Martensita

1. ScriptaMetallurgica et Materialia Vol. 32. No. 8, pp.1197-1201.1995
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ON 1NTE:RGRANULAR TEMPERED MARTENSITE EMBRITTLEMENT
K. B. Lee, S. 1% Yoon, S. I. Hong and H. Kwon
Department of Metallurgy and Materials Engineering
College of Engineering, Kookmin University
Seoul 136-702, Korea
(Received June 27, 1994)
(Revised November 8, 1994)
Tempering of martensitic alloy steels is generally required to impart adequately high toughness
instead of brittleness in the as-quenched state. When hardened steels are tempered in the range
of 250 - 400 “C. however. a loss in toughness can occur in suite of the decrease in strennth with
increasing temgerin
f
temperature. Tiis phenomenon is referred to as tempered muartensite
embrittlement (TME
TME can be classified into two types according to fracture mode: intergranular and
transrrranular TME. Transeranular TME was observed generally at test temueratures near the
duct&-brittle transition temperature. It was suggested to- be ca;sed by the f&nation of coarse
carbides at martensite lath boundaries, following Ihe decomposition of retained austenite (l-7).
However, there were studies indicating that ‘I‘ME would be correlated closely with the carbide
coarsening rather than the decomposition of retained austenite G-10). On the other hand, the
intergranular type of TME has been observed generally at test temperatures below the critical
temperature (e.g., transition temperature). It has been associated with the combined action of
coarse carbides .and impurities at the prior austenite grain boundaries (11-16).
In addition, it was suggested that the intrinsic toughness of the matrix, compositionally
modified by the alloying addition, affects the fracture behavior (17-22). The microstructures
combined with intrinsic toughness play a fundamental role detennining the intragranular toughness
(i.e., the resistance to transgranular brittle fracture), whereas, the impurity segregation acts as a
significant determiner of intergranular toughness (i.e., the resistance to intergranular fracture). The
fracture type of TME can be influenced by the intrinsic toughness as well as by the impurity
segregation since it is determined by the competition between the intergranular and intragranular
toughness.
In this study, the intergranular type of TME has been analyzed in terms of impact toughness
and fracture behavior on isothermal and isochronal tempering. The alloy systems chosen are the
commercial 4140 and 4340 steels. The 4340 steel contains the alloying element Ni which enhances
the intrinsic toughness.
The 4340 and 4140 steels as-received were made by air-melting and hot-forging. The 4340
steel had a coarse-grained structure, probably due to the relatively higher melting and/or forging
temperatures, compared with the 4140 steel. The chemical compositions of alloys are listed in
Table 1. Charpy V-notch impact specimens of the standard size were machined from the forgings.
The impact :,pecimens were austenitized in a flowing argon atmosphere at 1200 “C for 1 h, and
then oil-quenched. The different initial structures resulted in a variation in prior austenite grain
size, about 200 and 50 ,umin the 4340 and 4140 steels, respectively. Austenitized specimens were
1197

2. 1198 TEMPEREDMARTFNSITE EMBRITTLEMENT Vol. 32, No. 8
tempered in a neutral salt bath at 200 to 450 ‘C for 1 to 400 h and water-quenched.
The fracture surfaces were examined in a Jeol scanning electron microscope, operated at 25 kV.
The fractographs were taken from the vicinity of notch since the crack initiation may become the
primary stage in the fracture of the impact specimens of the relatively low toughness.
ExDerimental He_s&
The hardness values of both steels were similar and decreased from HRC 53 to 43 with
increasing tempering temperature in the range of 200 to 450 “C. The hardness decrement with
tempering time at a given temperature was slight, as compared to tempering temperature.
Fig. 1 shows the impact toughness variations with tempering time. In the 4340 steel, at 200 “C,
the impact toughness increased in the 1 h tempered condition, but decreased down through a
medium value of 13 J in the 10 h tempred condition to a low value of 5 J in the 400 h tempered
condition. At 250 ‘C, the impact toughness increased in the 1 h tempered condition, but rapidly
decreased to a low value of 6 J already in the 10 h tempered condition, and showed no recovery
even in the 400 h tempered condition. The impact toughness level of the 4140 steel was lower
than that of the 4340 steel. At 200 “C, it increased in the 1 h tempered condition, but continuously
decreased and already reached a low value of 4 J in the 100 h tempered condition, earlier than 400
h in the 4340 steel. At 250 “C, the impact toughness already exhibited a low value of 5 J with
little increase even in the 1 h tempered condition, earlier than 10 h in the 4340 steel, and a slight
decrease was retained without any recovery even in the 400 h tempered condition.
The impact toughness variations with tempering temperature in the 1 h tempered condition
were compared in Fig. 2. The 4340 steel showed the maximum at 200 “C, the rapid decrease at
300 “C, the minimum at 350 “C, the slight increase at 400 “C, and the rapid increase at 450 “C. In
the 4140 steel, there was a maximum at 200 “C, a rapid decrease at 250 “C, a minimum at 300 “C,
and a relatively rapid increase at 400 “C. In the 4140 steel with prior austenite grains of about
100 pm, austenitized at 1250 “C, an increase in grain size gave little influence on impact toughness
variation with tempering temperature, despite the lowering of impact toughness (23).
Fracture surfaces are shown in Figs. 3 and 4. In both steels, the intergranular area increased
with decreasing impact toughness (i.e., increasing tempering time and temperature), and dominated
the specimens with the low impact toughness of about 5 J, consistent with the impact toughness
results. Hence, more tempering time and the higher temperature are required for extensive
intergranular fracture to occur in the 4340 steel, as compared to the 4140 steel.
Discussion and Summary____
TME on isothermal tempering was detected by the continuous decreases in impact toughness
with increasing time in the range of l-400 h conducted in this study at 200 and 250 ‘C. The 4340
steel exhibited the slower decrease in impact toughness to the low value of about 5 J, which was
associated with the occurrence of the mostly intergranular fracture, as compared to the 4140 steel.
In addition, during isochronal tempering for 1 h, there were the TME troughs with the minimum
at 300 and 350 ‘C, in the 4140 and 4340 steels, respectively. Thus, it is seen that TME of the 4340
steel proceeds at a slower rate, as compared to the 4140 steel.
The intergranular type of TME has been associated with the combined action of coarse
carbides and impurities at the prior austenite grain boundaries (11-16). Almost all segregation of
impurities occurs during austenitizing, but the coarse boundary-carbides are formed during
tempering, as a result of the decomposition of retained austenite and/or of the independent
precipitation at all types of boundaries containing the martensite lath and packet boundaries and
the prior austenite grain boundaries, irrespective of the fracture type of TME. This means that the
toughness (or strength) of the grain boundaries, which is dependent upon the impurity segregation
before tempering, may determine the fracture type of TME. On the other hand, in recent years,
Kwon and his coworkers (17-22) suggested that the intrinsic toughness of the matrix
compositionally modified by the alloying addition affect the fracture behavior. If the grain
boundaries act as the slip barriers, the blocked slip bands can induce the crack nucleation at the
grain boundaries. Dislocation pile-ups at the grain boundaries needed for intergranular cracking
also may be affected by the relaxation process at the grain boundaries which is associated with
the inherent slip behavior. This means that the intergranular fracture can be influenced by the
intrinsic toughness which is dependent upon the inherent dislocation motion, as well as the
transgranular fracture.

3. Vol. 32. No. 8 TEMF’EREDMARTENSITEEMEJ~ 1199
Even though the higher impurity content, of course, lowers the impact toughness overall
tempering temperature range, it has little influence on variation in fracture mode with tempering
temperature (4,i!4), or extends the TME temperature range, that is, the embrittlement initiates at
lower tempering temperature and ends at higher temperature (12). In contrast, the 4140 steel
having more S, and similar P compared with the 4340 steel, exhibited the TME minimum at lower
temperature than the 4340 steel. However, the impact toughness presented a recovery at lower
temperature and the TME range was not widened. Hence, the impurity effect, by itself, cannot
reasonably account for the impact toughness variations in the TME region, that is, both the rapid
decrease and increase in impact toughness with tempering in the 4140 steel and the slow decrease
and increase in the 4340 steel.
Here, let us consider the TME process in view of intrinsic toughness. The relatively high
intrinsic toughness due to the Ni-addition in the 4340 steel can cause the relaxation of stresses
concentrated at the grain b0undarie.s to be easier (e.g., by means of the relatively easy cross slip),
in comparison with the 4140 steel. Since the coarser carbides at the grain boundaries are required
to establish the stress concentration for the occurrence of intergranular fracture in the 4340 steel,
the TME slowly proceeds. Furthermore, the coarsening rate of carbides of the 4340 steel, in
which the alloying element Ni has low solubility within cementite and high activity of carbon in
the matrix (25), may be somewhat slower. In order to clearly elucidate the effects of Ni-addition
on intrinsic toughness and microstructures (the retained austenite and the interlath and intralath
carbides), the 41.30+(0-6wt%)Ni system, which may indicate the various fracture modes containing
ductile fracture due to a relatively low carbon content, will be systematically analyzed, using the
instrumented im:pact testing machine.
Thus, the tendency of intergranular TME to slowly proceed in the 4340 steel can be understood
by the following two factors: 1) the delay of the formation of coarse boundary-carbides, and/or 2)
the high intrinsic toughness requiring the coarser boundary-carbides for the activation of brittle
intergranular fracture.
This study was supported by the Non-Directed Research Fund, Korea Research Foundation,
1992.
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References_____
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4. 1200 TEMF’ERFB MARTENSITE EMBRIlTLEMENT Vol. 32, No. 8
TABLE 1. Chemical Composilion of Experimental Alloys. (wt%)
25
=;‘
- 20
z
g 15
ti
I TEMPERING TEMP
~200°c-4J40
EEE1250°c-4340
~200°c-4140
B25O"C-4140
AQ 1 10 100 1000
TEMPERING TIME (h)
25 , 1
FIG. 1 - Impact toughness
variations with tempering time.
FIG. 2 - Impact toughness
variations with tempering
temperature in the specimens
tempered for 1 h.
TEMPERING TEMP(“C)

5. Vol. 32, No. 8 TEMPEREDMARTENSlTE EMBRITIZEMENT 1201
FIG. 3 - Fractographs of the 4340 steel:
(a) 200 “C, 100 h; (b) 200 “C, 400 hi
(c) 250 “C, 10 h
ITIc. 4 - Fractographs of the 4140 steel:
(a) 200 ‘C, 1 hi (b) 200 “C, 100 11:
(c) 250 “C, 1 b.