RESEARCHES REGARDING THE EFFECT OF PRESSURE ON SURFACE QUALITY DURING ABRASIVE WATERJET CUTTING OF AUSTENITIC STEELS

Transcription

1 RESEARCHES REGARDING THE EFFECT OF PRESSURE ON SURFACE QUALITY DURING ABRASIVE WATERJET CUTTING OF AUSTENITIC STEELS Ion Aurel PERIANU a, b, Ion MITELEA a, Viorel Aurel ŞERBAN a a Politehnica University of Timisoara, Timisoara, Romania, EU b National R&D Institute for Welding and Material Testing ISIM, Timisoara, Romania, EU perianu.ion.aurel@gmail.com; ion.mitelea@mec.upt.ro Abstract In this paper research elements are presented in the field of abrasive water jet cutting of materials hard to process by machining such as austenitic stainless steels, in this case with a thickness of 20 mm. Selection of the optimal cutting process based on technical and economic criteria takes into consideration the type and thickness of the targeted material and also the physical and geometrical quality requirements. The present paper contains experimental research results regarding abrasive water jet cutting of austenitic stainless steel EN (ASTM 304 L) at different values of water pressure. The abrasive material used is Garnet with particle granulation 80 Mesh. By making roughness measurements and hardness examinations of the cut surface an evaluation will be made of the surface quality defining the optimal pressure values. Keywords: water jet cutting, roughness, austenitic stainless steel 1. INTRODUCTION Abrasive water jet cutting is a process used in various applications in different fields due to the advantages it offers namely limiting thermo-mechanical affected zone, high machining versatility, minimum stresses on the work piece, advantages that proved this type of cutting as an effective technology for cutting engineering materials [1] and especially difficult to process materials [2] like thick austenitic steels. The process parameters that can be adjusted by the abrasive water jet cutting equipment user has influence over the surface quality by altering the Ra roughness of the processed surface [3,4] and also causes some surface defects to appear that are placed usually in the lower part of the work piece. This paper presents an analysis on the influence of variation of a single abrasive water jet cutting parameter, namely the water pressure [bars] over the processed surface quality studying the surface roughness [5] obtained in the entrance zone, median and lower zone of the water jet in the work piece. Apart from the roughness measurements of the cut surfaces a SEM examination was performed on samples cut and also micro-hardness HV 0,2 was measured along the thermo-mechanically affected layer in the water jet entrance and exit surfaces of the samples. Surface cut roughness [6] was chosen as an object of study because it is considered as a performance marker for many industrial applications and is a condition required for the functionality of the parts. 2. EXPERIMENTATION 2.1. Material The material used in experimentation is an austenitic stainless steel EN (ASTM 304L; JIS SUS 304L) with a thickness of de 20 mm and the chemical composition and mechanical properties of this steel are presented in table 1.

2 This type of steel is corrosion resistant due to the high content of Cr and Ni and is used in the automotive, chemical, food processing industries, decorative elements and cutlery, electronic equipment and petrochemical industry (especially for producing and storing nitric acid). Table 1 Chemical composition and mechanical properties of EN (ASTM 304 L) stainless steel Chemical composition (%) C Cr Ni Mn Si P S Mo Fe 0,06 19,19 8,29 1,75 0,63 0,028 0,024 0,08 69,948 Mechanical properties Yield strength Rp 0,2 [N/mm2] Mechanical strength Rm [N/mm2] Elongation A5 [%] Hardness HB Impact test ISO V at 25 C [J] Equipment The equipment used consists of a high pressure pump Streamline SL-V OEM made by KMT that is fitted on a CNC cutting portal with an abrasive feeding system that can vary the feed rate in the range of g/min [7]. Abrasive material used was chosen according to the nozzles used in the cutting process namely the water nozzle and the abrasive nozzle (focusing tube). The water nozzle used had an inner diameter Ø water of mm and the abrasive nozzle inner diameter was Ø abrasive 0.78 mm. Thus an abrasive material mesh 80 was chosen (the values of abrasive particles granulation varies between μm) with a density of 2300 kg/m3. The mixture between the water jet (the water is the accelerating work agent) and abrasive particles performs the actual cutting. In figure 1 the technological work parameters are presented together with the abrasive water jet cutting head. From this image the water jet delay can be observed in the lower part of the work piece due to variations of the water pressure and cutting speed parameters which generate the cut surface quality depending on its roughness. This delay of the water jet is characterized by a distance obtained from the difference between the entrance and exit points of the water jet in the material being cut. Fig. 1. Abrasive water jet cutting water jet cutting head and process parameters 2.3. Experimental conditions In the abrasive water jet cutting process the most important parameter in terms of operation productivity is the cutting speed [mm / min] which is inversely proportional to the quality obtained after cutting using maximum speed values depending on the nature and characteristics of the cutting material. As the cutting speed increases cut surface roughness increases (especially in the lower part of the cut). In order to obtain

3 superior quality of cut surfaces while maintaining productivity another important process parameter must be varied namely the cutting pressure [bar]. In order to demonstrate this, experimental cutting tests were performed varying the value of the pressure parameter while maintaining all the other parameters values constant. The values of the parameters used during experimentations are presented in table 2. Table 2. Experimental abrasive water jet cutting process values Sample no. Cutting speed [mm/min] Abrasive rate [g/min] Nozzle standoff Z [mm] Water pressure [bars] 1 80 (const) 300 (const) 1,5 (const) (const) 300 (const) 1,5 (const) (const) 300 (const) 1,5 (const) 3800 Only three different water pressure values were chosen, placed in the range bars due to the fact that the tests were made on a material with 20 mm thickness and water pressure values below 3400 bars are not relevant when a high productivity is desired. The maximum value for water pressure at 3800 bars for tests was chosen because of the technical limitations of the cutting installation. 3. RESULTS AND DISCUSSION After performing the cutting tests according to the established parameters three roughness profiles were obtained and in figure 2 a), b) and c) the surface aspect of the cut surfaces are presented according to samples 1, 2 and respectively mm a) b) c) Fig. 2. Surface aspect for cut samples: a) sample 1; b) sample 2; c) sample 3 Roughness measurements were carried out in the three areas of interest on the cut surfaces, respectively the upper, median and lower part of the samples and the values obtained are presented in table 3. The measurements were performed twice for each area on a length of 10 mm using a roughness measurement device type Mitutoyo SJ 201 P. Table 3. Measured roughness values in the three areas of samples cut Cr. No. Ra1 Upper Ra2 Upper Ra1 Median Ra2 Median Ra1 Lower Ra2 Lower 1 4,8 4,37 9,2 9,15 26,56 26,65 2 4,4 4,52 8,26 9,70 22,33 29,58 3 4,01 3,69 7,48 7,28 19,73 19,35

4 In order to establish the influence of abrasive water jet cutting on the thermo-mechanically affected layer HV 0,2 micro-hardness examination tests were carried out along the length of the water jet entrance and exit layer. For this purpose samples cut were embedded and prepared metallographic. In table 4 are presented the measured values for HV 0,2 micro-hardness in the entrance and exit areas for each sample. Table 4 Measured values for HV 0,2 Cr. No. Vickers Hardness HV 0,2 Sample 1 entrance Sample 1 exit Sample 2 entrance Sample 2 exit Sample 3 entrance Sample 3 exit D[µm] HV D[µm] HV D[µm] HV D[µm] HV D[µm] HV D[µm] HV , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,5 226 In table 4, D represents the vertical distance between the first hardness mark that was measured 100 µm from the edge of the cut sample having a horizontal distance of 150 µm between them. The microscopic aspect of the hardness examination marks are presented in figure 3 and the HV 0,2 microhardness graphic variations, depending on the entrance and exit areas of the cut samples, are presented in figures 4 and 5.

5 a) b) c) d) e) f) Fig. 3. HV 0,2 micro-hardness examinations: a) sample 1 entrance; b) sample 2 entrance; c) sample 3 entrance; d) sample 1 exit; e) sample 2 exit; f) sample3 exit; From the micro-hardness graphs differences can be observed between the measured values in the thermomechanically affected zone in the entrance area of the water jet in the sample with maximum values obtained on sample no. 3 which had the highest value for the work agent, respectively the water pressure and the lowest hardness values were measured on sample no. 1 at which the lowest water pressure was used at cutting the sample. Fig. 4. HV 0,2 in the entrance area of the water jet in the sample Fig 5. HV 0,2 HV 0,2 in the exit area of the water jet in the sample For the micro-hardness measured values in the lower part of the smaples differences and inversions were observed in comparison to those measured in the upper part. Thus, the maximum microhardness values in the thermomechanically affected area in the samples lower area were detected in sample no 1. It can be seen that there are influences on the layer adjacent to the cut area using this process with impact on its microhardness. The microscopic aspects of cut surfaces, in the upper and lower part of the samples are presented in figures 6,7 and 8 where SEM examinations can be observed for the three samples.

6 a) b) Fig. 6. SEM examination Sample no. 1 - a) entrance; b) exit a) b) Fig. 7. SEM examination Sample no. 2 - a) entrance; b) exit a) b) Fig. 8. SEM examination Sample no. 3 - a) entrance; b) exit

7 After the SEM analysis it can be seen that in all the three samples ductile material breaking is present, the cut surface morphology being significantly different in the entrance in comparison with the exit areas. It is ascertained that the appearance of cut surface in the impact areas with the technological agent has a finer roughness, material removal is uniform, while exit areas have a more grainy aspect, with a much higher roughness. 4. CONCLUSIONS Increased pressure of the technologic agent is manifested by an increase of depthness in the hardened layer and of maximum surface hardness especially in the impact area of material cut. Independent of the pressure values, the cut surface roughness in the cut starting area is superior than the roughness specific to the lower cut ending area. The investigations carried out at the SEM microscope have demonstrated that for all pressure values the material removal is uniform in the surface impact zones and uneven in the lower final exit areas of the material. ACKNOWLEDGEMENTS Support from the Romanian National Grant No 166/ , PN II ID PCE and learning agreement no. 27/11/2011 is gratefully acknowledged. LITERATURE [1] Andreas, W.M., KOVACEVIC, R., - Principle of Abrasive Water Jet Machining, Springer Verlag, Berlin, 1998; [2] M. Hashish: Material properties in abrasive-water jet machining. Trans. ASME, Journal of Engineering for Industry, Vol. 117, pp , 1995 [3] MYERS, R.H., Montgomery, D.C., - Response Surface Methodology, John Wiley&Sons, Inc. New York, [4] Kovacevic R. Monitoring the depth of abrasive water jet penetration. International Journal of Machine Tools & Manufacture, vol 32(5), pp , [5] F.L Chen, E. Siores: The effect of cutting jet variation on striation formation in abrasive waterjet cutting, IRIS, Swinburne University of Technology, Melbourne, Australia, 2001 [6] Hlaváček, P. et al. Measurement of fine grains copper surface texture created by abrasive water jet cutting. Strojarstvo, 51, 4(2009), pp [7] I.A. PERIANU, A-V. BÎRDEANU, I. MITELEA, - Optimizing abrasive water jet cutting for special materials. Case study: cutting austenitic stainless steels, 21nd International Conference on Metallurgy and Materials, 2012, pp