A R C H I V E S o f

F O U N D R Y E N G I N E E R I N G

Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences

ISSN (1897-3310)Volume 9

Issue 4/2009

81 – 86

15/4

A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 9 , I s s u e 4 / 2 0 0 9 , 8 1 - 8 6 81

The influence of heat treatments on

cavitation erosion resistance of BA1055 alloy

R. Jasionowski a*, W. Przetakiewicz a, D. Zasada b, S. Czorna a

a Institute of Basic Technical Sciences, Maritime University of Szczecin, Szczecin, Poland b Department of Metallurgy and Material Technology, Military University of Technology, Warszawa, Poland

* Corresponding author. E-mail address: rjasion@am.szczecin.pl

Received 19.06.2009; accepted in revised form 06.07.2009

Abstract

The cavitation erosion is a process of material deterioration as a result of materialization, increase and decrease of the cavitation bubbles in different types of liquid. The cavitation erosion materials are used to prevent the devastating effect of imploding bubbles. The aluminium bronze BA1055 is the most commonly used material among the cooper alloys used on the parts of machines exposed to cavitation erosion phenomenon. The following article brings up the study of the effects of bronze BA1055 heat treatment for its cavitation erosion resistance performed on a flux-impact measuring device. The conducted studies confirmed the extension of the incubation process of BA1055 alloy after the hardening in relation to the moulded alloy. It has resulted in the increase of the resistance to cavitation erosion. Keywords: Cavitation erosion; Cavitation; Copper-base alloy; Aluminium bronze; Heat treatment.

1. Introduction One of the forms of destruction of construction materials is

cavitational erosion. The phenomenon of cavitational destruction occurs mainly in machine elements that are washed by rapid flows of liquid or work in ultrasonic field of high intensity. They include the elements of pumps, water turbines, steam turbines, marine diesel engines, screw propellers, and sonic sounders. The basic reason of cavitational destruction process are sudden changes in flowing liquid pressure – pulse reduction of liquid pressure below the critical pressure, which is close to liquid evaporation pressure, followed by formation of vapour-gas bubbles and implosion of these bubbles in the zone of higher pressure. In the situation described, the developing micro-bubbles play the part of cavitational nuclei having such a property that they are capable of cyclic reproduction due to liberation of gases contained in the liquid volume. The recurrent phenomenon of compression and implosion of gas bubbles in the working liquid is accompanied by oscillating pressure pulses that even reach several thousands MPa, which are in particular dangerous in case

of the grouping of bubbles in a cavitational cloud, where the bubbles implode simultaneously, inducing at the same time a larger pulse of final pressure than the implosion of single bubble. Under critical conditions, a cumulative streamlet of liquid may be formed which moves at a speed exceeding 100 m/s [1-6].

On the surface of material exposed to the effect of liquid, the cavitation phenomenon induces local destruction of the surface layer as a consequence of the resultant effect of liquid micro-stream blows with high hydrodynamic parameters as well as pressure waves. Due to the nature of loading, destruction of material surface can be compared to the fatigue process. Quantitative and qualitative description of cavitational destruction depends first of all on the type of material and the conditions under which the process of cavitational erosion takes place. In the first case, the structure of material is in question (i.e. grain size, type of inclusions, impurities and phases, their morphology, arrangement, etc.), whereas in the second one it is about the distribution of cavitational loading and the possibility of additional occurrence of chemical, electrochemical and thermal processes within the implosion area [7-8].

Soft construction materials, such as copper, nickel and aluminium, undergo the plastic strain under the influence of cavitational loading as early as in the initial stage, with their surface becoming unevenly waved. Farther effect induces formation of elevations and cavities, which change with time into deep pitting and craters. To the following materials including the cooper alloys which are used to produce the parts of machines prone to cavitation erosion such as propellers, sliding elements, pump components and aqua fittings. These materials to oppose the cavitation deteriorating are the most commonly characterized by: high resistance standards, cracks and dents prevention, high cavitation erosion resistance, high corrosion resistance and decent technological properties (castability – propitious casting defects, weldability - advantageous in welding repairs) [9-10]. The aluminium bronzes BA1032, BA1044 and BA1055 are the most promising among the cooper alloys. They are ranked by PRS in the Cu3 category at endurance of Rm = 600÷770 MPa and toughness of 170÷200 HB, also characterized by additional high attrition endurance, high atmospheric corrosion and sea water corrosion. Those properties the aluminium bronzes owe to the following Fe (2,0÷5,7 wt%), Ni (3,0÷5,5 wt%), Mn (0,5÷4,0 wt%) and Si (0,2 wt%) alloy supplements. The increase of the endurance properties of the aluminium bronzes is obtained by using the heat treatment. The hardening at 950oC or hardening and drawback treatment at temperature of 400-600oC for 2÷3 hours. The aim of the following article was to determine the influence of the heat treatments of casting alloy BA1055 on resistance to cavitational erosion.

2. Experimental material

The study covered alloy BA1055 mostly used on shipping screw propellers, pump impellers and water turbines was tested in four different states: casted (A), hardened at temperature of 950oC (B), hardened and drawback treatment at temperature of 400oC (C) and hardened and drawback treatment at temperature of 600oC (D). The study of the chemical constitution and the microstructure of BA1055 alloy after the casting was conducted by the use of Phillips XL30LaB6 microscope equipped with the DX4-EDAX X-ray apparatus analyser. The chemical constitution samples were not altered in the consequence of conducting the heat treatment (Fig. 1).

The bronze BA1055 structure after the casting is typical for aluminium-nickel bronzes. That means the α + γ2 structure with the present secretions of aluminium–iron κ phases. The structure contains the bright grains of solid solution α, Cu rich, with RSC crystal lattice. There are unimportant quantities of β phase eutectoidal transformation in the alloy – eutectoid α+γ2. Isolated intermetallic phases of Fe-Al structure are visible with the morphology corresponding with κ1 phases (dentritic), κ2 (rose), κ4 (lamellar). Occasionally the isolated κ3 phase (based on Ni-Al phases) incorporated in the eutectoid is also observed (Fig. 2).

BA1055 Element %Wt %At

Al 9,18 19,01

Total 100,00 100,00

Fe 5,16 5,16 Ni 5,09 4,84

Mn 1,18 1,20

Cu 79,40 69,80

Fig. 1. EDAX analysis results on the BA1055

Fig. 2. Typical structure of BA1055 alloy after cast

The BA1055 structure after the hardening as a result of

diffusionless β phase transformation has an acicular structure with the needle length of 100 μm looking like a martensite in hardened steel (Fig. 3).

Fig. 3. Structure of BA1055 after the hardening

at temperature of 950oC

The alloy structures after the hardening and drawback treatment are similar. They also have an acicular structure but with the diffused needles (Fig. 4).

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a)

b)

Fig. 4. Structure of BA1055 after hardening and drawback

treatment at temperature of a) 400oC, b) 600oC

The heat treatments apart from altering the structure in the examined samples caused the growth of the grain to the size 2-6 mm (Fig. 5) and alteration of toughness showed on drawing no. 6.

1 mm

Fig. 5. Structure of BA1055 after hardening at temperature of 950oC

0

240

200

160

120

80

40

280

A

B

C

D

176

272

160210

HA

RD

NE

SS H

B

Fig. 6. Comparison of hardness

3. Research methods The examination of cavitational erosion was carried out on a

flux-impact measuring device (Fig. 7).

1

4

2

5

3

86

7

9

10

11

12

1314

F F

M

M

t

t

main water circulation systemauxiliary water circulation systemliquid coolant circulation system

sample: 20x6 [mm]

Fig. 7. A scheme of flux-impact measuring device: 1-rotor, 2-sample, 3-nozzle, 4-flow-meter, 5-pomp,

6-self-rinsing filter,7-circulating tank, 8-pomp of the cooling system, 9-cooler, 10-equalizing tank, 11-coolant pomp,

12-refrigerator, 13-elektric motor, 14-rotor casing The test stand includes the following closed-loop circulation

systems: • main water circulation system, where the water purified by a

self-rinsing filter (6) is drawn in by an impeller pump (5) and led to the rotor casing (14), from where it flows off to the tank (7);

• auxiliary water circulation system to cool down the water in the tank (7); includes a pump (8) and a cooler (9);

• liquid coolant circulation system includes an air conditioner (12) and a cooler (9) with a linking pipe system.

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The test stand enables two pieces (2) to be tested concurrently. The pieces are fastened in a rotor driven by an electric motor through a belt transmission.

Samples for the examination were of the cylindrical shape, 20 mm in diameter and 6 ± 0,5 mm height. Sample surface roughness, measured by means of PGM-1C profilographometer, ranged 0,010÷0,015 μm. The samples were mounted vertically in rotor arms, parallel to the axis of water stream pumped continuously at 0,06 MPa through a nozzle with a 10 mm diameter, 1,6 mm away from the sample edge. The rotating samples stroke against the water stream. Water flow intensity was constant and amounted to 1,55 m3/h. The samples were examined for the period of 30 minutes, took out from the fixtures, degreased in an ultrasonic washer for 10 minutes at 30°C, dried in a laboratory drier for 15 minutes at 120°C and weighed, than mounted again in the rotor arms, maintaining the initial position in relation to the water stream. The analyses included 4 samples of each alloy, examined for the period of 2040 minutes.

4. Study results and their analysis

The course of cavitational deterioration of bronze BA1055 in the state after the casting begins from rinsing out the κ phases from the material surface and plastic deformation of α and β phases (Fig. 8).

Fig. 8. The plastic deformation and mass decrement of the surface

of BA1055 alloy after cast

The crumbling up of κ phase causes the exposure of the grain boundary and furthermore the progressive prolapse of the whole grains or the whole sections. In consequence of further cavitational deterioration deep fissile cracks arise crossing through the α to κ phases. The κ phases isolation mostly come out when their surrounding from the α phase corrodes and weakens and then erodes. It results in the separation of the whole grain groups and increase of material deterioration speed (Fig. 9).

a)

b)

Fig. 9. Effects of cavitational erosion of BA1055 alloy after cast

a) in the incubation period b) in the acceleration erosion rate period

The bronzes undergoing the heat treatment have a similar

course of cavitational deterioration. The surface is exposed to a cyclic water stream bursts simulating the microstream impact on a closing cavitational bubble. It provokes in the early stage of process of deterioration an even erosion. Arising beneath the surface little cracks along the smallest needles result in separation of the whole clusters (Fig. 10).

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a)

b)

c)

Fig. 10. Effects of cavitational erosion of BA1055 alloy

in the acceleration erosion rate period after: a) hardening at temperature of 950oC, b) hardening and drawback treatment

at temperature of 400oC, c)hardening and drawback treatment

at temperature of 600oC

The diagrams of cavitational erosion of studied materials have been shown on the drawing no. 11.

0 500 1000 1500 2000

0

100

200

300

time [min]

mass loss Δm [mg]

D

B

A

C

Fig. 11. Comparison of the test result of cavitational erosion

of BA1055 alloy: after cast (A), after hardening at temperature of 950oC (B),

after hardening and drawback treatment at temperature of 400oC (C), after hardening and drawback treatment

at temperature of 600oC (D)

5. Conclusion

The bronze BA1055 is a material of the highest cavitational erosion resistance among the currently used cooper alloys on screw propellers. Its high resistance is as a result of high corroding-fatigue material endurance. Conclucted studies proved that the cavitational deteriorating resistance may be enhanced by the heat treatments. The highest cavitational erosion resistance was found in hardened bronze BA1055. This alloy has the longest incubation period (around 1500mins) and it is five times longer than to the casted one in the comparison. The cooper alloy its high resistance for cavitational erosion owes to high toughness and structure. This material’s resistance may be compared to the resistance of casted intermetallic FeAl alloys [11-12]. Performed studies also showed that the positive results for enhancing the cavitational deteriorating resistance may be achieved by hardening and drawback treatment at temperature of 600oC. For this alloy the period of accelerated use starts after 1000 minutes of water stream influence and it is four times longer than the incubation period for BA1055 casted bronze. The least benefits may be observed in the hardening and drawback treatment at temperature of 400oC. The diagram of deterioration of this material is shifted for about 180 minutes and in the period of accelerated deteriorating it is parallel to the course of bronze diagram after casting. The higher resistance of alloy after hardening and drawback treatment at temperature of 400oC (C) results in diffused aciculat structure and other source from material deterioration despite a slight toughness loss.

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References [1] C.E. Brennen, Cavitation and Buble Dynamics, Oxford

University Press, 1995. [2] L.J. Briggs, The Limiting Negative Pressure of Water,

Journal of Applied Physics, Vol. 21 (1970) 721-722. [3] D.H. Trevena, Cavitation and tension in liquids,

IOP Publishing Ltd, 1987. [4] M.S. Plesset, R.B. Chapman: Collapse of an Initially

Spherical Vapour Cavity in the Neighbourhood of a Solid Boundary, Jour. Fluid Mech., Vol. 47, Part 2 (1971), 283-290.

[5] R. Hickling, M.S. Plesset: Collapse and rebound of a spherical bubble in water, Phys. Fluids, No 7 (1963), 7-14.

[6] C.F. Naude, A.T. Ellis: On the mechanism of cavitation damage by non-hemispherical cavities collapsing in contact with a solid boundary, Journal of Basic Engineering, No. 83 (1961), 648-656.

[7] A. Karimi, J.L. Martin, Cavitation erosion of materials, International Metals Reviews, Vol. 31, No. 1 (1986), 1-26.

[8] A. Sakamoto, T. Yamasaki, M. Matsumura, Erosion-corosion test on Copper alloys for water tap use, Wear, Vol. 186-187, No. 2 (1995), 548-554..

[9] A. Al-Hashem, P.G. Caceres, W.T. Raid, H.M. Shalaby, Cavitation corrosion behavior of cast nickel-aluminium bronze in seawater, Corrosion, Vol. 51, No. 5 (1995), 331-342.

[10] R. Jasionowski, J. Chmiel, D. Zasada, The cavitational erosion resistance of copper-base alloys used for marine propellers, 10th Congress of Technical Diagnostics, Stare Jabłonki, 2005, 179-188 (in Polish).

[11] R. Jasionowski, W. Przetakiewicz, Cavitational erosion resistance of Fe-Al intermetallic alloys, Mechanical Engineering of The Baltic Region, Kaliningrad (2003), 31-33.

[12] R. Jasionowski, D. Zasada, The Effect of Aluminium Content on the Cavitational Erosion Resistance of Fe-Al(B2) Alloys, 15th International Conference on the Properties of Water and Steam, Berlin, 2008.

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