Search

The experiment described in this paper used vacuum induction melting (VIM) to coat Ni alloyNi alloy onto AISI 4140 steel. X-ray diffraction (XRD), scanning electron microscope (SEM), electron probe micro analyzer (EPMA), and transmission electron microscope (TEM) were used to identify the different phases of VIM Ni alloyNi alloys. Then, a ball-on-disc system and different dry sliding wear conditions were used to test VIM Ni alloyNi alloys. Results indicate that Ni 3 Si particles were scattered inside the γ-Ni solid solution, Ni 3 B and dendrite Ni 3 Si appeared in the lamellar structures, and Cr 7 C 3 and CrB were scattered in the lamellar structures of Ni 3 B and dendrite Ni 3 Si. Cr 7 C 3 and CrB were more wear resistant compared to other phases. The wear loss of VIM Ni alloys was lower when the sliding velocity was 0.1 m/s compared to when the velocity was 0.7 m/s. In the former case, the formation of a continuous oxidation layer protected the original surface from direct wear. A further increase in sliding distance caused, in both the cases, the formation of tumorous oxides, which led to delamination. Regardless, wear loss at 0.1 m/s was still lower than at 0.7 m/s.

An induction-melted Ni-based alloy (NiCrBSi) coating on AISI 4140 steel was prepared using induction melting under different melting conditions. To determine the microstructure and bonding behavior of their interfaces, the techniques of micro-diffraction, X-ray mapping and wavelength dispersive spectrometry (WDS) were employed. The experimental results showed that a white zone appears on the Ni-based alloy side of the final interface, becoming wider and wider with increasing temperature and time, which was identified as a solid solution of γ-Ni at 1000 °C and 1180 °C. A higher heating temperature and time caused a more severe iron dilution in the Ni-based alloy and decreased the microhardness.

A vacuum induction-melted (VIM) Ni-based alloy was immersed in 60% H2SO4 solution to investigate its corrosion behaviour and resistance. The results indicate that the microstructure contains a γ-Ni solid solution + Ni3Si particles, dendrite Ni3Si, Ni3B, Cr7C3, and CrB. The corrosion started at the zones of the γ-Ni solid solution + Ni3Si particles and dendrite Ni3Si. These zones transformed to oxide films and protected the alloy from significant attack. However, the pitting corrosion created paths for acid solution and/or SO 4 2 - to further attack. Therefore, the corrosion rate decreased and then stabilised at a high value as the immersion time increased.

A bimetal of Co-based alloy/AISI 4140 steel was fabricated by induction melting. The microstructure of the Co-based alloy was examined and the influence of on the alloy of acid solutions, the temperature of these solutions, and the immersion time was investigated. The results show that the microstructure of Co-based alloy consisted of a Co-rich phase, a Cr-rich phase, and W 3 CoB 3 . In a solution of 20% HCl and 6% FeCl 3 , the Co-rich phase was attacked to form porous channels but transformed to a Si-rich passivation film to prevent further attack in the solution of 72% H 2 SO 4 . Therefore, the relative corrosion resistance of Co-based alloy to acid solution is 72% H 2 SO 4 > 20% HCl > 6% FeCl 3 .

The effects of nitrogen content and the cooling rate on the reformation of austenite in the Gleeble simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The variation of stress corrosion cracking (SCC) behavior in the HAZ of 40 wt% CaCl2 solution at 100 °C was also studied. Grain boundary austenite (GBA), Widmanstatten austenite (WA), intergranular austenite (IGA) and partially transformed austenite (PTA) were present in the HAZ. The types and amounts of these reformed austenites varied with the cooling rate and nitrogen content in the DSS. U-bend tests revealed that pitting corrosion and selective dissolution might assist the crack initiation, while the types and amounts of reformed austenite in the HAZ affected the mode of crack propagation. The presence of GBA was found to promote the occurrence of intergranular stress corrosion cracking. WA, IGA and PTA were found to exhibit a beneficial effect on SCC resistance by deviating the crack propagation path.

The effects of nitrogen contents and cooling rates on the microstructure and pitting corrosion in the simulated heat-affected zone (HAZ) of 2205 duplex stainless steels (DSSs) were investigated. The results showed that the grain size of the α phase and the amount of Cr2N precipitates in the simulated HAZ decreased as the content of the γ phase was increased. Electrolytic extraction analysis demonstrated that Cr2N precipitation in the α phase became pronounced with a reduction in the nitrogen content and an increase in the cooling rate in the temperature range of 800–500°C. The results of cyclic polarization tests indicated that the pitting corrosion resistances of simulated HAZs in 3.5 wt.% sodium chloride (NaCl) solution were inferior to those of the solution annealed DSSs. Pitting corrosion was not found in regions where the precipitation of Cr2N was less likely to form, e.g. the precipitate free zone (PFZ) and the grain boundary austenite (GBA). Clearly, the presence of Cr2N in the simulated HAZs was the main cause of the deterioration in the pitting resistance of DSSs.

The effects of austenite stabilizers, such as nitrogen, nickel, and manganese, and cooling time on the microstructure of the Gleeble simulated heat-affected zone (HAZ) of 22% Cr duplex stainless steels were investigated. The submerged are welding was performed for comparison purposes. Optical microscopy (OM) and transmission electron microscopy (TEM) were used for microscopic studies. The amount of Cr2N precipitates in the simulated HAZ was determined using the potentiostatic electrolysis method. The experimental results indicate that an increase in the nitrogen and nickel contents raised the δ to transformation temperature and also markedly increased the amount of austenite in the HAZ. The lengthened cooling time promotes the reformation of austenite. An increase in the austenite content reduces the supersaturation of nitrogen in ferrite matrix as well as the precipitation tendency of Cr2N. The optimum cooling time from 800 to 500 °C (Δt8/5) obtained from the Gleeble simulation is between 30 and 60 s, which ensures the austenite content in HAZ not falling below 25% and superior pitting and stress corrosion cracking resistance for the steels. The effect of manganese on the formation of austenite can be negligible.

The effects of alloying elements on the microstructure, mechanical properties, and corrosion behaviors of duplex stainless steels (DSSs) have been investigated in this study. Experimental alloys were prepared by varying the concentrations of the constituent elements in DSSs. Hot ductility test, tensile test, charpy impact test, and corrosion test were performed to evaluate the properties of the experimental alloys. The results showed that the extent of edge cracking of DSSs increased with the increasing value of the crack sensitivity index (CSI). The higher the hot ductility index (HDI) was, the better the hot ductility of DSSs achieved. Austenite (γ) stabilizer generally caused a decrease in the strength and an increase in the charpy impact absorbed energy of the stainless steel. On the contrary, ferrite (α) former exerted its beneficial effect on the strength but became detrimental to the toughness of DSSs. The presences of sulfur and boron also caused a decrease in the impact energy, but nitrogen and carbon hardly affected the toughness within the concentration range tested in this study. The value of pitting nucleation potential (E np ) of different nitrogen contents in 3.5 wt.% NaCl solution at room temperature was almost the same, but the value of pitting protection potential (E pp ) among these alloys was increased with increasing the content of nitrogen. The susceptibility to stress corrosion cracking (SCC) of DSSs was high when tested in boiling 45 wt.% MgCl2 solution. On the other hand, the time to failure of the experimental steels in 40 wt.% CaCl2 solution at 100 °C was longer than that in MgCl2 solution. Nitrogen could affect the SCC behavior of DSSs in CaCl2 solution through the combinative effects by varying the pitting resistance and the slip step dissolution. An optimum nitrogen (N) content of 0.15 wt.% was found where the highest SCC resistance could be obtained. Although γ phase exhibited better resistance to SCC, cracks were found to penetrate through α and γ grains or to propagate along the α/γ interface. As a result, a mixed transgranular plus intergranular mode of fracture surface was observed.

The effects of minor elements and shielding gas on the penetration of TIG welding in type 304 stainless steel have been studied. The bead-on-plate test was performed, then the depth and width of the weld were measured using an optical projection machine. The arc voltage was measured with an arc data monitor. In addition, the metallurgical characteristics of weld were examined using OM and SEM. The results show that oxygen and sulfur are beneficial in increasing a depth/width ratio because of the increased surface tension/temperature gradient. Elements, such as aluminum, that have a deleterious effect on the depth/width ratio will combine with oxygen and reduce the soluble oxygen content in the weld pool. On the other hand, silicon and phosphorus have a minor effect on the depth/width ratio. Shielding gas using Ar + 1% O2 or Ar + 5% H2 can significantly promote the depth/width ratio. The former contains increased soluble oxygen content in the weld pool, and the latter produces an arc that is hotter than that produced by pure argon.