THE INFLUENCE OF MICROALLOYING WITH BORON ON PROPERTIES OF AUSTENITE STAINLESS STEEL X8CrNiS18-9

More recently a modified stainless steels have been used to produce various structural elements that work in complex operating conditions. Stainless steel X8CrNiS18-9 (standard EN 10088-3: 2005) is the most commonly used austenitic stainless steel due to its good machinability. This steel has high mechanical and working properties thanks to a complex alloying, primarily with the elements such as chromium and nickel. The content of sulphur present in the steel from 0.15 to 0.35% improves machinability. However, while sulphur improves machinability at the same time decreases the mechanical properties particularly toughness. The addition of sulphur, which is the cheapest available additive for free machining, will impair not only the transverse strength and toughness, but also the corrosion resistance.The aim of this work is to determine the influence of microalloying with boron on the machinability, corrosion resistance and mechanical properties the mentioned steel, but alsoto determine the effect of microalloying


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In steel, boron can be dispersed in a matrix in the form of Fe 2 B, boride with a size of20-30 x 10 -8 cm, and free which mainly segregates surrounding the primary boundaries of the austenite grain. This small amount of soluble boron distributed along the grain boundary obviously slows down the γ- transformation by diffusion, or prevents a ferrite reaction that increases the hardness of the steel [4]. Boron is considered as another possible element which improves the ductility at hot processing with similar types of steel. At relatively low temperatures, where austenitic and ferrite phases coexist, boron is thought to have a beneficial effect due to the precipitation of Fe 23 (B, C) 6 in the matrix. These precipitates act as preferential sites for intragranular nucleation of ferrite. In this way a smaller amount of ferrite is formed at the grain boundaries.This not only reduces the number of empty spaces and cavities formed at the grain boundaries, but also makes the interior of the austenitic grains more deformable due to the soft intragranular ferrite. The final result is an improvement in ductility in hot processing [5].

Experimental Research and Test Results
The melting and casting of austenitic stainless steel X8CrNiS18-9 was carried out in a vacuum induction furnace with a capacity of 20 kg, with a maximum power of 40 kW, and is located at the Department for melting and metal casting of the Institute "Kemal Kapetanović". Eight meltings were done. This means that we will, in addition to the melt without alloying elements and melt microalloyed with boron, produce six more melts and that microalloyed with zirconium, tellurium and zirconium and tellurium, after which we will also add boron to these variants. The ingots were processed by forging, hot rolling and heat treatment.
In preliminary research is planned that after primary processing (approx50 mm) samples will be tested by cutting forces, in order to determine to what extent the modification of the chemical composition affects the machinability of this material, and corrosion resistance. Of particular importance is to determine the behavior of nonmetallic inclusions in the process of developing structural parts and in a later exploitation. For this reason it is planned to simulation processing of austenitic stainless steel by plastic processing and by forging and rolling with two different degrees of processing. After that, the samples will be taken and laboratory testing of mechanical properties will be performed.Chemical analysis of the eight melt variants are given in Table 1.

Machinability
In the Laboratory for metal cutting and machine tools of the Faculty of Mechanical Engineering in Zenica, the machinability test of the ingots was done, based on the estimation of parameters of the cutting force. Testing on both samples was performed under the same treatment regime. The results of the cutting force tests (individual forces F x , F y , and F z as well as the resultant force F R ) are given in Table 2.
The melt microalloyed with boron, and even melt microalloyed with boron and zirconium have better machinability compared to the melts without alloying elements and melt microalloyed with only zirconium, respectively. However, this is not the case for the melts microalloyed with tellurium. Melts microalloyed with tellurium and zirconium and tellurium have significantly better machinability compared to those microalloyed with boron and tellurium, and boron, zirconium and tellurium, respectively.
697 Table 2:-The results of the cutting force tests [6].

Corrosion Resistance
General corrosion tests for X8CrNiS18-9 stainless steel samples were performed on a potentiostat / galvanostat PAR 263A-2 device in an electrochemical cell prescribed by ASTM G5-94.The samples were tested in a solution of 1% HCl at room temperature. The solution was previously deaerated with argon for 30 minutes as provided by ASTM G5-94. To test the general corrosion of the X8CrNiS18-9 stainless steel samples, the Tafel Directional Extrapolation Method described by ASTM G3-89 was used.The results of testing the general corrosion rate of these samples are given in Table 3. Melts microalloyed with boron, boron and zirconium, boron and tellurium, and boron, zirconium and tellurium have lower corrosion rate of the melt without the alloying elements, and the melts microalloyed with zirconium, tellurium, or tellurium and zirconium, respectively.Particularly significant is the reduction in corrosion rate for melt microalloyed with tellurium from 10.390 to 2.728 mm/year for the melt microalloyed with boron and tellurium.

Mechanical Properties
After the rolling process was completed, specimens were prepared for mechanical testing (tensile properties and impact toughness testing). The tests were performed at the Mechanical Laboratory of the Institute "Kemal Kapetanović" in Zenica. The results of the tensile properties and impact toughness testing are given in Table 4.
The impact toughness value is even slightly higher for melts microalloyed with boron and zirconium, and boron and tellurium compared to those microalloyed with zirconium, and with tellurium, respectively.

Metallographic Testing of Rolled Samples
Upon completion of the second stage of deformation (rolling to dimensions 14 x 50 mm), samples were taken to perform metallographic testing for the rolling condition. BAS EN 10088-1 does not specify limit values for the content of nonmetallic inclusions. The content, size and distribution of nonmetallic inclusions in the unetched state were analyzed, and the test results are given in Table 5. Many small sulphide inclusions with thickness less than 2µm have been observed. Complex inclusions of size 500 µm were also observed.
The test for the rolling condition was performed in accordance with Standard Test Methods for Determining the Contents of Inclusions in Steel -ASTM E45-11.In experimental melts after rolling and after heat treatment, the presence of type A inclusions (sulphides) according to ASTM E45-11 was detected. The largest number of inclusions and the biggest inclusions were determined for tellurium alloyed melt, and for variants of melts alloyed with boron and zirconium and zirconium and tellurium elements.
Sample imaging under a certain magnification (x50) was performed on an OLYMPUS PMG3 type optical microscope, and one image was given for each sample (Figure 1 [6].