Surface hardening treatment of C.P. titanium, α+β type Ti-4.5%Al-3%V-2%Mo-2%Fe (SP-700 alloy) and β type Ti-15%V-3%Al-3%Cr-3%Sn alloy in use of Ar-20%CO2 gas was studied at temperatures between 973 to 1123K in comparison of Ar-20%O2 gas. Weight gain of the specimen during heating was measured by an electric balance, and surface oxide layer thickness was also measured. Surface hardening was evaluated by measurement of micro-hardness distribution profile in the surface layer. Surface hardening due to Ar-20%O2 gas yielded the larger maximum surface hardness value and thicker hardening layer thickness in all three titanium materials compared with Ar-20%O2 gas. The largest surface hardness value was obtained in C.P. titanium, and β type titanium alloy resulted in the most thick hardening layer thickness although the surface hardness value was the smallest among three titanium materials. The maximum surface hardness and hardening layer thickness obtained in use of Ar-20%O2 gas was larger than these of Ar-20%O2 gas in C.P. titanium, nevertheless the former gas yielded higher oxidation rate over the latter gas. The oxidation rate was the smallest in SP-700 alloy among three titanium materials in both gases. Surface hardening due to Ar-20%O2 gas in C.P. titanium was brought about by solid solution hardening due to oxygen and carbon.
The removal of scales on hot rolled and annealed stainless steel coils is achieved only by the dissolution of matrix metal in the pickling process where the commercial pickling solution for Type 430 stainless steels is the sulfuric acid solution. It was found that the addition of nitric acid to the sulfuric acid solution increased the dissolution rate of the matrix metals 2 to 4times. Also, the addition of nitric acid to the pickling sulfuric acid solution improved the surface smoothness by the decrease of grain boundary corrosion due to the segregation of P.
Zn-Nb films were formed on the steel sheet by helicon plasma sputtering under argon gas atmosphere of 1.0Pa, with RF power of 50W and DC power of 50mA to study the structure and the polarization property of the films using EPMA, X-ray diffraction and polarization curve measurement. Zn plate like crystals in Zn-Nb films were decreased with increasing Nb content, resulting in smooth surface all over the films at Nb content of 30mass% and above. The preferred orientation plane of Zn plate like crystals was (0002) at 0mass% Nb, while the orientation indices of (1011) and (1010) Zn were increased with increasing Nb content. At 30-60mass% Nb, the X-ray diffraction spectra showed halo pattern characteristic of anomalous structure. The corrosion current of Zn-Nb films, which was determined by polarization curves in 3% NaCl solution, decreased with increasing Nb content to reach the minimum value at 30-60mass% Nb of anomalous structure.
High strength steels have been developed in order to improve fuel economy as well as to increase safety. The conventional Si-Mn bearing high strength steels have the drawback of poor galvanisability due to the presence of complex Si-Mn oxides on their surface. In order to improve the galvanisability of Si-Mn steels, the effect of the addition of Ni and Cu on the wettability and galvannealing kinetics has been investigated. It was show that the addition of Ni and Cu improved wettability and increased the galvannealing rate. This improvement was due to the change of the sub-micron oxide morphology under a deoxidized Fe layer just before the galvanizing treatment (oxidized and deoxidized). In the case of C-Si-Mn steel a film of Si-Mn oxide was formed under a deoxidized Fe layer. The addition of Ni and Cu made Si-Mn oxide intermittently and internal oxide formed remarkably.
Type 316LN stainless steel is a candidate material for jacketing materials of Toroidal Field (TF) coil for International Thermonuclear Experimental Reactor (ITER). It was known very well that superconducting reaction heat treatment, i.e., aging caused intergranular sensitization and decreasing of ductility and toughness of stainless steels at 4K. In this paper, the effect of N, Nb, P and C on the 4K mechanical properties of aged 316LN stainless steels was discussed. In order to prevent decreasing of 4K ductility and toughness of aged 316LN stainless steels with P content of an industrial level, Nb free, C content less than 0.01mass% and N content less than 0.18mass% were recommended.
The strain path dependence of forming limit strains and stresses of a steel tube subjected to combined axial load and internal pressure are investigated for linear and combined stress paths using a tension-internal pressure testing machine. The combined stress paths consist of two linear stress paths and include unloading between the first and second loadings. The strain hardening behavior of the steel tube for both linear and combined stress paths is observed in terms of the equivalent stress-equivalent plastic strain (σ-ε) curves. Forming limit stresses in stress space drop on a single curve irrespective of the strain paths when the σ-ε curves for the given linear and combined stress paths are on a single curve (isotropic hardening), while the forming limit stresses for other combined stress paths generally become less than those for the linear stress paths when the material exhibits the cross effect in the second loadings. It is therefore concluded that the strain path dependence of forming limit stresses is much affected by the strain hardening behavior of the material for the given loading path. A new method for evaluating forming limits for combined loadings is proposed utilizing a limit curve in an equivalent plastic strain-tress ratio space.
Fatigue tests were conducted using a series of ultrafine ferrite-cementite steels with the ferrite grain size below 1μm in order to investigate the effects of precipitation, solid solution and dislocation strengthening on the fatigue properties. Some steels contained 0.15% of carbon and others 0.45% to investigate the effects of precipitation strengthening since the carbon addition increased the number of cementite particle in case of the ultrafine ferrite-cementite steels. 0.1% of phosphorus was added to some steels to discuss the effects of solid solution strengthening. The effects of dislocation strengthening were investigated by comparing the as-rolled steels with the annealed versions. As the result, tensile strength of the prepared ultrafine ferrite-pementite steels ranged from 721 to 1048MPa. All of the prepared ultrafine ferrite-cementite steels showed high fatigue strength and the fatigue limit ratios (fatigue limit/tensile strength) exceeded 0.5. The fatigue strength of the ultrafine ferrite-cementite steels was higher than that of ferrite-cearlite steels, and as high as that of tempered martensite steels. In conclusion, all of the above strengthening mechanisms effectively improved the fatigue strength of the ultrafine ferrite-cementite steels. The reasons why the ultrafine ferrite-cementite steels stably showed high fatigue strength would be because the microstructures were not only ultrafine but also uniform.