Heat transfer from the workpiece to the die surfaces causes thermal gradients in the workpiece. The cooler areas at the die surfaces undergo less plastic flow than in the hotter core areas, so that plastic flow is not uniform. This is termed die chilling. In conventional forging practice, dies for steel forgings are typically heated to a maximum temperature range of 400 to 500°F (205 to 260°C), depending on equipment, to reduce chilling. The effects of chilling can also be reduced by using fast-acting forging machines, such as hammers, screw presses and mechanical presses, to reduce the contact time. The use of glass lubricants assists by forming a thermal barrier between the workpiece and die surfaces and reduce the die chilling effect.
Die chilling can be reduced by heating the dies nearer to the actual forging temperature. Die chilling can be eliminated entirely by heating the dies to essentially the same temperature as the workpiece. The former is called hot die forging; the latter isothermal forging.
Aluminum alloys are usually hydraulic press forged under isothermal or near isothermal conditions at around 800°F (425°C). In this range, conventional die materials do not undergo any significant loss of strength or hardness.
However, steels and alloys of titanium and nickel are forged in the range of 1700 to 2300°F (925 to 1260°C). Isothermal forging of these alloys requires special tooling materials, such as nickel-based superalloys and molybdenum alloys for dies, and lubricants that can perform adequately at these temperatures. Special attention to the surrounding atmosphere is also important, such as the use of an inert gas or vacuum to protect both the dies and the workpiece from oxidation.
Hot die and isothermal forging offer advantages and disadvantages. The primary advantages are closer forging tolerances resulting in reduced machining and material costs, a reduction in the number of preforming and blocking operations resulting in reduced processing and tooling costs, and the use of slow ram speeds resulting in lower forging pressures and the use of smaller machines.
The primary disadvantages are the requirements for more expensive die materials, uniform and controllable die heating systems, and an inert atmosphere or vacuum around the dies and workpiece to avoid oxidation of the dies. The typical production rates are very low to permit proper die filling at the low forging pressures.
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Heat transfer from the workpiece to the die surfaces causes thermal gradients in the workpiece. The cooler areas at the die surfaces undergo less plastic flow than in the hotter core areas, so that plastic flow is not uniform. This is termed die chilling. In conventional forging practice, dies for steel forgings are typically heated to a maximum temperature range of 400 to 500°F (205 to 260°C), depending on equipment, to reduce chilling. The effects of chilling can also be reduced by using fast-acting forging machines, such as hammers, screw presses and mechanical presses, to reduce the contact time. The use of glass lubricants assists by forming a thermal barrier between the workpiece and die surfaces and reduce the die chilling effect.
Die chilling can be reduced by heating the dies nearer to the actual forging temperature. Die chilling can be eliminated entirely by heating the dies to essentially the same temperature as the workpiece. The former is called hot die forging; the latter isothermal forging.
Aluminum alloys are usually hydraulic press forged under isothermal or near isothermal conditions at around 800°F (425°C). In this range, conventional die materials do not undergo any significant loss of strength or hardness.
However, steels and alloys of titanium and nickel are forged in the range of 1700 to 2300°F (925 to 1260°C). Isothermal forging of these alloys requires special tooling materials, such as nickel-based superalloys and molybdenum alloys for dies, and lubricants that can perform adequately at these temperatures. Special attention to the surrounding atmosphere is also important, such as the use of an inert gas or vacuum to protect both the dies and the workpiece from oxidation.
Hot die and isothermal forging offer advantages and disadvantages. The primary advantages are closer forging tolerances resulting in reduced machining and material costs, a reduction in the number of preforming and blocking operations resulting in reduced processing and tooling costs, and the use of slow ram speeds resulting in lower forging pressures and the use of smaller machines.
The primary disadvantages are the requirements for more expensive die materials, uniform and controllable die heating systems, and an inert atmosphere or vacuum around the dies and workpiece to avoid oxidation of the dies. The typical production rates are very low to permit proper die filling at the low forging pressures.
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Heat transfer from the workpiece to the die surfaces causes thermal gradients in the workpiece. The cooler areas at the die surfaces undergo less plastic flow than in the hotter core areas, so that plastic flow is not uniform. This is termed die chilling. In conventional forging practice, dies for steel forgings are typically heated to a maximum temperature range of 400 to 500°F (205 to 260°C), depending on equipment, to reduce chilling. The effects of chilling can also be reduced by using fast-acting forging machines, such as hammers, screw presses and mechanical presses, to reduce the contact time. The use of glass lubricants assists by forming a thermal barrier between the workpiece and die surfaces and reduce the die chilling effect.
Die chilling can be reduced by heating the dies nearer to the actual forging temperature. Die chilling can be eliminated entirely by heating the dies to essentially the same temperature as the workpiece. The former is called hot die forging; the latter isothermal forging.
Aluminum alloys are usually hydraulic press forged under isothermal or near isothermal conditions at around 800°F (425°C). In this range, conventional die materials do not undergo any significant loss of strength or hardness.
However, steels and alloys of titanium and nickel are forged in the range of 1700 to 2300°F (925 to 1260°C). Isothermal forging of these alloys requires special tooling materials, such as nickel-based superalloys and molybdenum alloys for dies, and lubricants that can perform adequately at these temperatures. Special attention to the surrounding atmosphere is also important, such as the use of an inert gas or vacuum to protect both the dies and the workpiece from oxidation.
Hot die and isothermal forging offer advantages and disadvantages. The primary advantages are closer forging tolerances resulting in reduced machining and material costs, a reduction in the number of preforming and blocking operations resulting in reduced processing and tooling costs, and the use of slow ram speeds resulting in lower forging pressures and the use of smaller machines.
The primary disadvantages are the requirements for more expensive die materials, uniform and controllable die heating systems, and an inert atmosphere or vacuum around the dies and workpiece to avoid oxidation of the dies. The typical production rates are very low to permit proper die filling at the low forging pressures.
Return to Table of Contents
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