Influence of MnS Inclusion on Ductility and Ductile Fracture Process of High Strength Steels

Abstract

The influence of MnS inclusion on ductility and ductile fracture process was investigated using quenched and tempered 70kg/mmmm2 class weldable high strength steels with a variation of sulfur content of 0.005 to 0.045wt%.
The influence of sulfur content on the tensile fracture strain and impact shelf energy was greater in the cross direction than in the longitudinal direction, and shelf energy was more affected than the fracture strain in the low sulfur level. In the case of materials with higher ductility which were tempered at higher temperatures, the influence became more pronounced. Shelf energies of 2mm V notched specimen were nearly the same as those of 2 mm deep fatigue notched and lower than those of 2mm V notched. The difference among shelf energies was independent on sulfur content.
In Charpy test, MnS inclusion affected the crack propagation rather than the crack initiation. On the other hand, in the tesile test, the true strain to crack initiation was pronouncedly affected by MnS inclusion.
The ductile fracture process of weldable high strength steels consists of two processes: relatively large void formation around inclusions which is detectable by an optical microscope, and coalescence of a void with other voids or notch or crack tip through “sheet of voids” as Rogers proposed. The size of void concerning “sheet of voids” in the present paper was very small and of electron microscopic order. A quantiative analysis made on the fracture surface proved that there existed a fairly good correlation between the shelf energy and area of small dimples due to “sheet of voids”, suggesting that the coalescence process was strongly affected by the matrix properties such as hardness, size of carbide and so on. The carbide particles larger than approximately 0.1 micron played an important role to the formation of small voids.