INVESTIGATION CORROSION AND MECHANICAL PROPERTIES OF CARBURIZED LOW CARBON STEEL

In this research, Taguchi method is used (S/N) ratio for measuring the variations in experimental design. Taguchi designs used in converting the multi-performance problem into a singleperformance problem for experiments which will are in building (Taguchi (L27) orthogonal array) for carburization operation. The main variables that had a great effect on carburizing operation are Carburization temperature ( o C), carburization time (hrs.) and tempering temperature ( o C). It was focused also on calculating the amount of carbon penetration, the value of hardness, wear rate, corrosion rates and optimal values obtained during the optimization by (Taguchi) approach method for multiple parameters. In this study, the carburization process was done in temperature between (850 to 950 oC) for (2 to 6 hrs.). Quenching process was done for the specimens after heat treatments in furnace chamber by different quench solution (water, salt and polyvinyl alcohol). Taguchi design used to achieve maximum hardness and depth penetration, Minimum wear and corrosion rates.


Introduction
Mild steel is considered the most form of steel that used because its price relatively low. It is widely used for many industrial applications; surface hardness can be increased through carburizing [1,2]. Carburizing process is heat treatment in which carbon can be dissolve in the surface layers of mild steel part at austenite temperature, carburizing process is followed by quenching and tempering in order to form a  [3,4]. The mechanical properties of mild steel samples were found to be strongly influenced by the process of carburization treatment [5]. Formation of martensite phase leads to increases the micro hardness with increasing cooling rate and carbon content [6]. Carburized process has been improved properties including surface hardness, wear resistance, and corrosion resistance [7].
Belete Kefarge Azmite et.al [8] studies the mechanical properties of mild steel samples for achieving better performance. The samples subjected to pack carburization process at (850 °C, 900 °C and 950 °C). The observation was showed that the mechanical properties strongly influenced by the carburization process and concludes that the properties like hardness and abrasive wear resistance are improved. Jitendra Prasad et.al [9] the investigation on the mechanical properties of (mild steel) was carburizing at temperature range (890, 920 and 950 0 C). It was found that the heat treatment process improved the hardness and wears resistance for hand tool. Paul Aondona Ihom [10] in this study, the experiment was carried out using a muffle furnace at (900 °C) for (8 hrs.). The result showed that (60 wt. % charcoal / 40 wt. % cow bone) had the best result. Fatai Olufemi Aramide et. al. [11] the investagation on studied the effects of carburized temperature and time on the mechanical charcteristic of mild steel. It was carburized at temperature (850, 900 and 950 ºC), for (15 and 30) minutes and quenched in oil. It was found that the optimum conditions of mechanical properties at temperature (900 ºC) followed by quenching in oil and (tempering at 550 ºC). Hesham elzanaty [12] investigation on effect of carburizing process on mild steels, then the samples subjected for kinds of testing such as abrasive wear, hardness, tensile and toughness.
The results appear that the process of carburization improves the mechanical properties .Amarishkumar J. Patel and Sunilkumar N. Chaudhari [13] investigation focus on carburized mild steel samples at (900 ᵒC) temperature and socking time up to (120 min). Then quenching in oil bath and tempered at different temperature like (200 ᵒC, 250 ᵒC and 300 ᵒC) for ninety minutes. The results appear improve the wear, hardness, tensile strength value and compromise in toughness at higher tempering process in mild steel material.
Mohammed abdulraoof [14] in this study, the surface for mild steel was carburized then quenched in oil solution. The microstructure, hardness and wear resistance properties have been studied at different carburized temperatures; (850, 900, and 950 ˚C) with constant time (2 hr). The result showed that at carburizing temperature (900 ˚C and 950 ᵒC) is higher than at (850 ᵒC). K. palaniredja et.al [15] Optimize the surface hardness and case depth of steel materials. Which is consist of gas carburizing process at temperature range between (870 to 930 ᵒ C) .The Taguchi method is design for the optimization process for nine number of experiment, the results appears optimal parameter of carburized part to improve properties. R. Singh [16] studies the optimization process for parameters in order to produce best characteristic in carburized low carbon steel. Taguchi method was used for optimization parameters. The results showed that the low carbon steel was carburized under the different temperature range and investigates suitable temperature at which the mild steel can gave the best results for the mechanical properties. Sorin and Adriana [17] focused on determine hardness of steels in different moments of deep carburizing and demonstrate an improvement in outcomes deep carburizing. From this study observed that the decrease in residual austenite content that located on the surface of the piece after annealing process, which is leading to increase hardness in the area.

Preparation of Sample
The chemical composition for the mild steel sample is explain in Table 1 (According to ASTM/E/415-14) and also the mild steel sample with diameter and length is( 10 mm 30 mm respectively). Carburizing sample can be used in several applications such as gears, ball bearings, railway wheels.so that to successful carburization treatment, at first the sample is cleaned with alcohols solution (CH 3 COCH 3 ) then it will be ready to start carburization steps.

Carburization Process
Which is consisting that the sample embedded in carburization box that contain graphite within activator (BaCO 3 ). The conditions under which pack carburizing experiments have conducted are given in Table 2. Then the box sealed with clay to prevent reaction of air with this mixture, generally the sample quench with different solution (water solution, salt solution and polymer solution) and the specification of these solutions is generally shown in Table 3.

Microstructure examination
Sample surface needed to prepare for microstructure examination this can be done by grinding and polishing using (silicon carbides (SiC) paper ) (320, 400, 600, 800, 1000 and 1200) followed by etching process by using Nital (2% nitric acid & 98% alcohol), then the specimen examined with microscope and calculate amount of depth penetration that shown in Figure 6. By calculating average amount of carbon penetration in different area from outer surface was done by the same device also.

Microhardness test
Micro-hardness instrument was used of a diamond Vickers pyramid. Which it can be produce a square impress and also the micro-hardness determine by the size of the impress diagonal by the used of optical microscopy so that the hardness (H) has defined as the ratio of the indentation force to the area of the impress after unloading. And also is the angle of the pyramid sides For a Vickers pyramid ∂ equal 136 o So that (Hv) will be VHN = (4) Where p = amount of Applied load (kg) D = represent the length of the diagonals impression

Corrosion rate test
The majority of metallographic specimen mounting is done by mixing the resin with hander in order to form mounting compound. Then the specimen is place in compound (which is considered self-curing compound) and let it to dry. This specimen is place in container full with solution (Nacl+Water), the salt (Nacl) concentration in water is about (0.6M).
The corrosion rate (C.R.) was convenient and customary to express corrosion rate as (mille inches per year-mpy). This unit gives an indication of penetration. Dividing the above equation by the electrode area and the density gives: Where: W = weight loss in milligrams D = metal density in (g /cm 3 ). A = area of sample in (cm 2 ). T = times in hours,

Results and Discussion
The result of tests for all samples is shown in Table 4. The S/N ratios for case depth, micro hardness, wear rate, corrosion rate have been calculated from experimental values in Table 4. The S/N ratio corresponding to each experimental run was given in Table 5. The main effects plot for S/N ratios is shown in Figure 1. The optimal conditions of these control factors was easily determining from graphs. The response graph was showed the change of the S/N ratio for various control factor levels. The best case depth value was at the higher S/N values in the response graphs. It will be seen that the initial optimumal condition for the tested specimens becomes (A2B2C2D1E1F1) for main control factors. The carburizing time is the important factor that affecting on the wear rate i.e. the minimum value of wear rate. Activator (%) has a lower effect. While the carburizing temperature shows the lowest effect among those factors.
For (S/N) ratios suggested that those levels of variables would minimized the wear rate, also were robust against variability due to noises as shown in Figure 2. The optimum conditions for wear rate at (A1B1C1D2E2F2). Since "smaller is the better" was selected for corrosion rate, the smallest values were depended to calculate the optimal combination of carburization temperature, carburization time, activator (%), tempering temperature, tempering time and quenching media. Therefore, the optimum combinations of corrosion resistance were determined as A1B1C1D2E3F2. The calculated optimal values were proposed for (27) trials is shown in Figure 3. The graphical representations of factors effect at different levels are shown in Figure 4. The optimum parameter level is the level corresponding to maximum average S/N ratio for a control factor. The predicted value for microhardness at the optimum condition is (A2B2C2D1E1F1).

Conclusions
 The optimum parameters level predicted in (S/N) optimization for maximum values of microhardness and case depth and for minimum value of wear rate and corrosion rate are A1B1C1D2E3F2, A1B1C1D2E2F2, A2B2C2D1E3F1 and A1B1C1D2E3F2 respectively.