TABLE OF CONTENTS The traditional route to high strength in steels is by quenching to form martensite which is subsequently reheated or tempered at an intermediate temperature, increasing the toughness of the steel without too great a loss in strength. Therefore, for the optimum development of strength, a steel must first be fully converted to martensite. To achieve this, the steel must be quenched at a rate sufficiently rapid to avoid the decomposition of austenite during cooling to such products as ferrite, pearlite and bainite. The effectiveness of the quench will depend primarily on two factors: the geometry of the specimen, and the composition of the steel.
A large diameter rod quenched in a particular medium will obviously cool more slowly than a small diameter rod given a similar treatment. Therefore, the small rod is more likely to become fully martensitic. With the exception of cobalt and aluminium, the addition of common alloying elements to a steel usually moves the time-temperature-transformation ( TTT) curve to longer times, thus making it easier to pass the nose of the curve during a quenching operation, i.e. there is a reduction in the critical rate of cooling needed to make a steel specimen fully martensitic. If this critical cooling rate is not achieved a steel rod will be martensitic in the outer regions which cool faster but, in the core, the slower cooling rate will give rise to bainite, ferrite and pearlite depending on the exact circumstances.
The ability of a steel to...
