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Fracture of case-hardened steels
Scripta METALLURGICA
Vol. 9, pp. 563-568 Printed in the United States
Pergamon Press,
Inc.
FRACTURE OF CASE-HARDENED STEELS
H. L. Marcus and J. M. Harris Science Center, Rockwell International Thousand Oaks, California 91360
(March 6, 1975)
Very often the fracture mode of case-hardened carburized steel is found to be intergranular (1,2).
Thermal mechanical processing which upsets the continuous prior austenite
grains tends to minimize the intergranular f a i l u r e mode. In this paper a study of the fracture mode and surface chemistry of two case-hardened steels is made. These are the lOl8 carbon steel, and the 4340 low alloy steel.
Of particular interest was to determine
whether the intergranular fracture modewas related to the chemistry of the grain boundaries The samples were case hardened using the carburizing and aging schedules shown in Table I.
The case depths were varied from 0.04 cm to a total carburization of the samples. TABLE l Carburizing and Aging Schedule lOl8 and 4340 Steels
Group l
Carburized endothermally in CO2 lO/15 for 3-I/2 Hrs @900°C - a i r cooled Austenized endothermally in CO2 lO/15 for 30 min. @815°C - o i l quenched Tempered in a i r 2 Hrs @150°C
Group 2
Carburized per MiI-S-6090A endothermally in CO2 lO/15 + NH3 for l hour @900°C - chamber cooled
Group 3
Carburized in H2 + 1.4% CH4 (with graphite over gas i n l e t tube) for 7 hours @lO00OC - o i l quenched
Group 4
Carburized in H2 + 1.4% CH4 for 2 hours @I025°C - water quenched
The sample geometry was 0.25 cm square and 2.5 cm long with a notch present before carburization to ensure fracture under the impact loading used. The samples were fractured in situ in a lO-9 t o r r high vacuum system adapted for Auger electron spectroscopy (AES) (3).
The
fracture surfaces were chemically analyzed using AES starting from the outer edge of the
563
$64
FRACTURE
OF CASE-HARDENED
STEELS
Vol.
9, No.
S
case and at various locations to the center of the specimen. The surfaces were also analyzed in depth using the combined AES and argon ion sputtering techniques (3,4).
Subsequent to
the AES analysis, the fracture surfaces were characterized with a scanning electron microscope. The results for each material w i l l be disucssed separatley. lOl8 The fracture surface of the lOl8 steel with a .050 cm case fractured at room temperature was found to consist of two regions.
The case region showed an intergranular fracture, with
the center region the classical ductile dimple rupture mechanism. AES results before and after sputtering of the intergranular region are shown in Figure I.
No particular chemistry
difference localized at the grain boundary is seen. Although some S and P is noted, they do not sputter away and, therefore, are most l i k e l y associated with particles in the plane of the fracture.
More S is present in the other region, implying that the sulfur bearing
particles serve as part of the fracture path in a l l regions.
An interesting feature of AES
was the detection of As in the ductile region of samples carburized in Group l (ref. Table l ) as shown in Figure 2.
The As was removed by argon ion sputtering o f f less than 20A from
the fracture surface.
This result w i l l be discussed later.
At low temperatures the fracture
was transgranular cleavag~ interior to the case. 4340 The 4340 did not always f a i l in an intergranular mode at the case. When the fracture was intergranular the AES results show the grain boundaries did not have a significant change in chemistry, Figure 3.
As noted for lOl8, As was detected in the ductile fracture
region of the 4340 steel in samples carburized with the Group l thermal history. Discussions The AES study indicates that grain boundary segregation does not play a major role in the carburized related intergranular failure.
The results indicate that at very high
carbon levels and corresponding matrix strength the grain boundary fracture strength is lower than the cleavage or yield strength.
When the carbon content is s l i g h t l y (locally)
lower, the fracture either proceeded into the cleavage mode at low temperatures or the material yielded resulting in the normal ductile mode.
In a l l cases no direct dependency
of the intergranular failure on the presence of an excessive amount of trace elements was found. Of particular interest was the result that As was present in the ductile fracture path of both the lOl8 and 4340 when the material was o i l quenched from 815°C and tempered 2 hours at 150°C. In this condition small carbides w i l l be formed and grow a limited amount and the presence of the As on the intergranular fracture surface can be interpreted using the solute rejection from the carbide model (5,6).
In the case of temper embrittlement (4,5) the
rejected elements concentrate at the grain boundaries.
In this study the rejected As would
be present at the carbide matrix interface and the fracture path would follow this weakened
Vol.
9, No.
5
FRACTURE
OF CASE-HARDENED
STEELS
565
space. The local nature of the As as determined by the sputtering profiles to be less than 20A indicate i t s presence in a thin, concentrated 2-dimensional layer.
This may help
explain why the presence of As can reduce the fracture strength of steels even without a change to an intergranular fracture mode. Conclusions I.
Trace elements do not play a predominant role in the grain boundary fracture in carburized steels.
2.
The solute rejection of As to the matrix carbide interface as carbides are formed can influence the transgranular fracture strength of steel.
Acknowledgement This work was supported by the Air Force Office of Scientific Research, AFSC contract F44620-71-C-0059. References I.
Metals Handbook, Eighth Edition 2, 74 (1974).
2.
C. A. Apple and G. Krause, Met. Trans. 4, I195 (1973).
3.
H. L. Marcus and P. W. Palmberg, Trans. AIME 245, 1664 (1969).
4.
P. W. Palmberg and H. L. Marcus, ASM Trans. Quart. 62, lOl6 (1969).
5.
J. R. Rellick and C. J. McMahon, J r . , Met. Trans. 5, 2439 (1974).
6.
H. Ohtani, H. C. Feng and C. S. McMahon, J r . , Met. Trans. 5, 516 (1974).
566
FRACTURE
OF CASE-HARDENED
STEELS
Vol.
9, No.
5
Vol.
9, No.
5
FRACTURE
I
OF CASE-HARDENED
I
STEELS
I
I
567
I
1018 AS CARBURIZED TG
F-20
SPUTTERED IOX
IOX
(b)
Alr~ • ELECTRON ENERGY ( e V )
FIG. 2 AE$ of transgranular ductile fracture o f ca~burlzed I018 i n t e r i o r to case: (a) as fractured showing
$68
FRACTURE OF CASE-HARDENED STEELS
4340 AS CARBURIZED IG
dN~
~
11
SCl
200
I
, C
),, ,.: ,, 0 600 800 ELECTRON ENERGY(eV)
FIG. 3 AES of intergranular fracture of c ~ r l z e d 4340 steel.
Vol.
9, No.
S
Vol. 9, pp. 563-568 Printed in the United States
Pergamon Press,
Inc.
FRACTURE OF CASE-HARDENED STEELS
H. L. Marcus and J. M. Harris Science Center, Rockwell International Thousand Oaks, California 91360
(March 6, 1975)
Very often the fracture mode of case-hardened carburized steel is found to be intergranular (1,2).
Thermal mechanical processing which upsets the continuous prior austenite
grains tends to minimize the intergranular f a i l u r e mode. In this paper a study of the fracture mode and surface chemistry of two case-hardened steels is made. These are the lOl8 carbon steel, and the 4340 low alloy steel.
Of particular interest was to determine
whether the intergranular fracture modewas related to the chemistry of the grain boundaries The samples were case hardened using the carburizing and aging schedules shown in Table I.
The case depths were varied from 0.04 cm to a total carburization of the samples. TABLE l Carburizing and Aging Schedule lOl8 and 4340 Steels
Group l
Carburized endothermally in CO2 lO/15 for 3-I/2 Hrs @900°C - a i r cooled Austenized endothermally in CO2 lO/15 for 30 min. @815°C - o i l quenched Tempered in a i r 2 Hrs @150°C
Group 2
Carburized per MiI-S-6090A endothermally in CO2 lO/15 + NH3 for l hour @900°C - chamber cooled
Group 3
Carburized in H2 + 1.4% CH4 (with graphite over gas i n l e t tube) for 7 hours @lO00OC - o i l quenched
Group 4
Carburized in H2 + 1.4% CH4 for 2 hours @I025°C - water quenched
The sample geometry was 0.25 cm square and 2.5 cm long with a notch present before carburization to ensure fracture under the impact loading used. The samples were fractured in situ in a lO-9 t o r r high vacuum system adapted for Auger electron spectroscopy (AES) (3).
The
fracture surfaces were chemically analyzed using AES starting from the outer edge of the
563
$64
FRACTURE
OF CASE-HARDENED
STEELS
Vol.
9, No.
S
case and at various locations to the center of the specimen. The surfaces were also analyzed in depth using the combined AES and argon ion sputtering techniques (3,4).
Subsequent to
the AES analysis, the fracture surfaces were characterized with a scanning electron microscope. The results for each material w i l l be disucssed separatley. lOl8 The fracture surface of the lOl8 steel with a .050 cm case fractured at room temperature was found to consist of two regions.
The case region showed an intergranular fracture, with
the center region the classical ductile dimple rupture mechanism. AES results before and after sputtering of the intergranular region are shown in Figure I.
No particular chemistry
difference localized at the grain boundary is seen. Although some S and P is noted, they do not sputter away and, therefore, are most l i k e l y associated with particles in the plane of the fracture.
More S is present in the other region, implying that the sulfur bearing
particles serve as part of the fracture path in a l l regions.
An interesting feature of AES
was the detection of As in the ductile region of samples carburized in Group l (ref. Table l ) as shown in Figure 2.
The As was removed by argon ion sputtering o f f less than 20A from
the fracture surface.
This result w i l l be discussed later.
At low temperatures the fracture
was transgranular cleavag~ interior to the case. 4340 The 4340 did not always f a i l in an intergranular mode at the case. When the fracture was intergranular the AES results show the grain boundaries did not have a significant change in chemistry, Figure 3.
As noted for lOl8, As was detected in the ductile fracture
region of the 4340 steel in samples carburized with the Group l thermal history. Discussions The AES study indicates that grain boundary segregation does not play a major role in the carburized related intergranular failure.
The results indicate that at very high
carbon levels and corresponding matrix strength the grain boundary fracture strength is lower than the cleavage or yield strength.
When the carbon content is s l i g h t l y (locally)
lower, the fracture either proceeded into the cleavage mode at low temperatures or the material yielded resulting in the normal ductile mode.
In a l l cases no direct dependency
of the intergranular failure on the presence of an excessive amount of trace elements was found. Of particular interest was the result that As was present in the ductile fracture path of both the lOl8 and 4340 when the material was o i l quenched from 815°C and tempered 2 hours at 150°C. In this condition small carbides w i l l be formed and grow a limited amount and the presence of the As on the intergranular fracture surface can be interpreted using the solute rejection from the carbide model (5,6).
In the case of temper embrittlement (4,5) the
rejected elements concentrate at the grain boundaries.
In this study the rejected As would
be present at the carbide matrix interface and the fracture path would follow this weakened
Vol.
9, No.
5
FRACTURE
OF CASE-HARDENED
STEELS
565
space. The local nature of the As as determined by the sputtering profiles to be less than 20A indicate i t s presence in a thin, concentrated 2-dimensional layer.
This may help
explain why the presence of As can reduce the fracture strength of steels even without a change to an intergranular fracture mode. Conclusions I.
Trace elements do not play a predominant role in the grain boundary fracture in carburized steels.
2.
The solute rejection of As to the matrix carbide interface as carbides are formed can influence the transgranular fracture strength of steel.
Acknowledgement This work was supported by the Air Force Office of Scientific Research, AFSC contract F44620-71-C-0059. References I.
Metals Handbook, Eighth Edition 2, 74 (1974).
2.
C. A. Apple and G. Krause, Met. Trans. 4, I195 (1973).
3.
H. L. Marcus and P. W. Palmberg, Trans. AIME 245, 1664 (1969).
4.
P. W. Palmberg and H. L. Marcus, ASM Trans. Quart. 62, lOl6 (1969).
5.
J. R. Rellick and C. J. McMahon, J r . , Met. Trans. 5, 2439 (1974).
6.
H. Ohtani, H. C. Feng and C. S. McMahon, J r . , Met. Trans. 5, 516 (1974).
566
FRACTURE
OF CASE-HARDENED
STEELS
Vol.
9, No.
5
Vol.
9, No.
5
FRACTURE
I
OF CASE-HARDENED
I
STEELS
I
I
567
I
1018 AS CARBURIZED TG
F-20
SPUTTERED IOX
IOX
(b)
Alr~ • ELECTRON ENERGY ( e V )
FIG. 2 AE$ of transgranular ductile fracture o f ca~burlzed I018 i n t e r i o r to case: (a) as fractured showing
$68
FRACTURE OF CASE-HARDENED STEELS
4340 AS CARBURIZED IG
dN~
~
11
SCl
200
I
, C
),, ,.: ,, 0 600 800 ELECTRON ENERGY(eV)
FIG. 3 AES of intergranular fracture of c ~ r l z e d 4340 steel.
Vol.
9, No.
S