Pros and Cons of Cold Forging and Hot Forging

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Cold forging is an important process used to shape and strengthen metals. In order to best understand how it works, it&#;s important to first understand what forging is, and what each type of forging offers.

What is Forging?

Forging is a manufacturing process through which a solid metal workpiece is deformed and then re-shaped using compression. Unlike other methods of shaping metal, forging gives the creator more control over the final result because the grain of the metal deforms to follow the new shape. This means that the forger can decide which parts of the new metal object will be the strongest. As a result, a forged piece is stronger than the same piece created through casting or machining.

Different tools are used to accomplish forging, including the more traditional hammer and anvil, as well as the industrial use of hammers powered by electricity, steam, or hydraulics. Today, forging is largely done by machines at the industrial level and is a worldwide industry.

Forging is done either, &#;hot,&#; &#;warm,&#; or &#;cold.&#; No matter the temperature, the method and machines used can be classified as one of the following:

• Drop Forging: Use of forging hammers and screw presses

• Pressure Forging (rotational motion): Use of hydraulic and mechanical machines

• Pressure Forging (translational motion): Use of rolling mills

• Pressure Forging (a combination of translational and rotational motion): Flospinning and orbital forging

What is Cold Forging?

Also called cold forming, cold forging is a process that takes place near room temperature, rather than at higher temperatures like warm and hot forging. It&#;s done by placing the workpiece in-between two dies, and pounding the dies until the metal assumes their shape. Because of the friction created by the process, the temperature of the metal being forged can actually rise to 250° C or 482° F. Cold forging has its advantages and disadvantages. 

Advantages of Cold Forging

Because this process takes place without heating the metal beforehand, it provides many advantages and is often used in the automotive industry. For example, it&#;s often more economical than other processes, as the final product doesn&#;t require much finishing if any at all. There&#;s also little to no excess material left afterward, as the net weight of the original metal is about the same as the weight of the finished product. In addition to having no excess material, the dies used in cold forging last longer than in hotter processes, meaning fewer replacements. 

Another advantage is that although the ductility of the metal decreases during the cold forging process, the metal sees a great increase in both yield and tensile strength. Because it&#;s not heated, the grain of the metal retains its size and changes direction to match the change in the shape of the piece itself, resulting in this increased strength.

Disadvantages of Cold Forging

The main disadvantage of cold forging is that it can&#;t be used for every metal because certain types are much more likely to crack or break during the process. For example, while some steel can be forged at room temperature, steel types with a carbon content of .5% or above cannot. 

Another disadvantage is that cold forging can only create certain shapes. Those shapes are usually more basic and mass-produced. If you&#;re looking for a custom metal piece, a different process will probably work better for your desired outcome.

How is it Different From Hot Forging?

The most obvious difference between cold and hot forging is the temperature. Unlike cold forging that starts at or near room temperature, hot forging happens when metal is heated above its recrystallization temperature. This is usually an extremely high temperature, for example, steel is heated to °C or °F, and copper alloys to 700-800°C or -°F, which equals about 75% of the melting temperature. These high temperatures are necessary to keep the metal from strain hardening during the forging process. At these temperatures, the metal is in a plastic-like state, still technically a solid, but much more malleable.

Hot forging is usually done through a process called stamping, which means the heated metal is put into a press, and then squeezed in between a die and a tool. 

Advantages of Hot Forging

Because the forging process occurs at high temperatures, the plastic metal is ductile and easily molded. Intricate shapes and designs can be created during hot forging, unlike cold forging where this can be extremely difficult. If you&#;re looking to manufacture customized metal pieces, hot forging is the best option because of its malleable state before hardening. This method is ideal for creating 3-D and complex shapes. 

Unlike cold forging, which sometimes requires the extra step of heating the outside of the metal afterward to give it the right properties, the surface of the metal that has been forged through high temperatures is ideal for most types of finishing work.

Hot forging is also ideal for metals with a high formability ratio, because the integrity of the metal isn&#;t compromised by the high heat. These metals can be easily shaped without seeing any defects in the metal, making them both strong and ductile.

Disadvantages of Hot Forging

A disadvantage of hot forging, when compared to cold forging, is that some metals can be warped if not carefully watched when heated, forged, and then cooled. This means less precise tolerances than metal forged through cold forming.

Hot forging is also often more expensive than cold forging because of the heat treatment necessary to start the forging process, as well as the cooling process that prevents warping. This heat treatment is best done when automated, especially on the industrial scale, which may result in additional up-front costs to purchase tools. 

Ultimately, the process you choose is dependent on the product you need and the type of metal you&#;ll be using. Both forging methods can be effective ways to strengthen and form your metal workpieces into the final product you need.

For more information on how GK can support your forging process check out our foundry equipment or talk to our experts today!

If you are looking for more details, kindly visit Block forging.

The Cold Forging Manufacturing Process

       Advantages of Cold Forging

       Disadvantages of Cold Forging

The Hot Forging Manufacturing Process

       Advantages of Hot Forging

       Disadvantages of Hot Forging

The Cost Factor

       Cost Efficiency in Cold Forging

       Cost Efficiency in Hot Forging

The Main Difference Between Cold and Hot Forging

Forging is a manufacturing process, which shapes a solid-phase metal workpiece by applying compressive forces on it. According to the temperature at which it is performed, forging is classified in &#;hot&#;, &#;warm&#; and &#;cold&#;. The machines used to squeeze and deform the material into high strength parts can be divided in the following categories:

• Drop forging machines: forging hammers, screw presses, axial and incremental forging

• Pressure forging machines with translational motion: mechanical and hydraulic presses

• Pressure forging machines with rotational movements: longitudinal and radial rolling mills

• Combination of pressure machines with translational and rotational movements: orbital forging and flospinning

Grain Structure of Forged Components

It is essential to note that the forging manufacturing process is completely different from the casting one, where the molten material is poured into a mold. Another significant difference to the other manufacturing methods as casting and machining is that forging improves the mechanical properties of the material by refining its grain structure, providing a good grain flow, and making it tougher and stronger.

The cold forging manufacturing process is performed at room temperature. The workpiece is squeezed between two dies until it has assumed their shape. To deliver a finished, ready to fit component, the technique includes rolling, drawing, pressing, spinning, extruding and heading. Cold forging extrusion is one of the most common manufacturing techniques, widely used in the production of automotive components.

Cold Forged Components

Manufacturers may prefer cold over hot forging process; since cold forged parts require very little or no finishing work. Since all bars are annealed before being forged, there is no need for a secondary heat treatment prior to machining. Another significant advantage is the material savings achieved through near net shapes. The initial weight of the workpiece equals the final weight of the cold forged component.  Cold forged parts offer a good level of attainable dimensional accuracy and excellent surface quality.

The economic advantages combined with high production rates and long die life are more than enough to convince many manufacturers that cold forging is the best option for them.

Depending on the manufacturer&#;s requirements, some of the cold forging characteristics may turn out disadvantages; e.g. only simple shapes in high volumes can be shaped. The achievable deformation levels and shaping grade are much more limited compared to stamping. Therefore, if the customer is searching for a specific customized component, cold forging won&#;t be the best alternative.

A second more significant disadvantage is that cold forged metals are less ductile, which makes them inappropriate for certain configurations. Also, because of the grain structure that gives the material its strength, residual stress may occur.

It is important to mention that cold forging extrusion for example requires also a heat treatment to eliminate possible cracks, creeks hardening or rafters.

As a consequence of the above statements, cold forging cannot be used on every steel grade, particularly on those with low ductility or sensitivity to strain hardening. For instance, low alloy steel with carbon content higher than 0,05% is not appropriate for cold forging.

The hot forging manufacturing process is performed at extreme high temperature (up to °C for steel, 360 to 520 °C for al-alloys, 700 to 800 °C for cu-alloys). This temperature is necessary in order to avoid strain hardening of the metal during deformation. Stamping is the most widespread hot forging manufacturing process, where the material is squeezed in a press, between a tool and a die surface.

Hot Forged Components

The high temperature during hot forging enables the highest possible level of material deformation and the access to complex 3D geometries. Hot forged components possess increased ductility which makes them desirable for many configurations. Also, as a technique hot forging is more flexible than cold forging, since customized parts can be manufactured.

The excellent surface quality allows a wide range of finishing work as polishing, coating or painting, tailored to customers&#; specific need. Hot forging materials are available all over the world which has a positive impact on their final price.

The properties of hot forged metals are obtained by subsequent heat treatment, where the material has been quenched to ° C and then returned to 500 ° C. This requires additional cost, which can be avoided if using cold forging (except in some cases as softening, annealing or relaxation).

Less precise dimensional tolerance is another possible disadvantage of hot forged components compared to the cold forged ones. The cooling process should be also performed under special conditions; otherwise there is a risk of warping. Also, the grain structure of forged metals may vary and there is always a possibility of reactions between the atmosphere and the workpiece.

The manufacturing process is a complex one that can be impacted by many factors as suppliers, equipment, management styles, customers&#; demand, etc. The more intense there are, the greater the possibility for disruption in the process and changes in the final price. However, there are 4 important factors that determine the final price of a forged component. These are:

1. The amount of the used material;
2. Forging costs as electricity, investments, tools, etc;
3. Post-forging heat treatments;
4. Additional operations as shot blasting or quality control tests.

Cost optimization in cold forging starts with the raw material. This one can be directly purchased, if necessary, with an included annealing and a peeling, which makes the slugs ready for the process. Since the number of raw materials for cold forging is limited, they are consequently more expensive. To minimize manufacturing cost, cold heading wire is preferred for small pieces.

To obtain best cost efficiency the manufacturing process itself should be managed with an excellent know-how of all involved tools and techniques. For example, large batches of cold extruded components are manufactured on high speed mechanical presses, but complex parts and small series rather on hydraulic presses.

An efficient lubrication system is of critical importance for the successful application of any cold forged part. All workpieces should undergo a zinc-phosphate or polymer coating. Additional lubrication with special forging oil is required on the multi-station presses. A good lubrication system guarantees the high-quality of the final piece and eliminates additional costs for eventual failures during the process.

The minimization of energy costs during hot forging is an ongoing concern of every manufacturer. Those can be optimized when using an inductor and a billet with matching diameters and installing a flexible multi-stage heating system.

The automatization of all involved forging presses is essential for the cost efficiency of the hot forging manufacturing process, especially for industries that require high volume production runs as the automotive one.

All applied tools should be optimized for the following innovative technologies:

Metallurgy: consolidated vacuum, powder metallurgy
Reloading
Heat treatment
Surface treatment: nitriding, carburizing and duplex treatment

The cold forging manufacturing process increases the strength of a metal through strain hardening at a room temperature. On the contrary the hot forging manufacturing process keeps materials from strain hardening at high temperature, which results in optimum yield strength, low hardness and high ductility.

Finally, a manufacturer would choose one process over the other because of economic and not quality reasons. The decision is based on the required functions of the desired component, the industry and if it&#;s a mass production or a low volume of customized parts.

Setforge provides cold, warm and hot forging or combination of different processes to address various customer demands, both performed in excellent conditions with state-of-the-art machinery for perfect results. We work close with our customers to understand their specific needs and find the best possible solution.

If you want to learn more, please visit our website Custom Universal Joints.