JOURNAL OF MATERIALS SCIENCE LETTERS 8 (1989) 277-278 Grain size effect on the oxidation behaviour of 21 Cr-1 Mo steel R. K. SINGH RAMAN, R. K. DAYAL, A. S. KHANNA, J. B. GNANAMOORTHY Metallurgy Programme, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India 2¼ Cr-1 Mo steel, a common construction material for steam generators, is used in normalized and tempered or in quenched and tempered or in annealed con- dition. High-temperature mechanical and corrosion properties are generally improved by thermomechan- ical treatments given to the alloy. These treatments cause changes in the microstructure of the alloy, including the grain size. Therefore it is important to consider the effect of grain size on the oxidation behaviour of this alloy. The corrosion behaviour of Inconel 600 as influenced by grain size in high- temperature steam has been studied by Abe et al. [1]. They reported a decreasing tendency in weight-gain with increasing grain size. This letter reports on the effect of grain size on the oxidation behaviour of 2¼ Cr-1 Mo steel in air at 873, 973 and 1073 K. Rectangular coupons (10mm x 10mm x l mm) of 2¼ Cr-1 Mo steel (chemical composition given in Table I) were subjected to the annealing treatments listed in Table II in order to obtain specimens of different grain sizes. The average grain size in each specimen was measured by the standard linear inter- cept method using an optical microscope. Oxidation tests at 873,973 and 1073 K were carried out in air for 6h in a Mettler TA1 thermobalance. 100 90 z 7O Q " 60 ? E E m 50 E <~ 40 1( .I '~) i -(i) o . --0-" -'~ - ° 0--' i I i i 0.03 0.04 0.05 0.06 d-1 ( gin-l} Figure 1 Inverse of grain diameter as a function of weight-gain at various temperatures of oxidation of 2¼Cr-I Mo steel. Oxidation temperature: ( ) 873 K, (- - -) 973 K, ( ...... ) 1073K. TABLE I Chemical composition (wt%) of the 2¼ Cr-1 Mo steel used C Mn Si S P Cr Mo Ni Fe 0.07 0.42 0.019 0.025 0.19 2.28 0.95 0.09 Balance Oxidation resistance was measured in terms of weight- gain (details of the starting material, preparation of sample coupons and other experimental steps have been given elsewhere [2]). For the correlation of oxidation or corrosion be- haviour with grain size, though there are examples of the use of grain diameter as such [1] or the inverse root of grain diameter (d I/2) [3], the inverse of the grain diameter (d -1) has been suggested [4-6] as the most appropriate parameter (the inverse of grain diameter is proportional to the average grain boundary length per unit area or to the grain boundary area per unit volume). Fig. 1 shows plots of d ~against the weight- gain per unit area after oxidation for 6 h. At the three temperatures of oxidation (873, 973 and 1073 K) the weight-gain for 6 h has been found to increase linearly with the inverse of grain diameter. In other words, oxidation resistance increased with grain size. Using the least-square-fit method, the following relationship between weight-gain A W and grain size d has been obtained for oxidation for a given duration and temperature: AW = AWo + nd I where A W0 and n are constants. The values of A W0, n and the corresponding correlation coefficients for oxi- dation for 6h at various temperatures are given in Table III. It is seen from Table III and Fig. 1 that a definite linear relationship (correlation coefficient > 0.95) exists between weight-gain and the inverse of grain size. The constant term zX W0 is a d-independent term and the values indicate the weight-gain when there are no grain boundaries, i.e. the weight-gain values for a single crystal or a grain of infinite diameter. The other constant, n, is dependent on the extent of grain bound- ary area where enhanced oxidation takes place. It is TABLE II AnneaIing treatments of 2¼Cr-I Mo steel Annealing treatment Temperature (K) Time (h)* Average grain dia. (~m) 1223 2 16.7 1273 0.5 21.6 1273 2 23.9 1323 2 28.2 t373 2 31.5 *Furnace-cooled after this duration. 0261-8028/89 $03.00 + .12 © 1989 Chapman and Hall Ltd. 277