The damage of austenite--ferrite stainless steels by cavitation erosion.

Bordeasu, Ilare ; Ghiban, Brandusa ; Popoviciu, Mircea Octavian 等

1. INTRODUCTION

Martensitic stainless steels are the most used materials for manufacturing hydraulic turbine components, such as runners and blades (Bordeasu 2006). But, different pieces made of stainless steels, with two phases austenite and ferrite, is compulsory analyzed in order to establish the cavitation erosion behavior. The present paper aim is to study some aspects of austenitic-ferritic microstructure, damaged by intense cavitation attack produced after testing them in the vibrator magnetostrictive device of Timisoara Hydraulic Machinery Laboratory.

2. TESTED MATERIAL

The experimental steel was elaborated in Resita Siderurgic Plant, after a receipt not standardized. The local composition determined by EDAX analysis in Laboratory of Special Materials Research Center from University Politehnica Bucharest is: C = 0.1%, Si = 0.76%, Mn =3.28%, Cr = 20.23%, Ni =3.98%, Al =0.63%, Fe=rest. In according with Schaffler diagram ([E.sub.Cr] = 21.37 %, [E.sub.Ni] = 8.62 %), the steel structure is formed by austenite =50 %, ferrite =45 % and martensite [congruent to] 5 %.

The mechanical characteristics were obtained in The Material Strength Laboratory of the Timisoara "Politehnica" University, and are: ultimate strength [[sigma].sub.m] = 610 MPa, yielding strength [[sigma].sub.0,2] = 338 MPa, and Brinell hardness 185 HB.

3. TEST FACILITIES AND TESTING METHOD

The cavitation erosion tests were carried out in the magnetostrictive facility, of Timisoara Hydraulic Machinery Laboratory, in accordance with ASTM G32-85 (Bordeasu et.al., 2007), using as cavitation liquid drinking water at 20 [+ or -] 10[degrees]C.

The microstructure, analyzed by a scanning electron microscope, figure 1-3, put in evidence the following aspects of the cavitation damages after 165 minutes:

* Fractures by cavitation, with brittle grain aspect. The cause of brittle behavior is due to the high amount of manganese, which makes the austenite grain and the needles of martensite more roughness (with a more fibrous structure).

* Mixed surface with brittle aspect, by cleavages having transcrystalline and intergrain propagation. The explanation is connected with the fact that microstructure damages may begin at the grain boundary between ferrite and austenite, respectively austenite and martensite needles. By continuing the cavitation attack, the damage is propagated in the ferrite matrix till massive breakdown. Because the bonds between austenite and martensite needles became softer, even the complete expulsions of austenitic grains and martensite needles occur during the attack. Also, almost complete damages of ferrite islands were locally put in evidence by scanning electron microscopy.

* Cavitations with intergrain propagation. These cavitations are specific to ferrite mass damage, which from the three constituents of the steel structure, is the phase with the lowest resistance to cavitation erosion (Mitelea et al., 2005)

[FIGURE 1 OMITTED]

For a correct analysis of the steel behavior at cavitation erosion, in figures 4 and 5, there are presented the characteristics curves by cavitation erosion (cumulative mass and erosion rate) compared with the martensitic stainless steel OH12NDL (C = 0.1%, Si = 0.3%, Mn =0.04%, Cr = 12.75%, Ni =1.25%, P =0.085%, S =0.025%, Cu =0.9%, Fe=rest, [[sigma].sub.m] = 610 MPa, [sigma]p.sub.0,2] = 225 MPa, Brinell hardness 400 HB, martensite [congruent to] 74 %, ferrite [congruent to] 26%) generally used for hydraulic turbine blades in our country and being considered with a good resistance to cavitation erosion.

The evolution of eroded volume, respectively eroded rate versus attack time, from figures 4 and 5 show clearly that the studied austenite- ferrite steel (A-F) has a better behavior at cavitation attack than the OH12NDL steel, usually used for hydraulic turbines blades in Romania.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The evolution of eroded volumes, respectively eroded rate versus attack time, in figures 4 and 5 clearly show that the studied austenite--ferrite steel (A-F) has a better behavior at cavitation attack than OH12NDL steel, usually used for casting hydraulic turbines blades in our country.

[FIGURE 5 OMITTED]

The dispersion of the experimental points in figure 5, in comparison with the average curve, may confirm the previous affirmations concerning the manner of ferrite breakdown, with expelling of ferrite, austenite grains and martensite needles.

4. CONCLUSIONS

The stainless steel having austenite and ferrite as predominant constituents is appreciated as a material whose structure may resist very well to cavitation erosion. The comparison with the usually steel OH12NDL, based on erosion characteristics, recommend both of them as materials, which can be used to cast different hydraulic devices (turbine and pumps) and also as a repair material for cavitation damaged zones.

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

Bordeasu I. (2006). Eroziunea cavitationala a materialelor (Cavitation Erosion of Materials)..Editura Politehnica Timisoara, ISBN: (10)973-625-278-7; (13)978-973-625278-5

Bordeasu, I., Popoviciu, M.O., Mitelea, I., Anton, L.E., Bayer, M., Funar, S.P. (2007). Cavitation Eroded Zones Analysis For G-X 5CrNi13.4 Stainless Steel Proceedings of The 18th International DAAAM Symposium "Intelegant Manufacturing & Automation: Focus on Creativity, Responsibility and Ethics of Engineers", Zadar Croatia, 24-27.10.2007, pp.105-106, ISSN 17269679, ISBN 3-901509-58-5.

Mitelea, I., Bordeasu, I., Hadar, A. (2005). The Effect of Nickel Content Upon Cavitation Erosion for Stainless Steels with 13% Chromium and less than 0,1% Carbon, Revista de Chimie, Chemical Abstracts RCBUAU 56(11), pp.1169-1174, ISSN: 0034-7752.

*** (1985). Standard method of vibratory cavitation erosion test, ASTM, Standard G32-85.