Baking Treatment Research Articles

溶融亜鉛めっき高力ボルトの遅れ破壊

At present, Hot-dip Galvanized high strength hexagon bolts having high strength levels of above 95kg/mm2 are frequently used for fastening structural members of outdoor structures such as a transmission steel tower in Japan.Though Hot-dip Galvanizing is very superior in corrosion resistance in general, delayed fracture has occurred presumably due to hydrogen embrittlement. Therefore, we thought the following two reasons for the delayed fracture occurance: Hydrogen enters into the steel by acid pickling in the Hot-dip Galvanizing process and stays in. Or the cracks occur in the Galvanized layer under fastening, and then, hydrogen enters into the steel from the environmental atmosphere.To ascertain the above consideration, we performed the static bending test and delayed fracture test etc. on the Cr-steel specimens subjected to the same heat treatment as that for the bolt. As the result, we obtained the following conclusions.(1) Delayed fracture occured even in a short dipping time.(2) The thick Galvanized layer makes transmission of hydrogen atoms difficult, so that it is not possible to remove hydrogen from the steel by the usual baking treatment.(3) Delayed fracture almost never occurs when the acid-dip process is not incorporated.(4) Delayed fracture often occurs because cracks nucleate in the Galvanized layer by adding impact energy on a bolt.

無電解ニッケルメッキ材の疲労強度

The rotating bending fatigue tests were performed on the electroless nickel plated steels at room temperature, and the effects of thickness of plating and baking temperature after plating on the fatigue strength were investigated. Furthermore, the characteristics of deposited film itself were discussed.The electroless nickel plating of the specimens used was carried out at 90°C, pH 6, in a plating bath of the following compositions; NiSO4·6H2O: 30g/l; NaH2PO2·H2O: 10g/l; NaC2H3O2·3H2O: 10g/l.The results obtained are as follows:(1) When the electroless nickel plating is carried out on carbon steels, their fatigue strength rather increases, contrary to the case of electro-deposit steels. This improvement is independent of the thickness of deposited film and carbon content of steels. The rate of increase in fatigue strength against non-plated specimens is about 16∼17% for both steels.(2) The electrolessly deposited nickel film shows very satisfactory adhesion to the substratum under fatigue fracture condition. The fatigue damage of plated steels is suppressed by deposited film. Consequently, the nucleation of fatigue crack is delayed, resulting in the increase in fatigue strength.(3) By baking treatment, the micro-hardness of deposited film increases, accompanied with structural changes. This increase is due to the precipitation hardening of Ni3P. At the same time, the tensile residual stress in deposited film is reduced. The rotating bending fatigue strength, however, is not affected. Therefore, it is not expected that the fatigue strength is improved by means of the baking treatment after the electroless plating.

X-ray diffraction approach to the mechanics of fatigue and fracture in metals

The effect of different thermo-mechanical processing parameters on the baking response, and intergranular corrosion (IGC) resistance of an Al–4.6 wt% Mg–0.54 wt% Cu alloy was investigated. It has been shown that the high temperature anneal at 530 °C resulted in superior baking response of pre-deformed material after short annealing at 140–180 °C, compared to the annealing at 280 °C. Specimens pre-deformed 2% had positive baking response for both initial annealing temperatures, however, pre-deformation of 5% induced softening during the baking treatment of the specimens annealed at 280 °C. Prior to sensitization of 7 days at 100 °C, specimens at all stages of thermo-mechanical treatment were IGC resistant. After the sensitization specimens that underwent annealing at 530 °C remained IGC resistant, but specimens that underwent annealing at 280 °C became IGC susceptible. Better corrosion properties and baking response are related to the higher Cu content in solid solution after annealing at 530 °C than at 280 °C.

The Effect of Nickel Plating on Fatigue Strength of Steel

It is well known that the fatigue strength of machine parts is generally reduced by electroplating. This paper describes the summary of the data of fatigue strength for nickel plated steels obtained recently. The main results are as follows:(1) When carbon steel is nickel plated, the fatigue strength decreases as the current density or the thickness of plating increases.(2) The baking treatment at 200∼550°C after plating causes the reduction in the hardness of plating, but does not affect the fatigue strength.(3) The corrosion fatigue strength in 3% NaCl solution is slightly smaller than in air. Nickel plating gives only a little protective effect under the repeated stress condition, but prolongs the fatigue life of plated steel as compared with that of bare steel.(4) The fatigue strength of notched steel is also reduced by nickel plating, and the degree of loss can be estimated from the product of the individual reduction effects caused by plating and by notch.(5) The loss of fatigue strength of plated steel can be decreased by diffusion treatment at 700∼850°C.(6) Shotpeening, either before or after nickel plating, increases the fatigue strength considerably.These results suggest that the main cause for fatigue strength loss by nickel plating is not the internal stress nor absorbed hydrogen in the plated layer, but the low fatigue strength of the plated layer itself. The fatigue crack tends to form easily in plated layer, and acts as the source of stress concentration for crack propagation into steel similar to the double notch effect.

The stress corrosion cracking mechanism of α-titanium alloys at room temperature

The role of H in causing rapid transgranular s.c.c. in α-Ti alloys is discussed in detail inrelation to the hypothesis that H enters the unfilmed surface at the crack tip and causes slow strain-rate embrittlement. The protective role of cathodic protection in neutral solutions, the relatively slow rate of H diffusion and similar cracking in media that do not contain H are each discussed. The ratio of activating to passivating species in the environment is important in promoting film formation and accounts for the protective effect of cathodic polarization. It is not effective in non-film-forming conditions. Fractographic comparison with hydrided specimens indicates that some form of “long range” effect is possible, i.e. rapid fracture can occur into regions that do not contain large amounts of hydrogen. Results in CCI 4 suggest that the residual H 2O content (200 ppm) is responsible for cracking. In CH 3OH-HCI environments the degree of embrittlement increases as the strain-rate is lowered. Evidence for H embrittlement is provided by (i) pre-exposed unstressed specimens which exhibit cleavage to considerable depths when subsequently broken in air, an effect that can be removed by an intermediate baking treatment, (ii) quantitative fractographic measurements of specimens anodically polarized in CH 2OH-HCI, in which the cleavage markings obtained upon subsequent. fracture in air are not seen if the dissolving front progresses at a rate comparable to H diffusion, and (iii) fractographic comparisons with broken hydrided specimens. Physical, mechanical and chemical aspects of the mechanism are considered.