Prakt. Met. Sonderband 38 (2006) 341 
Je crack An ultrasonic testing system (Telsonic-Ultrasonics, Switzerland) was employed for fatigue 
steel. In testing at 20 kHz in fully reversed mode (R = -1). Figure 2 shows its setup, which consists 
s and of of a generator, a transducer, an amplifier, and a coupling piece. Since direct strain 
f carbide measurement at the tested specimen was not possible due to specimen cooling, 
since as calibration was performed in such a way that strain gauges were attached to the coupling 
ion and piece and to a calibration specimen (inserted in place of the test specimen), and the 
as of this corresponding values of both gauges were recorded. Then, during fatigue experiments the 
des. strain at the coupling piece was measured and the sample strain calculated using the 
calibration data. Non corrosive coolant was used during fatigue testing in order to prevent 
heating through damping effects and resulting microstructural changes. Fracture surfaces 
were examined by means of scanning electron microscopy using a Zeiss DSM 962, and 
sizes of crack initiation sites and optical dark areas, and their distances from surface were 
red from determined from the SEM images. Residual stresses at the narrowest section of the 
nnesied fatigue samples have been measured employing X-ray diffraction [Co- 
hown for Ka; A=1,78897 Angström; 0=99,23°, lattice plane: (211)]. 
eometry, 13.0 or wonsducer  omother” Mp ~coupligpiece 
ore heat ' 
2d in oil. M10x1 > : 
ture was — aA 
ce. The : 3 specmen 
2s in the z kHz Sh 
ere (5.0 ; 125 Generator N iipscope dn 
ns using i ol 
finish in 3,0 - a | 
; 30,0 768 
te. Figure 1: Dimensions of fatigue test Figure 2: Scheme of the ultrasonic fatigue 
— specimens (in mm). testing system employed [8]. 
—— 3. Results and Discussion 
zo 
12 3.1 Microstructures 
: All images shown are sections perpendicular to the rolling direction, i.e. cross sections, 
which corresponds with the orientation of the fracture surfaces. Figure 3 shows the optical 
micrograph of the as-quenched structure. The image reveals prior austenite grain 
ur glass boundaries, which have been examined after Snyder-Graff. They were found to be 
the initial 13+ 2 pum. Figure 4 reveals the fine-structured tempered martensite matrix after 
S with 8 tempering. Massive white particles represent the primary alloy carbides. At this point it 
nation of should be noted that the amount of retained austenite has to be taken into account at this 
2 Zwick medium carbon content, since it is responsible for lower hardness, and also its influence 
e utilized on fatigue behaviour cannot be neglected [9]. But Fukaura et al [4] found for a quite similar 
ned on a steel that the retained austenite was totally transformed after tempering at 520°C. Thus, 
f optical since tempering was performed at 530°C in this work, it can be assumed that no retained 
package austenite is present after the heat treatment, which is also supported by the micrograph 
structural presented in Figure 4 showing a uniform tempered martensite structure. Furthermore the 
NOs and microstructure was then examined i.e. with respect to the numerous carbides existing in 
| etchant this tool steel, since they may act as defects in fatigue crack initiation [2,4,10]. Figure 5 
ons, and shows the uniform distribution of the carbides in the area perpendicular to the rolling 
6 H20, direction. The carbides appear coloured under Murakami etch. In rolling directions they are 
ng. For elongated due to the recasting operations, and the microstructure reveals the typical 
f 0,25N. alignment of the carbides. Thus, the mechanical properties are anisotropic. which should