L13: Heat Treatment Of Plain Carbon Steels Flashcards
What is the lower critical temp?
A1
Eutectoid temp
What is the upper critical temp?
A3 for Hypoeutectoid
Acm for hypereutectoid
Describe process annealing.
What temp?
What wt% C?
What does it do?
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)
Describe spheroidise annealing?
What alloys are used?
What temp?
What does it do to the alloy?
Hypereutectoid alloys
Improves ductility and softens metal ready for forming
30C below A1 for 15-25
hours
Cementite layers restructure into spheres
Describe normalising?
What temp?
What happens to the alloy?
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
Describe full annealing
What temp?
What does it do?
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
How is steel hardened?
What does this create?
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
What is martensite?
A body centred tetragonal structure
What does the hardness of hardened steel depend on?
The carbon content as this affects the amount of martensite made
What is the test used to determine harden ability?
The jominy end-quench test
What is hardenability?
What is it affected by?
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
What is a disadvantage if hardened steel?
Often too brittle to be of use
Describe tempering
What temp?
What does it do?
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
Why is it important not to overheat the component during tempering
To avoid further tempering and weakening
What are the properties of tempered martensite
Still Strong
Greater ductility and toughness
When is bainite formed?
When cooling is too fast for Pearlite but too slow for martensite
Describe bainite and it’s properties
Extremely fine Fe3C and ferrite phases (distributed differently to Pearlite)
Stronger than Pearlite and has reasonable ductility
What are the limitations of plain carbon steels and the advantages of alloys?
Poor hardenability
Poor strength compared to alloys
Advantages: corrosion resistance
Increased strength or hardness
Better heat treatment capabilities
Better processing
What is the advantage of high carbon content in plain carbon steel?
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
What are the main alloy metals of stainless steel and what do they do?
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
What are the 3 main groups in stainless steel?
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
Describe cast iron and its properties
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
What are the 4 main types of cast iron?
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
