Laser cladded steel - microstructures and mechanical properties of relevance for railway applications

Doctoral thesis, 2005

Owing to increased requirements like increased axle load for railway wheels and rails, new material have to be employed. However, these requirements are mostly surface related, which makes it appropriate to change only the surface layer while keeping the bulk unchanged. Laser cladding offers the possibility to produce surface coatings that are metallurgically bonded to the substrate with minimum dilution. As only small layers are applied, also high cost materials such as hardfacing alloys are at hand. The present study is concerned with the examination of two different alloys laser cladded onto a railway steel, giving a composite material consisting of the clad, the heat-affected zone (HAZ), and the unaffected substrate. The clad alloys are Co-Cr and Fe-Cr materials pre-selected for their promising wear properties. The substrate material is a medium carbon steel already employed in railway wheels. Microstructure analyses have been performed on the HAZ and the Co-Cr clad material as well as on the interface between the clad and the substrate. The HAZ consisted of tempered martensite near the clad, bainite further away and a partially transformed zone near the unaffected substrate. The clad consists of colonies of dendrites in elongated grains stretching from the interface to the clad surface. At the interface, there is a high compositional gradient. The integrity of the interface, however, is given by dilution of substrate elements into the clad. Hardness measurements from the clad through the HAZ into the unaffected substrate showed smooth transitions. This is advantageous as it implies a smooth transition of other mechanical properties. Tensile behaviour of the composite materials showed high yield strength and strong work hardening. Both materials showed advantageous fatigue behaviour in low-cycle fatigue. The number of cycles to failure is comparable to the substrate material alone, but with considerably higher applied stresses. Neither of the mechanical tests led to delamination of the clad material. The tempered martensite in the HAZ could even sustain fatigue straining without premature cracking.