Sometimes the physical properties of hardness and hardenability are confused. Hardness represents an existing condition after processing by heat treatment. It is a direct indicator of the mechanical strength of the steel. Hardenability denotes the potential of a steel to develop a particular value of hardness after a particular heat treatment. Hardenability is primarily dependent on the chemical composition of the steel. Hardenability is associated with a steel’s ability to develop hardness to a given desired depth in thickness of the finished product. Developing high hardness throughout thick cross sections is usually more difficult to attain without adding expensive elements (compared to carbon) to the composition.

During a typical heat treatment the steel is exposed to an initial high temperature in a range where the metallurgical phase known as austenite exists. It is then cooled down through progressively lower temperatures during which austenite may be transformed to other phases – pearlite or banite or mixtures of these. Finally the creation of the metallurgical phase known as martensite starts (and can come to completion) during the lowest temperatures in the cooling sequence. The rate of cooling from the austenite temperature range is critical along with the steel’s composition – especially its carbon percentage – in establishing how much martensite versus higher temperature, weaker phases are formed. A rapid cooling rate, i.e., quenching, and higher levels of carbon plus other elements in the composition such molybdenum, chromium, silicon and cobalt promote martensite formation. Small amounts of other elements often are added for other reasons.

Stainless steels differ substantially in their compositions versus the present steels. Mild or plain carbon steels, contain primarily iron and carbon. Low-alloyed steels contain small quantities of the above elements in addition to carbon.  However, the various grades of stainless steels may contain much higher levels of chromium, nickel, molybdenum or other elements and differ in several ways from mild and low-alloyed steels.

Martensite is the most desirable metallurgical phase to result from heat treatment because it provides the highest hardness and thus the highest strength. However, this completed phase is brittle and has very low ductility. Therefore fracture with little or no elastic deformation under load in the finished product can be the result. This characteristic is overcome by the heat treatment process of tempering. In tempering the steel in its martensitic state is reheated (to a selected level below the austenitic temperature range) and held at that temperature for a selected period of time before being allowed to slowly cool. The result is tempered martensite which provides some loss of hardness (and strength) from its un-tempered condition but with a gain in ductility and resistance to brittle fracture. The extent of hardness loss (and ductility gained) increases as the tempering temperature and holding time at that temperature are increased. The results are the familiar quenched and tempered (Q & T) mild and low-alloy steels that can include a range of grades and associated properties.

Trade-offs among the levels of carbon and other elements in the composition of a steel coupled with the cooling rate in the initial heat treatment and the tempering parameters used are the basis for making the optimal steel for a given application. Clearly many effects need to be considered.         

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