Elements such as copper and tin that get mixed into steel scrap, are hard to remove by refining, and accumulate in steel with each recycling. They cause quality problems such as surface cracking during hot working.
Japanese name: トランプエレメント
Tramp elements appears in 1 practice question across 1 topic, including Materials selection and design.
When steel scrap is melted in an electric arc furnace, elements mixed in with the scrap dissolve along with it. In refining, oxygen is blown in to remove elements more easily oxidized than iron, such as silicon, manganese, phosphorus and carbon, as oxides or gases, but elements less easily oxidized than iron, such as copper, tin, nickel and molybdenum, remain in the steel. Elements that cannot be removed by refining in this way and end up in the steel unintentionally are called tramp elements. The problem is that, because they cannot be removed, they build up little by little each time steel is made from scrap and that steel becomes scrap again.
The best-known harm is surface cracking during hot working caused by copper. When steel is heated before rolling, iron at the surface oxidizes preferentially to form scale, and copper, which oxidizes less easily, becomes concentrated at the interface with the scale. Above the melting point of copper (about 1085°C), the concentrated copper becomes liquid and penetrates the grain boundaries, and the surface cracks during rolling. Tin reduces how much copper dissolves in iron and also lowers the temperature at which liquid forms, so even small amounts make this cracking worse. Nickel, conversely, suppresses the cracking, and some steels contain copper and nickel added together.
Besides hot shortness, copper and tin lower the ductility of steel and make demanding forming such as deep drawing difficult. Tin, antimony, arsenic and similar elements tend to segregate to grain boundaries and also cause temper embrittlement. Products with strict quality requirements, such as automotive sheet steel, therefore tolerate only small amounts, and steel from scrap tends to go to products with relatively relaxed requirements, such as reinforcing bars for construction. This lowering of the grade of use with each round of recycling is called downcycling. On the other hand, copper is also a useful element when added on purpose, as in weathering steels.
Countermeasures fall into three kinds: not mixing them in, removing them and diluting them. Copper-rich parts such as motors and wiring harnesses from end-of-life vehicles are removed at the dismantling stage, and copper is removed after shredding by magnetic separation and hand sorting. What remains is diluted by adding pig iron or direct reduced iron low in impurities. Methods for removing copper in refining have been studied, but none is yet in wide practical use. In aluminum recycling too, iron and silicon are similarly hard to remove and are a problem as elements that build up with each remelting, so collecting scrap sorted by alloy type is important.
Answer: A. Copper and tin. They oxidize less readily than iron, so refining cannot remove them, and they cause surface cracking in hot working (hot shortness)
In steel refining, oxygen is blown in to oxidize elements that oxidize more readily than iron, and they are removed as slag. Copper and tin oxidize less readily than iron, so this method cannot remove them; when copper wire from motors and the like gets mixed into scrap, they become more concentrated in the steel with every recycling. During reheating before hot rolling, iron is oxidized preferentially, so copper becomes enriched at the interface with the scale, and above the melting point of copper (about 1085 °C) it becomes liquid, penetrates the grain boundaries and causes cracking. Tin aggravates this. Carbon can be removed as carbon monoxide by oxygen blowing, so the carbon content can be adjusted after recycling. Aluminum oxidizes far more readily than iron and is in fact used as a deoxidizer; it goes into the slag during refining. Tramp elements are controlled by sorting scrap and by dilution with cleaner iron sources.
In steel, copper, tin, nickel, molybdenum and others are listed, and copper and tin are particular problems. What they have in common is that in refining by blowing oxygen they are not oxidized before iron and cannot be moved into the slag. Which elements are controlled, and to what level, depends on the type of steel and its use.
Steel refining removes impurities by oxidizing them with oxygen into slag or gas, so elements less easily oxidized than iron cannot be removed without oxidizing the iron first. Evaporating them in vacuum is also very slow when the copper concentration in the steel is low and is considered hard to put into practice. Keeping them out at the scrap stage is therefore the basic approach.
Not necessarily. Steel made from iron ore by blast furnace and converter contains almost no copper or tin, but electric arc furnace steel can also be of high quality if the scrap is selected and diluted with reduced iron or similar. Because electric arc furnaces emit less CO₂ in production, the challenge is to widen the range where they can be used while controlling tramp elements.
Tramp elements are controlled by sorting scrap and by dilution with cleaner iron sources.
When steel scrap is remelted in an electric arc furnace for…Ashby chart (materials selection chart) Life cycle assessment Material index (performance index) Pareto optimality Specific stiffness Yield before fracture