Investigation and characterization of coating and carburizing AISI 1011 steel

In this research work, the effect of coating layer thickness of nickel, which electrodeposited on low carbon steel AISI (1011), on mechanical properties has been studied. It also studied the mechanical properties of the carburized alloy at 950˚C at the time of carburizing five hours. It was found that fatigue strength for this alloy increased as thickness of the nickel-coated layer increased. Also, the carburized specimen has a fatigue strength higher than the untreated metal. The metal that was treated with the carburizing process and nickel plating for 15 minutes has the best fatigue strength. The fatigue strength improvement against non-plated specimens was between 3.93% and 16.87 % . Furthermore, hardness of metal increased for specimens treated with electrodeposition, and it became 219.7 Hv for metal coated with nickel at 15 minutes and had a coated layer of 20.42 µm. The hardness value became 431.65 Hv for carburized alloy treated with plating for 15 minutes. However, there was an improvement in the surface roughness due to electroplating.


Introduction
Steel is a very important industrial material, as it has many more applications than any other engineering material [1]. Steel with carbon content less than 0.25 is termed as low carbon steel, while that with carbon content between 0.25% and 0.65% is known as medium carbon steel. If the content of carbon is high and ranges from 0.65% to 1.5%, then the steel is termed as high carbon steel [2]. Low carbon steel has high ductility and moderate strength and is used for its extensive fabrication properties for structural purposes, in buildings, bridges, ships, and cars [3]. Fatigue is the permanent, localized, and progressive structural change that occurs in metals and alloys subjected to fluctuating or repeated strains at values of stresses less than the tensile strength of metal [4]. Fatigue strength is an important mechanical property. However, the failure of fatigue starts on the metal surface [5]. Fatigue failure has four different stages: crack initiation, crack growth, crack propagation, and final rupture [6]. Most surface treatments produce compressive stresses in the metal surface, which reduce the probability of crack initiation and its expansion at the interface between the surface and core, thus increasing resistance to fatigue [7]. Carburization is an important and widely used process for surface hardening. This process I performed with the addition of carbon to the surface of the metal (low carbon steel) at a temperature between 850 and 950°C [8]. The nickel electroplating can be used for three purposes: decorative, functional, and electroforming. However, the appearance and other properties of the elecrodeposited nickel make this type of plating suitable for many applications by controlling the composition and the operating parameters of the plating solution [9].

Experimental Methods
A low carbon steel AISI (1011) has been used. The chemical analysis of this alloy was performed by the Thermo ARL 3460 optical emission spectrometer. The results can be summarized in the table below. This chemical analysis was carried out at 21 ºC and moisture rate 44%. Fatigue specimens were prepared according to DIN, as illustrated in Figure (1). To get suitable and accurate dimensions for the fatigue specimens according to the standard dimensions of (HSM20 rotating bending fatigue machine), specimens were manufactured using the conventional lathe machine (Harrison 600, M350, EW700). Stress relief process was carried out for all specimens at 200 ºC for three hours by using an eclectic furnace.  samples were treated by using the pack carburization process as shown in Figure (2). Samples packed were with carburizing compounds in a metal box, which was sealed with clay and then placed in an electric furnace, where it was heated to 950°C and held for five hours. However, samples were quenched in oil. Table (2) shows the carburizing components used in the carburizing process.  Figure 2: Carburizing box with fatigue specimens and carburizing components Surface preparation of metals for electroplating was done in order to give good bonding of coating to the substrate and obtained finished parts. Surface preparation process consists of the following steps: 1-Alkaline cleaning: To remove fats from the outer surface.Sodium hydroxide is the best alkali in cleaners, so samples were immersied in hot solution of 75 gm/l NaOH and held for 5 minutes at 50°C.

2-Rinsing with water.
3-Acid cleaning: To immerse samples in dilute hydrochloric acid bath holding for two minute at room temperature, in order to remove oxides and rust from the surface.

4-Rinsing in water.
After the surface preparation process for coating, samples were coated with nickel by electrodeposition . The coating with nickel was done for three groups of samples with different coating times. Group one, specimens of the base metal was subjected to electroless nickel plating for five minutes. while ; the electroless nickel plating performed to the second group of specimens for ten minutes. However, group three consist of metal specimens subjected to electroless nickel plating for fifteen minutes. The nickel electroplating was done at 55 to 60 °C, current 7 A, voltage 3 volt, 4.5_5 pH, and the electroless nickel bath contained distilled water, 300 g/L of NiSO4,30 g/L of NaCL, 40 g/L of H3-BO3, and polishing materials. After the nickel electroplating, the specimens were immersed in sodium dichromate composite in order to fixing color and prevent rust. After finishing the plating process, the specimens were dried. Ten samples of the selected alloy were manufactured as fatigue test specimens for each test except one sample for the purpose of the tensile test, as shown in Table (2). Figure (3) shows number of specimens for each group that is used for fatigue test.

Fatigue Test
The fatigue test machine of type (HSM20) rotating bending fatigue machine is used to do all fatigue tests, with the constant and variable amplitudes, as illustrated in Figure (4). The samples are exposed to an applied load from the right side of the perpendicular to the axis of the samples, developing a bending moment. The surface of the samples is under tension and compression stresses when it rotates. Fatigue testing machine components are illustrated in Figure(4). Value of the load (P) is measured by Newton (N) and applied to a specimen for known value of stress (σ) is measured by (MPa) and can be obtained from applying relation below: σ = (32 × L P) / (π × d³) where P = force in Newton. L = arm of the force which is equal to (125) mm. d = diameter of the specimen in (mm). The constant fatigue test was accomplished in the laboratory air (the relative humidity was 25_30%) at the room temperature on (HSM 20) rotary fatigue bending machine as illustrated in figure (4), with the stress ratio of R= -1. Cycle frequency was 50 Hz and rotating speed used is 3000 cycle/min.

Microscopic examination results
From Figure (5), it can be noticed that the microstructure of low carbon steel AISI (1011) after being subjected to stress relief process at 200 ºC for three hours using an eclectic furnace consists of granules dark region (pearlite) and other light region (ferrite).

Results of XRD test
XRD analysis was carried out on the tested samples, which are untreated (as received) and treated samples (nickel coating, carburizing), to examine the phases that exist in the material before and after surface treatment. Figure 6 illustrates XRD pattern for all the cases.

SEM test results
The coating thickness and coating solution temperature between (55-60º C) is influenced by many factors like (Temperature, plating time and current density). From table (5) it can be seen that the plating thickness increases with increasing time and this is due to increasing the rate of deposition as current density increased.

Microhardness results
Vickers hardness method was performed for specimens , and it has been found that the hardness values increased for all surface treated specimens. The hardness values for specimens treated with nickel plating for (5, 10, 15 min) were higher than those of untreated and this is due to the formation of nickel phosphide precipitates [10]. Furtheremore, hardness of the treated specimens by carburizing process (at 950 ˚C for 5hr) have higher values than plating specimens by nickel and those of untreated this is due to the formation of carbides, however, carbon diffusion (diffusivity) during the carburizing process increased with time as the relation: Case depth = k√t where K is constant and t is time So as time increase the diffusivity of carbon increased [11]. The higher carburizing temperature and the higher soaking time will result a large carbide layer and the harder case becomes, while core still tough.   Figure 9: values of microhardness for all groups

Surface roughness results
The surface roughness decreased for samples that treated with nickel coating, As plating time increased, thickness of nickel layer increased and it will be 20.42 μm within 15 minutes time of plating which leads to surface finer roughness. While, after the carburization process the surface roughness of samples increased. So the surface roughness of carburised specimens became (1.98 µm) at carburising temperature 950˚C and time of 5 hours.

Tensile test results
It can be seen from figure (11) the stress-strain diagram for low carbon steel AISI 1011. This test was carried out for this alloy after has subjected to stress relief process. It can be remarked, that the ultimate tensile strength was (550) MPa and the total percentage elongation became 30 mm.

Rotating Bending Fatigue Tests
S-N curves for all groups were obtained from the fatigue test as shown in figure (12). It can be seen that the fatigue strength for low carbon steel AISI (1011) increased for specimens subjected to carburising process as compared with the same metal which don't treated by this process. This improvement of fatigue limit was due to the compressive residual stresses and carbides formation at the surface of steel which may be stopping and blocking the crack. From S-N curves for specimens plating with nickel by electrodeposition process at room temperature, it can be seen that the effects of thickness of coating with nickel after coating process on the fatigue strength and value of fatigue limit  11 Furthermore, the characteristics of deposited film itself were examined. As the electroless nickel plating is performed on the metal, their fatigue strength rather increases, as compared with uncoated metal. The fatigue limit for this alloy coated with Ni at 5 minutes became ( 82.89)Mpa. While the the fatigue limit for this alloy coated with Ni at 15 minutes became (93.21) Mpa. And for untreated alloy was (79.75 ) Mpa. So It can be seen that fatigue strength improvement against non-plated specimens was between (3.93-16.87) % .
The fatigue limit for carburisied alloy at 950˚C for five hours was (98.12) MPa, while the fatigue limit for carburisied alloy at 950˚C for five hours and plating with nickel for time of plating 15 minutes became (109.30) MPa. It can be remarked that the electroless deposited nickel film shows a good adhesion to the substrate. The fatigue damage of plated steels prevented by deposited film. Consequently, the nucleation of fatigue crack is delayed, and there is an improvement in fatigue strength. In electroplating process, the micro-hardness of deposited film increases, beside that there is an structural changes. This improvement is due to the precipitation hardening of Ni3P. Furthermore, the tensile residual stress in deposited film is reduced. So the improvement in fatigue limit may be due to compressive residual stresses and largest ductility/ toughness exist in the electroless nickel plating which delayed the crack propagation. The surface fractures could be recognize by using scanning electron microscope TESCAN, VEGA 3 LMU device. Figure (13 ) shows surface fracture. Which occurred in the specimens after sufficient cycles of the bending load.