TECHNICAL SERVICE REPORT 122
FAILURE ANALYSIS OF CAST RUDDER
CG-6MMN (NITRONIC 50)
MARCH 1995
ISSUED TO:
STEEL FOUNDERS' SOCIETY OF AMERICA
SUBMITTED BY:
C.D. LUNDIN
K.K. KHAN
MATERIALS JOINING GROUP
MATERIALS SCIENCE & ENGINEERING
THE UNIVERSITY OF TENNESSEE, KNOXVILLE, TN 37996
STEEL FOUNDERS' SOCIETY OF AMERICA
FAILURE ANALYSIS OF CAST RUDDER
CG-6MMN (NITRONIC 50)
INTRODUCTION
A section of a failed cast rudder was submitted by the Steel Founders'
Society of America (SFSA) for analysis of the cause of failure. The failure was
reported to have occurred at the rudder vane-to-shaft transition during
machining/straightening of the casting. A schematic sketch of the rudder vaneto-shaft intersection showing this failure location is presented in Figure 1. A
circular section encompassing the fracture surface was submitted for
evaluation. The casting conforms to CG-6MMN (Nitronic 50). The casting mold
was prepared using the "Pepset" binder system. The heat chemistry reported
by the foundry along with the nominal composition is presented in Table 1. It is
to be noted that, for the elements reported, the chemical composition of the heat
conforms to the specification.
EXAM I N ATI ON
A preliminary examination of the fracture surface at the failure location
was conducted using a stereo-zoom microscope at magnifications to 70X. The
fracture surface morphology was further studied using the scanning electron
microscope. Following examination of the fracture surface, the sample was
sectioned, and metallographically prepared for optical light microscopic
examination. The ferrite content was measured using a Magne-gage.
RESULTS AND DISCUSSION
From the preliminary examination, it was evident that no or little ductility
was associated with the fracture. Initially, visual observation suggested interdendritic hot tearing. However, no evidence of oxidation was present on the
fracture surface and the appearance of the fracture was generally "rough" at low
magnification. An optical photograph of the fracture surface is shown in Figure
2. It is to be noted that hot tears (solidification cracks) generally encountered in
austenitic stainless steel castings are intergranular/interdendritic and normally
reveal "smooth or flowed" fracture faces. This characteristic was not clearly
evident at low magnification. Thus, SEM observations were undertaken at
higher magnification and resolution.
The fracture surface was cleaned with a solution of soap and water in an
ultrasonic bath for 15 minutes. This cleaning was followed by a rinse in distilled
water, followed by a methanol wash and warm air drying before SEM
examination.
SEM fractographs are presented in Figures 3 and 4. Figure 3 is from a
region where the dendrite pattern is clearly evident. Examination at the higher
magnification clearly reveals that the fracture is inter-dendritic. The microfractographic feature are generally of a cleavage-nature with small pockets
revealing a minor extent of microvoid coalescence. Secondary cracking is also
evident. Thus, it is apparent that the crack propagation leading to failure is by a
brittle fracture mechanism. Figure 4, from a location where dendrite pattern is
not as clearly evident, confirms that the fracture is primarily cleavage. No
evidence of hot tears characterized by a "flowed" surface indicative of presence
of a liquid film at the time of crack formation could be seen in any region of the
fracture surface.
The specimen was sectioned along the lines shown in Figure 2 for cross
sectional metallography. Micrographs at 50X, adjacent to the fracture location
and remote (approximately 5 mm) from the fracture location, are shown in
Figure 5. The fracture is inter-dendritic but not necessarily intergranular. It is
evident from these micrographs that the fracture path follows a secondary
microconstituent in the austenite matrix (Figure 5a). In addition, this
microconstituent is virtually completely interconnected.
It is to be noted that cast austenitic stainless steels are designed to
include delta ferrite to avoid solidification tearing problems. Delta ferrite is not
considered to be a brittle constituent at ambient temperatures. Examination of
the microstructure at higher magnification revealed that the microconstituent
along which the fracture took place is, in reality, a mixture of two phases (see
Figure 6). From a comparison of the micrographs presented in Figure 6 with the
SFSA published "Atlas of Austenitic Steel Casting Micrographs", it is evident
that the interdendritic microconstituent mixture, consists of delta ferrite + sigma
phase. It is to be noted that sigma phase is an intermetallic compound that
forms from delta ferrite in austenitic stainless steels during slow cooling from
high temperatures or by extended hold times at intermediate temperatures. In
addition, sigma phase is extremely brittle. Occasional pools of delta ferrite
which had not transformed to sigma phase were noted in the microstructure as
shown in Figure 7. In fact, from Figure 7b, it is to be noted that sigma phase
formation has just begun to form in the delta ferrite pool shown.
Ferrite measurements, using a Magne-gage, revealed that the ferrite
content is 2 FN, this suggests that most of the original solidification delta ferrite
has been transformed to sigma phase (it is to be noted that sigma phase is not
magnetic).
Thus, from the fractographic and metallographic examination it is evident
that the failure occurred by brittle fracture through the interconnected network of
sigma phase in the inter-dendritic regions of the cast structure. The presence of
sigma phase is most likely due to an improper or incomplete solution heat
treatment. In this instance, all evidence points to the fact that the casting was
not solution heat treated (which would re-dissolve sigma phase formed during
cooling). The significant sigma phase formation is most probably the result of a
slow cool after casting. No evidence of hot tearing could be found in any region
of the fracture.
It is recommended that a proper solution heat treatment be conducted
prior to any machining or straightening operations for all heavy walled castings
(or slow cooled castings) of similar compositions.