L13: Heat Treatment Of Plain Carbon Steels

Question 1

What is the lower critical temp?

Answer 1

A1

Eutectoid temp

Question 2

What is the upper critical temp?

Answer 2

A3 for Hypoeutectoid

Acm for hypereutectoid

Question 3

Describe process annealing.
What temp?
What wt% C?
What does it do?

Answer 3

Low temp- 80-170 C below the eutectoid temp A1

Less that 0.25-0.3 wt%

Removes effects of cold work

Recrystallises the ferrite (no effect on Pearlite)

Question 4

Describe spheroidise annealing?

What alloys are used?
What temp?
What does it do to the alloy?

Answer 4

Hypereutectoid alloys

Improves ductility and softens metal ready for forming

30C below A1 for 15-25
hours

Cementite layers restructure into spheres

Question 5

Describe normalising?
What temp?
What happens to the alloy?

Answer 5

Heated to 55-85C above A3 or Acm

Converts alloy to austenite
Produces smaller grains(grain refinement) - corrects distortion and growth
Gives uniform properties
Improved strength and toughness

Question 6

Describe full annealing
What temp?
What does it do?

Answer 6

Heated to 50C above A3- hypoeutectoid

50C above A1- hypereutectoid until they’re equilibrated as austenite

Cooled v slowly producing coarse pearlite structure

Soft and ductile

Question 7

How is steel hardened?

What does this create?

Answer 7

Heated into austenite region then rapidly quenched

Non-equilibrium cooling so unexpected steel phases are made

Insufficient energy for difFusion with iron and C so no Pearlite forms

Lots of distortion so martensite is created - hard for dislocations to slip -steel becomes hard and brittle

Question 8

What is martensite?

Answer 8

A body centred tetragonal structure

Question 9

What does the hardness of hardened steel depend on?

Answer 9

The carbon content as this affects the amount of martensite made

Question 10

What is the test used to determine harden ability?

Answer 10

The jominy end-quench test

Question 11

What is hardenability?

What is it affected by?

Answer 11

The extent to which alloys can be hardened uniformly.

Composition of steel
Whether the whole object (core too) can be cooled quick enough to be quenched- the inside may not become martensite as it cools too slowly

Question 12

What is a disadvantage if hardened steel?

Answer 12

Often too brittle to be of use

Question 13

Describe tempering
What temp?
What does it do?

Answer 13

Metal is heated to below the eutectoid temp- 230-650C depending on treatment

Turns some martensite into cementite and ferrite - dispersion strengthening

Process determines particle size and metal strength

Question 14

Why is it important not to overheat the component during tempering

Answer 14

To avoid further tempering and weakening

Question 15

What are the properties of tempered martensite

Answer 15

Still Strong

Greater ductility and toughness

Question 16

When is bainite formed?

Answer 16

When cooling is too fast for Pearlite but too slow for martensite

Question 17

Describe bainite and it’s properties

Answer 17

Extremely fine Fe3C and ferrite phases (distributed differently to Pearlite)

Stronger than Pearlite and has reasonable ductility

Question 18

What are the limitations of plain carbon steels and the advantages of alloys?

Answer 18

Poor hardenability
Poor strength compared to alloys

Advantages: corrosion resistance
Increased strength or hardness
Better heat treatment capabilities
Better processing

Question 19

What is the advantage of high carbon content in plain carbon steel?

Answer 19

Enables more effective heat treatment (hardening and tempering) to be carried out- good for hard surfaces and sharp edges

Also enable them to be used as tools

Question 20

What are the main alloy metals of stainless steel and what do they do?

Answer 20

4-22wt% Cr
Encourages ferrite formation and promotes grain growth - promotes grain growth

0-26wt% Nickel
Stabilises austenite so it exists even at room temp- promotes fine grains

Highly resistant to corrosion- thin Cr0 layer on surface

Question 21

What are the 3 main groups in stainless steel?

Answer 21

Ferritic- solid solution strengthened and work hardened

Austenitic- most common- not responsive to heat treatment- strengthened and work hardened 7-14wt%Ni

Martensitic- can be hardened or tempered - often used for cutlery

Question 22

Describe cast iron and its properties

Answer 22

Above 2.2wt%C (usually 3 to 4.5wt%

Lower mp than steels- good for casting processes

Carbon exists as graphite not cementite- formation of graphite promoted by adding 1wt% Si

Question 23

What are the 4 main types of cast iron?

Answer 23

Grey iron- 2.5-4wt%C, 1-3 wt% Si and other elements - weak and brittle in tension, stronger and ductile in compression (good damping)

Nodular- aka ductile iron- Mg/Cerium alters structure- stronger +more ductile than grey iron-valves,machine components

White cast iron- <1wt%Si - v hard, v brittle, non machinable- start for malleable iron (not much use)

Malleable iron- produced by heating white iron to 800-900C then cooling - connecting rods, pipe fittings etc