Risto Ilola

Effects of processing and manufacturing of high nitrogen-containing stainless steels on their mechanical, corrosion and wear properties

Abstract The effects of N alloying through different processing and manufacturing routes on metallurgical, mechanical, corrosion and wear properties of high N-containing stainless steels have been studied. Nitrogen alloying has a well defined and marked improving effect on mechanical and corrosion properties of these steels until the solubility limit of N is reached and nitride precipitation exhibits its deleterious effects on these properties. It is also demonstrated that N alloying contributes significantly to sliding wear and cavitation erosion resistance of stainless steels. The processing and manufacturing technologies of high N-containing stainless steels are evaluated with main emphasis in P/M fabrication including solid-state nitriding of the metal powder.

Photocatalytic Activity of Atomic Layer Deposited TiO2 Coatings on Austenitic Stainless Steels and Copper Alloys

Photocatalytic activity of TiO 2 -coated austenitic stainless steel (AISI 304) and copper alloys [deoxidized high phosphorus (DHP) copper and Nordic Gold] was studied by means of decomposition of methylene blue model waste water and open-circuit electrochemical potential measurements, and the photoinduced hydrophilicity was studied by means of contact angle measurements of water under ultraviolet irradiation. The TiO 2 coatings were prepared by an atomic layer deposition technique from TiCl 4 and H 2 O. The thicknesses of the prepared coatings were 5, 10, 50, 100, 150, and 200 nm. Morphology and crystal structure of the TiO 2 coatings were studied using scanning electron microscope and X-ray diffraction techniques. Photocatalytic activity of the studied coatings was low with a coating thickness of 5 and 10 nm. When the coating thickness was 50 nm or higher for AISI 304 stainless steel, and 100 nm or higher for DHP and Nordic Gold copper alloys, the photoactivity was good, but no saturation or systematic effect of coating thickness or surface finish was observed. The photoinduced hydrophilicity was good with all studied coating thicknesses (50, 100, 150, and 200 nm), with some exceptions.

On irradiation embrittlement and recovery annealing mechanisms of Cr-Mo-V type pressure vessel steels

Abstract Surveillance specimens of 15Kh2MFA reactor pressure vessel (RPV) steel and Sv-10KhMFT weld metal were investigated using internal friction, electrical resistivity and Mossbauer spectroscopy measurements in order to obtain information on their irradiation embrittlement and recovery annealing mechanisms. Internal friction measurements showed that carbon becomes trapped by irradiation-induced defects (fluence=4.1×10 19 n/cm 2 , E >1 MeV, T =265 °C). During the recovery annealing (450 and 475 °C/1 h) carbon does not redissolve into the solid solution. Although irradiation usually increases the electrical resistivity of the RPV steels, in this study irradiation decreased the electrical resistivity of both the base and the weld metals. This can be due to a decrease in the dislocation density or precipitation under irradiation. The annealing temperature range for recovery was 300–600 °C for the base and the weld metals according to the electrical resistivity measurements. Some evidence of irradiation-induced carbide formation was achieved by the Mossbauer spectroscopy measurements.

Dynamic strain aging of austenitic high nitrogen Cr-Ni and Cr-Mn steels

Dynamic strain aging (DSA) of solution annealed Cr-Ni and Cr-Mn type austenitic high nitrogen steels (HNS) was investigated by means of tensile tests. Nitrogen content of the steels varied from 0.26 to 1.00 wt. %. Tensile tests were performed at strain rates from 0.00005 to 0.03 1/s between room temperature and 800 °C. Austenitic HNSs exhibit DSA at elevated temperatures, which is caused by interactions between diffusing solute atoms and moving dislocations. Low activation energies for plastic deformation in the studied steels indicate that the diffusion mechanism is pipe diffusion below 600 °C, and also bulk diffusion at 800 °C. An activation energy for nitrogen diffusion of about 1.95 eV was determined by internal friction measurements.

Hot and Cold Rolling of High Nitrogen Cr-Ni and Cr-Mn Austenitic Stainless Steels

Behavior of austenitic Cr-Ni-(0.14-0.50)N and Cr-Mn-(0.78-1.00)N steels in hot and cold rolling was investigated by rolling experiments and mechanical testing. Structure of the steels in the as-cast condition and fracture surfaces after the rolling experiments were investigated using optical and scanning electron microscopy (SEM). Resistance to deformation was calculated using rolling forces in hot rolling. Increase in strength in the rolling experiments was related to the nitrogen content of the steels. Resistance to deformation during hot rolling increased with decreasing rolling temperature and with increasing nitrogen content. In some steels, hot rolling led to edge cracking, which was more a function of impurity than nitrogen content. Microscopy revealed that the edge cracking occurred along grain boundaries and second phase particles. For the cold-rolled steels, the highest achievable reductions were limited due to a “crocodiling” phenomenon, that is, opening of the strip end. Fracture type at the opened strip end was a brittle-like fracture.