Introduction to Forging
Forging = Compressive forming process where material is shaped by applying compressive forces through dies or tools
Key Characteristics
- One of the oldest metal forming processes
- Compressive forces applied
- Material flow in lateral directions
- Grain flow follows part contour → excellent mechanical properties
- Hot or cold working
Types of Forging
1. Open Die Forging (Smith Forging)
Process:
- Workpiece compressed between two flat or simple-shaped dies
- Material flows laterally (not constrained)
- Multiple blows and repositioning required
- Operator skill critical
Features:
- Simple tooling (low cost)
- Flexible (various shapes possible)
- Low production rate
- Poor dimensional accuracy
- Suitable for large parts, low volumes
Operations:
- Upsetting/Heading: Increasing cross-section, reducing height
- Drawing out: Increasing length, reducing cross-section
- Fullering: Creating grooves
- Edging: Gathering material
- Bending: Changing shape angle
- Punching: Creating holes
Applications: Large shafts, discs, rings, custom parts, prototypes
2. Impression Die Forging (Closed Die Forging)
Process:
- Material compressed in shaped die cavities
- Dies contain impression of desired shape
- Material flows to fill cavity
- Flash forms at parting line (excess material)
Features:
- Complex shapes possible
- Better dimensional accuracy than open die
- Higher production rates
- Expensive tooling
- Flash must be trimmed
Flash:
- Excess material squeezed out at parting line
- Purpose: Creates back pressure to fill die cavity completely
- Must be removed in trimming operation
Forging Force: F = K × A × Yf
Where:
- K = Multiplying factor (3-12, depends on complexity)
- A = Projected area including flash
- Yf = Flow stress of material
Applications: Automotive parts (crankshafts, connecting rods), hand tools, hardware
3. Flashless Forging (True Closed Die Forging)
Process:
- Material completely contained in die cavity
- No flash formed
- Precise volume control critical
Features:
- No material waste (no flash)
- Excellent dimensional accuracy
- Higher forces required
- Precise blank volume essential
- More expensive dies
Applications: Precision parts, gears, aerospace components
4. Upset Forging
Upset Forging = Increasing cross-sectional area by compressing length
Process:
- Bar or rod held in die
- End is struck, material upsets (bulges)
- Used to form heads on fasteners
Applications:
- Bolt heads, screw heads
- Valve stems
- Gear blanks
Limitations:
- Maximum upset length ≈ 3 × diameter
- Beyond this, buckling occurs
Forging Temperature
Hot Forging
- Temperature: Above recrystallization temperature (typically 1000-1300°C for steel)
- Lower forces required
- Large deformations possible
- Poor surface finish (scale)
- Less dimensional accuracy
- No strain hardening
Warm Forging
- Temperature: Between cold and hot (700-900°C for steel)
- Intermediate properties
- Better surface finish than hot
- Lower forces than cold
Cold Forging
- Temperature: Room temperature
- Higher forces required
- Excellent surface finish
- Better dimensional accuracy
- Strain hardening (increased strength)
- Limited deformation
Forging Defects
1. Incomplete Filling (Underfill)
- Die cavity not completely filled
- Causes: Insufficient material, low temperature, complex shape, inadequate force
2. Cold Shut
- Fold or lap on surface where two material flows meet but don't fuse
- Causes: Low temperature, insufficient force, poor die design
3. Scale Pits
- Oxide scale pressed into surface
- Causes: Inadequate descaling, hot forging
4. Die Shift
- Misalignment between upper and lower dies
- Causes: Die wear, machine problems
5. Cracks
- Surface or internal cracks
- Causes: Excessive deformation, low temperature, material defects, sharp corners
6. Laps
- Surface folds
- Causes: Improper material flow, die design issues
Forging Equipment
1. Forging Hammers
- Drop hammer: Gravity-driven
- Power hammer: Steam or compressed air driven
- Impact energy: 300 J to 400 kJ
- Multiple blows required
- Good for open die forging
2. Forging Presses
- Mechanical press: Crank or eccentric driven
- Hydraulic press: Hydraulic cylinder driven
- Force: 1 MN to 500 MN
- Slow, continuous pressure
- Single stroke usually sufficient
- Better dimensional control
3. Upset Forging Machines
- Horizontal machines
- Specialized for upsetting operations
- High production rates
4. Roll Forging
- Material shaped between rotating rolls
- Continuous process
- Used for tapered parts, preforms
Forging Design Considerations
Draft Angles
- Draft: Taper on vertical walls to facilitate part removal
- Typical: 3-7° for steel, 1-3° for aluminum
- Larger draft for deeper cavities
Fillet and Corner Radii
- Sharp corners cause stress concentration and incomplete filling
- Minimum radius: 3-5 mm typical
- Larger radii improve material flow
Parting Line
- Location where dies meet
- Should be at maximum cross-section
- Affects flash formation and trimming
Web Thickness
- Minimum thickness between features
- Too thin → incomplete filling
- Typical minimum: 3-6 mm
Rib Design
- Ribs for stiffening
- Height/thickness ratio limited
- Typical: height ≤ 3 × thickness
Advantages of Forging
-
Superior mechanical properties
- Grain flow follows part contour
- No internal voids or porosity
- High strength and toughness
-
Material efficiency
- Minimal waste (especially flashless)
- Near-net shape
-
Reliability
- Consistent properties
- Suitable for critical applications
-
Wide range of sizes
- From small fasteners to large shafts
-
Good surface finish (cold forging)
Limitations of Forging
- High tooling cost (impression die)
- Limited to relatively simple shapes
- High forces required (especially cold)
- Not economical for small quantities (closed die)
- Secondary operations often needed (trimming, machining)
Applications
Automotive: Crankshafts, connecting rods, gears, axles, steering components
Aerospace: Landing gear, turbine discs, structural components
Hand tools: Wrenches, pliers, hammers
Hardware: Bolts, nuts, hooks, chains
Oil & Gas: Valves, flanges, fittings
Heavy equipment: Crane hooks, lifting eyes, shafts