|Sažetak (engleski)|| |
Ductile iron is a quasi-ternary alloy of iron and carbon that is predominantly extracted in the form of spherical graphite. The microstructure of ductile iron can be ferritic, pearlitic, ferritic-pearlitic or austenitic. The proportion of individual phases in the metal matrix depends on the chemical composition, the cooling rate through the area of eutectoid conversion and the volume fraction and number of graphite nodules. Previous research on ductile iron has shown that it can be widely used in mechanical engineering and that it can be a quality and cheaper replacement for some steels.
Subsequent isothermal conversion of ductile iron to obtain better mechanical properties yields austempered ductile iron (ADI) a material of better quality than conventional ductile iron, many other castings and even some steels. The combination of such good properties cannot be obtained by classical quenching and tempering, but is achieved by isothermal conversion (austempering process). The properties ADI achieves after heat treatment depend on the time and temperature of austenitization and isothermal conversion and the chemical composition of the starting material. Depending on the austempering parameters, different auspherite microstructures can be obtained. ADI is a material that offers a good combination: low cost, construction flexibility and good machinability. Austempering ductile iron forms an ausferite matrix composed of bainite ferrite and carbon-rich austenite (15-40 %). With cold deformation, austenite can be transformed into martensite, which brings another favourable characteristic of ADI material, the possibility that after isothermal conversion the surface is further hardened by shot peening or hammering processes. Increased hardness contributes to greater wear resistance of the surface exposed to variable cyclic stress, which expands the possibility of applying ADI material.
The shot peening process selected for this research is a controlled technological process which, under normal environmental conditions, achieves plastic deformation, in other words, it introduces compressive stress into the surface layer of the metal. The main purpose of shot peening is to increase the surface hardness and dynamic durability, and it also has a surface cleaning effect. The shot peening achieved cold plastic deformation changes the stress distribution in the surface layer, which was created by earlier technological process. The applied compressive stress reduces the possibility of the formation and spread of surface micro cracks that can be caused by corrosion, stress corrosion, fatigue and cavitation erosion, which will increase the dynamic endurance.
The effect of the shot peening is influenced by the intensity (strength) of the shot peening. The intensity of shot peening is an indicator of the energy transfer of the shot jet to the surface layer of the sample. The same depends on the size, shape, hardness, material, speed and angle of incidence of the shots. It is accepted that a method based on measuring the deformation of the test strip (Almen strip) after shot peening is used to check the strength of shot peening. The means used in shot peening are made of cast steel, glass, ceramics and cut steel wire. Of these, the most commonly used are cast steel shots. Steel shots with a diameter of 1 mm were used in this study.
The aim of this doctoral thesis is to determine the possible influence of shot peening on the mechanical properties of austempered ductile iron by scientific research methods, based on laboratory tests. It is to be assumed that the cold deformation of the surface will achieve the transformation of austenitic into martensitic structure, which increases the hardness of the material, and thus the resistance to some wear mechanisms.
Wear is the process of gradual loss of material from the surface of a solid body due to dynamic contact with another solid body, fluid and/or particles. There is no unambiguous connection between friction and wear in one tribosystem. In most cases, the increase in friction is accompanied by intensive wear, but there are also cases when it is the other way around. Wearing always involves the process of crack formation and the process of crack progression. The appearance of worn surfaces and the shape of wear particles are the basic indicators for identifying the wear mechanism.
For the wear resistance test, in this thesis, three procedures were selected: the abrasion test procedure, the erosion test procedure and the FZG (Forschungsstelle für Zahnräder und Getriebebau) gear test method.
Abrasion wear is the extrusion of material caused by hard particles or hard protrusions. About 50% of all wear cases are due to abrasion. A characteristic of this type of wear is the presence of hard abrasive particles of mostly mineral origin, so it is often called mineral wear. For the experimental determination of abrasion resistance in this paper, the standard method according to ASTM G56 – 94, " dry sand/rubber wheel ".
Erosion is the loss of material from the surface of a solid body due to the relative motion (flow) of the fluid in which the erosive is located (solid particles). Erosion in which the flow is at a small angle (up to 30°) in relation to the surface is called abrasive erosion, and when the erosive strikes the surface at larger angles (60-90°) the dominant wear mechanism is surface fatigue and this form of erosion is called impact erosion. There are several methods and devices for experimentally determining the resistance of materials to erosion. In this case, a method was
used in which the sample rotates and strikes a jet of falling erosive. The angle of impact of the erosive can be changed.
For the needs of the work, according to the "Tundisch cover" procedure, samples of castings EN-GJS-600-3 were made, from which samples for testing mechanical properties, samples for testing the resistance of materials to abrasion and erosion and actual gear samples were made. During the process of casting the samples, the chemical composition of the melt was controlled, so that in the end, castings of standard quality EN-GJS-600-3 were cast.
Prior to the use of castings for the production of samples, their homogeneity was examined in order to detect possible defects caused by casting. The samples were tested using an X-Ray device.
Samples were made to test the microstructure of ductile iron EN-GJS-600 (cast state) and later to test the microstructure of the austempered EN-GJS-600 (ADI). Light microscopy and FE SEM (Field emission scanning electron microscope) analysis of the microstructure were performed. Based on the performed metallographic analysis of the samples, it was concluded that the graphite was mostly excreted in the form of nodules. Graphite nodules are of form VI, size 5/6, in a large percentage regular (80-85%) and evenly distributed in the structure. Ferrite is distributed around graphite nodules. Surface hardness and fracture impact work were examined. The obtained measurements and images of the structure are in accordance with the standard for EN-GJS-600-3.
In the first cycle of the ADI test, samples for microstructure testing, impact fracture testing, abrasion and erosion wear test specimens, and gears were austempered. Of all the samples, part of the total was austempered at 240°C and the rest at 380°C. Heat treatment in both cases consisted of heating to the austenitization temperature (900 °C), holding for one hour at that temperature, and rapid cooling to the isothermal conversion temperature. The samples were rapidly cooled and kept for one hour in an AB1 salt bath at 240°C and 380°C, respectively, followed by gradual cooling in air to room temperature.
The microstructure of ADI was analyzed by light and FE SEM microscope. Samples EN-GJS-600-3, that were austempered at 240°C, have higher sub cooling of austenite with lower carbon diffusion rate resulting in ausferite structure.
Bainite ferrite has the characteristic appearance of "lower" bainite - needle-shaped, and the rest of the structure consists of high-carbon stabilized austenite and graphite nodules. No perlite and carbides were observed in the structure, which is a confirmation of a well-performed isothermal conversion.
Metallographic analysis of austempered ductile iron at 380°C (ADI 380) revealed a change in the structure of the matrix from perlite-ferrite to auspherite structure. The ausferite structure of ADI 380 consists of: bainite ferrite of fluffy shape characteristic for the area of the so-called "upper" bainite, high-carbon stabilized austenite and graphite nodules. No formation of perlite and carbide was observed, which confirms that the heat treatment by austempering was performed correctly.
By changing the microstructure by the austempering process, there was an increase in hardness. The ADI 240 samples have a high hardness of the "lower" bainite which is in accordance with the EN 1564:2011 standard. While the ADI 380 samples have a lower hardness since it is a fluffy bainite ferrite - "upper" bainite. The hardness of this iron matrix is in accordance with the standard EN 1564:2011.
The isothermal conversion process for both temperatures significantly increased the values of the fracture impact work, with the fact that in the ADI 380 samples the fracture impact work is significantly higher than in the ADI 240.
The reason for this is the difference in the form of bainite ferrite, which was confirmed by light and FE SEM analysis. The impact fracture performance of these two ADI materials is inversely proportional to their hardness.
Austempering in ADI 240 samples resulted in a significantly higher increase in hardness compared to ADI 380 samples, and samples of this condition were selected for further wear resistance tests. In the first test cycle, samples of ADI 240 were subjected to the shot peening process. The samples were shot peened with three different intensities, "1.32 A", "1.11 A" and "0.99 A". The intensity of "1.11 A" proved to be the most economical.
By measuring the hardness with the Brinell method on samples for testing the resistance to wear of ADI 240 and shot peened ADI 240 (ADI 240 K), an increase in hardness was found in shot penned samples. In the thin surface layer there was an introduction of deformations, crushing of grains, and thus an increase in hardness. The increase in hardness was found to depend on the intensity of shot peening.
Abrasion wear - the lowest weight loss was achieved with the highest intensity shot samples. This can be explained by the highest surface hardness achieved by the highest shot peening intensity. From the test results it can be concluded that the shot peening process is a surface hardening mechanism that has a favourable effect on increasing the resistance of ADI materials to abrasion.
Erosion wear by higher granulation quartz sand shows that the wear resistance of non shot peened ADI samples is better than shot peened ADI materials, for the same applied
conditions. Softer material has a better ability to flow, compared to harder and more fragile ones. By flowing, parts of (non shot peened) material move but without breaking off. In the case of harder material, this is not the case, but the loss of microparticles begins much earlier. Based on the average mass losses per test cycle, it can be observed that the lowest total mass loss after five test cycles has non shot peened samples, while the largest mass loss is shown by samples shot peened with an intensity of "1.11 A". It follows from the above that shot peening has no significant effect on sand erosion resistance. Compared to sand erosion, steel shot eroded specimens show less overall loss in weight, which means that this wear mechanism has less detrimental effect on the test material. The reason for this can be found in the dimensional and geometric diversity of erosives.
The results of the FZG gear wear test showed a significantly higher durability of austempered gears compared to a ductile iron gear. Gears from ADI withstood a significantly higher number of cycles until complete wear of the tooth surface compared to NL (EN-GJS-600-3) gear. The ADI 240 gear showed the greatest durability. For the ADI 380 and ADI 380 K samples, there is a clear difference in wear resistance due to the shot peening effect. The surface layer of the teeth was hardened by the shot peening process by crushing the crystal grains and forming martensite. This has resulted in significantly higher wear resistance, and thus the possibility of achieving more cycles until wear. The test showed less durability of the ADI 240 K gear compared to the ADI 380 K gear. This is also caused by the difference in crystal grain size and toughness in the subsurface layer, where cracks form.
On all gears, the measurement of microhardness in the surface layer to the depth was 600 μm. From the obtained measurement results, two quantitatively different groups of hardness distribution by cross section are derived. In the case of a sample in the raw (cast) state and in the case of austempered samples, regardless of the temperature of the austempering (380°C or 240°C), a decrease in hardness along the edge of the samples is noticeable. This drop in hardness along the very edge of the samples extends to a depth of about 60 μm.
To expand the impact of different media on ADI wear, new abrasion and erosion test specimens were developed. They were austempered in another austempering procedure. The samples spent one hour in the oven at an austenitization temperature of 900°C, after which they were immersed in an AS 140 salt bath, the temperature of which was 240°C, where they also spent one hour. They were then cooled to room temperature.
After the austempering samples were shot peened, Almen intensities "1,32 A", "1,11 A" and "0,99 A".
In order to determine the effect of shot peening after the second cycle of isothermal conversion, the surface hardness of ADI 240 (non shot peened) was tested. There was an increase in hardness on all shot peened samples, regardless of the intensity of shot peening. The obtained results once again confirmed that the surface hardness increased more in samples that were shot peened with higher intensity.
In order to determine the influence of shot peening, micro hardness was measured in the edge layer of the samples. For comparison, the micro hardness of the non shot peened sample was also measured. In non shot peened austempered samples, a decrease in hardness along the edge of the samples is noticeable while it is uniform in depth above 200 μm. In austempered and shot peened specimens, there is no drop in edge hardness, and the hardness is slightly lower than in the non shot peened specimen. All shot peened samples show an increase in micro hardness from surface to depth 600 μm, followed by a gradual decrease.
After measuring the micro hardness, tribological tests were performed.
From the analysis of the obtained test results on abrasion wear by different abrasives in the second test cycle, it is concluded that shot peening and its intensity has a positive effect on abrasion wear with SiO2 – high. The intensity of shot peening has no significant effect on abrasive wear SiO2 – low as well as in abrasive markings Al2O3.
Considering the type, mass (size) and shape of the abrasive: SiO2 – high (0,25-0,50 mm); SiO2 – low (0,15-0,25 mm); Al2O3 (0,063-0,2 mm) it can be seen that the wear intensity of ADI 240 K material is more influenced by the granulation of the abrasive, than the chemical composition (type).
The intensity of shot peening has a positive effect on the reduction of abrasion wear in abrasives of higher granulation. This is not the case with smaller granulation abrasives.
In the case of erosion wear by different erosives, the intensity of shot peening does not have a significant impact on the tribological properties of ADI material, if the erosion takes place under the same parameters (type of erosive, impact angle). However, a difference in wear with regard to erosive is observed. In erosion of the erosive marked SiO2 – high, wear is significantly higher compared to erosion of the erosive marked Al2O3 or steel shots. The reason for this is the shape and mass of the erosive.
For all erosives, the lowest wear was shown at an angle of 60°, which indicates that the impact angle affects the erosion effect of ADI and ADI K materials.