INFLUENCE OF THE STRUCTURE OF NODULAR CAST IRON ON HARD CHROMIUM PLATING QUALITY

Transcription

1 INFLUENCE OF THE STRUCTURE OF NODULAR CAST IRON ON HARD CHROMIUM PLATING QUALITY Ondřej CHOCHOLATÝ, Antonín KŘÍŽ Department of Material Engineering and Engineering Metallurgy, ZČU Plzeň, Univerzitní 22, Plzeň, Czech Republic Abstract This paper deals with the application of nodular cast iron as the substrate for hard chromium plating. There is a major impact of the structure and especially graphite spherical nodules on the quality of the resultant chromium coating. The presence of spherical graphite on the substrate's surface causes that nodules on the surface of the chromium coating after electroplating. Two different samples of nodular cast iron were compared. The first sample was with nodules after chromium plating and the second one was without. In the metallographic cross section of cast iron coated with chromium with nodules there was found residual austenite in the martensitic structure and ferrite around spherical nodules of graphite. This ferrite probably cause that the machining will not pull the graphite ball out of the surface of substrate. After that the graphite causes creation of nodules on the chromium coating. 1. INTRODUCTION Hard chromium plating is still widely used technology. This surface treatment is used to increase wear resistance, increase corrosion resistance and to improve tribological properties of plated products. Hard chromium layer is usually thicker, than decorative chromium. Thickness is usually from 0.05mm to 0.25mm. Hard chromium can be plated to many kinds of materials (for example iron, steel, cast iron, aluminium, brass, etc.). Some of the materials are electroplated without problem some not. To chrome cast iron is usually not easy. There is a lot of carbon in cast iron and it can cause problems with adhesion of layer and with the quality of surface. The aim of this work is to compare two different nodular cast iron samples as a substrate for hard chromium plating. Both types of nodular cast iron were chrome plated. Plating parameters and conditions are the same for both samples. There was big difference in in the quality of quality of coatings. These coatings are described in Fig 1 and Fig 2. There are a lot of nodules on the surface of the sample 2 and there are not on the surface of the sample 1. These nodules are not acceptable in terms of quality. Because plating parameters were the same it can be expected, that creation of nodules is caused by different structure of individual samples. Objective was to determine the cause of modules creation. Samples were evaluated by optical microscopy confocal microscopy and electron microscopy. Cast iron without nodules on the chromium surface is sample 1 and cast iron with nodules is sample 2. 1

2 Fig 1 Hard chromium coating without nodules Fig 2 Nodules on the chromium layer 2. EXPERIMENT 2.1. Cross section of nodule Fig 3 shows cross section of nodule (sample 2). It is clear that the beginning of the nodule is based on the surface of substrate on the place where is situated sphere of graphite. All studied nodules began in graphite sphere. Sample 1 was without nodules. Fig 3 Cross section of nodule (sample 2) 2.2. Chemical composition and hardness of cast iron Samples were analysed on a spark spectrometer. Results are shown on Table 1. The concentration of C and Si shows that both irons were placed near eutectic point. Contents of element correspond to standards of nodular cast iron. 2

3 , Brno, Czech Republic, EU Tab. 1 C Mn Si P S Cr V Ti Sn Cu Al Sb Sample Sample Hardness of sample 1 is HV5= and hardness of sample 2 is HV5= Both samples can be considered to be the same in terms of hardness Metallographic analysis Both cast irons were evaluated by metallography. Structure of sample 1 is hardened and contains certain amount of residual austenite in addition to martensitic needles Fig 4 and Fig 5. Martensitic structure is also near graphite spheres. Fig 4 Structure of the sample 1 Fig 5 Structure of sample 1 in details Structure of sample 2 is different. Material contains martensitic needles residual austenite and there are ferrite places around graphite spheres in the sample Fig 6 and Fig 7. 3

4 , Brno, Czech Republic, EU Fig 6 Structure of sample 2 Fig 7 The sample 2 details of ferrite There are some differences between sample 1 and 2 resulting from these Figs. There are pieces of residual austenite and martensitic needles are tinny and close to graphite spheres in the sample 1. The structure of sample 2 is martensitic too but contains the austenite as well as ferrite. This ferrite surrounds the graphite Fig Surface analysis In connection with the results of metallography analysis there was investigated sample surface after machining before the chromium was plated. There was used scanning electron microscope and laser confocal microscope for surface analysis. Fig 8 Machined surface of the sample 1 (confocal microscope) There are pieces of graphite presented on the surface of sample 1 Fig 9 but they are uprooted by machining Fig 10. Those pieces of graphite are not protruding on the surface of substrate. 4

5 Fig 9 Machined surface of sample 1 Fig 10 Machined surface of sample 1 in detail graphite is broken out Fig 11 Machined surface of the sample 1 (confocal microscope) There are pieces of graphite presented on the surface of sample 2 Fig 12 and not are ripped out by machining Fig 13. Those pieces of graphite are protruding on the surface of substrate. 5

6 Fig 12 Machined surface of sample 2 Fig 13 Machined surface of sample 2 in detail graphite remains on the surface 3. CONCLUSION It was found that the nodules on the chromium coating deposited on nodular cast iron are due to graphite globules on the surface of the substrate. There are formations of residual austenite in the sample 1. Martensitic needles are very fine and are even close to graphite balls. Soft graphite and hard martensitic subsoil may have the effect that graphite can be jabbed out into the subsurface level or full ruptured of the machined surface. Then on the surface of subsequently deposited chromium layer there are no nodules. Sample 2 has also a hardened structure, which includes, in addition of residual austenite also ferrite. The ferrite surrounds the graphite. This causes the graphite remains on the surface after machining. On the hard chromium layer deposited on this surface there are nodules. The creation of nodules on the chromium layer is affected by the residual ferrite around the globular graphite. The ferrite causes that the graphite is not erased from surface by machining but remains on the surface of cast iron and causes creation of nodules on the chromium. REFERENCES [1] LAUSMANN, G. A., UNRUH, J. N. M., Die galvanische Verchromung, 1. Auflage, Bad Saulgau, Eugen G. Leuze Verlag, 1998, Schriftenreihe Galvanotechnik und Oberflächenbehandlung, ISBN [2] Kříž A., Technická zpráva, Plzeň