According to available information on the fracture toughness of high-strength alloys at low temperatures, the effect of low temperatures on toughness is generally dependent on the alloy base.
Alloy steels normally exhibit decreasing fracture toughness as the testing temperature is decreased through transition temperature range, when the structure contains ferrite or tempered martensite. The transition temperature is influenced by the alloy content, grain size and heat treatment.
Current and developing applications for materials at low
temperatures include structures, vehicles, and pipeline
equipment for arctic environments, storage and transport
equipment for liquefied fuel gasses, oxygen and nitrogen,
and superconducting machinery, devices and electrical
transmission systems. Most of these applications
relate to the production and distribution of
energy and have attained greater prominence because
of the current energy shortage.
According to available information on the fracture
toughness of high-strength alloys at low
temperatures, the effect of low temperatures
on toughness is generally dependent on the alloy base.
For many aluminum alloys, the fracture toughness tends
to increase or remain generally constant as the testing
temperature is decreased. Titanium alloys tend to have
lower toughness as the testing temperature is decreased,
but the effect is influenced by the alloy content and heat
treatment. Certain titanium alloys retain good toughness
at very low temperatures. Alloy steels normally exhibit
decreasing fracture toughness as the testing temperature
is decreased through transition temperature range, when
the structure contains ferrite or tempered martensite.
The transition temperature is influenced by the alloy
content, grain size and heat treatment.
Carbon and low alloy steels represent body-center-cubic
(bcc) atomic lattices and exhibit toughness transition
temperature ranges either above, at, or below room
temperature depending on a number of factors.
At temperatures above the transition temperature, the
alloy has substantially better toughness than at lower
temperatures. Furthermore, the lower strength steels
generally are strain-rate sensitive, while the higher
strength steels are not strain-rate sensitive
The curves for parent metal and welds in ASTM A517F steel
plate indicate that the weld metal and heat-affected
zones have lower transition temperatures than the parent
metal. However, the weld metal in the specimens
of A542 steel had higher transition
temperatures than the parent metal. The fracture
toughness of A533 Grade B Class 1 steel has been
studied extensively for nuclear reactor pressure vessels.
This study has shown that for testing temperatures above
-100oF (approx. -70oC), the toughness
increases substantially as the testing temperature is
increased.
Thus the required thickness of the specimens
must be increased in order to increase the constraint that
is necessary at the crack tip to simulate plane-strain
conditions at the initiation of fracture.
Results of fracture toughness tests on three
ASTM forging steels may have similar general trends in
the toughness data, but the compositions, grain sizes,
and other factors have marked effects on the transition
temperatures. Test results for HY-130 steel indicate that
this steel in the temperature range
down to -320oF (approx. -195oC) is
not strain-rate sensitive.
These steels are not intended for use at temperatures
in or below the transition temperature range, and there
is no accepted method for indicating the specific
transition temperature from a transition temperature curve.
Furthermore, there is no accepted method relating the
transition temperature to a safe minimum service
temperature for structural components. However, if Kic
data are obtained for given alloy at low temperatures,
the critical crack sizes may be estimated in the
low-temperature range at the maximum service stress
of the structure.
The effects of variations in composition on
a series of Ni-Cr-Mo-V steels has been studied
in order to show the effects of the alloying
elements on the low-temperature fracture toughness.
Bars of these steels were quenched and tempered
to about 170 ksi yield strength (approx. 1170 MPa) and
tested as precracked bend specimens. The effects
of carbon and nickel content were the most
significant. An increase of carbon content
and nickel content from 0.28 to 0.41 raised the transition
temperature based on KQ data. Increasing the nickel content
from 1.26 to 6.23 percent decreased the KQ transition
temperature. This represents one of the major
attributes of nickel additions to the alloy
steels.
The specimens of D6ac steel were austenitized
at about 1650oF, furnace cooled to
975oF, and quenched in oil or molten salt
according to several different procedures, to simulate
quenching of the welded forging that comprise
the F111 wing cary-through structure. The high-toughness
specimens were quenched in oil, while the medium-toughness
specimens were quenched in salt. Regardless of
the quench, the yield strength of the specimens
was approximately 217 ksi (1495 MPa) after tempering twice
at 1000 to 1025oF. The fracture roughness tests
were very sensitive indicators of the effect
of the variation in quenching rate on the
toughness. The specimens that had the highest toughness
at room temperature also had the highest toughness at
-65oF (-54oC).
Available fracture toughness data at low temperature for
other alloy steels: AISI 4340, 300M, HP9-4-20, HP9-4-25,
and 18 Ni (200) maraging steel usually have the trend
of decreasing toughness as the testing
temperature is decreased. The one exception is HP9-4-25 in
the temperature range +75 to -75oF
(+24 to -59oC). At lower temperature, the expected
trend would be for the toughness to drop as indicated for
HP9-4-20 in the range from -100 to -320oF
(-73 to -195oC). The data obtained by
Steigerwald for AISI 4340 steel and by Wessel for
the HP9-4-20 alloy steel were obtained before ASTM
Method E 399 was available and are designated as KQ values.
The 18Ni (200) grade maraging steel also exhibits
considerable reduction in toughness as the testing
temperature is reduced from -100 to -320oF
(-73 to -195oC), but at -320oF,
this heat of the 200 grade retained a toughness
of about 80 ksi in.1/2. From limited
information on the toughness of the 200 grade,
it appears that there is considerable range in results
of KIc tests at room temperature. This level
of toughness at -320oF probably can
be achieved only if the alloy has a toughness of
about 160 ksi in.1/2 or over at 75oF
(+24oC).
The effect of low temperatures on the static
and dynamic fracture toughness of bend specimens
of 18Ni (200) maraging steel is a straight
line relationship between the Kic values and the testing
temperature in the range from 75 to -320oF.
At -320oF, the KIc value was about 40 ksi
in.1/2, and the alloy is not strain-rate sensitive in
the low-temperature range. Tests results on part-through
surface-crack specimens of 200 grade maraging
steel has shown that these heats had high toughness at
75oF and also retained relatively good
toughness at -320oF. With optimum welding
conditions, the weld metal also retains good strength
and toughness at -320oF.