FIGURE 1: Cutaway view of a typical railroad bearing showing the types of clamping forces required for proper operations. Lateral clamping forces are shown in green. Radial clamping forces are shown in red.

Radial clamp (represented by red arrows in Figure 1) results largely from the force caused by the interference fit between the cone assemblies of the bearing (shown in yellow) and the axle journal (shown in gray). Radial clamp is thus affected by proper axle journal size and proper cone bore diameter size. Radial clamp is maximized when the axle journal diameter is at the upper end of the AAR specified range and when the cone bore diameter is at the lower end of the AAR specified range. In recent years, Brenco has made continuous improvements to its manufacturing operation resulting in cone bore diameters consistently at the lower end of the AAR diameter tolerance range. In addition, Brenco has encouraged car owners to target axle journal diameters at the high end of the AAR specification range. We are pleased to see that many car owners have adopted this recommendation for both new car construction and maintenance.

Radial clamp can be diminished in service due to a phenomenon known as cone bore growth. The link between cone bore growth and the level of retained austenite in cone races has been understood for some time. In recent years, Brenco, in cooperation with the University of Nebraska, has conducted extensive research in this area which has led to improved understanding of the ways that retained austenite influences the performance of railroad bearings in the field.

Retained austenite is a specific crystalline form of iron and steel. To explain how retained austenite affects bearing performance, it is necessary to understand some basic metallurgical principles. The world of matter exists in various phases. A phase is defined as a physically distinct form of a substance. Many different phases surround us and are part of our everyday experience. Examples of familiar phases are liquids and solids (FIGURE 2).

FIGURE 2: Liquid water and ice are familiar examples of how a material can exist in various forms. Steel also exists in various forms, including several different solid forms.

FIGURE 3: Detail of the Iron-carbon phase diagram. Each colored zone represents a combination of composition and temperature that will result in a particular phase or mixture of phases.

This diagram indicates what phase or mixture of phases will exist at a given temperature and chemical composition under equilibrium conditions. As you move from one zone to the next, the type of phase that makes up the steel will change.

The solid phases of steel that are most significant to our discussion are ferrite and austenite. To illustrate what happens during the phase transformation of steel, consider the simplified example of a plain carbon steel with a carbon content of 0.6% by weight. When this steel exists in the green zone of Figure 3 (Point A), the iron will naturally assume the ferrite structure. As the steel is gradually heated into the orange zone of Figure 3 (Point B), some of the ferrite begins to transform and assumes the structure of austenite. This results in a mixture of phases (ferrite and austenite together) in the orange zone. As the steel is heated beyond the orange zone and into the yellow zone (Point C), it will completely assume the structure of the austenite. The ferrite phase will disappear. This process can be reversed by slowly cooling the steel back down into the green range.

Associated with each phase is a characteristic volume. As steel is heated up and down through the different phase ranges, it’s crystalline structure shifts back and forth from one type of crystal to another resulting in a corresponding change in volume. The net result is that when a piece of steel changes from ferrite into austenite, it shrinks slightly. When austenite transforms back into ferrite, the steel will expand slightly. This change takes place in addition to any thermal expansion or contraction that is taking place.

The transition between ferrite and austenite is relatively slow, requiring time to complete, just as ice takes time to melt and water takes time to freeze. When steel is heated and cooled slowly, a phase diagram such as Figure 3 does a good job of describing the changes that take place. However, when steel is cooled very quickly (as is required to achieve the desired hardness in the final product) the situation changes and Figure 3 is no longer adequate. In steel that is heated to austenite and cooled down very quickly, the crystals don’t have the time they need to shift from austenite into ferrite. Rapid cooling results in a new crystalline form that is called martensite. In high-carbon steels, martensite has a greater volume than both austenite and ferrite. Martensite is created in a special reaction that occurs during rapid cooling and begins at a relatively low temperature specific to the chemistry of the steel. For typical case-carburized bearing steels this temperature is around 500 degrees F. As the steel cools below this martensite start temperature martensite begins to form. As the temperature decreases, the amount of martensite in the steel will increase until a temperature is reached where practically all of the available austenite is transformed into martensite. This is known as the martensite stop temperature. Typically, the martensite stop temperature is impracticably low (usually well below 0 degrees F). Railroad bearing races are not usually quenched to such a low temperature. Since the martensite stop temperature is not achieved during quenching, some of the austenite in the bearing race remains untransformed. This austenite, which is left over from when the steel was hot, is now mingled with the newly formed martensitic structure. Since this austenite is "retained" from the heat treating process, it is commonly referred to as retained austenite.

Austenite is the normal phase of steel at high temperatures, but not at room temperature. Because retained austenite exists outside of its normal temperature range, it is metastable. This means that when given the opportunity, it has a characteristic tendency to change from austenite into martensite. This metastability is an important characteristic of retained austenite, and combined with the volume shift associated with phase changes, it is responsible for many of retained austenite’s unique properties.

The demands that railroad service place on a freight car bearing are formidable. Railroad operators expect a bearing to operate maintenance free for hundreds of thousands of miles while supporting a fully loaded freight car in extremes of weather. These demands require a robust bearing that can take tremendous punishment. To make such a bearing, Brenco starts with the finest raw material: super clean bearing-quality steel. This steel is specially formulated with alloying elements and a low carbon content. During manufacture, each bearing race is heated and cooled in a special process that takes advantage of the phase changes in steel to obtain the specific properties required in a heavy-duty bearing. This multi-step process is called heat treating (FIGURE 4).

FIGURE 4: A diagram representing a typical carburizing and hardening cycle suck as that used to heat treat a railroad cone race. See test for explanation.

The first step in the heat treatment of a Brenco bearing race is carburizing. In the carburizing process, the steel is heated to a very high temperature (usually over 1700 degrees F) and flooded with high carbon gas (FIGURE 4a). The carbon in the gas will gradually soak into the surface of the bearing race and create a case of high carbon steel surrounding the low carbon core (FIGURE 5).

FIGURE 5: A section through a carburized and hardened cone race. The cone has been treated with an acid etch to reveal the high-carbon case that is a result of the carburizing process. The dark outer layer will be hard and strong, the light inner core will be softer and tougher.

The second step of the heat treating process is called hardening. In this step, the component is heated to a temperature of approximately 1600 degrees F (FIGURE 4b). This process refines the grain structure of the steel and changes the phase of the steel into austenite in preparation for transformation into martensite. Next, the race is rapidly cooled below the martensite start temperature by immersion into a special oil. This technique is known as quenching (FIGURE 4c). Quenching causes the steel in the case to transform primarily into martensite. Because the steel does not cool to the martensite stop temperature, the case will also contain some retained austenite. After quenching, the steel is hard, but the impact strength can be greatly enhanced through the application of one last process.

In the final step of the heat treating process, the steel is tempered. Tempering involves re-heating the steel to a lower temperature of approximately 350 degrees F (FIGURE 4d). This process reduces the stresses in the race and changes the martensite into yet another form called tempered martensite. Tempered martensite has the strength, hardness and the toughness needed to produce heavy-duty bearings for one of the most demanding applications on earth, your railroad.

If you look at the etched case of a railroad bearing race under a special metallurgical microscope, you will see that the structure consists of a dark-colored framework of "needles" surrounded by a light-colored material (FIGURE 6).

FIGURE 6: Photomicrograph at 900x of a steel specimen containing retained austenite. The dark needle-shaped material is martensite, while the surrounding light-colored material is austenite..

The structure created in the case of a Brenco cone race is a mixture of martensite and austenite which is uniquely suited to the application of heavy-duty railroad bearings. Each part of this mixture supplies some property that promotes longer bearing life. Austenite is soft and tough, whereas martensite is hard, strong and brittle. An example of an object that is made primarily from austenitic steel would be a stainless steel tablespoon. An example of an object made primarily from martensitic steel would be a high-speed drill bit. Imagine placing such a spoon in a vice and smashing it with a hammer. It would bend, but it would be difficult to break! Now imagine the same experiment with the drill bit. If struck with enough force it would snap in half without bending. In contrast, try drilling a hole through a steel plate with the soft austenitic spoon! These two forms of steel have dramatically different properties and uses. However, when combined, the mixture of austenite and martensite creates a composite material that has some of the benefits of each, while compensating for the shortcomings of both (FIGURE 7).

FIGURE 6: Photomicrograph at 900x of a steel specimen containing retained austenite. The dark needle-shaped material is martensite, while the surrounding light-colored material is austenite..

Dimensional Stability: Since retained austenite normally only exists at high temperatures it is a metastable product at room temperature. Retained austenite will transform to martensite if the temperature drops significantly below the lowest temperature to which it was quenched or if the austenite is subjected to high levels of mechanical stress. Freshly created martensite has a larger volume than the austenite that it replaces. Where transformation occurs, there will be a localized 4-5% increase in the volume of the microstructure at room temperature and a resulting dimensional change in the geometry of the component. If great enough, this dimensional change could lead to cone bore growth.

Fatigue Strength: There are many ways for engineers to measure fatigue strength. The type of fatigue strength that is of interest to bearing users is called rolling contact fatigue life. Two aspects of retained austenite can improve rolling contact fatigue life. First, the inherent ductility of retained austenite helps to delay crack growth by blunting the tips of cracks as they form. Second, as retained austenite transforms during service, compressive residual stresses increase in the case. These compressive stresses delay crack growth by acting like a vise and clamping cracks closed. These benefits are not present in a bearing with insufficient retained austenite.

Impact Strength: Impact strength is the measure of the steel’s ability to resist fracture when subjected to a sharp blow. Austenite is very tough and has a higher impact strength than martensite. As the austenite content increases, the impact strength of the steel increases. A higher impact strength can provide extra protection against cracking if brinelling develops in service. This is turn helps prevent another source of spalling.

Retained austenite transforms during service due to the application of stress from carrying the load of the car (FIGURE 8)

FIGURE 8: This graph shows how the retained austenite content in a cone race declines with service. In order to preserve the minimum amount of retained austenite necessary for beneficial effects throughout the service life of the bearing (about 10%), the level must be higher when the bearing is new.

Under consistent car loading, this transformation will gradually level off and a stable retained austenite level will be achieved. To realize the maximum benefit, sufficient retained austenite must be present in the raceway surface prior to installation to insure that an effective amount remains after this transformation is complete.

Through laboratory research, field testing and the experience of being the leading producer of tapered roller bearings for railroads in the United States, Brenco has come to understand the intricacies of the martensite/austenite system. Because of this knowledge, Brenco is able to control the martensite/austenite ratio to deliver the highest performance railroad bearings available.

Some of the confusion regarding the subject of retained austenite in a railroad bearing results from the misapplication of knowledge developed in other industries. The toolmaking industry does not regard retained austenite favorably. Retained austenite is recognized as a cause for many premature failures of expensive tools and fixtures. Retained austenite’s low hardness is also incompatible with an application that demands the maximum attainable hardness to resist wear. The mere presence of retained austenite in a tool steel is generally regarded as a sign of improper heat treatment.

The gear industry has a more favorable view of retained austenite. While some of the same mechanisms that effect tooling applications also effect gears, there are some major differences. Tools and dies are predominately manufactured using through-hardened steel and as a result have high hardness combined with low impact strength. Gears are typically made from case-hardened steel that has high impact strength. Where most tools fail by wear or fracture, most gear failures are the result of spalling on the teeth. Spalling (FIGURE 9) occurs when the surface of a metal component is subjected to repeated cyclic loads. A crack will form and grow until a small portion of the surface breaks loose, damaging the surface and adding debris to the system (for greater detail on the subject of spalling, see Brenco Tech Forum 94-1). The gear industry has learned that having the proper amount of retained austenite in a gear tooth can delay spalling damage by suppressing crack growth. While railroad bearings experience a different form of loading and fatigue than gears do, the principle that retained austenite inhibits crack growth is common to both industries.

FIGURE 9: A typical spall such as might be encountered on the surface of a fatigued cone race. Such damage can be delayed by controlling retained austenite content to beneficial levels.

In the mid-1980’s, the railroad industry recognized that approximately 50% of premature bearing removals involved either loose components or a loss of bearing clamp. Loose cones are especially troublesome to the railroad industry because the spinning cone damages both the bearing and the axle (FIGURE 10).

FIGURE 10: A railroad axle showing damage from a cone that "spun" in service. Controlling cone bore growth is necessary to prevent this type of damage.

The inherent dimensional instability of retained austenite is the characteristic that is primarily responsible for cone bore growth. As the bearing accumulates service miles, it is subjected to high levels of stress. This stress causes a thin layer of the retained austenite near the raceway surface to deform and work harden, eventually transforming into martensite. This work hardening and transformation results in an increase in the volume of the material near the surface. The effect of all of this material growing against itself is a dramatic increase in the compressive residual stresses near the surface of the raceway. If the retained austenite level is too high, the geometric displacement caused by the phase transformation can be too great and will actually pull the cone bore larger, reducing the interference fit on the axle. However, when the retained austenite content is controlled to moderate levels, the creation of residual compressive stresses in the case is beneficial. Brenco exploits compressive residual stresses to prolong the service life of railroad bearings. Compressive residual stresses in the race surface act like a powerful vise that prevents crack growth by clamping cracks shut. Reduced crack growth rates result in improved rolling contact fatigue life.

Some in the industry have proposed that the solution to cone bore growth is to reduce retained austenite to it’s lowest possible levels. This approach would be very similar to what is done in the toolmaking industry. However, the railroad bearing has more in common with a gear tooth than it does to a metalworking tool. While bearings with low levels of retained austenite (less than 5%) might have good dimensional stability, they would suffer from higher rates of removal due to spalling because they would lack the beneficial effects that moderate levels of retained austenite have on impact strength and rolling contact fatigue life.

Brenco’s solution to the problem is BALANCE. We have found that by controlling retained austenite to optimum levels, the beneficial effects of retained austenite can be realized without experiencing excessive dimensional growth. Brenco has found that there is a "sweet spot" for the retained austenite content of a railroad bearing race. By maintaining this balance between dimensional stability and the many beneficial effects of retained austenite, Brenco delivers the longest lasting , lowest life cycle cost bearing in railroad history.

In order to obtain the optimum level of retained austenite, Brenco has developed proprietary process controls that govern numerous variables in our raw material and heat treatment operations. These variables include steel chemistry, carbon content, hardening temperature, quenching rate, and tempering temperature. These proprietary process controls have enabled us to consistently achieve the desired balance of fatigue/impact strength and dimensional stability in our products that only a handful of world class bearing manufacturers have been able to achieve.

IS IT WORKING?

Absolutely! Brenco is in a unique position to monitor product performance due to our leadership in freight car bearing reconditioning. By collecting data from bearings coming out of service at our Quality Bearing Service reconditioning subsidiary, we are able to see how various process improvements have impacted product performance in the field. Data from those reconditioning shops clearly shows that cone bore diameter stability has steadily improved over the last several years. (FIGURE 11)

FIGURE 11: Reconditioning facilities such as Brenco’s Quality Bearing Service inspect each cone for adherence to original equipment specifications. Since control of retained austenite was introduced in the early 1990’s, cones having large cone bores after service have been virtual;ly eliminated, proving that cone bore growth is being controlled with current retained austenite levels.

The Technical Forum is an information resource for the rail industry and is provided as a courtesy of Amsted Rail Group. Suggestions, inquiries or comments are welcomed and should be directed to:

Editor, Technical Forum
BRENCO, Incorporated
P.O. Box 389
Petersburg, Virginia 23804
804-863-1713

Additional copies provided upon request.

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