MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LOW CARBON LOW ALLOY MARTENSITE

Abstract

A critical survey of the literature,on low alloy high strength (LAHS) steels revealed that they are most usefully employed in the proof strength range of 775-850 MN/m2 where ever a combination of high strength coupled with good ambient - temperature and sub-zero temperature toughness is an essential requirement. Steels which normally cater to this range of proof strength are,(a) aircooled bainitic steels and, (b) quenched and tempered (Q and T) steels alloyed with Ni, Cr, Mo, V and Mn. Ausformed steels which could also be employed have been excluded since ausforming is confined only to those steels having transformation characteristics leading to 'bay' formation. The main drawback with the bainitic steels is that the stipulated level of toughness may not be attained because of the formation of unintended microconstituents such as (i) elongated oarbides surround-ing the bainitic ferrite regions and, (ii) the austenite-martensite (A M) constituent. Although the toughness can be improved upon by tempering, the effective proof strength is lowered to /4700 MN/m2. The major problem with the Q and T steels is the necessity of making adjust-ments in the composition for toughening the ferrite matrix and for eliminating temper-embrittlement. Further, the difficulty in, exercising control over carbide size, shape................