Search Results for Alloy cast iron

An impact breaker bar showed signs of rapid wear. The nominal composition of this chromium alloy cast iron was Fe-2.75C-0.75Mn-0.5Si-0.5Ni-19.5Cr-1.1Mo. The measured hardness of this bar was 450 to 500 HRB. The desired hardness for this material after air hardening is 600 to 650 HRB. The microstructure consisted of eutectic chromium carbides (Cr7C3) in a matrix of retained austenite and martensite intermingled with secondary carbides. Analysis (visual inspection and 500x view of sections etched with Marble's reagent) supported the conclusion that the low hardness resulted from an excessive amount of retained austenite. This caused reduced wear resistance and thus rapid wear in service. Recommendations included avoiding an excessive austenitizing temperature and excessive cooling rates from the austenitizing temperature and controlling the chemical composition to avoid excessive hardenability for the section size involved.

Two damaged impellers made of austenitic cast iron came from a rotary pump used for pumping brine mixed with drifting sand. On one of the impellers, pieces were broken out of the back wall in four places at the junction to the blades. The fracture edges followed the shape of the blade. Numerous cavitation pits were seen on the inner side of the front wall visible through the breaks in the back wall. The back wall of the as yet intact second impeller which did not show such deep cavitation pits was cracked in places along the line of the blades. The microstructure consisted of lamellar graphite and carbides in an austenitic matrix and was considered normal for the specified material GGL Ni-Cu-Cr 15 6 2. It was concluded that the cause of the damage was porosity at the junction between back wall and blades arising during the casting process. Cavitation did not contribute to fracture but also could have led to damage in the long term in the case of a sound casting. It is therefore advisable in the manufacture of new impellers to take care not only to avoid porosity but also to use alloy GGL Ni-Cu-Cr 15 6 3, which has a higher chromium content and is more resistant to cavitation.

A die-cast zinc adapter used in a snowthrower failed catastrophically in a brittle overload manner. The component had a chemical composition similar to standard zinc alloy ZA-27 (UNS Z35840), although the iron content was much higher and the copper slightly lower. The mechanical properties and alloy designation were not specified. Investigation (visual inspection, 187x SEM images, unetched 30x images, hardness testing, and chemical analysis) of both the failed adapter and an exemplar casting from known-good lot supported the conclusion that the casting failed as a result of brittle overload fracture due to excessive iron-zinc phase and gross porosity. These conditions acted synergistically to reduce the strength of the material. The composition was nonstandard, and the inherent brittleness suggested that it was unlikely that this material was an intentional proprietary alloy. No recommendations were made.

This article focuses on the general root causes of failure attributed to the casting process, casting material, and design with examples. The casting processes discussed include gravity die casting, pressure die casting, semisolid casting, squeeze casting, and centrifugal casting. Cast iron, gray cast iron, malleable irons, ductile iron, low-alloy steel castings, austenitic steels, corrosion-resistant castings, and cast aluminum alloys are the materials discussed. The article describes the general types of discontinuities or imperfections for traditional casting with sand molds. It presents the international classification of common casting defects in a tabular form.

The information provided in this article is intended for those individuals who want to determine why a casting component failed to perform its intended purpose. It is also intended to provide insights for potential casting applications so that the likelihood of failure to perform the intended function is decreased. The article addresses factors that may cause failures in castings for each metal type, starting with gray iron and progressing to ductile iron, steel, aluminum, and copper-base alloys. It describes the general root causes of failure attributed to the casting material, production method, and/or design. The article also addresses conditions related to the casting process but not specific to any metal group, including misruns, pour shorts, broken cores, and foundry expertise. The discussion in each casting metal group includes factors concerning defects that can occur specific to the metal group and progress from melting to solidification, casting processing, and finally how the removal of the mold material can affect performance.

This paper describes an investigation on the failure of a large leaded bronze bearing that supports a nine-ton roller of a plastic calendering machine. At the end of the normal service life of a good bearing, which lasted for seven years, a new bearing was installed. However the new one failed catastrophically within a few days, generating a huge amount of metallic wear debris and causing pitting on the surface of the cast iron roller. Following the failure, samples were collected from both good and failed bearings. The samples were analyzed chemically and their microstructures examined. Both samples were subjected to accelerated wear tests in a laboratory type pin-on-disk apparatus. During the tests, the bearing materials acted as pins, which were pressed against a rotating cast iron disk. The wear behaviors of both bearing materials were studied using weight loss measurement. The worn surfaces of samples and the wear debris were examined by light optical microscope, SEM, and energy-dispersive x-ray microanalyzer. It was found that the laboratory pin-on-disk wear data correlated well with the plant experience. It is suggested that the higher lead content ~18%) of the good bearing compared with 7% lead of the failed bearing helped to establish a protective transfer layer on the worn surface. This transfer layer reduced metal-to-metal contact between the bearing and the roller and resulted in a lower wear rate. The lower lead content of the failed bearing does not allow the establishment of a well-protected transfer layer and leads to rapid wear.

A brackish water pump impeller was replaced after four years of service, while its predecessor lasted over 40 years. The subsequent failure investigation determined that the nickel-aluminum bronze impeller was not properly heat treated, which made the impeller susceptible to aluminum dealloying. The dealloying corrosion was exacerbated by erosion because the pump was slightly oversized. The investigation recommended better heat treating procedures and closer evaluation to ensure that new pumps are properly sized.

A high-chromium white cast iron shell liner installed in an ore crusher sustained impact damage in the course of operation. Visual-optical examination revealed horizontal cracks on the surface of the liner along with particles that had fractured off. Metallographic examination indicated a heavily deformed surface layer with chip formation at the wear surface. The chemical composition of the liner was found to be Fe-2.74C-0.75Mn-0.55Si-0.51Ni-19.4Cr-1.15M. This alloy is highly resistant to abrasive wear, yet at the same time, prone to chipping because little plastic displacement will occur at the surface. The liner failed as a result of severe abrasion caused by the impact of taconite rock. This was a material-selection problem in that the wrong alloy was used for a condition not anticipated in the original choice.

A sand-cast LM6M aluminum alloy sprocket drive wheel in an all-terrain vehicle failed. Extensive cracking had occurred around each of the six bolt holes in the wheel. Evidence of considerable deformation in this area was also noted. Examination indicated that the part failed because of gross overload. Use of an alloy with a much higher yield strength and improvement in design were recommended.

Two oil-pump gears broke after four months of service in a gas compressor that operated at 1000 rpm and provided a discharge pressure of 7240 kPa (1050 psi). The compressor ran intermittently with sudden starts and stops. The large gear was sand cast from class 40 gray iron with a tensile strength of 290 MPa (42 ksi) at 207 HRB. The smaller gear was sand cast from ASTM A536, grade 100-70-03, ductile iron with a tensile strength of 696 MPa (101 ksi) at 241 HRB. Analysis (metallographic examination) supported the conclusion that excessive beam loading and a lack of ductility in the gray iron gear teeth were the primary causes of fracture. During subsequent rotation, fragments of gray iron damaged the mating ductile iron gear. Recommendations included replacing the large gear material with ASTM A536, grade 100-70-03, ductile iron normalized at 925 deg C (1700 deg F), air cooled, reheated to 870 deg C (1600 deg F), and oil quenched. The larger gear should be tempered to 200 to 240 HRB, and the smaller gear to 240 to 280 HRB.

One of three valves in a dry automatic sprinkler system tripped accidentally, thus activating the sprinklers. Maintenance records showed that the three valves had been in service less than two years. The valve consisted of a cast copper alloy clapper plate that was held closed by a pivoted malleable iron latch. The latch and top surface of the clapper plate were usually in a sanitary-water environment (stabilized, chlorinated well water with a pH of 7.3) under stagnant conditions. Process make-up water that had been clarified, filtered, softened, and chlorinated and had a pH of 9.8 was occasionally used in the system. Analysis (visual inspection and 250x micrograph) supported the conclusions that failure of the latch was caused by plastic deformation from extensive loss of metal by galvanic corrosion and the sudden loading related to the tripping of the valve. Failure in some regions of the contact area was by ductile (transgranular) fracture. Recommendations included changing the latch material from malleable iron to silicon bronze (C87300). The use of silicon bronze prevents corrosion or galvanic attack and proper adjustment of the latch maintains an adequate contact area.

Cast iron bearing caps in tractor engines fractured repeatedly after only short operating periods. The fracture originated in a cast-in groove and ran approximately radially to the shaft axis. The smallest cross section was at the point of fracture. The core structure of the caps consisted of graphite in pearlitic-ferritic matrix. Casting stresses did not play a decisive role because of the simple shape of the pieces that were without substantial cross sectional variations. Two factors exerted an unfavorable effect in addition to comparatively low strength. First, the operating stress was raised locally by the sharp-edged groove, and second, the fracture resistance of the cast iron was lowered at this critical point by the existence of a ferritic bright border. To avoid such damage in the future it was recommended to observe one or more of the following precautions: 1) Eliminate the grooves; 2) Remove the ferritic bright border; 3) Avoid undercooling in the mold and therefore the formation of granular graphite; 4) Inoculate with finely powdered ferrosilicon into the melt for the same purpose; and, 5) Anneal at lower temperature or eliminate subsequent treatment in consideration of the uncomplicated shape of the castings.

After operating for six months, a pump impeller (of nickel-containing cast iron) showed considerable corrosion. Cross sections showed substantial penetration of the wall thickness without loss of material. The observed supercooled structure implied low strength but would not affect corrosion resistance. Etching of the core structure showed a selective form of cast iron corrosion (spongiosis or graphitic corrosion) which lowered the strength of the cast iron enough that a knife could scrape off a black powder (10.85% C, 1.8% S, 1.45% P). Analysis showed that some of the “sulfate” found in the scrubbing water was actually sulfide (including hydrogen sulfide) and was the main cause of corrosion.

A commercial hybrid-iron golf club fractured during normal use. The club fractured through its cast aluminum alloy hosel. Optical analysis revealed casting pores through 20% of the hosel thickness. Mechanical properties were determined from characterization results, then used to construct a finite element model to analyze material performance under failure conditions. In addition, a full scale structural test was conducted to determine failure strength. It was concluded that the club failed not from ground impact but from a force reversal at the bottom of the downswing. Large moments generated during the downswing aggravated by manufacturing defects and stress concentration combined to create an overload condition.

Cluster bomb tailcone assemblies each containing two aluminum die-cast components were rejected because of the poor surface condition of the die castings. Numerous heat checks were found on the surfaces of the tailcones and radiographic inspection revealed inclusions, gas holes, and shrinkage defects in the castings. Most of the components failed to meet required mechanical properties because of these casting defects.