QUENCHING & HARDENING
“The heat source is shut down and the remaining liquid mass is allowed to cool without intervention”.
The key phrase here is “without intervention”, this means that the metal not cooled at an artificially accelerated rate. Cooling metals at a rapid rate is known as QUENCHING; often done in water, though other substances can be used (these include CAUSTIC SODA, OIL, SAND and SALT according to purpose or speed of cooling required). The temptation, especially for the inexperienced, is to rapidly cool a design from a high temperature in order to more quickly continue the working process or complete the piece. One of the potential problems of doing this is that the metallurgical condition of the design can be modified, which in turn can cause problems later on.
The process of QUENCHING typically has the effect of HARDENING the rapidly cooled metal. Whilst this can be a desirable outcome (especially when toolmaking), the metal tends to become more BRITTLE making it harder to work or use without causing a FRACTURE. Hardening also means that the metal is likely to become more difficult to cut, this means that procedures like DRILLING and TAPPING a THREAD can be made more problematic. Rapid quenching can also stress a metal design to the point where it can crack or split and not be recovered.
How does the process of hardening come about? Materials like MILD STEEL have a range of very specific temperature points (which can be plotted on a HEATING/COOLING CURVE GRAPH), referred to as CRITICAL POINTS. The UPPER CRITICAL POINT for steel varies according to it’s CARBON content, however it is normally greater than 720°C (this temperature being the LOWER CRITICAL POINT CONSTANT for carbon steels. [ref 1] To harden a steel alloy, it must be raised to a temperature at least 20°C above that specific alloy's upper critical point, and this temperature maintained until the workpiece is SOAKED through.
At these elevated temperatures (cherry red heat), the metallurgical structure and physical properties of steel change. Normally, mild steels are made up of a mixed structure of FERRITE and PEARLITE crystals. Ferrite is composed of IRON crystals; PEARLITE is a 87:13 combination of FERRITE and CEMENTITE (or IRON CARBIDE [IRON-CARBON]), this last substance is very hard and brittle. These two crystal types can be seen distinctly in steels viewed at magnification, as shown in the diagram. At temperatures above the UPPER CRITICAL POINT this structure changes and the CARBON content of PEARLITE dissolves in the iron content to become a SOLID SOLUTION referred to as AUSTENITE. When allowed to cool normally, the process reverses and the CARBON comes out of SOLUTION and the FERRITE/PEARLITE crystalline structure reforms.
By rapidly QUENCHING steel, the process of CARBON coming out of SOLUTION occurs at a much lower temperature (some 400°C lower), than when cooled under ambient conditions. The quenching process prevents the reformation of the normal FERRITE/PERLITE structure, and instead a shard like substance MARTINSITE forms. This substance produces a very hard material, especially so in LOW ALLOY STEELS (also sometimes called HIGH TENSILE STEELS) and SILVER STEELS & TOOL STEELS; these alloys possess a higher CARBON content than the more familiar MILD (LCS) STEELS. Mild steel still hardens when quenched, but the lower carbon content prevents the material from achieving a level of hardness suitable for tool making and cutting operations.
Diagram of typical mild steel grain structure:
Digaram of quenched steel grain structure