Cavitation eroded zones analysis for G-X5 CrNi 13.4 stainless steel.

Abstract: The present research is concerned with the erosion mechanisms of the stainless steel G-X5 CrNi 13-4, used in manufacturing Kaplan turbine blades. The analysis was performed on specimens subjected to cavitations attack, 30 respectively 120 minutes, in the Timisoara Hydraulic Machinery Laboratory, using a magnetostrictive facility with nickel tube. Keywords: erosion, cavitations, stainless steel, electronic microscopy.

1. INTRODUCTION

For scientists and producers of hydraulic equipments (as well as maritime or fluvial ships), the cavitation erosion phenomena remain a permanent concern, because it creates unexpected geometry modifications of the elements in contact with the liquid, with serious implications upon the performance and the reliability of the equipment [Bordea[degrees]u (1997), Frank & Michel (1995)].

The paper is concerned with investigations upon cavitations erosion of the martensitic stainless steel G-X5CrNi13.4 used recently for casting the blades of numerous Kaplan turbines. The erosions were obtained in the magnetostrictive vibratory cavitations facility of the Timisoara Hydraulic Machinery Laboratory.

2. TESTED MATERIAL

The tested material is the martensitic stainless steel G-X5CrNi13.4 used in manufacturing the blades for the Iron Gates turbines [Grant CNCSIS 154 (2005)]. There were investigated two kinds of specimens, both manufactured from samples extracted directly from the blades, namely one from a zone near the blade disk and the other from a zone near the blade periphery. From the same samples there were manufactured also specimens for tensile tests. The chemical composition was determined in the Chemical Analyses Laboratory of the factory U.C.M. RESITA and the mechanical characteristics in Material Strength Laboratory of the Timisoara "Politehnica" University. The results are tabulated in Table 1 and 2.

3. TEST FACILITIES AND TESTING METHOD

The cavitation erosion tests were carried out in a magnetostrictive facility (Figure 1), in Timisoara Hydraulic Machinery Laboratory. The testing procedure is established by the ASTM standards (1985), and as working liquid it was used double distillated water at a temperature of 20 [+ or -] 1 0C. The main characteristics of the device are:

* the immersion depth of the specimen: 3-5 mm;

* the oscillation amplitude: 47 im;

* the oscillation frequency: 7000 [+ or -] 3% Hz;

* the working fluid: drinking water at 20[+ or -]1[degrees]C

The total duration of the cavitation attack was 120 minutes, divided in 9 periods: one of 5, one of 10 and seven of 15 minutes. After each testing period the specimens were washed successively in drinking and distilled water, alcohol and acetone. The examination of the eroded surface and of the microstructure of the influenced layer was made with a scanning electron microscope.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In order to see the profile of the eroded area, the specimens (figure 2) were sectioned along the diameter and the cross sections were polished. The photographs were taken with an optic microscope. As we can see in Figure 3 the general view of the eroded area is the same. Although, the mean depth for both specimens is around 0.04 mm, the eroded relief is dissimilar. On the specimen P1 the relief is characterized by great irregularities, which suggest the simultaneous expelling of grain groups.

[FIGURE 3 OMITTED]

With the view to analyze the microrelief of the cavitation eroded areas, the replicas after 30 and 120 minutes of attack time were examined by the electron transmission microscope Tesla BS 613 (Figure 4). From Figure 4 it becomes evident that after the erosion process, the surface presents both material slip steps and fractures of grain clusters.

[FIGURE 4 OMITTED]

4. CONCLUSIONS

In the images of the longitudinal cross section of the specimen there has not been observed microcracks directed from the attacked surface towards the middle of the specimen. The microcracks absence can be explained by an insufficient increase of stresses after the implosions in correlation with the material strength. The stress relaxation can be attributed to the heating up of the superficial layer of the eroded specimen both as a result of bubble implosions and ultrasound generation process. The electronic microscope examinations of the eroded surface indicate that microscopic parts, of the affected area, are subjected to strong sliding process. This relatively soft layer, of about 0.04 mm, is continuously subjected to sliding, as a result of the cavitation implosions and only after that, same particles are expelled. In this way the biggest part of the implosion energy is consumed in the sliding process, the remaining contribute to the heating of the superficial layer.

5. REFERENCES

Bordeasu, I., (1997). Eroziunea cavitationala asupra materialelor utilizate in constructia ma[degrees]inilor hidraulice si elicelor navale. Efecte de scara, Teza de doctorat, Timisoara (Cavitational erosion upon materials used in manufacturing hydraulic machinery and ship propellers) Doctoral degree thesis

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

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

*** (2005). Grant CNCSIS 154, Proiect TIP A, Tema 4, Contract nr. 32940/22.06.2004, Studiul deformarii si fisurarii produse prin eroziune cavitationala la otelurile inoxidabile austenitice folosite la turnarea paletelor de turbina hidraulica (Studies of deformations and cracks produced by cavitation erosion to austenitic stainless steels used in manufacturing hydraulic turbines blades)

Table 1 Chemical composition

                                             Blade near
            Customer's                        periphery    Blade near
Elements    specifications                         (Pl)     disc (P2)

C           [less than or equal to] 0.05%         0.05%         0.05%
Mn          [less than or equal to] 1.5%          0.22%         0.22%
P           [less than or equal to] 0.03%         0.01%         0.01%
S           [less than or equal to] 0.025%        0.02%         0.02%
Si          [less than or equal to] 1.0%          0.44%         0.44%
Ni          3.5-5.0%                              3.45%         3.50%
Cr          12.0-13.5%                              13%           13%
Mo          [less than or equal to] 0.7%          0.35%         0.38%
N           --                                    0.02%         0.02%
Fe          Rest                                   Rest          Rest

Table 2 Mechanical characteristics

Mechanical          TS                                      Vickers
characteristics    <MPa>             Elongation            Hardness

Customer's        760-960   [greater than or equal to]15      --
specifications

Blade near          730                 14.8               371HV5/30
periphery (PI)

Blade near          740                 14.9               392HV5/30
disc (P2)