HEXAGONAL BAR SPLITTING
Information Available:

Failure of the bar was experienced during cold forming operation.
The bar was produced by hot forming.
The shape was hexagonal, the initial bar size was about 20mm.
The material used in the product was mild steel.

Fig (1): Fracture mode, split section in the bar
The composition of mild steel:
According to the chemical analysis report that was done by atomic emission spectrometry (AES) the sample contains the following elements:

Element

Wt %

Fe

bal

C

0.143

Mn

0.73

S

0.043

Si

0.345

P

0.035

Nb

0.015

metallurgist analysis indicated to presence of 0.04% S which is considered high content of Sulphur to use in steel. Sulphur lowers ductility and notched impact toughness and has little effects on the longitudinal mechanical properties.
The parameters of forming process: the compressive forces applied by the rolls to reduce the thickness of the material or changes its cross sectional area, the contour of the roll gap and roll configurations.
The requested tests:
More specific and precise information should be available to determine the basic cause of the failure. The following techniques could be useful:

X-Ray Diffraction (XRD): to get enough data about the chemical composition of the bar material and the crystallography structure. As well, Scanning Electron Microscope (SEM).
Ultrasonic testing: to estimate the size, orientation, shape and nature of defects.

Manufacturing process: Hot Rolling:
Cooling
In all rolling processes, cooling the steel is a critical factor. The speed at which the rolled product is cooled will affect the mechanical properties of the steel. Cooling speed is controlled normally by spraying water on the steel as it passes through and/or leaves the mill, although occasionally the rolled steel is air-cooled using large fans.
(internal defect in steel during manufacturing) Centreline segregation occurs within continuously cast slab and this influences the type of the localised microstructure. Elements which are prone to segregate in structural steels include C, Mn, Si, P, S and Nb and an accurate measure of the segregation intensity can be obtained using electron probe microanalysis. It is known that such segregation can influence the fracture toughness together with other properties. There is growing concern among customers and regulating bodies that present specifications do not take sufficient account of this fact. It is, therefore, important that a quantitative relationship be established between the intensity of the segregating elements and the fracture toughness so that compositional adjustments to the parent plate can be made in order to optimise properties. Since it is not possible to obtain commercial samples of plate having controlled segregation characteristics it is intended to replace the central segregated region in commercial slab with inserts of known composition and thickness. It is also intended to carry out a limited assessment of the weldability of the segregated samples including the influence on the HAZ toughness. (1)…
Crocodiling:
The prediction of defect during rolling process is so complex because that is related to multiple parameters including cool rating, composition, distribution of carbides, deformation, etc.
In this case study there is a rolling defect which in known as split end or “crocodiling”. This phenomenon initiates as a crack forming along the center plane of the deformed bar.
The cracks in this situation are formed by cleavage fracture at the centerline of the bar, resulted from the stresses during drawing. In the drawing process the hydroststic stress components are basically tensile stresses. The fracture mechanism of cracking was primarily cleavage in the direction perpendicular to the rod plane. (2)
The major variables that affect mechanical properties and formability are working temperature, temperature of anneals between operations, percentage of reduction after the final anneal, and temperature of final heat treatment.
1. Effect of centreline segregation on the toughness of structural steels
2. Wla R. A. Toughness of Ferritic Stainless steels. 1980.