P 998 – New evaluation concept for appraisal of mechanical properties in high strength fine grained structural steel weldments based on the temperature-time-curve
Nowadays the cooling time concept t8/5 is widely accepted for the determination of suitable heat treatment strategies for low alloyed steels. Nevertheless it could be seen that for high strength steels the accuracy of the correlation between cooling time and mechanical properties are reduced so that a high effort has to be put into the qualification of welding procedure specifications.
In this research project experimental work has been carried out to investigate evaluation concepts for the mechanical-technogical properties of joint welds on the basis of the cooling curve. Therefore a comparison between the established cooling time concept t8/5 and the concepts t8/4 and the temperature time integral T8/4-400° has been carried out.
The results show that for steels with a mixed microstructure, consisting of ferrite, bainite and martensite, the correlation between heat input and characteristic value is improved.
The temperature time integral shows the best correlation also for low alloyed welding filler metals with low yield strength. For decreasing lower limit temperatures, the temperature dependence of the thermophysical properties becomes more important (phase transformation retardation). At the same time the minimal distance between weld seam and cooling time affecting heat sinks around the weld zone increases. This makes it more difficult to calculate the cooling time.
Based on the experimental investigation, it is possible to ensure the mechanical properties when welding high strength low alloyed structural steels through consideration of the cooling curve. The project objective has been achieved.
Only available in german language.
FOSTA – Research Association for Steel Application