contributions regarding cavitation … · the classification used the four classes (super...

machine design, Vol.5(2013) No.4, ISSN 1821-1259 pp. 157-162

*Correspondence Author’s Address: “ Polytechnica“ University of Timisoara, Faculty of Mechanical, Bvd. Mihai Viteazul no.1, 300222 Timisoara, Romania, [email protected]

Original scientific paper

CONTRIBUTIONS REGARDING CAVITATION EROSION HIERARCHY FOR STAINLESS STEELS Ilare BORDEASU1, * - Mircea Octavian POPOVICIU2

1 “Polytechnic“ University of Timisoara, Timisoara, Romania 2 Academy of Romanian Scientists, Timisoara, Romania

Received (10.09.2013); Revised (23.10.2013); Accepted (27.10.2013) Abstract: Paper presents a method for classifying the stainless steels from the point of view of cavitation erosion resistance. From this purpose was used the T2 laboratory vibratory facility, realized in the Cavitation Laboratory of Timisoara Polytechnic University (UPT), based on piezoelectric crystals. This laboratory device respects all the recommendations of the ASTM G32-10 ASME Standard. The classification used the four classes (super resistant, excellent, very good, and good resistance), established previously for the same steels tested in the old T1 facility with nickel tube. As defining parameter of the cavitation erosion resistance was chosen the value 1/MDER (the inverse ratio of Mean Depth Erosion Rate). This value is correlated with the principal mechanical properties (ultimate resistance, yield limit and Brinell hardness) and the rate between the chromium and nickel equivalents (Cre/Nie). There have been used laboratory results for 17 stainless steels having various structural constituents. There are presented diagrams in which the four mentioned resistance categories are confined through the curves 1/MDER (), 1/MDER (1), 1/MDER () and 1/MDER (1) were is a parameter grouping the principal mechanical properties and contains supplementary the influence of the structural constituents; the same parameters having the index 1 are dimensionless ones, obtained by splitting with the values of the standard steel OH12NDL used in the past, on a large scale, for manufacturing blades and runners for hydraulic turbines. The curves are constructed with new relations obtained using the method proposed earlier by Sakai/Shima (1987) and Bordeasu (2006). We think that this method is consistent not only for the tested steels but also for anticipating the behavior of other steels subjected to cavitation. Key words: stainless steels, cavitation erosion resistance, mechanical properties, chemical composition, vibratory device 1. INTRODUCTION Previous researches [8], [9], [10], [11], [12], [16], [18] regarding an eventual indestructible material to cavitation conducted only to increase the resistance and the running duration for different cavitation conditions. So, it appears a supplementary condition, requiring the possibility of repair works after a given time of service. In present there are a lot of tables, diagrams and relations permitting the anticipation of the cavitation erosion behavior for various materials, especially stain less steels, by considering their mechanical properties, chemical composition or even metallographic structure. Most laboratory tests use vibratory devices because their intense cavitation erosion. Some researches put into correlation the parameter 1/MDER (the reciprocal of mean depth erosion rate which expresses the resistance to cavitation erosion) with the important mechanical properties (Brinell hardness, tensile strength, yield limit, etc.) [2], [3], [5], [8], [14], [17]. Upon our opinion, the microstructure plays also an important role so, in the present work it was introduced also the chemical composition through the equivalent contents of chromium and nickel which in conformity with the Schäffler diagram [2], [6] determines the degree of the structural constitutive (austenite, martensite and ferrite).

2. TESTED MATERIALS, RESEARCH FACILITIES AND METHODS

The laboratory tested materials are stainless steels used for manufacturing details of hydraulic machineries, especially turbines runners and blades as well as pumps impellers. The material samples were heat treated to obtain the principal characteristics equal to those of actual machineries [13], [15]. The microstructures were established from the Schäffler diagram using the equivalent quantities of chromium and nickel [6]. The mechanical characteristics were measured in the testing departments of the University (UPT). Table 1 presents the actual chemical content, the equivalent content of chromium (Cre) and nickel (Nie) as well as the structure while Table 2 gives the principal mechanical characteristics for the 17 tested steels. The name of the steels was established with the help of:

the actual content of nickel,

the actual content of chromium,

the actual content of carbon,

To obtain dimensionless figures, OH12NDL stainless steel was chosen as standard. This material was used on a large scale to realize hydraulic machinery in Romania, URSS and other countries [1], [2].

Ilare Bordeasu, Mircea Octavian Popoviciu: Contributions Regarding Cavitation Erosion Hierarchy for Stainless Steels; Machine Design, Vol.5(2013) No.4, ISSN 1821-1259; pp. 157-162

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Table 1. Principal Chromium and nickel content, equivalent content of chromium and nickel and the structure of tested steels [13], [15]

No. Steel Rough carbon content [%]

Cr [%] Ni [%] Cre [%] Nie [%] Structure

1 Ni05Cr12C01 0.1

12.08 0.5 14.26 4.81 M+F 2 Ni2Cr12C01 12.02 2.15 14.62 6.23 M+F 3 Ni6Cr12C01 12.07 9.97 14.9 10.14 M+A 4 Ni10Cr12C01 12.02 10.09 14.66 14.74 A 5 Ni2Cr12C0036

0.036 11.957 10.105 13.27 3.15 M+F

6 Ni4Cr12C0036 11.84 10.105 13.1 5.25 M+F 7 Ni6Cr12C0036 12.059 10.105 13.16 6.69 M 8 Ni8Cr12C0036 12.206 10.105 13.54 9.16 M+A

9 Ni10Cr6C01

0.1

6.48 0.5 11,924 15,173 M+F

10 Ni10Cr10C01 10.62 2.15 14,919 14,854 A

11 Ni10Cr18C01 17.91 5.95 22,414 14,138 A+F

12 Ni10Cr24C01 23.86 10.28 30,362 15,101 A+F

13 Ni10Cr13C0036

0.036

12.705 1.97 13,209 11,454 M+F

14 Ni10Cr14C0036 14.208 4.009 15,022 11,4935 M+F

15 Ni10Cr16C0036 16.515 5.597 17,824 11,515 A

16 Ni10Cr18C0036 18.275 7.847 19,610 11,508 A+F

17 OH12NDL 0.1 12.8 1.25 13.2 4.45 M+F

The T2 vibratory device was used for laboratory tests. This facility use piezoelectric crystals and was realized in conformity with ASTM G32-10 Standard [19]. The principal characteristics are: oscillation double amplitude 50 µm, vibratory frequency 20 kHz, power of generator 500 W, specimen diameter 15.8 mm. The testing liquid was water at 21 ±10C. All the values were maintained constant during the cavitation erosion tests which was 2.75 hours. The procedure recommended by ASTM Standard was respected both for tests and results presentations. Table 2 Principal mechanical properties of tested steels

No. Steel Rm

[MPa] Rp0.2

[MPa] HB

1 Ni05Cr12C01 1450 1020 411

2 Ni2Cr12C01 1336 935.2 369

3 Ni6Cr12C01 1540 1083 434

4 Ni10Cr12C01 835 626 254

5 Ni2Cr12C0036 968 678 280

6 Ni4Cr12C0036 989 695 287

7 Ni6Cr12C0036 1035 725 297

8 Ni8Cr12C0036 1002 701 288

9 Ni10Cr6C01 1550 1120 489

10 Ni10Cr10C01 1450 1020 447

11 Ni10Cr18C01 1335 934 372

12 Ni10Cr24C01 1280 901 302

13 Ni10Cr13C0036 856 618 276

14 Ni10Cr14C0036 341 240 346

15 Ni10Cr16C0036 996 700 309

16 Ni10Cr18C0036 527 369 375

17 OH12NDL 650 400 225

3. CORRELATION BETWEEN CAVITATION EROSION RESISTANCE AND MECHANICAL PROPERTIES

Using a few stainless steels and various liquids, Garcia [9] and Hammitt [11] correlated 1//MDER, the cavitation erosion resistance, with principals mechanical properties (hardness, resilience, ultimate strength, etc.). For the first time was reached the conclusion that it exist the possibility to approximately correlate the cavitation erosion resistance with the material characteristics. Sakai and Shima [17] had the idea to group the principal mechanical characteristics into a single indicator and correlating it with 1/MDER. Bordeasu [2] using the Sakai-Shima method extended the correlation to more materials (non allied steels, ship propeller bronzes and brasses). Bordeasu tested those materials into T1 laboratory facility with nickel tube (double oscillation amplitude 94 μm, vibration frequency 7 kHz, power of the ultrasonic generator 500 W and specimen diameter 14 mm). In the present work, taking into account the mentioned researches, there were established correlations (see Fig. 1 and 2) for a lot of stainless steels between the parameter 1/MDER AND . Taking into account the ASTM recommendations we used also the dimensionless parameter 1. For this purpose was used the stainless steel OH12NDL employed on a large scale, in Romania, URSS and other countries, for manufacturing the hydraulic turbines details exposed to cavitation. During long running time this steel presented good/very good resistance to cavitation erosion, showing also very good repair properties [1], [2]. Fig. 1 and 2 presents five types of curves: those symbolized with I, II and III show the areas of different cavitation erosion resistance, the curve IV establishes the


159

reasonable limits of materials (both for cavitation erosion and material strength) and the curve V expresses the variation tendency of the cavitation erosion against the values of the material parameters. The four hierarchy classes areas (super, excellent, very god and god

resistance) are separated by three curves I, II and III which were obtained with new relations, developed using the models established previously by Sakay-Shima [17] and Bordeasu [2].

Fig.1. Hierarchy upon cavitation erosion resistance and mechanical properties

Fig.2. Hierarchy upon cavitation erosion resistance and mechanical properties, with dimensionless figures obtained by

using the properties of the standard steel OH12NDL The equations of the Fig. 1 curves are:

- Curve I 1/MDER = 900 (1-e-0,031) + 9000,031 e-0,031 (1)

- Curve II 1/MDER = 325(1-e-0,025)+3250,025 e-0,025 (2)

- Curve III 1/MDER = 2300,02 (1-e-0,02)+2300,02 e-0,02 (3)

The equations of the Fig. 2 curves are:

- Curve I 1/MDER = 900 (1-e-1.2

1)+900 1,2 1 e-1,21 (4)

- Curve II 1/MDER = 325 (1-e-0,9

1)+3250,9 1 e-0,91

(5)

- Curve III 1/MDER = 260 (1-e-0,3

1)+2600,3 1 e-0,31

(6)

The difference of these relations and those given previously by Sakai-Shima [17] and Bordeasu [2] is the exponential term. Using these results, in Table 3 is given the hierarchy of the tested steels.


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Table 3. Classification after the Rns parameter (by using the standard steel OH12NDL)

Cavitation erosion resistance

Steel

Super resistant Ni6Cr12C01

Excellent Ni10Cr6C01

Very good

Ni10Cr12C01 Ni10Cr10C01 Ni10Cr18C01 Ni10Cr24C01 Ni10Cr13C0036 Ni10Cr14C0036 Ni2Cr12C01 Ni6Cr12C0036 Ni8Cr12C0036

Good

OH12NDL

Ni10Cr16C036T1 Ni10Cr18C0036 Ni05Cr12C01

Ni2Cr12C0036 Ni4Cr12C0036

The curve V shows that through the increase of the mechanical properties the cavitation erosion resistance became greater. So, the researches concentrated upon various treatment methods (thermal, thermo chemical, mechanical, non conventional, etc.) are justified because in this way it can be obtained a prolonged life time between two repair works. From table 3 it can be observed that two of the tested steels (Ni6Cr12C01 and Ni10Cr6C01) have outstanding cavitation erosion resistance. Both of them have martensite as the principal structure constituent, great values of hardness and ultimate resistance. 4. CORRELATION OF CAVITATION

EROSION RESISTANCE WITH BOTH MECHANICAL PROPERTIES AND Cre/Nie RATE

Besides the mechanical properties the cavitation erosion resistance is influenced also by the structure of the steels. In order to detect this influence, in the relations and 1 was supplementary introduced the rate Cre/Nie. In this way it was obtained the more complex parameters and 1 and their variations is presented in Fig. 3 and 4.

Fig.3. Hierarchy upon cavitation erosion resistance, using both mechanical properties and chemical constitution

Fig.4. Hierarchy upon cavitation erosion resistance, mechanical properties and chemical constitution, with

dimensionless figures obtained by using the properties of the standard steel OH12NDL


161

The equations of the Fig. 3 curves are: - Curve I

1/MDER = 1150 (1-e-0,012)+11500,012 e-0,012 (7)

- Curve II 1/MDER = 360(1-e-0,014)+3600,014 e-0,014 (8)

- Curve III 1/MDER = 210 (1-e-0,008)+2100,008 e-0,008 (9)

The equations of the Fig. 4 curves are: - Curve I

1/MDER = 1200 (1-e-0.41)+12000.4 1 e-0.41 (10)

- Curve II

1/MDER = 360 (1-e-0.51)+3600.5 1 e-0.51 (11)

- Curve III

1/MDER = 240 (1-e-0,21)+2400,2 1 e-0,21 (12)

It can be observed that these relations (7-12) are similar with the previous ones (1-6) but the introduction of the rate Cre/Nie modifies the values of the scale and shape parameters. This aspect demonstrates the complexity of the dependence between cavitation erosion and various parameters of the steels. Differing from Fig. 1 and 2 the curve V in Fig. 3 and 4 show that by increasing , or 1 values, the cavitation erosion resistance decreases. The explanation is the fact that the increase of the rate Cre/Nie increases the ferrite content (constituent with reduced erosion resistance [2]). Also from these diagrams results the same conclusion obtained using the parameters , and 1 namely there exists combinations of structures and mechanical properties which allow to include a given material in two classes good or very good. 5. CONCLUSIONS 1) An efficient method for classifying the stainless steels

was obtained by using the complex parameters Φ and .

2) The use of dimensionless parameters Φ1 and 1 is recommendable for the future researches. As standard parameter must be chosen a material, used in the past for manufacturing hydraulic turbines and well known by the research community. We use the OH12NDL stainless steel employed on a large scale for hydraulic machinery details.

3) The cavitation erosion resistance, for a given material, can be increased using modern material treatments (thermal, thermo chemical, mechanical, non conventional, etc.).

4) Increasing the parameter exclusively through the rate Cre/Nie can reduce the cavitation erosion resistance because ferrite the weak structural component is increased.

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Academiei RSR, Romania [2] Bordeasu, I., (2006). Eroziunea cavitaţională a

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[3] Bordeasu, I., Anton, M.I., (1998). Correlation Between Cavitation Rate with Both Parameters of the 6. Vibratory Apparatus and the Phisico-mechanical Properties of the Material, Third International Symposium on Cavitation, pp. 199-202, Grenoble, 7-10 April, France

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[5] Bordeasu, I., Popoviciu, M.O., (2013). Cavitation erosion resistance for a set of stainless steels having 10 % nickel and variable chromium concentrations, Hidraulica, Magazine of Hydraulics, Pneumatics, Tribology, Ecology, Sensorics, Mechatronics, no.1, pp.79-85, ISSN 1453-7303

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[14] Jurchela, A.D., Bordeașu, I., Mitelea, I., Karabenciov, A., (2012). Considerations on the Effects of Carbon Content on the Cavitation Erosion Resistance of Stainless Steels with Controled Content of Chromium and Carbon. METAL 2012, 21st International Conference on Metallurgy and Materials, May 23-25, pp.718-723, Brno, Czech Republic, ISBN 978-80-87294-31-4

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