Chapter 6: Heat Treatment Flashcards
definition of heat treatment
combination of heating & cooling operations applied to a metal or alloy in the solid state to produce desired properties
general heat treatment cycle parts
1) heating
2) holding at temperature (soaking)
3) cooling
what can happen to a steel if the heating rate is too fast?
steel that is highly stressed can:
distort & crack
benefits of pre-heating
good for treating steels with:
variation on section thickness
low thermal conductivity
what is the objective of holding steel at heat-treating temperature?
ensure uniformity of temperature throughout its entire volume
factors that affect cooling rate
size of workpiece
quenching media used
what does unstable austenite mean?
the austenite can change into any other phase
how does alloying elements affect the TTT diagram? and which element does not?
alloying elements shift the TTT curve to the right, martensite will form at slower cooling rate
steel becomes easier to harden (less drastic quenching can be used)
cobalt does not make it easier
how are austenitising temperature affected by alloying elements?
raised by carbide formers (Cr, tungsten)
lowered by Ni, Mn
what does alloying elements do to Ms and Mf temperatures? and what will happen?
Ms and Mf temperature will be lowered
austenite will not be able to transform fully in to martensite at rtp, leading to retained austenite
how does complex carbide affect austenite transformation?
elements like (tungsten, titanium, vanadium) for very strong, stable complex carbide
which are difficult to dissolve in austenite
leading to higher austenitising temperature
causes of retained austenite
presence of large amount of alloying elements in austenite will lower Ms and Mf temperature
if Mf is below rtp, not all austenite will form into martensite
effects on properties by retained austenite
1) reduce overall hardness of steel (austenite is softer than martensite)
2) spontaneous cracking may occur (when austenite transform into martensite)
3) steel is dimensionally unstable (localised deformation may occur when RA transform into martensite)
ways to eliminate RA?
double tempering
sub-zero treatment
what is double tempering method to eliminate RA?
1st tempering:
tempers martensite
cooling from temperating temp transform some RA to martensite
2nd tempering:
temper newly formed martensite
may be repeated if needed
what is sub-zero treatment method to eliminate RA?
refrigerating the steel, temp as low as -98°C
allowed to warm up to rtp afterwards
how does distortion occur in heat treatment?
when steel is cooled, thermal contraction occurs, surface cools faster than the core
stresses are produced by the non-uniformed contraction and are relieved by plastic deformation resulting in distortion
how does cracking occur in heat treatment?
temperature below Ms, surface is martensitic and brittle, stresses generated will lead to crack
measures to prevent distortion or cracking
1) ensure component is well support in furnance
2) remove stress raisers in design
3) quenching rate not too drastic
4) ensure steel is selected correctly
martempering process
1) quench in salt or metal bath above temp Ms
2) hold for short time to equalise the surface and core temp
3) cool through Ms to Mf
4) temper the steel
limitations of martemping
only for small components of alloy steels
austempering process
1) quench in salt or metal bath above temp Ms
2) hold for sufficient time to allow isothermal transformation of austenite to bainite
3) air cool steel
limitations of austempering
only for small components
when to use what kind of heat treatment?
components need to withstand loading:
use through hardening
components need to withstand wear and fatigue (surface properties):
use surface hardening
purpose of surface heat treatment
improve wear & fatigue resistance
how is surface obtained?
diffusing an element into the steel
element must be soluble in the the steel at the temp used
what happens when an element is diffused into the steel surface?
- surface volume increase
- residual compressive stress induced on surface
types of surface treatment process and their description
1) thermochemical - surface chemistry of steel is modified by adding carbon and/or nitrogen
2) selective surface hardening process - only the surface is hardened and the core is left in pre-treated condition
where is carburising applied and the process of it
applied to low carbon steel (<0.2%C)
process:
- heat steel to temp above 880°C
- quench to develop hard martensitic case (>0.8%C)
when is gas carburising used and what is the process?
used for treating large quantities of small compoments
process:
heat component to carburising temp for 3-4hrs
- controlled atmosphere consists of mixture of hydrocarbon gases
2CO –> C + CO2
advantages and disadvantages of gas carburising
adv:
- neat and clean
- carbon content of surface layers can be controlled accurately and easily
disadv:
- more complex furnace or equipment
- high skilled work
when is gas nitriding used? and what is the process and reaction equation?
used for hardening finish-machined and heat-treated steels
process:
heat component at 500°C for 2-4 days in a furnace with ammonia gas being circulated
2NH3 = 2N + 3H2
advtanges & disadvantages of gas nitiriding
adv:
- very hard case obtained
- good corrosion resistance
- clean, simple to use
disadv:
- service temp cannot exceed 500°C
- nitriding reduces toughness
which steels are carbonitriding used for?
mild steel and low carbon steel
process of carbonitriding
- C & N are introduced to the surface together
- heat steel to 830°C for up to 3hrs
- quench in oil
advantages of carbonitriding
- N stabilises austenite (austenite transform slowly, no distortion)
- amount of C & N can be varied
how is selective hardening produced?
rapid application of heat to steel surface for austenitisation, followed by quenching
steels used for selective hardening
steels with 0.3 - 0.6%C
steels are heat treated for core properties before processing
flame hardening process
selected area is heating by torch to red hot and then quenched in water
case depth from flame hardening depends on?
gas mixture used
burner distance - short burner distance means thicker case
what is induction hardening?
selected area is to be surrounded by an inductor coil, high freq current is pass through the coil
induction hardening process
- area surrounded by inductor coil
- high freq current pass through the coil
- induces eddy currents, heating it up
- surface is then quenched with water pressure jets
pattern of heating of induction hardening depends on?
shape of coil
number of turns in the coil
operating frequency
power input
what does case depth depend on in induction hardening? and what is the approximate depth?
current frequency
higher freq = thinner case
approx. 7-8mm can be obtained by induction hardening
