Investigation into failure of engine parts

Result code in IS VaVaI

<a href="https://www.isvavai.cz/riv?ss=detail&h=RIV%2F49777513%3A23210%2F20%3A43961465" target="_blank" >RIV/49777513:23210/20:43961465 - isvavai.cz</a>

Result on the web

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DOI - Digital Object Identifier

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Result language

angličtina

Original language name

Investigation into failure of engine parts

Original language description

The company supplied failed parts of an operating bus engine. The investigation was carried out in three stages. The first one primarily involved the screws. Their fracture surfaces and threads were documented. The second stage focused on the cast iron bearing bracket. Metallographic characterization and measurement of chemical composition. The last stage focused on the structural condition of a failed bearing bracket. Hardness was measured on the part. The failed screws exhibit characteristic fatigue fracture with a relatively small final separation region. The crack propagated slowly and final separation was induced by a relatively small force. Metallographic examination revealed that the screws had been quenched and tempered, as required. Neither these inclusions nor the surface decarburization had impact on the failure. Chemical analysis of screws confirmed that the material was 30CrMnB steel. Recommendations: - use screws with thicker shanks and larger heads, as larger diameters provide a larger area to transmit the load; - reduce the diameter of the drilled hole to the size of the screw shank; - notify the vendor of the slight surface decarburization and minute carbides along grain boundaries. Examination of the cast iron bearing bracket provided important information on the cause of the defect in the engine. With the exception of carbon concentration, the chemical composition conforms to the specifications of the cast iron. The carbon content was at the upper specified limit. In some measurements, it even exceeded the specification. The microstructure of cast iron and the morphology of graphite were assessed according to EN ISO 945-1. The form of graphite particles has a major impact on mechanical properties. Once the material has been cast with graphite of adverse morphology, there is no heat treatment to correct it. If graphite takes an incorrect form, the part must either operate under controlled reduced stress (both mechanical and thermal) or be scrapped and remanufactured. To produce the right type of cast iron with appropriate graphite distribution, specific metallurgical procedures are necessary (chemical composition, pouring temperature, chills and others) for particular thickness of the casting. It was grey cast iron with flake graphite type I. Graphite was of mixed form, type C. Large graphite particles are particularly adverse, as they become weak spots which may lead to failure. The mean graphite size, depending on its type, was between 4 and 5. Graphite in this microstructure has various sizes and is not distributed uniformly, particularly with respect to the depth below the surface. This graphite structure and size is only adequate for parts under mild loads. In some regions, graphite flakes embedded in cementite were found. The presence of cementite makes the cast iron even more brittle. The cast iron under analysis does not meet the hardness specifications under GH190. It is clearly a brittle fracture which requires relatively little energy to propagate. Starting cracks were found in the dismantled parts (bearing brackets). Brinell hardness was, again, around 163 HB which does not conform to GH190 specifications. Carbon equivalent (Ce) of this cast iron is 1.04. This carbon equivalent is a limit value for grade GG15 (if it were 1.05, the cast iron grade would be GG10). According to specifications, grade GG20 should have a hardness up to 250 HB. Grade GG15 (or GG10) has a much lower hardness and is not adequate for heavy-duty machine parts. The microstructure revealed by analysis is not adequate for these loads either, particularly in terms of graphite size. Recommendations: – alter the metallurgical procedure (use inoculation) to modify the graphite size, distribution and form; – check chemical composition to ensure that the appropriate carbon equivalent is obtained (0.93-0.96); – measure Brinell hardne ss (according to ČSN EN ISO 6506-1) in the engine blocks and include this measurement in acceptance conditions (HB should be in the range of 200 to 230, a narrower interval would be desirable and should be agreed); – should failures continue to occur in the engines, focus on the condition of the cast iron bearing brackets or the engine blocks.

Czech name

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Czech description

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Type

V_souhrn - Summary research report

CEP classification

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OECD FORD branch

20501 - Materials engineering

Project

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Continuities

N - Vyzkumna aktivita podporovana z neverejnych zdroju

Publication year

2020

Confidentiality

C - Předmět řešení projektu podléhá obchodnímu tajemství (§ 504 Občanského zákoníku), ale název projektu, cíle projektu a u ukončeného nebo zastaveného projektu zhodnocení výsledku řešení projektu (údaje P03, P04, P15, P19, P29, PN8) dodané do CEP, jsou upraveny tak, aby byly zveřejnitelné.

Number of pages

38

Place of publication

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Publisher/client name

IVECO Czech Republic a.s.

Version

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