218 Prakt. Met. Sonderband 50 (2016) 
2. EXPERIMENTAL PROCEDURE Sample 
prepare 
The current measurements were performed with a special measurement cell that enables in situ paper t 
electrical resistance measurements by a four probe technique at high temperatures up to 1100°C [9]. polishit 
The measurement cell consists of a ceramic cylinder where the four prismatic contacts made from of diffe 
pure platinum are fixed. The outer contacts (current contacts) are designed for charging the sample K2S,O; 
with current and the inner contacts (voltage contacts) enable the measurement of the voltage change delinea 
in the sample. The contacts are connected to the measurement instrument with Pt wires. The high- section 
temperature resistant junction between the sample and the contacts is attained with springs and with as 
specially designed coupling holders made from high temperature resistant stainless steel. During the Analys 
experiments, the electrical resistance was measured by a four-probe method with a bidirectional 
current of 100 mA with a resolution of 10 pQ and accuracy of £35 nQ. In addition, the temperature Table 1 
changes were measured at the central part of the sample with a Ni-NiCr thermocouple with an outer 
diameter of 1 mm and accuracy +£1.5°C. The Pt contacts are spring loaded to maintain good contact JES 
during the whole thermal cycle. In certain cases, copper pads were put in between the sample and Pt 
contacts to ensure a stable connection. Specimens of different geometries were tested to illustrate 
the methods versatility; the respective shapes range from small vires to square shaped rods. The 
measurement cell is schematically depicted in figure 1. 
3. RE 
AL We beg 
soft anr 
1 Measured sample There i 
2 Pyramidal Pt contacts figure 
3 Ceramic holder 
7 4 Upper fixure 
1 5 Spring loaded clamps 
. u. 6 Type K thermocouple 
| 
ST“ 
Fig. 1: Schematic depiction of the measurement cell for “in-situ” electrical resistance measurements, illustrating a 
mounted sample 
The thermal cycle consisted of slow heating at an approximate rate of 15°C/ min, to a suitable 
austenitization temperature between 850°C and 950°C, depending on the material studied. Samples 
were austenitized for 30 minutes and then oven cooled to room temperature at a rate of 1°C/min. Fig. 1: N 
All experiments were conducted in an atmosphere of pure argon, since even slight oxidation of the 
test pieces can reduce the quality of the measured data by introducing noise. The low carbon steel Measur 
was soft annealed by annealing at 1100°C for 12 hours in an atmosphere of pure argon, followed by change: 
slow cooling. The sample of Ck45 was received in the form of a bright drawn wire and grinded ferritic 
down to a thin strip of 1.5 mm thickness. Prior to measurements the samples were ground to P1000, The ten 
and ultrasonically cleaned in ethanol. The measurement cell was also cleaned after each structut 
measurement to avoid contamination at the contacts. the cur 
to parai 
begins 
comple