Forging Materials: Microalloyed Forging Steels
Like all steels, microalloyed steels are iron-based metal alloys. Normally, they are plain carbon or low-alloy steel with small additions (i.e. a microalloy) of one of three special elements. These steels were developed in the 1960s and used for plate and pipeline applications. It was not until the 1980s, however, that forgers began to produce microalloyed steel components in significant quantities. Microalloyed steels have higher strength and higher toughness as compared to low-alloyed steels with the same microstructure. They do not have the same high strength/toughness combination of quenched-and-tempered alloyed steels, but their properties can be quite suitable in many applications.
The real advantage of microalloyed steels is in the cost savings realized by the elimination of heat treatments, since a properly designed and forged microalloyed steel part will not require any subsequent heat treatment. Thus, the elimination of the need for austenization, quenching and tempering after forging helps offset the additional cost associated with a microalloyed steel. Figure 1 shows a typical time-temperature diagram for a low-alloy steel and for a microalloyed steel. It can be seen that for a low-alloy steel the subsequent hardening, tempering and stress relief after straightening requires that the part be reheated several times, accruing energy costs in the process. In contrast, the microalloyed steel, if forged and cooled properly, can be used without additional heat treatments and attendant energy costs.
Chemistry and Microstructure
Figure 2. A high-magnification picture showing the very fine precipitates in a vanadium microalloyed steel. Note that they are dispersed throughout the microstructure.
Microalloyed forging steels typically have carbon contents of 0.15-0.55%, with manganese ranging from 0.60-1.65% and silicon 0.15-0.65%.
The three microalloy elements that are added in small quantities to form microalloyed steels are vanadium, niobium and titanium. Most microalloyed steels have a ferrite-pearlite microstructure. Some steel producers also add a small amount of molybdenum to these steels to produce a bainitic microalloyed steel directly after forging. These are sometimes referred to as the third generation of microalloyed steels.
Vanadium (V), added in the range of 300 to 1,000 ppm (0.03-0.10%) to the steel, has a very high solubility in the austenite phase of the steel. Thus, when the steel is heated to forging temperatures, all of the V dissolves into the austenite phase (similar to sugar dissolving into water when the water is hot). When the steel is forged and subsequently cooled in a controlled manner, the V will react with carbon and nitrogen to form vanadium carbonitrides, which precipitate out as fine particles in the microstructure. These particles provide a significant boost in the strength of the room-temperature steel. This strengthening process is called precipitation, or dispersion, strengthening. Figure 2 shows a transmission electron microscope (TEM) image of a vanadium microalloyed steel. The finely dispersed white particles in the image are the vanadium carbo-nitrides (V(CN)). They are nano-sized particles, indicating that microalloyed steels are really part of the current nanotechnology effort.
The other benefit vanadium provides is that it can form vanadium carbonitride precipitates on MnS particles that are within the steel. These MnS particles with V(CN) on their surface form nucleation sites for ferrite, causing ferrite to form inside the grains (intragranular) of austenite and be present inside the final pearlite colonies. The intragranular ferrite can contribute to a small increase in the toughness of the steel. Figure 3 shows a microstructure of a vanadium-microalloyed steel with intragranular ferrite.
Niobium (Nb), sometimes referred to in the U.S. as columbium (Cb), is not as often used in forging steels. It is a very common microalloy element in high-strength controlled-rolled plate steels. In forging steels, Nb is added at the levels of 200 to 1,000 ppm (0.02-0.10%). The solubility of Nb in austenite is very temperature sensitive. At high forging temperatures, most of the Nb will dissolve and precipitate out when cooled in a fashion similar to vanadium. At lower forging temperatures, the Nb will not fully dissolve. If it is present as fine precipitates at these lower forging temperatures, these precipitates will provide grain-boundary pinning of the austenite and not allow the austenite grain size to grow too large.
Figure 4 shows a TEM image of a niobium-microalloyed steel. Notice, in contrast to Figure 2, the fine niobium-rich precipitates are not dispersed throughout the structure but primarily reside along prior austenite grain boundaries. The size of these prior austenite grains is on the order of about 1 micron, which is indeed very fine. If fine austenite grains are present, then the transformation products (ferrite, pearlite or bainite) upon cooling will also have a very fine grain structure. Small grains provide not only an increase in the strength of the steel, they can also increase the toughness at the same time. This grain refinement is the only known mechanism that increases strength and toughness simultaneously. We would get an increase in strength but a decrease in toughness with all the other known strengthening mechanisms in metals. The control of these niobium-rich precipitates is much more difficult in a forge shop. The processing window for good forging product with a niobium-microalloyed steel is much smaller than with vanadium. So, in spite of the lower cost of niobium as compared to vanadium, vanadium is the normal microalloy element of choice for forgings.
Titanium (Ti) is the third major microalloying element used in forging steels. The amount of Ti in forging steels is very low – 100 to 200 ppm (0.01-0.02%). Titanium has very low solubility in austenite even at high temperatures. It reacts readily with any nitrogen in the steel and forms titanium nitrides (TiN). These TiN particles usually form during the solidification process in the steel mill. If TiN precipitates are very finely dispersed, they will pin the austenite grain boundaries during heating and forging, creating a very fine austenite grain size. Similar to the low-temperature forging of niobium microalloyed steels, the fine austenite grain size leads to a fine grain structure in the final product, increasing both the strength and toughness of the steel.
Figure 4. A high-resolution micrograph of a niobium-microalloyed steel. The precipitates (white particles) are not dispersed throughout the microstructure but occur primarily along the prior austenite grain boundaries. These precipitates on the grain boundaries prevent the austenite grain size from growing too large when at high temperatures.
Forged microalloyed steel components are used in a number of applications. They are used extensively in automotive applications including crankshafts, connecting rods and a variety of drivetrain components. They are also used in hand tools. The vast majority of microalloyed steel forgings are high-volume, moderately sized (1-10 pounds) products, which is the “sweet spot” for automotive parts.
Table 1 gives a summary of the behavior and effect of the three primary microalloy elements in steels.
In non-forged products, microalloyed steels are used extensively in high-strength plates, high-strength pipe and in structural components for ships, cars and trucks. In the U.S., the use of microalloyed, forged components in automotive applications lagged behind their use in Japan and in Europe. This delay was not due to technical limitations of the material but primarily due to potential legal issues that the U.S. automotive companies were not willing to assume. For many years from their initial development until the early 1990s, these microalloyed forging steels were classified as experimental grades of steel. It was not until 1992 that they received an official designation as ASTM A-909 and were not long in the experimental category. U.S. automotive companies were reluctant to use steels that were classified as “experimental” no matter how well proven they were.
Forging of Microalloyed Steels
The hot-forging temperature required for microalloyed steels is the same as that used for the plain-carbon or low-alloyed steels. Some companies have successfully coupled warm forging (temperatures about 1800°F or less) with microalloyed steels to produce a high-quality product. The forging loads may be a little higher than for the non-microalloyed grades. If warm forging is employed, then the loads can be significantly higher. Figure 5 shows the temperature range for these microalloyed steels. Virtually all applications are induction heated because they involve high production volumes and moderate to small part size.
Process-control requirements for microalloyed steels are significantly greater than those required for parts that will be subsequently heat treated. In addition to the temperature control offered by an induction heating line for heating of the billets, a critical production aspect of forging microalloyed steels is the controlled cooling of the parts after they come off the hammer or press. In order to obtain the proper size and distribution of the precipitates, the cooling rate needs to be faster than standard cooling in a bin but slower than quenching in oil or water. The correct cooling rate is critical in maximizing the properties of microalloyed steel forgings, especially components made with vanadium additions. Cooling on a fan-cooled belt conveyor or on a conveyor with a fine-mist spray are typical methods of achieving the proper cooling rate.
If the cooling rate is too slow, precipitates will form, but their size will be too large to be maximally effective. If the cooling rate is too quick, precipitates will not form, and the extra cost of the microalloyed steel will be wasted because you will not obtain the strength boost from having the precipitates in the microstructure. The important temperature range is from the forging temperature to the end of the austenitic transition temperature (typically 1350°F). Below this level, the amount of precipitation is greatly diminished.
Initial process development can be expensive, as forging temperature, conveyor speed and heat-transfer coefficient from fans are optimized to achieve the required precipitation after forging. The current trial-and-error development is not only expensive but can converge on a workable non-optimal set of conditions. This situation is an ideal opportunity to deploy process modeling to optimize cooling rate as a function of heat-transfer coefficient, as is done for forging in aerospace applications. Dropped billets, mishandled forgings or other delays result in scrapped parts. Occasionally, rework through subsequent heat treatment may be possible, but such rework defeats the advantage of microalloyed steels in eliminating extra heat treatments. The process control to successfully and optimally forge microalloyed steels is similar to the requirements associated with aerospace alloys. Forgings of microalloyed steels need to be engineered; they cannot be produced by a simple “heat it and beat it” process!
Figure 5. Typical temperature ranges for microalloyed steels.
One criticism that occurs with microalloyed steel parts is that they are more difficult to machine after forging. Studies have shown that the machining is not worse as compared to plain-carbon and low-alloy steels, but it is a bit different. If you operate the machining equipment with the same feeds, speeds and depths of cut as you would with a non-microalloyed steel of the same grade, then the tools will indeed wear more rapidly. The machining parameters need to be adjusted in order to obtain the same cutting-tool life.
Microalloyed steels are a relatively new class of forging material that can provide steel forgings of added strength and adequate toughness for a variety of applications. Their properties depend strongly on the control of the fine precipitates in the steel. This control requires special cooling conditions as the forging comes off of the hammer or press. Although microalloyed steel is more expensive than a plain-carbon or low-alloy steel equivalent, cost savings can occur by the elimination of post-forging heat treatments. Microalloyed steels should certainly be considered in discussions with your customers when they are trying to obtain a higher-strength steel component.
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.