HEAT TREATMENT – AN INTRODUCTION
                                    
The preceeding metallurgy pages have provided an insight into what metals are chemically. Usable workshop material, be it in a near ELEMENTAL form (like HIGH CONDUCTIVITY COPPER and WROUGHT IRON which are typically ≥99.9% pure), or else an ALLOY (BRONZE, BRASS, STEEL etc), can be prepared not just to a particular shape (plate, angle, bar etc), but also to a specific metallurgical CONDITION (HARD, HALF HARD, ANNEALED etc). In the workshop, the metalworker can further modify both the shape of the material and it's metallurgical condition as they progress with their work. Both forming and the changing the metal’s metallurgical structure often as not requires the application of heat; though there are exceptions such as COLD FORMING, a process which will be looked at later.

The next few articles discuss the common HEAT TREATMENTS that can be applied to metals; what they are, how they are applied and the effect they have on a design. The examples focus on the common fabrication material STEEL, which is essentially an alloy of IRON and CARBON. Most of the steels used for creating art and design works are MILD STEELS; technically, these steels are also known as LOW CARBON STEELS (LCS), so called because they typically contain a very small quantity of CARBON (typically <0.3%). These steels also contain other alloying elements including SULPHUR, PHOSPHORUS and occasionally COPPER in even smaller quantities (≤0.05%) [ref 1].

As with CAST IRON, it is the addition of CARBON that contributes much to the specific working properties of steel. A high purity iron material such as WROUGHT IRON is very soft and DUCTILE, this makes it ideal for forming (in a FORGE for instance), however this metal lacks much of the STRENGTH found in CAST IRONS which have a higher CARBON content (2-4% typically, dependent of alloy type) [ref 2]. The ‘trade off’ here is that most cast iron materials lack the DUCTILITY and MALLEABILITY of steel and can fracture all too easily when worked or welded. MILD STEEL is something of a working median between WROUGHT IRON and CAST IRON. This is a typical example of how relatively small adjustments in the proportion of a critical alloying element (in this case CARBON), can produce a range of materials with markedly distinct working properties.

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