The Methods of Preparing Stock or Billets
The forging process generally begins with the bar stock. The process requires the barstock to be cut into billets of the proper weight and length for our partners’ projects. Providing the proper billet size may be done by one of several cutting methods:
Cold Shearing: Cold shearing is the most common because it is the most economical and highly productive method.
Hot Shearing: Hot shearing requires a heating source to bring bars to proper temperature prior to cutting. It can offer the advantage of n in line process when done as part of the forging operation itself.
Sawing: Sawing is used when the design or tolerance is less than that allowed for cold shearing, or when the material is not cold shearable, even in an annealed condition. It is also the means used when only a few pieces are needed, or when the shear capacity is not adequate.
Torching: Torch cutting of billets can be used if it does not affect the metallurgical properties of the material. Low carbon steels fall in this category. Low productivity, and high costs generally limit this method of cutting.
Abrasive Cut Off Machines: Abrasive cut off machines are generally used for materials which cannot be easily cut by another method. Exotic materials, such as titanium, require abrasive cutting. Since the abrasive blades wear away during cutting, costs are high. Other environmental concerns also must be considered with this method of cutting.
Heating the Steel Billets
Prior to forging, it is necessary to heat the billets until they are in a “plastic” condition in order to enhance the material flow. This also reduces the force that is required to move the steel into the shape of the forging dies. The maximum forging temperatures for steels are determined by the material grade and chemistry. Generally, for carbon and alloy steels, the temperature range will fall between 2,200 deg, F and 2,350 deg. F.
Furnaces for heating steel to forging temperature may be either bath type or continuous, and may be fueled by natural gas, propane, oil, or electricity.
Batch type furnaces that are fired with natural gas are presently the most common for forge plants that run order sizes that take less than 4 or 5 hours of production. Most batch types are open across the front to provide the maximum flexibility for short run jobs. Such furnaces are referred to as “slot type.” Other batch types are rotary hearth, box, and car bottom.
Continuous types may use a conveyorized belt transfer system, a “walking beam” transfer system, or billets may be pushed through the furnace. These are generally not as flexible as the batch type.
Fossil fuel fired furnaces generate gases from combustion of the fuel. These are carbon monoxide, carbon dioxide, nitrogen, and water vapor. Carbon dioxide and water vapor are oxidizing agents which cause decarburization and scale on the billets. Carbon monoxide is a carburizing agent. Setting the furnaces to an optimum ratio of 10 parts air to 1 part fuel (called stoichiometric combustion) will result in an atmosphere that is essentially neutral – neither carburizing nor decarburizing – to the steel.
Electric heating systems for forging furnaces may be induction, direct resistant, or radiant type. Induction is the most common. This system consists of a water cooled copper coil (or series of coils) that conduct power of the correct frequency and voltage level but insulated from direct contact with the workpiece while it passes through. Induction heating systems can be completely automated and are the best systems for high volume applications.