Metalworking forms the foundation of numerous industrial sectors. Among various forming methods, hot forging stands out as an essential process for obtaining metal components with intricate geometries and superior mechanical properties. This high-temperature working technique plays a distinctive role, especially when the goal is to produce strong parts suitable for rigorous applications.
Those working in the precision mechanics sector are well aware of the relevance of these processes. Let's now delve into the mechanisms and distinctive features of hot forging.
What is hot forging?
Hot forging defines a plastic deformation process of metals performed at temperatures above the material's recrystallization threshold. At these temperatures, the metal's crystalline structure reorganizes autonomously. The metal gains greater ductility above this thermal threshold, consequently deforming with less force without undergoing strain hardening (or work hardening), which typically occurs with cold deformation.
The process unfolds in various stages. The material, usually an ingot or a billet, is heated to a predetermined temperature. Subsequently, the heated material is placed inside a mold. Significant pressure then deforms the metal. Once shaped, the part is extracted and then cooled in a controlled manner. The primary objective of the process is to give the metal the desired shape. It also aims to refine its mechanical properties to achieve a fine-grain structure and reduce the material's internal porosity.
Hot working
The concept of "hot working" extends beyond forging, as it includes any process that plastically deforms a metal at high temperatures. A part is shaped with forging, by deforming the metal with impacts or pressure. Open-die forging shapes the part without fully containing molds, while closed-die forging forms it by insertion into predefined cavities.
Another example is hot rolling. Here, the thickness of plates, sheets, and profiles is reduced by passing heated metal through a pair of rolls. With hot extrusion, a profile is produced by pushing heated material through a die. This obtains bars, tubes, or complex profiles. Hot drawing, although less common than cold drawing, involves reducing the cross-section of wires or bars by pulling the heated metal through a die. The unifying principle of the various methodologies seen is the increased ductility of the material exploited at high temperatures, which thus facilitates deformation and prevents excessive hardening.
Hot deep drawing
Deep drawing is a process that forms hollow parts from flat sheets; hot deep drawing is a particular case of deep deformation, whereas if done cold, it has limitations in terms of depth and complexity arising from strain hardening and the risk of material fracture.
Hot deep drawing is used when there is a need to form parts with accentuated depths or complex geometries, or for processing materials with poor cold formability, such as high-strength steels or special alloys. It also allows for the reduction of necessary forces and the number of intermediate passes, including annealing. It will suffice to heat the metal sheet, place it on the die, press it for forming, and finally extract the component. This is how tanks, complex sumps, and various other parts for the automotive sector are produced.
Characteristics and advantages of hot forging for precision mechanics
Hot forging brings distinct advantages to precision mechanics, translating the benefits of the process into high final product quality. Mechanical properties are significantly improved, with a fine and homogeneous grain structure. The grain direction follows the flow lines, which provides a structural advantage, as porosity and internal defects are eliminated, and the material is given greater integrity. This will result in increased toughness and fatigue resistance.
A high deformation capacity translates into the production of complex shapes with a single or few passes. The process is well suited for processing difficult materials such as alloy steels, superalloys, and titanium. Production efficiency is evident in the reduction of necessary forces and less mold wear compared to cold cycles, although temperature adds complexity to the overall process.
In the precision mechanics sector, hot forging is used to produce engine components (connecting rods, crankshafts), high-strength gears, parts for the aerospace and defense industries, and high-performance tools. Good expertise in hot forging will serve to produce high-quality and complex parts to best meet customer demands.
Differences between hot forging and cold forging
Understanding the distinctions between hot and cold forging helps in selecting the most appropriate process. Hot forging operates at temperatures above recrystallization, requiring less force for metal deformation. This thermal condition prevents or minimizes strain hardening, and the deformations achievable are complex and significant. The resulting surface finish is less precise and rougher, with wider dimensional tolerances. Although heat increases mold wear, the reduced force requirement for deformation compensates for this aspect.
Cold forging, on the other hand, takes place at temperatures below recrystallization and, consequently, requires greater force to shape the material. Strain hardening is an intrinsic characteristic of the process, and intermediate annealing is often necessary to continue processing. The possible deformations are limited and are ideally suited for small modifications or details. The process imparts an exceptional surface finish and precise dimensional tolerances. Mold wear is lower, although the high forces exerted stress the tools differently. A positive aspect is that there is no need to heat the material, and strain hardening, if well managed, can improve the final mechanical properties of the part.
Hot forging is used in the production of structural components, gears, and connecting rods. The cold process is used for the production of screws, nuts, washers, and electronic parts.