Cavitation behavior of unalloyed ADI materal

used as replacement components

D. Rajnovic1, S. Balos

1, M. Dramicanin

1, P. Janjatovic

1,

D. Labus-Zlatanovic1, L. Sidjanin

1, O. Eric-Cekic

2

1. Faculty of Technical Sciences, University of Novi Sad, Trg Dositeja Obradovica 6, 21000 Novi Sad, Serbia

2. Innovation Centre, Faculty of Mechanical Engineering, University of Belgrade, Kraljice Marije 16, Beograd, Serbia

Cavitation is material damage caused by the formation and collapse of bubbles, in a liquid. The shock waves and microjets emitted during the collapses of vapor structures interact with neighboring solid surfaces and may cause the material damage and surface erosion.

The cavitation is a source of concern in machine parts that are subject to vibratory forces while in contact with a liquid. Different materials offer different levels of resistance to cavitation. In this kind of extreme working conditions, some highly alloyed (Cr, Ni, Mo, Vo, Co, Mn) steels are usually used. For that reason, is of great importance to find a suitable alternative for those CRM elements.

As possible replacement material, a unalloyed or low alloyed ADI (Austempered Ductile Iron) could be used, due to a wide range of mechanical properties (high strength, ductility, toughness, good wear and fatigue resistance) which could be produced by appropriate heat treatment.

What is cavitation?

Finally, to conclude, microstructure transformation of unalloyed ADI

material give rise to possibilities of replacing CRM alloyed steels in

some cavitation sensitive applications.

Summary The authors gratefully acknowledge research funding from the Ministry of Education, Science and Technological Development of the Republic of Serbia under grant number TR34015.

The Austempered ductile iron (ADI) was subjected to intensive cavitation in water, by the application of ultrasonic equipment. The frequency of vibration and the peak-to-peak displacement amplitude of the horn were 20±0.5 kHz and 50 µm, respectively, with separation of 0.5 mm between the specimens and the horn tip. The liquid test was done in water at 25±0.5 °C

Experiment

The cavitation damage was as-sessed by a common mass loss technique, as well as metallograph-ic examination of eroded surfaces by means of light microscope and scanning electron microscopes (SEM) JEOL JSM 6460LV and JSM 5800 operated at 20kV.

Methods

31,4% Retained austenite 16%

The microstructure of the unalloyed ductile iron is consisted of spheroidal graphite in a predominantly ferrite matrix with 10% pearlite. The morphology of graphite was fully spherical with average graphite volume fraction of 10.9%, nodule size of 25 to 30 µm, nodule count of 150 to 200 per mm

2

Materials:

Consisting of a mixture of

ausferritic ferrite needles and

high retained austenite

(stringer type retained

austenite).

ADI 300

Consisting of mixture of a

plate like ausferritic ferrite

(feathery) and fine retained

austenite.

30 min 60 min 240 min

It can be seen, that in ADI 300, cracks are nucleated at the edge of the nodule cavity.

This behavior was not present in ADI 400.

In the last stage, after 240 min of cavitation testing, numerous craters and grooves with intense plastic deformation can be observed on the damaged surface.

In ADI austempered at 400°C, a low carbon, retained

metastable reacted austenite transforms trough stress assisted

phase transformation (SATRAM) effect into martensite hence

promoting cavitation resistance.

Acknowledgment

Graphite nodules become

separate from the matrix The eroded surface

The SEM results were correlated to the results obtained by mass loss analysis, performing a 3rd degree polynomial dependence between mass loss and cavitation duration.

Tem

per

atu

re

300 °C

Time

1 h

Austenitisation Austempering

400 °C

2 h

900 °C

Heat treatment

AD

I—400

A

DI—

300

0 min

C Si Mn Ni Cr Mg P S Fe

3.53 2.53 0.347 0.045 0.055 0.031 0.018 0.015 balance

Chemical composition of as-cast material [mass %]

ADI 400

a)

b)

Significantly more severe

damage may be observed

It can be noticed that ADI 300 suffers a more serious peeling in the matrix arey next to the pit rim compared to ADI 400. This is in conjunction with the presence of microcracks propagating radially, as well as induce the development of new microcracks under the peeled-off surface layer (a — white arrow).

RESULTS

SATRAM