ALLOY STEELS

Alloy steels may be defined as steels to which elements other than carbon are added in sufficient amounts to produce improvements in properties. The more common alloying elements added to steel for alloying are chromium, nickel, manganese, silicon, vanadium, molybdenum, tungsten, phosphorus, copper, titanium, zirconium, cobalt, columbium and aluminium.

Alloying is done for one or more of thafollowing reasons :-

1. To improve wearing resistance.

2.      To improve corrosion resistance.

3.      To improve electrical properties.

4.      To improve tensile strength, elastic limit, ductility etc.

5.      To improve hardenability and hardness.

6.      To improve weldability.

7.      To retain physical properties at high temperatures.

8.      To improve machinability.

9.      To produce fine grained steel.

The principal effects which these alloying elements have on the microstructure and properties of steel can be classified as follows :

a)    The Effect on the Allotropic transformation Temperatures -

The allotropic transformation temperatures are :- A, temperature is at 723°c when the austentite------► pearlite change takes place in plain carbon steels, A₂ temperature is at 769°c, above which pure iron ceases to be magnetic, this point has no structural significance, A₃ temperature is at 910°c where on heating, the body centred cubic (a) structure of pure iron changes to face - centred cubic (y) and A₄ temperature is at 1400°c, where the face - centred cubic structure changes back to body-centred cubic (5) iron.

Some elements notably, nickel, manganese, cobalt and copper, raise the A₄ temperature and lower the A₃ temperature, thus when added to a carbon steel, tend to_ stabilise austentftW(y) and increase the range of temperature over which austentite can ' exist as a stable phase.

Other elements, such as chromium, tungsten, vanadium, molybdenum, aluminium and silicon, have the reverse effect, in that they tend to stabilise ferrite (a) by raising the A₃ temperature and lowering the A₄ temperature. Such elements restrict the field over which austentite may exist and thus form what is known as "gamma loop". Thus with the addition of more than 30% chromium to a steel containing 0.4% carbon would lead to complete suppression of the allotropic transformation and such a steel would be no longer be amenable to normal heat - treatment.

b)    The effect on the Stability of Carbides -

Some of the alloying elements such as chromium, tungsten, vanadium, molybdenum, titanium and manganese form very stable carbide when added to plain carbon steel. This generally has ahardening effect on the steel, particularly when the carbides formed are harder than iron carbide itself.

Other elements such as silicon, nickel, aluminium and cobalt have a pronounced graphising effect on carbides, that is they make the carbides unstable, so that carbides break up releasing free graphite carbon. Therefore if it is necessary to add appreciable amounts of these elements to a steel, it can be done only when the carbon content is extremely low. Alternatively, if the carbon content needs to be high, one or more of the first group, namely the carbide stabilisers, must be added in order to counteract the effects of the graphilising element.

c)    The Effect on grain growth -

The rate of crystal growth is accelarated, particularly at high temperatures, by the presence of some element, notably chromium. Thus steels containing chromium should not be overheated or to be kept at an elevated temperature, otherwise brittleness will result which is usually associated with coarse grain.

Fortunately, grain growth is retarded by some elements such as nickel, whose presence thus produces a steel which is less sensitive to the temperature conditions of heat treatment.

d)    The Displacement of Eutectoid Point -

The addition of an alloying element to carbon steel displaces the eutectoid point towards the left of the equilibrium diagram. That is a steel can be completely pearlitic even though it contains less than 0.83% Carbon. For example, addition of 3% manganese to a steel containing 0.7% carbon will cause the latter to be completely pearlitic. At the same time alloying elements raise or lower the eutectoid temperature of steel.

e)    The Retardation of Transformation Rates -

The Time - Temperature - Transformation curves for a plain carbon steel are displaced  to the right by the addition of alloying element. Thus by adding alloying elements, the critical cooling rate can be reduced which is necessary for the transformation of austentite to martensite to take place.

In order to obtain a completely martensitic structure in the case of 0.83% carbon steel, it must be cooled from above 723°c to room temperature in approximately one second. This method of drastic quench, generally leads to distortion or cracking of the component. By addition of small amounts of suitable alloying elements, such as nickel and chromium, oil quenching is sufficient enough to produce a totally martensitic structure. Further addition of alloying element will reduce the rate of transformation so much that such a steel can be hardened by cooling in air.

f)    The Improvement in Corrosion - resistance -

The corrosion - resistance of steels is substatially improved by the addition of elements such as aluminium, silicon and chromium. These elements form this but dense and adherent oxide films which protect the surface of the steel from further attack. 

g)   Effects on the Mechanical Properties -  

One of the main reasons for alloying is to effect improvements in the mechanical

properties of steel. The improvements are generally the result of physical changes, for example, hardness is increased by stabilising the carbides, strength is increased when alloying elements dissolve in the ferrite and toughness is improved due to refinement of grain.

Approximate tensile strength may be estimated from its composition for an alloy steel by Walter's factors. For example, Walters takes a basic tensile strength for pure iron of 36,000 16 Ib/sq. in and multiplies this by factors for each alloying element present as shown in the curves. They give best results for pearlitic steels with carbon content below 0.25% and within intermediate alloy range. -

IMPORTANT ALLOY STEELS

NICKEL STEELS :-

Nickel is a non - carbide forming element which is soluble in iron in all proportions. The addition of nickel to a plain carbon steel tends to stabilise austentite phase over an increasing temperature range, thus lowers the critical temperature and makes the heat treatment a little less severe. At the same time, nickel makes the carbides unstable and tends to cause then to decompose to graphite. For this reason it is inadvisable to add nickel by itself to a high - carbon steel, and most nickel steels are low - carbon steels. If a higher carbon content is desired, then the manganese content is usually increased, since manganese acts as a stabiliser of carbides.

In the range of 2 - 5%, nickel contribute, great strength and toughness, with high elastic limit, good ductility, good resistance to-corrosion and decreases machinability. Nickel has a grain - refining effect which makes the low nickel, low carbon steels very suitable for case - hardening, since grain - growth will be limited during prolonged period of heating in the region of 900°c. Those with the lower carbon content are used mainly for case - hardening, while those with upto 0.4% Carbon are used for structural purposes, shafting, gears etc, which are subjected to alternate stresses, impacts and shocks.

In the range of 20 - 30% nickel, it offers maximum toughness, greatest resistance to rusting, corrosion and burning at high temperatures and non - magnetis. Used for steam turbine blades, internal combustion engine valves etc.

In the range of 30 to 40% nickel, it lowers the coefficient of thermal expansion. "Invar" having 35% nickel, 0.1% carbon, has negligible thermal expansion and is useful in the manufacture of pendulum rods for master clocks, measuring tapes and delicate sliding mechanism for use at varying temperatures.

In the range of 50% and above, it increases magnetic permeability, large amountsgive resistance to oxidation at high temperatures. This, makes them common as components in communication equipment.                                 

•CHROMIUM STEELS -

The main function of chromium when added in relatively small amounts to a carbon steel is to cause a considerable increase in hardness, tensile strength and elastic limit. At the same time there is some loss in ductility, toughness and machinability, though not noticable when less than 1%-chromium is added. It also imparts corrosion -resistance property to steel.

The increase in hardness is due mainly to the fact that chromium is a carbide stabiliser and forms hard carbide Cr₃C₂ or alternatively, a double carbide with iron carbide, chromium tends to stabilise alpha iron, thus causing transformation to come about slowly on heating to the upper critical temperature of steel. If the compositioon of the steel falls to the left of y - loop, chromium will give rise to a much greater depth of hardening.

The main disadvantages in the use of chromium as an alloying element, is its tendency to promote grain growth i.e. to increase in brittleness.

Steel containing small amounts of chromium and upto 0.45% Carbon are used for axle shafts, connecting - rods and gears, while those containing more than 1% Carbon are extremely hard and useful for manufacture of ball - bearings, drawing dies and parts for grinding machines chromium when added in larger amounts - upto 21% - has a pronounced effect in improving corrosion resistance, due to a thin protective layer of oxide. These steels take a very high polish. They are used extensively in the chemical -engineering industry, for domestic purpose, such as stainless steel sinks and in food containers, refrigerator parts, beer barrels, cutlery and table ware.

The most common chrome steels contain from 0.7% to 11.0% chromium and 0.15 to 0.5% carbon. These chrome steels find application in the automobile and tractor industry for valves, tappets, wrist pins, idler studs etc. In the machine tool industry, they are used for gears operating at high speeds and medium loads, claw clutches, bushings, worms and other similar parts.

NICKEL - CHROMIUM STEELS -

When both nickel and chromium are added to low carbon steel as alloying element, the tendency of chromium to cause grain - growth is nullified by the grain - refining effect of the nickel, while the tendency of nickel to favour graphitisation of the carbides is counteracted by the strong carbide - forming tendency of the chromium.

At the same time, other physical effects of each element are additive, sothat they combine in increasing strength, corrosion - resistance and the retardation of transformation rates during heat treatment.

When nickel chrome steels are slowly cooled after high tempering (150° - 650°c), then steels suffer "temper - brittieness" when tempered in the range of 250° - 400°c. Thus when tempered above 400°c, it is necessary to quench them quickly in oil, through the range in which* temper - brittieness is produced.

For mild nickel - chrome steels containing 0.2 to 0.3% Carbon, 0.3 - 0.6% manganese, 3.0 - 4.0% nickel and 0.4 to 0.8% chromium, used for highly - stressed parts such as crankshaft, axles and parts requiring strength and lightness.

Steels Containing 0.25 to 0.35% Carbon, 0.25 to 0.35% manganese, 3.0 to 3.75% nickel and 0.5 to 0.8% chromium are used for highly stressed parts subject to shock such as axles, connecting rods, tubes and plates.

When nickel content is increased to 4.0% and chromium about 1.5%, an air- hardening steel is obtained such steels are very tough, hard and strong and is very useful for the manufacture of complex shapes of machine parts operating under vibrating and dynamic loads.

When tungsten and molybdenum are added to nickel chrome steels, they increase the tensile strength and yield point without affecting the toughness. They also increase the resistance to softeningon heating and reduce the susceptibility to temper brittieness.

NICKEL - CHROME - MOLYBDENUM STEEL -

The.temper brittieness of nickel - chrome steels is eliminated by the introduction of about 0.3% molybdenum. It also makes the mass effect much less pronounced so that the large pieces can be more easily heat treated, as the transformation rates of nickel -chrome steels being further reduced by the presence of molybdenum. The addition of molybdenum to a nickel - chrome steel also enables an increased percentage of manganese to be (jsed without producing any reduction of impact strength.

Among alloy steels, the nickel - chrome molybdenum steels possess the best all -round combination of properties, particularly where high tensile strength combined with good ductility is required in large components.

NICKEL - CHROME - VANADIUM STEEL -

Vanadium is normally used as an alloying element in conjunction with nickel and or chromium. Its direct effect is to harden steel and it is not normally used in excess of about 0.2% except in tool steel. It is a very effective deoxidising element and it improves the mechanical properties generally and the fatigue strength in particular.

Vanadium has a strong carbide - forming tendency, forming V₄C₃. It also stabilises martensite and troostite on heat treatment and increases hardenability. Like nickel, it restrains grain - growth of the austentite and since it combines readily with oxygen and nitrogen, it is often used as a "cleanser" during de - oxidation, to produce a gas - free ingot.

MANGANESE STEELS -

Nearly all steels contain some manganese residual from the deoxidation and desulphurisation process, which are usually less than 1.0%. When the manganese content exceeds this amount, then it is regarded as an alloying element.

Like nickel, manganese stabilises austentite by raising A₄ point and lowering A₃ point, but unlike nickel, it has a stabilising effect on the carbides, itself forming carbide Mn₃C.

Manganese steel containing 10 -16% manganese and 1.0 - 1.4% carbon are remarkable for their extreme hardness, wear resistance and increase of surface hardness under repeated impact. This is because of the soft, tough core of shock - resistant austentite and a very hard case of wear - resistant martensite. These qualities make them useful for such parts as railway points, stone and ore - crushing rolls, dredger buckets, safe vaults, cover plates for lifting magnets and screens for coke and granel. Manganese steels with a low carbon content possess good plastic properties and respond easily to elongation and rolling in both hot and cold conditions. Good weldability is one of the advantageous features-of low carbon manganese steels:

SILICON STEELS -

Small amounts of silicon are frequently used in many alloy steels but the percentage does not usually exceed about 0.8%, except heat resisting stainless steels. A silicon content of 1.25 to 2.5% with 0.5 to 0.65% carbon and 0.6 to 0.9% manganese is frequently used in steel springs.

Steel containing from 0.5 to 1.0% silicon and 0.7 to 0.95% manganese is used for structural purposes. These steels like molybdenum steels, give a greater strength and higher ductility than plain carbon steels.

An iron containing less than 0.05% carbon, about 0.3% manganese and 3.4% silicon possess extremely low magnetic hysteresis and is widely used for laminations of electrical machines, (transformer core, motor and alternator cores).

CHROME SILICON MANGANESE STEELS -

Chrome silicon manganese steel containing 0.17 to 0.39% carbon, 0.9 to 1.2% silicon, 0.8 to 1.1% manganese and 0.8 to 1.0% chromium, have good weldability and high ductility. It can be easily formed and bent. These steels are used to make shafts, axles and weldments that to operate under alternating loads.

Steels with a higher carbon content is used as plates, pipes, bars, shapes and forgings. They have satisfactory weldability not withstanding the high hardness.

STEEL CONTAINING TUNGSTEN -

Tungsten raises A₃ temperature and like chromium, forms a closed y- loop. It combines readily with carbon, forming stable carbides WC and W₂C and in steel, a double carbide Fe₄W₂C. These carbides dissolve very slowly in steel when the latter is heated and then only at elevated temperature. Once in solution, the carbides can be made to precipitate only by cooling very slowly from a high temperature. At the same time, tungsten renders transformation very sluggish and also inhibits grain - growth, so that it is an essential constituent of high - speed steel. Tungsten Carbide is extremely hard, so that tungsten has pronounced hardening effect on steel. It is commonly used in other tool steels and die - steels. It is also used in heat - resisting steels, in which it raises the strength and limiting creep stress at high temperatures.

HEAT RESISTING STEEL -

Steels required for service at elevated temperatures are used for the manufacture of such components as exhaust valves for aero engines, racks for enamelling stoves, conveyor chains, furnace arch and floor plate, motors of gas turbines etc.

The main requirement of such steels are :

a) resistance to oxidation and attack by vapours and gases existing in the working
temperature.

b)       a sufficiently high strength at the working temperature.

In general, the resistance to oxidation is effected by the addition of chromium (and sometimes silicon), while the inclusion of nickel will toughen the alloy by restricting grain - growth at high temperatures. The necessary strength at high temperature (between 850 -1150°c) is developed by "stiffening" the alloy with additions of carbon, tungsten, titanium, molybdenum or aluminium. Thus the limiting creep - stress is raised.

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