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Failure of Cold Drawn Steel Wires due to Formation of Friction Martensite
The C-Mn-Si steel wires with a typical composition of C-0.81, Mn-0.78, Si-0.27, S-0.006 and P-0.016 were cold drawn to various shapes and sizes for fabricating steel ropes. The steel wires thus drawn were reported to have failures/ breaks, sometimes during drawing itself and the other times during service of ropes. In order to identify the cause of failure, investigation was carried out on both the wire samples broken during wire drawing as well as during the service of rope. The diagnostics consisted of the observations under optical microscope and scanning electron microscope including EDS, measurement of micro hardness and evaluation of tensile strength. Based upon the study, it was revealed that both the wire samples, in general, contained ferrite pearlite microstructure and tensile strength ranging between 1450-2050 MPa, coupled with a total elongation between 5-9%. On careful examination of the microstructure it was revealed that a white colored layer having fine structure was present near to the wire surface/ edges at locations where failures occurred. The fine structure at higher magnification was identified as untempered martensite, which was found to have micro hardness of 754 VHN as compared to 628 VHN in the centre. This confirmed the presence of harder micro-constituent in the surface, which most probably caused failure of the wires due to its brittle nature. As far as the transformation of this type of martensite is concerned, it could be inferred that during wire drawing as well as during service of ropes, excessive wear/ friction takes place due to abusive operating conditions of improper lubrication. Thus, under the sliding wear conditions a large amount of localized heat is generated, raising the surface temperature to austenitizing levels. The heated surface subsequently cools down rapidly, resulting in transformation of harder microstructure called the friction martensite. The structure remains untempered because of the faster cooling rates, there by promoting embrittlement of the surface layer.
Friction Martensite, Steel Ropes, White Layer, Wire Drawing.
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