Characterization of electrochemical properties of steel obtained by cryogenic treatment

Za izboljšanje mehanskih lastnosti kovinskih materialov se lahko uporabljajo različni tehnološki postopki. V tem diplomskem delu smo preverjali lastnosti jekel, pridobljenih s posebno toplotno obdelavo, in sicer postopkom globokega podhlajevanja. Uporabili smo različne metode za določanje lastnosti jeklenih materialov. Z elektrokemijskimi metodami merjenja korozijskega potenciala, linearne polarizacije, elektrokemijske impedančne spektroskopije in potenciodinamskih meritev smo določili korozijske lastnosti. Uporabili smo tudi metalografske in spektroskopske metode za mikrostrukturno analizo materialov. V raziskavi smo testirali jeklene vzorce treh različnih sestav z oznakami A, B in C. Vzorce sestav A in B smo testirali pri dveh različnih toplotnih obdelavah in z dodanim podhlajevanjem. Vzorci jekla B so bili pripravljeni s praškasto metalurgijo. Elektrokemijske metode smo izvajali v raztopini natrijevega tetraborata s pH = 10. Ugotovili smo, da pri jeklih sestave A, z industrijsko oznako M2, podhlajevanje korozijskih lastnosti ne spremeni. Martenzitne iglice se zaradi podhlajevanja zmanjšajo, opazna sprememba v trdoti pa je posledica različnih temperatur popuščanja. Če tej sestavi dodamo kobalt – jekla C, industrijske oznake M35, se korozijske lastnosti po podhlajevanju poslabšajo. Zaradi podhlajevanja ni opaznih razlik v mikrostrukturi, prav tako ni spremembe v trdoti materiala. Jekla sestave B, industrijske oznake M3:2, po podhlajevanju kažejo znatno izboljšanje korozijskih lastnosti. Število karbidov je v primerjavi z jekli A in C večje. Vzorec, popuščan pri nižji temperaturi, po podhlajevanju kaže povišanje trdote. Prav tako so vidne spremembe v trdoti med toplotnimi obdelavami zaradi različnih temperatur popuščanja.Various technological procedures may be used to improve the mechanical properties of metallic materials. In this thesis, we tested the properties of steels obtained by special heat treatment – deep cryogenic treatment. We used various methods to determine the properties of steel materials. Corrosion properties were determined with electrochemical methods for measuring corrosion potential, linear polarization, electrochemical impedance spectroscopy and potentiodynamic measurements. We also used metallographic and spectroscopic methods for the microstructural analysis of materials. We tested steel specimens of three different compositions, labeled A, B, and C. The samples with composition A and B were tested with two different heat treatments and with added cryogenic treatment. Samples of steel B were prepared by powder metallurgy. Electrochemical methods were performed in a solution of sodium tetraborate with pH = 10. We found that for steels of composition A, the industrial grade M2, the cryogenic treatment does not change the corrosion properties. The martensitic needles decrease due to cooling, the change in hardness is the result of different tempering temperatures. If we add cobalt to this composition – steel C (M35), the corrosion properties reduce after cryogenic treatment. There are no noticeable differences in the microstructure, nor is there any change in the hardness of the material. Steels of composition B, industrial grade M3: 2, exhibit a significant improvement in corrosion properties after deep cryogenic treatment. The number of carbides is visibly higher compared to steels A and C. The sample tempered at a lower temperature indicates an increase in hardness after cooling. Changes in hardness between the heat treatments are also evident due to different tempering temperatures