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Figure 1. Forged steel components are found in numerous applications ranging from aerospace products to automotive products, off-road vehicles, hand tools, etc.
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.
Figure 2. Typical temperature ranges for the forging of steel and use of steel forgings
Steels are typically composed of 95-99% iron with 0.005-1.0% carbon content. The carbon imparts strength and hardenability to the alloy. Other elements that are typically added to steel are manganese to improve hot workability; chromium and molybdenum to improve toughness and hardenability; nickel to increase strength and toughness; silicon, which is primarily a deoxidizer but can also increase strength; and aluminum, which is a deoxidizer.
Unwanted residual elements that are found in steels include phosphorus and sulfur, although sulfur is sometimes intentionally added to improve machinability. Sulfur-containing steels are more difficult to forge. Another residual metallic element that is increasing in content with the greater production of steel from scrap is copper. Copper in automobile electrical wiring is not removed before old cars are converted into scrap for electrical arc-furnace feedstock. Excessive amounts of copper can increase the tendency of hot shortness in steels.
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Figure 1 shows some typical applications for forged steel components. Steel is an excellent structural material with a wide range of applications. With excellent strength, toughness, stiffness and fatigue resistance at a reasonable cost, steel is the dominant material in transportation. Tables 1 and 2 present selected mechanical properties of normalized plain-carbon steels and quenched-and-tempered steels, respectively. Well over 50% of the weight of a typical passenger car is from its steel content. Most fasteners, construction and mining equipment, machine tools and large structures are predominately steel. Pipelines, railroads, agricultural equipment, ships and landing gear for aircraft are produced from steel. Finally, steel is the mainstay of defense applications ranging from ordnance to aircraft carriers. Figure 2 shows the range of temperatures in which steel is typically forged and components used.
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.
Figure 3. Flow stress curves for steel forged at various temperatures
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.
Figure 4. Photomicrograph of 1020 normalized steel showing regions of ferrite (white) and pearlite (black).
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.
Figure 5. Scanning electron micrograph of a 4140 quenched-and-tempered steel. Note the very fine structure that is produced by this processing.
Steels are a very forgeable class of materials that are often chosen by forging customers. The behavior of steel during the forging operation needs to be understood so that the best possible components are shipped to the customer. The next article in this series will continue the discussion about plain-carbon and low-alloy steels.
The support for this work from the PRO-FAST Program is appreciated. The PRO-FAST Program is enabled by the dedicated team of professionals representing both the Department of Defense and industry. These teammates are determined to ensure the nation’s forging industry is positioned for challenges of the 21st century. Key team members include: R&D Enterprise Team (DLA J339), Logistics Research and Development Branch (DLS-DSCP) and the Forging Industry Association (FIA). This work was originally prepared for the FIA Theory & Applications of Forging and Die Design course by Scientific Forming Technologies Corporation.
Co-author Dr. Chet Van Tyne is FIERF Professor, Department of Metallurgical Engineering, Colorado School of Mines, Golden, Colo. He may be reached at 303-273-3793 or firstname.lastname@example.org. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or email@example.com.