Hot Versus Cold Forging of Climbing Gear
Hot and cold forging are different means to a similar end. A manufacturer will choose one process over the other usually for economic reasons, or because one process more efficiently renders the product.
Forging is simply the working of metal by plastic deformation, and it can be done while the metal is either hot or cold, or even warm. If you watched the swordsmith hammer your samauri sword blade into shape, you have a grasp of what forging is all about, although in the case of climbing gear, it is a bit more complicated than that.
Do not confuse forging with machining, which is when you shape metal by removing material, usually by drilling, sawing or milling, a slow and expensive process reserved for items that must be precisely built, such as cam lobes and crankshafts.
Forging is also different from casting, where you pour molten material into a mold. Casting is cheap and can produce a complicated shape, one that would require numerous machining operations, in just one step. But, because casting randomly aligns the molecules, cast products typically aren’t as strong as forged ones. Iron and aluminum patio furniture, lawn jockeys and tin soldiers are often cast. Climbing protection is seldom cast, the micro brass nuts such as the RP may be the exception. The process is common for ice-axe heads and spikes, and ascender cams.
Hot forging involves heating the metal, then shaping it. Usually, a machine stamps the hot metal into a die, which might also be heated. Because the metal is hot, it is easy to move around, allowing for more elaborate shapes than cold forging. Hot forging is common for harder metals, such as steel, that would be difficult to shape when cold. The DMM Revolver carabiner is an example of a complex piece of equipment made by hot forging. Most aluminum belay devices and steel ice axe heads are hot forged, as are some modular crampon front points. Depending on the metal and the degree to which it was heated, the forging process itself might suffice to temper, or strengthen the material. If it isn’t, the product is heat treated after it is hot forged.
Cold forging is done when the metal, usually aluminum, which is soft and easy to work with, is cold. Cold forging is typically less expensive than hot forging, produces a precision product that requires little finish work, and is often used for symmetrical items such as carabiners. To cold forge a carabiner, aluminum bar stock is bent to shape, then smashed in a die to refine the shape. Because soft, non-heat-treated aluminum is weak, the carabiner is heat treated to strengthen it. Not too many years ago, almost (or all) carabiners were cold forged. Today, many carabiners are hot forged, as the process allows for a maximally strong design with minimal metal, i.e., hot-forged carabiners are usually lighter than cold-forged ones.
In a practical sense, only someone schooled in metalwork can visually inspect an item and determine how it was manufactured, and even then he is likely guessing because if labor and cost weren’t issues you could use just about any process to produce any product. As a rough rule, hot-forged gear usually has a more ornate design, a higher strength-to-weight ratio and is more expensive.