Heat Treatment of Steel

Question 1

Heat Treatment and Uses

Answer 1

Controlled heating and cooling of metals to alter their physical and mechanical properties without changing the product’s shape
Uses: increasing the strength of materials, improving machining and formability

Question 2

Basic steps in heat treatment

Answer 2

1. Heat the metal to a specific temperature
2. Hold the metal at that temperature for a specific amount of time
3. Cool the metal in a specific manner

Question 3

Why Heat Treated Steel

Answer 3

Efficient way to manipulate the properties by controlling rate of diffusion and rate of cooling within the microstructure
Achieve either softening, hardening, or material modification

Question 4

Types of Heat Treatment

Answer 4

Annealing
Normalizing
Quenching
Tempering

Question 5

Phase Diagram

Answer 5

Shows what phases exist at equilibrium and what phase transformation we can expect when we change one of the parameters of the system

Question 6

Phases

Answer 6

homogeneous portion of a system that has uniform physical and chemical characteristics

Question 7

Microstructure

Answer 7

characterized by the number of phases present, their proportions, and the manner in which they are distributed or arranged

Question 8

Solubility limit

Answer 8

Maximum amount of solute that can be dissolved in solvent

Question 9

Solid-solutions

Answer 9

Binary alloy: mixture of two metals and constitutes a two component system
Separate component: each metallic element in an alloy
Isomorphous systems contain metals which are completely soluble in each other and have a single type of crystal structure

Question 10

Eutectic Alloys

Answer 10

Eutectic composition: freezes at lower temperature than all other compositions
Eutectic temperature: minimum melting temperature for the alloy

Question 11

Eutectic point

Answer 11

liquid phase is transformed to two solid phases

Question 12

Eutectic vs Eutectoid

Answer 12

Eutectic: liquid in equilibrium with two solid phases
Eutectoid: solid phase in equilibrium with two solids

Question 13

Types of Microstructures

Answer 13

Pearlite: Ferrite + Cementite
coarse or fine
Bainite: Ferrite + Cementite
very fine, needle-like elongated Fe3C in ferrite matrix
Martensite: phase formed when austenitized Fe-C alloy is rapidly cooled (quenched)

Question 14

Martensite

Answer 14

Non-equilibrium single phase structure
Results from a diffusionless transformation of austenite
Body-centered tetragonal (BCT)
Hardest and strongest
Interstitial C atoms hinder dislocation motion effectively

Question 15

Annealing

Answer 15

Makes material softer producing uniform material properties
Heated slightly above the critical temperature (austenite phase) and cooled in oven removing all strain incidents to rolling or hammering
Expensive due to unusable oven during cooling stage
Very ductile pearlite microstructure

Question 16

Normalizing

Answer 16

Makes materials softer but stronger and less ductile than annealed materials
Heated slightly above critical temperature and cooled at room temperature outside the oven (air cooling)
Refines grains to improve ductility and increase toughness
Fine pearlite microstructure

Question 17

Quenching

Answer 17

Makes steel harder and stronger by creating martensite due to rapid cooling of the austenite phase
Maximum hardness achieve but becomes very brittle with high levels of residual stress

Question 18

Tempering

Answer 18

Substantially enhances ductility and toughness of martensite and relieves internal stress
Heated to create a solid solution of iron and carbon called austenizing, then quenched to produce martensitic microsturcture, heated again below the eutectoid until formation of tempered martensite composed of stable ferrite and cementite phases