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Km, 4, P. Box 44, Tabernas, Spain; i. After the heat treatment, the samples were analyzed in terms of hardness, microstructure performed by scanning electron microscopy , and corrosion resistance. The electrochemical measurements were performed by potentiodynamic and electrochemical impedance spectroscopy in liquids that simulate biological fluids NaCl 0.
Different corrosion behaviors according to the heat treatment type have been observed and a passivation layer has formed on some of the heat-treated samples. The samples, heat-treated by immersion quenching, exhibit a significantly improved pitting corrosion resistance. The heat treatments performed using solar energy applied on stainless steel can lead to good corrosion behavior and can be recommended as unconventional thermal processing of biocompatible materials.
Keywords: stainless steel; solar energy; hyper-hardening treatment; tempering; corrosion resistance 1. Introduction Biomaterials are synthetic materials that can be used to replace parts of a living system or to ensure the functionality of organs in close contact with living tissue, being able to replace functions lost due to a disease, as a support in the process of healing and for the improvement or correction of the functions of some organs. According to L. Kuhn [1], the most widely used class of biomaterials is that of metallic biomaterials, and stainless steels are among the material categories of this class.
This microstructural stability has enabled the use of this alloy for a long time in medical applications, for implant elements screws, nuts, or rods , for the purpose of temporary fixation until the healing of fractures or for the replacement of joints [2,3]. To improve the corrosion behavior of austenitic stainless steels, a number of technological procedures can be applied: electropolishing [3], heat treatment [4—6], surface treatment, or patterning [6,7].
Heat treatments are widely used to obtain mechanical or tribological characteristics adapted to the specific requirements of medical devices [8]. Concentrated solar energy used for heat treatment of metals exhibits increased interest in recent years, but only recently has been dedicated to industrial applications [9—12].
To emphasize the special effects of the solar energy treatments, heat treatments were performed in electric furnaces using the same values of the process parameters. The experiments carried out using samples from the same alloy showed that heat treatments using solar energy have led to reduced friction coefficient values, allowed the dissolution of the embrittling compounds and resulted in a decrease of the corrosion rate, all compared to the samples produced by conventional means.
This behavior may be due to the much higher heating rates obtained using the solar installation, the absence of direct interaction between the alloy and the furnace hearth, and the reduced dimensions of the treated samples [14]. The disadvantages of the solar energy treatment method are related to higher costs expensive installations , geographical positioning they should be located only in areas where solar irradiation is intense , and weather conditions [15]. A common method used to improve the mechanical, chemical, and functional characteristics of biocompatible highly alloyed steels is to modify the standardized chemical composition.
The addition of Mo confers better stability at high temperatures [16], while the simultaneous addition of Co, Cr and Mo leads to increased corrosion resistance at ambient or high temperatures [17—20]. Although a natural passive film forms in situ on the alloy surface, it cannot effectively protect the material, thus the need for further corrosion resistance improvement procedures, especially in the case of implant materials [20,21].
The corrosion behavior in various media is of great importance [21—23], as the release of chemical elements from the implant into the biological medium can lead to problems in the implantation tissues necrosis, local concentrations exceeding acceptable limits, the occurrence of tumors, allergic reactions, etc. In this paper, the results on the influence of the concentrated solar energy used for heat treatments on the characteristics of AISI-modified stainless steels are presented.
The electrochemical characterization of the samples was used to determine their corrosion resistance in NaCl 0. Microstructural changes related to the phase transformations obtained after heat treatments, as well as changes in mechanical characteristics microhardness , consistent with the electrochemical test results, highlight the beneficial effects of solar energy heat treatments, coupled with chemical composition alternations.
Its chemical composition was modified as compared to the standardized commercial grade by the addition of 0. Table 1. Table 2. Heat treatment parameters for modified AISI samples. The main components of the vertical axis solar furnace Figure 1 VSF are: heliostat, concentrator, attenuator, and test table. Figure 1. SF5 vertical axis solar furnace test room. The samples were placed on the test table, under Ar protective atmosphere Figure 2.
A thermocouple TC was placed into a machined hole of the metallic sample and a solar-blind IR camera was used for temperature control during the heating process. Materials , 13, 4 of 15 Figure 2. Sample placed on the heating table. The latches open at the desired temperature, and the sample falls into the quenching medium.
Figure 3. The specific diagram for the heat treatments applied to the AISI modified stainless steel samples. In the heating diagram Figure 3 the meaning of the codes used is as follows: - S red curve : evolution of the specific temperature during the heat treatment quenching followed by tempering.
The tests related to chemical composition, structural evaluation and microhardness tests were conducted at Transilvania University of Brasov Brasov, Romania. Several measurements were performed on each sample, the results were averaged, and the standard deviation was calculated. The electrochemical corrosion tests were conducted at the Laboratory of Biophysics, the Faculty of Medicine, from Transilvania University of Brasov. The measurements were performed for all stainless-steel samples in a conventional electrochemical cell, containing three electrodes: discs of modified AISI stainless steels, as working electrode with an active surface area of 0.
The working electrode was isolated by embedding it in epoxy-acrylic resin. Before electrochemical measurements, the samples were ultrasonically cleaned in distilled water and rinsed with ethanol. The potentiodynamic methods and electrochemical impedance spectroscopy EIS were carried out both in 0.
The electrochemical measurements were repeated three times in each case and then the average values of all corrosion parameters were calculated considering the computed standard deviation values. The potentiodynamic tests were recorded at a 0. The tests were interrupted at certain values of current density, indicated by the abrupt increase plots, where the samples reached the stable pitting regime.
Using the PSTrace 5. Results 3. Microstructure To reveal the microstructure aspects, the samples were mirror polished using abrasive grit paper and powders and then were subjected to etching using aqua regia for 10 seconds.
Figure 4. X-ray diffraction patterns of AISI modified stainless steel samples. For the as-cast sample, the carbides K morphology is represented in the SEM microscopy images Figure 5. The microstructural aspects of heat-treated samples B—E are presented in Figures 6—9. Figure 5. Figure 6. Figure 7. Materials , 13, 7 of 15 Figure 8. Figure 9. Vickers Hardness HV 0. The standard deviation reaches relatively large values, indicating the inhomogeneity of the metallic material after the heat treatments.
Table 3. Average HV0. The analysis of the hardness test results presented in Table 3 and Figure 10 shows that all heat-treated samples, regardless of the process parameter values, reach lower hardness values than those of the untreated as-cast A sample. Electrochemical Evaluation The electrochemical characterization of the samples was done through potentiodynamic studies and electrochemical impedance spectroscopy in NaCl 0.
A tendency of initial experimental growth of current density was observed initially, as can be seen in Figure Figure Polarization curves in NaCl 0. Materials , 13, 9 of 15 Table 4. Corrosion parameters in NaCl 0. Evans diagrams in: a NaCl 0. The potentiodynamic tests on both untreated and heat-treated AISI modified stainless steels in both saline solutions showed similar polarization behaviors, which is typical for the localized corrosion of stainless steel [23].
These tests performed in the two saline solutions simulating biological fluids lead to the conclusion that the Ebd and Epass are related to critical conditions which depend on the chemistry inside the pits and on their geometry.
Materials , 13, 10 of 15 Table 6. Critical electric charge densities, Qc, in NaCl 0. Thus, it can be observed that the smallest values of Qc were obtained for the two samples B and E, proving that these samples are more resistant to the corrosion process. The experimental results obtained from EIS measurements are summarized in Tables 7 and 8. This impedance technique has the advantage of using only very small signals which do not disturb the electrode properties to be measured, and which allows the determination in one experiment of double layer capacitance and polarization resistance [23].
Table 7. The inset presents the electrical equivalent circuit used to obtain the electrical parameters by fitting experimental EIS spectra that can be used to describe the electrical features of the electrochemical interfaces between the samples and the electrolytes. This consists of the solution cell resistance, R1, in series with one or two parallel R—CPE configurations, which are attributed to the electrode AISI stainless steel samples , the passivation layer formed on its surface and the charge transfer process.
R represents the charge transfer resistance of ions through electrochemical interfaces. Materials , 13, 11 of 15 By analyzing the EIS data, the Nyquist plots Figure 13 and the Bode plots Figure 14 , the corrosion mechanism of the system can be identified. Nyquist Impedance spectra in complex plane in: a NaCl 0. In the above paragraph describing the potentiodynamic studies, the existence of a passivation layer was highlighted, which formed on the surface of some samples when electrochemical measurements were performed.
Bode plots for all stainless-steel samples in NaCl 0. The data presentations in Bode plots Figure 14 for NaCl 0. The same capacitive behavior was noted for the low frequency region, with a resistive behavior in the intermediate frequency range. Materials , 13, 12 of 15 4. Discussion The microstructural analysis performed before and after the heat treatments with solar energy revealed changes in terms of the shape of intermetallic precipitates, which separated from the metallic matrix.
The carbides mainly precipitate at the interface of the delta ferrite phase. And its machining and welding is difficult than low carbon steel due to increased content of carbon. High: In high carbon steel the carbon content is in between 0. And it is the challenge for welding and machining this type of steel. Very high:. In very high carbon steel the carbon content is up to 1. Example: SAE in which 1 indicates plain carbon non modified steel and contains 0. Alloy steel: Alloy steel is a type of steel in which one or more elements other than carbon have been intentionally added, to produce a desired physical property or characteristic.
Common elements that are added to make alloy steel are molybdenum, manganese, nickel, silicon, boron, chromium , boron and vanadium. There are two types of alloy steel 1. By lowering the carbon content to 0. High alloy steel is highly corrosion resistant with high reliability, and is used extensively in petrochemical, pharmaceutical, and nuclear power plants, heat exchangers, centrifugal separators, driers, pipelines, couplings, valves, bolts, salt manufacturing, exhaust gas desulfurizers, and semiconductor cleaning equipment.
Last two digits: Last two digits indicate carbon concentration in 0. According to application: According to application steel can be classified into two types: 1. Stainless steel 2. Stainless steel is more resistant to stains , corrosion ,and rust than ordinary steel. It is also called a corrosion resistance steel when the alloy type and grade are not detailed, particularly in the aviation industry.
Stainless steel is commonly used in table cutlery , jewelry , watch bands , watches , handgun model , pistol , storage tanks , tankers ,food processing plant ,surgical instruments as well as in the aviation industry. Designation system of alloy steel: AISI has established three-digit system for the stainless steels: 2XX series — chromium-nickel-manganese austenitic stainless steels 3XX series — chromium-nickel austenitic stainless steels 4XX series — chromium martensitic stainless steels or ferritic stainless steels 5XX series — low chromium martensitic stainless steels Tool and die steels: Tool and die steels are high carbon steels either carbon or alloy possessing high hardness, strength and wear resistance.
With carbon content between 0. Tool steels are heat treatable. In order to increase hardness and wear resistance of tool steels, alloying elements forming hard and stable carbides chromium, tungsten, vanadium, manganese, molybdenum are added to the composition. Tool and die steels are used to shape other metals by cutting, forming , machining and die casting.
Tool and die steel is used to make chisels, forging dies, hummers, drills, cutters, shear blades, drills, razors. Tool and die steels can be classified on their use, mechanical properties ,composition and method of heat treatment.
Tool steels are made to a number of grades for different applications. Choice of grade depends on whether a keen cutting edge is necessary or not, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements or not. The letter means; W- water hardened plain carbon tool steel O- oil hardening cold work alloy steel A- air hardening cold work alloy steel D- diffused hardening cold work alloy steel S- shock resistance low carbon tool steel T- high speed tungsten tool steel M- high speed molybdenum tool steel H- hot work tool steel P- plastic mold tool steel According to de-oxidation practice: De-oxidation of steel is a steel making technological operation, in which concentration of oxygen dissolved in molten steel is reduced to a required level.
On de-oxidation degree base steel can be divided into four groups 1. Rimmed steel 2. Killed steel 3. Semi-killed steel 4. Capped steel Rimmed steel: Rimmed steel is also known as drawing quality steel. Rimmed steels are low carbon steel which are partially deoxidizes or non-oxidized.
Rimmed steels evolve sufficient amount of carbon mono-oxide during solidification. These steel are, therefore ideal for rolling, large number of applications, and is adapted to cold- bending, cold-forming and cold header applications.
A wide variety of steels for deep drawing is made by the rimming process, especially where ease of forming and surface finish are major considerations. This type of steel can be classified on the base of their good surface quality and the amount of blowholes on their ingots.
Their solidification does not cause formation of carbon monoxide CO. Ingots and castings of killed steel have homogeneous structure and no gas porosity blowholes Killed steel, because of greater uniformity in chemical composition and soundness is used for forging, carburizing, heat treatment and other applications.
Symbol K stands for killed steel. Semi - killed steel : semi-killed steel are incompletely deoxidized steels. Structural steels containing 0. Semi-killed steel containing some amount of excess oxygen, which forms carbon monoxide during last stages of solidification. In semi-killed steel, the aim is to produce metal free from surface blowhole and pipe. They are used for general structural applications.
This type of steel is suitable for drawing operation except severe drawing. Capped steel: Capped steels are partially de-oxidized steels. Capped steel has characteristics similar to those of rimmed steels but to a degree intermediate between those of rimmed and semi-killed steels. The good surface and mechanical properties of capped steel make it ideal for the production of sheet and strip products. The four basic forms of deoxidized steel in descending order of oxygen removal are killed, semi-killed, capped, and rimmed steels.
According to manufacturing methods: Steel is produced from pig iron by processes which involves reducing the amounts of carbon, silicon and phosphorous. There are three main steel manufacturing processes as under: 1. Electric-arc furnace 3. This rotation is necessary for charging raw materials and fluxes, sampling the melt and pouring the steel and the slag out of the furnace.
The Basic Oxygen converter is equipped with the water cooled oxygen lance. The basic oxygen converter uses no additional fuel. The pig iron impurities carbon, silicon, manganese and phosphorous serve as fuel.
The steel making process in the oxygen converter consists of charging steel scrap, pouring liquid pig iron into the furnace, charging fluxes, oxygen blowing, sampling and temperature measurement, tapping the steel to a ladle, de-slagging. The iron impurities oxidize, evolving heat, necessary for the process. The forming oxides and sulfur are absorbed by the slag. The oxygen converter has a capacity up to t and production cycle of about 40 min. Electric-arc furnace : Structure of electric-arc furnace: The electric-arc furnace employs three vertical graphite electrodes, one for producing arcs, 2nd is for striking on to the charge and 3rd one is for heating it to the required temperature.
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