Ercan Balikci

Investigation on liquid bath nitriding of selected steels

Abstract The influence of experimental parameters, such as temperature, time, alloy composition and nitriding bath composition, on nitriding case formation on three steels has been investigated. Two of the steels are of low alloy type, one with low carbon (DIN 1·6523) and the other with medium carbon (DIN 1·7225). The third alloy is a nitriding alloy (DIN 1·8550). Two cyanate baths are used, i.e. type 1 (80 wt-% potassium cyanate) and type 2 (40 wt-% potassium cyanate). Higher temperatures, times and bath compositions result in a thicker case. As it thickens, the compound layer becomes composed of ϵ type nitrides. In general, DIN 1·6523 has developed thicker cases, and DIN 1·8550 with Al and Cr has formed thinner cases. Porosity has been observed in the cases, and the roughness of the nitrided surfaces has been measured to be greater for the higher carbon content alloy (DIN 1·7225) in this study.

A Comparative Study on the Single Crystal Growth of CMSX-4 via the Vertical Bridgman and Vertical Bridgman with a Submerged Baffle

Convective cells due to thermal and solutal gradients above the solid/liquid (s/l) interface lead to instability or freckle formation which generally results in the breakdown of single crystallinity while solidifying superalloys directionally. To overcome this problem, a novel modification of the traditional Vertical Bridgman (VB) technique is utilized. This new Vertical Bridgman technique with a supmerged baffle (VB-SB) makes use of a ceramic baffle that is supmerged into the melt near the s/l interface to decrease the melt height which leads to reduction of natural convection. Crystals of commercial CMSX-4 superalloy have been grown by both the traditional VB technique and VB-SB technique with various withdrawal velocities. Results are evaluated to establish a comparison basis between VB and VB-SB in terms of interface morphology, dendrite dimensions, and segregation.