FORGING DIES

• Proper die design is important in the success of a forging operation.

• Parts to be forged must be designed based on knowledge of the principles and limitations of the process.

• Note : Design of open die is generally straight forward because the dies are relatively simple in shape.

Impression Die in Forging

TERMINOLOGIES FOR CONVENTIONAL IMPRESSION DIE

• Parting line : The parting line is the plane that divides the upper die from the lower die.

(Called as flash line in impression die forging) Its selection by the designer affects grain flow

in the part, required load and flash formation.

Contd…

• Draft : is the amount of taper on the sides of the part required to remove it from the die.

• The term also applies to the taper on the sides of the die cavity.

• Typical draft angles are 3o on aluminium and magnesium parts and 5o to 7o on steel parts.

• Draft angles on precision forgings are nearly zero.

Contd…

• Webs & Ribs : A web is a thin portion of the forging that is parallel to the parting line, while a rib is a thin portion that is perpendicular to the parting line.

• Note : These part features cause difficulty in metal flow as they become thinner.

Contd…

• Fillet & Corner Radii : As shown in figure, these are the small radii that needs to be given to reduce hindrance to metal flow and hence, to reduce stresses on the die surfaces during forging.

• Note : Absence of these radii will cause a defect in forging, called as cold shut or lap.

Contd…

• Flash : Flash formation plays critical role in impression die forging by causing a pressure build up inside the die to promote filling of cavity.

• The land determines the surface area along which lateral flow of metal occurs, thereby controlling the pressure inside the die.

• The gutter permits excess metal to escape without causing the forging load to reach extreme values.

Defects in forgings

1. Unfilled sections : In this, some sections of the die cavity are not completely filled by the flowing metal.

• Cause : Improper design of forging die or using faulty forging techniques.

Contd…

2. Cold shut : This appears as a small crack at the corners of the forging.

• Cause : Small fillet and corner radii results in improper metal flow and ends up as a cold shut.

Contd…

3. Scale pits : This is seen as irregular depressions on the surface of the forging.

• Cause : Improper cleaning of the stock used for forging. Oxide and scale present on the stock surface gets embedded into the finished forging surface

Contd…

4. Die Shift : This is caused by the misalignment of the two die halves, making the two halves of the forging to be of improper shape.

5. Flakes : These are basically internal ruptures caused by the improper cooling of the large forging.

• Cause : Rapid cooling causes the exteriors to cool quickly causing internal fracture.

Contd…

6. Improper grain flow : This is caused by the improper design of the die which makes the flow of metal not following the final intended directions.

Assignment No. 2

• Powder Metallurgy Forging.• Isothermal & Radial Forging.• Roll passes (Roll Pass Sequence).

Die material : Requirement

1. Strength and toughness at elevated temperature.

2. Hardenability and ability to harden uniformly.3. Resistance to mechanical and thermal

shocks.4. Wear resistance – to resist abrasion wear due

to scales present on work piece.

Factors on which die material depends

• Die size• Composition and properties of work piece• Complexity of shape- no of performing steps• Forging temperature• Type of forging operation• Cost of die material• No. of forgings required• Heat transfer from work piece to dies

Common Die material

• Tool and die steels with Cr, Co, Ni, Mo, W, Va.

One important property behind addition of alloying elements mentioned above?

RED HARDNESS(tool retains its hardness and strength when it

becomes extremely hot)

Designation system of tool steel:

• The letter means;• W- water hardened plain carbon tool steel• O- oil hardening cold work alloy steel• A- air hardening cold work alloy steel• D- diffused hardening cold work alloy steel• S- shock resistance low carbon tool steel• T- high speed tungsten tool steel• M- high speed molybdenum tool steel• H- hot work tool steel

Forgeability of Metals

• Forgeability is a combination of the following characteristics:

i) The flow stressii) The ability to fill a dieiii) The degree of deformation that can be

carried out without failure (due to surface or internal cracking).

Order of decreasing forgeability and approximate hot forging temperature range in oC

Forgeability v/s TEMPERATURE

• Although in general the forgeability of metals increases with increasing temperature, for certain metals there is a maximum temperature above which some undesirable phenomena occur, such as fast grain growth or melting of a phase.

NOTE : Fine grain metals have better forgeability. Metals with insoluble inclusions tend to be

brittle and have low forgeability.