FATIGUE BEHAVIOR OF NI-ZN COMPOSITE COATING ON EN8 STEEL
BY PULSE ELECTROPLATING
M. Senthil Kumar
1, S. Ragunathan
2and M. Suresh
11
Department of Mechanical Engineering, Sona College of Technology, Salem, India 1
Department of Mechanical Engineering, AVS Engineering College, Salem, India E-Mail: [email protected]
ABSTRACT
EN8 steel has an important application on production of rotary elements like transmission shafts, crankshafts and gears. The base metal EN8 was exposed to Nitriding in Cyanide salt bath with 5600C temperature. Ni-Zn coating was done on both Nitrided and Non-Nitrided EN8 specimens by pulse electrodeposition. The results of Tensile, Hardness and microscopic tests were studied. Fatigue behavior of each category was studied through Cantilever type Rotary Bending Fatigue machine. Results showed that coating on nitrided specimens produces high fatigue resistance than coating on non-nitrided specimens. Failure mechanism was investigated through SEM analysis.
Keywords: nitriding, Ni-Zn coating, pulse plating, tensile, fatigue, SEM and microhardness.
INTRODUCTION
Most of the engineering components in automobiles such as Crankshafts, gears are manufactured by EN8 steel which is generally subjected to wear and corrosion due to cyclic loading. The enhancement of surface properties of EN8 can be obtained by Hard metallic coating.
Fatigue failure of crankshafts in automobiles is led by Dynamic Loading on the machine parts. [1] Fatigue failure are characterized in three stages say, i) Crack Initiation (ii) Crack Propagation & iii) Fracture. Initiation and propagation of fatigue failure in crankshaft is started from critical locations in general [2].
Uniform thickness deposition of fine grained particles which provides the lower porosity with reduced stress and improvised adhesion property can be produced by Pulse electroplating [3]. The uniform distribution of particle with higher amounts of particle incorporation is possible attain through Pulse Current plating than DC Plating. Results say that high mechanical properties can be achieved through Pulse Current plating than DC plating [4].
The Fatigue resistance of materials can be improved through the process of Heat treatment, Coating & Shot peening. The crack initiation and propagation can further be resisted by combining any two or more methods [5].
Wear resistant hard chromium plating deposits lead to the fatigue strength decrement in aeronautical steel [6].
The under layers of electroless nickel plating are responsible for the increment of fatigue strength of steel. The electroplating induces the compressive residual stress [7]. A thin film Zinc coating was provided on steel using electro deposition method that increased mechanical properties [8]
The Zinc coating offers the low cost and fine protection of iron and steel.
The fine protection of iron and steel components with lower price can be offered by Zinc coatings. From
pure metal baths plating, it is impossible to get some required change in the electrodeposits which is the main reason for plating by alloy bath. On mild steel, the electro-deposition of nano crystalline zinc-nickel coatings were carried those substrates from an electrolyte containing zinc bromide, nickel chloride and boric acid to get good mechanical properties. [9]
Nickel/nano-Al2O3 composite coatings are produced by the pulse electro deposition method. An AISI 1018 mild steel specimen electroplated were investigated in a Watt’s type bath. The investigations are on the influence of pulse parameters, say, voltage and duty cycle, current density on the microstructure, hardness, pulse frequency and wear resistance. The effect of pulse parameters were considered to maximize the specimen hardness [10].
The fatigue life was increased on the material by the diffusion coating. Initiation of surface crack is more homogeneous. Nucleation period of the principal crack in the specimen is prolonged in the coated material. Principal crack propagation path is coincided both in coated and in uncoated superalloy with the interdendritic regions [11].
The investigation on the effect of mechanical properties were carried out tensile and fatigue tests of Ni-Co thin film on dog bone specimen[12].
Results and analyses showed that coating effects were not unique, but they depended on the specific technology and parameters.
Results and analyses showed that coating effects were not unique, but they depended on the specific technology and parameters. Coating technologies with nanostructured particles is a method to promising increase of wear resistance and contact fatigue life of machinery parts.[13].
Different types of coating material have experimentally investigated and nickel zinc based coating had improved in fatigue strength with the optimized input parameters. The surface properties have been studied using metallurgical microscope [15].
Nickel zinc coating were carried out using chloride bath by electroplating process, the effect of parameter had discussed and also provided optimized input parameters. Optimization was done with the help of Taguchi method. [16].
Failure reasons were identified in this paper and also by using software fatigue analysis is carried out for coated and uncoated crankshaft; coated crankshaft had higher fatigue strength. Microscopic behavior also investigated. [17].
EXPERIMENTAL PROCEDURE
Materials and Mechanical Properties
EN8 steel is mostly used in aerospace and automobiles industries, the chemical composition of EN8 steel measure from Optical Emission Spectroscope (OES) is shown in the Table-1.
Table-1. Chemical composition of EN-8 steel.
The mechanical properties of EN8 steel are shown in tab. 2.
Table-2. Mechanical properties of EN8 steel.
Fatigue Specimens
The test specimens used for the rotating fatigue tests shown in Figure-1, by using CNC turning lathe specimen was prepared.
Figure-1. Fatigue testing specimen.
Four groups of fatigue specimens were prepared to obtain S-N curve from rotating bending fatigue tests.
B.1. Base metal
Figure-2. Specimen prepared for coating.
32 specimens out of 30 from each type of coating are used for fatigue test and the remaining 2 specimens are used for tensile test.
15 smooth specimen of base metal.
15 smooth specimens of base metal with Nickel-Zinc Electroplating.
B.2. Nitrided Specimen
Figure-3. Nitrided specimen for coating.
32 smooth specimens of base metal with nitriding were selected for electroplating.
B.3 Pre-treatment for coating
The steel specimens were undergone three pre-treatment processes before coating.
Mechanical polishing,
Ultrasonic cleaning with acetone, Anodizing (reverse plating).
Electrode Position
Figure-4. Pulse plating set up.
Polished EN8 steel rod specimen was a cathode and a high purity Nickel act as an anode (Ni plate was a soluble electrode). Chloride electrolyte bath has selected to get high mechanical properties. Coating processes was carried out by Dynatronix pulse rectifier.
Table-3. Pulse parameter values.
Electrolytic bath compositions for different coating materials are shown in Table-5.
Table-4. Electrolytic bath composition.
RESULTS AND DISCUSSION
Tensile testing
Specimen is fitted in the Universal Testing Machine (UTM) and the hydraulic load was applied gradually and yielding load and ultimate load are measured.
Graph-1. UTM result of base metal.
Graph-2. UTM result of coated base metal.
Graph-4. UTM result of coated nitrided base metal.
Table-5. Tensile testing results.
Micro Hardness
The hardness of specimens after coating was measured with a Vickers micro hardness testing machine using a diamond indenter on the coated surface by applying 500g load in 15 seconds.
The results of micro hardness of different types of coating are represented in Table-6.
Table-6. Micro hardness test results.
Graph-5. Micro hardness of all categories.
Microscopic Results
The coated specimen was placed on the Metallurgical Microscope (METSCOPE) and the suitable magnifications were adjustable.
Figure-5. Microscopic image of Ni-Zn coated specimen with 500X.
Coating was uniform throughout the coating surface; it was obtained from the metallurgical microscopic image.
Fatigue Testing
One end of the Specimen was inserted into the collet and another end was pushed into the load cell. The load value was given to the load cell and the corresponding number of cycles of failure was noted from the counter.
Figure-6. Fatigue testing machine.
Figure-6 shows the cantilever type fatigue testing machine.
D.1.Fatigue testing calculation L=130mm
Mb=P*L (load should be in Newton) Z=5.0265E-8 (D=8mm)
Bending stress= Mb/Z
Stress and number of cycles to failure are listed in table 7 and table 8 for the base metal and base metal with Nickel-Zinc coating.
Table-7. S-N values for base metal.
Table-8. S-N values for base metal with Ni-Zn coating.
The S-N curves for the rotary bending fatigue tests for the base metal and different types of coating on steel are represented in Graph-6.
Graph-6. S-N curve for base metal and Ni-Zn coating.
The endurance limit of base metal and Nickel-Zinc coating are measured using the S-N curve and it is listed in table
Stress and number of cycles to failure are listed in following table for the Nitrided base metal and Nitrided base metal with Nickel-Zinc coating.
Table-10. S-N values for Nitrided base metal.
SEM Analysis
Figure-7. Fracture surface of base metal from a bending fatigue specimen.
Figure-8. Crack-initiation sites around the specimen periphery.
Figure-9. Fracture surface from a bending fatigue specimen electroplated Ni-Zn coated base metal.
Figure-10.Fracture surface from a bending fatigue specimen electroplated Ni-Zn coated base metal.
Figure-12. Fracture surface from a bending fatigue specimen electroplated Nitrided base metal.
The Nickel-Zinc electroplating layer is responsible for the increase in fatigue strength of EN8 Steel. This compressive residual stress was induced which probably arrested and also delayed the micro crack propagation from the coating external layer to base metal.
Radial micro cracks are along in the thickness direction of coating, which did not allow the micro cracks to grow in direction to the substrate.
Figure-7 shows the stable and unstable cracks of the failed specimen of steel.
Figure-8 shows the general fracture surface with several crack-initiation sites around the specimen periphery.
Figure-9 & 10 shows a fatigue crack nucleation Site at the coating/substrate interface. Fatigue crack nucleation occurs at the interface with propagation occurring inside the substrate. The cracks initiation from the surface is less compare compared with base metal due to Ni-Zn electroplating.
Figure-10 shows fracture surfaces from bending fatigue Ni-Zn electroplated specimens. It is possible to observe the micro cracks density distributed along thickness.
Figure-11 & 12 shows the Fracture surface from a bending fatigue specimen electroplated Nitrided base metal
CONCLUSIONS
Experimental fatigue testing result shows coated specimens has more fatigue strength compared to the uncoated specimens, either base metal or nitrided base metal. The endurance limit was higher for the coated specimens.
Ni-Zn coated specimen has higher Tensile strength and micro hardness by comparing base metal specimens.
Coating is uniform and homogeneous throughout the surface of the specimens; it was concluded based on Metallurgical microscopic results.
The Nickel-Zinc electroplating layer is responsible for the increase in fatigue strength of EN8 Steel. This compressive residual stresses were induced which probably arrested and also delayed the microcrack propagation.
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