Transcript CHE 333 Class 10 - Chemical Engineering

CHE 333 Class 10
HEAT TREATMENT OF STEEL
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.
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
0.6%C and 1080 has 0.8%C.
Quench media, grain size, bar
diameter affect the measurements.
APPLICATION OF
HARDENABILITY
Applications of Hardenability Include
1.
Choosing steels that need to have a uniform microstructure after quenching
2.
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.
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.
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.
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.
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.
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 0.2%C, XX80 is 0.8%C.
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 0.08%C – 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.
STEEL MICROSTURCTURES
• Martensite – after quenching, produced only from g.
• Tempered martensite – after thermal treatment of
martentsite, consists of a Fe and small carbides, such as
Fe3C or carbides from alloy additions such as Cr, Mo, W,
V.
• Pearlite – equilibrium eutectoid product of platelets of a
Fe and Fe3C
• Bainite – after quenching to a temperature above Ms,
small carbides are formed directly in ferrite.
• Spheroidized – held for long times, 24hrs, just below
eutectoid temperature, spheroidal carbides are formed.