Cold working is any process such as rolling, wire drawing, swaging, bending, stretching, or other processes that are used to harden and work a material to its final shape. To cause this shape change, a force must be applied which exceeds the yield point of the material. Cold working a metal results in an increase in strength or hardness and a decrease in ductility. It is an important industrial process that is used to harden metals or alloys, which do not respond to heat treatment. Microstructurally, cold work produces elongated grains in the principal direction of the work.
The formation and movement of dislocations allows a gold alloy to be deformed. Dislocations are submicroscopic imperfections in the crystalline structure of the material, which are not visable to the naked eye. Plastic deformation causes the number of existing dislocations and other structural defects to move and multiply. The accumulation of dislocations and defects causes an alloy to become harder. If the density of dislocations and defects becomes so great that plastic deformation can no longer occur, the material being cold worked will crack or fracture.
Dislocation movement can be compared to stretching a carpet by creating a ripple or ridge in the carpet and moving the ripple to the opposite end. The ridge is equivalent to a dislocation. By repeating this procedure, the carpet can be moved a considerable distance as the total effect of all of the ripple movement accumulate at one end of the carpet. Unlike the carpet, the strained gold alloy can be returned to a structure that is ready for more cold work by annealing it.
The driving force for annealing is the elimination of the stored energy that accumulates in the piece during cold working (plastic deformation). About 5% of the energy used for plastic deformation is stored in the material while the remainder is converted into heat. A good question to ask at this point is "How much cold work should I or can I do to my product?"
The amount of cold work that can be done to a product is dictated by several major factors. Some of the most important factors are:
- Type of alloy
- Degree of cold work already in the product
- Presence of as-cast, or dendritic structure
Most of the standard gold alloys can be cold worked up to 70% reduction without risking failure. Dendritic, or as-cast structures can have a dramatic impact on the degree of deformation which some alloys can tolerate. Some alloys require special handling during or after casting to achieve good cold working characteristics.
Gold alloys such as nickel whites, age hardenable colored gold alloys and palladium white gold alloys modified with copper, nickel or zinc should not be allowed to slow cool past the red color or a decrease in ductility can be experienced. 18 karat red and 18 karat white gold are especially sensitive and may crack immediately when cold worked if allowed to slow cool past the red color. This is especially true for gold cast into investment molds. Determining the degree of cold work in a purchased product would be difficult without information either from the vendor, a metallographic cross section or experience.
When producing sheet, plate or shaped wires it is best to develop a schedule for the cycles of cold work and annealing that will be required. A schedule consists of a series of reductions totaling 50-70 percent followed by annealing that will be required. It is the total percentage of reduction between anneals that is important, and it is independent of whether the process is rolling flat sheet, square wire or drawing round wire. These processes are only different ways to achieve the total reduction required to produce a desired product.
The ultimate goal of any cold working process is to produce a final product with the desired shape and the smallest possible grain size. A logical starting point is to determine what thickness and temper are required for the final product and work back up to the initial starting size required. Some basic steps are as follows:
- Determine the thickness and temper of the final product.
- Reduction of an as~cast, "dendritic" structure should be a minimum of 45% but should not exceed about 55%. This will prevent damage to less ductile dendritic structure.
- Total reduction at the intermediate stages between anneals should be a minimum of 55% and should not exceed 70%.
- Best results are achieved if at least two intermediate stages of cold work followed by annealing are scheduled prior to the final stage.
- Unless a dead soft product is desired, more consistent results are obtained if a product is cold worked to its final temper instead of trying to anneal the final product to a desired temper.
At this point, some formulas will be required to assist in calculating the percentage reduction between desired points where annealing should take place. Since it would be difficult to cover the variety of shapes that could be created, we will focus on the three most common mill products, flat sheet, square and round wires.
The thickness of sheets and diameter of wires are usually described by Brown and Sharpe (B&S) or American Wire Gauge (AWG) measuring systems. Tables of gauge numbers with English and metric dimensions are given in the appendix.
In strict mathematical terms, the percent reduction that occurs during any type of cold working process, be it sheet rolling or wire drawing, is calculated based on the change in cross sectional area. It is calculated using the equation given below:
% Reduction = (Initial area - Final area) / Initial area
For sheet materials, the reduction during rolling is usually based on the decrease in sheet thickness for simplicity:
% Reduction = (Initial thickness - Final thickness) / Initial thickness
Note, however, that this equation is strictly valid only if the width of the sheet does not change during the rolling operation.
For wire drawing the equation can be simplified to:
% Reduction = (Initial diameter)² - (Final diameter)² / (Initial diameter)²
For Square rod rolling use the following equation:
% Reduction = (Initial thickness)² - (Final thickness)² / (Initial thickness)²