Maciej Hajduga, Dariusz Jędrzejczyk

Transcription

1 Hradec nad Moravicí THE OXIDATION S KINETICS OF SOME HIGH ALLOYED STEELS Maciej Hajduga, Dariusz Jędrzejczyk University of Bielsko-Biała, Material s Engineering Department, Willowa 2, 4-0 Bielsko-Biała, Poland, djedrzejczyk@ath.bielsko.pl Abstract The results of surface oxidation and decarburization of (Fe,C,Mn,Si,Cr,Ni) steels are reported. The oxidation anneals were carried out at temperatures T= 80, 020, 060, 00 C. The aim of presented experiment was to compare typical alloyed steel behavior in the same high temperature corrosion conditions. The estimated oxidation kinetics in connection with later microstructure investigations and measurements of decarburized thickness layer will allow for complex evaluation of examined steels. It follows from the experiment that according to expectations steel H8NS that belongs to the heat-resisting group characterizes with greatest resistance against influence of high temperature. On special attention deserve fact that the measurement indicate decreasing of the sample weight. This process is systematical and with increasing of oxidation time and temperature the weight of oxidized samples decrease. Among the rest of materials the high resistance against the high temperature shows steel: NC0 and SW8. Steel SK is situated in the middle of the range of weight increase, whereas steel: G2 and ŁH show weight increases nearing to the greatest.. INTRODUCTION The high temperature atmospheric corrosion is the most frequent example of chemical corrosion and brings serious loses in chemical industry, power engineering, air and road transport. The corrosion wear has essential influence on the lifetime both steel constructions and machine elements. Atmospheric corrosion cause also the considerable loses during metals production treatment. According to the statistical data about / of metallic materials is withdrawn from uses in consequence of damages caused corrosion [, 2]. Naturally part of these materials is used again among other things as the charge materials in metallurgical process, however about 0% of material is irrevocably lost. During the production of hot working elements about % of treated material creates the scale layer and makes in this way receiving of high quality product more difficult. Total elimination of losses caused by corrosion is not possible. However advisable is aspiration for it s limitation both by suitable protection and by exact recognition of rights ruling corrosion. The most of papers and monographs published till now refers to the oxidation process of the samples corresponding to semi-infinitive plane. Results of these investigations find practical application in relation to some constructional elements and mainly to thin steel sheet. Considering the practical application of cylindrical shape products (rods, wires), authors from many years lead research regarding the oxidation of the cylindrical shaped samples with diameter -0 mm [-]. It was stated among others thing that the oxidation process of the sample with small diameter differs essentially from the oxidation of the flat samples, and it s kinetics could be described by exponential equation, whereas the kinetics of flat samples is usually described by parabolic equation. One of the methods of enlarging the steel resistance against corrosion is the proper chemical composition choice of alloys working in corrosion circumstances. Most often introduced alloyed elements are chromium and nickel.

2 Hradec nad Moravicí The aim of presented experiment was to compare typical alloyed steel behavior in the same high temperature corrosion conditions. The estimated oxidation kinetics in connection with later microstructure investigations and measurements of decarburized thickness layer will allow on complex evaluation of examined steels. 2. THE OWN INVESTIGATIONS The experiment was realized using cylindrical shape samples turned from typical alloyed steel presented in table. The example of speciment appearance after oxidation is presented at Fig.. a) b) Figure. An example of specimens appearance after oxidation steel SWMo; a T = 80 0 C, t = 000 min, b T = C, t = 000 min. Table. Chemical composition of materials used in the experiment, %. Steel Chemical composition, % design. C Mn Si P S Cr Ni Cu ŁH 0, 0,2 0,4 0, 0, 0,02 0,02,40 0, 0,2 G2 0,, 0,4 0, ,0 0 H8NS 0,0 2,0 0,8,,0 20,0,4 - SW8 0,80 0,4 0, 4,0 ab. 8%W;,2%V 0, 0,0 0,80 0, 0, 0,040 0,040 0,2 0,0 0,0 NC0,60 ab. 0,4 ab. 0,4,0 - - SWMo 0, ab. 0,0 ab. 0,2 0 0 ab. 4,00 ab.6,%w; ok. 2,00%V;,% Mo SK,06 0,40 0,40,80 4,80 6,6,%W; 0,40%Ni;,2,%V; 4,%Co Sample with diameter φ=0 mm and length l =20 mm after polishing were oxidized in silite chamber furnace PKS 600/2. The experiment was conducted in four different 2

3 Hradec nad Moravicí temperature levels 80, 020, 060 and 00 0 C. Samples were taken from the furnace one by one after: 20, 400, 00 and 000 min. After cooling samples were exactly measured and weighted, then the scale layer was removed and the metallographic specimens were prepared perpendicularly to the cylinder axis to enable structure observation and micro-hardness measurement. For these research the optical microscope NEOPHOT-2 and durometer DURIMET-20K were used.. RESULTS ANALYSIS Because investigations relating to the micro-hardness measurements and structure analysis are in the course of executing, the presented analysis will refers only to the oxidation kinetics of tested materials. Data regarding internal materials structure, micro-hardness measurements and estimated diffusion coefficients will be published gradually depending on the progress of research work. a) b) H8NS SWMo ,2 0-0,4-0,6-0, Time, 600 min Time min Figure 2. The kinetics of oxidation of steel H8NS and SWMo At the Fig. 2 the results of the weight change for two extreme materials: steel H8NS, in chance of which in every oxidation temperature the weight decreasing was stated and steel SWMo, which shows the greatest weight increase. Only in chance of steel H8NS the weight decrease was stated, the rest of materials showed systematical weight increase. The maximal relative weight increase of steel SWMo was about 4% (T = C, t = 000 min.), considering the initial sample weight about 4g. The final confrontation of obtained weight measurements is presented at Fig.. Not typical maximal weight increase of steel SWMo was measured at temperature C (see Fig.). Such material behaviour could be explained by scale layer structure covering sample oxidized at this temperature level. The observed scale was cracked and distorted. There was no similar scale layer both in higher and lower temperature of oxidation. It follows from presented results that according to expectations steel H8NS that belongs to heat-resisting group characterizes with greatest resistance against influence of high temperature. On special attention deserve fact that the measurement indicate decreasing of the

4 Hradec nad Moravicí sample weight. This process is systematical and with increasing of oxidation time and temperature the weight of oxidized samples decrease. Although differences between the initial and final weight are not large and reach about 0.g, which mean decreasing about 0.%, the stated trend shows rather, that this dependence it is not result of measuring error. Among the rest of materials the high resistance against the high temperature corrosion shows steel: NC0 and SW8. Steel SK is situated in the middle of the weight increase range, whereas steel: G2 and ŁH show weight increases nearing to the greatest. a) b) - 80 o C ŁH G2 H8NS SW8 c) d) NC0 SWMo SK Weight increase, g 020 o C o C 00 o C Figure. The kinetics of oxidation the investigated alloyed steel at different temperature level: a C, b C, c C, d 00 0 C. 4. CONCLUSIONS. Among the investigated steels the greatest resistance against influence of the high temperature expressed by weight increase reveals steel H8NS (in the range of temperature C, within time min). 2. Unexpectedly, the relatively large weight increase (about 4% of initial value) was measured for steel SWMo in oxidation temperature C. Both in lower and higher temperature of oxidation such surrosion was not stated. 4

5 Hradec nad Moravicí. The complex resistance estimation of examined materials against the high temperature corrosion demands further investigation - measurements of: scale layer thickness, decarburized layer thickness, micro-hardness of subsurface layer and X-ray microanalyze. BIBLIOGRAPHY. KUčERA J., HAJDUGA M.: High-temperature and long-time oxidation of iron and steels; Edt. PŁ Filia w Bielsku-Białej, 8, p KLESNIL M., LUKAS P.: Fatigue of Metallic Materials, ACADEMIA Praha, 2. HAJDUGA M., JĘDRZEJCZYK D., JURASZ Z.: Influence of Fe-samples diameter on the process of oxidation at high temperature. METAL. 8 th International Metallurgical Symposium Ostrava Czech Republic. Proceedings V.4., pp HAJDUGA M., JĘDRZEJCZYK D., JURASZ Z.: High temperature time dependence of Fe-C samples oxidation. EDEM. International Conference on Environmental Degradation of Engineering Materials Gdańsk-Jurata. Poland. Proceedings pp.-2.. 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 0 September 2, Zakopane.