SEM ANALYSIS OF NONMETALLIC INCLUSION OF THE STAINLESS STEEL MODIFIED WITH ADDITION OF Zr AND Te

Dr. Sc. Derviš Mujagić 1 , Dr. Sc. Aida Imamović 2 And Dr. Sc. Mirsada Oruč 2 . 1. University of Zenica, Institute "Kemal Kapetanović" 2. University of Zenica, Faculty of Metallurgy and Technology. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History Received: 02 April 2019 Final Accepted: 04 May 2019 Published: June 2019


…………………………………………………………………………………………………….... Introduction
The research was carried out on an austenitic stainless steel labelled X8CrNiS18-9, as defined in standard EN 10088-3: 2005, which was produced at semi-industrial plants at the Institute "Kemal Kapetanović", University of Zenica, in order to determine the influence of zirconium and tellurium in austenitic stainless steel [1]. Grade X8CrNiS18-9 is the most readily machinable of all the austenitic grades of stainless steel. The machinable nature of grade X8CrNiS18-9 is due to the presence of sulphur in the steel composition. The X8CrNiS18-9 stainless steel referred to as "free-machining" stainless steel has the following nominal chemical composition, table 1 [2][3][4][5]. The manganese sulphide (MnS) stringers help to promote the easy breakup of metal shavings during machining. Whilst the sulphur improves machining, it also causes a decrease in the corrosion resistance and a slight lowering of the toughness and general decrease of the mechanical properties.
The intention is to make higher machinability of this steel grade but with good mechanical properties.The results show that after microalloying by tellurium or zirconium the modification of nonmetalic inclusionmanganese sulphide of X8CrNiS18-9 stainless steel can be significantly changed. The machinability varies with the inclusion shape, which means that it is desirable that the manganese sulphide inclusions in steel must be as spherical as possible 6.

ISSN: 2320-5407
Int. J. Adv. Res. 7 (6), 63-69 64 The aim of the research was to examine the possibility of reducing the effect of Te and Zr on the mechanical properties of X8CrNiS18-9 by microalloying by tellurium and zirconium, which can modify the MnS and improve machinability, with mechanical properties in the limits prescribed for X8CrNiS18-9 standard grade.

Experimental work
Detrimental effects of inclusions in steel do not only depend on their sizes, shape, distribution, but also on their chemical composition and mechanical properties. For this reason, the control of formation of nonmetallic inclusion and the characterization present the basis of improvement of steel product properties and lead to sustainable development in design of new steel grades. Thermo-Calc software package is used to show the temperature region where in theoretical way it is possible to form non metallic inclusions. Also it have to take into consideration that Thermo-Calc calculation is related to equilibrium state and is also related to chemical steel composition, in this case of standard X8CrNiS18-9.
The aim of the research was to examine the possibility of increasing the effect of machinability of X8CrNiS18-9 stainless steel microalloying by tellurium and zirconium. They seem to exert beneficial effects by promoting the retention of globular-shaped sulphide type inclusions.
The intention is to make better machinability of X8CrNiS18-9 stainless steel but to keep good mechanical properties. Tellurium or zirconium are considered to have a less deleterious effect than sulphur on mechanical properties [7].

Results and discussion
Thermo-calc calculation Thermo-Calc calculation of characteristically equilibrium phases for standard X8CrNiS18-9 depending on temperature is shown that characteristically nonmetallic inclusions in these steels are MnS types by which precipitation starts under liquids temperature, figure 1 [6].

Analysis of nonmetallic inclusions
In accordance with the program of testing at the Department for melting and casting metals of the Institute "Kemal Kapetanović", several melt of different chemical composition was made [8]. Production of X8CrNiS18-9 stainless steel microalloyed by tellurium and zirconium was performed in a vacuum induction furnace with capacity of 20 kg at the Institute "Kemal Kapetanović" in Zenica. The ingots were processed by forging, hot rolling and heat treatment.
The first melt was made without the addition of alloying elements, while in the remaining melts, the various contents of zirconium and tellurium were added individually. In this paper, the tests were performed for three variants (three different melts), with the mentioned alloying elements being added within the boundaries according 65 to the literature [8]. The samples from variant 1 (excluding alloying elements) were tested, followed by variants 2 with the addition of Zr, and variant 3 with the addition of Te  In order to prepare a sample, grinding and polishing of the testing samples was carried out. Than the analysis of the content, size and distribution of non-metallic inclusions in non-etched state was carried out at the Institute of Zenica, Bosnia and Herzegovina. For each of the above samples, line and point analysis were done, and also mapping [1,8].
The detailed SEM/EDS analyses of modified nonmetallic inclusions were performed by scanning electron microscope (SEM) Jeol JSM 5610 with attached energy-dispersive x-ray spectroscopy (EDS) system. Examination of the inclusions on the scanning electron microscope gives the highest data on the type of inclusions and their composition. Figure 2 shows a linear analysis of characteristic non-metallic inclusions for a sample of austenitic stainless steel X8CrNiS18-9 without additives of alloying elements (variant 1). The shape and composition of inclusions in the standard X8CrNiS18-9 grade are typical MnS stringers, [8].   (Figures 5b and 5c). Also, on the basis of the comparison of the SEM image with an image indicating the zirconium distribution, it can be clearly concluded that the zirconium is at its maximum in two different locations within the two different inclusion (Figure 5d).  Figure 6 gives a linear analysis of the pattern with the addition of tellurium (variant 3) [8]. Figure 6, i.e. the distribution diagram of the contents of S, Mn and Te shows that the inclusion shown in the SEM figure (Fig. 6a), in addition to the already conventional manganese sulphide, contains to a certain extent tellurium. The picture also shows, and the diagram confirms that the tellurium practically surrounds the inclusion of manganese sulphide, and is located at its ends.  Figures 7b and 7c). From figure 7d it can be clearly concluded that, as already mentioned above, the tellurium is surrounds the inclusion of manganese sulphide, and is located at its ends. The steel -variant 3 with the addition of tellurium has characteristically manganese sulphides combined with a tellurium which are a typical globular shape and specifically improve the machinability of this steel.

Conclusions:
Detrimental effects of inclusions in steel do not only depend on their sizes, shape, distribution, but also on their chemical composition and mechanical properties. The aim of the research was to determine the effects of zirconium and tellurium on types of non-metallic inclusions in austenitic stainless steel X8CrNiS18-9.
Thermo-Calc calculation of characteristically equilibrium phases is shown that characteristically nonmetallic inclusions in these steels are manganese sulphides types.
Metallographic tests on the SEM were performed and the influence of the chemical elements of the zirconium and the tellurium on the change in the type and chemical composition of non-metallic inclusions was monitored.
It has been found that the basic types of inclusions in this steel are manganese sulphides and that the nonmetallic inclusions of austenitic stainless steel X8CrNiS18-9 can be modified by addition of zirconium and tellurium. Especially nonmetallic inclusions of manganese sulphides types in combination with a tellurium can be translated into a suitable form, and on one side they will provide good machinability and, on the other hand, will not adversely affect the properties of austenitic stainless steel X8CrNiS18-9.