Structural features of cavitation damages in some stainless steels.

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

1. INTRODUCTION

The new technological solutions regarding the life time increasing of some requested machine elements (pumps, navy and hydraulic turbine (air) screw) have imposed the materials analyzing towards phenomena induced by cavitation attack. In this way becomes very important the establishment of all factors influencing the cavitation erosion like chemical composition, macro and microstructural constitution, elaboration technology, heat and mechanical treatments, etc (Franc & Michel, 1995).

2. MATERIALS AND EXPERIMENTAL PROCEDURE

Four types of austenitic stainless steel were analysed--manufactured at Siderurgic Plant, Resita. These types of stainless steel alloys are classified by Cr and Ni equivalent. Chemical composition, estimated at CEMS laboratory--Bucharest and Cr & Ni equivalent quantities necessary to structure determination, based upon Schaffler diagram are: Steel 10/06: 0.07% C; 10.51% Cr; 6.81% Ni; 1.30% Mn; 0.1% Mo; 1.55% Si, rest Fe - (Cr)e = 10,266%, (Ni)e = 15,173%; Steel 10/10: 0.1% C; 10.32% Cr; 10.22% Ni; 1.96% Mn; 0.1% Mo; 1.56% Si, rest Fe - 14,486%, (Ni)e = 14,854%; Steel 10/18: 0.07% C; 10.07% Cr; 17.88% Ni; 1.08% Mn; 0.1% Mo; 1.27% Si, rest Fe - (Cr)e = 21,448%, (Ni)e = 14,138%; steel 10/24: .07% C; 10.51% Cr; 24.81% Ni; 1.30% Mn; 0.1% Mo; 1.55% Si, rest Fe- (Cr)e = 29,145%, (Ni)e = 15,101%. Cavitation destruction and surface microscopically study there were performed in magnetostrictive vibrating apparatus at Cavitation Laboratory (Polytechnic University of Timisoara). Stereomicroscopy and SEM analysis were performed after 165 minutes of cavitation erosion at University Politehnica Bucharest at Center of Expertise of Special Materials (UPB-CEMS).

3. RESULTS AND INTERPRETATIONS

Optic metallography of the experimental steels are given in figure 1.

[FIGURE 1 OMITTED]

As one may remark from figure 1a, steel 10/6 has a structure which consists of 98% martensite and 2% austenite. SEM, fig. 2a, reveals at this steel the following aspects:

* surface with cavitation and big dimples bigger than 200[micro]m with cleavage propagations;

* fracture has a brittle aspect with ingtergranular and cleavage propagation;

* many secondary intergranular cracks, cleavage planes and a propagation of fracture along the slip lines.

As one may remark from figure 1b, steels 10/10 has a structure which consists of fully and austenite. SEM, fig. 2b, reveals for this steel the following aspects:

* surface with cavitation and big dimples bigger than 200[micro]m with cleavage propagations;

* fracture has a brittle aspect with ingtergranular and cleavage propagation;

* many secondary intergranular cracks, cleavage planes and a propagation of fracture along the slip lines.

As one may remark from figure 1c, steels 10/18 has a structure which consists of 98% austenite and 2% ferite. SEM, fig. 2c, reveals for this steel the following aspects:

* brittle aspect of fracture for cavitation corrosion;

* mixt aspect of propagation front through intergranular cracks and cleavage planes;

* corrosion aspect by cavitation with intergranular propagation.

As one may remark from figure 1d, steels 10/24 has a structure which consists of 81% austenite and 19% ferite. SEM, fig. 2d, reveals at this steel the following aspects:

* cavitations with big dimples bigger than 200[micro]m along intergranular corrosion may be developed;

* both trancrystalline and intercrystaUine propagation of the fracture front;

* surface with many twin boundaries, intergranular cracks and numerous corrosion pits on twin boundaries.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

In comparison with our previous papers (Bordeasu et al., 2007), (Bordeasu et al., 2008), (Bordeasu et al., 2009) and (Ghiban et al., 2009), where the experiments were made on austenitic stainless steels, in present paper the structure of stainless steels varies from a mixture of martensite and austenite, fully austenite to mixture of austenite and ferrite. Considering the structural damages, which differ from a steel to another, we may appreciate from the point of view of cavern that the cavitation phenomena is deeply influenced not only from chromium of nickel contents but from their equivalents, after Schaeffler diagram. So, at steel 10/24, with the highest equivalent in chromium, (about 29), the highest values of damage of cavitation is observed (about 330[micro]m). At steel s 10/18, respectively 10/10, with smaller equivalents in chromium (about 21, and 14), the damage is intermediate, respectively 228[micro]m and 136[micro]m. The smallest values of the cavitation damage is about 97[micro]m, at steel 10/6, with the smallest value of equivalent in chromium, about 10.

4. CONCLUSIONS

The behavior of cavitation erosion depends on the chemical constitution of the studied austenitic stainless steel. There were studeid four types of austenitic stainless steels with the following ratios of [E.sub.Cr]/[E.sub.Ni]: 10/6; 10/10; 10/18 and 10/24. Depending on the concentration in alloying elements (either alpha-type or gamma type), the destroying of the material could be fragile and with bigger dimples, or mixed fragile/ ductile. The highest values of the equivalent in chromium is for a stainless steel, the deeper penetration of the cavitation damage may be.

5. 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).

6. REFERENCES

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