Medium Carbon Martensitic

13-17% Chromium (Types 403, 410, 414, 416, 420, 431, 440) - These steels resist oxidation scaling up to 825°C but are difficult to weld and, thus, are used mainly for items that do not involve welded joints. They are thermally hardened and useful for items that require cutting edges and abrasion resistance in mildly corrosive situations. However, they should not be tempered in the temperature range of 450 to 650°C. This reduces the wear resistance and hardness and also lowers the corrosion resistance because of the depletion of chromium in solution from the formation of chromium carbides. 

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Without these advances in hard, strong materials based on abundant, and therefore low-cost iron ore, there could have been no industrial revolution in the nineteenth century. Long bridges, sky-scraper buildings, steamships, railways, and more, needed pearlitic steel (low carbon) for their construction. Efficient steam engines, internal combustion engines, turbines, locomotives, various kinds of machine tools, and the like, became effective only when key components of them could be constructed of martensitic steels (medium carbon). 

Conunon base metals include cast iron, low-carbon steel, medium-carbon steel, alloy steel (including tool and bearing steels), stainless steel (austenitic, martensitic, or ferritic), aluminum alloys, titanium alloys or other nonferrous metals, for example, bronzes, copper, and brasses. 

Medium carbon steels have carbon contents up to 0.6 wt% C and may contain Ni, Cr, Mn, and other trace elements. With the carbon content closer to the eutectoid, these steels can be heat treated or tempered to improve their performance. Heat treatment consists of austenizing, heating above the eutectoid temperature to convert the cementite to austenite, then quenching to form martensite. 

A final consideration is the presence of unintended or alternate processes. Often reactions or phase changes can operate along a number of different pathways. One of the oldest commonly known cases of this type (not understood in detail until the mid 20 th century) was the martensitic transformation in steel. Martensite forms when a blacksmith heats a piece of medium-carbon steel to orange heat (above 740°C) and quickly quenches it (usually in water). A slow cooling leads to a relatively soft but thermodynamically-stable two-phase material (pearlite), while the sudden quench produces a hard brittle phase known as martensite. The issue of whether pearlite or martensite forms is one of diffusion kineties for earbon in the steel. 

The austenitic-ferritic steels, because of their two structural components also known as duplex steels, are chromium-nickel steels with chromium contents of about 21%-27%, and nickel contents of 4%-5%. They are usually made with about 3% molybdenum, nitrogen additions and a carbon content of < 0.03%. They reach the category of temperable martensitic steels with values for the 0.2% yield point of > 450 N/mm and are thus clearly above the austenitic steels. Worth mentioning are the good viscosity parameter values and the favourable fatigue strength properties of these steels, even in corrosive mediums. 

Chromium Increases strength and hardness, forms hard and stable carbides raises the critical temperatures increases hardenability amounts in excess of 12% render steel stainless 1.0-1.5% Cr in medium- and high-carbon steels for gears, axles, shafts, and springs, ball bearings and metal-working rolls 12-30% Cr in martensitic and ferritic stainless steels also used in conjunction with nickel... 

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