The research of cracking in the fabrication of the internal-combustion engine exhaust valve

The final set of properties of the exhaust valve is formed in the course of the entire set of processes: electro-forging, heat treatment, surfacing, machining and electroplating, the most important of which is the build-up of the valve chamfer which is subjected to constant high temperatures, gas flow and thermal cycling, therefore strict adherence to the whole complex of properties is necessary, otherwise a defect of the working chamfer will lead to its destruction, which will inevitably lead to the violation of the engine and the need to repair it. In the study two materials were investigated: the deformable part of the base material (steel grade 40X10C2M (EN steel grade X40CrSiMo10)) and surfacing (alloy EP616A), used to manufacture the exhaust valve of the internal-combustion engine. The rejection of surface defects is carried out on finished products by the method of luminescent control. However, as practice shows, not all details that passed a visual inspection are suitable. Destructive methods for studying the continuity of the material can detect unacceptable internal defects (shrink-holes, cracks). The structural components of the material such as the size and shape of the grain and inclusions, the nature of their distribution, the location of the shrink-hole, its length, the dimensional characteristics of the heat-resistant layer, the transition zone, the method of crack propagation were studied with optical electron microscopy. The study was carried out at different stages of valve manufacturing: after electro-forging during the formation of a valve head from a rod, subsequent heat treatment (quenching + high tempering), after surfacing a ring with the heat-resistant alloy. In the present work, the chemical composition of phases, inclusions, and sections of EP616A alloy around the shrink-hole were determined by the method of micro-X-ray analysis. The results indicate a shortage of visual inspection, and that conducting a selective destructive method does not ensure the full probability of detecting internal defects. It is advisable instead of destructive control to introduce ultrasonic flaw detection.