Alloying Elements and Steel Properties
Alloying Elements and Steel Properties
Steel is essentially iron and carbon alloyed with certain additional elements. The chemical composition of steel is of great importance since it determines the potential mechanical properties of the finished steel product, controls the degree of corrosion resistance, and weldability of the steel material.
Alloying elements are added to the steel to change the chemical composition of steel and improve its properties over carbon (C) steel or adjust them to meet the requirements of a particular application. Different alloying elements each have their own affect 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 high temperature range, (v) improved corrosion resistance, and (vi) improved high temperature performance.
More than twenty elements are used in various proportions and combinations in the production of both C and alloy steels. Some are used because they impart specific properties to the steel when they alloy with it (i.e. dissolve in the iron), or when they combine with C, 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.
Chromium (Cr), vanadium (V), molybdenum (Mo), and tungsten (W) when added to steels improve strength by forming second phase carbides. Manganese (Mn), silicon (Si), nickel (Ni), and copper (Cu) are added to increase strength of the steels by forming solid solutions in ferrite. Addition of small quantities of Ni and Cu improve corrosion resistance. Mo addition in steel helps to resist embrittlement. Zirconium (Zr), cerium (Ce), and calcium (Ca) increase toughness in alloy steels by controlling the shape of inclusions. Machinability of the steel is increased be manganese sulphide (MnS), lead (Pb), bismuth (Bi), selenium (Se), and tellurium (Te) increase machinability.
The alloying elements in the steels can be found (i) in the free state, (ii) as inter-metallic compound with iron (Fe) 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. As to the character of their distribution in steel, alloying elements can be divided into two groups. The first group of elements does not form carbides in steel. Examples are Ni, Si, Cu, cobalt (Co), aluminum (Al), and nitrogen (N2). The second group of elements forms stable carbides in steel. Examples are Cr, Mn, W, V, Ti, Zr, and niobium (Nb).
The principle effects of addition of alloying elements to steel include the following.
- Alloying elements can encourage formation of graphite from the carbide. Only a small proportion of these elements can be added to the steel before graphite forms and starts destroying the properties of the steel, unless elements are added to counteract the effect. Elements which encourage the formation of graphite include Si, Co, Al, and Ni.
- Alloying elements can go into solid solution in the Fe, enhancing the strength. Elements which go into solid solution include Si, Mo, Cr, Ni, and magnesium (Mg).
- Hard carbides (cementite) associated with Fe and C can be formed with alloying elements. Elements which tend to form carbides include Cr, W, Ti, Nb, V, Mo, and Mn.
- Alloying elements which stabilize austenite include Mn, Ni, Co, and Cu. These increase the range over which austenite is stable e.g. by lowering the eutectoid temperature, and this retards the separation or carbides.
- Alloying elements which tend to stabilize ferrite include Cr, W, Mo, V, and Si. They reduce the amount of C soluble in the austenite and thus increase the volume of free carbide in the steel at a given C content. This effectively reduces the austenite phase by raising the eutectoid temperature and lowering the peritectic temperature. Intermediate compounds with Fe can be formed e.g. FeCr. Alloying elements can adjust the characteristics such as eutectoid content, quenching rate which produces bainite or martensite.
The combined effect of alloying elements results from many 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) Ni has reduced carbide forming tendency than Fe and dissolves in the alpha ferrite, (ii) Si combines with oxygen (O2) to form non-metallic inclusions or dissolves in the ferrite, (iii) most of the Mn in alloy steels dissolves in the alpha ferrite and any Mn which form carbides result in (Fe,Mn)3C, (iv) Cr spreads between the ferrite and carbide phases with the spread depending on the amount of C and other carbide generating elements present, (v) W and Mo form carbides if sufficient C is present which has not already formed carbides with other stronger carbide forming elements, (vi) V, Ti, and Nb are strong carbide forming elements and are present in steel as carbides, and (vii) Al combines with O2 and N2 to form Al2O3 an AlN. The following gives an overview of some of the influences of alloying elements on the properties of steel.
Aluminum – Al is widely used as a deoxidizer. It can control austenite grain growth in reheated steels and is hence added to control grain size. It is the most effective alloying element in controlling grain growth prior to quenching. As a deoxidizer, upto 0.05 % Al can be added to steel. For increasing fine grain characteristics or sub-zero impact properties, upto 0·1 % of Al can be added. Nitriding steels contain around 1 % of Al for promoting a high surface hardness when heated in ammonia (NH3). Still larger additions made to heat resisting steels promote resistance to scaling. Around 5 % of Al added to Cr steel increases electrical resistivity.
Al improves oxidation resistance, if added in considerable amounts. It is used in certain heat resistant alloys for this purpose. In precipitation hardening steels, Al is used to form the inter-metallic compounds which increase the strength in the aged condition.
Boron – It is added to fully killed steel to improve hardenability. B treated steels are produced to a range of 0.0005 % to 0.003 %. Whenever B is substituted in part for other alloying elements, it is to be done only with hardenability in mind since the lowered alloying content can be harmful for some applications. The addition of around 0.003 % of B ensures increased hardenability to steels in the quenched and tempered condition. Further, it has been found that the addition of 0·003 % of B to low C, 0·50 % Mo steel in the normalized condition doubles the yield strength and gives a 30 % increase in tensile strength, but the advantage due to B is only small when Mo is less than 0.35 %. It causes difficulty in hot working. As much as 2 % can be added to steels used in nuclear engineering.
B is a powerful alloying element in steel. A very small amount of B (around 0.001 %) has a strong effect on the hardenability. B steels are generally produced within a range of 0.0005 % to 0.003 %. Boron is most effective in lower carbon steels.
B in the range of 0.003 % to 0.005 % is normally added to previously fully killed, fine-grain steel to increase the hardenability of the steel. The yield ratio and impact are definitely improved, provided advantage is taken of the increased hardenability obtained and the steel is fully hardened before tempering. In conjunction with Mo, B forms a useful group of high tensile bainitic steels. B is used in some hard facing alloys and for nuclear control rods.
Calcium – Ca in the form of calcium silicide is sometimes added to steel as a deoxidizer and degasefier. Ca additions are made during steelmaking for refining, deoxidation, desulphurization, and control of shape, size and distribution of oxide and sulphide inclusions. Ca is not used as alloying element since its solubility in steel is very low.
Carbon – C has a major effect on steel properties. It is the primary hardening element in steel. Hardness and tensile strength increases as the C content increases upto around 0.85 %. Ductility and weldability decrease with increase of the C content. C is essential in steels which have to be hardened by quenching and for example, in austenitic Mn steel which is required to have high resistance to wear. The maximum hardness obtainable in any C steel is a function of the C content which can vary upto around 2 % according to the purpose for which the steel is to be used. It occurs in varying forms according to the percentage present, and the heat treatment to which the steel has been submitted.
C is a strong austenite former and strongly promotes an austenitic structure. It also significantly increases the mechanical strength. C reduces the resistance to inter-granular corrosion. In the ferritic stainless steels, C strongly reduces both toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels, C increases hardness and strength. In the martensitic steels, an increase in hardness and strength is generally accompanied by a decrease in toughness and in this way C reduces the toughness of these steels.
C is present in all steels. It is the most important hardening element. It also increases the strength of the steel. C has by far the greatest influence of any of the elements. Martensite, along with bainite gives steel a microstructure of hard, tough carbide. None of the other elements so dramatically alter the strength and hardness as do small changes in C content. C-iron crystalline structures have the widest number and variety known to exist in the metallurgy. C also combines with other elements to furnish steel with an assortment of Fe alloy carbide systems.
Cerium – Ce is a rare earth metal which in many respects resembles the alkali metals. The hot working properties of high alloy corrosion resistant and heat resistant steels can be improved by the addition of Ce. Ce in cast iron, acts as a deoxidizer and desulphurizer but when the sulphur (S) content has been reduced to a value of around 0·015 %, Ce enters into solution in the cast iron and functions as a powerful carbide stabilizer. In amounts above 0·02 %, Ce is the operative factor in the production of nodular graphite structures in cast iron.
Ce when added in small amounts to certain heat resistant steels increases the resistance to oxidation and high temperature corrosion.
Chromium – Cr is normally added to steel to increase corrosion resistance and oxidation resistance, to increase hardenability, or to improve high temperature strength. As a hardening element, Cr is frequently used with a toughening element such as Ni to produce superior mechanical properties. At higher temperatures, Cr contributes increased strength. Cr is strong carbide former. Complex Cr-Fe carbides go into solution in austenite slowly, hence, sufficient heating time is to be allowed for prior to quenching.
Cr is added for wear resistance, hardenability, and (most importantly) for corrosion resistance. As with Mn, Cr has a tendency to increase hardness penetration. When 5 % Cr or more is used in conjunction with Mn, the critical quenching speed is reduced to the point that the steel becomes air hardening. Cr can also increase the toughness of steel, as well as the wear resistance.
As an alloying element in steel, Cr increases the hardenability and in association with high C gives resistance to abrasion and wear. 4 % is present in high speed steel and upto 5 % is present in hot die steels. In structural steels it can be present in amounts upto around 3 %. Simple Cr-C steels are used for ball bearings having high elastic limit and high uniform hardness due to the uniform distribution of the hard carbide particles, but for most structural purposes Cr is used in conjunction with upto 4 % Ni and small amounts of Mo or V, In heat-resisting steels, Cr is present in amounts upto 30 %, and it is an important element in many of the highly alloyed heat-resisting materials, whose Fe contents are so low that they cannot be regarded as steel.
Cr is the most important alloying element in stainless steels. It is this element which gives the stainless steels their basic corrosion resistance. The corrosion resistance increases with increasing Cr content. It also increases the resistance to oxidation at high temperatures. Cr promotes a ferritic structure. It is unique in its effect on resistance to corrosion and scaling and is an essential constituent in all stainless steels, where in many grades it is associated with Ni, and small amounts of other elements. Steel with at least 13 % Cr is deemed ‘stainless’ steel. Despite the name, all steel can rust if not maintained properly.
Cr can dissolve in either alpha iron or gamma iron, but, in the presence of C, the carbides formed are cementite (FeCr)3C in which Cr can increase to more than 15 %, Cr carbides (CrFe)3C2, (CrFe)7C3, and (CrFe)4C, in which Cr can be replaced by a few percent, by a maximum of 55 % and by 25 % respectively. Stainless steels contain Cr4C. The pearlitic Cr steels with, say, 2 % Cr are extremely sensitive to rate of cooling and temperature of heating before quenching. It increases the depth penetration of hardening and also the responsiveness to heat treatment.
Cr is normally added with Ni for use in stainless steels. Most of the Cr bearing alloys contain 0.50 % to 1.50 % Cr. Some stainless steels contain Cr as much as 20 % or more. It can affect hot working, causing a tendency in the steel to crack. The reason is that the Cr carbides are not readily dissolved in the austenite, but the amount increases with increase in the temperature. The effect of the dissolved Cr is to raise the critical points on heating (Ac) and also on cooling (Ar) when the rate is slow. Faster rates of cooling quickly depress the Ar points with consequent hardening of the steel.
Cr imparts a characteristic form of the upper portion of the isothermal transformation curve. The percentage of C in the pearlite is lowered. Hence the proportion of free cementite (hardest constituent) is increased in high C steel and, when the steel is properly heat-treated, it occurs in the spheroidize form which is more suitable when the steel is used for ball bearings. The pearlite is rendered fine. When the Cr exceeds 1.1 % in low C steels, an inert passive film is formed on the surface which resists attack by oxidizing reagents. Still higher Cr contents are found in heat resisting steel.
Cr steels are easier to machine than Ni steels of similar tensile strength. The steels of higher Cr contents are susceptible to temper brittleness, if slowly cooled from the tempering temperature through the range 550 deg C/450 deg C. These steels are also liable to form surface markings, generally referred to as ‘chrome lines’. The Cr steels are used wherever extreme hardness is needed, such as in dies, ball bearings, plates for safes, rolls, files and tools. High Cr content is also found in certain permanent magnets.
Cobalt – Co increases strength and hardness, and permits quenching at higher temperatures. In some steels used for nuclear engineering, Co is an undesirable impurity, even in amounts as low as 0·02 %. Unlike most other alloying elements, Co reduces hardenability. It raises the red hardness of the steel and this is the reason 5 % to 10 % of Co is added to certain types of high speed steels, developed for the specific purpose of cutting exceptionally hard materials. Heat resisting alloys with high Co contents have been developed for use in gas turbines. Co is added to the extent of upto 40 % to magnet steels requiring high coercive force and it is used in electrical- resistance alloys. In the sintered hard metals Co acts as the binding metal.
Co is only used as an alloying element in martensitic steels where it increases the hardness and tempering resistance, especially at higher temperatures. It has a high solubility in alpha iron and gamma iron but a weak carbide forming tendency. It decreases hardenability but sustains hardness during tempering. It is used in ‘Stellite’ type alloys, gas turbine steel, magnets and as a bond in hard metal.
Copper – Cu is an austenite former. It decreases hot working. It is used as precipitation hardening alloying element. It dissolves in the ferrite to a limited extent, not more than 3.5 % is soluble in steels at normalizing temperatures, while at room temperature the ferrite is saturated at 0.35 %. It lowers the critical points, but insufficient to produce martensite by air cooling. The resistance to atmospheric corrosion is improved with Cu. Cu steels can be temper hardened.
Cu, in significant amounts is detrimental to hot-working of steels. It negatively affects forge welding, but does not seriously affect arc or oxy-acetylene welding. It can be detrimental to surface quality. It is beneficial to atmospheric corrosion resistance when present in amounts exceeding 0.20 %. Weathering steels are having more than 0.20 % Cu. The addition of around 0.20 % Cu to low C steel can increase its resistance to atmospheric corrosion by as much as 20 % to 30 %. In amounts of around 0·50 % Cu appreciably increases the tensile and yield strengths. The addition of increasing amounts of Cu leads to defects in rolling.
High yield point structural steels containing Cu, in association with Cr and appreciable percentages of Si and P (phosphorus) have been developed. Cu is also added to some stainless steels to improve corrosion resistance. Cu enhances the corrosion resistance in certain acids and promotes an austenitic structure. In precipitation hardening steels, Cu is used to form the inter-metallic compounds which are used to increase the strength.
Hydrogen – H2 in steel is an undesirable impurity which is introduced from moisture in the atmosphere or the charge during melting. If a large amount of H2 is present in the liquid steel, some can be liberated on solidifying giving an unsound cast product. Evolution of H2 subsequently when the solid steel cools can cause hair line cracks. H2 can be reduced to safe proportions by vacuum treatment of liquid steel or by prolonged annealing. It can also be introduced into the steel by electrolytic action or by pickling and can then cause brittleness.
Lead – Pb is an undesirable element. It increases the machinability of steel. It impairs ductility, toughness, and creep strength. It has no effect upon the other properties of the steel. Pb is virtually insoluble in liquid or solid steel. However, it is sometimes added to C and alloy steels by means of mechanical dispersion during pouring to improve the machinability.
The addition of around 0.25 % Pb improves machinability. It also causes a reduction in fatigue strength, ductility and toughness but this only becomes serious in the transverse direction and at high tensile levels. In creep resisting alloys, very small amounts of Pb can be harmful.
Manganese – Mn fulfills a variety of functions in steel. It is used as a deoxidizing agent in nearly all steels. It forms manganese sulphide inclusions (Fig 1) which are spherical in the cast product. In the absence of Mn, S forms inter-dendritic films of iron sulphide causing brittleness at hot working temperature (hot shortness). It effectively increases hardenability and upto 1.5 % and hence it is added for this purpose. In larger amounts, it is used to stabilize austenite, as in 14 % Mn steel.
Fig 1 Non metallic inclusions
Mn is normally present in all steel and functions as a deoxidizer. It also imparts strength and has responsiveness to heat treatment. It is normally present in quantities of 0.5 % to 2 %. In the range 0.3 % to 1.5 %, it is always present in steels to reduce the negative effects of impurities carried out forward from the production process e.g. S embrittlement.
It promotes the formation of stable carbides in quenched-hardened steels. Alloy steels containing Mn are pearlitic. Upto 1 %, it acts as hardening agent and from 1 % to 2 % improves strength and toughness. Alloy steels containing more than 5 % are non-magnetic. Alloy steels containing large proportions of upto 12.5 % Mn have the property that they spontaneously form hard skins when subject to abrasion (self-hardening properties).
All commercial steels contain 0.3 % to 0.8 % Mn, to reduce oxides and to counteract the harmful influence of iron sulphide. Any Mn in excess of these requirements partially dissolves in the Fe and partly forms Mn3C which occurs with the Fe3C. There is a tendency nowadays to increase the Mn content and reduce the C content in order to get steel with an equal tensile strength but improved ductility. If the Mn is increased above 1.8 %, the steel tends to become air hardened, with resultant impairing of the ductility. Upto this quantity, Mn has a beneficial effect on the mechanical properties of oil hardened and tempered 0.4 % C steel. The Mn content is also increased in certain alloy steels, with a reduction or elimination of expensive Ni, in order to reduce costs. Steels with 0.3 % to 0.4 % C, 1.3 to 1.6 % Mn and 0.3 % Mo have replaced 3 % Ni steel for some purposes.
Non-shrinking tool steel contains upto 2 % Mn, with 0.8 % to 0.9 % C. Steels with 5 % to 12 % Mn are martensitic after slow cooling and have little commercial importance. Hadfield`s Mn steel is a special steel which is austenitic and normally contains around 12 % Mn. It is used in mining, earth- moving equipment and in railroad track work. Hadfield`s Mn steel containing 12 % to 14 % of Mn and 1 % of C is characterized by a great resistance to wear and is hence used for railway points, rock drills and ore crushers. Austenite is completely retained by quenching the steel from 1,000 deg C, in which soft condition it is used, but abrasion raises the hardness of the surface layer from 200 VPN (Vickers pyramid number) to 600 VPN (with no magnetic change), while the underlying material remains tough.
Annealing has embrittling effect on the steel by the formation of carbides at the grain boundaries. Ni is added to electrodes for welding Mn steel and 2 % Mo sometimes is added, with a prior carbide dispersion treatment at 600 deg C, to minimize initial distortion and spreading.
Mn is normally beneficial to surface quality especially in resulphurized steels. It contributes to strength and hardness, but less than C. The increase in strength is dependent upon the C content. Increasing the Mn content decreases ductility and weldability, but less than C. Mn has a significant effect on the hardenability of steel.
Mn aids the grain structure, and contributes to hardenability, strength and wear resistance. It improves the steel (e.g. deoxidizes) during the steel’s production and processing (hot working). Mn slightly increases the strength of ferrite, and also increases the hardness penetration of steel in the quench by decreasing the critical quenching speed. This also makes the steel more stable in the quench. Steels with Mn can be quenched in oil rather than water, and hence are less susceptible to cracking because of a reduction in the shock of quenching.
Mn is normally used in stainless steels in order to improve hot ductility. Its effect on the ferrite / austenite balance varies with temperature. At low temperature Mn is an austenite stabilizer but at high temperatures it stabilizes ferrite. Mn increases the solubility of N2 and is used to get high N2 contents in the austenitic steels.
In general, Mn increases strength and hardness, forms carbide, increases hardenability, lowers the transformation temperature range. When in sufficient quantity produces austenitic steel and is always present in steel to some extent because it is used as a deoxidizer.
Molybdenum – It is carbide former, and prevents brittleness. It maintains the steel strength at high temperatures. It is present in many steels. Air-hardening steels always have 1 % or more of Mo. Mo provides the ability in steel to harden in air.
Mo adds greatly to the penetration of hardness and increases toughness of an alloy steel. It causes steel to resist softening at high temperatures, which defeats the purpose of hot working. If the alloy steel has below 0.02 % Mo, then steel can be hot worked with little difficulty.
Mo is used very widely because of its powerful effect in increasing hardenability and also because in low alloy steels, it reduces susceptibility to temper brittleness. It forms stable carbides, raises the temperature at which softening takes place on tempering, and increases resistance to creep. In high speed steel, it can be used to replace around twice its weight of W. The corrosion resistance of stainless steel is improved by the addition of Mo.
Mo increases the hardenability of steel. It can produce secondary hardening during the tempering of quenched steels. It enhances the creep strength of low alloy steels at high temperatures. It considerably increases the resistance to both general and localized corrosion. It increases the mechanical strength somewhat and strongly promotes a ferritic structure. It also promotes the formation of secondary phases in ferritic, ferritic-austenitic and austenitic steels. In martensitic steels it increases the hardness at higher tempering temperatures due to its effect on the carbide precipitation.
Mo dissolves in both alpha iron and gamma iron and in the presence of C forms complex carbides (FeMo)6C, Fe21Mo2C6, and Mo2C. Mo is similar to Cr in its effect on the shape of the time- temperature-transformation (TTT)-curve but upto 0.5 % appears to be more effective in retarding pearlite and increasing bainite formation. Additions of 0.5 % Mo have been made to plain C steels to give increased strength at temperatures of 400 deg C, but the element is mainly used in combination with other alloying elements.
Ni-Cr-Mo steels are widely used for ordnance, turbine rotors and other large articles, since Mo tends to minimize temper brittleness and reduces mass effect. Mo is also a constituent in some high speed steels, magnet alloys, heat-resisting and corrosion-resisting steels.
Mo is a strong carbide forming element, and also improves high temperature creep resistance; reduces temper-brittleness in Ni-Cr steels. It improves corrosion resistance and temper brittleness.
Nickel – Ni increases strength, and improves toughness. It is ineffective in increasing the hardness. It is a ferrite former. It is normally added in amounts ranging from 1 % to 4 %. In some stainless steels it is sometimes as high as 20 %. It is used for strength, corrosion resistance, and toughness. Ni increases the strength of ferrite, and hence increasing the strength of the steel. It is used in low alloy steels to increase toughness and hardenability. Ni also tends to help reduce distortion and cracking during the quenching phase of heat treatment.
Ni is a ferrite strengthener. It does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase. It increases the hardenability and impact strength of steels. In the range of 0.2 % to 5 %, it improves strength, toughness, and hardenability without seriously affecting the ductility. It encourages grain refinement. Ni and Cr together have opposing properties and are used together to advantage in Ni-Cr steels. The resulting steels have their advantages combined and their undesirable features cancel each other. Ni at 5 % provides high fatigue resistance. When alloyed at higher proportions significant corrosion resistance results and at 27 % a non-magnetic stainless steel results.
The addition of Ni, in amounts upto 8 % or 10 %, to low C steel, increases the tensile strength and considerably raises the impact resistance. 9 % Ni steels are useful at very low temperatures. In engineering steels, it is widely used, often with Cr and Mo. High Ni increases resistance to corrosion, and in combination with Cr, is used in the austenitic corrosion-resisting steels. Certain Fe-Ni alloy steels have unique properties. 25 % nickel steel is practically non-magnetic. Alloys with around 36 % Ni have very low coefficients of expansion, whilst with 50 % to 78.5 % Ni; the alloys are obtained having very high magnetic permeability in low fields. An alloy containing 29 % Ni, 17 % Co is used for sealing with certain boro-silicate glasses.
For stainless steel, the main reason for the Ni addition is to promote an austenitic structure. Ni normally increases ductility and toughness. It also reduces the corrosion rate and is thus advantageous in acid environments. In precipitation hardening steels, Ni is also used to form the inter-metallic compounds which are used to increase the strength.
Ni and Mn are very similar in behaviour and both lower the eutectoid temperature. This change point on heating is lowered progressively with increase of Ni (around 10 deg C for 1 % of Ni), but the lowering of the change on cooling is greater and irregular. The temperature of this change (Ar1) is plotted for different Ni contents for 0.2 % C steels in Fig 2. It can be seen that the curve takes a sudden plunge around 8 % nickel. Steel with 12 % Ni begins to transform below 300 deg C on cooling, but on reheating, the reverse change does not occur until around 650 deg C. Such steels are said to show pronounced lag or hysteresis and are called irreversible steels. This characteristic is made use of in maraging steels and 9 % Ni cryogenic steel.
Maraging steels are a class of high-strength steel with low C content and the use of substitutional (as opposed to interstitial) elements to produce hardening from formation of Ni martensites. The name maraging has resulted from the combination of ‘Martensite + Age hardening’. Maraging steels contain 18 % Ni, along with amounts of Mo, Co, Ti, and Al, and almost no C. These alloy steels can be strengthened considerably by a precipitation reaction at a relatively low temperature. They can be formed and machined in the solution-annealed condition but not without difficulty. Maraging steels are strengthened by inter-metallic compounds such as Ni 3Ti and Ni 3Mo which precipitate at around 500 deg C.
The addition of Ni acts similarly to increasing the rate of cooling of a C steel. Thus with a constant rate of cooling the 5 % to 8 % Ni steels become troostitic, at 8 % to 10 % Ni, where the sudden drop appears, the structure is martensitic, while above 24 % Ni the critical point is depressed below room temperature and austenite remains. The lines of demarcation are not as sharp as indicated by Fig 2, but a gradual transition occurs from one constituent to another.
Ni strengthens steel, lowers its transformation temperature range, increases hardenability, and improves resistance to fatigue. Strong graphite forming tendency stabilizes austenite when in sufficient quantity. It creates fine grains and gives good toughness.
Fig 2 Effect of Ni on change points and mechanical properties of 0.2 % C steels cooled at a constant rate
The mechanical properties change accordingly as shown in the right hand side of the Fig 2. Steels with 0.5 % Ni are similar to C steel, but are stronger, on account of the finer pearlite formed and the presence of Ni in solution in the ferrite. When 10 % Ni is exceeded, the steels have a high tensile strength, high hardness, but are brittle, as shown by the impact strength and elongation curves.
When the Ni is sufficient to produce austenite, the steels become non-magnetic, ductile, tough, and workable, with a drop in strength and elastic limit. C intensifies the action of Ni and the change points as shown in Fig 2 and the values vary according to the C content. The influences of C and Ni on the microstructure are shown in the Guillet diagram in Fig 2, for one rate of cooling.
Steels containing 2 % to 5 % Ni and around 0.1 % C are used for case hardening and those containing 0.25 % to 0.40 % C are used for crankshafts, axles and connecting rods. The superior properties of low Ni steels are best brought out by quenching and tempering (550 deg C to 650 deg C).
Since the Ac3 point is lowered, a lower hardening temperature than for C steels is permissible and also a wider range of hardening temperatures above Ac3 without excessive grain growth, which is hindered by the slow-rate of diffusion of the Ni. Martensitic Ni steels are not utilized and the austenitic alloy steels cannot compete with similar Mn steels owing to the higher cost. Maraging steels have fulfilled a high tensile strength requirement in aero and space fields. High Ni alloys are used for special purposes, owing to the marked influence of Ni on the coefficient of expansion of the metal. With 36 % Ni, 0.2 % C, 0.5 % Mn, the coefficient is practically zero between 0 deg C and 100 deg C. This alloy steel ages with time, but this can be minimized by heating at 100 deg C for several days. The alloy is called Invar (FeNi36) and it is used extensively in clocks, tapes and wire measures, differential expansion regulators, and in Al pistons with a split skirt in order to give an expansion approximating to that of cast iron. A C free alloy containing 78.5 % Ni and 21.5 % Fe has a high permeability in small magnetic fields.
Nickel and chromium – Ni and Cr are used together for austenitic stainless steels; each element counter-acts disadvantages of the other. Ni steels are noted for their strength, ductility and toughness, while Cr steels are characterized by their hardness and resistance to wear. The combination of Ni and Cr produces steels having all these properties, some intensified, without the disadvantages associated with the simple alloy steels. The depth of hardening is increased, and with 4.5 % Ni, 1.25 % Cr and 0.35 % C, the steel can be hardened simply by cooling in air.
Low Ni-Cr steels with low C content are used for case hardening, while for many construction purposes the C content is 0.25 % to 0.35 %, and the steels are heat-treated to give the desired properties. Considerable amounts of Ni and Cr are used in steel for resisting corrosion and oxidation at high temperatures.
Tempering has effects on Ni-Cr steel. The impact strength reaches a dangerous minimum in the range 250 deg C to 450 deg C in steel containing 0.26 % C, 3 % Ni, and 1.2 % Cr steel with 0.25 % Mo added and many other steels. This is known as 350 deg C embrittlement. N2 and P have a significant effect while other impurities such as arsenic (As), antimony (Sb) and tin (Sn), and Mn in larger quantity can also contribute to the embrittlement.
Temper brittleness is normally used to describe the notch impact inter-granular brittleness (grain boundaries are revealed in temper brittle samples by etching in 1 gm cetyl tri-methyl ammonium bromide, 20 gm picric acid, 100 cc distilled water, and 100 cc ether) induced in some steels by slow cooling after tempering above about 600 deg C and also from prolonged soaking of tough material between about 400 deg C and 550 deg C. Temper brittleness seems to be due to grain boundary enrichment with alloying elements (Mn, Cr, and Mo) during austenitizing which leads to enhanced segregation of embrittling elements P, Sn, Sb, As, by chemical interaction on slow cooling from 600 deg C. The return to the tough condition, obtained by rehearing embattled steel to temperatures above 600 deg C and rapidly cooling, is due to the redistribution and retention in solution of the embrittling segregation. Sb (0.001 %), P (0.008 %), As, Sn, Mn increase, while Mo decreases the susceptibility of steel to embrittlement. 0.25 % Mo reduces the brittleness as shown by the impact.
Niobium – It increases the yield strength and to a lesser degree the tensile strength of C steel. The addition of small amounts of Nb can significantly increase the yield strength of steels. Nb can also have a moderate precipitation strengthening effect. Its main contributions are to form precipitates above the transformation temperature, and to retard the recrystallization of austenite, thus promoting a fine-grain microstructure having improved strength and toughness.
Nb occurs in association with Ta (tantalum), to which it is closely related. Nb is a strong carbide-forming element and as such is added to certain austenitic corrosion-resistant steels of the 18/8 Cr-Ni type for the prevention of inter-crystalline corrosion. Where Nb is used as the stabilizer, it is normally specified that it is to be present in an amount at least 8 times that of the C content. Further, Nb is frequently used as a constituent of the electrodes used in the welding of such steels. Nb is added to heat-resisting steels and enhances creep strength. In small amounts, of the order of 0·05 %, it increases the yield strength of mild steel.
Nb is both a strong ferrite and carbide former. As Ti, it promotes a ferritic structure. In austenitic steels, it is added to improve the resistance to inter-granular corrosion but it also enhances the mechanical properties at high temperatures. In martensitic steels Nb lowers the hardness and increases the tempering resistance.
Nitrogen – N2 can combine with many metals to form nitrides and is thus applied to the case hardening of steel, the usual source for this purpose being NH3. The incorporation of N2 in austenitic Ce-Ni steels stabilizes the austenite and increases the strength. In C steels it has an influence on creep.
N2 is a very strong austenite former and strongly promotes an austenitic structure. It also considerably increases the mechanical strength. N2 increases the resistance to localized corrosion, especially in combination with Mo. In ferritic stainless steels, N2 strongly reduces toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels, N2 increases both hardness and strength but reduces the toughness.
Phosphorous – P has embrittlement effects. It increases machinability. It is an undesirable element. It increases strength and hardness and decreases ductility and notch impact toughness of steel. The adverse effects on ductility and toughness are greater in quenched and tempered high C steels. P levels are normally controlled to low levels. Higher P is specified in low C free-machining steels to improve machinability.
Although P has been used to increase the tensile strength of steel and to improve resistance to atmospheric corrosion, it is normally regarded as an undesirable impurity because of its embrittling effect. In several specifications for steels, the maximum permitted P content is 0.05 %, but in steel for nitriding, it is to be restricted to a maximum of 0·02 % since during the nitriding treatment P has a temper embrittling effect.
Selenium – Se is a metalloid closely resembling S in its properties. It is sometimes added to steels to the extent of 0·2 % to 0·3 % to improve machinability.
Silicon – It is a ferrite former and encourages brittleness. It is one of the principal deoxidizers used in steelmaking. Si is less effective than Mn in increasing as-rolled strength and hardness. In low C steels, Si is normally detrimental to the surface quality. Si increases the resistance to oxidation, both at high temperatures and in strongly oxidizing solutions at lower temperatures. It promotes a ferritic structure. It has a beneficial effect upon tensile strength and improves hardenability of the steel. It has a toughening effect when used in combination with certain other elements. Si is normally added to improve electrical conductivity of the steel. Its average concentration is between 1.5 % and 2.5 %.
Si slightly increases the strength of ferrite, and when used in conjunction with other alloys can help increase the toughness and hardness penetration of steel. It is a powerful deoxidizer, and as such is used in steelmaking processes in amounts upto around 0·8 %. When used as an alloying element, Si in small percentages increases the tensile strength and yield point of structural steels. It is used in amounts of 1·5 % to 2 % in Si-Mn spring steels and ultra-high tensile steels due to its effect in raising the limit of proportionality and resistance to tempering. Si upto 4 % in heat resisting steels improves scale resistance owing to the formation of a protective layer.
The higher is the Si content, the higher is the temperature at which protection against further atmospheric oxidation is given. Water vapour and carbon dioxide (CO2), however, attack the layer. Alloys of Fe and Si, containing 15 % of the element, are used as acid-resisting materials, but have the properties of cast irons rather than of steels. C free alloys with upto 4 % Si have a high electrical resistance and low hysteresis loss, and are used as transformer steels.
Si dissolves in the ferrite, of which it is a fairly effective hardener, and raises the Ac change points and the Ar points when slowly cooled and also reduces the gamma-alpha volume change. Only three types of Si steel are in common use, one in conjunction with Mn for springs, the second for electrical purposes, used in sheet form for the construction of transformer cores, and poles of dynamos and motors, which demand high magnetic permeability and electrical resistance, and the third is used for the automobile valves.
Si contributes oxidation resistance in heat-resisting steels and is a general purpose deoxidizer. In general Si strengthens ferrite and raises the transformation temperature temperatures, and has a strong graphitizing tendency. It is always present to some extent, because it is used with Mn as a deoxidizer.
Sulphur – S increases the machinability. It is an undesirable element. It has embrittlement effects. It decreases ductility and weldability. It is normally regarded as an impurity in most steels and its addition to steel is held to a minimum as it is damaging to the hot forming characteristics of steel. It is, however added to increase machinability.
S decreases ductility and notch impact toughness especially in the transverse direction. Weldability decreases with increasing S content. S is found primarily in the form of sulphide inclusions. S levels are normally controlled to low levels. The only exception is free machining steels, where S is added to improve machinability.
S is a non-metal, which combines with Fe to form iron sulphides, in which form its effect is to make the steel hot short but combined with Mn its influence is less harmful. In steels, the S content is normally specified as less than 0.05 % but it can be added deliberately to improve machinability. S is added to certain stainless steels, the free-machining grades, in order to increase the machinability. At the levels present in these grades, S considerably reduces corrosion resistance, ductility and fabrication properties, such as weldability and formability.
Tantalum – Ta metal is associated with Nb and is very similar to it chemically. As an alloying addition to steel, Nb is preferred.
Tellurium – Te is added to steel either alone or together with Se to promote machinability. It is a powerful carbide stabilizer and has been also added to cast iron where it is said to increase the depth of chill and to prevent shrinkage. It can be added in small amounts to the liquid iron or by the use of cores dipped or painted with washes containing Te in suspension.
Tin – Sn owing to its good resistance to corrosion in many conditions is used for coatings of steel. It is an undesirable impurity in steel giving rise to temper brittleness, but is less harmful than P.
Titanium – Ti is used to retard grain growth and thus improve toughness. It is also used to achieve improvements in inclusion characteristics. It causes sulphide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.
Ti is a strong ferrite former and a strong carbide former, thus lowering the effective C content and promoting a ferritic structure in two ways. In austenitic steels, it is added to increase the resistance to inter-granular corrosion but it also increases the mechanical properties at high temperatures. In the ferritic stainless steels, Ti is added to improve toughness and corrosion resistance by lowering the amount of interstitials in solid solution. In martensitic steels Ti lowers the martensite hardness and increases the tempering resistance. In precipitation hardening steels, Ti is used to form the inter-metallic compounds which are used to increase the strength.
The principal use of Ti is to stabilize C by forming titanium carbide. In austenitic stainless steels, it is used in this way to prevent inter crystalline corrosion, the Ti addition being at least four times the C content. It is also added to low C steels to prevent blistering during vitreous enameling. Titanium carbide is used with tungsten carbide in the production of hard metal tools.
Tungsten – Tungsten is also known as wolfram. It is used as an alloying element in tool steels, as it tends to impart a tight, small, and dense grain pattern and keen cutting edges when used in relatively small amounts. It also causes steel to retain its hardness at higher temperatures and hence has a detrimental effect upon the hot working of steel.
W dissolves in gamma iron and in alpha iron. With C it forms WC and W2C, but in the presence of Fe it forms Fe3W3C or Fe4W2C. A compound with Fe, Fe3W2 provides an age-hardening system. W raises the critical points in steel and the carbides dissolve slowly over a range of temperature. When completely dissolved, W renders transformation sluggish, especially to tempering, and use of this is made of this in most hot-working tools (high speed) and die steels. It increases wear resistance. When combined properly with Cr or Mo, W makes the steel to be high-speed steel. The high-speed steel has a high amount of W. W refines the grain size and produces lesser tendency to de-carburization during working. W is also used in magnet, corrosion resisting and heat resisting steels.
The effect of the addition of W to steel is to increase the strength at normal and high temperatures. Owing to the hardness of tungsten carbide and its influence on secondary hardening, W is used as the main alloy addition in high speed tool steels, Mo being its only substitute. In addition, W finds considerable application in general tool steels, die and precipitation hardening steels. It has been found a useful application in valves and other steels required for use at high temperatures. W is an essential constituent in the sintered hard metals.
W forms hard and stable carbides. It raises the transformation temperature range, and tempering temperatures. Hardened W steels resist tempering upto 600 deg C.
Vanadium – V is a ferrite promoter and carbide and nitride former. It acts as a scavenger for oxides, forms vanadium carbide (VC), and has a beneficial effect on the mechanical properties of heat treated steels, especially in the presence of other elements. It slows up tempering in the range of 500 deg C to 600 deg C and can induce secondary hardening. Cr-V (0.15 %) steels are used for locomotive forgings, automobile axles, coil springs, torsion bars and creep resistance.
V increases the yield strength and the tensile strength of C steel. The addition of small amounts of V can considerably increase the strength of steels. V is one of the primary contributors to the precipitation strengthening in micro-alloyed steels. When thermo-mechanical processing is properly controlled, the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature also increases when V is added.
All micro-alloyed steels contain small concentrations of one or more strong carbide and nitride forming elements. V, Nb, and Ti combine preferentially with C and/or N2 to form a fine dispersion of precipitated particles in the steel matrix. The presence of V in steel raises the temperature at which grain coarsening sets in and under certain conditions increases the hardenability. It also lessens softening on tempering and confers secondary hardness on high speed and other steels. VC is intensely hard and as much as 5 % V can be added to high speed and high Cr tool steel where it improves abrasion resistance. V is an important constituent in many types of steel, for widely varying applications, e.g., nitriding, heat-resistance, tools, wearing plates and other fully hardened parts. In conjunction with Mo, V has a marked effect in enhancing creep resistance.
V increases the hardness of martensitic steels due to its effect on the type of carbide present. It also increases tempering resistance. V stabilizes ferrite and at high contents, it promotes ferrite in the structure. It is only used in hardenable stainless steels. It retards grain growth within steel even after long exposures at high temperatures, and helps to control grain structures while heat treating. It is normally present in small quantities of 0.15 % to 0.2 %. Most of the tool steels which contain this element seem to absorb shock better than those which do not contain V.
V contributes to wear resistance and hardenability. As carbide former, it helps produce fine-grained steel. V being a strong carbide forming element has a scavenging action and produces clean, inclusion free steels. It can cause reheat cracking when added to Cr-Mo steels.
Zirconium – Zr can be added to killed high strength low alloy steels to achieve improvements in inclusion characteristics. Zr causes sulphide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending. It acts as a deoxidizing element in steel and combines with the S.