Change of Condition

1
Change of Condition

• Chapter 12

2
Competencies

• Describe the different methods of softening steel
  
• Describe the different methods of hardening steel
• Describe the difference between Martensite and
  Austinsite

3
Phase Diagram

• Is a graphical means of representing the phases
  of a metal allow system as a function of
  composition and temperature
• Discuss the Water phase system (O/H)
• Discuss the Copper-nickel Alloy system (O/H)

4
Phase Diagram

• Discuss the Tin-Lead Allow system
• widely used in soldering for making electrical
  connection.
• The addition of two solid phases alpha (a) and
  beta (ß).
• Alpha phase is a solid solution of tin in lead
• Beta phase is solid solution of lead in tin that
  occurs only at elevated temperatures around 200
  degrees C
• Between these solid solutions lies a mixture of
  the two solid phases, (a) (ß).
• Two liquidus lines that begin at the melting
  points of the pure metals and meet at a
  composition of 61.9 Sn.
• Point is called the eutectic composition for the
  tin-lead system

5
Phase Diagram (Tin-Lead Allow system)

• A eutectic alloy is a particular composition in
  an allow system for which the solidus and
  liquidus are at the same temperature.
• The corresponding eutectic temperature, the
  melting point of the eutectic composition is 183
  deg C
• Eutectic temperature is always the lowest melting
  point for an alloy system.

6
Discuss Iron-Carbon Phase Diagram

• Steels with less than 0.3 carbon cannot be
  hardened effectively, while the maximum effect is
  obtained at about 0.7 due to an increased
  tendency to retain austenite in high carbon
  steels
• The ferrous metals of engineering importance are
  alloys of iron and carbon.
• These alloys divide into two major groups steel
  and cast iron.

7
Iron-Carbon Phase

• Pure iron melts at 1539 degrees C (2827 deg F)
  during the rise in temperature from ambient, it
  undergoes several solid phase transformations
• Starting at room temperature the phase is alpha
  iron or ferrite. With less than 0.025 carbon at
  temperatures below 894 deg C
• At 912 degrees C, ferrite transforms to gamma
  iron, called austenite. With less than 2 carbon
• At 1394 degrees C, austenite transforms to delta
  iron, which remains until melting occurs at 1539
  degrees C

8
Iron-Carbon Phase

• Solubility limits of carbon in iron are low in
  the ferrite phase only about 0.022 at 723 deg
  C. Austenite can dissolve up to about 2.1
  carbon at 1130 deg C. The difference in
  solubility between alpha and gamma leads to
  opportunities for strengthening by heat treatment
• The eutectoid point is the lowest temperature at
  which austenite can exist (722 deg C).
• Eutectoid the temperature and composition (0.77
  - 0.81 Carbon) at which a single-phase solid
  goes directly, on cooling, to a two-phase solid.
  Steels below 0.77 Carbon are considered
  hypoeutectoid steels those above up to 2.1 are
  considered hypereutectoid steels.
• The eutectoid composition of the Iron-Carbon
  system is called pearlite.

9
Iron-Carbon Phase

• Even without head treatment, the strength of iron
  increases dramatically as carbon content
  increases, and we enter the region in which the
  metal is called steel.
• More precisely, steel is defined as an
  iron-carbon alloy containing from 0.02 to 2.1
  carbon.
• Another prominent phase in the iron-alloy system.
  Is Fe3C also known as cementite. Which is a
  metallic compound of iron and carbon that is hard
  and brittle Carbon content of about 6.7.

10
Iron-Carbon Phase

• Above a carbon content of 2.1 up to about 4 or
  5 is defined as cast iron
• If sufficient time is allowed for cooling of the
  austenite
• it will revert completely to pearlite
• however, if the steel is cooled quickly from the
  austenite, martensite is formed
• Martensitic steel has Rockwell C hardness of
  about 66 and pearlite is very soft in comparison.

11

• Discuss the TTT curve (12-11)
• Three major categories of heat treatments
• Methods of softening steels
• Methods of hardening steels
• Methods of modifying the properties of steels

12
Methods of Softening Steels

• Annealing is the softening of a metal to its
  softest possible condition. For steels, the metal
  must be heated into the austenitic range and
  cooled very slowly.
• Normalizing is a heat treatment used to give
  steel an even GRAIN size. It is used prior to
  machining or other heat treatments.

13
Methods of Hardening Steels

• Can be done by flame, induction, electron beams,
  and laser beam
• Quenching is the rapid cooling of a metal to
  harden it.
• Cryogenics, or deep freezing
• done to make sure there is no retained Austenite
  during quenching.
• When steel is at the hardening temperature, there
  is a solid solution of Carbon and Iron, known as
  Austenite.

14
Methods of Hardening Steels

• The amount of Martensite formed at quenching is a
  function of the lowest temperature encountered.
• At any given temperature of quenching there is a
  certain amount of Martensite and the balance is
  untransformed Austenite. This untransformed
  austenite is very brittle and can cause loss of
  strength or hardness, dimensional instability, or
  cracking.

15
Methods of Hardening Steels

• Quenches are usually done to room temperature.
  Most medium carbon steels and low alloy steels
  undergo transformation to 100 Martensite at
  room temperature.
• High carbon and high alloy steels have retained
  Austenite at room temperature. To eliminate
  retained Austenite, the temperature has to be
  lowered.
• In Cryogenic treatment the material is subject to
  deep freeze temperatures of as low as -185C
  (-301F), but usually -75C (-103F) is
  sufficient.
• The Austenite is unstable at this temperature,
  and the whole structure becomes Martensite.

16
Methods of Hardening Steels

• Surface Hardening
• If steel is hardened all the way through the
  part, it will be brittle. In parts that have
  wearing surfaces such as gear teeth, shafts,
  lathe beds, and cams, only the surface of the
  part should be hardened so as to leave the inside
  soft and ductile.
• Flame hardening is widely used in deep hardening
  for large substrates.
• Induction hardening is suitable for small parts
  in production lines.
• These processes are applicable only to steels
  that have sufficient carbon and alloy content to
  allow quench hardening.

17
Methods of Hardening Steels

• Case Hardening
• If low-carbon steel is used and toughness is need
  in the workpiece, its surface cannot be
  significantly hardened. Therefore a process to
  add carbon or nitrogen to the surface is done.
• Done by carburizing, nitriding, carbonitriding or
  cyaniding
• These elements diffuses into the outer layers of
  the steel to increase hardness.
• The steel surface can then be hardened by
  QUENCHING.

18
Hardness

• Is a function of the Carbon content of the steel.
  
• Requires a change in structure from the
  body-centered cubic structure found at room
  temperature to the face-centered cubic structure
  found in the Austenitic region.
• Steel is heated to Autenitic region. When
  suddenly quenched, the Martensite is formed. This
  is a very strong and brittle structure.

19
Hardenability

• The ease with which full hardness can be achieved
  throughout the material.
• A measure of the depth of full hardness achieved
• Is related to the type and amount of alloying
  elements.
• Different alloys, which have the same amount of
  Carbon content, will achieve the same amount of
  maximum hardness however, the depth of full
  hardness will vary with the different alloys.
• The reason to alloy steels is not to increase
  their strength, but increase their hardenability

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21
Methods Of Modifying The Properties Of Steels

• Tempering is the removal of internal stresses
  in a metal by heating the part back to a
  temperature between 200 1200 deg F, for an
  appropriate time based on part size and desired
  tempering.
• Spheroidizing done by heating the steel to a
  temperature just under 1300 deg F and held for a
  period based on size. Grains will be changed
  into small spheres

22
Methods Of Modifying The Properties Of Steels

• Martempering
• steel is quenched from the austenitic temperature
  to just above the MARTENSITE start temperature
• held there for a few seconds to a few minutes
• and then quenched.
• It is used to provide an even-sized martensite
  throughout the part.

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• Austempering
• steel is quenched to just above the MARTENSITE
  start temperature
• held there for several hours before lowering the
  temperature to room conditions.
• The grain structure of the steel will be
  entirely bainitic
• Bainite has some of the hardness properties of
  martensite and some of the toughness properties
  of pearlite

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