An In-Situ Scanning Electron Microscopy Study of the Bonding between MnS Inclusions and the Matrix during Tensile Deform
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NTRODUCTION
MANGANESE sulfide inclusions appear regularly as a constituent of structural steels as determined by the sulfur content. The MnS inclusions improve the machinability of the material due to their beneficial influence on chip embrittlement, tool protection, and flow zone improvements.[1] Although the influence of MnS on machinability is appreciated, the amount, shape, size, and distribution of these inclusions are subject to restrictions. This occurs because the sulfides can act detrimentally on the mechanical properties of the material.[2] The MnS shows a hardness of 170 HV at room temperature and its Young’s modulus ranges between 70 and 140 GPa.[3] Different from most other nonmetallic inclusions, MnS has the property to be softer than the steel matrix during hot working operations.[1] This results in deformation of the originally equiaxed MnS inclusions to flattened shape upon a hot rolling operation. The consequences of pancake or discoid shapes are high stress concentrations on the sharp edges of the deformed MnS inclusions and therewith strong mechanical anisotropy.[4] Anisotropy in the mechanical behavior of forged components can be experienced, e.g., in ductility, yield strength, and tensile strength, but also in fatigue properties. For instance, the tensile strength of a material is lower when inclusions are oriented transverS. B. HOSSEINI, Failure Analyst, Researcher, C. TEMMEL, Researcher, and N.-G. INGESTEN, Manager Forged Materials, are with the Materials Technology, Volvo Powertrain Sweden, SE-405 08, Gothenburg, Sweden. B. KARLSSON, Professor, is with the Department of Materials and Manufacturing Technology, Chalmers University of Technology, SE-412 96, Gothenburg, Sweden. Contact e-mail: [email protected] Manuscript submitted June 16, 2006. Article published online May 19, 2007. 982—VOLUME 38A, MAY 2007
sally to the principal stress as compared with material with longitudinally oriented inclusions.[5] Also, the fatigue strength properties show distinct anisotropy depending on inclusion orientation.[2] An inclusion that cannot maintain bonding to the matrix will act as a crack within the matrix material.[6,7] It is therefore the aim of the present investigation to evaluate the bond strength between MnS inclusions and the surrounding steel matrix in order to draw conclusions on inclusion behavior in stressed materials. II.
EXPERIMENTAL
Tensile deformation of miniature tensile specimens has been carried out in a tensile stage mounted in the specimen chamber within a scanning electron microscope (SEM). A. Material As a test material, a medium carbon steel of grade 42CrMo4 (EN 10083-1) (Table I) was used. The steel was melted and cast in vacuum in experimental amounts with tight requirements regarding cleanness. Ingot casting was used in order to avoid initially textured material as often experienced in continuous casting. Deformation and therewith orientation of the material was introduced by a hot cross rolling operation. Rolling deformation occurred along longitudinal (L) and transverse (T
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