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