(Steels)

Outline

1. Introduction to steels.2. Classifications of Steels. 2.1 Based on composition. 2.1.1 Carbon steels. 2.1.1.1 Low carbon steel. 2.1.1.2 Medium carbon steel. 2.1.1.3 High carbon steel. 2.1.1.4 Ultra high carbon steel. 2.1.2 Low alloy steel. 3. High strength low alloy steel. 3.1 Classification of HSLA.4. Heat treatment of steel. 4.1 Carburizing.5. AISI 5130. 5.1 Mechanical properties.

Introduction To Steels

Alloys of Iron and Carbon that contains up to 2.00 wt% C are classified as steels while those containing over 2.00 wt% C are classified as Cast Irons.

Steels are the most complex and widely used engineering materials because of:

The abundance of Iron in the earth crust

The high melting temperature of Iron(1534 C).⁰

A range of mechanical properties can be obtained.

Associated microstructures can be produced by solid state phase transformation by varying cooling rates from austenitic conditions.

IIron Carbon Diagram

Classifications of Steels

Steel can be classified by different systems depending upon:

Compositions.

Manufacturing methods.

Finishing methods.

Product shape.

The deoxidation practice employed.

Microstructure.

The Heat Treatment.

Based on Composition

Carbon steels

Low Alloy steels

High Alloy steels

Carbon Steels

Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Carbon steels are generally categorized according to their carbon content.

Continue………..

Carbon steels contain up to 2% total alloying elements and can be subdivided into

Low-carbon steels.

Medium-carbon steels.

High-carbon steels.

Ultrahigh-carbon steels.

Low Carbon SteelsGeneral: Contain less than about 0.30 wt% C. Produced in the greatest quantities .Microstructure: ferrite and pearlite.Properties: Unresponsive to heat treatments intended to form martensite

(strengthened by cold working) but for the improvement in ductility. Relatively soft and weak but have outstanding ductility and toughness; in addition, they are machinable, weldable, and, of all steels, are the least expensive to produce. They typically have a yield strength of 275 MPa (40,000 psi), tensile strengths between 415 and 550 MPa (60,000 and 80,000 psi), and a ductility of 25%EL. Applications: Typical applications include automobile body components, structural shapes (I-beams, channel and angle iron), and sheets that are used in pipelines, buildings, bridges, and tin cans.

Medium Carbon SteelsGeneral: Carbon concentrations between about 0.30 and 0.60 wt%. May be heat treated by austenitizing, quenching, and then tempering to

improve their mechanical properties. Microstructure: They are most often utilized in the tempered condition, having

microstructures of tempered martensite.

Properties: The plain medium-carbon steels have low hardenabilities and can be

successfully heat treated only in very thin sections and with very rapid quenching rates.

These heat-treated alloys are stronger than the low-carbon steels, but at a sacrifice of ductility and toughness.

Applications: Applications include railway wheels and tracks, gears, crankshafts, and

other machine parts, garden tools and high-strength structural components (having a combination of high strength, wear resistance, and toughness).

High Carbon SteelsGeneral: Normally having carbon contents between 0.60 and1.0 wt%, They are almost always used in a hardened and tempered condition Properties: The hardest, strongest, and yet least ductile of the carbon steels. Especially wear resistant and capable of holding a sharp cutting edge.

(The tool and die steels are high carbon alloys, usually containing chromium, vanadium, tungsten, and molybdenum.)

Applications: These steels are utilized as cutting tools and dies for forming and

shaping materials, as well as in knives, razors, hacksaw, blades, springs, and high-strength wire.

Ultra High Carbon Steels

General:

Ultrahigh-carbon steels are experimental alloys containing approximately 1.25 to 2.0% C.

Microstructure:

Ultrafine, equiaxed grains of ferrite and a uniform distribution of fine, spherical, discontinuous proeutectoid carbide particles.

Low Alloy Steels

Low-Alloy Steels: Low Alloy Steel is one that contains specified amount(s) of alloying

element(s) and /or more 1.65 % Mn,0.60%Cu and 0.60%Si. Nature and amount (s) of alloying elements (s) dictate the properties of these

steels unlike plain carbon steels in which mainly C govern the properties of steel.

Total alloy content can range from2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr.

Low-alloy steels can be classified according to:

• Chemical composition,

• Heat treatment

High Strength Low Alloy Steels

High-strength low-alloy (HSLA) steels, or micro alloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels.

The chemical composition of a specific HSLA steel may vary for different product thickness to meet mechanical property requirements.

These are primarily low carbon (C≤0.20%)steels with about 1% Mn and small quantities (<0.50%) of other elements.

They are not considered to be alloy steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition (HSLA steels have yield strengths of more than 275 MPa).

They exhibit very good formability and Weldability.

These steels have been basically developed with main emphasis on mechanical properties in order to reduce weight(by increasing strength).This is why these are referred to as High Strength Low Alloy Steel.

HSLA Classification Weathering steels:

Designed to exhibit superior atmospheric corrosion resistance

Control-rolled steels:

Hot rolled according to a predetermined rolling schedule designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on cooling

Pearlite-reduced steels:

Strengthened by very fine-grain ferrite and precipitation hardening but with low carbon content and therefore little or no pearlite in the microstructure.

Micro alloyed steels:

With very small additions (generally <0.10% each) of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardening.

Acicular ferrite steel:

Very low carbon steels with sufficient hardenability to transform on cooling to a very fine high-strength acicular ferrite (low-carbon bainite) structure.

Dual-phase steels:

The microstructure of these steels consists of mainly hard martensite embedded in soft ferrite matrix. This is why these steels are known as dual-phase steels.

The strength and ductility are governed by martensite and ferrite respectively.

Heat Treatment of Steels

Steels can exhibit a wide variety of properties depending on composition as well as the phases and micro-constituents present, which can be achieved by heat treatment.

Various heat treatments process used for steels are following

Annealing

Normalizing

Tempering

Carburizing

Carburizing

Surface hardening method for low carbon steel

Temperature range is 900-930 C.⁰

Carbon diffused by heating above transformation temperature .

Carbon layer is enriched 0.7-0.9 %.

C is absorbed in solid solution in austenite.

Carburizing is done by following methods:

1. Pack carburizing.

2. Liquid carburizing.

3. Gas carburizing.

4. Vacuum carburizing.

5. Plasma carburizing.

Pack carburizing

Oldest method in which we used 80 % granular coal & 20 % Barium carbonate as energizer in heat resistant boxes at 930 C.⁰

Time depends upon case depth required.

Depth of penetration is dependent on diffusion & can be related to time by this equation:

Case depth = k√ t

Time varies 6-8 hours for case depth of 1-2 mm.

AISI 5130

AISI 5130 is an alloy steel which fall in the category of chromium steels. The chemical composition of AISI 5130 is shown in the table.

Element %

C 0.28-0.33

Mn 0.80-1.10

Cr 0.80-1.10

Si 0.90-1.20

Ni <0.30

P <0.030

S <0.030

Fe Rest

Mechanical Properties

At room temperature 25 C⁰Density 7.7-8.3 Kg/m3

Poison ratio 0.27-0.30

Elastic Modulus 190-210 GPa

Tensile strength 1275 MPa

Yield strength 1207 MPa

Elongation 12 %

Reduction in area 51 %

hardness 379 HB

References:

1. ASM Handbook, Volume 1, Properties and Selection: Irons, Steels, and High Performance Alloys.

2. Askeland Science Engineering Materials 6th Edition’

3. Callister - Materials Science and Engineering - An Introduction 7e (Wiley, 2007)

4. Engineering Materials(Properties & Applications of Metals and Alloys) by C.P SHARMA