METALWORKING

Post 120 - by Gautam Shah

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Metals are very important materials, because these can be easily deformed into useful shapes, and several times over and over again. The shape, function, and appearance of metal objects are largely determined by the type of metal used. Metals as pure material and as alloys or in mixed states offer innumerable variation of properties. Metals show a wide range of unique characteristics when ‘worked’ over. These ‘working over the metal’ are related to temperatures and stress inductions. Metals also show varied behaviour under different use conditions.

Rusting is an indication How a metal is stressed

Metals begin to change during the conversion processes like casting, forging or rolling. Casting is a melt process but its environment, and cooling-period affect the quality of material. Forging and Rolling, are hot as well as cold processes, which stress the metal and cause alteration in its constituents and their arrangements. In cold working some temperature rise is inevitable. In hot conversion processes the reheating conditions, in-line scale removal, temperature maintenance, and cooling rate, all determine the quality of the product.

Welding heat can cause stresses in metals

There are several processes where unintentional heating of metal occurs. The heating conditions, local or total, include heavy stamping, shaping, forming, shearing, grinding, cutting, welding, and exposure to fire, change the quality of metal material. As these processes are unintentional, the effects cannot be foreseen, and realized in later part of the life of a metal. At such a late stage the section or component is not detachable and replaceable. Unintentional stresses are induced in existing structures when 1. one tries to cutout or attach a component, 2. allow consistent exposure of a structural member (like beam, truss, column, bracket) to fire (of kiln, stove, etc.), and 3. when rusted or damaged parts (such as in automobiles) are removed and replaced with fusion welding.

Toyota Tundra Chassis -is de stressed after assembly

 

Stresses in metal, existing (leftover of earlier processes) or newly set-in, affect the rate of corrosion, paint-ability, metal plating. Sometimes the final pass in hot-rolling generates specific surface patterns, for example, the protrusions on reinforcing bars or checkers on floor plates, ribs. In cold-rolling a specific surface roughness is rolled into strips at the temper mill to improve the deep-drawing operations and to assure a good surface finish on the final product.

Retro-fitting of Oakland bridge

 

Hundreds of metalworking processes have been developed for specific purposes, but these can be divided into SIX broad groups: casting, rolling, extrusion, drawing, forging, and sheet-metal forming. The first five processes subject a metal to large amounts of strain. However, if deformations occur at a sufficiently high temperature, the metal will re-crystallize, that is, its deformed grains will be consumed by the growth of a set of new strain-free grains. For this reason, a metal is usually rolled, extruded, drawn, of forged above its re-crystallization temperature. This is called hot working, and under these conditions there is virtually no limit to the compressive plastic strain to which the metal can be subjected. Other processes are performed below the re-crystallization temperature. These are called cold working. Cold working hardens metal and makes the part stronger. However, there is a definite limit to the strain that can be put into a cold part before it cracks.

 

Scrape Containers

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