Forging Materials: Plain-Carbon and Low-Alloy Steels (Part II)
As we indicated in the first article in this series, more forgings are made from the various grades of steel than any other type of metal. Steels have a range of compositions and properties that make them the ideal choice in many critical forging applications. They are also one of the most economical materials in meeting the requirements for numerous components. In the first article, we examined the chemistry, typical applications and some operational issues for forging plain-carbon and low-alloy steels. In this second article, we will discuss metallurgical issues, post-forging processing and typical properties of forged components
Steel Alloy Designation
Figure 1. 5120 steel normalized. The white area is ferrite and the dark area is pearlite. Pearlite consists of very fine bands of ferrite and carbide.
Steel grades follow a reasonable classification scheme. The plain-carbon steels are designated as 10XX grade, where the XX indicates the weight percentage of carbon in the steel. For example, 1050 contains 0.50% carbon. The 15XX grades are plain carbon but with higher amounts of manganese. The 40XX grades are steel alloys with molybdenum. These steels can be hardened more easily and can achieve higher strengths than the plain-carbon steels. The chromium-molybdenum steels are designated 41XX. The nickel-chromium-molybdenum steels are the 43XX or 86XX steels. 51XX are steels with chromium. All of these steel alloys can be forged without a great deal of difficulty. Other specialized grades of low-alloy steels exist, but the ones listed here are the most commonly used grades.
Normally, the steels selected for various components are based on the alloys’ ability to achieve higher strength by alloying or heat treatments. Often, the alloying and heat treatments are synergistic so that with the proper steel grade and appropriate heat treatment, excellent final properties can be achieved.
Figure 2. High magnification of the pearlite region of a 5120 normalized steel. The structure shows alternating bands of ferrite and carbide.
Post-forging heat treatments that involve quenching to form martensite cause a significant increase in strength, where strength is the material’s resistance to loads. Often, an additional temper heat treatment is needed to improve toughness without a significant loss in strength. Toughness is the material’s resistance to fracture upon impact. In most cases, an increase in strength causes the material to have a lower toughness. The two-step quench-and-temper (Q&T) process produces steels with the best combination of strength and toughness in a component.
Fine-grained steels also improve strength and can simultaneously improve toughness. In fact, producing a fine grain structure is the only means by which both strength and toughness can be increased. To create a fine-grained steel structure usually requires forging at lower temperatures and heat treating into the lower austenite region so that excessive grain growth is avoided. Multiple cycles (up to about three) into the lower-temperature austenite region can often result in a finer-grained product.
Another means of increasing the strength of the steel is to increase its carbon content. Higher-carbon steels are stronger (but less tough) than the lower-carbon grades. Alloy additions, especially carbide formers such as chromium, molybdenum, vanadium and tungsten, can be added to increase strength, though at additional cost.
Steel strength can also be increased by deforming the metal at cold-working temperatures (below 500˚F). This is the advantage of cold-forged components. The major disadvantage is that the equipment needed for cold-forging steels of equivalent sizes must be of significantly larger capacity because the steel strengthens during the forging process.
Figure 3. In this photomicrograph of 4140 Q&T steel, fine martensite platelets with very fine carbides are just barely visible at this magnification.
Heat treating can be used to control strength and toughness property combinations. Slow cooling after heating into the austenite region creates a ferrite plus pearlite microstructure, which is soft but cold-formable. This slow-cool process produces normalized steel. Figure 1 shows a slow-cooled 5120 steel that has a ferrite-pearlite microstructure. Pearlite is a mixture of ferrite and cementite (i.e. iron carbide). Figure 2 shows a high-magnification scanning electron microscopy (SEM) image of the pearlite region in the 5120 steel. The white plates in this micrograph are the carbides and the background is the ferrite. This structure is called lamellar (like a laminate) due to the alternating plates of carbide and ferrite. The spacing of these carbide plates is often able to cause reflected light to separate into colors giving rise to a “pearl”-like glistening, hence the name pearlite. Normalized steels are used in applications in which there is a need for high ductility but strength requirements are not large.
Quenching from the austenite region produces a martensitic microstructure. Quenching can be done in water, oil or water with dispersed polymer. Water provides the most rapid cooling and causes the most severe quench conditions. The martensite structure that forms is very strong, but it lacks toughness and ductility. Hardenability is the term that describes the ability of steel to form martensite during a quench. The addition of alloying elements is required to increase the hardenability of a steel alloy.
Tempering is a low-temperature heat treatment often performed after quenching. It produces a tempered martensite structure that exhibits increased toughness with only a small decrease in strength. This two-stage heat treatment produces Q&T steel. Figure 3 shows a Q&T 4140 steel with tempered martensitic microstructure. The steel was austenitized, then water quenched, then followed with a one-hour temper.
Figure 4. 5120 steel that has been spheroidized for 20 hours at 1275˚F. Note the difference in microstructure as compared to Figure 1, which is the same steel after a normalized heat treatment.
The very fine structures seen in the photomicrograph (Figure 3) are platelets of tempered martensite. In a higher magnification of this view you would see some very fine carbide. This combination of fine martensite and very fine carbides gives the metal both the strength and toughness indicative of Q&T steel, which are used in applications where there is a need for both high strength and high ductility. The disadvantage of these steels is their higher cost, which is due to both the extra alloy content and the multiple heat treatments required to produce the final properties.
Most forged steels are machineable. The most machineable condition would be a soft spheroidized microstructure, which is ferrite with spherical carbides dispersed throughout the microstructure. Such a structure is obtained by a lengthy heat treatment at temperatures just below the formation of austenite. If a spheroidized structure is formed, subsequent heat treatments are usually needed to convert the microstructure into structures that will produce higher strength in the final steel component. Figure 4 shows the same 5150 steel as in Figures 1 and 2 but after a spheroidizing heat treatment. Notice that the carbides are now spherical and are very fine in size. Such a structure gives the steel very low strength but good machineability.
Sulfur can be added to the steel to improve its machinability. Additional manganese must also be added to prevent hot shortness, but the forging of these sulfurized steels is more challenging.
Typical Physical Properties
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Table 1 shows some typical properties of normalized plain-carbon steels. As the carbon content increases from 0.15% to 0.80%, the strength of the steel more than doubles. In contrast, the tensile elongation decreases as the carbon content increases.
Table 2 lists some mechanical properties for steels with 0.40% carbon that have been quenched and tempered. The properties for the Q&T plain-carbon steel (i.e. 1040) are not significantly greater than the normalized 1040. In order to obtain the large increases in strength, the addition of alloy elements are required.
While steel is an excellent structural material, it is normally used at temperatures well below 1000°F, as extended exposure to high temperatures will result in over-tempering that will significantly reduce its strength. Other nonferrous alloys are available for higher service temperatures (e.g., superalloys), and they will be examined in a future article.
Steel is not a corrosion-resistant material. The reddish product that forms on the surface of the steel is iron hydroxide, commonly called rust. If the rust layer becomes large, it will crack or flake off. This exposes more of the base steel to the corrosion, which leads to further degradation of the component. A corrosive environment can also degrade a steel component’s fatigue properties. If, for example, a steel component is under load in an environment containing hydrogen-sulfide gas, it can fail by stress corrosion cracking. If steel is to be used in a corrosive environment, it needs to be painted, plated or coated to prevent its degradation in service. The use of alloy elements such as silicon, chromium and nickel can slow the rate of corrosion. Steels with high levels of chromium and nickel are classified as stainless steels, the forging of which will be examined in the July issue of FORGE.
Steel is a very useful and versatile metal that is used in an extensive range of applications. Forged steel can have superior properties and provide customers with excellent performance in a large number of components. Although steel is relatively easy to forge, there are certain features about it that need to be understood so that mistakes are not made during forging or post-forging processing.
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 email@example.com. Co-author John Walters is vice president of Scientific Forming Technologies Corporation, Columbus, Ohio. He may be reached at 614-451-8330 or firstname.lastname@example.org.