Is Steel an Alloy | What is Alloy Steel | Example Alloys |Properties Alloy Steels

Is Steel an Alloy | What is Alloy Steel | Example Alloys |Properties Alloy Steels

Is Steel an Alloy | What is Alloy Steel | Example Alloys |Properties Alloy Steels

Is Steel an Alloy | What is Alloy Steel | Example Alloys |Properties Alloy Steels

Is Steel an Alloy

Steel is an alloy of iron and carbon and has a carbon content proportion of up to 2%. When elements metals and non-metals are combined with carbon steel, then alloy steel is formed. Various alloy steel types have different chemical, environmental, and physical properties, which vary with the specific elements used to alloy. The proportion of alloying elements produce varied mechanical properties.

Alloy Steels

What is Alloy Steel

An alloy steel can be defined as a type of steel with one or more alloying elements in addition to carbon. The alloying elements are added in varying amounts and/or in combination with one another to modify the properties of the steel. Alloy steels cover such a wide range of alloys that the classification is very difficult.

Example Alloys

Examples of Alloys include materials such as;

  1. Amalgam (Dental)
  2. Brass
  3. Bronze
  4. Duralumin
  5. Pewter
  6. Solder
  7. Solder (Lead Free)
  8. Steel
  9. Steel (Stainless)
  10. Steel (Tool)

 Examples of Alloys (Table)

Alloy Example

Elements of Alloy Example

Proportion and Details

Amalgam (Dental)silver, mercury, copper, tinAmalgam, Hg content is about 50%, Ag is 22-32%. About 1% Zn may also be added.
BrassCopper and zincCu content is about 65% in the softest brasses and 35% in the hardest brasses. May also contain Pb, Si, Sn, Fe, Al, and Mn.
BronzeCopper and tinSn content is about 12%. Other elements such as Al, As, Mn, Ni, P, Si, and Zn may be added to improve strength or ductility.
Duraluminmagnesium, copper, aluminum, manganese,Al content is 95%, Cu 4%, Mg 0.5%, and Mn 0.5%. May also contain Si. The alloy is stronger than pure Al for structural use in cars, aircraft, etc.
Pewtercopper, tin, antimony, bismuthSn content is 85-99%. Lead and silver may also be added.
SolderTin and leadSn content is 5 – 70%. More Sn leads to higher shear and tensile strength. For specialist applications, other elements including Bi, Ag, and Zn can be added.
Solder (Lead Free)zinc, Tin, copperSn content is 50 – 95%. Cu may be replaced by Ag.
SteelCarbon and IronC content is usually lower than 2%.
Steel (Stainless)chromium, iron, carbonCr content is at least 12%; Ni may also be added to inhibit corrosion.
Steel (Tool)carbon, iron, manganese, chromium, vanadium, tungsten, siliconC content is 0.6 – 1.3%. Other elements vary from 0 – 2%.

 

Types of Alloys Steel

Broadly alloy steels can be classified into three categories, namely:

Low Alloy Steels

Low alloy steel is one that possesses micro-structure and requires heat treatment similar to carbon steel. These generally contain 5.0 percent of one or more alloying elements to improve the strength, toughness, and hardenability. The low alloy steels are generally used for an application similar to plain carbon steel, having the same amount of carbon content.

Medium Alloy Steels

  Contain alloying elements between 5-10 percent.

High Alloy Steels

High alloy steels possess micro-structures and require heat treatment different from those of plain carbon steel. High alloy steels contain more than 10 percent alloying elements.

Properties Alloy Steels

Based upon the applications in the engineering industry, alloy steels are known by the following names: –

Stainless Steel

 Stainless steel derives its name from the remarkable resistance it has to atmospheric corrosion. This is due to the alloying element chromium, which forms a protective oxide film over the metal surface. Stainless steels are low carbon steels containing at least 13.0 percent chromium with or without other alloying elements such as Nickel, manganese, molybdenum, silicon, phosphorus, sulfur, nitrogen. The very common types are SS 304 and SS 316. According to the structure, there are three types of stainless steel, namely.

 Martensitic Stainless Steel

These are magnetic. The chromium content is limited to 13 percent, the carbon content 0.1 to 1.5 percent. With more chromium content, the steel becomes ferritic. When Nickel has added it becomes austenitic. They can be hardened by heat treatment.

 Ferritic Stainless steel

It contains 13-27% chromium and less than 0.1 percent carbon. They are magnetic and ductile and therefore, can be drawn easily. They cannot be hardened by heat treatment but may be strengthened by work hardening.

 Austenitic stainless steel

Contains chromium and Nickel as the main alloying elements. They are non-magnetic in the fully annealed condition, but many become slightly magnetic during cold working. They have excellent corrosion resistance because of their higher chromium content. They are tough and ductile. The most important austenitic stainless steels are:

(a) 18: 8 stainless steel (18.0% chromium and 8% nickel)

(b) 18:10:3 stainless steel (18% chromium, 10% nickel and 3% molybdenum).

The carbon content is kept less than 0.15 percent minimize the formation of chromium carbides in the structure, as this would result in reduction in corrosion resistance. Carbide may form in these steels if they are allowed to cool slowly from high temperature or if they are reheated in the range 500-7000C. As this latter condition may apply in the heat-affected zones adjacent to welds, the type of corrosion failure that can occur owing to the presence of carbide particles is known as weld decay.

Austenitic steels that are required for welding contain small stabilizing additions of titanium or niobium. These stabilizers prevent intercrystalline corrosion, weld decay.

Tool and Die Steels

The essential requirement for the tool and die steel is that they must be hard. In the case of cutting tools, they retain hardness and cutting edge at high speed and at high temperature due to friction. The plain carbon tool steels are either oil or water hardened, having 0.5 to 0.8% and 0.2 to 0.3% manganese, respectively. IF the manganese in water hardened tool steels exceeds 0.35%, they are liable to crack during hardening. The harmful impurities like sulfur and phosphor are restricted to a minimum. Tool and die steels are classified into the following based upon quenching methods and other special characteristics.

High-speed steels:

They are either tungsten or molybdenum type. Cobalt is added in certain cases to improve cutting qualities. They are the best-known tool steels. They possess high hardness, high compression strength, and outstanding wear resistance in addition to heat resistance at high speeds. Cobalt is added to increase cutting efficiency at high temperatures. Molybdenum grade high-speed steels are used for drilling and tapping operations. Tungsten high-speed steels are used for all-purpose tool steels.

Cold work tool steels:

They possess high wear resistance and hardenability, average toughness, and resistance to heat softening. Oil hardening tool steel possesses good machinability as compared to high carbon high chromium steels.

Special-purpose tool steels:

 They contain low carbon, low alloy, carbon-tungsten percentage and are used for special-purpose tools. Composition of widely used high-speed tool steels is given in

(a)            Maraging steels: Maraging steels contain 18% nickel, 7% cobalt, and small amount of other elements such as titanium. The carbon content is generally less than 0.05%. These are very high strength materials that can be hardened to give strength up to 1900 N/mm2. To produce a uniform austenite structure to develop maximum properties, it requires solution treatment at 800-8500C, followed by a rapid quench.

After solution treatment, they are soft enough to be worked and machined with comparative ease. It is used for the manufacture of undercarriage components for aircraft. These steels have good weldability.

(b)            Hadfield’s manganese steel: It contains 12-14% of manganese and 1-0% of carbon. The high manganese content makes this steel austenitic at all temperatures and non-magnetic. It has a high resistance to abrasion. It is therefore used for pneumatic drill bits, rock crusher jaws, excavator bucket teeth, railways points, and switches.

(c)             Heat-resisting alloys: Heat-resisting alloys are used to avoid plastic deformation creep during continuous loading and in corrosion processes such as oxidation above 5400C temperature and attack by flue gases containing sulphur dioxide and hydrogen sulphide.

By alloying steel with chromium and silicon, resistance to corrosion can be improved because of the hard protective film formed on the surface of the alloy. Creep resistance is improved by the addition of molybdenum, tungsten, and vanadium.

(d)     Magnetic alloys: There are two types of magnetic alloys, namely – hard and soft. The hard type is used for permanent magnets, while the soft type is used for transformer cores, motor, and generator armatures. Hard type contains chromium, tungsten, cobalt or Nickel, aluminum as alloying elements. Soft gets magnetized quickly and has high magnetic permeability.

(e)   Spring steel: Spring steel possess a high elastic limit, good elongation, and high fatigue resistance. They are used either in cold rolled condition or after heat treatment. Two types of spring steels are used for this purpose, namely:

  • Medium and high carbon steels
  • Medium carbon alloy steels with manganese, silicon, and other alloying elements

(f)   High strength low alloy steels: These are used for structural members of the bridge, buildings, railroad, cars, etc., They possess high strength, toughness, resistance to corrosion, good weld ability, and workability, which is due to their very fine grain size, i.e., approx. 10 micrometers. This is achieved by the highly controlled addition of alloying elements that combine during the hot rolling of cast metals to produce an extremely fine distribution of tiny precipitates.

These precipitates are very stable intermetallic compounds usually based on at least two metallic components such as niobium, aluminum, titanium, or vanadium and two non-metallic components like carbon and nitrogen. These low alloying additions comprise only about 0.15 percent of the total weight of the steel. The mean size of the particles is about 50 atoms in diameter. They can be seen only under a powerful microscope.

(g)    Concrete reinforcement steelIt is available plain as well as tor-steel. Tor-steel possesses high yield strength and greater surface area, which results in higher bond strength than the plain steel bars.

(h) Rail steel: Rail steel possesses good strength ductility, high impact, and fatigue resistance. Medium carbon steels with heating treatment having manganese and chromium up to 1.0% are used for this purpose.

 Bearing Alloys

The selection of materials for sliding and rolling contacts depends on the matching of materials properties with demand for low friction and wear in the machinery involved. Properties of lubricants combined with mating bearing materials determine the level of frictional forces and the nature of wear processes. For compatibility, good bearing material must have:

(i) Hardness and modulus of elasticity or oil film and boundary lubricated bearing materials should be as low as possible while providing sufficient strength to carry the applied load.

(ii) Ability to absorb foreign dirt particles to prevent scoring and wear.

(iii) For sliding bearings, materials of intermediate compressive strength are usually desirable.

(iv) Higher fatigue strength in applications where load changes direction, e.g., in reciprocating engines.

(v) Should not be readily attacked by lubricants or any other corrosive media contacting the bearing.

(vi). It should have high thermal conductivity.

(vii). It should retain a thin film of lubricating oil so that there will be no metal-to-metal contact.

(i) White metal alloys:  These are high lead and tin based alloys, which are known as babbits. They offer an unsurpassed combination of compatibility, conformability, and embed ability for use as bearing surfaces. Because of their good rubbing characteristics, babbits are used as a thin surface coating under severe operating conditions, i.e., high loads, fatigue problems, or high temperatures. Babbits are used in electric motors. A typical percentage composition and properties of tin and lead base bearing alloys are given in

(ii) Copper lead alloys:  Copper lead alloys consists of a copper matrix with 20-40% lead dispersed in pockets. The copper lead alloys owe their frictional properties to the spreading of a thin film of soft lead over the surface of harder copper. These alloys are stronger than babbits. These are used extensively in reciprocating engines where high fatigue strength and improved high-temperature performance is required, e.g., connecting rod bearings in I.C engines for automobile, aircraft, commercial vehicles, etc.,

(iii) Bronze bearing alloys: Bronze bearing alloys possess adequate bearing properties, good compressive and fatigue strength, excellent casting, and machining characteristics. Bronze can be grouped into the leaded, tin, and high strength bronzes, each having successively higher hardness and strength. Leaded bronzes are used for bearings in machine tools, home appliances, farm machinery, and pumps.

Tin bronzes are used in high loads, low-speed applications such as trunnion bearings, gear bushings for farm equipment, earth moving machines, and rolling mill bearings.

(iv) Aluminum bearing alloys:   Aluminum bearing alloys usually contain 7% tin and a small percentage of copper and Nickel. Bearings of aluminum alloys have high fatigue strength, excellent corrosion resistance, and high thermal conductivity. They are used in connecting rod and main bearings in automobile and diesel engines, reciprocating compressors.

(v) Silver lead bearings:      A layer of silver, 0.5 to 0.7 mm thick, is electro deposited on the steel backing. Silver has ductility, good figure resistance, and high thermal conductivity. However, it has a high coefficient of friction and is difficult to lubricate.

(vi) Porous metal bearings (Self-Lubricating bearing):    Porous metal bearings are easily shaped to final dimensions and contain their own lubricating oil up to 35% volume. No external source of lubrication is required in the porous metal bearing. They are popular in home appliances, small motors, machine tools, business machines, farms, and other types of equipment. Porous bronze containing 90% copper and 10% tin is a commonly used material. No external lubrication is required for this type of bearings.

(vii) Graphite bearing: Graphite bearings are used extensively with water and other low viscosity fluid in dry operation at temperatures up to 4000C. Common applications are in food, drug, and textile equipment where contamination with oil and grease must be avoided and in chemical pumps where boundary lubrication properties permit operation on low viscosity, non-lubricating fluids.

However, precautions should be observed in using graphite, as the material cracks and chips when struck on the corner are subjected to high thermal, tensile, and bending stresses.

(viii) High-temperature bearing materials: Nuclear systems, supersonic aircraft, guided missiles, gas turbines, and compact compressors have created the need for materials with improved tolerances for higher temperature. Selected graphites and solid film coatings are the usual choices up to 5000C. Hard metals and superalloys such as Mo alloys (TZM), Hastelloy c, Stellite 6, Inconel x, Stellite 19, Triballoy T-400 are some of the materials which are employed up to 8000C. For still high temperatures ceramics such as x-AI2O3, B4C, Si3N4-SiC, Haynes LTIB (19% AI2O3, 59% Cr, 20% Mo, 2% Ti) K162B 64-70% Tin, 25-30% Ni, 5-6% Mo) and graphite are used.

Alloying Elements  and Effect on Steel Properties 

Most of the elements used in alloy steels form solid substitution solutions. This increases the tensile and impact strength of the alloy formed. In general, the alloying elements have the four following effects on the various properties of steel.

Strength and Ductility

An increase in the carbon content increases the strength of the steel with a loss of ductility. Many of the commonly used alloying elements enter into the solution with ferrite and increase the strength of the steel with little or no reduction in ductility. Silicon is an example of such an alloy.

Carbide formation

The alloying elements may form stable, hard carbides or nitrides and increase the strength and hardness. Manganese, chromium, and tungsten have this effect.

 Graphite formation

The alloying elements may cause the breakdown of cementite and result in the presence of graphite in the structure of steel, resulting in the decrease in strength and hardness. Silicon and Nickel are therefore not added with high carbon steels.

 Corrosion resistance

Alloying elements can improve corrosion resistance. Some elements from the oxide layer on the surface of the steel and thus improve the corrosion resistance. Chromium, when added more than 12%, has such an effect.

Critical cooling rate

Most alloying elements reduce the critical cooling rate. The effect of this is to make air or oil quenching possible, rather than water quenching. It also increases hardness.

Machinability

They affect the machinability of the steel. Sulphur is added to improve the chip formation of steel.

 Grain growth

Faster grain growth leads to large grain structure and, therefore, to a degree of brittleness. The slower grain growth leads to smaller grain size and thus improves the properties. Nickel and vanadium decrease grain growth. Chromium increases grain growth. Therefore care should be taken in heat treatment of steels to avoid excessive grain growth.

Transformation temperature

The alloying elements may increase or decrease the alpha to gamma transformation temperatures of iron. Chromium, molybdenum, tungsten, vanadium, silicon, and aluminum increase the temperature at which austenite is formed on heating the steel. This increased the temperature to which the steel has to be heated for hardening. Manganese, Nickel, copper, and cobalt lower the temperature at which austenite is formed on heating the steel. Lowering this temperature means reducing the temperature to which the steel has to be heated for hardening by quenching.

 Alloying Elements and their Effects

Alloying elements can be classified into two groups, namely:

(i) Austenite stabilizers – Ni, Mn, Cu, C, AI.

(ii) Ferrite stabilizers – Cr, W, Mo, V.

Commonly used Alloying Elements

Chromium

Small amount of chromium stabilizes the formation of stable carbides, which are much harder than iron carbides. A large amount of chromium improves the corrosion and heat resistance of stainless steel. The disadvantage of chromium is, it promotes grain growth. Therefore it is most important to heat treat chrome steels to avoid grain growth and brittleness due to grain growth.

Nickel

Nickel improves the corrosion resistance. It promotes fine grains but tends to unstabilize (Graphitise) the carbides. Thus Nickel and chromium work as complementary to each other when used as alloying elements. Hence with the controlled amount of Nickel and chromium, it is possible to manufacture steel that has both stable carbides and fine grains.

  Molybdenum

It raises the creep strength of alloy steels at high temperatures, stabilizes their carbides, improves the red hardness of cutting tools, and minimizes temper brittleness in Ni-Cr steels.

Tungsten

Tungsten promotes the formation of very hard carbides and induces sluggishness into the heat treatment transformation in alloy steels. This helps alloy steels to retain their hardness at high temperatures. This also enables alloy steels and die steels to be hardened by oil or air blast quenching with a corresponding reduction in the cracking and distortion. It is inert and finds its application in Electronic-tuber Electrodes for are welding, electrical contacts.

 Cobalt

Cobalt improves the ability of tool steels to operate at high temperatures without softening. It induces sluggishness in the heat treatment transformation.

 Aluminium

Aluminum forms a homogeneous dense oxide film, which protects the steel from the environment and prevents further corrosion. However, when maximum corrosion resistance is required along with high mechanical strength, alloying elements like chromium, Nickel together provide required properties.

Tantalum

It is inert to practically all organic and inorganic compounds under 1500 except MF, fuming H2SO4, oxalic acid.

Alloying Elements Table

ElementEffects
ChromiumIncrease strength, hardenability. Improves corrosion resistance. Forms hard and stable carbides such as Cr4C; (Fe Cr)3C, (Fe Cr)3 C2
NickelImproves strength, toughness and resistance to fatigue. Lowers the critical cooling rate hence increases hardenability. Promotes fine grains but tends to unstabilise the carbides.
MolybdenumInhibits grain growth. Improves strength and toughness. Improves creep resistance at high temperature. Forms complex carbides Mo2C; (Fe Mo)6C.
TungstenImproves machinability. Retains hardness at high temperature. Produces less tendency to decarburization during working. Raise the critical temperature range. Forms hard and stable carbides like WC, W2C, Fe2W3C, Fe4W2C
VanadiumRestricts grain growth. Promotes formation of carbides. Improves strength and hardness. Enhances hot hardness of tool and die steels. Strong deoxidizer.
SiliconIncreases hardenability. Removes oxygen in steel making. Improves resistance to corrosion and oxidation.
PhosphorusImproves machinability.
SulphurImproves machinability. Causes embrittlement (due to Fe S formation)
CarbonIncreases strength, hardness, wear resistance with increase in carbon content but decreases ductility and weldability.
ManganeseCombines with the sulphur to reduce brittleness. Improves strength and ductility. Improves hardenability by lowering the critical cooling rate. Used as a deoxidizer in steel. Lowers the eutectoid temperature. Forms hard carbides (Mn3C)
TitaniumStrong carbide forming element. 2% Ti renders 0.5% carbon steel un-hardenable. Generally resistant to corrosive effect of seawater, Brine solution.

 

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