Alloying elements of special steels

Alloying elements of special steels

The alloying constituents are generally divided unto carbide, austenite and ferrite forming elements. Furthermore, the purpose for which they are added to steel must be taken into consideration. According to its content, each alloying element imparts specific properties to steel. When several elements are present the effect may be increased, a fact which is fully utilized in modern alloying technology. There are, however, alloy compositions for which the individual elements do not exert their influence regarding a certain properties in the same direction, but rather counteract one another.

The presence of alloying elements merely creates a prerequisite for desired properties, but it is the processing and heat treatment operations which permit us to achieve them. The principal effects of alloying constituents and trace elements upon steels will be outlined below.

Aluminum-Al

Aluminum is the strongest and most common used deoxidizing and dentriding agent. It has therefor a favorable effect on the intensivity to ageing and it promotes a fine grain formation, when added in small amounts.

As aluminium combines with nitrogen to form very hard carbides, it is used as alloying constituent in most nitriding steels.

It increases the resistance to scailing and is therefore often added to ferritic heat resisting steels. In unalloyed carbon steels altizing (diffusion of aluminium into the surface) improves the scailing resistance. Aluminium narrows greatly the gamma-range. Owing to its ability to increase the coercive force, Al is used as alloying constituent in irin-nickel-cobalt-aluminium permanent magnet alloys.

Antimony-Sb

Antimony has a detrimental effect in steel. It reduces considerably the toughness properties and narrows the gammy-range

Arsenic-As

Arsenic, too, narrows the gamma-range and has detrimental effects in steel because it shows a strong tendency towards segregations, similar to phosporus. The elimination of segregations by homogenizing is, however, even more difficult than in case of phosporus. Besides, it increases temper brittlenes, reduces considerably toughnes  and effects weldability.

Beryllium-Be

Beryllium narrows substantially the gamma-range. The addition of Be my cause precipitation hardening, entailing, however, a decline in toughness. It has a strong deoxidizing effect and a great sulphur-affinity: it has so far a very seldom been used in steels.

Boron-B

As boron has a large neutron absorption cross-section, it is usually added to steels intended for the manufacture of controllers and screens for nuclear power plants. In austenitic 18/8CrNi steels, the addition of boron raises their strength and yield point through precipitation hardening, while at the same time it lowers corrosion resistance. Precipitations caused by boron improve mechanical properties of austenitic creep resisting steels at elevated temperatures. In constructional steels, this element improves through-hardening and thus entails on increase in the core strength of case-hardening steels. A reduction of weldability is to be expected in boron alloyed steels.

Calcium-Ca

Calcium is used together with Si in the form of silico-calcium for deoxidation. It increases the scaling resistance of heating conductor materials.

Carbon-C

Carbon is inseparable from steel and is therefore usually not defined as alloying constituent. Carbon is the most important element for the majority of steels and has the strongest influence on their properties. In unalloyed and alloyed steels, properties can vary within a wide range by choosing different carbon contents (from a few hundredths to two percent) and a suitable heat treatment. Strength and hardenability increase with increasing C content, while elongation values forming properties, weldability and machinability are reduced.

Cerium-Ce

It is usually used together with lanthanum, neodymium, praseodymium and other rare earth metals as "mischmetal". It has a cleaning effect, because it is a strong deoxidizer and it promotes desulphurization. In highly alloyed steel, it has a favourable influence on the hot forming properties, andin heat resisting steels it enhances scaling resistance. Fe-Ce alloys containing approx. 70% Ce are pyrophorous (spark metals). Cerium is also used as addition to spheroidal graphite castiron.

Chromium-Cr

Chromium makes steel oil and air hardenable. By lowering the critical cooling rate necessary for the formation of martensite, it increases hardenability and thus improves heat treating properties while at the same time it reduces impact strength. Chromium is a strong carbide former. Its carbides increase edge-holding property and wear resistance. High-temperature strength and resistance to high-pressure hydrogen are improved by the addition of chromium. The resistance to scale formation increases with increasing Cr contents, while a minimum content of approx. 1 3% Cr dissolved in the matrix is required for making steel resistant to corrosion. Chromium narrows the gamma-range and thus extends the ferrite field. However, it stabilizes austenite in austenitic CrNi steels. Thermal conductivity and electric conductivity as well as thermal expansion (glass sealing alloys)are reduced by chromium. In amounts of up to 3%, chromium-combined with elevated carbon contents-increases remanence and coercive force.

Cobalt-Co

Cobalt does not form carbides. It inhibits grain growth at elevated temperatures and improves substantially retention of hardness and high-temperature strength. It is therefor often used as alloying constituent in high-speed steels, hot-work tool steels, high-temperature and creep resisting materials. This element promotes the formation of graphite. When present in large amounts, it enhances remanence, coercive force and thermal conductivity and therefore serves as base element for high-grade permanent magnet steels and alloys. When exposed to neutron radiation, it forms the highly radioactive 60 Co isotope. For this reason, Co is an undesirable element in steels intended for use in nuclear reactors.

Copper-Cu

Copper is only added to a few steel grades, because it builds up below the oxide layer and produces high surface sensitivity in hot forming operations due to penetration into the grain boundary. It is therefore sometimes considered to be detrimental to steels. Yield point and ratio of yield point to tensile strength are increased by copper. Cu contents above 0.3.0% may cause precipitation hardening. Hardenability is improved. In unalloyed and low-alloy steels, a marked improvement in resistance to weathering is achieved by the addition of Cu. in acid resisting high-alloy steels, Cu content above 1% increases the resistance to hydrochloric and sulphuric acids

Hydrogen-H

Hydrogen is considered to have detrimental effects in steels. It produces brittleness due to a decline in elongation and reduction of area, without increasing the yield point and tensile strength. It causes the dreaded flake formation and promotes the formation of ghost lines. Active hydrogen produced during pickling penetrates into the steel and forms blowholes. Humid hydrogen causes decarburization at elevated temperatures.

Lead-Ph

Lead is added in amounts of 0.2 to 0.5% to free-cutting steels because its very fine suspens dispersion (lead is undissolvable in steel) permits to obtain short chip lengths and clean cut surfaces and thus it guarantees improved machinability. Lead contents within the range indicated above have practicaly no influence on the mechanical properties

Magnesium-Mg

Magnesium promotes graphite formation in cast iron.

Manganese-Mn

Manganese has a deoxidizing effect. It combines with sulphur to form manganese sulphides and hus lowers the untavourable effect of iron sulphides. This is of particular importance for free- cuting steels because itreduces the risk of red shortness. The Ar3 and Art points are lowered; Mn reduces substantially the critical cooling rate and thus increases hardenability Yield point and strength are increased by the addition of Mn. Mn contents greater than 4% lead to a britte martensitic structure, even at a slow cooling rate, so that the alloy range is hardy utilized. Steels containing more than 1 2% Mn in combination with elevated carbon contents are austenitic because Mn enlarges considerably the gamma-range. Such steelis experience severe strain hardening of surface, when exposed to impact stresses, while the core remains tough. They are therefore highly wear resistant in case of impact stresses. Steels with Mn co tents greater than 1 8% remain non-magnetizable, even after relatively heavy cold forming, and are used as specialty steels and as steels intended for low-temperature service. Mn increases the coefficient of thermal expansion, while it reduces thermal conductivity and electric conductivity.

Molybdenum-Mo

Molybdenum is mostly added together with other alloying elements. By reducing the critical cooling rate, it improves hardenability Mn reduces to a large extent temper brittleness, e.g. in CrNi and Mn steels, and promotes fine-grain formation. It increases yield point and strength. Being a strong carbide former, it improves the cutting efficiency of high speed steels. Mo belongs to those elements which increase resistance to corrosion and is therefore often used as alloying constituent in highly alloyed Cr steels and austenitic CrNi steels. Elevated Mo contents lower susceptibility to pitting. Mo narrows substantially the gamma-range, increases high- temperature strength and reduces resistance to scale formation.

Nickel-Ni

In constructional steels, Ni causes an increase of impact strength, even at subzero temperatures is therefore added to case-hardening steels, heat treatable steels and steels intended for low temperature service to increase theirtoughness properties All transformation points (A1-A4) are lowered by nickel. It does not form carbides enlarging substantially the gamma-range, Ni-in amounts of more than 7%-imparts an austenitic structure to chemically resistant high-chromium steels down to far below room temperature. Ni as single alloying element, oven in large percentages, merely slows down corrosion processes. In austenitic CrNi steels, however, it induces resistance to the attack cf reducing chemicals; the resistance of these steels in oxidizing substances is achieved by the addition of Cr. Austenitio steels show elevated strength at temperatures above 600 °C owing to their high recrystallization temperature, they are practically not magnetizable.Thermal conductivity and electric conductivity are reduced considerably. High Ni contents within precisely defined analysis limits result in steels with special physical properties, e.g. low thermal expansion (Invar grades).

Nobium-Nb (columbium-Cb) and tantalum-Ta

These elements occur nearly almost in combination and are very difficult to seperate from one another, so that they are usually used together. They are strong carbide-forming elements and are therefor, added as stabilizers to chemically resistant steels. Both elements form ferrite and thus reduce the gamma-range. Owing to its ability to increase high-temperature strengh and creep rupture strengh, Nb is often added to creep resisting austenitic boiler steels. Ta has a large neutron absorption cross-section; for nuclear reactor steels, only Nb poor in Ta is used.

Nitrogen-N

Nitrogen may occur both as element having detrimental effects and as alloying constituent. It is considered detrimental because it reduces toughness as a result of precipitation processes causes susceptibility to ageing and blue brittleness (deformation in the range of blue temper heat 300-350 C) and involves the risk of initiating intergranular stress corrosion cracking in unalloyed and low-alloy steels. As alloying constituent, nitrogen enlarges the gamma-range and stabilizes the austenitic structure. In austenitic steels, it increases the strength, particularly the yield strength, and improves mechanical properties at elevated temperatures. By causing the formation of nitrides, it allows us to obtain high surface hardness (nitriding).

Oxygen-O

Oxygen has adverse effects in steels. Its specific influence depends largely on the type and composition of its compounds in steels as well as on their shape and distribution. The mechanical properties, especially impact strength, are lowered, particularly those in transverse direction, while the susceptibility to ageing brittleness, red shortness, fibrous fracture and flaky fracture is increased.

Phosphorus-P

Phosphorus is considered to be detrimental to steels, because it causes heavy primary segregation during solidification and involves the risk of secondary segregation in the solid state by substantial reduction of the gamma-range. Owing to the relatively low diffusion rate in both the gamma- and alpha-solid solution Crystal phases segregations if any, are very difficult to compensale for. As it is hardly possible to obtain homogeneous distribution of phosphorus, its contents should be kept to a minimum. The extent of segregation cannot be determined with sufficient cortainty P increases susceptibility to temper brittieness, even in smallest percentages brittleness due to phosphorus gets higher with increasing carbon content, increasing Temperature, increasing grain size and decreasing ratio of reduction by forging It manifests itself Cold shoriness and sensitivity to impact stresses (susceptibility to britte fracture), In low-alloy onstructional steels with C contents of approx O 1%, phosphorus causes an increase in strength and resistance to atmosphere corrosion. Cu promotes corrosion resistance (steels with low susceptibility to corrosion, In austenitic CrNi steols, P additions may cause a yield point increase and produce precipitation effects.

Selenium – Se

Selenium is added to free-cutting steels where its effects are similar to those of sulphur. It is, however, more effective in improving machinability. In corrosion resisting steels, it effects corrosion resistance properties less severly than sulphur.

Silicon – Si

Silicon has a deoxidizing effect. It promotes graphite precipitation and narrows subtantially the gamma-range. It increases strength and wear resistance (heat treatable Si-Mn steels). Silicon causes a considerable increase of the elastic limit and is therefore most suitable as alloying constituent in spring steels. Owing to its ability to improve substantially the resistance to scaling, Si is added to heat resisting steels. Its contents are, however, limited, as it impairs the hot and cold forming properties. With a content of 12% Si resistance to the attack of acids is attained, but such steel grades are only available as very hard and brittle castings which can be only be machined by grinding.

Owing to the considerable reduction of electric conductivity, coercive force and power losses brought about by silicon, this element is used in steel for electric quality sheets.

Sulphur – S

Among all trace elements, sulphur produces the most serious segregations. Iron sulphide leads to red shortness because the low melting sulphide eutectics surround the grains like a net, resulting in a low coherence of the latter and in breaking up of grain boundaries during hot forming. This phenomenon is intensified by the effect of oxygen.

As sulphur has an extremely great affinity to manganese, it is combined with manganese to from Mn sulphide  which is, among the usually existing inclusions, the most harmless one, being distributed point-like in steel and having a high melting point.The  toughness properties in transverse direction are substantially reduced by sulphur. S is added to free-cutting steels, as its lubrication effect on the cutting edge reduces the friction between workpiece and tool, thus permitting to obtain a prolonged tool life. Moreover, short chips are obtained in machining operations.

S increases susceptibility cracks.

Tellurium – Te

Tellurium influences steel properties in a similar manner as selenium. Contens up to 0,2% improve machinability.

Tin – Sn

Tin is detrimental to steels. Like cooper, it builds up bellow the oxide layer, penetrates into the grain boundaries and produces cracks and solder brittleness. Sn exhibits a strong segregation tendency and narrows the gamma –range.

Titanium – Ti

Owing to its great affinity to oxygen, sulphur and carbon, titanium is strongly deoxidizing, denitriding and carbide forming and combines with sulphur.

In corrosion resisting steel, it is used as carbide former for stabilization to ensure resistance to intergranular corrosion. Besides, titanium has a grain refining effect and narrows substantially the gamma-range.

In higher amounts, it leads to precipitation and because of the high coercive forces obtained, it is added to permanent magnet alloys. Titanium increases creep rupture strength by the formation of special nitrides.

Titanium, however, exhibits a strong segregation and banding tendency.

Tungsten – W

Tungsten is strong carbide former (its carbides are very hard) and it narrows the gamma-range. It improves toughness and inhibits grain growth. It increases high-temperature strength and retention of hardness as well as wear resistance at elevated temperatures (red heat) and thus cutting efficiency. It is therefore predominantly added to high-speed steels, hot work tool steels, high-temperature steels and steels featuring maximum hardness. Tungsten increases considerably the coercive force and is therefore used as alloying constituent in permanent magnet alloys. It impairs the scaling resistance. Its high specific gravity becomes particularly noticeable in high-tungsten high-speed steels and hot work steels.

Vanadium-V

Vanadium refines the primary grain and thus the as cast structure. It is a strong carbide forming element, thus causing an increase in wear resistance, edge holding properly and high-temperature strength. It is therefore a preferred alloying constituent in high-speed steels, hot work tool steels and high-temperature steels. It improves considerably the retention of hardness and reduces overheating  sensitivity. As vanadium refines the grain and inhibits air hardening by the formation of carbides, it has a favorable influence on the welding properties of heat treatable steels. Due to carbide formation, it increases resistance to high-pressure hydrogen. Vanadium narrows the gamma range and shifts the Curie point to higher temperatures.

Zirconium-Zr

Zirconium is a carbide forming element and is used metalurgically as deoxidizing, denitrding and desulphurating agent because it leaves only a few deoxidation products. Zr additions to fully killed sulphur-containing free-cutting have a favorable influence on sulphide formation and prevention of red shortness. By forming special nitrides, it improves high-temperature strength and creep repture strengh in high temperature steels and alloys. It increases the service life of heating conductor materials and causes narrowing of the gamma-range.