Materials Science Forum Vols. 539-543

Abstract: Large amount of nitrogen addition into an austenitic stainless steel can improve the mechanical properties and corrosion resistance remarkably as far as the nitrogen is in solid solution. However, once the nitrogen precipitates as nitride, it results in deteriorations in the properties of the high nitrogen austenitic stain steel. During welding, a high nitrogen austenitic stainless steel is ready to precipitate rapidly immense amounts of chromium nitride in the heat affected zone (HAZ), as intergranular or cellular morphologies at or from grain boundaries into grain interiors. The nitride precipitation reduces seriously the local mechanical properties and corrosion resistance. The present authors have demonstrated that a thermomechanical-processing as grain boundary engineering (GBE) inhibited intergranular chromium carbide precipitation in the HAZ of a type 304 austenitic stainless steel during welding and improved the intergranular corrosion resistance drastically. In the present study, the thermomechanical-processing was applied to a high nitrogen austenitic stainless steel containing 1 mass% nitrogen to suppress the nitride precipitation at or from grain boundaries in the HAZ during welding by GBE. GBE increases the frequency of coincidence site lattice (CSL) boundaries in the material so as to improve the intergranular properties, because of strong resistance of CSL boundaries to intergranular deteriorations. The optimum parameters in the thermomechanical-processing brought a very high frequency of CSL boundaries in the high nitrogen austenitic stainless steel. The GBE suppressed the intergranular and cellular nitride precipitation in the HAZ of the high nitrogen austenitic stainless steel during welding.

Abstract: Investigation into the effect of carbon and nitrogen contents of steels with an addition of chromium, nickel and manganese on their structure and properties after thermoplastic treatment has been carried out within the present study. For the physical modeling of thermoplastic treatment processes, a DIL 805A/D dilatometer with a deformation capability and a Gleeble 3800 simulator were used. The effect of carbon and nitrogen contents and the sum of these elements (C+N) on the hardening of the material during hot plastic working. The softening of the material depends on the C/N element ratio. The influence of the constituents under analysis on the dynamics of removal of cold work effects is different; C speeds up, while N slows down this process. The influence of the sum of the elements C and N depends on the type of nonmetallic phase precipitates. The structure and properties of such steels are determined by quenching and tempering cycles.

Abstract: We produced low carbon and high nitrogen martensitic stainless steels that contain less than 0.1 mass% C and more than 0.45 mass% N, through the pressurized induction melting process, in which nitrogen is introduced from a pressurized N2 atmosphere. The hardness and corrosion resistance of these steels were investigated under various heat treatment conditions. The hardness of these steels after spheroidal annealing treatment is approximately 95HRB and the cold workability is superior to that of AISI440C. The hardness of these steels after quenching and sub-zero treatment is from 53 to 56HRC. In the tempering process, however, high nitrogen steels show secondary hardening at approximately 4 points in HRC compared with the quenched hardness after subzero treatment and have the maximum tempered hardness of 56 to 60HRC around 723K. The corrosion resistance of quenched and tempered materials under 723K is better than AISI304 evaluated by the pitting potential in 3.5% NaCl aqueous solution. Both remnant Cr2N in hardening and precipitated Cr2N in tempering degraded the corrosion resistance of high nitrogen martensitic stainless steels. The best balanced developed steel has a hardness of 60HRC and better corrosion resistance than AISI304 under optimal heat treatment conditions.

Abstract: The influence of shot peening on the fatigue properties of duplex stainless steel reinforcing bars manufactured using both hot and cold rolled processes was studied. From determination of the S-N curves, the experimental results show that shot peening improves the fatigue behaviour of the re-bars, but that the improvement is much greater for the hot rolled bars. A more severe peening action capable of promoting greater plastic deformation of the bar surface needs to be used to improve the fatigue resistance of cold rolled corrugated bars.

Abstract: Internal friction of nitrogen in α iron has known as Snoek peak of N atom resolv ed in the octahedral interstitial site of bcc. When M atom ( Mn, Mo, Si, et.al) which has the affinity bigger than Fe were added ,another peaks due to the jump of N from Fe-Fe site to Fe-M site appear in the upper temperature side and complicate the Snoek peak curve of N. In this paper, the Snoek peak curve was studied in Fe-0.4wt%Nb-0.02wt%N alloys. This alloy showed no other peaks in the upper side to 373K and had only the single peak of N in Fe-Fe site. Internal friction measured by torsion pendulum method at about 1 Hz. After that this alloy was cooled from 373K to room temperature and reheated to 373K, but Snoek peak of N which showed on the first measurement just after nitriding disappeared completely and internal friction was only background. One of the reasons of it is the precipitation of Fe16N2 under heating and resolved N atoms disappeared. So these specimens were reheated to 873K and quenched, but Snoek peak of N was not measured. This shows the disappearance of N atom from Fe-Fe site. The mixture gas of a few % NH3 and H2 was used in this study. Therefore after nitriding much of H atoms were resolved in α iron alloys. Because the diffusion rate of H atom in α iron is bigger than N atom , it expected that H atoms can combine with Nb atoms before coming N atoms. And the appearance of Snoek peak of N in these alloys is considered. After that, in the heating to 373K H atom leave Nb and go away from the surface, and N atoms combine with Nb in place of H atoms. Nb has the very strong affinity with N, so these alloys have no peaks. When they reheated to the nitriding temperature N atoms can not diffuse from Fe-Nb site to Fe-Fe site. Then Snoek peak of N can not appears again in these alloys. Therefore, when Fe-Nb alloys were nitrided in NH3 and H2 mixture gas H atom plays very important part and it needs that the interaction of N-H atoms will be considered in these nitrided Fe-Nb alloys.

Abstract: Measurements of conduction electron spin resonance (CESR) in steel allow to separate the contributions from free electrons which provide the metallic character of interatomic bonds and from localized electrons involved in the covalent bonds. The data of the CESR study carried out on austenitic CrMn steels alloyed with carbon, nitrogen or carbon+nitrogen are presented. It is shown that, in contrast to carbon, nitrogen enhances the metallic character of atomic interactions with a maximum of the concentration of free electrons at some critical content of nitrogen (about 2 at.%). The combined alloying with carbon+nitrogen leads to two effects: (i) a larger concentration of free electrons and (ii) a shift of the critical content of interstitials towards higher values. The experimental data are supported by theoretical ab initio calculations of the electron properties of austenitic CrMn steels alloyed with carbon, nitrogen or carbon+nitrogen. Using the full-potentialfull- electron-linearized-augmented-plane-wave (FLAPW) method, the total energy per primitive crystal cell, the density of the electron states (DOS) and the distribution of the electron density over the crystal lattice were calculated by means of the computational program WIEN-2k. The total electron energy decreases due to alloying in the sequence of carbon→nitrogen→carbon+nitrogen, which suggests a corresponding increase in the thermodynamic stability of the austenite. The obtained results of the theoretical and experimental studies of the electron structure were used for the development of super-high–strength stainless austenitic steels.

Abstract: This paper overviews our recent investigations on the processing of net-shaped Fe-based nanoparticulate materials and their related material properties such as mechanical and corrosion properties. The key-process for fabricating fully densified net-shaped nanopowder by pressureless sintering is an optimal control of agglomerate size of nanopowder. Enhanced mechanical property of powder injection molded Fe-Ni nanopowder could be explained by grain refinement and uniformity of microstructure.

Abstract: Nanoscale studies of four important phenomena – in-situ development of high temperature (HT) wear resistant nanostructured surface glaze, the initial stages of oxidation of TiAl intermetallics, the high temperature degradation of DLC coatings and the property change promoted by nano-patterning of a TiO2 surface are described.

Abstract: Copper sheet samples composed of nanometer scale lamellar twins was produced by electrodeposition. The coherent lamellar twin boundaries were within 20˚ of being parallel to the sheet plane in more than 60% of the grains. The electrodeposited sample was cold rolled to 30 and 85% reductions in thickness and the structural evolution during cold rolling was examined by transmission electron microscopy (TEM) and high resolution TEM (HRTEM). Extensive activity of partial dislocations along twin boundaries and of perfect dislocations within twins (in particular in coarse twins >100nm) were identified. Moreover, it was found that shear banding occurred, which locally destroyed the lamellar twin structure. A dislocation structure developed within the shear bands, and such a structure evolved with strain and gradually replaced the lamellar twin structure. After 85% deformation, a large volume fraction of the lamellar twin structure was replaced by a lamellar dislocation structure characteristic of high strain rolling where the lamellar dislocation boundaries are almost parallel to the rolling plane. It was also found that the structural scales are coarser in the lamellar dislocation structure than in the initial lamellar twin structure.

Abstract: The radiotracer technique was applied to measure self- (Fe, Ni) and solute- (Ag) grain boundary diffusion in nanocrystalline Fe-40wt.%Ni alloy. The nanocrystalline material was prepared by pressureless sintering of the nanoalloy powders. The nano-sized crystallites were found to be clustered in micrometer-large agglomerates. Two types of internal interfaces with fundamentally different properties exist in the nanomaterial: the grain boundaries between the nanocrystallites and the interfaces between the agglomerates. A complete and consistent model of the diffusion processes in such material is elaborated. Whereas the nanocrystalline boundaries reveal diffusivities, which are similar to those in coarse-grained material, diffusion along interagglomerate interfaces occurs faster by orders of magnitude. This behavior is explained by a nonrelaxed structure of the inter-agglomerate interfaces.