Scripta METALLURGICA Vol. 26, pp. 1303-1308, 1992 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved INFLUENCE OF LONG-TERM ISOTHERMAL EXPOSURES UPON M7C3 CRRBIOE CHANGES IN 2.7Cr-O.6MO-O.3V STEEL 3ozef 3ANOVEC - * V l a d i m i r MAGULA -*Anton HOL~ - Anna V~ROSTKOVQ I n s t i t u t e o f E x p e r i m e n t a l M e t a l l u r g y , S t o v a k Academy of S c i e n c e s , Ko~ice , SoLovjevova 47, CSFR *Welding Research I n s t i t u t e , Ra~ianska 71, 832 59 B r a t i s l a v a , CSFR (Received December 3, 1991) (Revised February 21, 1992) I n t r o d u c t i o n Although carbide p a r t i c l e s represent only a small volume f r a c t i o n in the microstructure they l a r g e l y determine the mechanical properties of low alloyed steels and r e f l e c t the microscopic changes taking place in t h e i r surroundings. From the view points of mechanical properties and f r a c t u r e behaviour, the morphology, size, and l o c a t i o n of carbide p a r t i c l e s in the microstructure are of great importance. I t is generally known that dispersive p a r t i c l e s are ef- f e c t i v e obstacles against plastic flow in the matrix and they are responsible for the strength and the brittleness increase. One- and two-dimensional parti- cles (e.g., needle and foil-shaped) act as stress concentrators and hence the sites with preferred crack nucleation. The shape, size, and distribution of carbide particles and also their structural stabilitv depend on various factors, the most important of which are: - chemical composition of the steel - thermal and deformational h i s t o r y of the steel - carbide type (given by crystallography, stoichiometry, and chemical compo- sition). Modern experimental methods (X-ray and electron diffraction, EOXSISTEM etc.) enable one to exactly define the carbide type and to find its relation to the size, morphology and distribution of particles. The works of Pitting and R i d t e y [ 1 ] , H r i v ~ & k [ 2 ] , GOth e t a t . [ 3 ] and o t h e r s a r e e x a m p l e s . The i n f l u e n c e of t h e t h e r m a l h i s t o r y o f t h e s t e e l on t h e p r o p e r t i e s o f carbide p a r t i c l e s in a 2.7Cr-O.GMo-0.3V steel was q u a n t i f i e d e a r l i e r [4] in terms of type of crystallographic Lattice, carbide stoichiometry, and chemical composition. While the chemical compositions of M3C and MC carbides changed with the tempering conditions, that of M7C3 was stable. The influence of chemical composition of the steel is also very important. For example Stevens and F l e w i t t [5] found that the weight f r a c t i o n of carbides in the high tempered 2.25Cr-1Mo steel depended upon P content. An increase of P content in the steel led to an increase in the amount of MGC and a decrease for M23C6 carbide. The t o t a l carbide portion also increased, explained by a higher value of the MIC r a t i o in MGC carbide in comparison with two other carbides. Yu and Mc Mahon [6] showed that the measure of the segregation b r i t t l e n e s s in 2.25Cr-XMo steels ( x - v a r i a b l e Mo content) depends in part on the weight p o r t i o n of M2C carbide in the s t e e l . The presence of individual carbide tyDes in the microstructure and the contents of Cr and Mo in ferrite matrix were influenced by the chanqes of X. Grabke et at. studied the k i n e t i c s and mechanism of segreqation in several model steel a l l o y s [7, 8, 8]. They found that carbide-forming elements influence C and P a c t i v i t i e s in the f e r r i t e s o l i d s o l u t i o n . An increase of P segregation in the model system Fe-Cr-C-P (in comparison to Fe-C-P system) they explained by the decrease of C activity in ferrite, as the result of the creation of MTE3 stable carbides on the grain boundaries and by the Locking of carbon atoms in the particles [73. 1303 0036-9748/92 $5.00 + .00 Copyright (c) 1992 Pergamon Press Ltd.