Basic Metallurgy 3f3l1m

Basic Metallurgy • Extractive Metallurgy deals with processing of the natural occurring raw materials for converting those in to useful metals and alloys • It also deals with reaction of metal with slags and gases. Thus reflects on the compositional changes in the molten metal before solidification of the weld pool. • Physical Metallurgy deals with identifying, controlling and varying the properties of the metals and alloys as per requirements of the mankind.

• It takes into consideration the phase changes with temperature and/or composition. It reflects on the causes of the change in mechanical and physical properties.

Metals and Alloys Alloys are consisting of two or more elements with major element being metal. Pure metals normally do not impart the highest possible properties therefore are generally alloyed with other metallic or non-metallic elements

Physical Metallurgy. • Metals are crystalline in nature

Type of Crystal Structure • Cubic • Hexagonal • Tetragonal • Triclinic • Rhombohedral • Orthorhombic • Monoclinic

Common Crystal Structures

Simple

Body - Centered

Face - Centered

Phase

Homogeneous, physically distinct (Co-existing) entity in an alloy, characterised by crystal structure and lattice parameter

Single Phase Binary System Physical Metallurgy. Liquid Only

Liquid

Temperature

a3 Solids

Liquid + Solid

m.pt. Of B

a2 b3 a1

m.pt. Of A

C1

b2 b1

Solid Only

C3

C2

Time-Temperature Cooling Curves 0% B 100% A

Composition The construction of a simple equilibrium diagram of the solid solution type using cooling curves of a series of alloys.

100% B 0% A

Two Phase Binary System

Allotropic Modifications of iron • Iron has a distinction of existing in different forms at different temperatures. • This property gives rise to many different alloys and unique ability to alter properties of iron alloys by heating and cooling at different rates. • Heating and cooling of various metals and alloys for attaining the desired mechanical properties is known as “Heat Treatment”

Phase of Iron Iron exist as Delta iron from melting temperature to 1410 C and form solid solution with carbon known as delta-Ferrite Gamma iron from 1410C to 910 C and form solid solution with carbon known as Austenite and Alpha iron from 910 C to room temperature and form solid solution with carbon known as alpha- ferrite

Iron Carbon Diagram

Phases in Steel Cementite • It is an intermetallic compound of iron and carbon and exist as a phase. Pearlite • This is consisting of thin alternate layers of cementite and ferrite. • When no other alloying elements are present, its composition will be nearly 0.8 % Carbon. • Pearlite imparts strength and toughness to the steel.

Non-equilibrium phases • Iron carbon diagram does not give complete picture of the phases forming under faster cooling rates. Additional phases which can form are Martensite, Upper bainite , Lower bainite , Side plate ferrite or Widmanstatten ferrite and Acicular ferrite Formation of these phase can be explained by TTT Diagrams

TTT DIAGRAMS

CCT Diagram

Critical Cooling Rate • Critical Cooling Rate is defined as the lowest cooling rate which produces 100% Martensite. • If hard martensitic structure is to be avoided, cooling rate shall be slower than critical cooling rate

Structures Forming in Steel

Heat-Treatment Various Heat Treatments • • • • • •

Annealing Normalising Spherodising Hardening Age Hardening Stress Relieving

Annealing

• A term denoting a treatment, consisting of heating to and holding at a suitable temperature, followed by cooling at a slow rate, used primarily to soften, but also to simultaneously produce desired changes in other properties or in microstructure.

Stress Relieving A process to reduce internal residual stresses in a metal object by heating the object to a temperature below lower critical and holding for a proper time at that temperature. This treatment may be applied to relieve stresses induced by casting, quenching, normalizing, machining, cold working or welding.

Hardening of Iron • Martensite is very hard phase when containing some carbon. • It can be obtained by fast cooling of steel from austenitic condition. • To impart some toughness, it needs to be softened by tempering treatment

Tempering • The object of tempering is to reduce the brittleness of the hardened steel and to remove the internal strains caused by the sudden cooling in the quenching bath. • The tempering process consists in heating the steel by various means to a certain temperature and then cooling it. On reheating to a temperature of about 300 to 750°F., a softer and tougher structure is formed.

Effect of Tempering on Mechanical Properties

Effects of Alloying Elements Effect of alloying elements in steel Purpose of alloying. * Strengthening of ferrite. * Improved corrosion resistance * Better hardenability * Better grain size control greater strength * Improved machinability * Improved high or low temp. Stability * Improved ductility * Better wear resistance * Improves cutting ability

Factors Affecting Martensite Formation • The hardenability increases with increasing austenite grain size. • Most metallic alloying elements slow down the ferrite and pearlite reactions, and so also increase hardenability. • This means that addition of alloying elements increases the tendency in the steels to form martensite. • In other words even slow cooling welds will have increasing possibility of martensite formation.

CE and Process Controll • For CE%<0.14 Excellent weldability, no special precautions necessary 0.14 <0.45 Martensite is more likely to form, and modest preheats with low hydrogen electrodes become necessary CE%>0.45 Extreme complications, weld cracking is very likely, hence preheat in the range 100-400°C and low hydrogen electrodes are required