Temper embrittlement is inherent in many steels and can be characterized
by reduced impact toughness. The state of temper embrittlement has practically
no effect on other mechanical properties at room temperature.
Figure 1 shows schematically the effect of temperature on impact toughness of
alloy steel which is strongly liable to temper embrittlement. Many alloy steels have
two temperature intervals of temper embrittlement. For instance, irreversible
temper brittleness may appear within the interval of 250-400°C and reversible
temper brittleness, within 450-650°C.
The impact toughness of quenched steel after tempering at 250-400°C is lower
than that obtained on tempering at temperatures below 250°C. If brittle steel
tempered at 250-400°C is heated above 400°C and transferred into a tough state,
a second tempering at 250-400°C cannot return it to the brittle state. The rate
of cooling from the tempering temperature within 250-400°C has no effect on
Steel in the state of irreversible temper embrittlement has a bright
intercrystalline fracture at boundaries of former austenitic grains.
This type of brittleness is inherent to some extent to all steels, including
carbon grades. For that reason medium-temperature tempering is, as a rule not
employed in practice, though it can ensure a high yield limit.
Irreversible temper embrittlement is thought to be due to the formation of
carbides on decomposition of martensite, in particular, precipitation of carbides
in the form of films at grain boundaries. At higher temperatures of tempering, this
film disappears and cannot be restored on repeated heating at 250-400°C. Silicon in
low-alloy steels can prevent irreversible temper embrittlement by retarding the
decomposition of martensite.
The embrittlement on high-temperature tempering may manifest itself in two
- as a result of heating at 450-600°C (irrespective of the rate of
subsequent cooling) and effect of temperature, and
- as a result of tempering at temperatures above 600°C with
subsequent slow cooling within the range of 600-450°C.
A high-rate cooling from a tempering temperature above 600°C, for
instance, water-cooling, can prevent the appearance of temper embrittlement.
On the other hand, a quick cooling on tempering at 450-600°C cannot prevent
temper embrittlement. Thus, entering the dangerous temperature interval
from either "below" (on heating and holding at that temperature) or from
"above" (on slow cooling) can produce the same result.
The most important feature of embrittlement on high-temperature tempering is
that the process is reversible. If a steel embrittled through tempering at a
temperature above 600°C with subsequent slow cooling or through tempering at
450-600°C (with any rate of cooling) is again heated above 600°C and cooled
quickly, its impact toughness will restore to the initial value. If the steel
then again enters the dangerous interval of tempering temperatures, it is again
embrittled. A new heating at a temperature above 600°C, followed with quick cooling,
can eliminate the embrittling effect, and so on. This is why the phenomenon discussed
is called reversible embrittlement.
Carbon steels with less than 0.5% Mn are not prone to reversible temper
embrittlement. The phenomenon can only appear in alloy steels. Alloying elements
may have different effects on steel after tempering at the steel proneness to
temper embrittlement. Unfortunately, the most widely used alloying elements, such
as chromium, nickel, and manganese, promote temper embrittlement. When taken
separately, they produce a weaker effect than in the case of combined alloying.
The highest embrittling effect is observed in Cr-Ni and Cr-Mn steels. Small
additions of molybdenum (0.2-0.3%) can diminish temper embrittlement, while
greater additions enhance the effect.
A fundamental fact is that alloy steels of very high purity are utterly
unsusceptible to temper embrittlement which is caused by the presence of various
impurities, in the first place of phosphorus, tin, antimony and arsenic, in
The rate and degree of development of temper embrittlement depend on the
temperature and time of holding steel within the dangerous temperature interval
(450-600°C). With a certain temperature of tempering within this interval, the
initial stages of embrittlement appear appreciably sooner than at a higher or a
Many scientists adhered for a long time to the "solution precipitation" hypothesis,
according to which the loss in impact toughness was caused by precipitation of
some phases, such as phosphides, at grain boundaries. These phases were thought
to pass into the á-solution on heating up to approximately 650°C and to precipitate
from the solution and embrittle the steel on slow cooling; quick cooling should
prevent the precipitation of embrittling phases. As has been found by
electron-microscopic analysis, however, there are no special precipitates at grain
boundaries in embrittled steel, so that the "solution precipitation" hypothesis
turned to be inconsistent.
Another hypothesis explained temper embrittlement by an increased concentration
of impurities in boundary layers of the solid solution. This was proved by an
increased etchability of grain boundaries in embrittled steel by picric acid.
The hypothesis on the leading role of impurity segregates has been fully
confirmed in the recent years by a brilliant series of research work using
Auger spectroscopy, a method enabling determination of concentrations of
elements in monatomic surface layers. Using this method makes it possible to
detect segregations of phosphorus and other impurity elements at the fracture
surface in embrittled steel and measure their concentrations (as also the
concentrations of alloying elements) at the fracture surface. It has also been
shown that the development of temper embrittlement is directly linked with the
rise of impurity concentration near the prior austenite boundaries.
Owing to equilibrium segregation, the concentration of harmful impurities at
the surface of a fracture may exceed tens or hundreds times their average
concentration in the steel. The concentration of impurities in commercial purity
steels is usually a few thousandths or hundredths of a percent, but amounts to a
few percent at the surface of fracture.
As the temperature increases, the diffusion process of grain boundary segregation
is accelerated, with the absolute value of equilibrium segregation being
simultaneously decreased owing to thermal motion. At temperatures above 600-650°C,
the segregation of impurities either disappears fully (Sb) or drops to a very
low level (P). On subsequent cooling of the steel in water, the segregates have no
time to restore.
The role of alloying elements in the development of temper embrittlement
is not less than that of impurities. The segregation of harmful impurities in
iron-carbon alloys is so small that causes no temper embrittlement. In the presence
of alloying elements (Ni, Cr or Mn), the segregation of impurities increases
appreciably. In this process, the alloying elements themselves, which cause no
equilibrium segregation in high-purity steels, segregate at grain boundaries in
the presence of harmful impurities.
Therefore, we can assume that an alloying element and impurity interact
with each other in the á-solution and thus mutually promote their segregation.
It can be also assumed that if atoms of an impurity and alloying element attract
one another stronger than atoms of that impurity and iron, the segregation of the
impurity and alloying element will be mutually enhanced. Namely in this way behave
P and Ni, P and Cr, Sb and Ni, Sb and Mn
and other "impurity - alloying element"
pairs. A second alloying element can additionally enhance segregation of an impurity.
For instance, nickel and chromium, when present together in steel, can cause a greater
segregation of antimony than might be expected from simple summation of their
An increased concentration of harmful impurities in boundary layers of the solid
solution, which may be caused by the effect of alloying additions, weakens the
intergranular bondage and is one of the main causes why alloy steels containing Ni,
Cr or Mn are highly susceptible to temper embrittlement. The main measures to prevent
temper embrittlement are as follows:
- of the content of harmful impurities in steel;
- accelerated cooling from the temperature of high-temperature tempering
- alloying of steel with small additions of molybdenum (0.2-0.3%); and
- subjecting the metal to high-temperature thermo-mechanical treatment.