Effect of Thermo-Plastic Treatment on ...

Effect of aging on structure and corrosion properties in high strength austenitic Cr-steels alloying with nitrogen Cr21N1 and Cr18Ni2N1 has been investigated. Structure of the steels and kinetics of austenite decomposition during the treatment was studied by X-ray diffraction, TEM-analyses and electric resistance measurements. The highest values of strength, micro hardness, corrosion rate and relative strength reduction as far as brittle intergranular fracture were observed at the early stage of austenite decomposition at the aging. The formation of nitrides CrN as well martensite stress induced took place in the steels. According to resistormeter investigations of nitrogen steels in compare with carbon steel Cr18C1 at the early stage of aging (350 ?C) the metastable ordered clusters enriched with chromium and nitrogen were formed. Growths of aging temperature up to 500 ?C led to dissolve the carbides or nitrides but at 600 ?C discontinuous precipitations were observed nearby the grain boundaries. Effect of aging combined with cold plastic deformation (CPD) at an 8-24 % reduction by two schemes, before and after aging at 350 ?C, on structure and related corrosion properties of steel Cr18Ni2N1 has been studied. CPD at an 8-12 % reduction before aging at 350 ?C was shown to thrice decreasing of corrosion rate of the steel as compared to the undeformed state. A thermoplastic treatment by the second scheme with using CPD at an 20% reduction after aging at 350 ?C is found to be more effective: the corrosion rate decreased by an order and strength reduction decreased more than by fourfold as compared to the undeformed state.