Steel is the most widely used category of the metallic materials. It is mainly since it can be produced relatively in an economic manner in large quantities to very precise specifications. Steels also provide a wide range of mechanical properties, from moderate yield strength levels (200 MPa to 300 MPa) with excellent ductility to yield strengths exceeding 1,400 MPa with fracture toughness levels as high as 110 MPa under-root meter. Steels form one of the most complex group of alloys in common use. The synergistic effect of alloying elements and heat treatment in steels helps in producing a tremendous variety of microstructures and properties (characteristics).
Steel is basically an alloy of iron and carbon. The plain carbon steels are relatively cheap, but have a number of limitations with respect to their properties. These limitations are (i) they cannot be strengthened above 690 MPa without loss of ductility and impact resistance, (ii) they are not very hardenable i.e., the depth of hardening is limited, (iii) they have low corrosion and oxidation resistance, (iv) they are to be quenched very rapidly to achieve a fully martensitic structure, leading to the possibility of quench distortion and cracking, and (v) they have poor impact resistance at low temperatures.
Alloying elements are added to the steel to change the chemical composition of steel and to improve its properties over carbon steel or adjust them to meet the requirements of a particular application. Different alloying elements can be added with each having its own effect on the properties of steel. Steels having alloying elements in the composition are called alloy steels. The advantages of adding alloying elements to steel include (i) increase of the maximum tensile strength, (ii) availability of thick sections of steels with high hardness throughout the section, (iii) more controllable quenching with minimum risk of shape distortion or cracking, (iv) improved impact resistance at higher temperature range, (v) improved corrosion resistance, and (vi) improved high temperature performance.
The term ‘alloy steel’ is used for those steels which have got in addition to carbon other alloying elements in their composition. Alloy steels are made by combining steels with one or more other alloying elements. There are a large number of alloying elements which can be added to steel. These elements are normally metals like nickel, manganese, tungsten, aluminum, copper, and molybdenum etc. The elements are intentionally added in steel to incorporate certain properties such as mechanical, electrical, thermal, magnetic or corrosion resistance properties which are not found in the plain carbon steels. Total quantity of alloying elements in alloy steels (other than micro-alloyed steels) can vary between 1 % and 50 %. Alloy steels covers a broad range of steels and end use markets.
The difference between the low alloy steels and high alloy steels is somewhat arbitrary. Some people define low alloy steels as those steels which contain alloying elements up to 4 %, while in second definition low alloy steels contain alloying elements up to 8 %. Steels having alloying elements higher than this quantity come under the category of high alloy steels. Common alloying elements include manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron. Less common alloying elements include aluminum, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, antimony and zirconium.
The main reason of adding alloying elements in steel are (i) to increase the hardness and wear resistance, (ii) to increase corrosion resistance, (iii) to increase resistance to abrasion and wear, (iv) to increase ductility and machinability, and (v) to improve the properties of steels.
Advantages of alloy steels include (i) increased strength at high temperatures, (ii) high hardenability, (iii) improved ductility and resistance to cracking, (iv) higher elastic ratio and endurance strength, (v) better machinability at high hardness, and (vi) lesser internal stresses. Limitations of alloy steel are (i) higher cost than plain carbon steels, (ii) care is needed to be taken during handling, (iii) tendency to have retained austenite, (iv) certain grades show temper brittleness.
Since their invention in 1865, alloy steels have found broad application in multiple industries such as automotive, aerospace, industrial machinery, heavy equipment, energy sector, and pipeline industries to name a few. Alloy steels include a tremendous variation in alloying content. As an example, chromium in the alloy steel ranges from the 1 % to 2 % in some low alloy steels to 15 % to 18 % of chromium in several stainless steels. The topic of alloy steels contains both the common grades AISI 4140 (a low alloy steel containing chromium, molybdenum, and manganese) alloy steel and AISI 316 stainless steel to more exotic alloys such as the Hadfield steels. These steels can form a wide variety of micro-structures such as pearlite, bainite, or martensite, which result in an equally broad range of properties. It is this range which has made them useful to several industries. In some cases, these are the only alloy steels which can provide the required combination of properties. Their use in the automotive industry has been key to the development of safer vehicles and improved fuel efficiency. The modern world would not be possible without the advanced alloy steels employed to safely transport oil / natural gas through the pipelines.
Alloy steels are normally of three types. They are (i) micro-alloyed steels, (ii) low alloy steels, and (iii) high alloyed steels.
Micro-alloyed steels – These steels are also sometimes called high strength low alloy (HSLA) steels. Micro-alloyed steels are a type of alloy steels which contains small quantities of alloying elements (normally 0.05 % to 0.15 %). The carbon content of micro-alloyed steels can range from 0.05 % to 0.25 % and manganese content up to 2 % in order to provide adequate formability and welding properties. In these steels, small quantities of chromium, nickel, molybdenum, copper, vanadium, niobium, titanium, zirconium, and nitrogen are also used in different combinations. The micro-alloyed steels can also have small additions of calcium, or rare earth elements for sulphide inclusion shape control.
Micro-alloyed steels are designed to provide better mechanical properties and / or higher resistance to atmospheric corrosion than normal carbon steels. These steels are not considered to be alloy steels in the normal sense since they are designed to meet specific mechanical properties rather than a chemical composition. These steels have yield strengths of more than 275 MPa. This high strength is achieved by refinement of grain size as well as for precipitation hardening. The chemical composition of specific micro-alloyed steel can vary for different product thickness to meet mechanical property requirements. Micro-alloyed steels are normally produced in the as-rolled condition. They can also be produced in a controlled-rolled, normalized, or precipitation-hardened condition to meet specific property requirement.
Primary applications for micro-alloyed steels include oil and gas line pipe, ships, offshore structures, automobiles, off-highway equipment, and pressure vessels. The types of micro-alloyed steels normally used are (i) weathering steels or atmospheric corrosion resistant steels which normally contains copper content of around 0.35 %, (ii) control rolled steels which are hot rolled as per predetermined rolling schedule designed to develop a highly deformed austenite structure which then transforms into a very fine equiaxed ferrite structure on cooling, (iii) pearlite reduced steels in which the strength is achieved by very fine grain ferrite and by precipitation hardening but with low carbon content and hence these steels have little or no pearlite in the microstructure, (iv) acicular ferrite steels which are very low carbon steels with sufficient level of hardenability and in which the structure of steel transforms on cooling to a very fine high strength acicular ferrite (low carbon bainite) structure instead of the normal polygonal ferrite structure, and (v) dual-phase steels which are processed to a micro-structure of ferrite containing small uniformly distributed regions of high carbon. Dual phase steels have low yield strength and a high rate of work hardening and have high strength with superior formability.
Low alloy steels – These steels are those steels which show mechanical properties superior to the properties of plain carbon steels because of the additions of alloying elements such as nickel, chromium, or molybdenum. Total content of alloying elements in these steels can vary from 2.07 % up to the levels just below that in the stainless steels which contain a minimum of 10 % of chromium. In several low alloy steels, the primary function of the alloying elements is to increase the hardenability of the steel so as to optimize the mechanical properties and toughness after heat treatment. However, in some cases the addition of alloying elements is done to reduce environmental degradation under specified service conditions. The term low alloy steel, rather than the more general term alloy steel, is being used to differentiate the steels from the high alloy steel.
As with steels in general, low alloy steels can be classified as per chemical composition, and heat treatment. In case of chemical composition, the classification is based on the alloying element such as tungsten steels, nickel steels, nickel chromium steels, molybdenum steels, and chromium molybdenum steels etc. In case of heat treatment, low alloy steels can be classified such as quenched and tempered steels, normalized and tempered steels, or annealed steels and so on.
Since there are wide variety of chemical compositions which are possible in low alloy steels and also because of the fact that some steels can be used in more than one heat treated condition, there exist certain overlap in the classification of low alloy steels. Hence, it is rather difficult to classify low alloy steels. There are four major groups of low alloy steels which are normally used. These groups are (i) low carbon quenched and tempered steels, (ii) medium carbon ultra-high strength steels, (iii) bearing steels, and (iv) heat resistant chromium-molybdenum steels.
Low-carbon quenched and tempered steels combine high yield strength (from 350 MPa to 1,035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The different types of low carbon quenched and tempered steels have various combinations of these characteristics based on their intended uses. Some of these steels are used in military for armour purpose mostly as plates. Some of these steels are also produced as forgings or castings.
Medium carbon ultra-high strength steels are structural steels which are having yield strengths that can exceed 1,380 MPa. Some of these steels are covered by designations given in various standards while some other steels are having proprietary compositions. Product forms for these steels include billets, bars, rods, forgings, sheets, pipes, and welding wires.
Bearing steels are used for ball and roller bearing applications. These steels consist of low carbon (0.1 % to 0.2 %) case hardened steels and high carbon (around 1 %) through hardened steels. Some of these steels are covered by designations given in different standards.
Chromium-molybdenum heat-resistant steels contain 0.5 % to 9 % chromium and 0.5 % to 1 % molybdenum. The carbon content is normally below 0.2 %. The chromium provides improved oxidation and corrosion resistance while the molybdenum increases strength at high temperatures. These steels are normally produced in the normalized and tempered, quenched and tempered, or annealed conditions. Chromium-molybdenum steels are extensively used in the oil and gas industries and in fossil fuel and nuclear power plants.
High alloy steel – These steels include steels with a high degree of fracture toughness. High alloy steels are ultra-high strength steels and consist of corrosion resistant steels, heat resistant steels, and wear resistant steels. High alloy steels also include maraging steels, austenitic manganese steels, tool steels, and stainless steels.
Effects of alloying elements
More than twenty elements are used in various proportions and combinations in the production of both carbon and alloy steels. Some are used since they impart specific properties to the steel when they alloy with it (i.e., dissolve in the iron), or when they combine with carbon, wholly or in part, to form compounds known as carbides. Others are used since they are beneficial in ridding the steel of impurities or rendering the impurities harmless. Still another group is used to counteract harmful oxides or gases in the steel. The elements of this last group act only as fluxes or scavengers and do not remain in the steel to any extent after solidification occurs. Some elements fall into more than one of the above groups.
Alloying elements addition to the steels improves a range of properties in alloy steels as compared to carbon steels. These properties are strength, hardness, toughness, corrosion resistance, wear resistance, hardenability, machinability, heat resistance, fire resistance, and hot hardness etc. Alloy steels can need heat treatment to achieve some of these improved properties.
Chromium, vanadium, molybdenum, and tungsten when added to steels improve strength by forming second phase carbides. Manganese, silicon, nickel, and copper are added to increase strength of the steels by forming solid solutions in ferrite. Addition of small quantities of nickel and copper improve corrosion resistance. Molybdenum in steel helps to resist embrittlement. Zirconium, cerium, and calcium increase toughness in alloy steels by controlling the shape of inclusions. Machinability of the steel is increased by manganese sulphide, lead, bismuth, selenium, and tellurium.
The alloying elements in the steels can be found (i) in the free state, (ii) as inter-metallic compound with iron or with each other, (iii) as oxides, sulphides, and other non-metallic inclusions, (iv) in the form of carbides, or (v) as a solution in iron.
The principal effects of addition of alloying elements to steel include (i) they can encourage formation of graphite from the carbide, (ii) they can go into solid solution in the iron for improving the strength, (iii) hard carbides (cementite) associated with iron and carbon can be formed with alloying elements, (iv) some alloying elements such as manganese, nickel, cobalt, and copper increase the range over which austenite is stable e.g. by lowering the eutectoid temperature, and this retards the separation or carbides., and (v) some alloying elements such as chromium, tungsten, molybdenum, vanadium, and silicon reduce the quantity of carbon soluble in the austenite and hence increase the volume of free carbide in the steel at a given carbon content which effectively reduces the austenite phase by raising the eutectoid temperature and lowering the peritectic temperature.
The combined effect of alloying elements results from several complex interactions resulting from the processing history, the number and quantities of constituents, the heat treatments, and the section shape etc. Some basic principles with respect to the effect of the alloying elements on the steel are (i) nickel has reduced carbide forming tendency than iron and dissolves in the alpha ferrite, (ii) silicon combines with oxygen to form non-metallic inclusions or dissolves in the ferrite, (iii) majority of the manganese in alloy steels dissolves in the alpha ferrite and any manganese which form carbides result in (Fe,Mn)3C, (iv) chromium spreads between the ferrite and carbide phases with the spread depending on the quantity of carbon and other carbide generating elements present, (v) tungsten and molybdenum form carbides if sufficient carbon is present which has not already formed carbides with other stronger carbide forming elements, (vi) vanadium, titanium, and niobium are strong carbide forming elements and are present in steel as carbides, and (vii) aluminum combines with oxygen and nitrogen to form aluminum oxide, and aluminum nitride.
Chromium, vanadium, molybdenum, and tungsten when added to steels improve strength by forming second phase carbides. Manganese, silicon, nickel, and copper are added to increase strength of the steels by forming solid solutions in ferrite. Addition of small quantities of nickel and copper improve corrosion resistance. Molybdenum addition in the steel helps to resist embrittlement. Zirconium, cerium, and calcium increase toughness in alloy steels by controlling the shape of inclusions. Manganese sulphide, lead, bismuth, selenium, and tellurium increase machinability.
Alloying elements also have an effect on the eutectoid temperature of the steel. Manganese and nickel lower the eutectoid temperature. These alloying elements are known as austenite stabilizing elements. With enough of these elements the austenitic structure in alloy steels can be attained even at room temperature. Carbide forming elements raise the eutectoid temperature of alloy steels. These alloying elements are known as ferrite stabilizing elements.
Alloying elements (except cobalt) reduce the critical cooling velocity by making the transformation to the equilibrium phase slower. Alloy steels can hence be hardened by an oil or even air quench, reducing the risk of cracking or distortion which can result from a rapid water quench. Majority of the elements also lower the Ms (martensite start) and Mf (martensite finish) temperatures to below room temperature, leading to some ‘retained austenite’ in the quenched structure. The effect of alloying elements on the yield stress of steel is shown in Fig 1.
Fig 1 Effect of alloying elements on the yield stress
Categories of alloy steels
There are several categories of alloy steels. Some of them are described below.
Alloy tool and die steels – These steels are used in making cutting and forming tools. The total content of alloying elements in alloy tool steels ranges from 0.25 % to over 38 %. There are several categories of tool steels. These categories can be further classified according to their basic properties. Each category has a large number of grades. These steels are used for high quality drills, reamers, milling cutters, threading tools, punches, plastic moulds, punch press tooling, and wrenches. Majority of the alloy tool steels are hardened in oil and air. Hence, they are frequently referred as oil hardened or air hardened tool steels. When compared with plain carbon tool steels, alloy tool steels harden more deeply and are more shock resistant.
Tool steels are mainly medium to high carbon steels with specific alloying elements added in different quantities to provide it special characteristics. The carbon in the tool steel is provided to help hardening of the steel to higher hardness for cutting and wear resistance while alloying elements are added to the tool steel for providing it higher toughness or strength. In some cases, alloying elements are added to retain the size and shape of the tool during its heat treat hardening operation or to make the hardening operation safer and to provide red hardness to it so that the tool retains its hardness and strength when it becomes extremely hot. Various alloying elements in addition to carbon are chromium, cobalt, manganese, molybdenum, nickel, tungsten, and vanadium. The effect of the alloying elements on the properties of tool steels is described below.
Chromium results into deeper hardness penetration during heat treatment and contributes wear resistance and toughness to the tool steel.
Cobalt is normally used in tool steels since it increases the red hardness so that these steels can be used at higher operating temperatures.
Manganese when used in small quantities, aids in making tool steel sound and further additions help tool steel to harden deeper and more quickly in heat treatment. It also helps to lower the quenching temperature necessary to harden steels. Larger quantities of manganese in the range of 1.20 % to 1.60 % allow tool steels to be oil quenched rather than water quenched.
Molybdenum in tool steels increases the hardness penetration during heat treatment and reduces quenching temperatures. It also helps increase the red hardness and the wear resistance.
Nickel adds toughness and wear-resistance to the tool steels. It is used in conjunction with hardening elements.
Tungsten is added to tool steel to increase its wear resistance. It also provides red hardness characteristics. Around 1.5 % of tungsten increases wear resistance and around 4 % of tungsten in combination with high carbon greatly increases wear resistance of tool steel. Tungsten in large quantities with chromium provides for red hardness.
Vanadium in small quantities increases the toughness and reduces grain size in tool steels. Vanadium in quantities higher than 1 % provides extreme wear resistance especially to tool steels. Smaller amounts of vanadium in conjunction with chromium and tungsten help in increasing red hardness properties in tool steels.
Constructional alloy steels – These alloy steels have relatively low content of alloying elements as compared to alloy tool steels. Total content of alloying elements in these steels range from 0.25 % to around 6 %. This class of alloy steels is used in the construction of bridges, buildings, ships, auto frames, and railroads etc. Construction alloy steels are used for such machine parts as shafts, gears, levers, bolts, springs, piston pins, and connecting rods etc.
COR-TEN steel – It is the trademark for weathering steel, and sometimes written without the hyphen as ‘CORTEN steel or even Corten steel’. It is a group of steel alloys which were developed to eliminate the need for painting, and form stable rust like appearance if exposed to the weather for several years. United States Steel Corporation (USS) holds the registered trademark for the name COR-TEN.
COR-TEN steel gets its properties from a careful manipulation of the alloying elements added to steels during the production process. It has a combination of chromium, copper, silicon, and phosphorus, the amounts depending on the properties required. COR-TEN steel works by controlling the rate at which oxygen in the atmosphere can react with the surface of the steel. The rusting of a steel takes place in the presence of air and water, resulting in the product of corrosion which is the iron oxide.
Electrical steel – It is a kind of special steel which is tailored to show certain specific magnetic properties such as small hysteresis area (small energy dissipation per cycle or low core loss) and high permeability. It is also called lamination steel, silicon steel, silicon-electrical steel, or transformer steel. The steel contains specific percentage of silicon in it which is responsible for its unique property.
Electrical steels have special physical properties which make them suitable for application in the production of electrical equipments and appliances with rotating magnetic fields. The utilization of the fully processed steels is also widespread for construction of electrical static devices. Electrical steels are used in the stacked cores of transformers and motors, which are rarely seen by the ordinary people.
Fire resistant steels – These steels have been developed for construction applications where increased high temperature strength provides improved protection to a building structure during a fire. Improved fire protection, in turn, helps to prevent building collapse caused by reduced load carrying capability of steel structures at high temperature, or provides the building occupants greater time to escape the building in the event of such a collapse. Fire resistant steel has higher yield strength at higher temperatures when compared to carbon steels.
Defining more precisely, according to the requirements developed by the Japan Institute of Metals, the yield point of the fire-resistant steel relating to the steel temperature equal to 600 deg C is to be ensured to be in the range of two thirds of that specified at room temperature. This restriction is much stronger than the analogous one, specified in the USA, as per which only at least one half of a room temperature yield strength is needed as the remaining in such cases. In addition to this basic requirement, the weldability of such a fire-resistant steel during the whole time of a fire exposure has to remain similar to that relating to the conventional steel not exposed to a fire.
Hadfield manganese steel – This is a high alloy steel which contains 12 % to 14 % manganese and 1 % carbon. It is austenitic at all temperatures and hence non-magnetic. Hadfield had done a series of test with the addition of ferro-manganese containing 80 % manganese and 7 % carbon to de-carbonized iron. Increasing manganese and carbon contents led to increasing brittleness up to 7.5 % manganese. At manganese contents above 10 %, however, the steel became remarkably tough. The toughness increased by heating the steel to 1,000 deg C followed by water quenching, a treatment which would render carbon steel very brittle. This alloy steel which was introduced commercially contained 1.2 % carbon and 12 % manganese in a 1 to 10 ratio.
Hadfield manganese steel is unique in that it shows resistance to impact, high toughness, high ductility, high work hardening ability, excellent wear resistance, and slow crack propagation rates, in comparison to other potentially competitive materials. The steel has a unique property in that when the surface is abraded or deformed, it greatly increases surface hardness while retaining a tough core. The steel has the ability to harden in-depth in service as well as by induced means. It is also non-magnetic and can be work-hardened during service or can be surface-hardened to as high as 500 HB (Brinell hardness) by mechanical or explosive means prior to service. Because of these properties, Hadfield manganese steel gained rapid acceptance as a useful engineering material.
Hadfield manganese steel has a proven high resistance to abrasive wear including blows and metal-to-metal wear, even though it has a low hardness. This steel is supposed to work-harden under use and hence gives a hard-wear resistant surface and has a good wear resistance in components even without heavy mechanical deformation.
Hadfield manganese steel is used in pneumatic drill bits, excavator bucket teeth, rock crusher jaws, ball mill linings and railway points and switches. Water quenched from 1,050 deg C to retain carbon in solution, the soft core has strength of 849 MPa, ductility of 40 % and a Brinell hardness of 200, but after abrasion the hardness rises to 550 BHN. The reason for the rapid rise in surface hardening is uncertain, though martensite formation or, more likely, work-hardening has been proposed.
Heat resistant steels – These steels are extensively used for high temperature components, and they cover a broad range of applications. The properties of steel and its yield strength considerably decrease as the steel absorbs heat when exposed to high temperatures. Heat resistance means that the steel is resistant to scaling at temperatures higher than 500 deg C. Heat resistant steels are meant for use at temperatures higher than 500 deg C since they have got good strength at this temperature and are particularly resistant to short and long term exposures to hot gases and combustion products at temperature higher than 500 deg C. These steels are solid solution strengthened alloy steels.
The heat resistant steels are normally classified into ferritic / martensitic steels and austenitic steels. The ferritic / martensitic steels have the same body centered cubic (bcc) crystal structure as iron. They are simply iron containing with relatively small addition of alloying elements, such as the main element chromium added from 2 % to around 13 %. These ferritic / martensitic grades also have a small percentage of manganese, molybdenum, silicon, carbon and nitrogen, mostly included for their benefits in the precipitation strengthening and encouraging high temperature behaviour. Ferritic grades are normally used since they are economical because of their low content of alloying elements. They also have some resistance to oxidation at red heat, and which is in direct proportion to the chromium content.
Heat resistant steels are extensively used for high temperature components, and they cover a broad range of applications. Various kinds of heat resistant steels are separately used according to their specific purposes. The heat resistant steels are normally ferritic steels and austenitic steels. Ferritic steels include carbon steels, low alloy steels (0.5 % molybdenum, 2.25 % chromium-1 % molybdenum), intermediate alloy steels (5 % to 10 % chromium) and high alloy steels (12 % chromium martensitic steels and 12 % to 18 % chromium ferritic steels of the AISI 400 series). Austenitic steels include 18 % chromium–8 % nickel steels and 25 % chromium–20 % nickel steels of the AISI 300 series, 21 % chromium-32 % nickel steels such as Alloy 800H, and chromium-manganese steels of the AISI 200 series. Ferritic steels normally do not contain nickel, and, since chromium compositions of 2 %, 9 % and 12 % are particularly high in strength, they are widely used.
Ferritic / martensitic steels used for high temperature service can be divided into two types based on their microstructure and composition. The first type is low alloy steels, which contain 1 % chromium to 3 % chromium and has total alloying elements content less than 5 %. The second type is called 9-12 chromium martensitic steels and normally contain alloying elements in the range 10 % to 20 %. As the requirements for the high temperature service becoming more stringent, pearlitic steels are being replaced with the martensitic steels.
In recent years, there is increasing demand for steels which can withstand higher pressure and higher service temperatures. There is development of new grades, which are modification of the existing 9-12 chromium grade with additions of vanadium, niobium, and nitrogen. The most advanced martensitic steels today can be exposed to temperatures which are less than 650 deg C.
High speed steels – These steels form a special class of highly alloyed tool steels, combining properties such as high hot hardness and high wear resistance. These are so named mainly because of their ability to machine materials at high cutting speeds. These steels have been widely adopted as the most basic material for cutting tools in mechanical processing since it was first developed. At present, high speed steel cutting tools still occupy the leading position of the tool market. These steels are primarily used for the manufacture of cutting tools such as taps, dies, tool bits, drill bits, milling cutter, reamer, broaches, and long run punches and dies etc.
High speed steels are used not only for cutting tools but also various forming tools which need higher wear resistance and toughness. Along with conventional type steels, there are some grades made by powder metallurgy (P/M) process which have outstanding wear resistance and toughness because of higher alloy content and uniform fine microstructure.
High speed steel normally are tool steels with high carbon and high alloying elements. They are complex iron-base alloys of carbon, chromium, vanadium, molybdenum or tungsten, or combinations thereof, and in some cases substantial quantities of cobalt. The first four are carbide formers and combine with carbon to form carbides. These carbides are very hard and wear resistant and hence make good cutting tools. The content of carbon is normally in the range of 0.7 % to 1.6 %, and the sum of the percentage of the major alloying elements, such as tungsten, molybdenum, chromium, vanadium and cobalt are between 10 % and 40 %. Another characteristic which is different from other types of steel is that the content of chromium is fixed at around 4 %. The carbon and alloy contents in high-speed steels are balanced at levels to give high attainable hardening response, high wear resistance, high resistance to the softening effect of heat, and good toughness for effective use in industrial cutting operations.
The three main classes of high-speed steels are (i) tungsten high speed steel, (ii) molybdenum high speed steel and (iii) tungsten-molybdenum high speed steels. The other classes of high sped steels can be made by addition of cobalt, and increasing the carbon and vanadium content to the three main classes. High speed steels with carbon content higher than 1.25 % and vanadium content higher than 2 % can be separately grouped by commercial designation ‘super high-speed steels’.
Interstitial free steel – The term ‘Interstitial free steel or IF steel’ refers to the fact, that there are no interstitial solute atoms to strain the solid iron lattice, resulting in very soft steel. Interstitial free steels have interstitial free body centered cubic (bcc) ferrite matrix. These steels normally have low yield strength, high plastic strain ratio (r-value), high strain rate sensitivity, and good formability.
Conventional interstitial free steels which were developed commercially in Japan during 1970s following the introduction of vacuum degassing technology contained carbon in the range of 40 parts per million (ppm) – 70 ppm and nitrogen in the range of 30 ppm – 50 ppm. Later, niobium and / or titanium were added to these steels to stabilize the interstitial carbon and nitrogen atoms.Interstitial free steel is termed as ‘clean steel’ as the total volume fraction of precipitates is very less. In spite of this, the precipitates appear to have a very significant effect on the properties of interstitial free steels.
Liquid steel is processed to reduce carbon and nitrogen to levels low enough that the remainder can be ‘stabilized’ by small additions of titanium and niobium. Titanium and niobium are strong carbide / nitride formers, taking the remaining carbon and nitrogen out of solution in liquid iron, after which these latter two elements are no longer available to reside in the interstices between solidified iron atoms.
Interstitial free steel has ultra-low carbon content, achieved by removing carbon monoxide, hydrogen, nitrogen, and other gasses during steelmaking through a vacuum degassing process. Interstitial elements like nitrogen or carbon are also in the form of nitrides and carbides due to the alloying elements such as niobium and / or titanium used for the stabilization of the residual interstitials. Hence, interstitial free steels possess typically non aging properties. Because of their non ageing properties, interstitial free steels are the standard base for hot dipped galvanized products.
Maraging steels – These steels are iron nickel alloy steels. A typical example of such steels is 18 % nickel, 8 % cobalt, 4 % molybdenum and up to 0.8 % titanium, with less than 0.05 % carbon. Heat treatment involves solution treatment at 800 deg C to 850 deg C followed by quenching of the austenite to give a BCC (body centred cubic) martensitic structure. This is less brittle than the BCT (body centred tetragonal) martensite found in plain carbon steels because of the low carbon. Ageing at 450 deg C to 500 deg C for 2 hours produces finely dispersed precipitates of complex inter-metallics such as TiNi3 resulting in tensile strengths around 2,000 MPa. They are soft enough to machine cheaply, before ageing, which can compensate for higher materials cost. They are relatively tough with good corrosion resistance and good weldability since they do not harden so rapidly as some steels. Uses of these steels include aircraft undercarriage components, dies, tools, and engine parts etc.
Special alloy steels – Special alloy steels are designed for extreme service requirements. They include steels with very high heat resistance, corrosion resistance, or wear resistance etc.
Spring steels – Technically spring is an elastic component which is able to store an applied force effect. A very high degree of quality, reliability, and service life is expected in springs since it is vital for the functioning of the mechanical system. While certain materials have come to be regarded as spring materials, they are not specially designed alloys. Spring materials are high strength alloys which frequently show the greatest strength in the alloy system.
Since springs are resilient structures designed to undergo large deflections, spring materials are to have an extensive elastic range. Other factors such as fatigue strength, cost, availability, formability, corrosion resistance, magnetic permeability, and electrical conductivity can also be important and are to be considered in the light of cost / benefit. Hence, careful selection is to be made to achieve the best compromise.
Besides medium and high carbon steels, spring steels are also low alloy steels, or stainless steels, produced to very high yield strengths. Spring steels are also used when there are special requirements on rigidity or abrasion resistance. These steels are also to meet the different requirements from the technical point of view. These requirements are (i) high elastic limit which is the tension that can be applied on the steel material without a plastic deformation, (ii) high ultimate strain which is the value of the extension until rupture in relation to the original length, (iii) high contraction at fracture which is the change of the original cross section in comparison to the cross section at rupture, (iv) good creep rupture strength which is a kind of tensile strength taking in account temperature and time, (v) good endurance limit which is the reaction of the material on constantly changing maximum stresses till the plastic deformation begins, and (vi) low surface decarburization and a clean, free from fracture surface makes the outer shell of the material soft hence it is to be avoided.
The above special requirements of spring steels are being met by adding different alloying elements in the steels. These are silicon, manganese, chromium, vanadium, molybdenum and nickel (in case of stainless steels). Most of the spring steels are hardened and tempered to around 45 HRC. The alloy steel springs are made from either alloy steels or from stainless steels.
Alloy spring steels are used for conditions of high stress and shock or impact loadings. These steels can withstand a wider temperature variation than high carbon spring steels and are used in either the annealed or pre tempered conditions. Silicon is the key element in most of the alloy spring steels. A typical example of alloy spring steel contains 1.5 % to 1.8 % silicon, 0.7 % to 1 % manganese, and 0.52 % to 0.6 % carbon.
The use of stainless spring steels has increased in recent times. There are compositions available which can be used for temperatures up to 300 deg C. All these steels are corrosion resistant but only the stainless steel of 18-8 composition is to be used at sub-zero temperatures.
Stainless steels – Stainless steels are characterized by corrosion resistance, aesthetic appeal, heat resistance, low life cycle cost, full recyclability, biological neutrality, ease of fabrication, cleanability and good strength to weight ratio. The selection of stainless steels is based on corrosion resistance, fabrication characteristics, availability, mechanical properties in specific temperature ranges, and product cost. However, corrosion resistance and mechanical properties are normally the most important factors in selecting a grade for a given application.
Stainless steels are iron base alloys containing at least 10.5 % chromium. Few stainless steels contain higher than 30 % chromium or less than 50 % iron. These steels have chromium as the main alloying element and are valued for high corrosion resistance. With over 11 % chromium, steel is around 200 times more resistant to corrosion than mild steel. These steels achieve their stainless characteristics through the formation of an invisible, self-healing, and adherent chromium rich oxide surface film. This oxide forms and heals itself in the presence of oxygen. Other elements added to improve particular characteristics include nickel, molybdenum, copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur, and selenium. Carbon is normally present in quantities ranging from less than 0.03 % to over 1 % in certain martensitic grades.
Stainless steels can be divided into three groups based on their crystalline structure. The first group of stainless steels consists of austenitic steels which are non-magnetic and non-heat treatable, and normally contain 18 % chromium, 8 % nickel and less than 0.8 % carbon. Austenitic stainless steels form the largest portion of the global market for stainless steel and are frequently used in food processing equipment, kitchen utensils, and piping. The second group consists of ferritic stainless steels. Ferritic steels contain trace amounts of nickel, 12 % to 17 % chromium, and less than 0.1 % carbon, along with other alloying elements, such as molybdenum, aluminum or titanium. These magnetic steels cannot be hardened with heat treatment, but can be strengthened by cold work. The third group consists of martensitic stainless steels. Martensitic steels contain 11 % to 17 % chromium, less than 0.4 % nickel, and up to 1.2 % carbon. These magnetic and heat treatable steels are used in knives, cutting tools, as well as in dental and surgical equipment.
Stainless steels are normally divided into five groups (Fig 2) which are (i) austenitic stainless steels, (ii) martensitic stainless steels, (iii) ferritic stainless steels, (iv) duplex (ferritic-austenitic) stainless steels, and (v) precipitation-hardening stainless steels. In each of the three original groups of stainless steels which are austenitic, martensitic, and ferritic, there is one composition which represents the basic, general-purpose steel. All other compositions derive from this basic steel, with specific variations in composition being made to impart very specific properties.
Fig 2 Types of stainless steels
Stainless steel is available in the form of (i) plate, (ii) sheet, strip, and foil, (iii) hot and cold finished bars, (iv) different kind of wire products, (v) structural shapes, (vi) pipes and tubes, and (vii) semi-finished products. Stainless steel can also be available as castings. The cast stainless steels, in general, are similar to the equivalent wrought steels. Majority of the cast steels are direct derivatives of one of the wrought grades.
Stainless steels are used in a wide variety of applications. Most of the structural applications occur in the chemical and power engineering industries. These applications include an extremely diversified range of uses, including nuclear reactor vessels, heat exchangers, oil industry pipes, components for chemical processing and metallurgical, pulp and paper industries, furnace parts, and boilers used in fossil fuel electric power plants.
Ultra-high strength steel – Structural steels with very high strength levels are normally called ultra-high strength steels. The medium carbon low alloy family of ultra-high strength steels include grade AISI 4130, the higher strength grade AISI 4140, and the deeper hardening, higher strength grade AISI 4340. Several modifications of the basic AISI 4340 grade steel have been developed. In one of the modification, silicon content is increased to prevent embrittlement when the steel is tempered at the low temperatures needed for very high strength. In certain steel grades, vanadium is added as a grain refiner to increase toughness, and the carbon is slightly reduced to promote weldability. One steel grade contains vanadium, slightly higher carbon, chromium, and molybdenum than grade AISI 4340, and slightly lower nickel. Other less widely used steels which can be included in this family are grades AISI 6150 and AISI 8640 steels.
Wear resistant steels – These steels are characterized mainly by high resistance to wear friction, weldability, good ductility, and machinability. The disadvantage of wear resistant steels is low corrosion resistance which can limit their application in aggressive environments.
The presence of alloy carbides improves the wear resistance of steels. Hence alloying elements such as chromium, vanadium, tungsten, and molybdenum contribute to wear resistance in steels. The carbides being the hardest component in the microstructure has a decisive influence on the wear resistance. Further the smaller is the size of the carbides in the steel, higher is its wear resistance.
Commercially available steels for wear resistance are normally marketed based on their bulk hardness and carbon equivalent (an indication of weldability). These steels are either in the quenched or quenched and tempered martensitic condition with Brinell hardness in the range 300 BHN to 550 BHN and carbon content in the range of 0.15 % to 0.4 %. These steels are alloyed with manganese, molybdenum, and chromium for hardenability, silicon for solid solution strengthening, and micro-alloying elements like niobium, vanadium, and titanium added for austenite grain refinement during hot rolling. Their impact energy is around 20 J (joules) to 40 J at -40 deg C and this is relevant for low-temperature applications.
Alloying elements, like manganese, chromium, molybdenum, and boron etc., are added to steel to improve hardenability so that a full martensitic structure can be obtained on quenching from the austenite phase field to room temperature. In general, the wear resistance steels are produced in thick sections and hence the addition of alloying elements is needed to increase the hardenability. Though silicon is a strong solid solution strengthening element, its addition is restricted to 0.5 % to avoid red scale formation during hot rolling. Micro-alloying elements, niobium, vanadium, and titanium are added to control the austenite grain size during hot rolling.
Besides micro-alloying elements, rare earth elements can be added to refine the austenite grain size. It has been reported that addition of rare earth elements improves impact energy and also the material performance against wear for a particular application. It has also been found that these elements also act as deoxidizers and desulphurizers which results in clean steels. However, rare earth elements are expensive to use on a large scale and are sparsely distributed in the world.
Three-body abrasion resistance of steels containing different quantities of carbon, boron, and chromium have been studied for agricultural tools and it has been found that steel containing 0.3 % carbon with either 0.4 % chromium or 25 ppm (parts per million) boron in quenched condition performed better than other combinations due to combination of martensite and fine carbides in the steels. Effect of carbon, chromium, nickel, and manganese on change in abrasion wear of line pipes by sand is shown in Fig 3. It seems that chromium is the most effective element to increase wear resistance after carbon. Further, it is seen that addition of at least 2 % chromium to 5 % chromium improves wear resistance. Chromium increases hardenability, along with carbon, form a variety of carbides, and it can replace part of iron to form composite cementite and form complex carbides which play a considerable role in increasing wear resistance of steel.
Fig 3 Effect of alloying elements on the wear rate
While developing very high wear resistance steel, it has been suggested that high strength medium carbon steels (0.3 % to 0.4 % carbon) which are alloyed with up to 2 % manganese, 2 % to 4 % chromium and 0.5 % molybdenum in quenched and tempered condition have high wear resistance in high stress abrasion. The steel also has high fracture toughness compared to commercially available steels and hence it is expected that this material performs better when exposed to impact damage besides abrasion.
Weathering steels – Weathering steels are high strength low alloy steels which can provide corrosion protection without additional coating. Increase in alloying elements, mainly copper, provides an arresting mechanism to atmospheric corrosion in the steel itself. The alloying elements in the steel produce a stable and durable rust layer which adheres to the base steel. This rust patina develops under conditions of alternate wetting and drying to produce a protective barrier, which impedes further access of oxygen and moisture. This patina acts as a skin to protect the steel substrate.
Weathering steels are corrosion resistant steels which work by controlling the rate at which oxygen in the atmosphere can react with the surface of the metal. Weathering steels are low‐alloy steels with a carbon content of less than 0.2 % to which mainly copper, chromium, nickel, phosphorus, silicon, and manganese are added as alloying elements to a total of no more than 3 % to 5 %. The enhanced corrosion resistance of weathering steel in relation to mild steel or plain carbon steel is because of the formation in low aggressive atmospheres of a compact and well‐adhering corrosion product layer known as patina. This definition of weathering steel, however, has not remained unchanged but has evolved as new weathering steel compositions have been developed to achieve improved mechanical properties and / or withstand increasingly aggressive atmospheric conditions from the corrosion point of view, especially in marine environments.