This Forge method starts with two dies. One is set on a stationary anvil and the other is connected to a moving ram. The raw metal is heated By Billet Heater & Billet put into the die connected to the anvil. Once it’s in place, the ram comes down onto the metal to shape it.
Drop forged steel varies depending on the metal used, the number of blows and the cooling method. As the name implies, the process typically uses gravity to drop the ram onto the anvil and connect the two die plates. The basic process of drop forging is the same as hand forging. Just like a master craftsman that forges steel one hammer swing at a time, drop forging carefully crafts a piece to fit your desired specifications. Timeless techniques are blended with modern technology to create high-quality, reliable components.
Forging Materials: Plain-Carbon and Low-Alloy Steels
The forging process creates components that have superior properties relative to those created by other manufacturing operations, such as casting and machining. The variety of metals that can be forged is quite large, but each class of materials has its own set of issues that need to be understood by the forger so that high-quality components can be produced. In this first of a series of articles, the forging of plain-carbon and low-alloy steels will be examined.
Since steels are so widely used,
General Description
Steel is an iron-based alloy with small additions of carbon and other elements that impart improvements in various properties. Steel is among the most useful of metals. It is the metal that has the highest strength per unit cost. There are a wide range of steel compositions as well as a wide range of microstructures that can be produced. These ranges allow a large choice of properties to be achieved within the steel-alloy family. At low temperatures, the microstructure of steel is often in the ferrite plus iron-carbide phase field, whereas at high temperatures, the microstructure of steel is in the austenite phase field. The different phases indicate differences in crystal structure. More importantly, they indicate differences in properties. Hot forging is conducted in the austenite phase field, and cold forging is done in the ferrite phase field.
Forging Temperatures
With the exception of the machinable grades, the behavior of steel during forging is excellent. It should be realized that forging in itself can have an effect on the ductility, impact toughness and fatigue life of the final component. This improvement in properties occurs because of the breakup of segregation, the closing of pores and the aiding of homogenization that forging provides to the steel. Forging can also reduce grain size and produce a fibrous grain structure (i.e. grain flow) in the component. Typical grain structures are shown in Figures 4 and 5. If the grain flow is oriented perpendicular to the crack that would be generated during use (due to either impact or fatigue loading), the grain flow can hinder the propagation of the crack and improve the forging’s impact and fatigue properties. While forged steel generally has superior fatigue and toughness properties, it must be noted that forging has only minor effects on the final hardness and strength of the component. Hardness and strength are normally controlled by the steel composition selected and the heat treatments.
Hot Forging – This is the most common process for steels. At high temperatures, the ductility is excellent and the flow stress is 10-20% of the room-temperature yield strength (Figure 3). The forging temperature that can be used primarily depends on the steel’s carbon content. Steels with higher carbon content or alloying elements have lower maximum allowable forging temperatures due to their lower melting temperature. If the temperature of the steel is too high, then “burning,” or incipient grain boundary melting, of the steel can occur.
While typical hot forging temperatures are between 2150°F and 2375°F – well below the melting temperature of more than 2500°F – deformation (adiabatic) heating results in local heating. Localized temperature increases of 200°F or more can result in localized melting, which will significantly reduce mechanical properties and forging ductility. At hot forging temperatures, the strain rate or speed influences how resistant the steel is to deformation. The higher the speed of deformation, the higher the strength of the steel and the more force required for its deformation. Figure 3 illustrates this point for hot forgings.
Warm Forging – This typically occurs in the 1500-1800°F temperature range and is used to shape many different steel grades. Warm forging reduces energy costs for heating as well as the amount of scale and thermal contraction that occurs during post-process cooling. The press loads required for warm forging can be significantly higher than forging at traditional temperatures due to higher flow stress. These increased loads can reduce die life. Warm forging can also produce better microstructures so that the forged component may not require subsequent heat treatment. The tooling for warm forming is generally more expensive because it is designed to withstand much higher stress levels. Warm forged parts are most common in high-volume mechanical-press applications, such as those used in automotive drivetrain components.
Cold Forging – Steels can also be cold forged at temperatures below 500°F. Cold forming is virtually always performed at room temperature because the benefits from heating a few hundred degrees are negligible, and the costs of heating are significant. The component needs to be fairly small since steel will greatly work-harden during cold forging, causing the strength of the material to significantly increase, thus increasing the already high forging loads. The flow stress is very high for cold-formed processes. The tooling cost and complexity is exponentially higher, with very sophisticated tooling assemblies required to absorb contact pressures well in excess of 100,000 psi. Cold-formed parts are limited to coining operations and high-volume mechanical-press applications such as fasteners, spark-plug bodies, bearing components and hand tools.
Forging Operations
The preheating of steel billets for hot and warm forging is most commonly performed in gas (or other) furnaces for large parts, small quantities, upsetting, open die and job-shop-type operations. For a large part, heating times can exceed 30 hours! This long time results in surface scale, which must be removed prior to forging or the part will sustain a very poor surface finish as the scale is forged into the surface. Scale can be removed by upsetting or mechanical removal. It is common to see an operator using high-pressure air to break scale off of large forgings during the process. Higher quantities of small- to medium-diameter forging stock (less than 10 inches) are frequently heated in horizontal induction units to provide fast heat-up time, improved process control and significantly less scale. Resistance heating is possible but fairly rare. Steel is very tolerant of surface chill from the environment during hot and warm forging processes. During hot forging, lubrication is applied to the tooling with very few exceptions. The most common lubricant is graphite. During most cold-forming processes, lubricants are applied to the workpiece in the form of a coating. Tool (die) temperatures are rarely critical in the production of a steel forging. It is not uncommon for a shop to forge product on tooling that is preheated to less than 300°F when forging a workpiece at 2350°F. For the most part, steel forgings are tolerant of a wide range of process conditions.
