The aim of the contribution is creation of transformation diagrams of hypoeutectoid 25CrMo4 steel that is used for tube production for the OCTG (Oil Country Tubular Goods) industry. For a comparison purpose, the CCT diagrams with and even without influence of previous deformation were assembled. In addition, the DCCT diagram (i.e. with previous deformation influence) was exposed to different austenitization conditions. The transformation diagrams were assembled on the basis of dilatometric tests which were confronted with the metallographic analysis and hardness measurement. Due to the influence of previous deformation and different austenitization conditions, peremptory movements of curves in the assembled transformation diagrams were occurred, which, of course, was reflected also on the structure and hardness, respectively. After the previous deformation, the structures of samples had considerably increased share of quenching phases, including of share of acicular ferrite. This, of course, was manifested by increased values of hardness.Keywords: Transformation diagrams, dilatometric tests, microstructure, deformation
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