WEB Features of weld metal brittle fracture in Charpy tests
The fracture energy measured in Charpy impact tests is usually divided into few components: elastic deformation, the growth of the initial ductile crack, the macro-deformation of the specimen, the cleavage formation and the energy of the ductile final fracture. When a low-alloy low-carbon steel is fractured in the ductile-to-brittle transition temperatures range, after formation of brittle cleavage surface, a region of ductile final fracture is often observed. For some specimens of weld metal, this area may have a significant size. If a brittle cleavage occurs at the early stages of specimen bending, followed by a ductile final fracture of the specimen, the resulting value of the impact toughness and the percentage of shear fracture may reflect the level of the material reliability incorrectly. The possibility of this outcome may differ for a metal with different microstructures.
In view of the above, the purpose of this work is to establish the possibility of fracture with high share of final fracture energy in total energy of fracture during Charpy impact tests of metal with different microstructures, as well as the frequency of this phenomenon occurrence and to develop methods for controlling this type of fracture.
Welded joints made by six different methods along with several types of low-alloy low-carbon steel were tested for impact bending with recording of load-displacement curves (total – 813 specimens). A quantitative fractographic study of the fractured specimens was carried together with micro-fractographic study using scanning electron microscopy. By analyzing load-displacement curves and fracture surface images, a partitioning of fracture energy between the fracture before the brittle cleavage was formed and the final fracture energy was studied.
It was found that in the ductile-to-brittle transition temperatures range, type of fracture in which the total work significantly exceeds standard value, but the energy is spent before the formation of brittle cleavage significantly lower than standard, occurs in the weld metal specimens with a probability of 2 to 25% for different welding methods. Also, this type of fracture is not typical for the base metal and the heat affected zone.