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