Optimization of Fe/Cr/C base structural Steels for improved strength and toughness
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INTRODUCTION
I T is well known that the martensitic transformations can be exploited to produce a wide variety of strength values by simply changing the alloy content, especially carbon (e.g., References 1, 2). However, increases in strength are usually associated with corresponding decreases in the toughness values limiting the extent of structural applications of steels. 3 An approach to overcome this problem by microstructural control (duplex structures) involving ternary and quarternary additions of substitutional alloying elements, 4-7 and changes and/or variations in the heat treatments 5'6'7 has, for example, led to the development of experimental Fe/4Cr/0.35C structural steels which have superior strength-toughness combinations over more complex commercial steels, and which are used as the base material for the present study. In the as-quenched condition, the microstructure of the base steel consists of heavily dislocated autotempered lath martensite and thin films of retained austenite at the lath boundaries. In fact, large increases in toughness in these and commercial steels have been attributed 4'7's to the presence of stable, retained austenite which is found in these duplex structures. To improve the toughness to strength ratio, steels are usually tempered following quenching. Retained austenite becomes thermally unstable following 300 to 400 ~ tempering which is associated with loss in toughness and ductility (tempered martensite embrittlement).l~ The objective of the current investigation is to improve further the properties of the base steel through careful microstructural control via modifications in the alloying additions and heat treatments, and also to explore the properties from air melted alloys since previous data 4-7 were all obtained from vacuum melted alloys. The Cr content was decreased from its original value of 4 wt pct4'7 to 3 wt pet, and 0.5 wt pet Mo was added to compensate for the loss in hardenability. 9 Mo, having the effect of temper resistance in steels, may limit and postpone the onset of tempered martensite embrittlement (TME). 10Cr can not entirely be replaced by Mo, since it has been obM. SARIKAYA and G. THOMAS are both with the Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. B.G. STEINBERG was a Graduate Student, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720. Manuscript submitted May 18, 1981. METALLURGICALTRANSACTIONSA
served that there is little or no retained austenite present in Fe/Mo/C steels. ~1C is necessary to increase strength and it is vital to obtain retained austenite. Mn and Ni additions were made mainly to stabilize the retained austenite in the presence of C. TM The level of the alloying additions followed previous considerations. 7 Higher austenitizing temperatures than 900 ~ can be beneficial to the mechanical properties of structural steels, s'7'8 By high temperature (>
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