Hot forging and cold forging are two different metal forming processes that can provide similar results. Forging is the process of using certain tools and equipment to deform a metal into a predetermined shape - deformation is accomplished using hot forging, cold forging, or even warm forging processes. Ultimately, manufacturers will consider many standards before selecting the type of forging that is most suitable for a specific application. In the case where the arrangement of grain structure endows the part with directional characteristics, forging is used to align the grains so that they can resist the highest stress that the part will encounter. In contrast, casting and mechanical processing typically have less control over the arrangement of grain structures.
Forging process
Forging is defined as the formation or deformation of a metal in its solid state. Many forging processes are completed through upsetting, in which the hammer or striker moves horizontally to press against the end of the rod or rod to widen and change the shape of the end. Before reaching the final shape, the part usually passes through continuous workstations. High strength bolts are 'cold headed' in this way. The engine valve is also formed by upsetting.
In drop hammer forging, the part is hammered into the shape of the finished part in the mold, which is very similar to the open die forging of a blacksmith. In this case, the metal is hammered into the desired shape against the anvil. There is a difference between open die forging and closed die forging. In die forging, the metal is never completely constrained by the mold. In closed or pressed molds, forged metal is limited between half molds. Repeatedly hammering the mold forces the metal into its shape, and the two halves of the mold eventually meet. The energy of a hammer can be provided by steam or pneumatic, mechanical or hydraulic means. In true drop hammer forging, gravity alone pushes the hammer downwards, but many systems use power assistance combined with gravity. The hammer provides a series of relatively high speed, low force blows to close the mold.
In pressure forging, high pressure replaces high speed, and the half of the mold is closed in a single stroke typically provided by a power screw or hydraulic cylinder. Hammer forging is usually used to produce smaller parts, while press forging is usually used for mass production and automation. The slow application of pressure forging often processes the interior of parts better than hammering, and is typically applied to large high-quality parts such as titanium aircraft bulkheads. Other specialized forging methods vary depending on these basic themes: for example, bearing races and large gear rings are made through a process called rolling ring forging, which can produce seamless circular parts.
Hot forging
When a piece of metal is hot forged, it must be significantly heated. The average forging temperature required for hot forging of different metals is:
Steel up to 1150 ° C
360 to 520 ° C for aluminum alloys
700 to 800 ° C (copper alloy)
In the hot forging process, steel billets or billets are induction heated or heated to a temperature above the recrystallization point of the metal in a forging furnace or oven. This extreme heat is necessary to avoid strain hardening of metals during deformation. Due to the plastic state of metal, it can be made into quite complex shapes. Metal maintains ductility and ductility.
In order to forge certain metals, such as superalloys, a type of hot forging called isothermal forging is used. Here, the mold is heated to approximately the temperature of the billet to avoid surface cooling of the parts during the forging process. Forging is sometimes carried out in a controlled atmosphere to minimize the formation of oxide scale.
Traditionally, manufacturers choose hot forging to manufacture parts because it allows the material to deform in a plastic state, making the metal easier to machine. Hot forging is also recommended for metal deformation with high formability, which is an indicator of how much deformation a metal can withstand without producing defects. Other considerations for hot forging include:
Production of discrete parts
Medium to low precision
Low stress or low work hardening
Homogeneous grain structure
Increase ductility
Eliminate chemical incompatibility and porosity
Possible drawbacks of hot forging include:
Less precise tolerances
The material may warp during the cooling process
Different metal grain structures
Possible reactions between the surrounding atmosphere and metals (scaling)
Cold forging (or cold forming)
Cold forging causes metal to deform below its recrystallization point. Cold forging significantly improves tensile strength and yield strength while reducing ductility. Cold forging is usually carried out near room temperature. The most common metal in cold forging applications is usually standard steel or carbon alloy steel. Cold forging is usually a closed die process.
When the metal is already a soft metal (such as aluminum), cold forging is usually preferred. This process is usually cheaper than hot forging, and the final product hardly requires precision machining. Sometimes, when the metal is cold forged into the desired shape, heat treatment is carried out after removing residual surface stress. Due to the improvement of metal strength by cold forging, lower grade materials can sometimes be used to produce usable parts that cannot be made from the same material through machining or hot forging.
Manufacturers may choose cold forging over hot forging for various reasons - because cold forged parts require little or no precision machining, and this step in the manufacturing process is usually optional, thereby saving money. Cold forging is also less susceptible to pollution issues, resulting in better overall surface finish of the components. Other benefits of cold forging include:
Easier to assign directional characteristics
Improve reproducibility
Increase size control
Handling high stress and high mold loads
Production of clean or nearly clean parts
Some possible drawbacks include:
The metal surface must be clean and free of oxide scale before forging
Poor ductility of metals
Residual stress may occur
Need heavier and more powerful equipment
Need more powerful tools
Warm forging
Warm forging is carried out at temperatures lower than the recrystallization temperature but higher than room temperature to overcome shortcomings and gain the advantages of hot and cold forging. The formation of oxide skin is not a problem, and compared to hot forging, the tolerance can be smaller. Compared to cold forging, the cost of the mold is lower and the force required for manufacturing is also lower. Compared to cold working, it reduces strain hardening and improves ductility.
Application
In the automotive industry, forging is used to manufacture suspension parts such as idler arm and axle, and Powertrain parts such as connecting rod and transmission gear. Forgings are commonly used for pipeline valve stems, valve bodies, and flanges, sometimes made of copper alloy to increase corrosion resistance. Hand tools such as wrenches are usually forged, with many wire ropes