Forging is a pretty big deal. For thousands of years, blacksmiths have hammered out tools, weapons, and various objects to advance civilization. Incredibly, the basic concepts still apply today, although they’ve taken massive leaps in efficiency.
Learning to forge takes patience and skill, but understanding the basics on paper is much easier. This guide outlines some common forging methods along with their pros and cons.
What is forging?
Forging is a manufacturing process that heats metal to high temperatures, then uses force to shape it into a practical shape. Popular metal fabrication techniques include pressing, hammering, or rolling.
Many types of metal are suitable for use in a forge, but alloy steel, carbon steel, and stainless steel are some good choices. They gain strength, conductivity, structure, and fatigue resistance when heated to extreme levels. Manufacturers can mass-produce items with minimal waste and specific mechanical properties.
What are some popular forging methods?
1. Cold forging
Cold forging is a process where metal is shaped below its recrystallization temperature. Shearing, drawing, squeezing, and bending are common techniques for working with cold metal.
Cold forming at ambient room temperature is a low-cost method that doesn’t require finishing. Metalsmiths typically use soft standard or carbon alloy steels to cold forge; aluminum suits this process.
- High-quality surface finish
- Easier reproducibility
- Reduced contamination problems
- High yield and tensile strength
- No heating is needed
- Excellent mechanical properties
- Need durable, heavy-duty tools and equipment
- Less ductile metal requires a lot of force
- Metal surface must be scale-free and clean
- May result in residual stress
2. Warm forging
Some metals, like steel alloys, are challenging to shape, so they need to be heated. The warm forging process heats metal to a range of 1202°F (650 °C) and 1832°F (1000 °C), depending on the type of component and metal used. Source.
Warm forging increases the elasticity of the steel, making it easier to shape and less taxing on tools and equipment. Compared to cold forging, the use of heat reduces the tolerance of the forged part and also impacts the surface finish.
- Reduced tooling load
- High rate of production
- Excellent dimensional tolerances
- Increased steel ductility
- Less precise tolerances than cold forging
- More expensive than cold forging
3. Hot forging
Hot forging takes the metal to a heat of 1742°F (950°C) – 2282°F (1250°C). The billet is plastically deformed above its recrystallization temperature, meaning the object forms new grains.
Using extreme heat, the workpiece is much easier to work with. Most manufacturers prefer this method over cold or warm forging.
Metals with a high formability ratio are best for hot forging as the metal’s integrity isn’t compromised.
- Much easier to mold metal
- Intricate designs are possible
- Suitable for most types of finishing work
- Low tooling load
- Hard to lubricate at extreme temperatures
- Low quality surface finish
- Metals can warp, lowering tolerances
- More expensive than cold or warm forging
4. Open die forging
Drop forging can be broken into two categories, open and closed die. The best method will depend on the desired final shape, equipment, tooling capability, and budget.
Open die forging places metal between two flat dies that don’t completely encase the piece. The metal is stamped until it reaches the desired shape.
Also known as smith forging, the open die approach allows the metal to flow as it deforms. This feature means the operator must use skill and experience to position the object correctly to achieve the desired result.
Open die forging is best for simple shaping of large parts. Shafts, rollers, and rings are commonly made using this technique. The aircraft and railway industries also rely heavily on open die manufacturing.
- Lower tooling cost than closed die
- Higher strength and resistance
- Quicker lead times in production
- Finer grain size and continuous grain flow
- Machining is usually needed after forging
- Can’t produce complex parts
5. Closed die forging
Closed die forging (impression forging) heats metal and then places it within two shaped dies. The metal is fully enclosed and compressed between an anvil and hammer until it reaches a desired form.
This method usually requires a series of cavities to take the piece from its basic form to an intricate shape.
Any excess metal is forced out of the die’s cavity, resulting in flash. This metal rapidly cools, resulting in more robust properties.
The closed die method is excellent for making complex parts used in the automotive, agricultural, aerospace, and mining industries.
- Create net shapes requiring minimal/no machining
- Suitable for a wide range of metals
- Cost effective for large scale production
- Make small parts with close tolerances
- High tooling and setup costs
- Potential warping during cooling
- Varying structure of metal grain
6. Roll forging
Roll forging places a piece of metal between two rollers and then compresses it to create a desired form. The rollers have grooves that shape the metal as it passes through them.
The roll forging process can produce various parts with exceptional efficiency and accuracy. It makes automotive parts like crankshafts and connecting rods. Leaf springs, knives, and other hand tools also rely on roll forging.
- Form different materials simultaneously
- Highly accurate with tight tolerances
- Excellent surface finish requires minimal finishing
- Able to produce very long parts
- Energy efficient and minimal material waste
- High tool and equipment investment
- Can only form on two sides of the piece
- Difficult to forge thin parts
7. Press forging
Rather than using impact like a drop hammer, press forging uses continuous force to shape metal slowly. The metal piece is placed between two dies and processed in a single stroke.
Press forging deforms the entire piece of metal while hammering style methods typically only shape the surface.
- Excellent control over the workpiece
- High levels of dimensional accuracy
- Economies of scale for large-scale production
- Potential to automate the process
- Crank press investment is relatively high
- Can be a challenge to descale the blanks
8. Upset forging
Upset forging is also known as heading or cold forging. It places metal in a die and then compresses it using a punch, resulting in a shorter, thicker section of metal. Upset forging is typically carried out at room temperature or with minimal heating.
Upset forging produces a variety of parts, including bolts, screws, and fasteners. It makes parts with impressive strength, surface finish, and dimensional accuracy.
- Aligns the grain flow and eliminates weakness
- Eliminates flash and forging draft
- An efficient manufacturing process
- Relatively high production rate
- The process can be automated
- High tooling costs
- Limited shape complexity
9. Isothermal forging
Isothermal forging is an advanced method of shaping metal. The metal’s temperature is kept constant during forging by heating the die to the same temperature.
The isothermal approach produces high-quality parts with excellent strength, accurate dimensions, and uniform metal flows. Aluminum is a popular choice of metal used to make precision components for the aerospace, automotive, and defense industries.
- High recrystallization temperature
- High creep resistance at elevated temperatures
- Relatively expensive die materials
- Need a controllable die heating system
A quick history of the forge
Smiths have practiced forging copper and bronze since 4000 BC. Source. As the technology evolved, temperature control allowed for iron smelting. Products like hand tools, weapons, and kitchenware were all forged using iron.
It wasn’t until the Industrial Revolution that forging became a mass-produced, efficient process. Over time, technology like automation and robotics have resulted in a vast array of metal shapes, finishes, and sizes.
Commonly asked questions
What is swaging?
Swaging is a forging process that alters the shape of metal by forcing it through a die. Although this compressive method typically uses cold metal, it is also possible to forge hot metal. Swage may refer to the tool or the actual process of swaging.
What products are made by forging?
Forging was traditionally used to make hardware, axe heads, and hammers. Modern technology allows manufacturers to produce many products, from small rivets and screws to massive aircraft components.
What is cogging?
Cogging uses open die forging technology to deform a metal bar along its length. The process manipulates the malleable metal until it reaches the desired thickness, and edging allows the operator to achieve the target width.
How does forging strengthen metal?
The forging process heats and presses a piece until the metal’s tiny cracks and spaces are fully sealed. This technique reduces stress points by breaking up and redistributing impurities in the metal. Forging also alters the grain flow of metal, significantly improving its strength.