COLD FORMING & WORK HARDENING

There are various ways of producing stock items in the mill then modifying these basic shapes in the workshop, this includes ROLLING, BENDING, DRAWING and FORGING for example. Whilst there is a large number of forming methods available to the metalworker, the way in which the material is treated during processing can be divided into one of two general categories, COLD FORMING and HOT FORMING.
 
As the name implies, COLD FORMING (also COLD WORKING, REDUCING, PROCESSING etc), involves working the metal whilst cold (or more specifically at temperatures below the LOWER CRITICAL POINT (LCP), which can be as high as 600˚C), HOT FORMING on the other hand, requires the metal to be heated to a temperature beyond the LCP. Lower working temperatures in cold forming means that the CRYSTALINE STRUCTURE of the metal is less PLASTIC than when hot, this causes the grains to become distorted (usually in the direction of working). Generally speaking, the more force applied during working, the more distorted the crystalline structure. As a rule therefore, cold forming is better suited to metals and alloys that are in a SOFT or ANNEALED condition, these plastic materials are more able to distort without failing as they are formed. Soft metals and alloys are essential for some of the more extreme forming processes such as DEEP DRAWING (see also PROPERTIES - STRENGTH).

As a rule HIGH PURITY METALS are better suited to cold forming than many ALLOYS, this is because alloying has a tendency to HARDEN the parent metal (this is one reason why alloyed GOLD is considered superior to fine gold for many jewellery items, the pure metal distorts too easily in use), GOLD, SILVER, LEAD, TIN, COPPER, ALUMINIUM in unalloyed states are easily cold formed, some alloys are also well suited to the process, this includes LOW CARBON STEELS (annealed), CARTRIDGE BRASS (a 70/30 alloy), PEWTER and MAGANESE ALUMINIUM combinations. Alloys like LEADED GUNMETAL becomes brittle when heated and can only be worked (after being cast), whilst cold.

One of the consequences of cold forming is that the distortion of the metal’s crystalline structure tends to HARDEN the worked material. The greater the force and distortion, the harder the material becomes and the more plasticity/ductility is lost. The progressive loss of plastic properties in this case is referred to as WORK HARDNEING.  If taken to an extreme, work hardened material fails and cracks, and this can mean scraping the design. Few metals can endure repeated or extreme cold forming without some significant EMBRITTLEMENT and hardening. Fortunately, as has been demonstrated before (see NORMALISING & ANNEALING), HEAT TREATMENTS can be applied to a design for counteract the effects of work hardening. A work hardened design can be RE-CRYSTALLISED [RE-CR] via a NORMALISING heat treatment process, full ANNEALING is undesirable because the re-growth of the grain structure becomes too extreme which can diminish the surface quality of the finished design. Re-crystallisation may sometimes be carried out in a REDUCING (low oxygen) atmosphere to minimise the development of scale of fine surfaces.

COLD FORMING in the mill produces a distinct product from the same metal when HOT FORMED. Typically, cold formed material has a refined visual quality; the surface of the finished product is usually very smooth and can even be described in some cases as SEMI-POLISHED. This quality can be very important in art and architectural design metalwork, especially when a very high standard of paint finish or polish is to be applied later; the rougher and sometimes even pitted surfaces associated with hot formed metals can make these high quality finishes more difficult to achieve. Cold formed material is also often selected when a high degree of accuracy is essential, it is possible to produce cold rolled material to specifications simply not achievable through hot forming. Antony Gormley’s ‘Blockwork’ series of sculptures for example, rely heavily on the accuracy of COLD DRAWN material to maintain both the visual and spatial integrity of each work.

Stock items such as PLATE and SHEET are MILL ROLLED, other items such as WIRE and PROFILES are usually DRAWN through a series of DIES. Welding rods are a common example of a COLD DRAWN material, welding rod diameters are traditionally based on IMPERIAL WIRE GAUGE and are produced in diameters to an accuracy of at least two decimal places (ie 3.25mm).

A final note; COLD WORKING is often used to refine FORGED and WELDED material. It is not unusual to see a BLACKSMITH carefully finish a forged section by lightly hammering the newly worked surface. This has the effect described above of slightly modifying the crystalline structure of the forging, both hardening/polishing the new form. Similarly, welders will sometimes work over a welded joint with a PEENING HAMMER or AIR HAMMER; again this action refines the METALLURGICAL CONDITION of the weld, and is especially beneficial if the heat input in the HEAT AFFECTED ZONE has had an softening effect on the metal’s structure or if the alloy is prone to some types of welding fault (see also GUNMETALS).

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