Hiroshi Yaguchi

Fatigue-damage evaluation in aluminum heat-transfer tubes by measuring dislocation cell-wall thickness

Evolution of dislocation structure during low cycle fatigue in 3003 aluminum alloy was investigated for the purpose of finding a possible parameter to indicate fatigue damage prior to crack initiation. In low cycle fatigue, it is observed that by increasing the number of cycles, the dislocation structure develops to random cells, then changes to clear cells and finally to sub-grains. In other words, the cell-wall thickness decreases with increasing number of cycles. It is also found that measuring the cell-wall thickness is a valuable method to evaluate fatigue damage when the cell structure is formed. The fatigue damage index (the number of cycles/cycles up to fracture) is found to be inversely proportional to the average cell-wall thickness, regardless of the applied stress level. This phenomenon can be explained by dynamic recovery of dislocation during fatigue, i.e. mutual annihilation of dislocations, which takes place during each half cycle.

The effects of Nb carbo-nitride precipitation conditions on abnormal grain growth in Nb added steels

In order to investigate the effects of Nb carbo-nitride precipitation conditions on abnormal grain growth behavior during high temperature carburizing, size of Nb carbo-nitride precipitates was controlled by precipitation treatment at 1173-1273K for 0.6-54ks, and the specimens were quasi-carburized at 1323K. Abnormal grain growth was enhanced when the size of Nb precipitates was fine or coarse, so there is a suitable size range in Nb precipitates to suppress abnormal grain growth. The reason why abnormal grain growth was enhanced is the lack of pinning force as the conventional theory proposed by Hillert or Gladman; however, it cannot be explained by this theory that small precipitates promote abnormal grain growth. It is considered that Ostwald ripening rate of precipitates is also an important factor in controlling abnormal grain growth in addition to the amount and size of precipitates and austenite grain size, which were parameters in the Gladman‘s theory on abnormal grain growth behavior.

Metal-induced embrittlement of low-carbon steel by indium in association with tellurium

Liquid metal embrittlement is harmful to mechanical properties such as ductility and toughness, and thus, should be prevented. However, there is one case for which liquid metal embrittlement is useful: it is believed to be at least one of the machinability improvement mechanisms by Pb and Bi in steel and Al alloys. It is known that tellurium compounds, such as PbTe and the MnTe-MnS eutectic, also cause liquid metal embrittlement around their melting temperatures of 923 C and 810 C, respectively. However, their melting points are believed to be too high for machinability improvement. The typical temperature range in the shear zones during conventional machining of low carbon steels is believed to be between 150 and 800 C. The In-Te binary phase diagram suggests that several intermetallic compounds could be formed. Thus, if any of these compounds are formed in steel, different machinability behavior could be expected. However, no thermodynamic data are available to indicate whether or not these intermetallic compounds can form in solid steel. Thus, this investigation was initiated to study (1) whether or not tellurium and indium form intermetallic compounds in steel and (2) if so, how these compounds affect liquid metal embrittlement behavior.

Long-Term Isothermal Aging Behavior of V-Modified 2.25Cr-1Mo Steels

Long-term isothermal aging behavior up to 30,000h has been investigated in V-modified 2.25Cr-1Mo steels. When impurity elements were added, FATT in Charpy impact test vs. aging time curves showed basically of parabolic type, indicating the importance of segregation of impurities as the critical factor in controlling temper embrittlement. On the other hand, when the amount of impurities was small, FATT showed complex behavior with aging time. This suggests the importance of other factors in controlling temper embrittlement such as recovery of dislocations and enlargement of various carbide precipitates. When the degree of temper embrittlement after aging is compared in terms of the increase in FATT due to aging between the modified steels and the conventional 2.25Cr-1Mo steels, it is found to be small in the modified steel at the same impurity level (J-factor). It was also found that the modified steels showed no hydrogen embrittlement. No change was observed in FATT after hydrogen charge on the contrast to the conventional steel, which showed significant increase in FATT. This effect has been attributed to the small amount of diffusible hydrogen content in the modified steel by the presence of hydrogen trapping sites, which is present even after long-term isothermal aging.Copyright