Properties and Uses of Carbon Steel | Metals | Industries | Metallurgy

Steel has got a variety of applications for engineering purposes due to the wide range of physical properties obtainable by changes in carbon content and heat treatment. First we shall study about the properties and uses of carbon steels.  

1. Low-Carbon Steels (0.05 to 0.25% C):

These are used where only moderate strength is required together with considerable plasticity. Steels with carbon content between 0.05 to 0.10% are used for sheet, strip, tubing, wire nails etc.; steels with carbon content between 0.10 to 0.20% are used for rivets, screws and parts to be case-hardened. Sheets can be produced by hot-rolling or by cold-rolling process. Cold-rolled sheets have a better surface finish, and improved mechanical properties.

Cold rolling also permits the rolling of thinner gauge material than hot rolling. Sheets for deep-drawing applications are made dead soft in order to have a maximum amount of plasticity; for better finish these should have a relatively fine grain size.

By the small additions of columbium or vanadium, high-strength hot-rolled, cold-rolled and galvanised sheets can be made. Sheets are available in coil form and are extensively used for welding into tubes for construction of furniture, automobiles, refrigerators etc.

2. Structural Steels (0.20 to 0.35% C):

These are available in the form of plates, different sections, and bars in the hot-rolled condition. A uniform strength over a range of section thickness is provided by varying the amount of carbon, manganese and silicon. These have high yield point (3300 kg/cm²) and are suitable for both welded and riveted construction.

3. Boron Steels:

These steels achieved special importance in times of alloy shortages. It is found that as little as 0.005% of boron increases hardenability of steels with 0.15 to 0.60% carbon, but increasing the boron content above 0.005% has adverse effect on hot workability. Boron sheets can replace such critical alloying elements as nickel, chromium, molybdenum and manganese and when properly heat treated, possess physical properties comparable to the alloy grades they replace.

Additional advantages of the use of boron in steels are decrease in susceptibility to flaking, formation of less adherent scale, greater softness in the unhardened condition, and better machinability.

4. Free-Cutting Steels:

These have high sulphur content present in the form of manganese sulphide inclusions causing the chips to break short on machining. Manganese and phosphorus harden and embrittle the steel which also contributes towards free machining. Lead (0.20 to 0.35%) is sometimes added to steel to further improve the machinability.

5. Forging Steels:

These contain carbon between 0.30 and 0.40%, and are used for axles, bolts, pins, connecting rods, crank shafts etc. These steels are readily forged and after heat-treatment, develop considerably higher mechanical properties than low carbon steels.

6. Tool Steels (0.60 to 1.30% C):

The selection of proper tool steel depends on the purpose or operation it is supposed to perform i.e. cutting, shearing, forming, drawing, extruding, rolling etc. Each of these operations requires in the tool steel a particular physical property or a combination of such metallurgical characteristics as hardness, strength, toughness, stability, wear resistance and resistance to heat softening.

Other important factors requiring due consideration in selection of proper tool steel are hardenability, permissible distortion, surface carburisation during heat-treatment and machinability. Tool steels have been identified and classified by SAE and AISI into six major groups, based upon quenching methods, applications, special characteristics and use in specific industries. These six classes are water-hardening, shock resisting, cold-work, high-speed and special-purpose tool steels.

Water-hardening tool steels (0.70 to 1.30% carbon) are widely used because of their low cost, good toughness and excellent machinability. These are shallow-hardening steels unsuitable for non-deforming applications because of high warpage and possess poor resistance to softening at elevated temperatures. These are used for files, twist drills, shear knives, chisels, hammers and forging dies.

Shock-resisting tool steels contain alloy combinations of chromium-tungsten, silicon-molybdenum, or silicon- manganese. These have good hardenability with outstanding toughness and wearing qualities. Their disadvantage is the tendency to distort easily, which can be minimised by oil quenching. The most common type has 0.6% carbon, and tungsten, chromium, or vanadium.

Cold-work tool steels are further classified as oil- hardening; medium-alloy air hardening; and high-carbon, high chromium. These possess high wear resistance and hardenability, develop little distortion, but at best are only average in toughness and in resistance to heat softening. Machinability varies from good in the oil-hardening grade to poor in the high-carbon, high-chromium steels.

Air hardening steels are used for larger sizes of tools and dies. These are air hardened in large and intricate sections with little distortion in hardening and easier to machine.

Oil hardening non-deforming die steels have a substantially reduced alloy content. These have wide range of used for all types of medium life tools and dies, easy to machine and harden uniformly.

Hot work tool steels (either chromium based or tungsten based) possess fine non-deforming, hardenability, toughness and resistance to heat softening characteristics, with fair machinability and wear resistance. These are used in blanking, forming, extrusion and casting dies, hot-blanking dies, hot punching dies, forging and die-casting dies, where temperature may rise to 540°C.

High speed steels possess all properties except toughness. These are either tungsten or molybdenum-base types. Cobalt is added sometimes to improve the cutting qualities in roughening operations. These retain considerable hardness at a red heat. They are used in metal cutting upto 593°C without softening below Rc 60.

Special-purpose tool steels are comprised of the low- carbon, low-alloy, carbon-tungsten, mould and other miscellaneous types.

7. Spring Steels: 

For manufacture of small springs, steel is often supplied in a form that requires no heat treatment except perhaps a low-temperature annealing to relieve forming strains.

For small helical springs, previously treated wire is supplied in any of the following three forms:

(i) Music Wire:

It is given a special heat-treatment called patenting and then cold-drawn to develop a high yield strength.

(ii) Hard-Drawn Wire:

It is of lower quality than music wire and is made of lower grade material and is seldom patented.

(iii) Oil-Tempered Wire:

It is quenched and tempered.

Depending on the application of the spring and the severity of the forming operation, the wire usually has a Brinell hardness of 350—400.

Steel for both helical and flat springs, which is hardened and tempered after forming, is usually supplied in an annealed condition. For small springs, plain carbon steel is satisfactory; whereas alloy steel (chrome-vanadium, or silicon-manganese steel) is used for large springs. This is in order to obtain a uniform structure throughout the cross- section. It is especially important for springs in whose case the surface of the steel is free from all defects.

8. Stainless Steels:

Certain alloys of iron and chromium known as stainless steel are highly resistant to corrosion and oxidation at high temperatures and maintain considerable strength at these temperatures.

Stainless steels may be classified according to their micro- structure into three categories:

(i) Martensitic (Hardenable Alloys Containing upto 16% Cr and 0.7%C which are Martensitic when Quenched):

These are very hard and possess strain-resisting properties and, therefore, are used for utensils, surgical and dental instruments, springs for high temperature operation, ball valves and seats etc. The proper hardening range depends on composition and size, but in general the higher the quenching temperature, the harder the article.

Oil quenching is preferable, but with thin and intricate shapes, which might warp on quenching, satisfactory hardening is obtained by cooling in air. Tempering around 500°C does not lower the tensile strength, and in this condition the steel shows remarkable resistance to weathering, to attack by fruits and vegetable acids, ammonia and other corrosive agents to which cutlery may be subjected.

(ii) Ferritic (Low Carbon, Non-Hardenable Alloys Containing 12 to 27% Cr):

These have very low carbon content and possess considerable ductility, ability to be worked hot or cold, excellent corrosion resistance and are relatively inexpensive. These can be hardened to a great extent by cold working, and are best suited for forming and medium-deep drawing operations.

These are used extensively for kitchen equipment, dairy machinery, heat exchangers, boiler tubing, screws, bolts, nuts, interior decorative work, automobile trimmings and for chemical equipment to resist nitric acid corrosion. The chromium content is increased to 25—30% for resisting oxidising conditions at high temperatures. Such alloys are use full for all types of furnace parts not subjected to high stress.

(iii) Austenitic (Chromium-Nickel Alloys):

These contain 16—26% chromium and 6 to 22% nickel. Carbon is minimum. The addition of substantial quantities of nickel to high chromium alloys lowers down the A₃ temperature and thus austenite exists at room temperature; cold working introduces some excellent properties. These are highly resistant to many acids (even hot or cold nitric acid); strong and scale less than any of the plain chromium alloys.

These are very useful for parts subjected to severe stress at elevated temperatures. Tungsten and molybdenum arc added to increase the strength at elevated temperatures; and silicon and aluminium to improve the resistance to scaling and selenium and sulphur are added to improve the machinability. These find uses in food processing, dairy industry, textile industry, pharmaceuticals.

These are not highly resistant to hot sulphurous gases and are sometimes subject to embrittlement and intergranular corrosion which may be overcome only by adding titanium and columbium. Normal corrosion resistance can be restored by heating the steel above 930°C and cooling rapidly.

Other two types of stainless steels found in common use are:

(i) Extra low-carbon stainless steel.

(ii) Precipitation-hardenable stainless steel.

(i) Extra Low-Carbon Stainless Steel:

18—8 austenitic steels have been developed having carbon content as low as 0.03%. These have higher intergranular corrosion resistance. The intergranular corrosion in stainless steel occurs because of attack on chromium carbides precipitated in the grain boundaries. Use of low carbon improves corrosion resistance by decreasing the potential amount of such carbide precipi­tation.

(ii) Precipitation-Hardenable Stainless Steel:

These can attain a strength and hardness formerly achievable only by cold working. In 18—8 chromium nickel steel, the effective age-hardening elements viz, titanium, aluminium, copper, molybdenum, columbium and tantalum in various combina­tions are added. The heat treating temperatures employed for precipitation hardening are low enough to eliminate or minimise the dangers of distortion, cracking and decarburisation inherent in quench-hardening grades.

One type contains 17% Cr, 7%. Ni, 1% Al, and a small amount of titanium and is martensitic at room temperature and is aged at 482°C. These are available in coils, sheets strips, plates, forgings, billets, bars, rods etc. Final machining operations can be performed before heat-treatment if allowance is made for slight growth that occurs. These can be classified into three types—martensitic, semi- austenitic and austenitic.

9. Special Property Alloys:

Iron-nickel alloys are used extensively in the electrical industry owing to their exceptional magnetic properties. Alloys containing 20 to 30% nickel are non-magnetic and are used to some extent for non­magnetic parts in electrical machinery. Alloys having a high permeability and low hysteresis loss have a composition between 55 and 80% Ni, (Permalloy). Perminvar (45% Ni, 20% Co) has a constant permeability over a range of flux densities.

Another important group of iron-nickel base alloys are those with low coefficients of expansion. Invar containing 36% Ni has an exceedingly low coefficient of linear expansion.

Elinvar (32% Ni with small percentages of Cr, W, Mn, Si and C) not only has a low coefficient of expansion but also has a constant modulus of elasticity over the temperature range of 0—40°C and is thus useful in hair springs for watches, and springs for other precision instruments.

Platinite (a 46% nickel alloy) has the same thermal coefficient as platinum.

Electrical sheet steels are alloys of iron and silicon with carbon, manganese, phosphorus and sulphur kept as low as possible. The silicon increases resistivity of iron and greatly decreases the hysteresis loss. Silicon-alloys are used in almost all magnetic circuits where alternating current is used. As silicon makes steel brittle, it is limited to 4% in structures subjected to vibrations, as in the case of motor armature, and is kept at 5% for transformers.

10. Austenitic Manganese Steel:

It is also known as Hadfield’s manganese steel and is a non-magnetic alloy containing around 12% Mn and 1% carbon. It is relatively soft but work hardens on the surface when subjected to severe abrasion and is thus extremely useful in crushing machinery, for railroad crossings, tractor shoes etc.

As cast, this alloy is party martensitic and, therefore, hard and brittle. By quenching from a high temperature of 1040°C, a homogeneous austenite is retained and the alloy has high toughness, strength and ductility characteristics of austenitic steels.