CHE 333 Class 10

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CHE 333 Class 10

• HEAT TREATMENT OF STEEL

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What and Why Heat Treat?

• HEAT TREATMENT is THERMAL PROCESSING to OPTIMISE
  MECHANICAL PROPERTIES.
• By heat treatment a 10 to 1 ratio can be achieved
  between maximum and minimum
• Strength levels.
• At the same time a 50 to 1 ratio of ductility can
  be achieved.
• Thermal Treatments range from quenching to long
  holds, 24 hours, at a fixed
• Temperature. In all cases the thermal processing
  controls the microstructure and so also
• The mechanical properties.

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Hardenability of a Steel
Hardenability is the ability of a steel to form
martensite. The greater the hardenabillity the
more martensite. Note the difference between
hardness and hardenabilty. Hardness is used to
measure hardenability. A steel rod is cooled
rapidly from one end in a Jominy test and the
hardness measured as a function of distance from
the quenched end. The decrease in hardness gives
the hardenability. For the three steels 1040,
4140 and 4340, the hardness drops rapidly after
5mm for the 1040 so it has low hardenabilty. The
4340 has much better hardenability. The hardness
of martensite depends on The carbon content as
1060 has 0.6C and 1080 has 0.8C. Quench media,
grain size, bar diameter affect the
measurements.
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APPLICATION OF HARDENABILITY

• Applications of Hardenability Include
• Choosing steels that need to have a uniform
  microstructure after quenching
• Components needing a dual microstructure, such as
  car axles, where a hard surface to withstand a
  bearing is combined with a softer tougher center
  so that failure will be ductile. Low
  hardenability can be used in this case to only
  form hard martensite on the surface. Another
  example would be gears. In this application,
  induction hardening followed by quenching surface
  hardens the gears and leaves a soft ductile core.

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TEMPERING OF MARTENSITE
After quenching to form martensite, a strong but
brittle material is produced. For many
Applications a weaker but more tough or ductile
is needed to quenched steels are Tempered to
reduce strength but increase ductility. During
tempering carbide particles Are formed as the
steel tries to go back to its equilibrium phases.
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Tempering Martensite.
Tempering is holding the steel below the
eutectoid temperature of 727C for a period of
time. During this period, the martensite,
transforms to two phase a carbide. The specific
carbide depends on the steel composition. Note
the tempering temperature controls the service
temperature of the steel. A 4340 steel is
austenitized at 1650F, quenched into oil and
tempered at 325F for 1 hour to give a yield
strength of 230,000 psi. Temper embrittlement is
a range of tempering where the steel becomes
brittle after tempering. The temperature range is
350 to 500F, which produces hardnesses of 48 to
42 Rockwell C scale. The higher the temperature
or the longer the time, the lower the strength,
the greater the ductility and the higher the
elongation to failure.
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Spherodized Structure
Holding pearlite for 24 hours at 650C leads to a
Spherodized structure as the carbides form large
particles. This is the softest and weakest steel,
Rc is 8.5, yield strength around 30,000. The idea
is to machine in the soft condition where
minimum effort is required, then heat treat to
reach the strength required of the component.
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Heat Treatment Terms.

• Annealing heat treating to produce a soft
  structure.
• Normalizing air cooling after high temperature
  exposure
• Full Anneal furnace cooling after high
  temperature exposure very slow cool
• Process Anneal an anneal conducted during
  processing
• Bright Anneal control atmosphere to stop
  oxidation process.
• Controlled atmosphere annealing control the
  atmosphere while heating. Produces
• 
  specific surface compositions.
• Cautions surface condition changes, due to
  oxidation and composition changes as
• elements diffuse from the
  surface e.g. decarburization.
• distortion piece changes
  shape during annealing, especially after working.

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Steel Compositions

• American Iron and Steel Institute (AISI), Society
  of Automotive Engineers (SAE), Unified
• Numbering System (UNS), and Mil Spec are all
  different methods of classifying steels.
• AISI is most common.
• Last two digits are the carbon content. For
  example XX20 is 0.2C, XX80 is 0.8C.
• The first two digits are the alloy additions, For
  example 1020 is a plain carbon steel,
• while a 4340 steel is the Nickel, Chrome
  Molybdenum series.
• All these steel have manganese added to pick up
  sulfur as MnS inclusions.
• Tool steels have a different AISI series
  depending how the steel is hardened.
• Stainless Steels have series, such as 300, 400.
  300 series is for steels that are austenitic
• at room temperature, 304 is common which is Fe 19
  Cr 9 Ni 0.08C note the very low
• carbon content. The 400 series are lower on
  nickel and so are ferritic unless quenched w
• when they become martensitic. 440A is Fe 17Cr 1Mn
  0.75Cr 0.7C. Grades of this are A,
• B and C with increasing carbon content. Also have
  17 4pH for precipitation hardening.