in significant costs if not adequately controlled. Erosive wear occurs in a wide variety of machineries and typical examples are the damage to gas turbine blades when an aircraft flies through dust clouds, hydro turbine under water parts when silt laden water flows though it and the wear of pump impellers in mineral slurry processing systems. It is seen that hydro power plants suffer a serious setback due to rapid erosion of underwater parts by silt laden water and cavitation. The I 3Cr-4Ni (termed as 13/4 or CA6NM) steel is currently being used for fabrication of underwater parts in hydroelectric projects. There are, however, several maintenance problems associated with the use of this steel. In this research attempts are made to develop an erosion resistant material called 21-4-N steel as an alternative to 13/4 steel. So far no investigation has been reported on the comparative study on erosion behaviour of 13/4 and 21-4-N steel. 21-4-N steel is a nitrogen strengthened austenitic stainless steel. The role of nitrogen in 21-4-N steel is to stabilize the austenitic structure at room temperature, strengthen the austenitic matrix by solid solution hardening and decrease the stacking fault energy, which results in improving the strain hardening ability. By alloying with nitrogen, the strength of the alloy can exceed than that of martensitic steel. Due to higher strain hardening ability, the strength of nitronic steels can be enhanced by mechanical deformation. Bars of 40 x 40 mm cross section of as cast 13/4 steel and 21-4-N steel in as cast and hot rolled conditions were received from M/S Star Wire (India) Ltd. Ballabhgarh (Haryana). The heat treatment of as cast 13/4 steel involved austenitizing at 1050°C, followed by oil quenching, followed by tempering at 620°C for 4 hrs. The heat treatment of as cast 21-4-N steel consisted of heating at various austenitizing temperatures (950°C, 1000°C, 1050°C and 1100°C) for 2 hr, followed by water quenching (WQ). A few specimens of 21-4-N steel were solution treated at 1100°C, followed by WQ, followed by aging at 700°C for 20 hrs. To investigate the effect of deformation on mechanical properties and erosion behaviour 8% reduction in thickness was given to as cast 13/4 and 21-4-N steels at 700°C. Studies were also conducted on 21-4-N steel, which was subjected to 89% reduction in cross sectional area by hot rolling at 1180°C. The erosion rate or weight loss for all the samples was determined by means of solid particle erosion using gas jet. The erodent particles used were SiC of size 500-700 pm. Particle velocity of 120 ms-1 at a constant feed rate of 5 g min has been employed. The erosion experiments were conducted at 30° and 90° impingement angles at room temperature. The samples were cleaned in acetone, dried, weighed to an accuracy of lx 10-4 g using an electronic balance, eroded in the test rig for 10 min and then weighed again to determine weight loss. The ratio of this weight loss to the weight of the eroding particles causing the loss (i.e., testing time x particle feed rate) was then computed as the dimensionless incremental erosion rate. The cavitation erosion tests were performed using ultrasonic vibration processor for total duration of 32 hr. The scanning electron microscopy was used to study the mechanisms of erosion. To study the effect of surface coatings on erosion behaviour, D-Gun spray coatings of stellte-6, Cr3C2-NiCr and WC-Co-Cr • were provided on the hot rolled 21-4-N steel, which has excellent erosion resistance. IV The microstructure of 13/4 steel consists of martensitic laths and 8-ferrite. However, these martensitic laths are observed to be thickened after tempering as well as after 8% rolling operation. The microstructure of 21-4-N steel in various conditions consists of precipitates of carbides in the matrix of austenite. During solution annealing treatment most of the carbides are dissolved in austenitic matrix. The extent of carbide precipitation is increased during aging of 21-4-N steel. Small deformation (8%) does not cause any appreciable change in the microstructure of as cast 21-4-N steel. The microstructure of hot rolled 21-4-N steel consists of fine grains of austenite and precipitates of carbides along the grain boundaries. The 13/4 steel in as cast condition possesses significantly higher values of impact energy, YS and UTS than the corresponding values in the 21-4-N steel in as cast condition. The hardness, ductility (% elongation), tensile toughness and strain hardening exponent in 13/4 steel in as cast condition are lower than that for as cast 214-N steel in as cast condition. Due to austenitizing and tempering of 13/4 steel (termed as tempered. 13/4 steel), there is significant increase in YS, UTS and impact energy, and the value of,. hardness is decreased, while minor changes occur in ductility, tensile toughness and strain hardening exponent. Drastic fall in the values of ductility, impact energy, tensile toughness and considerable increase in the values of YS, UTS and hardness are observed, while strain hardening exponent remains almost unchanged in 13/4 steel as a result of 8% rolling. The solution annealing treatment of as cast 21-4-N steel shows the increased value of YS, UTS, ductility and tensile toughness, whereas these are decreased as a result of 8% rolling. Hardness and strain hardening exponent of as cast 21-4-N steel are observed to decrease by the solution annealing treatment, but increase in hardness and V strain hardening exponent is seen after aging treatment; while minor change is seen in impact energy. After 8% rolling of 21-4-N steel, there is remarkable increase in YS, UTS and hardness; whereas the values of ductility, tensile toughness, impact energy and strain hardening exponent decrease considerably. It is seen that erosion damage in both the 13/4 and 21-4-N steels is lower at 30° impingement angle than that at 90° impingement angle, which corresponds to erosion of brittle materials. The erosion behaviour in 13/4 and 21-4-N steels are observed to be affected by heat treatments and deformation by rolling. Cumulative weight losses of all the samples eroded at 30° impingement angle give straight line relationship with time, which indicates that the mechanism of erosion is same throughout the erosion test. Maximum weight loss is observed at 30° impingement angle in tempered 13/4 steel; while at 90° impingement angle maximum weight loss is observed in 8% rolled 13/4 steel. The least cumulative weight loss is observed in hot rolled 21-4-N steel at both the impingement angles. The erosion resistance of as cast 21-4—N steel investigated by means of solid particle impingement is higher than that of as cast 13/4 martensitic stainless steel, because of its austenitic matrix, which is less prone to erosion damages as compared to the stressed and untempered martensitic matrix of 13/4 steel. Mechanical properties significantly affect the erosion resistance of target material. In 21-4—N steel, high resistance to erosion is due to (i) high hardness coupled with high ductility, (ii) high tensile toughness and (iii) high rate of strain hardening in comparison to 13/4 martensitic stainless steel. VI Remarkable improvement is observed at 90° impingement angle in the erosion resistance of as cast 13/4 steel; whereas erosion resistance slightly deteriorates at 30° impingement angle as a result of solutionizing at 1050 °C and tempering at 620 °C. At 90° impingement angle the increased erosion resistance of 13/4 steel as result of tempering treatment is due to relieving of internal stresses and thickening of martensitic laths. As a result of tempering at 620 °C there are certain changes in mechanical properties of 13/4 steel. Due to tempering the hardness decreases from 305 to 289 VHN and the tensile toughness increases slightly (from 68 to 71 M.Inf3). The values of ductility and strain hardening exponent (n) almost remain unchanged. The decrease in erosion resistance at 30° impingement angle is attributed to decrease in hardness whereas increase in erosion resistance at 90° impingement angle is correlated with slightly increase in tensile toughness. Substantial improvement in the erosion resistance of 21-4-N steel at both the impingement angles of 30° and 90° is observed as a result of solution annealing at 1100 °C. However, aging treatment of solution annealed 21-4-N steel causes deterioration in erosion resistance at both impingement angles. The improved erosion resistance of 21-4-N steel as a result of solution annealing at 1100 °C is due to substantial dissolution of carbides in the austenitic matrix. Again.reprecipitations of carbides in the aged 21-4-N steel are responsible for decreased erosion resistance. The mechanical properties resulting from solution annealing significantly affect the erosion resistance of 21-4-N steel. The increased values of ductility and tensile toughness as a result of solution annealing of 21-4-N steel increase the erosion resistance. VII However, lowering of hardness and strain hardening exponent of 21-4-N steel as result of solution annealing may cause reduction in erosion resistance; but cumulative effect of all mechanical properties results in increased erosion resistance. Mechanical working (by rolling) significantly affects properties and erosion resistance of 13/4 and 21-4-N steels. In 13/4 steel the martensitic laths are thickened whereas in 21-4-N steel no significant change is observed in the microstructure as a result of 8% rolling at 700 °C. The values of YS, UTS, and hardness increase and the values of tensile toughness, ductility, impact energy and strain hardening exponent in both 13/4 and 21-4-N steels as a result of 8% rolling at 700 °C. The values of hardness, impact energy, YS, UTS, tensile toughness and strain hardening exponent increase in 21-4-N steel as a result of hot rolling. As a result of 8% reduction in thickness by rolling, for both 13/4 and 21-4-N steels there is improvement in erosion resistance at 30° impingement angle, whereas at 90° impingement angle there is a deterioration in the erosion resistance. Hot rolling to 21-4-N steel causes improvement in erosion resistance at both impingement angles. By 8% rolling the improved erosion resistance of 13/4 and 21-4-N steel at 30° impingement angle is attributed to overwhelming effect of increased hardness; whereas, the deterioration in erosion resistance of both the steels at 90° impingement angle is attributed to decreased values of ductility, tensile toughness and strain hardening exponent, in addition to internal stresses induced during rolling. By hot rolling at 1180 °C the improved erosion resistance of 21-4-N steel at both 30° and 90° impingement angles is attributed to (i) fine grain structure of austenite and (ii) increased values of hardness, ductility, tensile toughness and strain hardening exponent. VIII The cavitation erosion resistance of 13/4 steel investigated by means of ultrasonic vibration processor is less than that of 21-4-N steel in both as cast and hot rolled conditions. However, hot rolled 21-4-N steel exhibits excellent cavitation erosion resistance. From the microstructure view point, the untempered martensitic laths of 13/4 steel exhibit more cavitation erosion than the austenitic structure of 21-4-N steel. The martensitic laths, already associated with internal stresses, are less able to absorb the strain energy due to transient stresses in the material induced by cavitation impact wave. It can also be inferred that the carbides present in 21-4-N steel are detrimental to cavitation erosion resistance leading to more erosion in as cast condition. In 21-4-N steel in both as cast and hot rolled conditions, the cavitation erosion damages occur along the grain boundaries and the carbides are the first to be dislodged from the specimens. In 21-4-N steel higher resistance to cavitation erosion is associated with high hardness coupled with high ductility, high tensile toughness and high strain hardening exponent in comparison to those in 13/4 steel. The stellite-6 coating on 21-4-N steel substrate exhibits maximum damage due to erosion at 30° and 90° impingement angles. Minimum erosion losses are observed in WC-Co-Cr coating at 30° impingement angle. The analysis of the influence of various parameters on erosion behaviour of surface coatings poses a complex problem. It is not possible to quantify the role of individual parameters like microstructural features, microhardness and porosity in affecting the erosion behaviour of coating. It is, however, observed that the nature and extent of porosity affect to a large extent the erosion rate of coatings. Maximum concentration of porosity is observed in stellite-6 coating. Scanning electron microscopic study of eroded surfaces on coated components reveals that at 30° IX impingement angle erosion occurs by a shear process involving formation of lips and ploughs on the target surface. Existence of deep craters in the SEM micrographs at 90° impingement angle indicate knocking out of chunk of material from the target surface resulting in high rates of erosion. The entire work has been presented over nine chapters in the thesis. Chapter 1 presents a critical review of the available literature on nitrogen strengthened austenitic stainless steels, solid particle erosion, erosion resistant engineering materials, cavitation erosion, cavitation erosion resistant engineering materials, and erosive wear of surface coatings. Chapter 2 deals with the formulation of problem and planning of experiments. Chapter 3 deals with the experimental study employed in the present study. Details of air jet erosion test rig, ultrasonic processor for cavitation erosion test have been described. Details of various treatments given to the steels under study have been described. Details of instruments/machines and the techniques employed in the mechanical testing and metallographic study are given. Chapter 4 consists of characterization (microstructural and mechanical properties) and solid particle erosion behaviour of as cast 13/4 and 21-4-N steels. Chapter 5 describes the effect of heat treatment on the solid particle erosion behaviour of as cast 13/4 and 21-4-N steels. Chapter 6 describes the effect of deformation on the solid particle erosion behaviour of as cast 13/4 and 21-4-N steels. The erosion test was also conducted on hot rolled 21-4-N steel and comparative study has been made with that of as cast 21-4-N steel. Chapter 7 gives the cavitation erosion behaviour of as cast 13/4 steel and 21-4-N steel in as cast and in hot rolled conditions. Chapter 8 describes the characterization and erosive wear behaviour of surface coated hot rolled 21-4-N steel. Chapter 9 presents the co