NORMALISING & ANNEALING
This process of modification can be continued further, with the same piece of steel returned to it’s original HALF HARD state, or reduced [in hardness] further to a QUARTER HARD or even SOFT condition by further treatments. The process of returning a metallic material to neutral metallurgical condition, is called NORMALISING, reducing hardness to a level below this is known as ANNEALING.
Why NORMALISE or ANNEAL steels and other metals? Often, when working metals into various forms and designs, stresses and strains are set up in the material which could later cause problems, either during further working or in use. This is especially the case when COLD WORKING. Cold working is a means of producing mill stock such as PLATE, as well as being a workshop process that will be looked at in more detail later, however as the name suggests, it involves manipulating unheated or moderately heated material. Working in this way often requires more force than HOT WORKING, leaving the material is less able to disperse the stresses imposed. Furthermore, COLD WORKING has the overall effect of HARDENING the material being worked, as we have seen earlier, hardening can lead to cracking and fracturing of a design or tool. Returning the material to a NORMALISED state removes much of the imposed stress of COLD WORKING and reduces hardness. NORMALISING is essential during extreme REDUCTION processes (beating out COPPER plate for instance), if the worked design is to be saved from STRESS CRACKING. In a similar vein, ANNEALING a metal prior to working makes it easier to manipulate and minimises the level of stress input and likelihood of cracking.
The processes of NORMALISING and ANNEALING are superficially similar. To NORMALISE a design, it is heated to a point slightly over the UPPER CRITICAL POINT (for most CARBON STEELS this is somewhere between 750-850°C or cherry to orange red). The design is held at this temperature just long enough to heat through, before being air cooled in short order. This has the effect of causing metal CRYSTALS to reform to a more REFINED structure, thus returning the material to a more DUCTILE condition.
ANNEALING requires the metal to be heated to a point significantly beyond its UPPER CRITICAL POINT, about 40°C or so is usual (slightly more for steels with a low CARBON content, slightly less for higher carbon tool steels). For most MILD STEELS, the optimum ANNEALING temperature is in the region of 900°C (light cherry to just orange red), at this point the steel is composed of AUSTENITE (the CARBON content of the steel is in SOLID SOLUTION). Unlike NORMALISING, the design is SOAKED at this temperature for some time to ensure thorough heating. This is best done in a KILN or FURNACE which is easily controllable. The heating element can be turned off in due course and the design left in the heater to cool down as slowly as possible. This has the overall effect of significantly reducing the proportion of harder PEARLITE to softer FERRITE as the CARBON comes out of SOLUTION during cooling.
Diagram illustrating the highly