Some aspects of cavitation damages in austenitic stainless steels.

Ghiban, Brandusa ; Bordeasu, Ilare ; Ghiban, Nicolae 等

1. INTRODUCTION

The life time increasing of some specific machine elements (like pumps, navy and hydraulic turbine screw) is deeply influenced by the cavitation attack phenomena. So, a high resistance material must be used in very aggressive sea water media. Present paper presents a structural investigation of some austenitic stainless steels exposed to cavitation attack. A correlation between chemical constitution--elaborating technology and mechanical behavior is made, in comparison with other researchers (Frank & Michel, 1995), (Szkodo & Giren, 2004).

2. RESEARCHED MATERIALS, DEVICES AND METHODS

In this paper three types of austenitic stainless steels are analyzed--manufactured at Siderurgic Plant, Resita. These types of stainless steel alloys are classified by Cr and Ni equivalent. Chemical composition, determinated at CEMS laboratory--Bucharest, including Cr & Ni equivalents for structure determination, after Schaffler diagram are: Steel 20/26: 0.07% C; 20.51% Cr; 26.81% Ni; 1.30% Mn; 3.37% Mo; 1.53% Si, rest Fe - [(Cr).sub.e] = 26,175% [(Ni).sub.e] = 30,16%; Steel 18/13: 0.1% C; 18.32% Cr; 13.22% Ni; 1.96% Mn; 2.84% Mo; 1.56% Si, rest Fe - [(Cr).sub.e] = 23,5 % [(Ni).sub.e] = 17,22%; Steel 22/27: 0.07% C; 22.07% Cr; 27.28% Ni; 1.08% Mn; 3.02% Mo; 1.27% Si, rest Fe - [(Cr).sub.e] = 26,995% [(Ni).sub.e] = 29,92%.

Cavitation destruction and surface microscopically study were performed in magnetostrictive vibrating apparatus at Cavitation Laboratory (Polytechnic University of Timisoara). 165 minutes cavitation exposed samples were analyzed by scanning electron microscopy at a Philips Microscope at Polytechnic University of Bucharest. Figures 1-3 show images of eroded microstructure of the experimental steels at different magnifications of the cross-section. Figure 4 indicates the depths of the erosion cavity in the investigated samples. The highest value of depth is present in case of sample 2, with about 400 [micro]m, in comparison with the other two, where the depth is about 80 [micro]m. Figure 5 shows the typical curve of the cavitation erosion (erosion rate vs cavitation attack time).

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3. DISCUSIONS

Even if the stainless steel alloys have the same structure, the figures 1-3, a-d show different evolutions of structural damages, from the point of view of caverns--on one hand, and deformations--on the other hand, previous grain sputtering (or fragments of grains). We consider, as proposed by other researchers (Bojin et al., 2005), (Bregliozzi et al., 2005), (Lebrun & Poirier, 2002), that these phenomena are due not only to Cr--Ni couple, but to other base elements, especially Mn and Si, which produce loosening and growth of grain dimensions. The size of sputtered grains during the cavitation attack is very well reflected in the distribution of the experimental points, face to approximation curves. SEM analysis evidenced the following aspects of the steel erosion: 20/26 alloy (figure 1); the eroded surface presents a mixed aspect of smooth cavitation and relatively big cavitation (30-80 [micro]m diameters); propagation of breaking front by intergranular cracks, the breaking propagation by intergranular cracks sliding paths; 18/13 alloy (figure 2); equal proportions of fine and big cavitations (30-140 [micro]m diameters); the surface with deep secondary intergranular cracks; fragile character breaking with intergranular propagation and sliding paths; 22/27 alloy (figure 3); fragile breaking aspect; mixed propagation aspect of the front by intergranular cracks and cleavage planes; corrosion aspects with intergranular propagation. From the curves evolution (figure 5) results a similar behavior of 22/27 and 20/26 steels, which quantities of [(Ni).sub.e] and [(Cr).sub.e] are very appropriate. This aspect leads to the idea that the austenitic structure, according to Schaffler diagram, is the same, even that there are major differences between chemical elements concentrations: Mn, Si & Mo.

4. CONCLUSIONS

The behavior of cavitation erosion depends on the chemical constitution of the studied austenitic stainless steel in the same mechanical testing conditions. Scanning electron analysis may offer spectacular observations of cavitation erosion attack for austenitic stainless steels. Present paper gives an explanation of cavitation attack by means of fine structure and fracture front propagation trough grains and grains bounderies. For the same structure, austenite, cavitation attack is deeply influenced by chemical composition, reflected in chromium and nickel equivalents. At 20Cr/26Ni propagation of breaking front is given by intergranular cracks. At 18Cr/13Ni breaking character is due to intergranular cracks and sliding path and at 22Cr/27Ni only brittle behavior may appear, with cracks both intergranular and cleavage surfaces.

ACKNOWLEDGMENTS

The present work has been supported from the National University Research Council Grant (CNCSIS) PNII, ID 34/77/2007 (Models Development for the Evaluation of Materials Behavior to Cavitation).

5. REFERENCES

Bojin, D.; Miculescu, I. & Miculescu, M. (2005). Microscopie electronica de baleiaj si aplicatii (Scanning electron microscopy and applications), Ed. AGIR, ISBN 973-720-019-5;

Bregliozzi, G.; Di Schino, A.; Ahmed, S.I.U.; Kenny, J.M. & Haefke, H. (2005). Cavitation wear behaviour of austenitic stainless steels with different grain sizes. Wear, vol. 258, No. 1-4 (704 p.), 2005, pp. 503-510, ISSN 0043-1648;

Frank, J. P. & Michel, J. M. (1995). La cavitation. Mecanismes physiques et aspects industriels (Cavitation. Physical mechanisms and industrial aspects), Presse Universitaires de Grenoble;

Lebrun, J.P. & Poirier, L. (2002). Solutions to improve the surface hardness of stainless steels without loss of corrosion resistance, ATTS Congress, Paris, June 2002;

Szkodo, M. & Giren, B.G. (2004). Cavitation erosion of steels processed by C[O.sub.2] laser beams of various parameters. Journal of Materials Processing Technology, Vol. 157-158, december 2004, pp. 446-450.