Steel

1. (Steels)

2. 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.

3. 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(1534⁰C).
 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.

4. IIron Carbon Diagram

5. 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.

6. Based on Composition
 Carbon steels
 Low Alloy steels
 High Alloy steels

7. 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.

8. 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.

9. Low Carbon Steels
General:
 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

10. 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).

11. High Carbon Steels
General:
 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.

12. 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.

13. 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

14. 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.

15.  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.

16. 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.

17.  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.

18. 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

19. Carburizing
 Surface hardening method for low carbon steel
 Temperature range is 900-930 ⁰ C.
 Carbon diffused by heating above transformation temperature .
 Carbon layer is enriched 0.7-0.9 %.
 C is absorbed in solid solution in austenite.

20.  Carburizing is done by following methods:
1. Pack carburizing.
2. Liquid carburizing.
3. Gas carburizing.
4. Vacuum carburizing.
5. Plasma carburizing.

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

22. 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

23. Mechanical Properties
At room temperature 25⁰CDensity 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

24. 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