Prakt. Met. Sonderband 38 (2006) 339 
Gigacycle fatigue fractography of high Cr alloyed cold work 
tool steel 
C. R. Sohar*, A. Betzwar-Kotas**, C. Gierl*, B. Weiss**, H. Danninger* 
* Institute of Chemical Technologies and Analytics, Vienna University of Technology, 
Vienna, Austria 
** Institute of Materials Physics, University of Vienna, Vienna, Austria 
Abstract 
Wrought cold work tool steel 1.2379 (X155CrVMo12.1) has been fatigue tested in the 
longitudinal direction up to 10E10 cycles, an ultrasonic resonance system operating at 
20 kHz in push-pull mode (R = -1) being employed. The fracture surfaces were studied by 
SEM. It showed that fatigue fracture occurred even at cycle numbers >10E9, indicating 
that the existence of a true fatigue limit is rather improbable. Internal fish-eye type crack 
initiation was observed in megacycle fatigue regime, carbide clusters and larger isolated 
carbide particles are definitely sites of fatigue crack initiation. In gigacycle regime, near- 
surface crack initiation was observed the origin of which was difficult to identify in many 
cases. At some fractographs several isolated large carbides were found near the initiation 
site close to the surface. The observed fatigue behaviour is discussed with regard to 
compressive residual surface stresses, size effects of crack origins, and possible 
cavitation and corrosion. 
1. Introduction 
Cold work tool steels are used for blanking, forming, and shearing applications. Further 
applications are cold forming, cold rolling, powder pressing, and wood and paper cutting 
knives, where working temperatures are below 200°C. There is a wide range of cold-work 
tool steels, having moderate to high carbon contents in order to satisfy the application 
demands in respect to desired hardness and wear resistance. The steel investigated here 
is one of the most popular grades in usage containing medium amount of carbon and high 
amount of chromium. The addition of chromium is of utmost importance for high abrasion 
resistance and hardness, since it forms a large amount of hard chromium carbides. 
Mechanical and also fatigue properties of such hard steels depend mainly on their 
chemical composition and their complex microstructure resulting from the applied heat 
treatment processes. Due to the contact between tool and workpiece the tools are 
subjected to stresses and strains during service. l.e. the demand of the tool's surface is 
high, since it is subjected to the highest stresses. Thus, failure can take place in form of 
micro chipping and fracture of the tool. Service operation imposes a repeated exposure to 
the above mentioned stresses. Thus, the failure that occurs is a consequence of fatigue of 
the tool material. The fatigue crack initiation in these hard materials can possibly take 
place at inclusions [1,2,3], carbide particles [2,4], and surface defects [5,6]. Non-metallic 
impurities in tool steels are aimed to be eliminated to large extents employing 
manufacturing processes like vacuum induction melting, and electro slag remelting. Thus, 
with these non-metallic inclusions more or less eliminated by the above-mentioned