On the analysis of cast structure and its changes during hot working of forging ingots
Sammanfattning: Forging ingots are hot worked to consolidate structural imperfections and refine the microstructure. Possible imperfections are segregations, porosities, cracks, and inclusions. In this thesis, techniques and methods for the characterisation of behaviour and properties of ingots and forgings. The root causes for quality issues in open-die forgings are shown to be numerous. Ingot structure in cast tool steel was analyzed, and the main imperfections were centre macrosegregation, and mid radius A-segregations.For overheated steel forgings and low reductions after reheating, a high ultrasonic attenuation and low Charpy-V toughness was found. It could be related to the coarse grain structure found. Only a high forging reduction after reheating will break down the coarse structure. Shorter reheating times and lower forging temperatures gave higher toughness and lower ultrasonic attenuation. Some reduction in toughness was also found from the inclusion field from the bottom of the ingot. Accurate attenuation measurements require a lathe turned surface, complicating in-between-measurements in heat treatment sequences. But on carefully surface prepared forgings, attenuation measurement can be used to determine the success of grain refinement in the heat treatment. A new method for ultrasonic macrography of cast ingot samples is presented. In addition, a new method for hot compression testing of cylindrical metal samples is presented.
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