Let’s investigate how high temperature is used in large industrial equipment and processes.
When we think of railroads, we typically think of our grandfather’s time. Certainly, railroads are an emblem of the past. The golden-spike ceremony in 1869 effectively joined the East Coast to the West Coast in Utah. The history is storied, and you will probably see a locomotive in your favorite Western movie.
Did you know that the railroads are a $100 billion industry? Investors like Warren Buffett invest often and heavily in the railroad industry.
The Timken Company has been serving this industry for many years and also has facilities for remanufacturing railway bearings. Railroad cone bearings are produced from carburized 8720 alloy steel.
AREMA (American Railway Engineering and Maintenance-of-Way Association) creates specifications for the industry. Railroad rail is commonly made from high-carbon (0.6-1.0%) steel. The average locomotive measures about 75 feet in length, at least 15 feet in height and weighs in at 420,000 pounds. Their massive diesel engines typically range in size from 4,000-6,000 horsepower.
Rail wheels are typically forged in a forged-and-rolled process. In this wrought process, a wheel blank is forged from a steel block. A second forging operation forms the rough contour of the wheel. The next process is a metal-forming operation called wheel rolling, which uses various rolls (typically eight) to configure different parts of this complicated geometric shape.
Steel used for railroad wheels depends on the wheel class. The most common types are AISI 1060 or 1070. Both grades are heat treated using a technique called rim treating, which involves heating the entire wheel in a furnace to around 1650°F followed by a water-spray quench of the rim of the spinning wheel. The wheels are also shot-peened to an industry standard for improved fatigue-crack resistance under rolling stresses. Final hardness typically ranges from 277-363 Brinell.
In addition to steel, other materials and thermal processes are involved in the manufacture of locomotives. Brazed copper brass, known as CuproBraze, is used in locomotive heat exchangers for North American diesel-powered commuter trains. The heat exchangers (radiators) are manufactured from flat brass tubes, which are mechanically bonded into steel headers. Similar heat exchangers are also used to cool the transformer oil in electric-powered locomotives.
Powder metals get involved in the action as well as nonferrous through the use of ultralight aluminum foam. The PM “foam” is worked into sheets and profiles. The material expands and becomes highly porous during heat treatment. It is used for locomotive collision posts, crash cages, crush buffer zones and railcar side-impact barriers.
You may have seen advertising touting the “green” nature of rail freight hauling. Trains are now more fuel-efficient than hybrid cars. They are able to travel huge distances on just 1 gallon of fuel and can haul more cargo than 300 trucks. Hybrid diesel-electric engines reduce fuel consumption by 15% and emissions by 50% over their diesel counterparts.
A tunnel boring machine (TBM) is an interesting piece of equipment. Just how does this 23-foot-diameter machine with a length of 200 feet and a weight of 1.2 million pounds cut 35 feet of tunnel every day? And how does thermal processing play a role?
From early hand digging to gunpowder and nitroglycerine, tunnel-digging technology advanced in relative baby steps until James Robbins invented the TBM in 1954. The front face of the TBM (cutter head) contains numerous cutting wheels or disc cutters that roll against the rock face, breaking it into small pieces as the cutter head rotates at 0.1-10 revolutions per minute. Hydraulic cylinders thrust the face and disc cutters into the rock wall.
Modern TBMs are customized for each project by matching the types and arrangement of the disc cutters to the site geology. These cutters are positioned to cut concentric tracks in the rock face – generally 2-4 inches apart. The diameter of the TBM must be equal to the diameter of the designed tunnel (including its lining).
With so many different TBM arrangements, there are also a wide variety of cutting discs. These discs are typically about 17 inches in diameter and are made from hardened alloy steels specifically developed for maximum efficiency in rock-tunneling applications. The steels – typically high-alloy tool steels – are heat treated to provide the ideal balance between hardness (for wear resistance) and toughness (to prevent premature breakage). This ideal balance, and consequently the heat treatment, will vary depending on the site geology. Cemented carbides and cutters with carbide insert rings are used for the most demanding hard-rock applications.
Materials and their thermal processes are involved in almost every design aspect of the TBM. The disc cutters – there might be 40 or more on a single TBM – rotate in bearing assemblies. The heat treatment of these bearings is another critical design element of the TBM. The TBM cutterhead is attached to the main body by a specially designed and hardened two-row, high-capacity tapered roller bearing.
Similar to tunnel boring, another interesting drilling tool is worth a look. This equipment has saved lines on more than one occasion – Quecreek in 2002 and Chile in 2010.
The type of drill used in Chile is called the LP drill. LP stands for “low-profile,” and it was developed by Center Rock (CR) in Pennsylvania following the Quecreek incident. Inside the 28-inch canister there are four air hammers with a 250-pound piston and drill bits that move in tandem to dig through the rock. As the piston strikes the rock, it is fractured, and the air moves the rock chips to the surface. The drilling rig stands almost 150 feet high, and it can bore through as much as 130 feet of rock in a day.
Most of the material used in the construction of the canister and the pistons is quench-and-tempered 4140/4340. The drill piston is carburized to create good wear and improved fatigue properties. They are designed to survive 10-15 bits and are also shot-peened to improve the fatigue strength.
The “expendable” bits/inserts are tungsten carbide (WC) or diamond-impregnated WC. For a 6.5-inch hole, an insert can last 1,000-25,000 feet, depending on the rock type. The WC inserts are produced using hot isostatic pressing (HIP) followed by sintering. The diamond bits make use of a tetrahedral press. A WC insert is used with polycrystalline diamond powder. Temperatures of 1800-2500°F and pressures to 1.5 million pounds fuse the diamond to the carbide, which improves the wear resistance of the insert 50 times.
Welding with a Bang
There are a number of ways to join two metals together. Most of these involve some form of welding or brazing with many variations in these techniques. One of the welding categories is solid-state welding, and one of the eight solid-state welding techniques – explosion welding (EXW) – meets a unique metal-joining need.
As a welding technique, EXW is a relative newcomer. Its origins go back to World War I when it was observed that shrapnel was sticking to armor plate. Since heat was not applied, the conclusion was that the explosive forces caused the welding, and this theory was tested and duplicated in the years following World War II.
DuPont applied for a patent on the explosion-welding process in 1962, and it was granted in 1964. Much of the early work identified the ability to bond different types of metal with very different crystal structures. This unique ability has led to most of the current applications of EXW.
An early application of this technology was the tri-layer bonded material – copper sandwiched between copper-nickel –
used for coinage in 1965. For this application, 6-inch billets about the size of a desktop were explosion welded and then hot rolled. In the following three years, DuPont produced 70 million pounds of blanks for dimes, quarters and half dollars that had previously been made from silver. When roll bonding of strip came into use, EXW was no longer a competitive way to produce the coin stock.
During the explosion, an energetic jet forms between the two metals that cleans the surfaces. Although heat is not applied in making an explosion weld, investigation shows that the interface metal is molten during the EXW process. This interface melt line is only 0.05-0.2 microns thick and cannot be seen with an optical microscope. Consequently, the generated heat does not affect the material properties. The strength of the EXW joint has been found to be equal to or greater than the strength of the weaker of the two joined metals.
The localized heat necessary to form the weld is generated from the shock wave associated with the collision. Heat is also released by the plastic deformation (think friction) at the interface. The EXW process is self-contained and portable, and it can occur quickly over large areas.
The ability to join large surfaces in short order has led to many of the applications of this technology. It is typically used for joining large, flat surfaces. Large plates – some up to 100,000 pounds and more than 40 feet long – are produced this way. These largest plates are produced for oil-refinery equipment, power generation and chemical-equipment manufacturers.
The largest applications and most exotic material combinations are the manufacturing areas best suited to EXW. In the 1980s, the dominant products were titanium and aluminum clads. In the mid-1980s, clad plates were made for the first zirconium-clad acetic-acid reactors. This has developed into a major application for EXW products. A large titanium-steel clad-plate application to produce an autoclave about 14 feet in diameter and almost 100 feet long is one of “the most successful large titanium-clad jobs ever.” Some of the smaller applications include ultrahigh-vacuum joints between aluminum, copper and stainless steel; corrosion-resistant claddings on mild steel; and alloy aluminum joined to low-expansion-rate metals for electronics.
By virtue of the explosive nature of the process, companies that perform EXW have experienced challenges in finding the best location. The largest EXW manufacturer uses a limestone mine in Pennsylvania as the site of its explosions. The 70-foot-thick limestone with a 60-foot-tall and 60-foot-wide entrance makes the perfect location.
While being somewhat limited by its manufacturing technique –
who wants an EXW facility next door – the flexibility in material size, variety of metal combinations and quick delivery are all areas in which explosion welding leads the way.
We hope you have enjoyed this look into the industrial thermal processes involved in producing large industrial equipment. For more interesting thermal-processing stories, check out our Everyday Metallurgy book in the online bookstore.