Forging. Types of forging process. 1. Open Die Forgings or Hand forgings. Lecture Notes on Manufacturing Process

Transcription

1 Forging Forging is manufacturing process where metal is pressed, pounded or squeezed under great pressure into high strength parts known as forgings. Heated metal to be shaped is placed on a mold. Pressure is applied to the metal with the help of a press or hammer and due to this impact the malleable metal conforms to the Die cavity shape. Extreme pressure is produced when the die halves are closed. The seam of the die as well as punch acts as a relief valve. Once the metal object is shaped, flashing is removed Use of a proper lubricant during the process helps to prevent sticking of the work piece with the die. It also acts as a thermal insulator and helps the wear and tear on the die Types of forging process 1. Open Die Forgings or Hand forgings In this process the forgings are made with the help of repeated blows in an open die. Here the operator manipulates the work piece in the die during the blow process, similar to the traditional manufacturing process used by a blacksmith. Figure- Open die drop hammer forging

2 2. Impression Die Forgings or Precision Forgings In impression die forging, two dies are brought together, squeezing the metal causing it to fill the die impression. Depending on the design, metal can be allowed to flow outside of the dies or can be allotted no escape. The impression die forging process usually requires one hit through a die cavity for the metal. The impression die forging process relies heavily on the work of the dies and less on the work of the operator. Figure- Impression die forging

3 These forgings are the refined form of blocker forgings. The finished metal part much more identical to the die impression. 3. Close Die Forging/Flashless forging Close die or close impression die forging overcomes the limitation of open die forging. Figure shows typical set of dies. Here the closed impression rather than open face dies are used. In this process there is a drastic flow of metal in the dies due to repeated hammer blows on the metal. For products of complicated shapes a preliminary shaping operation using more than one set of dies may be required. Close die forging also overcomes the problem of flash formation in impression die forging. 4. Upset Forgings These forgings increase the cross-section by reducing the length of the metal. The process is used to make heads on valves, bolts and fasteners, and other similar parts. Length of the stock to be upset should not be more than two to three times its diameter otherwise bending of stock occurs. Forging machine can produce 400 to 600 parts/hr.

4 (a) (b) Figure (a) Upset forging (b) Roll forging 5. Roll forging In roll forging thickness of round or flat bar stock is reduced and length is increased. Forging machine have two cylindrical or semi cylindrical rolls containing one or more grooves to impart a desired shape to the work piece. Main application of roll forging is to found in leaf spring, axles, lever, etc. 6. Swaging It is used to reduce the diameter of the rod or tube to impart a desired shape to the piece. In swaging die itself act as a hammer and metal is hammered from various angles due to which metal flows inward and take the shape of die

5 Forging operation 1. Drawing out: The operation in which cross-section area reduces and length of stock increases is known as drawing out operation. In this operation force is to applied in a direction perpendicular to the length axis 2. Upsetting: The operation in which cross-section area increase and length of stock reduces is known as upsetting operation. To perform the operation force applied in a direction parallel to the length axis 3. Heading: heading operation is done at one end of the stock to produce large crosssection. Head of bolts, nails and screw are formed by heading operation. 4. Fullering: It is the operation of displacing the metal away from centre to reduce thickness in middle and increase the length (a) (b) (c) Figure (a) Fullering, (b) edging and (c) bending operation. 5. Edging: It is the operation in which material is displaced from edge to inside i.e. between two cup shaped die to create bulge in middle. 6. Bending: In bending operation die are shaped to produce directional change in the work piece rather than changing the size of section. Forging Defects Main cause due to which defects occurs in forging are improper heating, incorrect die design, uneven cooling of stock, defect in original metal, improper forging operation, incorrect forging condition etc. Some common defects found in forging are

6 1. Cold shut: Cold shut are short cracks. They are caused due to folding of metal surface against itself during forging. It can be avoided by rounding the sharp corner. 2. Pitting: pitting is caused by scales which is not completely removed from the die cavities and is worked into the surface of forging. Scale produces shallow depression on the surface of forging. It can be avoided by cleaning of dies and proper control of forging. 3. Incomplete penetration of dies: Due to incomplete filling of closed compression die cavity these defects occurs. It occurs due to many reason such as poor forging design, poor heating, insufficient no of blows during forging etc. 4. Die shift: Die shift is caused due to upper and lower die misalignment. This defect is caused due to lose wedges. So wedges should be rigidly fixed. 5. Fins and rags: Fin and rags are small projection or piece of loose metal driven into the surface of forging. To control this defect proper die design is required. 6. Cracks: Both transverse and longitudinal cracks occur on forging surface. It occurs due to improper heating of forging, bad quality ingot and improper cooling of alloy steel. 7. Flakes: Flakes are internal breaks and ruptures caused due to rapid cooling of forging. 8. Hair line cracks: hair line cracks are very fine cracks on the surface of the forging caused due to rapid cooling of forging and defective metal piece