FORGING DIE DESIGN Die Design

Introduction Dies are employed for production of forgings in large numbers and or quantities. Correct design of die can affect both the economics of production and the properties of the forged component

Die Material and Processing Drop forging dies are made either of a plain carbon steel, some-times with a small % vanadium in addition, or of a nickel-chromium-molybdenum steel. A few typical composition are given below Carbon Manganese Nickel Chromium Vanadium Molybdenum 0.5-0.7 0.5-0.9 1 - 2 0.5-1.0 - 0.15-0.35 0.5-0.65 0.6-0.8 0.6-0.75 0.2-0.4 0.75-0.9 0.3

Die blocks should be thoroughly normalised before machining them Die blocks should be thoroughly normalised before machining them. After machining, the impressions are often checked by making lead cast of them. The die block is then heated up to the required temperature for hardening. And then tempering is carried out. The hardness for shallow impressions is about 58 and is about 45 for impressions 75 mm or more in depth.

Considerations in closed die design Draft: Draft is the taper put on all sides of the forging in order to be able to remove it from the dies. Outside and inside draft angles are 5-7 degrees and 7-10 degrees respectively. Parting line: A line usually at the largest cross-section of the part. For simple symmetrical shapes, the parting line is normally a straight line at the center of the forging, bit for more complex shapes the line may not lie in a single plane. The dies are then designed in such a way that , they lock during engagement, in order to avoid side thrust, balance forces, and maintain die alignment during forging.

Radii and Fillets: They indicate a change in the direction of metal flow in a closed die forging. Sharp radii and fillets increase the tendency toward forging defects and accelerate the die wear. Therefore, the size of all radii and fillets should be made as large as possible. Pockets and Recesses: For forming pockets and recesses in forgings, it requires corresponding raised sections in die impressions. For small holes, it is better to omit raised sections from the dies and machine the holes later in the forgings. However, in other cases, where raised sections in die impressions are must, it is desirable to make all connecting radii as generous as possible, as well as to permit generous draft angles.

Ribs and Thin sections: The production of both ribs and thin sections on the forgings require corresponding thin slots in the die impression. Therefore, in order to be filled properly the ribs should be kept as low and as wide as possible with in the design limit. Dimensional Tolerances: Dimensional tolerances ie., the variations permitted from the normal dimensions have been standardized for commercial drop die forgings. Weight limits, surface conditions, straightness, close-hardness range, close- composition range may also be incorporated.

Materials for Forging Die Block Factors in the selection of Die Material: Die materials are selected on the basis of the following characteristics:- 1) Ability to harden uniformly 2) Ability to resist the abrasive action of the hot metal while it is being forged 3) Ability to withstand high pressure and heavy shock loads 4) Ability to resist cracking and checking caused by heat

Selection of the most suitable combination of steel and hardness for the die blocks are influenced by:- 1) Shape, size, and weight of the forging 2) Composition of the metal to be forged 3) Temperature at which work piece metal is to be forged 4) Number of forgings to be made 5) Type of forging equipment (hammer or press) 6) Cost of the die steel 7) Sequence of machining the die impressions (before or after hardening) 8) Forging tolerances ( including draft angles)

Lubrication Lubrication is the one factor which greatly influence friction and wear and affects the forces required and also flow of metal in the die cavities. They can also act as a thermal barrier between the hot work piece and relatively cool dies, slowing down the rate of cooling of the work piece and improving the metal flow.

One of the important role it has during the forging operation is it acts as parting agent ie., which inhibits the forging from sticking to the dies and helps it in release from the die.

Varieties of Lubrication: For hot forging: Graphite, Molybdenum disulfide, glass is also used. For cold forging: Mineral oil and soaps

Forging Defects In addition to surface cracking during forging, other defects can develop as a result of material flow pattern in the die. If there is an insufficient volume of material to fill the die cavity, the web may buckle during forging and develop laps. On the other hand, if the web is thick, the excess material flow past the already formed portions of the forging and develops internal cracks.