In this part of our series on
forging materials we will discuss a relatively new class of materials –
microalloyed forging steels. As with our previous articles on low-alloy steels
and stainless steels, this article will provide a general description of microalloyed
steels followed by details of their chemistry and microstructures. Applications
of forged microalloyed steel components and special considerations for forgers
who handle microalloyed steels will complete the article.
Figure 1. Typical time-temperature plots for a low-alloy
steel that is quenched and tempered compared to a microalloyed steel.
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.
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.
Figure 3. The microstructure
of a medium-carbon vanadium microalloyed steel. The addition of vanadium allows
the ferrite (white) phase to form on the MnS particles (black bull’s eyes in
the ferrite) and break up the large pearlite (black) colonies in the
microstructure.
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.
Table 1 gives a summary of the behavior and effect of the
three primary microalloy elements in steels.
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.
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.
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.
Summary
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.
Acknowledgements
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 cvantyne@mines.edu. Co-author John
Walters is vice president of Scientific Forming Technologies Corporation,
Columbus, Ohio. He may be reached at 614-451-8330 or jwalters@deform.com.
by C.J.Van Tyne Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO.
