Hydrogen Embrittlement Cracking in a Batch of Steel Forgings
Handbook of Case Histories in Failure Analysis,
Vol 2, K.A. Esakul, Ed., ASM International, 1992
The repeated occurrence of random cracks in the fillet radius portion of low-alloy steel (38KhA) end frame forgings following heat treatment was investigated. Microstructural analyses were carried out on both the failed part and disks of the rolled bar from which the part was made. Subsurface cracks were found to be zigzag and discontinuous as well as intergranular in nature. A mixed mode of fracture involving ductile and brittle flat facets was observed. Micropores and rod-shaped manganese sulfide inclusions were also noted. The material had a hydrogen content of 22 ppm, and cracking was attributed to hydrogen embrittlement. Measurement of hydrogen content in the raw material prior to fabrication was recommended. Careful control of acid pickling procedures for descaling of the hot-rolled bars was also deemed necessary.
Inclusions; Subsurface cracks
(Chromium alloy steel)
Hydrogen damage and embrittlement; Surface treatment related failures
The repeated occurrence of random cracks in the fillet radius portion
of low-alloy steel and frame forgings following heat treatment was investigated.
A batch of 120 end frame parts forged and machined from a 200 mm (8
in.) diam medium-carbon chromium steel rolled bar was heat treated in accordance
with specification requirements. These parts are used for critical applications
in helicopters, and thus utmost care is necessary to ensure quality from the
melting stage onward.
Circumstances leading to failure
Magnetic-particles inspection of the initial batch of 50 heat-treated
parts after cadmium plating revealed several tiny radial cracks (Fig. 1
Fig. 1 Heat-treated end frame component after cadmium plating. Several tiny
cracklike indications, primarily in the base fillet radial zones, were detected
by magnetic-particle testing.
Russian alloy 38KhA was sued for the fabrication of the parts.
One cracked sample was selected for low-power optical and high-power
scanning electron microscopic (SEM) examination. Two 20 mm (0.8 in.) thick
disks sliced from the rolled bar stock used for forging were also examined.
Visual Examination of General Physical Features
Examination of the parts that had been inspected by magnetic-particle
testing showed several tiny radial cracks at random locations, but primarily
confined to the base edge and fillet radius zones (Fig. 1
Testing Procedure and Results
Scanning Electron Microscopy/Fractography.
One of the
tiny cracks was carefully opened. Because the depth of the crack was negligible,
it was difficult to retain the fracture zone intact. The fracture zone was
examined by SEM, which showed the predominantly dimple structure of a ductile
failure. No clue as to the nature and type of cracking could be established.
It was suspected that the hairline cracks could be due to hydrogen embrittlement.
Two disks were sliced from the rolled bar, one in the annealed (as-supplied)
condition and the other in the heat-treated condition. A 50 mm (2 in.) diam
hole similar to the one in the finished end frame part was bored in the center
of each disk. Magnetic-particle-inspection of both disks revealed tiny discontinuous
subsurface cracklike indications in the bore surface (Fig. 2
). One of the subsurface cracks was carefully
opened so that the very small zone of cracking could be retained undamaged
for SEM examination. The fracture clearly exhibited dimpled structures associated
with brittle (cleavage) flat facets, ductile hairline cracks, micropores (Fig. 3
), and rod-shaped manganese
sulfide inclusions (Fig. 4
All of these fracture characteristics are typical of hydrogen embrittlement.
Fig. 2 Curved path of the fatigue crack.
Fig. 3 High-magnification SEM micrograph of the fracture zone of a subsurface
crack in the bore, showing brittle flat facets, ductile dimples, micropores,
and hairline cracks-all indicative of hydrogen embrittlement
Fig. 4 High-magnification SEM micrograph of the same zone shown in
Fig. 3. Rod-shaped manganese sulfide
inclusions are visible, normally a preferred she for hydrogen accumulation.
Microexamination of sections
taken across the defect indications in both disks revealed subsurface discontinuous
hairline (zigzag) cracks (Fig. 5
which were intergranular in nature with no evidence of branching. The microstructure
of the defective part at higher magnification also showed discontinuous intergranular
cracks, with typical manganese sulfide inclusions along the path of cracking
Fig. 5 Optical micrograph of a polished, unetched section across a crack
indication shown in
showing the clear presence of subsurface cracks
Fig. 6 High-magnification optical micrograph showing intergranular cracks
with characteristic manganese sulfide and oxide inclusions in the crack path
of a section across the radial cracks in the heat-treated part
The chemical composition of the end frame material was found to conform
to specifications (Table 1
The cracked end frame part was also subjected to instrumental gas analysis;
the hydrogen gas content was found to be on the order of 22 ppm.
Table 1 Chemical specifications for 38KhA steel
Charpy impact properties determined
in locations both near and remote from the defective zone in the longitudinal
and transverse directions were generally low—44 and 34J, or 32 and 25
ft .lbf, respectively—compared with the specification requirement of
49J (36 ft . lbf).
SEM fractography clearly established that the random occurrence of tiny
cracks was caused by hydrogen embrittlement. This finding was also supported
by the presence of a high hydrogen content in the material prior to heat treatment.
Initially, from the nature and location of the cracks (mostly in the
fillet radius zones), faulty heat treatment was thought responsible. However,
when subsequent heat treatment of the second batch resulted in identical cracks
in the same location (despite satisfactory chemistry, microstructure, and
hardness), the random cracking was strongly suspected to be associated with
a hydrogen-induced phenomenon. Because the tiny cracks in the part could not
be successfully opened, a sub-surface crack in the heat-treated disk was forced
open. The fracture was examined by SEM, which revealed characteristic hydrogen
embrittlement features (Fig. 3
A 50 mm (2 in.) diam hole had been bored into the disk to develop residual
stresses sufficient to cause subsurface cracking. Under high stresses, hydrogen
gas pressure increases rapidly, causing flakes and fissures. A gas content
on the order of 22 ppm present in the raw material was high enough to cause
hydrogen embrittlement in high-strength steel without any external stresses.
The low impact properties measured on the failed part also indicated the brittle
nature of the material caused by hydrogen embrittlement.
Conclusion and Recommendations
Most probable cause
The basic cause of the development of tiny cracks was hydrogen embrittlement.
In the absence of records related to the manufacturing history of the
supplied raw material, hydrogen absorption was suspected to have occurred
during improper pickling of the hot-rolled bars. Careful control of the acid
pickling operation within specified process parameters was recommended. Measurement
of hydrogen content in the raw material prior to fabrication was also suggested.
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