THE HIGH TEMPERATURE CORROSION OF HARDENING FURNACE ELEMENTS MADE FROM STEEL. Maciej Hajduga, Dariusz Jędrzejczyk a Bogdan Sepiol b

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1 THE HIGH TEMPERATURE CORROSION OF HARDENING FURNACE ELEMENTS MADE FROM STEEL Maciej Hajduga, Dariusz Jędrzejczyk a Bogdan Sepiol b a Akademia Techniczno-Humanistyczna w Bielsku-Białej, Katedra Podstaw Budowy Maszyn, Zakład Inżynierii Materiałowej, ul. Willowa 2, Bielsko-Biała, Poland b Instytut Inzynierii Materiałowej, Uniwersytet Techniczny, Wiedeń, Austria Abstract The paper describes the corrosion process of the conveyor belt used inside the hardening furnace. The change of geometric dimensions and internal structure of the investigated material is presented. The high surface oxidation degree was stated. From the other side the RTG analysis revealed the increased carbon level in subsurface layer. As the destruction s reason of the conveyor too high carbon content is mentioned that in combination with high chromium contents and the elevated temperature results in chromium carbides precipitations and decrease the chromium contents in surrounding metallic matrix. 1. INTRODUCTION In many spheres of techniques application the temperature elevation is one of the main way of the technological processes intensification. However, simultaneously with the temperature increase processes causing deterioration of materials are accelerated. In this connection the fight with different kind of corrosion, especially with gas corrosion of metals and it s alloys becomes problem that posses larger and larger technical and economic meaning [1]. The gas corrosion processes have complicated character. Very often the aggressive atmosphere activity can totally change when small changes of chemical composition of environment or used material will appear. In the same way can influence the slight increase of temperature beyond the acceptable level, above which sometimes begins impetuous acceleration of corrosive processes. Corrosion brings serious losses in chemical and energetic industry, air and car transportation. Corrosive wear has essential influence both on steel constructions and elements of machines parts lifetime. The corrosion causes also considerable losses in processes of metals production [1,2]. Hitherto existing theoretical and practical achievements in the range of anticorrosion protection give in many cases the possibility of foresee materials behaviour and proper it s selection for the application in defined aggressive environments. The entire elimination of losses caused by corrosion is not possible. However, advisable is the aspiration to their elimination both by suitable protection of surface and by exact recognition of laws ruling corrosion. The most common way of the increase the steel resistance against the gas corrosion is the proper chemical composition choice of alloys working in corrosion circumstances. Usually the chromium and nickel are introduced. The typical example of cylindrical shape elements subjected to high temperature corrosion (authors from many years lead research regarding oxidation of this shape of materials [4-6]) is the conveyor belt used inside the hardening furnace made from Cr-Ni steel. The aim of present work is to describe reasons and mechanism of this type of elements wear and propose the way of decrease the losses caused by corrosion. 1

2 2. THE OWN INVESTIGATIONS The conveyor installed inside the hardening furnace was made from the wire of 2 and 4 mm diameter formed in the shape of interrelated meshes. (fig.1). Fig. 1. The view of the part of the conveyor being investigated The wear of conveyor consisted in gradually stiffness increasing as the result of the scale growing and increasing of wire diameter up to blocking of the conveyor movement. The reasons of corrosive wear were analysed using 9 samples cut from delivered material - wire of 2,5 mm and 4,4 mm diameter (dimensions after exploitation). Samples were investigated using both optical end electron microscopes. The observations of examined samples were realized using the optical microscope - NEOPHOT 2 and scanning microscope Jeol-J7. The main target of analyse was structure identification, estimation it s homogeneity and detection of surface defects. In research work also the X-ray microanalyser JCXA JEOL was used. The intensity of lines K α elements like: silicon, chromium, manganese, iron, nickel, molybdenym and carbon were measured. Additionally the carbon content was measured inside the microareas using the wave-dispersion method. Results of analysis are presented in Table RESULTS AND THEIR DISCUSSION The microscopic investigations revealed two phases inside the material structure. Relatively thick scale layer covered the examined wire the thickness reached sometimes 0,3mm (wire diameter 2,5mm). Structures visual using both optical and electron microscope are presented at Fig Table 1 The chemical analyze results of the investigated wire Chemical composition, % wt. Element Si Cr Mn Fe Ni Mo Content 1,84 23,942 1,708 48,182 19,342 0,352 Wire φ=2,5mm Standard 0,075 0,229 0,110 0,266 0,179 0,175 deviation Content 1,792 23,848 1,856 49,594 19,522 0,158 Wire φ=4,4mm Standard deviation 0,084 0,158 0,216 0,195 0,310 0,183 2

3 a b c Fig. 2. The structure of investigated material; a wire of diameter 2,5mm, not etched, x35, lengthwise section; b wire of diameter 4,4, mm; etched 4%HNO 3, x25, crosswise section; c wire of diameter 4,4mm, etched 4%HNO 3, x35, crosswise section. a b Fig.3. Structure observed under the scanning microscope in wire φ=4,4mm, not etched, with marked points of chemical composition analyssis, ; a subsurface layer, x1000; b transitory layer, x

4 Table 2 Examples of chemical analysis made in micro-areas of wire samples - diameter 4,4mm Point Chemical composition, % wt. designation Si Cr Mn Fe Ni Mo Edge 1 o 2,29 7,79 1,83 59,71 27,01 0,13 Fig.3a 2 o 0,70 5,56 0,21 62,68 27,80 0,54 3 o n.o. 2,22 0,01 63,44 33,98 0,29 4 w 0,17 60,63;11,7%C 1,43 23,22 2,04 0,66 5 w 0,19 58,68;13,02%C 1,47 23,82 2,53 0,29 Core Fig. 3b 6 w 0,25 50,36;12,16%C 1,63 28,90 6,23 0,27 7 t 1,75 39,77 3,50 17,48 0,42 0,27;35% O 2 8 t 5,53 32,08 3,04 17,31 1,21 0,11;39,16O 2 9 t 1,07 36,27 3,39 20,49 2,63 0,17;34,42%O 2 1 o 2,62 40,22 2,10 42,13 9,44 1,20 3 o 2,73 40,27 1,38 42,23 9,83 0,79 4 o 1,65 21,92 1,58 51,37 21,82 0,07 5 o 2,71 38,29 1,60 42,36 10,88 0,53 o matrix; w carbides, t - oxides 4. RECAPITULATION AND FINAL CONCLUSIONS Results of investigation enable to express the following conclusions: 1. In the examined material wire with diameter 2,5 and 4,4, mm the high level of oxidation was stated. The scale layer thickness reached even 0,3mm. 2. The chemical composition of investigated samples is very close to chromium-nickel steel H25N20S2. From the other side the chemical analysis revealed that the carbon content is much higher in subsurface layer (1,44±0,45%) then in the sample core (0,52±0,12%). 3. Inside the samples exists the structure composed of two phases contained a great number of complex chromium carbides in the metallic matrix. These two phases differs essentially regarding the chromium content about 2% inside the matrix up to 60% inside carbides. 4. Subsurface layers reveal greater amounts of carbides. When stepping towards the sample core the carbides amount gradually decrease. In the wire of diameter 2,5 mm carbides are also observed in the sample core whereas in wire of diameter 4,4mm there is no carbides in the sample middle. 5. The reason of gradually worsening conveyor functioning is very intensive corrosion process of the wire used for this application. The thick scale layer causes stiffening and blocking of the belt. This situation is caused by too high carbon content in the examined steel (introduced from surrounding atmosphere) that in connection with high chromium content results at elevated temperature in chromium carbides precipitations and essential decrease of chromium in metallic matrix. BIBLIOGRAPHY [1] DOBRZANSKI L.: Podstawy nauki o materiałach i metaloznawstwo; Wydawnictwa Naukowo-Techniczne, Gliwice Warszawa, 2002 [2] KUČERA J., HAJDUGA M.: High-temperature and long-time oxidation of iron and steels; Edt. PŁ Filia w Bielsku-Białej, 1998, p.88. 4

5 [3] KLESNIL M., LUKAS P.: Fatigue of Metallic Materials, ACADEMIA Praha, 1992 [4] HAJDUGA M., JĘDRZEJCZYK D., JURASZ Z.: Influence of Fe-samples diameter on the process of oxidation at high temperature. METAL th International Metallurgical Symposium Ostrava Czech Republic. Proceedings V.4., pp [5] HAJDUGA M., JĘDRZEJCZYK D., JURASZ Z.: High temperature time dependence of Fe-C samples oxidation. EDEM 99. International Conference on Environmental Degradation of Engineering Materials Gdańsk-Jurata. Poland. Proceedings pp [6] HAJDUGA M., JĘDRZEJCZYK D.: The influence of high-temperature oxidation on decarburization, hardness and on fatigue limit in Fe-C-Cr-Mn-Si steels. KSCS rd Kurt Schwabe Corrosion Symposium. August 30 September 2, Zakopane. 5