Prakt. Met. Sonderband 46 (2014) 267 
, which is and 2. EXPERIMENTAL 
al guide wires, 
phase exhibits 
ce of the B19’ The experiments were performed in a vertical laboratory vacuum furnace at 1000°C using 
prove the wear pure acetylene as carburizing gas. The construction of the laboratory device and principle 
riding could be of the low pressure vacuum carburizing process was described in detail in our previous 
1. To solve this publication [11]. Briefly, the experiments were performed following four steps: (i) vacuuming 
n conditions. In the retort to 102 mbar, (ii) heating the sample to the desired annealing temperature, (iii) 
oving the wear annealing the sample in a low pressure acetylene atmosphere (5 to 10 mbar) for the 
predetermined time ta — active stage of the process, and (iv) at the end of the active stage 
the retort was vacuumed again to 102 mbar, and the sample was annealed in the vacuum 
entally friendly for the predetermined time te — diffusion stage of the process. At the end of each experiment 
dustrial sector. the sample was cooled down to room temperature. 
drocarbon gas For the vacuum carburizing experiments we used NiTi wires with an equiatomic composition 
mbar and in and diameter of 0.35 mm. Before carburizing the wire surface was polished with Chemomet 
lilibrium boost- polishing cloth with alumina polishing paste 0.05 um and cleaned ultrasonically in acetone 
istenitized in a to minimize surface contamination. 
in low vacuum . . . . i” 
carburizing is The thicknesses of the carburized zones determined at different process conditions were 
here — usuall examined on the transversal cross-section of metallographic samples with an optical 
phere y microscope, Nikon Epiphot 300, equipped with a system for digital quantitative image 
eating the parts analysis (Olympus DB12 and software program Analysis). The metallographic preparations 
a constant part consisted of mechanical wet grinding on SiC paper to a final mesh size of 4000 and polishing 
uate pressure, with diamond suspension (1um). Additionally, the morphology and chemical microanalysis 
mpletion of the of NiTi samples after vacuum carburizing was examined with the scanning electron 
‘vacuum again. microscope FEI Siron NC equipped with an energy-dispersive X-ray (EDX) detector. 
x of the sample 
irburized zone. 
1g temperature. 
rixture of these 
of carbon are 3. RESULTS AND DISCUSION 
the matrix by 
the process [7- Co ; un i 
The vacuum carburizing process is triggered with admitting the acetylene gas in the retort 
of the furnace through the needle dozing valve. The acetylene dissociates at high 
ons such as in temperature in carbon and hydrogen immediately when coming in a contact with the metallic 
dition of carbon surface of the sample. The active stage of the process starts with the adsorption and 
and abrasion dissolution of carbon into the surface layer of the matrix. Dissolved carbon atoms diffuse 
inward through the metal matrix and after the solubility limit of carbon in NiTi matrix is 
exceeded the surface layer is supersaturated with carbon that leads to precipitation of the 
ig conditions of titan carbides (TiC) from the solid solution. The precipitates TiC form on the surface of the 
TiC film on the sample a thin, a few um thick compact and uniform film (fig. 1a). The morphology of formed 
microstructural TiC film is homogeneous and free of defects. No micro-porosity or cracks could be observed 
(fig. 1a and fig. 2 b). 
In this research work, we made several vacuum carburizing experiments with aim to find the 
optimal operation conditions for formation of compact TiC layer. The best results were 
obtained using shorter duration time of active stages at higher partial pressure of acetylene 
gas (fig. 1a). On other hand, lower partial pressure lead to formation of non-uniform TiC 
layer (fig. 1b). Moreover, in the vacuum carburizing experiments, were after active stage a