Alloying Elements: Distribution and Effects | Metallurgy

In this article we will discuss about:- 1. Classification of Alloying Elements 2. Distribution of Alloying Elements in Annealed Steels 3. Effects 4. Characteristic Effects.

Classification of Alloying Elements:

A. Based on Stablising Austenite or Ferrite:

(i) Austenite Stabilizers:

Mn, Ni, Co, Cu, Zn increase the range in which γ-phase, or austenite is stable and also tend to retard the separation of carbides.

These elements have γ-phase-FCC crystal structure (or similar structure) in which these elements are more soluble than ferrite, and that is why, in the (α + γ) two phase equilibrium, these segregate (dissolve) in austenite in preference to ferrite. Elements like carbon and nitrogen (interstitial solid solution forming elements) are also austenite stabilizers.

(ii) Ferrite Stabilizers:

Cr, W, Mo, V, Si, Al, Be, Nb, P, Sn, Ti, Zr increase the range of a-phase (by lowering A4 and raising A3 temperatures). These elements have a-phase-BCC crystal structure (or similar structure) and thus in (α + γ) two phase equilibrium, these elements segregate (dissolve) in ferrite in preference to austenite. These elements decrease the amount of carbon soluble in austenite, and thus, tend to increase the volume of free carbide in the steel for a given carbon content.

Majority of carbide formers are also ferrite formers. Chromium is a special case of these elements as at low concentrations, chromium lowers A3 temperature and raises A4, but at high concentrations raises A3 temperature. Overall, the stability of austenite is continuously decreased. Transformer steel with hardly carbon and around 3% silicon is ferritic steel.

B. Based on Carbide Forming Tendency:

(i) Carbide Forming Elements:

Important elements, in this class, are arranged in order of increasing affinity for carbon, and thus the carbide forming potential of the element (as compared to iron):

Fe → Mn → Cr → W → Mo → V → Ti → Nb → Ta → Zr.

If say, vanadium is added in steel having chromium and molybdenum with insufficient carbon, then vanadium first removes carbon from chromium carbide, the remaining vanadium then removes carbon from molybdenum carbide and forms its own carbide. The released chromium and molybdenum dissolve to form solid solution in austenite. Several ferrite formers are also carbide formers.

(ii) Graphitising Elements:

Si, Ni, Cu, Al are common graphitisers. Small amount of these elements in steel can graphitise it and thus, impair the properties of steel unless elements of group (i) are present to counteract the effect.

(iii) Neutral Element:

Co is the only element which neither forms carbide, nor causes graphitization.

Distribution of Alloying Elements in Annealed Steels:

An annealed steel consists of ferrite and carbide at room temperature.

Depending on the distribution of alloying elements in these phases, the alloying elements can be categorised in three classes:

(i) Elements which Enter Only the Ferrite Phase:

Ni, Si, Al, P, S, Cu can dissolve easily to form solid solution in ferrite. Their solid solubility in cementite or in alloy carbides is very less. These elements also dissolve in austenite easily.

(ii) Elements which Form Stable Carbide and also Dissolve in Ferrite Phase:

Most of the elements like Mn, Cr, Mo, V, Ti, W, Nb fall in this class. At low concentrations, they may form solid solution in cementite as well as form solid solutions in ferrite. At high content of these elements, these may form their own alloy carbides.

For example, Mn dissolves in cementite replacing some iron atoms as (Fe, Mn)3C. Strong carbide forming elements are generally present in amounts greater than that needed in carbide phase, and thus the remainder forms solid solution in ferrite. Some of these elements, notably Ti, W and Mo result in substantial solid solution hardening of ferrite.

The stability of the carbides is dependent on the presence of other elements in the steel and is dependent on how the element is partitioned between the cementite and the matrix. The ratio of the percentage by weight of the element in cementite (carbide) and matrix is called the partition coefficient, K.

Manganese carbide is generally not found in steels, but is a strong carbide stabliser. Chromium is used in practice as a strong carbide stabliser. Steels contain Cr to counteract the effect of Si or Ni. Cr is added to prevent growth of cast irons. Cr is not added in white cast iron used for malleable cast iron production.

(iii) Elements which Enter Predominately the Carbide:

Nitrogen forms carbonitrides with iron and with many alloying elements. Ti, Al are strong nitride forming elements, and these form separate alloy nitrides.

(iv) Elements which Form Nitrides:

It is important to classify elements in this category due to the importance of case-hardening of steel called, nitriding. All carbide forming elements are also nitride forming elements. Nitrogen is one element which enters the carbide phase and forms carbo-nitrides with iron and alloying elements.

The strong nitride-forming elements like Al, Ti, Cr, Mo ( < 1.5%) form separate alloy nitrides, which when finely dispersed in steel, block the motion of dislocations, resulting in high hardness (VPN 1100) of steel.The effect of alloying elements on hardness during nitriding. Ni is not a nitride-forming element and thus, does not increase hardness.

Most of the elements dissolve in ferrite to varying extent. The first five elements (and the last two) have no tendency to form carbides. The next seven elements have tendency to form carbides, the potency being indicated by (×). Al and Si act as deoxidisers, which control the grain size. Al-killed steel becomes inherently fine grained, while silicon-killed steels are inherently coarse-grained.

Even presence of carbide keeps the steels fine-grained. Elements dissolved in austenite, increase the hardenability of the steel. Intermetallic compounds increase the hardness. Elements dissolved in ferrite and austenite strengthens the steels. Insoluble elements like Pb increase the machinability.

Distribution of Elements in Austenite and Carbides:

Non-carbide forming elements (Ni, Si, Cu, P, S, Al) dissolve in austenite and not in carbide. Of the carbide-forming elements (Mn, Cr, Mo, W, Ti, Nb, V), Mn dissolves in cementite in which it substitutes the iron atoms to form the carbide (Fe.Mn)3C.

Manganese carbides are not seen in steels. Cr in low concen­trations is austenite-stabiliser, but in high concentrations is ferrite-stabliser. Cr preferentially dissolves (Cr > 2%) in to carbide phase (over austenite/ferrite), in cementite as (Fe.Cr)3C and with increased chromium con­centration, it forms two carbides, M7C3 and M23C6.

Molybdenum is a strong-carbide forming element and is seen to be present, apart from in cementite, as alloy carbides, M23Q6, M2C, M6C. Tungsten too is seen apart from in cementite, as MC and M6C carbides.

A small amount of tungsten replaces cementite by its own alloy-carbide, and the austenite phase is severely restricted. Vanadium, niobium and titanium which have no solubility in iron, and have hardly solubility in austenite, form respectively V4C3, NbC and TiC. Cementite is replaced by their carbides at small concentration of these elements. Solubility of NbC and TiC in austenite is even less than that of V4C3.

Effects of Alloying Elements:

When the total alloy content in iron is small, 5-10%, the steel is called low-alloy steel, but when the alloy content is more than 10%, it is called high-alloy steel.

The alloying elements may be present in the steel in the following forms and affect the steels:

1. They may form substitutional solid-solutions in ferrite or austenite, resulting in solid-solution strengthening effect of some of them on ferrite. Structural steels, because of their physical dimensions cannot be heat treated and, are used in hot rolled conditions, can be strengthened by solid-solution-hardening particularly, the weldable type of steels, where manganese content may be increased to 1.3-1.7% (with carbon 0.2-0.4%).

The alloying elements when present in solid-solution state produce ferritic or austenitic steels. Normally, expensive alloying elements are rarely used to derive only solid-solution-strengthening.

2. They may dissolve in carbide. Mn may dissolve in cementite to form alloyed cementite, (Fe Mn)3C. Even non-carbide forming element like silicon enter in epsilon (e) carbide to stablise it, to be present at tempering temperature of 400°C if silicon is in amounts 1-2% in steel.

3. Alloying elements may form their own carbides or nitrides like NbC, VN, WC, Cr23 C6, V4C3 etc., if sufficient carbon is present in steel. Niobium and titanium if present in amounts greater than 0.25% each, vanadium higher than 1-2%, molybdenum up to about 5% form their own carbides. These fine-carbides and nitrides are responsible for maintaining fine-grain size of the steels (thus toughness), and in tool steels, give a good cutting edge. Finely-dispersed precipitated carbides in steels are responsible for red-hardness of high speed steels-say 18.4.1 (W/Cr/V) with 0.7% carbon.

4. They may form intermediate phases like sigma phase FeCr in stainless steels-a brittle phase. They may form intermetallic compounds like Ni₃Ti, NiAl etc.

5. They may form non-metallic inclusions such as oxides, silicates, sulphides such as AI₂O₃, MnSiO₃, MnS etc.

6. They may be present as insoluble metals like copper or lead.

7. Alloying elements may influence the critical range in Fe-F₃C diagram in one, or more of the following ways:

(a) Change the Carbon Content of Eutectoid :

All the elements lower the eutectoid carbon content. Titanium and molybdenum are the most effective in lowering it. For example, a steel with 5% Cr has its eutectoid point at 0.5% C as compared to 0.77% in carbon steels. High speed steel (18/4/1) has eutectoid point at 0.25% carbon.

(b) Change the Eutectoid Temperature:

The eutectoid temperature in plain carbon steels is 727°C. Elements like Ni, Mn. i.e., the austenite stabilisers lower this temperature. Ferrite stabilisers (Cr, V, W etc.) raise the eutectoid temperature above 727°C as the concentration is increased. Ti, and Mo are most effective in raising the eutectoid temperature. For example, 3% Ni lowers the A1 temperature by 30°C. High speed steel (18/4/1) has eutectoid temperature raised from 727°C to 840°C. It is to be noted that manganese and nickel are the only common elements that lower the eutectoid temperature and all others raise it.

Fe-C-X Phase Diagrams:

The effect of alloying elements on the important austenite phase boundaries of Fe-Fe3C system is illustrated in Fig. 1.49 (a) & (b) using binary plots. Fig. 1.49 (a) illustrates effect of one of the austenite stabilisers, i.e. manganese. It illustrates Ac3 and Acm lines for different manganese contents. The dotted lines indicate normal positions in Fe-Fe3C diagram.

As the manganese content increases, the austenite region is enlarged, the eutectoid temperature is lowered and, the eutectoid carbon is shifted to lower contents that mean, 100% pearlite is obtained at lower carbon content in these alloy steels, leading to greater ductility in them. As the eutectoid temperature is lowered, the quenching can be done from lower temperatures, reducing the stresses which otherwise cause distortion and cracks.

Fig. 1.49 (b) illustrates the effect of one of the ferrite stabilisers-chromium. As its concentration increases the austenite field shrinks and ultimately it vanishes. The ferrite and δ-ferrite regions merge. The eutectoid temperature is raised, but the eutectoid carbon content is lowered. If an alloy containing 19% Cr, and 0.55% C is to be given heat treatment, it has to be raised to an almost fixed temperature ≈ 1225°C to form austenite.

With chromium greater than 19.5%, the austenite region disappears completely. Carbon being an austenite stabliser, requires the presence of higher amount of ferrite stabilising elements as compared to when carbon is absent. For example, 19.5% chromium is needed for Fe-C-Cr as compared to 12.7% chromium for Fe-Cr steels.

Andrews suggested the equation for Ac, for alloy steels as:

Ac₃ = 910 – 203 √% C- 15.2 (% Ni) + 44,7 (% Si) + 104 (% V) + 31.5 (% Mo) + 13.1 (% W) – 30 (% Mn) + 11 (% Cr) + 20 (% Cu) – 700 (% P) – 400 (% Al) – 120 (% As) – 400 (% Ti) … (1.43)

Andrews has given now another equation for steel compositions up to 0.7% C, 3% Mn, 0.87 Si, 5% Ni as:

Ae₃ (°C) = 913 – ΔT – 25 Mn – 11 Cr – 20 Cu + 60 (si %) + 60 (Mo %) + 40 W + 100 V + 700 P … (1.44)

where ΔT accounts for (C + Ni/10) combined, which varies from 24 for wt% of 0.05 to 173 for 0.70%.

Andrew’s equation for Ac₁ is:

Ac₁ (°C) = 727 – 10.7 (% Mn) – 16.9 (% Ni) + 29.1 (% Si) + 16.9 (% Cr) + 290 (% As) + 6.38 (% W) …(1.45)

(c) Effect on ‘Critical Cooling Rate‘:

The process of hardening depends on the cooling with sufficient rapidity to obtain microstructure consisting largely or entirely of martensite. There is a critical cooling rate that must be equalled or surpassed if the steel is to transform to martensite. This critical cooling rate depends on the carbon and the alloying elements, and grain size.

The maximum time allowed (to cool from Ae₃ to 500°C) if marten site is to be obtained C_B, is given by:

log C_B = 3.725 (%C) + 0.046 (% Si) + 0.626 (% Mn) + 0.706 (% Cr) + 0.520 (% Mo) + 0.026 (% Ni) + 0.675 (% Cu) – 1.818 …(1.46)

Of the alloying elements (apart from carbon), chromium and manganese are most effective (in that order) in decreasing the cooling rate, i.e., a thicker section can be hardened easily, i.e. hardenability is increased. All the alloying elements except cobalt shift the S curve of the steels towards the right.

The basic principle of increasing the hardenability is “slowing the time-dependent processes of nucleation and growth (i.e. austenite to ferrite, pearlite or bainite), the greater the time available to reach the Ms temperature and the larger the cross-section that can be transformed to martensite.Alloying elements also lower Ms, (start of martensite formation) temperature.

(d) Effect of Alloying Elements on Volume Change:

Structural change when martensite forms from austenite (volume also changes due to change of temperature) leads to increase of volume, which induces (structural) stresses, causing distortion and cracks. The volume change is effected by the presence of alloying elements. Choosing proper alloying elements, to reduce the volume increase, reduces the risk of cracking of the steel-parts during quenching.

(e) Effect on Grain-Growth:

Strong carbide and nitride forming elements play an important role in limiting the grain growth during heating. Al-killed steels, the inherently fine-grained steels are, universally used as case-hardening steels as compared to Si-killed steels. In the former-steel, A1N particles in fine dispersion are present at austenite grain boundaries and, thus render the steel resistant to grain-growth up to a temperature of heating of 950-1000°C (by particle drag-effect).

Vanadium too, even in small con­centrations of around 0.1%, prevents grain growth in steels on being heated for heat treatment. Vanadium as carbide (also as nitride) is present uniformly and finely-dispersed.

High temperatures are needed to dissolve them and, thus in normal hardening-temperature range, they act as inhibitor to grain-growth. High speed steel (18/4/1) can be heated to 1260-1290°C and still sustain a fine- grain size of austenite. Titanium and Niobium also act in similar manner in other high-alloy steels, or even in HSLA steels.

(f) Effect on Resistance to Softening on Tempering:

The hardened plain carbon steels soften rapidly with increasing tempering temperature. If a non-carbide forming element like silicon is added to 0.5-0.55% C steel, the effect of increasing tempering temperature and the silicon-content is illustrated in Fig. 1.53 (a). (There is a substantial retarding effect on softening due to dissolution of silicon in epsilon-carbide). Silicon inhibits the transformation of ε-carbide to cementite. Even, nickel has very small effect in increasing resistance to softening but a constant effect on tempered hardness at all temperatures due to weak solid-solution-strengthening effect.

Most effective elements in retarding softening are strong carbide-forming elements such as chromium, molybdenum or vanadium. If present in sufficient quantity, carbide-forming elements not only retard softening but also form fine alloy-carbide dispersion that produces an increase in hardness at high tempering temperatures. This increase in hardness is frequently called as secondary hardness. Fig. 1.53 (b) illustrates the effect of addition of molybdenum on hardness of 0.35% carbon steel due to tempering temperature.

Upto 0.50% molybdenum, there is no peak in curve, but significant retardation of softening is seen. Secondary hardening takes place at higher temperatures as diffusion needed of alloying elements takes place at 500°C, or more to form fine dispersion of carbides. Alloy-carbides resist softening up to 550°C or so, and later the finely dispersed particles start coagulating to show a drop in hardness, after the peak in curve.

(g) Effect on Corrosion and Oxidation Resistance:

Alloying elements like chromium, aluminium and silicon make the steel resistant to oxidation and corrosion, though chromium is most outstanding. A minimum of 12% chromium is needed for protection against oxidising atmospheres.

The amount of chromium needed, has to be increased to give resistance to scaling at high temperatures as the temperature of application increases. Stainless irons (virtually free of carbon) having 13% chromium are ferritic, but are used for furnace components.

Cutlery steels require high carbon to get hard martensite to have sharp, hard cutting edge and thus, are made of 0.6-0.75% carbon with 17-18% chromium. In 18/8 austenitic stainless steel (0.1% C), addition of nickel further improves corrosion resistance apart from converting the alloy steel to metastable austenite (FCC) to impart ductility, toughness and excellent cold-working properties, and the steel finds use in kitchen wares, surgical instruments and in chemical plants.

(h) Effect on Strength by Precipitation-Hardening:

Precipitation-hardening in steels is mainly by precipitating carbides as explained in section (f) above. Nitrides have also been used to increase hardness such as in nitralloys by nitriding heat treatment. Intermetallic compounds have been used for precipitation hardening in austenitic steels. In Fe-Cr-Ni alloys, titanium and aluminium are added. Intermetallic compounds such as NiAl, Ni3 Ti, Ni3 (Al, Ti) and Ni (Al, Ti) precipitate at 700-750°C, yielding strengths in the range of 700-1000 MNm-2 with good ductility and toughness. Even, marageing steels having 20-25% nickel (very little carbon) are air cooled to martensite.

Ageing causes hardening by precipitation hardening due to Mo-rich, or Ti- Ni intermetallic compounds, producing yield strength in the range 1400-1700 MNm^-2 with good toughness. These alloys have been used mainly in rockets and high-speed aircrafts.

Characteristic Effects of Elements:

1. Manganese:

Plain carbon steels invariably have 0.5 to 0.8% manganese due to the steel making practice, as it acts as deoxidiser and, takes care of the harmful effect of presence of sulphur. Manganese forms MnS, which is present as large solid globules irregularly distributed and does not allow FeS to form, which otherwise, would have been present as a low melting eutectic (with iron), predominantly present as a film at grain boundaries. Free cutting steels may have 1.5% manganese (and 0.2% sulphur) which as MnS, increases machinability of the steels.

The excess manganese, may dissolve in ferrite as well as form (Fe Mn)3C with cementite. It thus, increases yield strength, tensile strength, hardness and toughness. Manganese, being least expensive of the alloying elements, is, now a days, increased in steels to get the equal tensile strength with increased ductility by reducing carbon content of the steels. Structural steels invariably have increased manganese. These steels are also used for gears, spline-shafts, axles, rifle barrels.

Manganese increase the hardenability of steels, but causes quench cracking. Thus, water hardening high carbon steels has less than 0.5% manganese. Manganese steels with more than 0.6% carbon suffer from temperembrittlement. Higher manganese content (≈ 1.5 %) is used to advantage in steels having up to 0.4% carbon and, in oil-hardened and tempered state.

When manganese is higher than 1.8%, the steel tends to become ‘air-hardening’, causing loss of ductility. Being cheaper, manganese is used to replace expensive nickel in some expensive alloy steels. Manganese is weak carbide former. Non-shrinkage tool steels contain up to 2% manganese and 0.8 – 0.9 % carbon.

Manganese lowers A3 and A1 temperatures, i.e. is an austenite stabliser. Steels with more than 10% manganese become austenitic. Hadfield steel has 12-14% manganese and 1% carbon. It is heated to 1080°C and then, water quenched to retain tough and ductile austenite.

Slight abrasion to the surface layers of the component, made out of this steel, transforms a thin layer of austenite to martensite (hardness of surface increases from 200 VPN to 600 VPN), while the underlying core is still tough austenite. Such steel has high wear and abrasion resistance with excellent toughness. Hadfield steel is used to make railway crossings, rock-drills, Jaw plates for stone-crushing, power-shovel buckets and teeth.

2. Nickel:

Nickel resembles manganese in some respects, such as both lower A3 and A1 temperatures and tend to stabilise austenite. The carbon content of eutectoid point is also decreased, i.e. for the same carbon content, nickel steel has higher pearlite content (Ni dissolves in ferrite of pearlite which makes it stronger). Thus, steels can attain a given strength at lower carbon content with increased toughness and ductility as compared to plain carbon steels. Huge steel sections, which cannot be hardened, can be made of nickel steels, such as structural sections, which are used in hot rolled state.

Large forgings, bridge structures, railway axles are also made of nickel steels. Carburising steels (0.1% carbon) contain 2-5% nickel. Low nickel hardenable steels have superior properties in hardened and tempered state. Steels, having nickel higher than 5%, have increased resistance to corrosion and oxidation. Higher nickel contents produce austenitic steels, which are soft, ductile, tough and non-magnetic.

Nickel dissolves in ferrite (also in austenite), causing solid solution Strengthening to increase hardness, strength without drop in ductility. High strength and tough steels have around 5% Ni. Guillet diagram (Fig. 1.54) illustrates the effect of carbon and nickel in a simple way on the micro-structure obtained in steel for one rate of cooling. The basic idea is based on the effect of lowering A1 temperature and shifting of ‘S’ curve of the steel towards right as the nickel content is increased.

Steels with 0 to 5% nickel have peartitic structure similar to plain carbon steels, but ‘S’ curve being more towards right, transformation of austenite occurs at a lower temperature to yield a finer pearlite. The ferrite of pearlite has been strengthened due to dissolution of nickel in it.

As the eutectoid carbon has also decreased, the amount of pearlite is more. All these factors increase strength with increased toughness and ductility. Same rate of cooling of steel having 10% Ni, yields martensite in structure to give high hardness and strength but reduced ductility. Still higher contents of nickel retain austenite at room temperature.

Steels with 3.5% Ni (0.1% C) are used for carburising for applications like drive gears, connecting rod, bolts, studs and pins. 5% Ni steels (carburised) are used for heavy duty applications such as bus and truck gears, cams, crank-shafts. These parts have good toughness. The increase in hardenability due to the addition of nickel induces outstanding toughness particularly at low temperatures.

Nickel reduces coefficient of thermal expansion. Invar (36% Ni, 0.2% C, 0.5% Mn) has almost zero coefficient of thermal expansion in working range of 0°C to 100°C. Elinvar (36% Ni 12% Cr is another such alloy. These alloys are used for surveyor’s tapes, gauges, watch parts (hair springs and balance wheels), differential expansion regulators, and in aluminium pistons with a split skirt in order to give an expansion equivalent approximately to that of cast iron.

3. Chromium:

Chromium is less expensive than nickel and. is a strong carbide forming element. It is added to alloy steels, or alloy tool steels as a cheaper substitute to increase hardenability (4% Cr in high speed steel, 18/4/1). It dissolves in alpha as well as gamma iron to increase strength and toughness.

Chromium also forms carbides like (Fe Cr)3 C, Cr7C3, Cr23, C6. The fine dispersion of carbides increases hardness and wear resistance. These carbides are responsible for the resistance to softening on tempering as well as secondary hardening in chromium steels.

Steels with 0.15% C and 0.7-1.15 Cr are normally carburised. Chromium (as carbides) increases the wear resistance of the carburised case. Medium carbon chromium steels are oil hardenable (chromium increases hardenability) and, have been used for making springs, engine bolts, studs, axles, etc. As chromium reduces the eutectoid carbon content, more carbides form in high carbon steels (1% C, 1.5% Cr). Proper heat treatment produces these carbides in globular form, which is more suitable for applications like ball bearings and for-crushing machinery. A steel with 1% C and 2-4% Cr is used for making permanent magnets.

Chromium in amounts greater than 12% in low carbon steels forms, probably, an inert passive (Cr2 O3) oxide film on the surface of the part to resist further attack by oxidising atmospheres. Heat resisting and scale free steels require addition of increasing chromium content as the temperature of application of the part increases. Cutlery steel and surgeons blade needing sharp cutting edge has around 0.7% carbon and 17% chromium.

Chromium steels are susceptible to temperembrittlement, i.e. such steels when tempered in range 500-575°C, show intergranular brittle fracture, because the transition temperature is raised due Jo co-segregation of Sb, Sn, As with Ni, Cr, Mn. Chromium steels also show surface markings generally called ‘chrome lines’. Chromium steels find large applications due to wear resistance of chromium carbides, for example for dies, rolls, files, tools, plates for safes.

4. Nickel-Chromium:

The effect of nickel in imparting strength, ductility and toughness is combined with the effects of chromium in improving hardness and wear resistance with much superior hardenability such that a steel having 4.5% Ni, 1.25% Cr and 0.35% C becomes an air hardening steel.

Low carbon chromium-nickel steels are used for case-carburising as chromium provides wear resistance (due to hard chromium carbides) of the case and, both the elements, specially nickel improve the toughness of the core. Steel haying 1.5% Ni, 0.6 Cr is used for worm gears, piston pins. A steel with 4% Ni, 1.2% Cr, 0.2% Mo is used for crown wheels, bevel pins, aero-reduction gears, shafts, cams. Nickel-chromium (8/18) steel, the commonly used austenitic stainless steel (0.1% C max.) resist common corrosion. Ni-Cr steels are also used for high temperature applications. Nickel-chromium steels suffer from temperembrittlement.

5. Molybdenum:

Molybdenum is a ferrite stabliser (dissolves in a-as well as in γ-Fe) and relatively a strong carbide forming element and thus, apart from (Fe Mo), C, forms alloy carbides Mo₂₃ C₆, Mo₂ C, Mo₆ C.

The main important functions of adding molybdenum in steels are:

i. It is one important element which decreases temper-brittleness and is thus, invariable present, in Ni-Cr, Cr-Mn etc. steels (which suffer from temper-brittleness) in amounts 0.5%.

ii. It increases hardenability remarkably, even in amounts 0.5%, to make steels deep hardening. ‘S’ curve of the steel is shifted towards right-more of the pearlitic part than the bainitic part. When sufficient carbon is present in steel, and amount of molybdenum is 5%, the alloy carbides cannot be completely dissolved in austenite at the highest austenitising temperatures. Alloy carbides do keep the grain size fine. MoC particles present in finely dispersed state in the material enhance the high temperature (creep) resistance of steels.

iii. It increases the resistance to softening on tempering [Fig. 1.53 (b)]. Plain carbon steels with 0.5% Mo could be used for boiler parts for temperatures up to 400°C. Molybdenum causes secondary hardening in alloy steels and thus, tool steels contain molybdenum. Molybdenum carbides increase wear resistance. Molybdenum increases wear resistance of carburised case and toughness of the core, thus, is present in steels used for transmission gears, spline shafts.

Chrome-molybdenum steels are used for pressure vessels, air-craft structural parts, automobile axles. Ni-Mo steels, combining the effects of Ni (strength and toughness) and Mo (deep hardening and wear resistance) are used for transmission gears, chain pins, shafts and bearings. Ni-Cr-Mo are used for turbine rotors, structural parts of wing assembly, fuselage and landing gears.

Molybdenum is a bit expensive element. Heal and corrosion-resistant steels have molybdenum.

6. Tungsten:

Tungsten is an expensive metal. It dissolves in ferrite and austenite, but is a strong carbide former and forms WC and W6C carbides, which increase the wear and abrasion resistance, apart from maintaining fine grain size of steel. The dissolution of carbides in austenite requires higher temperatures of heating. The transformation of alloyed austenite in such steels becomes very sluggish to get martensite even by air cooling.

Hot-worked tools and die steels contain tungsten as it leads to resistance to softening on tempering (when present in small amounts), and causes secondary hardening. Being a strong carbide former, it reduces the tendency of steel to decarburisation and does not allow the carbon to diffuse away from it. Tungsten is a part of corrosion and heat-resisting steels.

7. Vanadium:

Vanadium is a ferrite-stabliser and a strong carbide former, forming V4 C, and VC. Vanadium acts as expensive deoxidiser and produces inherently fine grained steels even in amounts 0.1% as, V4 C, particles act as grain size refiner by drag effect. Thus, some carburising steels have vanadium (vanadium carbide shall increase the abrasion and wear resistance of carburised case).

When the amount of vanadium is 1-2% (with enough carbon in steel) its carbides cannot be dissolved completely at the highest solutionising temperature. High speed steels, normally have 1% V (18/4/1). Vanadium resists softening during tempering and produces finely dispersed carbides to give secondary hardening with hardness of Rc 65-66.

The super high-speed steels having 5% vanadium attain a hardness of Rc 72, which increases the life of the tools much more than the common high speed steels. Vanadium improves the hardenability of steels. Vanadium carbide enhances the high temperature (creep) strength by providing a fine but stable dispersion of particles in matrix. Leaf springs, coil springs, heavy duty axles, gears, valves, pinions, torsion bars use steels having vanadium.

8. Titanium:

Titanium is a ferrite stabliser and is a very strong carbide as well as nitride forming element. It forms Ti C which increases the wear and abrasion resistance of the steel. It is a strong grain refiner as titanium as small as 0.25% as Ti C does not dissolve completely in austenite. Titanium, being stronger carbide forming element, is added in stainless steels to prevent chromium to form chromium carbides which otherwise would have decreased the corrosion resistance particularly, during welding to cause ‘weld decay’.

9. Cobalt:

Cobalt dissolves in ferrite and austenite. It reduces the hardenability but increases resistance to softening on tempering at high temperatures by hardening ferrite. Cobalt is added in tool steels, high temperature materials, permanent magnets and acts as a bonding base for cemented carbide particles. Gas turbine steels have cobalt.

10. Boron:

Boron has no solubility in ferrite and, solubility in austenite is .001% at 912°C which increases to .005% at eutectic temperature. Thus, for getting the maximum boron effect, it is present in range 0.0005% to .003%. Main reason of adding boron in steels is to increase hardenability. Boron increases hardenability in hypo-eutectoid steels, no effect on eutectoid steel and decreases hardenability of hypereutectoid steels. As the austenite grains become finer, hardenability effect of boron increases.

Boron is largely used in C-Mn steels having 0.15% to 0.40% carbon and 0.80 to 1.65% Mn as these steels have superior cold-forming properties, less tendency to crack on quenching and better machinability. Molybdenum-boron form a useful group of high tensile bainitic steels. Boron is present in hard facing alloys and in nuclear control rods.