Effect of impingement angle on erosion of high strength low alloy steel (Shot peened, Heat treated)

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1 International Conference on Shot Peening and Blast Cleaning. Effect of impingement angle on erosion of high strength low alloy steel (Shot peened, Heat treated) G.Dixit, Asstt Prof., Applied Mech. Deptt., MACT, Bhopai Erosion of tubes by coal particles or coal ash impingement has caused serious problems in thermal power stations. So erosion behaviour is a vital parameter for selection of steel for tubing purpose. One of the most important characteristics of erosions is the variation of erosion damage with impingement angle. When considering materials to withstand erosion the properties of abrasive particles namely velocity, impingement angle, concentration, density hardness and surface contour are important. The influence of impact angle is dependent upon the type of material undergoing erosion. It can be seen from fig. (1) ductile materials the maximum erosion occurs at small impingement angle ( 5 45"). Whereas for britle materials it occur at normal impact (90"). According to Finnie erosion was proportional to velocity [I], Wood found that erosion rates were essentially constant for dust concent ration, which ranged from 1 x 1 O6 Kg/m3 to 3 x 3x Kg/m3 [2]. SO particles interference is not on important factor in erosion in this range of dust concentration. Finnic found that erosion resistance of annealed metallic materials at very small impingement angle (20") was proportional to their Vicker's hardness [4]. Shot Peening & Heat Treatment Rectangular plates (of dimensions 50x30~3 mmj'were cut from alloy steel (chemical composition given in appendix). Three types of samplers were taken for the purpose of erosion test. One was as received and other two were shot peened and heat treated. On sample was Shot peened by 0.8 mm diameter shots of steel of Hv hardness of 62C Second sample was annealed by solutionizing in three zone high temperature tubular vertical furnace at 860 C for 2 hours followed by

2 cooling in furnace. The surfaces of above prepared three samples were ground with 180 grit size paper, standard metallographic technique were employed for polishing the samples. After polishing roughness has reached the average value Ra = 0.42 pm flat tensile specimens (gauge length 15 mm, thickness 3 mm) were prepared from all three types of samples (as received, shot peened and heat treated). Fig. 1. Schematic represenlalion of Erosion rate on attack angle for ductile and briille materiels Fig.2. Variation d Erosion with angle d Impingement (as received)

3 Angle of lmplngemenf (Degree) Fln. 3 Fig. 3. Variation of Erosion with angle of lmpinyernel~t (Annealed) Arigle of Impingement (Degree) Fig. 4 dg. 4. Variation of Erosion with angle of Impingement (Shot Peened)

4 Test Procedure Ultimate tensile strength and % elongation was determined from the tensile specimen of above prepared three types of samples by using computer controlled UTM. hardness of the three types of samples was measured by hardness testing machine (Vickers Hardness tester). All the three types of samples were cleaned with flowing water and acetone, one sample was inserted in the specimen holder. Angle of impingement was adjusted by (+90 ) using pointer and scale. After closing the front chamber door the vacuum was created in the chamber by means of blower. The distance between nozzle and specimen was adjusted by dial indicator. Alumina particles (of average size 50 pm) was used as erodent. The test samples were exposed to a mixture of air and erodent of known velocity. The nozzle pressure was kept at 0.17 MPa. For rate of air and flow wheel r.p.m. were kept at 30 litlmin and 10.3 respectively. Erosion testing was done for 45 minutes. Erosion was measured in terms of weight loss using a precision electronic balance accurate to mg. All the experiments were conducted at room temperature, i.e. 25 C. Fig. 2 shows a plot of erosion damage vs angle of impingement of the as received steel. It may be noted that erosion increased with the increase in the angle of impingement attained a maximum followed by a decrease in the erosion damage was observed at an angle of 30". The erosion data of the annealed steel as a function of the angle of impingement can be seen in fig. 3. The trend observed was the erosion damage with the angle of impingement was identical in this case to the as received steel (fig. 2). The erosion damage was found to be maximum at an angle of 30" in this case also. The maximum erosion damage in the case of shot peened steel could be attained at 60" of impingement (fig. 4). A compression of the erosion of the steel in different conditions is shown in fig. 5. It may be noted that angle of impingement at which maximum erosion was caused shifted from 30" in the case of annealed and as received steels to 60" in the shot peened steel (Fig. 5).

5 Angb of lmplngemenl (Degree) Fig.5. Comparison of Erosion of Alloy steel [As received (l), Heat treated (2) and Shot peened (3) condition] The erosion damage of the annealed steel was noted to be significantly higher than the received one upto an angle of 45", beyond which it was comparable. On the other hand shot peened sample showed lower erosion damage upto 45" and attained a maximum at 60". At angle 75" and above the erosion damage of hardened plus tempered steel was comparable to as received and annealed steel. Condition As Received Shot Peened Annealed, Hardness UTS (MPa) % Elongation Mechanical properties such as tensile strength, % elongation and hardness of the steel in the various conditions are shown in table 1. The annealed steel showed minimum tensile strength (UTS) and hardness but maximum % elongation. The hardness and UTS value of as received sample was found to be less than the shot peened sample. While a reverse trend was observed for the % elongation. I

6 Conclusion Based on observations mode in this study following conclusion could be drawn: 1. Erosion damage first increase with the angle of impingement attained a maximum followed by a further decrease. The maximum erosion damage in the case of annealed and as received samples was observed at 30" which changed to 60" in the case of shot peened sample. 2. Ultimate tensile strength and hardness of the annealed steel was less than that of shot peened steel while a reverse trend was noticed by % elongation. Bibliography 1. Fimic, I. Wolak, and Kabil, Y. : Erosion of Metals by Solid Particles, Journal of, Materials, Vol. 2, No. 3, Sept. 1967, p Wood, C. D., 'Erosion of Materials by High Speed Impact of Dust Particles Proc. Inst. Envison, Sci. (1966) p Bitter, J. C. A.:'A study of Erosion Phenomenon, Part I and 77, Wear Vol6,(1963), p and Finnie, 1. : Proc. 3rd U. 5. Metal Congr. of Applied Mechanics (1 958) ASME, New York, 1958, p Appendix - I Chemical composition of High strength low alloy steel C % Cr - 65% Si % V % Mn - 1.6% Ti % S % Al %