Chromium molybdenum—its uses and fabrication requirements
Chromium molybdenum alloys have characteristics that make them good choices for many products used in construction and manufacturing. This article discusses some applications for these materials and the processes and equipment necessary to complete them.
Chromium-molybdenum alloys often are grouped in a single category. The names for this category are nearly as numerous as their uses. Some of the names are chrome moly, croalloy, chromalloy, and CrMo.
These alloys’ characteristics make them desirable in many areas of construction and manufacturing. The main characteristics are strength (creep strength and room temperature), rigidity, hardenability, wear resistance, corrosion resistance, fairly good impact resistance (toughness), relative ease of fabrication, and the ability to be alloyed in various ways that create “fitness for use” in some applications.
Often unnoticed by the general engineering populace, the value of these alloys in building construction has been brought to the forefront by the 9/11 event. Some innovative engineers realized that this material possibly could have saved hundreds of lives had the towers been constructed with the alloy in some form—most likely A387 Grade 11 or Grade 22.
The theory is that the CrMo would have retained approximately 50 percent of its strength at 1,000 degrees F and, therefore, would not have collapsed so quickly. It’s possible that the material could be used in constructing skyscrapers and other buildings in the future. Hopefully, a tragedy such as 9/11 will never occur again, but fires that produce similar results are likely to occur.
Automotive and Heavy Equipment
These alloys have many more uses than once was believed. For a young car racer, a chrome-moly crankshaft was a must-have if he wanted to be a winner. Some drivers had no idea why it was important to have such an alloy in that part of the engine. It was just thought to be the “thing” to have.
Cast-iron crankshafts served automobiles well for many years when the RPMs were relatively low and the highways did not stretch from coast to coast. The horsepower was not great enough to cause crankshaft failure in most cases. Heat didn’t build up under the hood the way it does today with modern engines.
The crankshaft is a very good example of the benefits of chrome moly in that wear, heat resistance, and strength are all needed properties for an efficient and long-lasting engine.
Modern diesel engines also need very strong crankshafts. Earthmoving machinery has higher horsepower and torque that enable the equipment to move massive amounts of earthen materials (rock, coal, and sand) very rapidly.
The hydraulic systems in this equipment also require good tensile strength and wear resistance to lift the large buckets and shovels that were unheard of just a few years ago. Most of the hydraulic shaft material is CrMo.
Axles and roller pins on track machines (bulldozers) are nearly always made of this material in some formula.
One of the major benefits of using this material for heavy-duty machinery is its ability to surface-harden (case-harden) or through-harden. Case hardening allows the surface to be wear-resistant and the interior to be relatively ductile and less brittle.
The torsional strength inherent in CrMo alloys is extremely important in modern high-performance engines. Some of today’s street automobiles now have more than 300 HP and torque that was unimaginable some years ago.
Today’s race cars have even much more horsepower. Their frames are not built by the automotive manufacturers; they are custom-built to meet the requirements of NASCAR® and other racing affiliates. Most of the frames (nearly all) are fabricated from CrMo tubing. This allows the frames to be made from thinner materials because the strength is greater than that of regular low-carbon steel. Because of its ductility, 4130 CrMo is the most popular for race car frames. Although it is stronger, higher-carbon 4140 is not as ductile. Brittle material renders race cars unsafe.
Note: For those who are unfamiliar with the AISI/SAE terms, such as 4130 and 4140, the first two digits refer to the alloy type chromium molybdenum, and the last two digits refer to the carbon content—in these cases, 0.30 percent and 0.40 percent, respectively, not 30 percent and 40 percent.
Fabricating CrMo frames has become an industry of its own, and some shops are dedicated to building and rebuilding them (Figure 1). The endeavor is very lucrative because the frames often survive only one or two races. Also, racers constantly redesign their cars to make them faster or stronger or more stable in the turns.
Perhaps the declared expert (by most motorsports people) for welding the frames is Denny Klingman of Lincoln Electric. Klingman’s specialty is gas tungsten arc welding. (My personal belief is that he should apply for the AWS A5 Committee for the CrMo wire and electrode definitions and usages. One of the pioneers in this field, he has the knowledge and the practical experience that not many experts can claim.)
The type of wire normally used for welding 4130 material is ER80S-B2. ER80S-B3 may be used, but is not usually recommended except for material with higher Cr and Mo content. It is a bit more brittle than the B2 wire.
Many CrMo fabricators do not use preheat. I can’t bring myself to weld any CrMo without preheat, not even the thin stuff. Professor Roy McCauley of Ohio State University placed the idea in my head years ago that if the alloy is CrMo, preheating is not optional, it is mandatory. I have observed many instances that proved his point. I am sure the slow heat input and hydrogen diffusion that the GTAW process provides help to slow-cool and prevent cracking of the material, but my professor’s instruction still sticks too firmly in my memory. Perhaps it is not absolutely necessary, but it will not be detrimental. The material’s composition lends itself to hydrogen attraction.
The tungsten type most frequently used with 4130 CrMo is EWTH-2 with the same sharpening that is used with stainless or carbon steel. I prefer the 1/8-in.-diameter tungsten electrode because it lasts longer than smaller diameters.
The gas is pure argon. Since the amperage is fairly low (75 to 95), an air-cooled torch is sufficient in nearly all cases. Cup size depends on the joint configuration. A tight joint may require a smaller cup and a square groove joint may need a larger cup. The larger cup tends to be easier to cup walk on an easily accessible butt joint, while the tighter joints allow for better cup walking with the smaller cup.
Probably the most important part of welding CrMo with GTAW is to take your time. Allow the material to slow-cool, and do not rush in and weld multiple beads without stopping.
The critical maximum temperature presented in the old Westinghouse handbook is 600 degrees F. I believe this is a realistic requirement. Once the material is brought up to welding temperature, the heat should be cycled as little as possible. Lunch and coffee breaks are not good for GTAW on CrMo.
Material Production Furnaces
Type 4130 is only one type of CrMo material and the example presented here is only one of the many ways CrMo alloys are used and fabricated. Many heavy industrial fabrications are 100 percent built from these alloys. Heat resistance and superior creep strength make this material suitable for parts in material production furnaces that utilize moderate to extreme heat.
An example of this usage is in the aluminum production plant. Furnace cars frequently are fabricated from A387 Grade 22, which is very strong at elevated temperatures. The melting point of aluminum is around of 1,200 degrees F. The melting point of the element chromium is approximately 2,768 degrees F. The melting point of the element molybdenum is about 4,700 degrees F. This does not mean that the CrMo melting point is as high as the elements’, but it is heat-resistant enough to make the material a good choice for transport cars that run in and out of the furnace when aluminum is being smelted.
Furnace car material usually is 1-1/2 in. thick or thicker. The welding procedure normally requires at least a 400-degree-F preheat and a postweld heat of 600 degrees F followed by a slow cool in still air or covered by a heat blanket.
Material of this thickness actually needs to be preheated before tacking. I have experienced 20 tacks placed and 20 tacks cracked without preheat. It is a mournful sound at the end of a shift, but a good lesson learned. The fit-up should be performed just before welding, and the tacks should be large enough to hold the parts securely together until most of the welding can be completed. This minimizes the amount of heat cycling, which is a major cause of delayed cracking.
The welding material in this application usually is the B3 grade because the nominal composition of the 387 Grade 22 alloy is 2.25 percent Cr and 1 percent Mo (1.25 Cr and 0.50 Mo in the Grade 11).
Some fabrications have multiple intersections and multiaxial stresses (Figure 2), which makes distortion control very difficult. Rather than reheat the whole weldment, it is much more efficient to use vibratory stress-relief equipment during and after all the welding is completed. This process, called weld conditioning, does not relieve mechanical stresses, but it does a good job with thermal stresses, especially if it is used during welding. In our factory, we have not yet experienced cracking due to the vibration.
The vibratory stress-relief process does not eliminate the need for preheating. It is very difficult to preheat the grids in such a part, but the heat must be distributed over as wide an area as possible. Welds should be examined by magnetic particle testing 48 hours after welding to ensure that no delayed cracking has occurred.
A whole book could be written about this family of alloys that also includes P91, P5, and others not discussed in this article. Of course, no one material will satisfy all requirements, but when wear resistance, strength, heat resistance, and certain other attributes are needed, chromium-molybdenum alloys will fit the bill.