technical insight
Notably, Duplex stainless steels have the potential for application, because of their high strength in larger thicknesses. However, they will not become popular in the production of railcars. They are more expensive than austenitic steels, and in lower thicknesses (up to about 5 mm), cold-worked austenitic materials are stronger than duplex steels. Where larger thicknesses are required, high- strength, low alloy (HSLA) steels with yield strengths up to 700 MPa are commonly used. The use of martensitic and ferritic steels is limited to non-structural applications. In typical descriptions of austenitic stainless steels, their mechanical properties are those in the annealed condition. However, the strength of these materials may be significantly increased by cold deformation, such as thickness reduction in cold rolling, forming, or bending. Deformation strengthening of austenitic steels results from the partial transformation of austenite into martensite. The strengthening efficiency of cold rolling depends on the material thicknesses. For example, in thicknesses up to 1 mm, tensile strength close to 1300 MPa and yield strength (0.2% proof), close to 1000 MPa, may be achieved. For 5-mm thick materials, the achievable values are 1000 and 750 MPa, respectively. The high strength-to-weight ratio allows for cold-worked stainless steel to be considered a lightweight material.
immediately dismissed these alterations as impractical. What caught his attention was the rail car’s stainless steel construction. The material was lightweight yet strong, and it lasted forever. The problem was that no one had been able to figure out a way to build stainless steel rail cars in a practical shop operation. Fortunately, the Burlington president’s timing was exactly right. The Budd Company had just patented its newly devised “Shotweld Process.” The system allowed rivet-less seams of stainless steel to be joined without reducing its corrosion-resistant qualities. At the same time, it provided a joint stronger than the steel it held together. Ralph Budd knew a good product when he saw one. He decided this would be the material for the new train he wanted to build to recapture the rail passenger market. On June 17, 1933, barely a year since he first stepped into office, Ralph Budd signed a contract with the Budd Co. to construct a train out of stainless steel. The firm was given free rein in the design of the train. Grade selection considering mechanical and physical properties: The first stainless steel railcars were made from an austenitic alloy produced by Allegheny and classified by Budd as 18-8 steel, consisting of 18% chromium and 8% nickel. High carbon content made this stainless steel susceptible to chromium carbide precipitation in the HAZ of welds and to subsequent intergranular corrosion. The need to limit dwell time, in the critical temperature range, motivated for Budd’s experts to invent the short-time spot welding process (‘shotweld’). In the 1950s, 201 and 202 steels were also applied. In their chemistries, a substantial part of the nickel is replaced with manganese. Later, 17-7 Type 301 steel was introduced. In the 1980s, the advent of argon-oxygen decarburization allowed the fabrication of low-carbon stainless steels containing less than 0.03% C. This carbon level prevents the sensitization of stainless steel caused by welding, either with resistance or fusion processes.
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Issue 2 – 2024
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