Effect of thermocyclic treatment on structure, length variation and fracture of armco-iron specimens

It is shown that maximal shortening of specimens is observed under thermocyclic treatment with rapid heating and cooling when Tmax=0.53 Tmelt. of iron, in this case the temperature elevation from 600 to 680 deg C shortens specimens some times. It is supposed that shortening of specimens embraces processes of formation of defects in the crystal structure, their displacement under the action of the field of thermal stresses which arises under thermocyclic treatment in the central zone of the sample, mainly to the grain boundaries perpendicular to the specimen axis as well as shifts of grains and subgrains relative to each other. Displacement of defects on the grain boundaries, perpendicular to the axis promotes here a formation of discontinuities and cracks in the central part of the specimen under the action of the field of thermal stresses specified by the specimen configuration. A value of the specimen shortening is affected by the initial structure. If a grain size in the initial specimens eight-ten times exceeds a grain size after preliminary annealing at T=960 deg C, the shortening of the specimen is maximal. Under thermocyclic treatment within 830-20 deg C with heating in a furnace and water cooling there is no a noticeable shortening of the specimens