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УДК 669.15-194.56:621.793
THE EFFECT OF PROCESS CONDITIONS IN HEAT-ASSISTED BORONIZING TREATMENT ON THE TENSILE AND BENDING STRENGTH CHARACTERISTICS OF THE AISI-304 AUSTENITIC STAINLESS STEEL
© 2015 г. Ali Gunen*, Bülent Kurt**, Ilyas Somunkiran***, Erdo gan Kanca****, and Nuri Orhan***
*Mustafa Kemal University, Faculty of Technology, Materials Engineering Department, Hatay, Turkey **Nevsehir University, Faculty of Engineering and Architecture, Materials Engineering Department, Nevsehir, Turkey ***Firat University, Faculty of Technology, Materials Engineering Department, Elazig, Turkey ****Mustafa Kemal University, Faculty of Engineering, Mechanical Engineering Department, Hatay, Turkey e-mail: aligunen2013@gmail.com Поступила в редакцию 10.04.2014 г.; в окончательном варианте — 05.08.2014 г.
In this study, AISI 304 austenitic stainless steel surface was boronized with nanoboron and ekabor-III powders at 950 and 1000°C for 2 and 4 hours period by solid-state box boronizing method. Then, behaviors of the boronized specimen in the microstructure, three-point bending, and tensile strength characteristics were investigated. As a result of the boriding process, the boride layer thickness in the range of 23—67 p.m and micro-hardness value in the range of1020—2200 HVhave been obtained according to the increase in processing time and temperature and to the particle size of the boron source (0.i). The coating layer on boronized specimens did not exhibit any sign of reaction caused by the tensile strength applied until the yield point was in both tests. Although the particle size of the boron agents was more effective on the boronized specimen's bending and tensile strength behaviors, it was observed that processing temperature and its duration are effective as well.
Keywords: nanoboron, ekabor, boride layer, tensile strength, bending strength. DOI: 10.7868/S0015323015090028
1. INTRODUCTION
Materials utilized in machinery, manufacturing, automotive industry, and construction are exposed to wearing, corrosion, static and dynamic loadings [1—3]. Therefore, it is vitally important to select appropriate material for machinery components. There are several factors that are considered during selection of suitable material such as stresses exposed by the element, working conditions, work environment, manufacturing method, and convenience to the required heat treatment. As well as suitable material selection, applied heat treatments are important criteria to extend the lifetime of the materials used. Considered heat treatment is supposed to develop internal structure of the material; to give appropriate characterization of the material surface; and to increase mechanical specifications of the material so that expected performance from the material can be achieved [4, 5].
Production of austenitic stainless steels constitutes primary portion of overall stainless production; and they are usually preferred for the applications in the
corrosive environments. These steels exhibit prefect ductility, toughness, and formability even at low temperatures. However, they have limited ability to be used in tribological applications because of their low surface hardness and low loading capacity. To decrease these limitations and to develop their surface characteristics, there are several methods applied in industry. The most convenient and the most economical ways for surface improvement are the surface hardening applications. These processes provide harder, strong, and corrosion resistant material surface and tough and energy absorbing inner structure. One of the prominent surface hardening processes is boriding applied extensively in industry [6—8].
Boriding or boronizing, is a thermochemical surface treatment process in which boron atoms, due to their small size and mobility, are diffuse into the interstitial spaces of the base atoms to produce hard layers at the surface [9]. This treatment can be applied to a wide range of materials such as ferrous metals, non-ferrous metals, and cermets [10]. The temperature of
Table 1. Chemical composition of the AISI 304 austenitic stainless steel used in the experiments
Chemical composition (wt %)
C Ni Cr Mn P S Si Cu Mo N
AISI 304 0.058 8.07 18.17 1.0 0.032 0.0005 0.41 0.24 0.1 0.043
thermal diffusion treatment is usually within the range of 973—1273 K [11]. Boriding can be performed in various boron mediums that allow application of different boriding techniques, such as molten salt boriding [12—14], paste boriding [15], and pack boriding [16]. The present study employs pack boriding, also called as the powder-pack-boriding, the method considered more versatile and lesser expensive than other boriding techniques.
Boron atoms can be dissolved in iron interstitially; and can react with it to form FeB and Fe2B intermetal-lic compounds. Depending on the potential of medium and chemical composition of base materials, a single or a duplex layer can be formed [8]. For industrial applications, the formation of Fe2B mono-phase layer is more desirable than FeB poly-phase layer at the surface while the Fe2B layer occurs underneath it because FeB has a very brittle structure with its hardness level around 2300 HV compared to hardness of Fe2B varying in the range of1500-1700 HV [17, 18]. The boride layers form a smooth structure in stainless steels, which results in the appearance of mono-phase or poly-phase iron borides. However, the occurrence of polyphase iron borides tends to make the components harder and more brittle in nature, which prohibits their use under impact and fatigue service conditions [8, 16, 19].
Boronized steels are characterized by their high surface hardness and high wear resistance. One of the most significant properties of the boride layer is that it can even maintain its high harness value in the temperature range of 900—1000°C. Thus, it can exhibit better resistance against wearing and corrosion without losing its tribological characteristics even at high temperatures [20]. There are extensive application areas for steels boronized with boron powder in industry because of their high resistance against wearing and corrosion. Accordingly, great majority of the researches have been focused on hardness and wearing characteristics. Although boronized austenitic stainless steel is the one applied most extensively in industry among the stainless steels varieties, there have not been found any studies in the relevant literature regarding their bending strength and there are only a few studies that were found about the tensile strength of steels subjected to surface treatment with boron.
Based on the most recent literature, Mathew and Rajendrakumar [3] showed that processing temperature was the most influential control factor that affects the tensile strength properties of boronized and boro-
carburized AISI 1015 steels. What concerns the tensile properties of boronized martensitic chromium-nickel stainless steels, they were studied by Tian et al. [21]. The study showed that the tensile properties of N80 steel can be enhanced by fan cooling with a graphite rod under the pack boriding method.
The purpose of this study is to investigate the mechanical behaviors and the effect of process conditions in heat-assisted boronizing treatment on the tensile and the bending strength characteristics of the AISI 304 austenitic stainless steel.
2. EXPERIMENTAL STUDIES 2.1. Material and Method
In this study, bending and tensile strength experiment were prepared according to ASTM B528-83a and TS EN ISO 6892 standards respectively from the AISI 304 austenitic stainless steel sheet with 3 mm thickness. The chemical composition of the AISI 304 austenitic stainless steel used in this study is presented in Table 1. As boriding agents, 99.7% pure, 10—50 nm sized elemental nanoboron and ekabor-III powders in sizes smaller than 1400 ^m were used.
Specimens were cut in the appropriate dimensions from the boronized parts and then were cold molded for metallographic investigation. Surfaces of the specimens were polished with 1200 grade sand paper finally. After that process, specimens were polished with 1 ^m diamond paste. Polished materials were then etched with the HNO3 and H2O (50-50%) solution for the microstructure observation. Coating layers was observed by scanning electron microscopy (SEM) and the chemical composition of coating layers were analyzed by energy dispersive spectroscopy (EDS) examination. Depending on processing conditions, the boride layer thickness was measured by Clemex analysis program, hardness values were measured by employment of Future Tech FM-700 microhardness equipment by using 100 gf load for 10 s. In addition, X-ray diffraction (XRD) analyses were carried to determine phases that occurred within the structure after boriding.
Three-point bending experiment was performed by 50 kN Shimadzu Brand tensile testing equipment at room temperature with 10 mm/min press rate. Specimens were prepared according to the ASTM B528-83a standard and the bending test system is shown in Fig. 1.
Fig. 1. (a) Bending diagram based on the ASTM B528-83a standard (ASTM B528 - 10, 2004). (b) Photograph of the bending apparatus.
All experiments were repeated three times for each condition and their average values were taken into account. Bending moment (1), moment of inertia (2) and bending strength (3) were calculated according to the formulas given below:
F T
Me = —. (1)
4
Me: Bending moment (N m) F: Maximum downforce applied (N) L: Length of the specimen (m)
bh2
We = — (2)
6
We = Moment of inertia (mm3) b: Width of the specimen (mm) h: Height of the specimen (mm) Bending strength is given by the rate of bending moment to moment of inertia.
0e = We (N/mmm2). (3)
2.3 Tensile Tests
Tensile tests were performed with using test specimens of 3 mm-thick. The specimens were prepared according to TS EN ISO 6892 from AISI 304 quality stainless steel sheet
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