During tempering, embrittlement can occurs. embrittlement is a serious problem for steel. further information is given in the slide.

1. A SICKNESS DURING
TEMPERING

2. Types of Embrittlement
 Temper embrittlement(TE)
 Tempered martensite embrittlement(TME)
 Hydrogen embrittlement
 Liquid metal embrittlement
Some of the embrittlement mechanisms are due to structural changes introduced during
processing and tempering, and some are due to the interaction of environment with the
quench and tempered microstructure. The characteristics of tempered martensite
embrittlement are well known.

3. Occurs after tempering between 260 to 370°C(500-700°F) and is also referred to as
350°C embrittlement or 500°F embrittlement.
 Also called as ‘irreversible embrittlement’ and ‘one-step embrittlement’ because it
does not occur again on tempering in this region.
 There are two important structural features in TME.
o segregation of minor elements, mostly P to prior austenite grain boundaries.
o decomposition of retained austenite into carbide upon tempering
 P weakens the adhesion at the austenite grain boundaries.
 The subsequent formation of cementite at the original austenite grain boundaries,
may increase the segregation of impurity atoms as they are rejected from the growing
cementite particles.
 Thus, cracking of cementite initiates inter-granular fracture.

4. The lower toughness of the higher phosphorus
steel was related to a substitutional amount of
intergranular fracture.
 Addition of silicon inhibits the formation of
Cementite in the critical range, silicon dissolved
in Ɛ-carbide makes it stable, so no TME.
Addition
of Si
Stabilize
Ɛ-carbide
No TME
P TME

5. Upon tempering, the retained austenite decomposed isothermally to ferrite and carbide.
Change in the structure of Ɛ-carbide to cementite in the form of a film at the grain
boundaries. The minimum in the impact energy-temp is due to the formation of thin
carbides from retained austenite.
In some low carbon steels, TME is associated with carbide morphology. It provides numerous
sites for microcrack initiation, their growth by microvoid coalescence.
Crack initiation and crack growth occurs by microvoid around carbide particles.
 The initiation of the translath cleavage attributed to cracking of relatively thick cementite
particle at the grain boundaries.
So, this greater amounts of interlath and grain-boundary carbides, showed greater
transgranular cleavage and intergranular fracture modes of TME.
Effect of tempering temperature on impact strength in
steels prone to embrittlement during tempering

6. Occurs when alloy steels are temped in the range of 450°C to 600°C
Also called reversible embrittlement, as de-embrittlement may occur on
heating to about 575°C after holding only a few minutes in atmosphere.
It decreases toughness and increases ductile brittle transition
temperature.
Small amounts(~100ppm or less) of Pb, P, Sn and As have been shown to
cause TE.
Plain carbon steels are not considered to be susceptible to TE, provided
the Mn content is below 0.5%.
Mo in 0.5% or in less added to steel to minimize TE.

7. Hardness, UTS, yield strength, %elongation and fatigue properties
are not much affected by TE.
Co-segregation(double equilibrium segregation) takes place.
TSTE develops with aging time.
If the steel, at the end of the initial tempering time is cooled too
slowly through the temp. range TE occurs.

8. TE
toughness
Ductile Brittle
Transition
Temp.
Intergranular
fracture
Co-segregation