Lecture- 5

Technological properties of metals

Steel and its classification

Property The term property may be defined as the quality, which defines the specific characteristics of a metal.

The different materials possess different properties in varying degrees and behave in different ways under given conditions.

Provides a sound basis for predicting its behavior in manufacturing shop and also in actual use.

Aids the design engineer to select the suitable material for various jobs.

Properties of importance

1. Physical properties2. Mechanical properties3. Electrical properties4. Thermal properties5. Chemical properties

6. Magnetic properties

7. Technological

properties

Mechanical properties of metal

Determine the range of usefulness

of the metal and establish the

service that can be expected.

Also used to help specify and

identify metals.

Strength

Hardness

Ductility and

Malleability

Importanat properties

i. Stress

ii. Strain

iii. Yield point

iv. Ultimate stress

v. Breaking stress

vi. Elastic Stress

vii. Linear and lateral strain

i. Stress - It is defined as the internal resistance set up by the molecules of a material to resist

deformation, due to the application of external forces. Stress is expressed load per unit area of cross-section. i.e., Where, Stress O = P/A

P = Force or load applied, and A = Cross-sectional area.

ii. Strain - The deformation or change in length per unit length, under the action of the external forces.

dL L- L0

Strain, e = ---------------- = ----------------- L0 L0

Where, L0 = Original length, L = Final length, and

dL = Change in length = L-L0

When a body is subjected to two equal and opposite axial pulls the stress induced at any section of the body is known as tensile stress. The ratio of the increase in length to the original length is known as tensile strain.

Tensile Stress

Pt = P/A. A = Cross sectional area of barP = Axial tensile forcePt = Tensile stress

When a body is subjected to two equal and opposite axial pushes, the stress induced at any section of the body is known as compressive stress.

The ratio of the decrease in length to the original length is known as compressive strain.

Compressive Stress

Pc = P/A. A = Cross sectional area of barP = Axial compressive forcePt = Compressive stress

Shear Stress

When a body is subjected to two equal and opposite forces, acting tangentially across the resisting section, the stress induced is called shear stress.

Strain is known, as Shear strain.

When a material is loaded within its elastic limit, the stress is proportional to strain.

Stress --------- = E = a constant Strain

Hooke's Law

Hooke’s law holds good for tension and compression stresses. The proportionality constant it is called modulus of elasticity or Young's modulus.

It is expressed as N/m2 or MPa (Mega Pascal).

Elastic ConstantsModulus of Elasticity or Young’s Modulus (E)

Ratio of direct stress to corresponding linear strain within elastic limit. If p is any direct stress below the elastic limit and e the corresponding linear strain,

then E = p / e.

Modulus of Rigidity or Shear Modulus (G)Ratio of shear stress to shear strain within elastic limit. It is denoted by N,C or G. if q is the shear stress within elastic limit and f the corresponding shear strain,

then G = q / f.

Bulk Modulus (K)

Ratio of volumetric stress to volumetric strain within the elastic limit. If pv is the volumetric stress within elastic limit and ev the corresponding volumetric strain,

we have K = pv / ev.

Yield stress =Yield load / original cross-sectional area

Yield pointThe point at which permanent deformation of a stressed specimen begins to take place. This is a point on the stress-strain curve at which the increase in strain is no longer proportional to the increase in stress.

Ultimate load is defined as maximum load which can be placed prior to the breaking of the specimen. Stress corresponding to the ultimate load is known as ultimate stress.

Ultimate stress= Ultimate load / Original cross-sectional area

Ultimate stress

Breaking stress or ultimate strength

The stress required to fracture a material whether by compression, tension, or shear.

A - Elastic LimitB - Upper Yield StressC - Lower Yield StressD - Ultimate StressE - Breaking Stress 

Linear strain of a deformed body is defined as the ratio of the change in length of the body due to the deformation to its original length in the direction of the force. If l is the original length and dl the change in length occurred due to the deformation, the linear strain e induced is given by e=dl/l.

Linear strain may be a tensile strain or a compressive strain according as dl refers to an increase in length or a decrease in length of the body.

Linear Strain

Lateral strain of a deformed body is defined as the ratio of the change in length (breadth of a rectangular bar or diameter of a circular bar) of the body due to the deformation to its original length (breadth of a rectangular bar or diameter of a circular bar) in the direction perpendicular to the force. 

Lateral strain

Mechanical properties of metals i. Strength

ii. Elasticity

iii. Stiffness, Rigidity

iv. Plasticity

v. Ductility

vi. Malleability

vii.Brittleness

viii.Toughness

ix. Hardness

x. Resilience

xi. Creep

xii.Fatigue

StrengthThe property of a material by virtue of which it can withstand or support an external force or load without rupture.

Depending upon the value of stress, the strength of a metal may be elastic or plastic.

Depending upon the nature of stress, the strength of a metal may be tensile, compressive, shear, bending or torsional.

There are three typical definitions of tensile strength:

Yield strength: The stress at which material strain changes from elastic deformation to plastic deformation, causing it to deform permanently.

Ultimate strength: The maximum stress a material can withstand.

The property of a material by virtue of which it is able to retain its original shape and size after the removal of the load.

The elasticity is always desirable in metals used in machine tools and other structural members.

Elasticity

Stiffness

The property of a material by virtue of which it resists deformation or deflection under stress.

It is also known as rigidity.

Plasticity

The property of a material by virtue of which it a permanent deformation (without fracture) takes place, under the action of external forces.

This property finds its use in forming, shaping and extruding operation of metals.

Ductility

The property of a material by virtue of which it can be drawn into wires or elongated with the application of a tensile force, before rupture takes place.It is measured by its percentage elongation and percentage reduction in cross sectional area before rupture.

The ductile materials commonly used in engineering practice are mild steel, copper, aluminium, nickel, zinc, tin and lead.

MalleabilityThe property of a material by virtue of which it can be rolled or hammered into thin sheets.

The malleable materials commonly used in engineering practice are lead, wrought iron, copper and aluminium.

Brittleness

The property of a material by virtue of which it will fracture or break without any appreciable deformation.

Cast iron, glass, and concrete ate brittle materials used in engineering.

ToughnessThe ability of a metal to deform plastically and to absorb energy in the process before fracture is termed toughness.

HardnessThe property of material by virtue of which it is able to resist wear, scratching, indentation (or penetration), deformation and machinability.

The term used to indicate the degree of hardness that the metal can acquire by the process of hardening is known as hardenability.

In one, test, a hardened steel ball is pushed into the material by a known force. How far the ball penetrates, as indicated by the diameter of the indent, is measurement of the hardness.

Another type of hardness test uses a diamond-tipped pyramid that is forced into the material.

Hardness refers to the resistance that the material presents to being penetrated by another material.

The ability of a material to absorb energy when deformed elastically and to return it when unloaded is called resilience

or

The property of a material by virtue of which it stores energy and resists shock and impact loads. It is measured by the amount of strain energy stored per unit volume within the elastic limit.

.

Resilience

The property of a metal by virtue of which it undergoes a slow and permanent deformation at constant stress. Generally creep occurs in a material when it is subjected to high temperature for a long period of time. The creep in a metal occurs in three stages. Stage-1:Material elongates rapidly at decreasing rate. Stage- 2: The elongation is constant Stage -3: The elongation increases rapidly until the material fails. The stress for a definite rate of strain at a constant temperature is known as creep strength.

Creep - High temperature progressive deformation of a material at constant stress is called creep.

Fatigue

Metal fatigue is caused by repeated cycling of load.  It is a progressive localized damage due to fluctuating stresses and strains on the material.  Metal fatigue cracks initiate and propagate in regions where the strain is most severe.     The process of fatigue consists of three stages:

• Initial crack initiation 

• Progressive crack growth across the part

• Final sudden fracture of the remaining cross section

The failure is caused by means of progressive crack formation, which is usually fine and microscopic size.

The failure may occur even without prior indication.

This property of a metal is of great importance in the deign of shafts, connecting rods, springs, gears etc.

Technological properties

The technological properties of a metal are those which completely define its behaviour in shaping, forming, and fabrication operation during the manufacturing processes.

The important technological properties of a metal are

i. Machinability ii. Weldabilityiii. Formability iv. Castability

MachinabilityIt is defined as the property of metal, which indicates the ease with which it can be cut or removed by cutting tools in various machining operations such as turning, drilling, boring, milling etc.

Machinability of a metal depends upon mechanical an physical properties of a metal, chemical composition of a metal, microstructure of a metal and cutting conditions.

Machinability is a term that includes several parameters: finish, integrity, tool life, cutting speed, force, chip formation, composition and properties of material etc.

In general, tool life and surface finish are measures of Machinability.

Assessing parameters

Machinability can be assessed based on the

following parameters:

Material removal rate (depends

primarily on the cutting speed) for a given

tool life

Cutting forces

Surface finish

Material's characteristics

Machinability is influenced by the material'scharacteristics:Mechanical characteristics (resistance, hardness, stretching, work hardening), which depend partly on the production method used and the heat treatment carried out. Thermal properties (evacuation of the heat cause by cutting). Chemical properties (inclusion, abrasive particle or lubricant content).

WeldabilityThe ability of a material to be welded under imposed conditions into a specific, suitable structure and to perform satisfactorily for its intended use.

Many metals and thermoplastics can be welded, but some are easier to weld than others.

It greatly influences weld quality and is an important factor in choosing which welding process to use.

Weldability is a measure of how easy it is to:•Obtain crack free welds •Achieve adequate mechanical properties •Produce welds resistant to service degradation.

Factors affecting weldability

1.Composition of the metal2.Brittleness of the metal at elevated temp.3.Thermal properties of the metal.4.Welding techniques, fluxing, and filler

material5.Proper heat treatment before and after

the deposition of the metal.

Formability

Capability of a material to be shaped by plastic deformation.

Formability is the amount of permanent deformation that a material can withstand before breaking.

Factors affecting formability

1) Crystal structure of the metal

2) Grain size of the metal

3) Hot and cold working

4) Alloying element present in the parent metal

Castability

It is defined as the property of a metal, which indicates the ease with which it is casted into different shapes and sizes from liquid state.

Castability is the ease of forming a casting.

Castability can be thought of as how easy is it to cast a quality part. A very castable part design is easily developed, incurs minimal tooling costs, requires minimal energy, and has few rejections.

Castability can refer to a part design or a material property.

Factors affecting castability

1.Solidification rate

2.Gas porosity

3.Segregation

4.shrinkage

Steel

Steel is fundamentally an alloy of iron and carbon, with carbon content varying upto a maximum of 1.5%.

The carbon is distributed as a compound with iron if carbon is increased above 1.5%, a stage soon arrives when no more carbon can be contained in the combined state and any excess must be present as free carbon.

It is at this stage that the metal merges into group termed cast irons.

Alloys with higher carbon content than this are known as cast iron because of their lower melting point and castability.It is essentially an iron carbon alloy modified (2-4%carbon) by the presence of small percentage of silicon, phosphorus and manganese.

Steel is also to be distinguished from wrought iron containing only a very small amount of other elements, but containing 1–3% by weight of slag in the form of particles elongated in one direction, giving the iron a characteristic grain. Wrought iron is highly refined iron with a small amount of slag forged out into fibers. The chemical analysis shows as much as 99% of iron.

It is more rust-resistant than steel and welds more easily. It is common today to talk about 'the iron and steel industry' as if it was a single entity, but historically they were separate products.

Types of steel

Steels are a large family of metals.

All of them are alloys in which iron is mixed with carbon and other elements.

Steels are described as mild, medium- or high-carbon steels according to the percentage of carbon they contain, although this is never greater than about 1.5%.

Classification of Steel

Based on carbon content, steel can be

classified as

Unalloyed steel or plain carbon steel ( 0.06

to 1.5%)

Alloy steel (Low alloy and High alloy)

Classification of unalloyed or plain carbon steel

a.Low carbon or mild steel (0.05 –0.3% of C)

b. Medium carbon steel (0.3 –0.6% of C)

c. High carbon steel (0.6- 1.5% of C)

d. Tool steel (0.9 – 1.5% of C)

Plain carbon steel products

Alloy steelA steel to which one or more alloying elements other than carbon have been deliberately added (e.g. chromium, nickel, molybdenum) to achieve a particular physical property. These steels have greater strength, hardness, , wear resistance, hardenability, or toughness compared to carbon steel. Low alloy steel is steel alloyed with other elements, usually molybdenum, manganese, chromium, vanadium, silicon, boron or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections.

Element Percentage Primary function

Aluminium 0.95–1.30 Alloying element in nitriding steels

Bismuth - Improves machinability

Boron 0.001–0.003 Powerful hardenability agent

Chromium0.5–2 Increases hardenability

4–18 Corrosion resistance

Copper 0.1–0.4 Corrosion resistance

Lead - Improves machinability

Manganese

0.25–0.40 Combines with sulfur to prevent brittleness

>1Increases hardenability by lowering transformation points and causing transformations to be sluggish

Molybdenum 0.2–5 Stable carbides; inhibits grain growth

Nickel2–5 Toughener

12–20 Corrosion resistance

Silicon

0.2–0.7 Increases strength

2 Spring steels

Higher percentages Improves magnetic properties

Sulfur 0.08–0.15 Free-machining properties

Titanium -Fixes carbon in inert particles; reduces martensitic hardness in chromium steels

Tungsten - Hardness at high temperatures

Vanadium 0.15Stable carbides; increases strength while retaining ductility; promotes fine grain structure

High-alloy Steels

Stainless steel - Steel with higher alloy contents are generally called stainless steel, tool steel, or simply high alloy steel depending on the alloying elements. Stainless steel is defined as a steel alloy with a minimum of 11.5 wt% chromium content.

Stainless Steel is a metal alloy popular as a surface for sinks and appliances. Stainless steel is a great surface that doesn’t corrode or rust easily.

Classification of steel alloysAccording to chemical composition1. three component steels, containing one alloying

element in addition to iron and caste iron2. four component steels, containing two alloying

elements etc.According to structural classa) Pearlitic b) Martensitic c) Austenitic d) Ferritic e) Carbidic

According to purpose1. structural steels2. tool steels3. steels with special physical properties