Cause of failure of carburized wheel disc

Kříž Antonín - ZČU v Plzni (Czech Republic)

The present article is based on multiple examinations of a blasting wheel. According to specifications, the wheel is to be carburized to obtain a case depth of 4 mm.This requirement was set out by the designer in order to improve the wheel’s life.On the other hand, there is an effort to keep the cost of the untreated part and the price of carburizing and quenching operations as low as possible, which in turn affects the selection of material suppliers and heat treating contractors.As a result of this cost policy, the products are carburized in one heat treating shop and then heat treated in another.If the resulting part fails to meet specifications, it is difficult to identify the culprit.Omitting the tempering operation after quenching, as expressly specified by the designer on the drawing, proved another deficiency.The quality of this product has been monitored on a long-term basis. Emphasis was laid on metallographic observation and on measuring hardness profiles.Results of these tests for the manufacturing route and the material in question are presented in the paper.The manufacturer of the entire assembly strives to tackle the problem but fails to find good contractors in the preferred price range. Consequently, no solution seems to be at hand.The objective of the paper is not only to present optical micrographs of interesting microstructures and detail scanning electron micrographs of hardening microstructures but mainly to point out the designer’s mistakes and the interrelationship between inadequate input material, incorrectly chosen manufacturing route and the process non-compliance on the part of the heat treating plant.

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