CONTROLLED SHOT PEENING OF ADI/AGI CASTINGS

Transcription

1 CONTROLLED SHOT PEENING OF ADI/AGI CASTINGS Z. ANDRSOVA 1, B. SKRBEK. 1, I. SEJNOHOVA 2 1 Technical University of Liberec, Studentska 2, , Liberec 1; zuzana.andrsova1@tul.cz, bretislav.skrbek@tul.cz 2 RESL, s.r.o., Obchodni 618, , Liberec 11, ivana.sejnohova@atlas.cz Abstract Austempered Ductile Iron (ADI) and Austempered Gray Iron (AGI) are the most progressive group of graphitic irons in terms of the mechanical properties. The matrix of ADI/AGI is bainite-like, composed of ferrite needles and of residual austenite (up to 40 %) which is oversaturated by carbon. By a suitably chosen plastic deformation (shot peening, rolling, gear mesh) is in the surface induced the martensitic transformation of the residual austenite. This paper deals with the conditions needed for this transformation on specific materials. It is a part of the work, which results should support the production of this very progressive material in the Czech Republic. Keywords: austempered iron, martensitic transformation initiated by plastic deformation 1. INTRODUCTION This paper discusses an experiment with controlled work hardening of castings made from austempered iron. Transformation hardening of ADI/AGI on specific parts is subject to corporate secrets, in the literature there s no description of this hardening on specific product. That s why there will be the subject of research and the task of further work. In this article it is a kind of "first sighting", which verifies in particular the martensitic transformation initiated by plastic deformation during selected terms of work hardening with help of X-Ray diffraction, metallography and measurements of microhardness. It's mainly about finding the optimal conditions for the diagnosis of this technology, so without anticipated phenomena and contexts. The experiment is a part of the doctoral thesis Non-destructive structuroscopy of austempered iron and also builds on the paper The potential of isothermally hardened iron with vermicular graphite. 2. BASIC INFORMATION ABOUT SHOT PEENING Shot peening is a cold working process used to produce a compressive residual stress layer and to modify mechanical properties of alloys. It involves impacting a surface with shot (metallic, glass, or ceramic particles - balls) with force sufficient to create plastic deformation. In mechanical engineering is the shot peening primarily used to fight with fatigue problems, to prevent stress corrosion cracking, assist in form or shape correction or provide a uniform texture. The first three rely on the beneficial compressive stresses, while the fourth item is used only for the uniform appearance produced by the strict control of shot. In the shot peening of austempered iron there s the added benefit besides of the fatigue characteristics improvement, ADI/AGI responds favorably to work hardening that can improve its wear characteristics. This is the result of cold working, causing transformation of residual austenite to martensite (martensitic transformation initiated by plastic deformation). The shot peening is a controllable process. The depth of the compressive stress layer is a function of the kinetic energy imparted to the peened surface, which is a function of the mass*velocity of the shot. This depth of compression (or the point at which compressive stresses change to tensile) also reflects hardness of the material at a given intensity (a measurement of the kinetic energy) the depth of compression is deeper on a softer material than on a harder material. The kinetic energy is measured by a

2 standard known as Almen strip. But to get all of the advantages of shot peening requires more controls than determining of the intensity there s also necessary to maintain the shot integrity and to check coverage of peened areas. Of course, it s preferred to use automated or computer controlled equipment. [8] 3. MATERIALS AND THEIR TREATMENT For the above-mentioned dissertation was created a set of reference samples of cast iron with lamellar (C 3,15; Si 2,24; Mn 0,19; P 0,02; S 0,016; Cu 0,02; Ni 0,01), vermicular (C 3,62; Mn 0,18; Si 3,5; S 0,015; P 0,024; Cu 0,21; Mg 0,014; Mo 0,35; Cr 0,04) and nodular graphite (C 3,3; Si 2,45; Mn 0,25; Mg 0,046; Cu 0,04; P 0,02; S 0,015). Chemical and structural composition of samples were chosen to represent the most suitable used types of default iron. For the production of austempered iron is necessary to use as default an iron with specific chemical composition (limited contents of elements - especially Mo, Ni, Cu) with ferritic or ferritic-pearlitic matrix (labor savings in machining, impact on austenizing) with the largest amount of the small, most regular particles of graphite, preferably equally spaced ( p/mm2) to achieve the desired effect of isothermal hardening. (The result of experiments should be the set of reference samples that can expand as needed for specific applications.) Heat treatment of the samples was chosen to cover the interval of prescribed parameters of isothermal hardening (temperatures and dwell time). Austenitizing temperature was chosen in the middle of interval 900 C. Dwell time of austenitizing was 30, resp. 90 minutes (to achieve a different saturation of matrix by carbon from graphite) in prescribed inert atmosphere (to avoid decarburization of the surface). Isothermal temperatures were chosen through the whole interval 240, 310 and 400 C. Isothermal dwell times were 2,10 and 60 minutes, where only the 60 is the right dwell. Hardening was performed into the salt bath (As140) with final cooling in air. [1], [2], [3], [4], [5] 4. EXPERIMENT To this experiment with surface hardening was selected 10 samples, which by its structure meet the requirements for transformation hardening, ie according to the terms of the heat treatment and the available measurements their matrix have contained only ferrite needles and residual austenite. Data on samples and their heat treatment are in Tab. 1. Tab. 1 The samples and their heat treatment Tab. 2 Conditions of shot peening The controlled shot peening is realized in 2 phases on VacuBlast device in Aero Vodochody, a.s. On this device is possible to determine the intensity of the peening (expressed by the pressure) and corresponding phase transformation, resp. depth of the hardened layer. It is realized by precise measurement of deflection of Almen strips. This ensures repeatability of the process. Peening head movement is done automatically by robot FANUC M16lb/10L, in strips (in this case 3-4 crossings). Before each peening with different pressure had been at first peened the Almen strip and was measured its deflection. As the shot have been used steel balls (55-62 HRC, tempered martensite) with diameters in the range of 0,5-0,6 mm. These balls are during

3 the peening continuously sorted distorted pieces are separated, so the integrity of the shot is ensured. The shot peening device is in Fig. 1. In the first stage have been used 5 pressures in the range 1-3 bar (see Tab.2). Initial pressure of 2 bar (medium) was chosen from the first test on a sample 9K1 310, where it was used a maximum pressure of 6 bar. This test was done primarily to verify the possibility of transformation of residual austenite to martensite. According to metallography the influenced layer was about 100 µm, however, the sample surface was almost destroyed - with Ra=12,5 and with torn out material. According to the experience of the operator is commonly used pressures max. 3 bar (steel), the soft materials only about 0.8 bar. The first sample 9K1 310 was not the best in terms of structure because, due to its heat treatment (short isothermal dwell 10 min), already contained a small amount of martensite. Yet the transformation of the contained residual austenite to martensite, initiated by plastic deformation, was confirmed by X-Ray diffraction, microhardness and metallographically. Therefore, the next test used only samples with the longest 60 min. dwell and medium and high temperature (310 a 400 C) to exclude the presence of martensite before hardening. Fig. 1 Controlled shot peening; a) VacuBlast device, b) used shot, c) Almen strips During the shot peening arises a layer of material, which is hardened thanks to the plastic deformation. The existence and the depth of the hardened material was determined metallographically (LOM Zeiss Axio Imager 2, software Axio Vision 4.8) and by measurement of the microhardness gradient (Fischerscope XY, HV 0,1). However, it is necessary to distinguish whether it is only a compressive stress layer, or also the desired transformation of residual austenite to martensite. To confirm this transformation were carried out qualitative and quantitative X-ray phase analysis (θ - θ diffractometer X Pert PRO MPD in Bragg-Brentan focusing geometry, software TOPAS) both on the surface of the samples and their inner structure (represented by metallographic samples). Since we can not completely distinguish ferrite from martensite (similar lattice Fe ), transformation can be deduced mainly from a quantitative difference between contents of residual austenite. [6], [7] Besides hardening have been also assessed surface condition of samples after shot peening. This is very important because these materials are due to its excellent properties often used for dynamically loaded components, which fatigue resistance is mainly conditioned by quality surface. Therefore it have been measured surface roughness Ra and Rz and surfaces were also scanned using macroscope (stereo Zeiss STEMI DV4 with LED illumination, software NIS Elements BR) and light optical microscope in Z-stack mode (Zeiss Axio Imager 2, software Axio Vision 4.8). In Figs see images, gradients and analysis of three selected samples. Fig. 2 9C6 400 after the shot peening; a) microstructure of the surface layer (BF, 500x) b) 3D image of the surface (BF, 25x), c) surface roughness

4 Fig. 3 9K6 400 after the shot peening; a) microstructure of the surface layer (BF, 500x) b) 3D image of the surface (BF, 25x), c) surface roughness Fig. 4 9L6 400 after the shot peening; a) microstructure of the surface layer (BF, 500x) b) 3D image of the surface (BF, 25x), c) surface roughness 5. DISCUSSION Samples were grinded on metallographic grinders before the peening, with removal of material min 0,5 mm. In the case of samples, which haven t been during the heat treatment protected against oxidation (deliberately created decarburization for the purpose of NDT diagnostics of unwanted layers), were previously milled and grinded layers of 1.5 mm. Yet on several samples occurred another, hidden decarburization, which reflected until during the measurement of microhardness (surface structure, which has not been enough saturated by carbon during austenitization had lower hardness). From the microhardness gradients is nevertheless apparent a hardening, however, values of hardness achieved in the decarburized layer do not exceed values of the internal structure. Due to an error in the preparation of some samples, thus did not improve their utility properties, although it would be presumable. Layers modified by peening reach thicknesses in the range of cca microns. The main influence has of course applied pressure (intensity) of the peening, of these results, however, is not yet possible to establish a clear dependence, since each sample is made of a slightly different material. Also determination of the dependence pressure/layer thickness for each material will be the task of further research. Tab. 3 Quantitative phase analysis of metallographic and bulk samples The qualitative and quantitative phase analysis shows that peened bulk samples contain a significantly higher proportion of Fe compared to the other phases in comparison with metallographic samples (see Tab. 3). The exception is the bulk sample 9K6 310 which still contains about 14% of austenite. However,

5 even in the case of this sample is the proportion of Fe higher than in the metallographic sample. It can thus be concluded that during the plastic deformation was realized the required transformation of austenite to martensite. Fig. 5 Diffraction records of 9C6 400 a) metallographic sample, b) bulk sample Fig. 6 Diffraction records of 9K6 400 a) metallographic sample, b) bulk sample Fig. 7 Diffraction records of 9L6 400 a) metallographic sample, b) bulk sample. Tab. 4 Roughness of peened surfaces

6 Fig. 8 Microhardness gradients of selected samples; a) 9C6 400, b) 9K6 400, c) 9L6 400 As the most suitable for peening of used samples from ADI seem to be pressures in the range of 2 bar 3 bar. At these pressures originated sufficient depth of hardened layer in the order of tens of microns, while the surface roughness Ra is not significantly worse, varies within the range of about Ra 1,6 (see Tab. 4). In case of samples from AGI with vermicular graphite could be applied pressures of 1,5 to 2 bar, when there also arises sufficient depth of the hardening and the surface quality is still satisfactory with Ra around 1,3. At higher pressure the surface roughness has increased to twice. At a pressure of about 1 bar the depth of hardened layer on ADI and AGI vermicular vary at minimum value about 30 microns, which is on the edge of measurability by microhardness and by available NDT methods, while surface roughness is not significantly better. Almost useless is probably this specific shot peening for samples from AGI with lamellar graphite even at lower pressures there was a significant surface damage (lamellae of graphite at the surface even "come out" of the matrix see metallography). Lamellar graphite by its nature also makes difficult the measurement of the gradient of microhardness, results are therefore not fully reliable. In the case of continuation of experiments could be further quality of the surface affected by the change of the shot (finer steel balls, or the use of glass particles). In the next procedure samples will be grinded in depth to remove all possible decarburization (the results of microhardness can be followed) and then the samples (except GJL) will be shot peened again in the 2nd phase on the selected pressure of 2 bar. After the peening will be performed again the metallography and the gradient of microhardness and in case of decarburized samples even the new diffraction. These samples will then be used as the default for future research of controlled surface hardening of ADI/AGI. 6. CONCLUSION We can say that this initial experiment despite the difficulties mentioned above have verified the possibility of use of the martensitic transformation initiated by plastic deformation during the controlled shot peening of austempered iron. Follow-up research will continue with more extensive tests of peening conditions, will closely investigate the properties of the surface layers and will develop methods of NDT detection /

7 measurement of the depth of these layers, to ensure the comprehensiveness of diagnostics of austempered castings following the original dissertation dealing just with NDT structuroscopy of these materials. ACKNOWLEDGEMENT This paper was created with support of the SGS project Modern trends in material engineering REFERENCES [1] PODRABSKY, T., POSPISILOVA, S.: Structure and properties of graphite cast irons. Study support, VUT Brno, [2] SENBERGER,J.: Austempered ductile iron (ADI) perspective material for Czech foundry. Foundry magazine for the foundry industry, 2003, vol. XLIX, no , ISSN [3] DORAZIL, E., VECHET,S., KOHOUT, J.: The iron with nodular graphite and its blast-furnace variant ADI. Foundry magazine for the foundry industry, 1998, vol. XLVI, no , ISSN [4] ANDRSOVA, Z., VOLESKY, L.: The potential of isothermally hardened iron with vermicular graphite. In COMAT 2012 Conference Proceedings. Plzeň, Tanger, Ostrava, CD-ROM. ISBN [5] ANDRSOVA, Z, SKRBEK, B.: The use of magnetic and ultrasonic structuroscopy for inspection of ADI/AGI castings. In Manufacturing Technology, vol. 12/2012, no. 13. ISSN [6] KRAUS, I., GANEV, N.: Technical applications of diffraction. Textbook, CVUT publishing, Praha [7] HAUK, V.: Structural and Residual Stress Analysis by Nondestructive Methods. In Elsevier, November 1997, ISBN [8] LAWERENZ, M.: Shot peening of ductile iron. In Modern Casting, February 1990, American Foundry Society. ISSN