Bronze

It is used for screws and wires. One is HMS 1, wich means Heavy Melting Scrap grade 1,and HMS 2, which means Heavy Melting Scrap grade 2. It is somewhat stronger than copper and it has equivalent ductility. For purposes of recycling, cast iron is classified into two types. Commercial bronze is 90% copper and 10% tin. The properties are similar to malleable iron but parts can be cast with larger sections. Alpha bronze alloys of 4-5% tin are used to make coins, springs, turbines and blades. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies.

Alpha bronze consists of the alpha solid solution of tin in copper. Tiny amounts of magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes. Bronze is typically 60% copper and 40% tin. A more recent development is nodular or ductile cast iron. Phosphor bronze is particularly suited to precision-grade bearings and springs. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron. Bronze is still widely used today for springs, bearings, bushings and similar roles, and is particularly common in the bearings on small electric motors. In general, the properties of malleable cast iron are more like mild steel.

Bronze also has very little metal-on-metal friction, which made it invaluable for the building of cannon where iron cannonballs would otherwise stick in the barrel. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. Common bronze alloys often have the unusual and very desirable property of expanding slightly just before they set, thus filling in the finest details of a mould. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. Bronze is the most popular metal for top-quality bells and cymbals, and also for cast metal sculpture (see bronze sculpture). Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Aluminium is also used for the structural metal Aluminium bronze. Malleable iron starts as a white iron casting, that is then heat treated at about 900 °C.

In the twentieth century, silicon was introduced as the primary alloying element, creating an alloy with wide application in industry and the major form used in contemporary statuary. High-chrome white iron alloys allow massive castings (for example, a 10t impeller) to be sand cast, i.e., a high cooling rate is not required, as well as providing impressive abrasion resistance. Some common examples are the high electrical conductivity of pure copper, the excellent deep-drawing qualities of cartridge case brass, the low-friction properties of bearing bronze, the resonant qualities of bell bronze, and the resistance to corrosion by sea water by several bronze alloys. White cast iron can also be made by using a high percentage of chromium in the iron; Cr is a stong carbide-forming element, so at high enough percentages of chrome, the precipitation of graphite out of the iron is suppressed. Copper and its alloys have a huge variety of uses that reflect their versatile physical, mechanical, and chemical properties. The resulting casting, called a “chilled casting”, has the benefits of a hard surface and a somewhat tougher interior. The cost of copper-base alloys is generally higher than that of steels but lower than that of nickel-base alloys. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron.

Bronzes also conduct heat and electricity better than most steels. It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. Bronzes resist corrosion (especially seawater corrosion) and metal fatigue better than steel. White iron is too brittle for most uses, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as balls for rolling-element bearings, the wear surfaces (impeller and volute) of slurry pumps and the teeth of a backhoe's digging bucket. Bronzes are softer and weaker than steel, and more elastic, though bronze springs are less stiff (and so storing less energy) for the same bulk. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a cermet. They are generally about 10 percent heavier than steel, although alloys using aluminium or silicon may be slightly less dense. These precipitates inhibit plastic deformation by impeding the movement of dislocations through the ferrite matrix, offering hardness at the expense of toughness.

Copper-based alloys have lower melting points than steel and are more readily produced from their constituent metals. With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe₃C, rather than graphite. Steel, of course, has wondrous properties that bronze cannot compete with. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware. It is considerably less brittle than iron and has a lower casting temperature. It is also difficult to weld. While it develops a patina, it does not otherwise oxidize into nothingness. Easier initiation of cracks can be a drawback once an item is finished, however: grey cast iron has less tensile strength and shock resistance than steel.

Excluding steel from the discussion, bronze is superior to iron in nearly every application. The sharp edges of graphite flakes also tend to concentrate stress, allowing cracks to form much more easily, so that material can be removed much more efficiently. As ironworking improved, iron became both cheaper and stronger, eclipsing bronze in Europe by the early to mid-Middle Ages. All of the properties listed in the paragraph above ease the machining of grey cast iron. Bronze was still used during the Iron Age, but for many purposes the weaker iron was sufficiently strong. In practical terms, this means that cast iron tends to “damp” mechanical vibrations (including sound), which can help machinery to run more smoothly. But the Bronze Age gave way to the Iron Age, perhaps because the shipping of tin around the Mediterranean (or maybe from Britain) became more limited during the major population migrations around 1200 – 1100 BC, which dramatically limited supplies and raised prices [1]. Since ferrite is so different in this respect (having heavier atoms, bonded much less tightly) phonons tend to scatter at the interface between the two materials.

Bronze was stronger than the era's iron; quality steels were not available until thousands of years later. The exceptionally high speed of sound in graphite gives cast iron a much higher thermal conductivity. The earliest tin-alloy bronzes date to the late 4th millennium BC in Susa (Iran) and some ancient sites in Luristan (Iran) and Mesopotamia (Iraq). Graphite acts as a lubricant, improving wear resistance. For Europe, the major site for tin was Britain. The graphite content also offers good corrosion resistance. The archaeologists suspect a serious disruption of the tin-trade led to the development of the Iron Age. The metal expands slightly on solidifying as the graphite precipitates, resulting in sharp castings.

Serious bronze has always involved trade. This structure has several useful properties. While copper and tin can natually co-occur, the two ores are rarely found together (an ancient site in Thailand does prove they can co-occur). Weak bonding between planes of graphite lead to a high activation energy for growth in that direction, resulting in thin, round flakes. In early use, the natural impurity arsenic created a superior natural alloy; this is termed arsenical bronze, which Ötzi's axe is made of. Silicon causes the carbon to rapidly come out of solution as graphite, leaving a matrix of relatively pure, soft iron. First used in the Bronze Age, it made tools, weapons and armor harder or more durable than their stone and copper ("Chalcolithic") predecessors. Silicon is essential to making of grey cast iron as opposed to white cast iron.

. . (See table below). The color of a fracture surface can be used to identify an alloy: carbide impurities allow cracks to pass straight through, resulting in a smooth, "white" surface, while graphite flakes deflect a passing crack and initiate countless new cracks as the material breaks, resulting in a rough surface that appears grey. Bronze is the usual English term for a broad range of copper alloys, usually with tin as the main additive, but other elements may be the main additive (e.g., phosphor, manganese, aluminum, silicon). Cast iron tends to be brittle, unless the name of the particular alloy suggests otherwise. Since cast iron has nearly this composition, its melting temperature of 1420 to 1470 K is about 300 K lower than the melting point of pure iron.

The iron-carbon eutectic point lies at 1403 kelvins and 4.3 mass % carbon. Other elements are then added to the melt before the final form is produced by casting. Carbon and silicon content are reduced to the desired levels, which may be anywhere from 2% to 3.5% for carbon and 1% to 3% for silicon depending on the application. It is made by remelting pig iron, often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulfur, which weaken the material.

Cast iron usually refers to grey cast iron, but can mean any of a group of iron-based alloys containing more than 2% carbon (alloys with less carbon are carbon steel by definition).
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