FERROUS AND NON FERROUS METALS
1 Effect of alloying elements on steel properties
2 Characteristics of alloying elements
3 Maraging steels
4 Heat treatment cycle
5 Classificaion of copper and its alloys
5.3 Tool and die steel
6 Effects of alloying elements on steel
1.EFFECT OF ALLOYING ELEMENTS ON STEEL PROPERTIES
Alloying is changing chemical composition of steel by adding elements with purpose to improve its properties as compared to the plane carbon steel.
The properties, which may be improved
Stabilizing austenite - increasing the temperature range, in which austenite exists.
The elements, having the same crystal structure as that of austenite (cubic face centered - FCC), raise the A4 point (the temperature of formation of austenite from liquid phase) and decrease the A3 temperature. These elements are nickel (Ni), manganese (Mn), cobalt (Co) and c opper (Cu).
Examples of austenitic steels: austenitic stainless steels, Hadfield steel (1%C, 13%Mn, 1.2%Cr).
Stabilizing ferrite - decreasing the temperature range, in which austenite exists.
The elements, having the same crystal structure as that of ferrite (cubic body centered - BCC), lower the A4 point and increase the A3 temperature.
These elements lower the solubility of carbon in austenite, causing increase of amount of carbides in the steel.
The following elements have ferrite stabilizing effect: chromium (Cr),
tungsten (W), Molybdenum (Mo), vanadium (V), aluminum (Al) and silicon (Si).
Examples of ferritic steels:transformer sheets steel (3%Si), F-Cr alloy
Carbide forming - elements forming hard carbides in steels.
The elements like chromium (Cr), tungsten (W), molybdenum (Mo),vanadium (V), titanium (Ti), niobium (Nb), tantalum (Ta),zirconium (Zr) form hard (often complex) carbides, increasing steel hardness and strength.Examples of steels containing relatively high concentration of carbides: hot work tool steels, high speed steels. Carbide forming elements also form nitrides reacting with Nitrogen in steels.
Graphitizing - decreasing stability of carbides, promoting their breaking and formation of free Graphite.
The following elements have graphitizing effect: silicon (Si), nickel (Ni), cobalt (Co), aluminum (Al).
Decrease of the eutectoid concentration.
The following elements lower eutectoid concentration of carbon:
titanium (Ti), molybdenum (Mo), tungsten (W), silicon (Si), chromium (Cr), nickel (Ni).
Increase of corrosion resistance.
Aluminum (Al), silicon (Si), and chromium (Cr) form thin an strong oxide film on the steel surface, protecting it from chemical attacks.
2.CHARACTERISTICS OF ALLOYING ELEMENTS
Manganese (Mn) - improves hardenability, ductility and wear resistance. Mn eliminates formation of harmful iron sulfides, increasing strength at high temperatures.
Nickel (Ni) - increases strength, impact strength and toughness, impart corrosion resistance in combination with other elements.
Chromium (Cr) - improves hardenability, strength and wear resistance, sharply increases corrosion resistance at high concentrations (> 12%).
Tungsten (W) - increases hardness particularly at elevated due to temperatures stable carbides, refines grain size.
Vanadium (V) - increases strength, hardness, creep resistance and impact resistance due to formation of hard vanadium carbides, limits grain size.
Molybdenum (Mo) - increases hardenability and strength particularly at high temperatures and under dynamic conditions.
Silicon (Si) - improves strength, elasticity, acid resistance and promotes large grain sizes, which cause increasing magnetic permeability.
Titanium (Ti) - improves strength and corrosion resistance, limits austenite grain size.
Cobalt (Co) - improves strength at high temperatures and magnetic permeability.
Zirconium (Zr) - increases strength and limits grain sizes.
Boron (B) - highly effective hardenability agent, improves deformability and machinability.
Copper (Cu) - improves corrosion resistance.
Aluminum (Al) - deoxidizer, limits austenite grains growth.
Maraging steels (from martensitic and aging) are steels (iron alloys) which are known for possessing superior strength and toughness without losing malleability, although they cannot hold a good cutting edge. Aging refers to the extended heat-treatment process. These steels are a special class of low- carbon ultra- high-strength steels which derive their strength not from carbon, but from precipitation of inter-metallic compounds. The principal alloying element is 15 to 25% nickel. Secondary alloying elements are added to produce intermetallic precipitates, which include cobalt, molybdenum, and titanium. Original development was carried out on 20 and 25% Ni steels to which small additions of Al, Ti, and Nb were made.
The common, non-stainless grades contain 17-19% nickel, 8-12% cobalt,3-5% molybdenum, 0.2-1.6% titanium. Addition of chromium and produces stainless grades resistant to corrosion. This also indirectly increases hardenability as they require less nickel: high-chromium, high-nickel steels are generally austenitic and unable to transform to martensite when heat treated, while lower-nickel steels can transform to martensite.
Due to the low carbon content maraging steels have good machinability. Prior t o aging, they may also be cold rolled to as much as 80- 90% without cracking. Maraging steels offer good weldability, but must be aged afterward to restore the properties of heat affected zone. When heat-treated the alloy has very little dimensional change, so it is often machined to its final dimensions. Due to the high alloy content maraging steels have a high hardenability. Since ductile FeNi martensites are formed upon cooling, cracks are non-existent or negligible. The steels can be nitrided to increase case hardness, and polished to a fine surface finish.
Non-stainless varieties of maraging steel are moderately corrosion- resistant, and resist stress corrosion and hydrogen embrittlement. Corrosion- resistance can be increased by cadmium plating or phosphating.
4.HEAT TREATMENT CYCLE
The steel is first annealed at approximately 820 °C (1,510 °F) for 15- 30 minutes for thin sections and for 1 hour per 25 mm thickness for heavy sections, to ensure formation of a fully austenitized structure. This is followed by air cooling to room temperature to form a soft, heavily-dislocated iron-nickel lath (untwinned) martensite. Subsequent aging (precipitation hardening) of the more common alloys for approximately 3 hours at a temperature of 480 to 500 °C produces a fine dispersion of Ni3(X,Y) intermetallic phases along dislocations left by martensitic transformation, where X and Y are solute elements added for such precipitation. Overaging leads to a reduction in stability of the primary, metastable, coherent precipitates, leading to their dissolution and replacement with semi-coherent Laves phases such as Fe2Ni/Fe2Mo. Further excessive heat- treatment brings about the decomposition of the martensite and reversion to austenite.
Newer compositions of maraging steels have revealed other intermetallic stoichiometries and crystallographic relationships with the parent martensite, including rhombohedral and massive complex Ni50(X,Y,Z)50 (Ni50M50 in simplified notation).
Maraging steel's strength and malleability in the pre-aged stage allows it to be formed into thinner rocket and missile skins than other steels, reducing weight for a given strength. Maraging steels have very stable properties, and, even after overaging due to excessive temperature, only soften slightly. These alloys retain their properties at mildly elevated operating temperatures and have maximum service temperatures of over 400 °C (752 °F)
They are suitable for engine components, such as crankshafts and gears, and the firing pins of automatic weapons that cycle from hot to cool repeatedly while under substantial load. Their uniform expansion and easy machinability before aging make maraging steel useful in high-wear
components of assembly lines and dies. Other ultra-high-strength steels, such as Aermet alloys, are not as machinable because of their carbide content.
In the sport of fencing, blades used in competitions run under the auspices of the Fédération Internationale d'Escrime are often made with maraging steel. Maraging blades are required in foil and épée because crack propagation in maraging steel is 10 times slower than in carbon steel, resulting in less blade breakage and fewer injuries. The notion that such blades break flat is a fencing urban legend: testing has shown that the blade-breakage patterns in carbon steel and maraging steel blades are identical.
Stainless maraging steel is used in bicycle frames and golf club heads. It is also used in surgical components and hypodermic syringes, but is not suitable for scalpel blades because the lack of carbon prevents it from holding a good cutting edge. Maraging steel production, import, and export by certain states, such as the
United States, is closely monitored by international authorities because it is particularly suited for use in gas centrifuges for uranium enrichment; lack of maraging steel significantly hampers this process. Older centrifuges used aluminum tubes; modern ones, carbon fiber composite.
Copper alloys are metal alloys that have copper as their principal component. They have high resistance against corrosion. The best known traditional types are bronze, where tin is a significant addition, and brass, using zinc instead. Both these are imprecise terms, and today the term copper alloy tends to be substituted, especially by museums.
The similarity in external appearance of the various alloys, along with the different combinations of elements used when making each alloy, can lead to confusion when categorizing the different compositions. There are as many as 400 different copper and copper-alloy compositions loosely grouped into the categories: copper, high copper alloy, brasses, bronzes, copper nickels, copper-nickel-zinc (nickel silver), leaded copper, and special alloys. The following table lists the principal alloying element for four of the more common types used in modern industry, along with the name for each type. Historical types, such as those that characterize the Bronze Age, are vaguer as the mixtures were generally variable.
5.CLASSIFICATION OF COPPER AND ITS ALLOYS
A brass is an alloy of copper with zinc. Brasses are usu ally yellow in color. The zinc content can vary between few % to about 40%; as long as it is kept under 15%, it does not markedly decrease corrosion resistance of copper. Brasses can be se nsitive to selective leaching corrosion under certain conditions, when zinc is leached from the alloy (dezincification), leaving behind a spongy copper structure.
A bronze is an alloy of copper and other metals, most often tin, but also aluminium and silicon.
Aluminium bronzes are alloys of copper and aluminium. The content of aluminium ranges mostly between 5-11%. Iron, nickel, manganes e and silicon are sometimes added. Th ey have higher strength and corrosion resistance than other bronzes, especially in m arine environment, and have low reactivity to sulfur compounds. Aluminium forms a thin passivation layer on the surface of the metal.
Carbon steels are iron-carbon alloys containing up to 2.06% of carbon, up to 1.65% of manganese, up to 0.5% of silicon and sulfur and phosphorus as impurities. Carbon content in carbon steel determines its strength and ductility. The higher carbon content, the higher steel strength and the lower its ductility.
Alloy steels are ir on-carbon alloys, to which alloying elements are added with a purpose to improve the steels properties as com pared to the Carbon steels. Due to effect of alloying elements, properties of alloy steels exceed those of plane carbon steels. AISI/SAE classification divide alloy steels
According to the four-digit classification SAE/AISI system: First digit indicates the class of the alloy steel:
2- Nickel steels;
3-Nickel-chromiu m steels;
4- Molybdenum steels;
5- Chromium steels;
7-Tungsten-chrom ium steels;
9- Silicon-manganese steels.
Second digit indicates concentration of the major element in percents (1 means 1%).
Last two digits indicate carbon concentration in 0,01%.
Example: SAE 5130 means alloy chromium steel, containing 1% of chromium and 0.30% of carbon.
3.TOOL AND DIE STEELS
Tool and die steels are high carbon steels (either carbon or alloy) possessing high hardness, strength and wear resistance. Tool steels are heat treatable. In order to increase hardness and wear resistance of tool steels, alloying elements forming hard and stable carbides (chromium, tungsten, vanadium, manganese, molybdenum) are added to the composition. Designation system of one-letter in combination with a number is accepted for tool steels. The letter means:W - Water hardened plain carbon tool steels
Applications: chisels, forging dies, hummers, drills, cutters, shear blades, cutters, drills, razors.
Properties: low cost, very hard, brittle, relatively low harden ability, suitable for small parts working at not elevated temperatures.
O, A, D - Cold work tool steels
Applications: drawing and forging dies, shear blades, highly effective cutters. Properties: strong, hard and tough crack resistant.
O -Oil hardening cold work alloy steels;
A -Air hardening cold work alloy steels;
D -Diffused hardening cold work alloy steels;
S - Shock resistant low carbon tool steels
Applications: tools experiencing hot or cold impact.
Properties: combine high toughness with good wear resistance.
T,M – High speed tool steels (T-tungsten, M-molybdenum)
Applications: cutting tools. Properties: high wear heat and shock resistance.
H – Hot work tool steels
Applications: parts working at elevated temperatures, like extrusion, casting and forging dies. Properties: strong and hard at elevated temperatures.
P - Plastic mold tool steels
Applications: molds for injection molding of plastics.
Properties: good machinability.
Chemical compositions of some tool and die steels
6.EFFECTS OF ALLOYING ELEMENTS IN STEEL
Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects various alloying elements in steel.
The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength by heat treatment but the addition of carbon enables a wide range of hardness and strength.
Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the A.I.S.I 200 Series of Austenitic stainless steels (e.g. A.I.S.I 202 as a substitute for A.I.S.I 304)
Chromium is added to the steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steel' has approximately 11% chromium and a very marked degree of general corrosion resistance when compared with steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment, thus improving hardenability and strength.
Nickel is added in large amounts, over about 8%, to high chromium stainless steel to form the most important class of corrosion and heat resistant steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form austenite is responsible for a great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.
Molybdenum, when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion especially by chlorides and sulphur chemicals. When added to low alloy steels, molybdenum improves high temperature strengths and hardness. When added to chromium steels it greatly diminishes the tendency of steels to decay in service or in heat treatment.
The main use of titanium as an alloying element in steel is for carbide stabilisation. It combines with carbon to for titanium carbides, which are quite stable and hard to dissolve in steel, this tends to minimise the occurrence of inter-granular corrosion, as with A.I.S.I 321, when adding approximately 0.25%/0.60% titanium, the carbon combines with the titanium in preference to chromium, preventing a tie -up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.
Phosphorus is usually added with sulphur to improve machinability in low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work shows that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.
When added in small amounts sulphur improves machinability but does not cause hot shortness. Hot shortness is reduced by the addition of manganese, which combines with the sulphur to form manganese sulphide. As manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present, the weak spots at the grain boundaries are greatly reduced during hot working.
Selenium is added to improve machinability.
Niobium is added to steel in order to stabilise carbon, and as such performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.
Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenite forming element. Yield strength is greatly improved when nitrogen is added to austenitic stainless steels.
Silicon is used as a deoxidising (killing) agent in the melting of steel, as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminium killed steels.
Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually aproximately 0.2% maximum. This problem is emphasised because there is residual cobalt content in the nickel used in producing these steels.
Chemically similar to niobium and has similar effects.
Copper is normally present in stainless steels as a residual element. However it is added to a few alloys to produce precipitation hardening properties.
Copyright © 2018-2021 BrainKart.com; All Rights Reserved. (BS) Developed by Therithal info, Chennai.