STEEL MATERIAL MECHANICAL PROPERTIES AFTER YEARS OF COMPLEX LOADING. Nermina Zaimovic-Uzunovic a Isak Karabegovic b Mirsada Rizvanovic c

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1 STEEL MATERIAL MECHANICAL PROPERTIES AFTER YEARS OF COMPLEX LOADING Nermina Zaimovic-Uzunovic a Isak Karabegovic b Mirsada Rizvanovic c a Faculty of Mechanical Engineering, Fakultetska 1, Zenica, Bosnia & Herzegovina, nzaimovic@yahoo.com b Technical Faculty, Bihac, B&H, c Faculty of Materials and Metallurgy, Zenica, B&H Abstract Blast furnace jacket is exposed to the continuous acting of the different influences and loads; mechanical, thermal and chemical. Quantitative relations of the chemical influences of the outside and inside acting factors are not possible to measure, as well as the other factors and disturbances. But the final effects of those influences are evident and they changed properties of the material of the jacket. This paper deals with changing of the mechanical and chemical properties of the jacket used for ten years under aggravated working conditions. The results of the testing materials show that the mechanical and chemical properties are not significantly changed after so many years of exploatation. 1. INTRODUCTION Constuction of the blast furnace has made of the fire resistant bricks. Blast furnace steel jacket is the last of the number of layers of a multilayer lining of a set in which chemical processes take place together with the high temperature raw material melting process which is technologicaly insufficiently investigated. The lining and the steel jacket cooling is provided for the purpose of protection from the high temperatures inside the furnace. For technical reasons it is not possible to provide cooling system continualy, which results in jacket local overheating. Besides, the blast furnace steel jacket is affected by the action of permanent static and periodical dynamic loads as well as the chemical action on an aggressive medium. The complex technological pig iron production in the blast furnace takes place continuously and the interventions and repairs are performed in a warm state. All these phenomena in the blast furnace steel jacket influence on the assurance of insufficiently known stresses. Situation become worse because all effects act simultinously and continuously changing mechanical and chemical properties of the material of the jacket. The main aim of this paper is discussion of the material properties given after more than ten years of exploatation. Thats why the chemical and mechanical examination of the properties has been done. 1

2 2. LOADS ACTING ON THE BLAST FURNACE JACKET Blast furnace jacket is exposed to the different influences, loads and disturbances. Most of them are variable and have different intensity at the different places. But not all of them have the same influence at the jacket. Dominant and permanent acting at the jacket caused: -mechanical loads, -increased temperature, -chemical influences Mechanical loads Mechanical loads can be static or dynamic. Static loads are result of permanent acting of the parts connected or supported at the top of the blast furnace and cooling equipment uniformly distributed and connennected with the blast furnace jacket, especiallyat the bottom of the blast furnace. Except permanent static loads, blast furnace jacket is loaded with a variable static loads as a result of inside gas pressure as well as material in crude and molten phase. Wind is a typical atmosphere influence but not cause dominant influence. Dynamical loads are stohastic in their nature and appears after unusual behaviour of the material inside the furnace. From time to time pieces of material impacts at the brick walls of the furnace and couse defects. Through partialary damaged walls impacts are transmitted onto the jacket and after a pariod of the same situations may cause initial cracks Termic loads This kind of load has working permanently with a different intensity at all levels of blast furnace. It s influence cause different consequences at different parts of the jacket. Termic loads are not easy to determine and temperature distribution field has to be find using mash of connected thermo elements Chemical influences Definitely, chemical factors are not possible to measure. But permanent acting of different influences like outside and inside factors slowely but permanently, change chemical properties of the material. Outside factors are first of all agressive atmosphere as a consequence of sulpur and nitrogen gases whose conncentration is high around the blast furnace jacket. Those gases are result of fuel combustion. In the combination with the humidity from the air, gases formed agresive layer and attach the surface of the jacket. Similar agresive processes are existed on the inside the surface of the jacket. Inside surface is heated at the approximatly 40 C and cooled simultineously. Evaporated water condensate at the surface from the heated air which is blowing into the blast furnace. Nitrogen oxides together with the humid have formed agresive chemical materials whose are acting at the outside of the jacket decreasing the reliability in the exploitation as well as the life cycle. Specially the acting is emphasized in the areas of welded zones. All influences are permanent and simultaneous and may change chemical and mechanical properties of the material. 2

3 3. MATERIAL TESTING AFTER A YEARS OF EXPLOITATION Chemical examination After ten years of exploitation of the blast furnace jacket the samples of the material of the jacket has been taken and examinated of chemical contet. Four years after that the same procedure was repeated. Content of chemical elements C, Si, Mn, P, S, Cr, Ni, Cu was not significantly changed and completely stayed between the limits prescribed in the standards. In the Table 1. the results of the chemical testing are shown for the steel 10Γ2C1 acording the GOST Table 1. Content of the chemical elements % C Si Mn P S Cr Ni Cu after 10 years 0,12 1,1 1,55 0,018 0,033 0,05 0,05 0,07 after 14 years 0,11 0,98 1,59 0,017 0,021 0,08 0,05 0,07 standard < 0,12 0,9-1,2 1,3-1,65 max 0,35 max 0,40 < 0,3 < 0,3 < 0, Examination of the mechanical properties Tensile mechanical examinations have done Rp0,2, Rm, A5 and examination of the used energy of impact KU300/2 at the +20 C and C. Averrage values of the results of the mechanical tests and comparisons to the standards are presented in the Table 2. Table 2. Mechanical properties of the tested material (average values) Thickness of the sample (mm) Rek (Rp0, 2) N/mm2 A5 % Impact energy (J) KU300/2 on+ 20 C samples samples ,6-64,8 GOST Values of the elongation A5 are not changed and the values are consistent to those given in the standards. Differences for Rp0,2 are determined in a case of all samples Examination of micro section Material of the samples taken from the blast furnace jacket has manufactured using roll forming. Samples were taken in the direction of rolling and examinated cross section is perpendicular at the direction of rolling. Macro corroding has shown typical defects (porosity, micro cracs, segregations etc.). There are sulphur segregations near non metalic inclusions. Micro structure testing has done at the drill test bars for impact energy. Results of testing are given in the table 5. 3

4 METAL 2003 Table 3. Results of testing samples slag number 3, 83 4, 32 4, 3 3, 58 4, 49 4, 46 - notice very long and thin sulph. very long and thin sulph. nests of undeformed inclusions nests of the undeformed inslusions Inclusions in the material are sulphides, oxides, a little bit of aluminate and somewhere silicate but not in the classical shape. The titanium carbides are found. Examination shows secundary ferite perlite structure The examination has done using scaning microscope in non-corroding state with the aim to determine kvalitative microanalysis of inclusions. This kindof testing proved presence of titanium carbide, some of the aluminate and silicate. At the figures axial and perpendicular cross sections are shown. Figure 1. Metalographic structure (perpendicular) Figure 2. Metalographic structure (axial) 4

5 METAL 2003 Figure 3. Micro analysis of non metalic inclusions (very long thin sulphid and nests of undeformed inclusions) 4. CONCLUSION Material properties were examinated after a long period of exploitation of the blast furnace jacket under the aggraviated conditions. It was necessary for making decision what to do with the jacket, to change and repair parts of jacket or not. Chemical analysis of the material show the values stayed between the allowed limits. Mechanical properties are changed and have different values for Rp0, 2 and A5 compares to standard values. But inspite of that fact values may be accepted and no activities about change of the jacket can be done. Structure of the used material is not changed significantly as well as slag number according to ASTM standard. 5