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W hen dealing with steel structures we engineers assume that our steel is homogenous, without impurities, and without any other defects. We assume that we are getting the raw material for our structure tomeet our needs for strength, elasticity, yield, etc.We use this information to build crane structures that we have calculated to be safe. Over the past 20 years, computers have developed to help us calculate safe loads for cranes, and enable us to predict the behaviour of our structure with greater certainty. Despite this, we are still seeing failures in our bridges, shafts, beams, and arms from defective steel. There is increasing evidence that cranes, particularly tower cranes, are not being spared from this problem. “The quality of the material is one of the most important factor in the stability of the crane,” said Christoph Schneider, head of project management at tower crane manufacturer Liebherr Biberach, at Cranes Today Crane Safety 2007. Steel is an alloy of mostly iron, with between 0.02% and 1.7% of carbon by weight. Carbon and other elements act as hardeners in the iron lattice. The alloying elements in the steel control properties such as hardness, elasticity, ductility (its mouldability), yield strength (the force required to permanently deform it) and tensile strength (the force required to break it). Common grades of steel include S960QL, which has a yield strength of 1,100 N/mm 2 , or ST 52, with an ultimate tensile strength of 5,200 kg/mm 2 . These properties can render a crane unsafe. For example, some tower crane manufacturers use steel that can become brittle and break when hit in cold temperatures, according to Manitowoc CraneGroup, the corporate owner of Potain tower cranes. "It is our understanding that some of the local Chinese crane manufacturers use grades Q235B and Q345B steel for their cranes," the company said in a statement. "A crane manufactured with B-grade steel would potentially be at risk of material failure when operating under 20°C (68°F)," it said. In contrast, Potain's plant in China uses Q235C, Q345C and Q345D steels, which are appropriate for operation in cold temperatures. C-grade steels are suitable for operation at 0°C (32°F), and D-grade steels are suitable for work at -20°C (-4°F), it said. There are two ways to make steel: by converting mined iron ore, or by recycling steel that has already been made. In an integrated steel mill, a blast furnace burns iron ore mined in its natural form. Then a basic oxygen furnace converts the pig iron intohigher-quality steel byadding carbon and removing impurities through chemical reactions inside a vessel. Some of the impurities escape as gas or form a slag, which floats on top of the steel and can be skimmed off. The concentration of the carbon in the molten iron can be seen from the colour of the flame emerging out from the tank cap; today photoelectric sensors perform this job. In contrast to the large integrated steel mills, there are many more mini-mills, which use electric arc furnaces to melt scrap steel: train rails, pressed cars, iron fence, rebar, or other reclaimed materials. Ingots produced using these sources have many metallic and non-metallic impurities. In theory it is possible to remove all of these impurities by pumping the right gas through the reaction vessel so that it chemically reacts with the impurity to form a slag. But with scrap steels, there are so many impurities that the process is more complicated. However the steel is made, it needs to be cooled and formed into a usable product. The traditional method is to pour the molten iron into ingots. As the steel cools and hardens, impurities will float to the top. Once the ingot has solidified, manufacturers cut off the impurities. It takes considerable technical expertise to know exactly where to cut the ingot. If the cut is too shallow, the steel risks being contaminated. If the cut is too deep, perfectly good steel could be wasted. Also, might commercial pressures force the cut line upward? In Europe and the USA, this dilemma has largely been superseded by new technology. Most steel is now poured into a so-called continuous casting machine that rolls a single long slab of steel, according to Emmanoel Lima, ThyssenKrupp steel technical marketing specialist. This process is more efficient and produces higher volumes, he said. He also said that continuous casting machines could squeeze impurities to the sides of a slab during the rolling process, for later trimming. The collection of scrap steel is popular and profitable, and has become an international business. About two thirds (65.4%) of steel in 2005 was produced by integrated steel mills, and a third (31.7%) Tower crane engineer Felix Weinstein argues that steel impurities are threatening the safety of cranes. Steel produced in ingots from recycled steel is most at risk to contamination. The only solution is extra testing. Dangerous steel Tower cranes www.engineers.org.il הנדסת מכונות 29

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