Increasing the Wear Resistance of Marine Diesel Engines Elements Made of Ductile Iron

Summary This paper examines the infl uence of austempering and shot peening on the wear resistance of ductile iron. Samples for further testing were made from mechanically processed casts. The hardness and microstructure of ductile iron were examined on the prepared samples. Metallographic analysis of ductile cast iron samples in the cast state determined the pearlitic-ferritic structure of the matrix characteristic of this type of cast iron. Pearlite predominates in the structure, and the rest consists of ferrite and graphite nodules. The samples were then austempered. The isothermal conversion was 240 °C. After austempering, the hardness was measured and the microstructure was characterized, and the changes that occurred in relation to the heat-treated ductile iron were studied. Austempering created a new austempered structure, harder than that in the cast state, which led to improved mechanical properties . A needle-like structure of lower bainite (ADI) with high-carbon austenite, untransformed austenite, martensite and graphite nodules was achieved. The selected parameters of the shot peening process deformed the surface of the ADI and achieved a mostly martensitic structure without the presence of a soft phase, which increased resistance to abrasion. Additional hardening of the substrate by the shot peening process resulted in a composite material with the


INTRODUCTION / Uvod
Due to its good properties, ductile iron is increasingly used in mechanical engineering, and thus in the construction of marine machinery and plants.
Below are some of the examples of the application of ductile iron in the construction of parts of marine engines (engine type is in brackets): piston sleeve, air cylinder -exhaust valve housing part, cylinder pressure connection extensions, crankshaft bearing supports, crankshaft bearing cover and crosshead bearing caps (60MC-C), fuel pump wheel guide and exhaust valve wheel -fuel pump and exhaust valve drive system (50MC-C), oil cylinder -part of the exhaust valve housing (60MC-C (M)), auxiliary marine engine housings, etc.
Since a large part of machine elements is exposed to wear, and in order to fi nd even wider application of ductile iron in their production, this paper is aimed at fi nding procedures for processing ductile iron in order to increase its wear resistance.
Ductile iron is an iron alloy that has very good mechanical properties, good castability and machinability. In addition, the cost of its production is lower compared to steel. Good mechanical properties are achieved already in the cast state. Precisely because of this favourable combination of properties and low production cost, the production of ductile iron is constantly growing and its application is expanding. Ductile iron is still the subject of numerous studies which have established that mechanical properties can be further improved by thermal and mechanical treatments [1,2]. Thus, previous research has shown that ductile iron can be widely used in mechanical engineering and that it can be a quality and cheaper replacement for some steels. This is especially true for the qualities of ductile iron whose mechanical properties are further improved by subsequent heat treatment -isothermal ductile iron (austempered ductile iron -ADI), which proved to be the most effi cient heat treatment of ductile iron, Figure 1. In the edition of the standard HRN EN 1564 from 2011, ADI is defi ned as austempered ductile iron, because the austempered structure can be achieved today without the application of isothermal conversion [3].
Austempering consists of three phases: austenitizing (A-C), quenching (C-D), and isothermal conversion (D-E). The austenitizing temperature ranges from 850-930 °C, and is determined for each alloy by thermal analysis. The quenching rate must be suffi cient to avoid the pearlite area before starting the isothermal conversion reaction. The most important phase is the isothermal conversion (between 200 and 400 °C) during which the conversion of metastable austenite to ausferrite (D-E) occurs. The isothermal conversion temperature and the holding time at that temperature are crucial for the mechanical properties of the ADI material. Lower temperature causes greater cooling of austenite and the formation of more ferrite dust and causes the formation of a fi ner ausferrite structure resulting in high strength, relatively high hardness and low ductility, while higher temperature (above 350 ° C) causes the formation of coarser ausferrite structure with lower strength, lower hardness and high ductility [5,6], Figure 2.  Figure 2 Schematic representation of austenite to ADI conversion at diff erent temperatures [5].
Slika 2. Shematski prikaz konverzije ausferita u ADI na različitim temperaturama [5] Today, diff erent types of austempered ductile iron with properties depending on the time and temperature of austenitization and isothermal conversion and the chemical composition of the cast material are used. By combining these parameters, very good properties of ADI material can be achieved, with tensile strength up to 1700 MPa, hardness up to 480 HB and elongation 1 -16% [7,8]. Figure 3 shows that ADI has twice the strength of ductile iron for the same level of ductility and its strength is comparable to alloy steels. Today, ADI is used for many components in mechanical engineering such as gears and crankshafts. Figure 3 Comparison of the properties of ADI materials with steels and standard ductile iron [7]. Slika 3. Usporedba svojstava ADI materijala s čelikom i standardnim nodularnim lijevom [7] Even better properties and wear resistance of ADI can be achieved by additional mechanical surface treatment procedures: hammering or shot peening.
In this paper, the selected mechanical surface treatment procedure was the shot peening procedure. It is a process in which shots in a jet hit the surface at high speed.
The shot peening process is a controlled technological process which, under normal environmental conditions, achieves plastic deformation, i.e., it introduces compressive stress into the surface layer of metal [9].
By the introduction of plastic deformations at room temperature, a surface microstructural change occurs, the residual austenite turns into martensite [10,11].
The eff ect of shot peening depends on the intensity (strength) of shot peening. The intensity of shot peening depends on the size, shape, hardness, material, speed and impact of the shots. To check the intensity of shot peening, the Almen method is used, based on the measurement of the test strip (Almen bow) formed after shot peening.
The eff ects of cold deformation caused by shot peening are observed at a depth of up to 1 mm [12].

EXPERIMENTAL SECTION / Eksperimentalni dio
Ductile iron EN-GJS-600-3 with a predominant pearlitic structure was selected for this study, due to good mechanical properties (already in the cast state) and the possibility of their further improvement by austempering and shot peening.  The primary melt for making ductile cast iron was prepared in a mains-frequency induction furnace. The cartridge consisted of low manganese pig iron, steel scrap, circular material and ferroalloys FeSi75 and FeMn75. Chemical analysis was performed before pouring into the ductile pot. After obtaining the melt of the required chemical composition, it was poured into a ductile pot at a temperature of 1520 °C, where vaccination was performed. Nodulation was performed by Tundish cover procedure. The melt was further grafted into the jet when poured into the mould. Samples measuring 180x130x35 mm were cast, from which abrasion test specimens were later machine-made.
The dimensions of the samples are 13x22x73 mm. Hardness was measured on the same samples and metallographic tests were performed.

Austempering / Izotermičko poboljšavanje
Austempering was performed in the Heat Treatment Laboratory, Department of Materials and Tribology, Department of Production Engineering, Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture in Split. Abrasion test samples were improved to a temperature of 240 °C, Figure 5. The process consisted of heating to an austenitization temperature of 900 °C, holding for one hour at that temperature, and rapid cooling to an isothermal conversion temperature. The samples were rapidly cooled and kept for one hour in an AS 140 salt bath at 240 °C, followed by gradual cooling in air to room temperature, Figure 6. After cooling, the samples were washed with water to remove salt residues.
The heating and cooling process is diagrammatically shown in Figure 7. The isothermal conversion was selected according to the literature [13,14].

Shot Peening / Kugličarenje
Shot peening was done using a shot peening device UZK-1, Figure 8. The shot peening agent used is a high-carbon heat-treated granulate Abrasiv Muta S 390 (Ø 1 mm). Prior to the shot peening of the wear test specimens, calibration of the shot peening device had to be performed.

Abrasion Resistance Test / Test otpornosti na abraziju
The standard method of "dry sand/rubber wheel" was used for experimental determination of abrasion resistance (Figure 9). Quartz sand SiO 2 with a granulation of 0.25-0.50 mm was used as the abrasive medium. The sample rests on a rubber-coated wheel (2) (hardness about 60 Shore A) and is loaded with weights over the crankshaft. The force F is 45 N or 130 N, depending on the process variant. The total number of wheel revolutions is variable and is registered by a counter. In the case of this test used force was 130 N. Sample mass loss was measured by weighing after 100, 200 and 300 revolutions. Figure 9 Sketch of the device "dry sand/rubber wheel" [15] Slika 9. Skica uređaja ,,suhi pijesak/gumeni kotač'' [15] 3. RESULTS / Rezultati Table 1 shows the chemical composition of ductile iron EN-GJS-600-3.

Hardness / Tvrdoća
The surface hardness was tested with the Rockwell method on three samples, using the device shown in Figure 10. The measured mean value was 18 HRC.

Microstructure (Cast State) / Mikro struktura (izljev)
Based on the performed metallographic analysis, Figure 11, it was concluded that the graphite was mostly excreted in the form of nodules. The shape and size of the nodules were determined according to the standard HRN EN ISO 945-1: 2011. Graphite nodules are of form VI, size 5/6, in a large percentage regular (80-85%) and evenly distributed in the structures. Upon cooling and solidifi cation of ductile cast iron with increased pearlite content, in the early stage of eutectoid conversion, a signifi cant amount of pearlite is formed at the boundaries of austenitic grains, after which the formation of pearlite dominates by eutectoid conversion. The growth rate of pearlite is signifi cantly higher than the growth rate of ferrite, so after the beginning of pearlite formation, little ferrite is formed [16]. The matrix is pearlite-ferrite, and the ferrite is distributed around graphite nodules, which is a consequence of the diff usion of carbon to graphite nodules. Copper is an element that infl uences the development of pearlite, settles around the nodule and prevents the diff usion of carbon from austenite to the nodule, and in eutectoid conversion austenite is largely transformed into pearlite.

Hardness / Tvrdoća
The surface hardness of ADI 240 was tested on the same device on which the samples of cast iron were tested, Figure 10. The average measured value is 35.2 HRC.

Microstructure after isothermal conversion at 240 °C / Mikro struktura nakon izotermalne konverzije na 240 °C
At an isothermal conversion temperature of 240 °C, the subcooling of austenite was higher, the diff usion of carbon was slower, and a structure of fi ner ausferrite was formed, Figure 12. Ferrite germination is favoured over ferrite growth due to high hypothermia. In this way only a small proportion of carbon can diff use from ferrite to austenite. Part of the carbon is excreted in the form of carbides within the ferrite grains. The remaining austenite is only slightly enriched in carbon, 0.9% and remains unstable, Figure  2 [5,6]. The proportion of metastable austenite can subsequently be converted to martensite when cooled to room temperature, which was obtained in this study at a temperature of 240 °C, Figure  12. Isolated white areas show plate austenite, while carbon-rich austenite is found around the ferrite needles of bainite ferrite. The needle shape is characteristic of the lower bainite ferrite. Figure 12 shows the ausferrite structure formed at lower isothermal conversion temperatures.

ADI Shot Peening (ADI 240 K) / ADI kugličarenje (ADI 240 K)
In order to achieve the set shot peening intensities, it was necessary to provide the required air pressure.
During shot peening, the pressure at the inlet to the shot peening device drops by a certain amount, which depends on the capacity of the compressor and its associated tank.
In order to achieve a continuous operating pressure of 7 bar (Figure 13), the pressure at the inlet to the device should have been min. 7.6 bar. At an inlet pressure of 6 bar, a drop of 0.4 bar was achieved in shot peening, which means that the continuous working pressure was 5.6 bar. For an inlet pressure of 5 bar, the pressure drop was 0.3 bar, i.e., an operating pressure of 4.7 bar was achieved.

Hardness ADI 240 K / Tvrdoća ADI 240 K
The surface hardness was tested on three samples, two samples with a shot peening intensity of "1.19 A" and one sample with a shot peening intensity of "0.94 A".
In the fi rst case, the average hardness is 39 HRC, and in the second 37.8 HRC.
The same measuring device was used as in the previous two cases. Table 2 shows the mass loss of non-shot-shaped and diff erentintensity shot-shaped ADI samples, measured after 100, 200 and 300 revolutions.  Figure 14 gives a diagrammatic representation of the results from Table 2.

CONCLUSION / Zaključak
Based on the tests performed in this paper, the following is concluded: -Microstructure analysis shows an orderly structure in heattreated samples. The structure of these samples consists of graphite nodule with a pearlite-ferrite matrix. -Austempering of ductile cast iron resulted in a new ausferrite structure that is also homogeneous. -Austempering at a temperature of 240 °C resulted in an ausferrite structure of needle-shaped lower bainite with metastable austenite, martensite and graphite nodules. -Isothermal conversion at a temperature of 240 °C resulted in a signifi cant increase in surface hardness. -With shot peening, the hardness increased even more.
Higher surface hardness was achieved with higher intensity shot samples. -By abrasion of non-shot samples, a higher weight loss was achieved compared to shot samples. -The highest total mass loss, after three test cycles (3x100 rpm), was achieved with the lowest intensity shot samples.
On the other hand, the highest intensity of shot peening did not achieve the highest resistance to abrasion wear, because the best results were given by shot peening with a medium intensity of "1.04 A". The smallest deviation of mass loss per cycle was achieved in the samples "1.04 A".