Browsing by Author "Pacyna, Jerzy Stanislaw"

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Rola cementytu w procesie pękania staliwa L200HNM
(Data obrony: 2008-07-08) Rożniata, Edyta
Wydział Odlewnictwa
The chromium - nickel - molybdenum cast steel of ledeburite class is mainly used for working mili rolls. These rolls are one of the most expensive instruments employed in mechanical working of metals. A very serious problem for large-size cast rolls (including rolls madę of cast steels of ledeburite class) is the risk of their fracture. Probable cause of cast steel rolls fracture is the presence in their structure of continuous network of hypereutectoid cementite and transformed ledeburite precipitated along prior grain boundaries of former austenite and precipitates of hypereutectoid cementite in structure of Widmannstätten type. The network, formed in the roll's structure just after casting is a way for easy propagation of the fractures and decreases the mechanical properties of the rolls, especially fracture toughness, so important for the users of that instrument. That is why, therefore within the framework of the paper an attempt of refinement of prior austenite grain, evaluation of the influence of the refinement and cooling rate on the hypereutectoid cementite morphology and fracture toughness of G200CrNiMo4-3-3 cast steel. To reach the aims of the work and prove formulated thesis investigated materials were underdone to wide researching and complex analysis. Research of G200CrNiMo4-3-3 cast steel included metallographic and fractographic analysis and fracture toughness research (impact strength, stress intensity factor $K_{Ic}$ and hardness). However the most important thing was to create fine substructure (advantageous from the tribologic properties) with subtle network of partially coagulated hypereutectoid cementite by application of incomplete „normalizing" annealing for test cast steel.