Full Steam Ahead: Railroad Wheel Manufacturing
How are railroad wheels made?
Did you know that railroads can haul three times the freight as trucks for the same amount of fuel?
The history of “rail roads” goes back to as early as 1550 when roads of rails – called wagonways – came into use in Germany. The first tram road was laid down in England in 1776 with cast-iron angle bars on timber ties. In 1789, Englishman William Jessup designed the first wagons with flanged wheels to run the rails.
One of the key developments that made railroading viable was the wheel for the cars and engines. Early railroad wheels were cast iron – eventually becoming cast steel. In 1895, Standard Steel (SS) designed and introduced the first bolted and cast-steel-tired railroad wheel. Nine years later they developed the “Standard Wheel” – a forged-and-rolled wheel that became the new railroad standard. It was not until 1930, with the SS development of the “Freedom Wheel,” that the first heat-treated wheel was used.
The forged-and-rolled wheel is commonly called a wrought-steel wheel. This process typically involves forging a wheel blank from a steel block. A second forging operation forms the rough contour of the wheel. The next stage is a unique metal-forming operation called wheel rolling. It is unique because of the simultaneous use of the various rolls to configure different parts of this complicated geometric shape. Although rolling operations vary, typically eight rolls simultaneously hold and shape the wheel to the desired diameter and contour. All rolls are movable and gradually change their position during rolling as the wheel transforms to the desired shape.
Between 1904 and 1979, forging was the wheel-manufacturing requirement of the American Association of Railroads (AAR). Beginning in 1979, the AAR began permitting pressure casting. Today, some wheels are forged and some are pressure cast. To manufacture wheels for the railroad, a company must be AAR-certified – requiring annual inspections/audits by the AAR.
Steel used for railroad wheels today is typically AISI 1060 or 1070. There are four classes of wheels, but only two – class B and C – are common today. Class B is 1060 steel, has a hardness requirement of 277-341 Brinell and is designated for high-speed service with severe braking conditions and heavier wheel loads. Class C is made from 1070 steel with a hardness requirement of 321-363 Brinell. Its intended service can include a range of braking conditions where off-tread brakes are employed.
Both classes are heat treated using a technique called rim treating. Rim treating involves heating the entire wheel in a furnace to the hardening temperature (usually around 1650°F) followed by a water-spray quench of the rim of the spinning wheel, as shown in Figure 1. Alternatively, while being fixtured vertically, the outer rim can be immersed in a water quench and spun. Tempering to the specified hardness is the final step. Throughout the process, slow cooling is incorporated as a precaution against the possible formation of minute internal cracks.
As part of the final wheel inspection, the rims are ultrasonically inspected to evaluate their internal quality. Because of its sensitivity, ultrasonic inspection is considered a key final-inspection tool.
Now you know what keeps trains rolling down the track. Rim treating provides the railroad wheel with the perfect combination of hardness for wear resistance with the strength and toughness to prevent fracture in this demanding application.