Influence of welded layers thickness on the cavitation erosion resistance

DOINA FRUNZAVERDE, CONSTANTIN VIOREL CAMPIAN, VASILE COJOCARU,

GABRIELA MARGINEAN, MARIAN BARAN, RELU CIUBOTARIU

Faculty of Engineering

“Eftimie Murgu” University of Resita

No. 1-4, P-ta Traian Vuia, 320085, Resita

ROMANIA

d.frunzaverde@uem.ro, v.campian@uem.ro, v.cojocaru@uem.ro,

gabriela.marginean@fh-gelsenkirchen.de, mrnbaran@yahoo.com,

r.ciubotariu@uem.ro, http://www.uem.ro

Abstract: - Hydraulic turbine components are exposed to cavitation. This phenomenon consists in formation

and collapse of vapor bubbles in a fluid due to decreasing of pressure under the equilibrium vapor pressure of

water. The repair work of the cavitation affected areas is done by welding. Through the high heat input, this

procedure generates important residual stresses and structural modifications of the base material.

The thickness of the layers deposited by repair welding depends on the depths of the eroded cavities. The goal

of this study was to point out the influence of the weld thickness on the cavitation resistance. The comparative

investigations, regarding also microstructure and hardness of the welded materials, were carried out on

samples with 7 mm, 10 mm and 15 mm austenitic layers, applied onto martensitic stainless steel substrates.

Key-Words: -cavitation, erosion, Kaplan hydraulic turbine, welding repair, austenitic stainless steel,

martensitic stainless steel, welded layers thickness

1 Introduction The term cavitation refers to the formation and

collapse of vapor bubbles cavities in a fluid due to

localized decreasing of pressure under the

equilibrium vapor pressure of water [1]. This

phenomenon affects the runner blades of hydraulic

turbines and causes material loss [2,3].

Up to now the best results regarding the repair

techniques applied to the cavitation affected areas of

runner blades were obtained by overlay welding of

cold hardening austenitic steels [4,5,6].

The thickness of the layers deposited by repair

welding depends on the depths of the eroded

cavities. When extensive welding is applied, high

residual stresses and structural modifications of the

base and filler material may appear [7,8].

The goal of this study was to point out the

influence of the thickness of welded austenitic layers

on the cavitation resistance. The research [9,10,11]

was carried out on three samples (Table 1), realized

by simulating an in-situ repair procedure.

The welding of the filler material was made

according to the ISO 4063 standard. Liquid

penetrant inspection, as described in ISO 4987, was

made on the base material (before welding) and on

the welded layers. The samples did not show any

cracks before or after welding.

All the welded samples were cut in four test

pieces for experimental investigations (light

microscopy, hardness measurements, bending tests

and cavitation tests).

Table 1. Samples

Sample Base

material

Filler

material

Weld

thickness

1 Martensitic

stainless

steel type

1.4313

(0.03% C,

12.64% Cr,

3.63% Ni)

Austenitic

stainless

(0.24% C,

16,24% Cr,

12,37% Ni)

7 mm

2 10 mm

3 15 mm

2 The research program The experimental program was as follows:

a) Tests for welding certification in

accordance with EN ISO 15614-7:

- Bending test;

- Hardness measurements;

- Metallographic

investigations.

b) Chemical analysis;

c) Determination of the cavitation

erosion rate.

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ISSN: 1792-5924 / ISSN: 1792-5940 316 ISBN: 978-960-474-237-0

3 Chemical analysis The chemical composition was determined using a

laboratory spectrometer with optical emission. With

the data obtained, the Creq and Nieq equivalents

were calculated (Table 2).

Accordingly to Figure 1, the base material is a

soft martensitic stainless steel (containing up to 10%

ferrite) and the filler material is an austenitic

stainless steel.

Fig. 1. Schaeffler diagram

4 Metallographic investigations The material used as substrate for the Samples 1, 2

and 3, as shown by the micrograph in Fig.2, was a

soft martensitic stainless steel grade 1.4313. The

content of ferrite, included in the martensitic basis,

was about 10%, as also revealed by the Schaeffler

Diagram.

The Figures 3-5 show microstructural images of

the 7mm, 10mm and respectively 15mm welds. The

micrographs marked with 3.1, 4.1 and 5.1 reveal the

structure of the surface, while the micro-graphs

marked with 3.2, 4.2 and 5.2 show the

microstructure of intermediary layers.

Fig. 2. Microstructure of the base material

3.1– surface (100x);

3.2 – intermediary layer (100x);

Fig. 3. Sample 1 (7 mm weld). Cross section.

As one can observe, especially comparing the

microstructures in the views captured at greater

magnitude (4.3 and 5.3, 200x), the enhancement of

the welded layer thickness conduces to coarse grain

sizes in the layers situated in intermediary positions.

This phenomenon is exceedingly in the case of the

15 mm thick weld, therefore unsatisfactory

mechanical properties have to be expected.

Table 2. Creq and Nieq equivalents

Material eqCr eqNi

1.4313 13,955 5,035

Filler

material

19,99 18,64

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ISSN: 1792-5924 / ISSN: 1792-5940 317 ISBN: 978-960-474-237-0

4.1 – surface (100x)

4.2 – intermediary layer (100x)

4.3 – intermediary layer (200x)

Fig. 4. Sample 2 (10 mm weld). Cross section.

5 Hardness The hardness was measured in three areas: on the

base material, on welded layers and on heat-affected

zone (HAZ).

The results didn’t show important differences

between the three samples:

- ~ 270 HB on the sample

surface;

- ~ 245 HB on the weld

layers;

- ~ 255 HB on the base

material;

After the two hours of cavitation, the hardness on

samples surface (welded layer) increased with ~120

HB.

5.1 – surface (100x)

5.2 – intermediary layer (100x)

5.3 – intermediary layer (200x)

Fig. 5. Sample 3 (15 mm weld). Cross section.

6 Bending The bending tests were made according to the EN

910-97 standard. Two test-pieces were prepared

from each type of sample (7 mm weld, 10 mm weld

and 15 mm weld). Figures 6-8 present the test-pieces

after performing the bending tests. The results are

presented in Table 3.

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6.1 – sample 1 (7 mm weld)

6.2 – sample 2 (10 mm weld)

6.3 – sample 3 (15 mm weld)

Fig. 6. Samples after the bending test

The bending test is necessary for welding

certification (EN ISO 15614-7). The sample with 15

mm weld thickness didn’t pass this test.

Table 3. Bending test result Sample Test-

piece

Bending test

Sample 1

(7 mm weld)

1.1 without crack

1.2 without crack

Sample 2

(10 mm weld)

2.1 without crack

2.2 without crack

Sample 3

(15 mm weld)

3.1 with crack

3.2 with crack

7 Cavitation erosion tests For the cavitation erosion tests was used the

ultrasonic method (frequency – 20 kHz, amplitude –

40 µm) [12]. The samples were tested for two hours.

The weight measurements were made every fifteen

minutes with a high precision balance.

The weight loss variation (Figure 7) proves that

the filler material has a very good resistance to

cavitation erosion. The total weight loss after two

hours of cavitation was about 0.8 mg for samples 1

and 2 and 0.6 mg for sample 3.

Fig. 7. Weight loss variation

After the first 15 minutes the cavitation erosion

rate became constant (about 0.01 mg/hour).

On the samples surface no erosion craters specify to

cavitation were observed.

For a correct conclusion about the differences

between the three samples, the cavitation tests will

be extended to a longer period.

8 Conclusions and further research The investigations regarding the influence of the

weld thickness on the cavitation resistance, carried

out on the 7 mm, 10 mm and 15 mm austenitic weld

deposits, conduced to the following conclusions:

- for relatively short cavitation exposures, the 15

mm weld (sample 3) had the best behavior;

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ISSN: 1792-5924 / ISSN: 1792-5940 319 ISBN: 978-960-474-237-0

- one possible explanation may be the hardening

process, which occurs as a consequence of the

precipitation of complex chromium carbides on

the austenitic grains limits, which probably

took place during the welding process in case

of sample 3;

- in order to obtain applicable results, the

cavitation erosion tests have to be carried out

for longer testing periods, because it is

expected that the behavior of the coarse grain

structure of the 15mm austenitic weld, as

shown by previous investigations, after longer

exposure periods conduces to unacceptable

high cavitation erosion rates.

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ISSN: 1792-5924 / ISSN: 1792-5940 320 ISBN: 978-960-474-237-0