Kazuo Toyama

Fracture toughness of medium-high carbon steel for railroad wheel

Railroad wheel brittle fracture is catastrophic, and can cause a derailment. Such usage conditions as load and speed of railroad vehicles have become severe in recent years. Therefore, more precise evaluation of wheel strength and safety has been desired. From recent development of fracture mechanics, it is clear that in order to prevent the wheel fracture, fracture toughness of medium-high carbon steel for railroad wheels should be raised. This paper describes the effect of carbon, manganese, aluminum solution on the fracture toughness. Ferrite volume fraction and grain refinement were analysed to obtain the relationship between fracture toughness and material characteristics in medium-high carbon steel.

Environmental effects on the crack growth properties of alloy 600

Abstract Double U-bend tests are usually applied for estimation of stress corrosion cracking in nickel alloys in pressurized water reactor environments. The test environment is often selected to be severe by the addition of an NaOH concentration higher than that of the actual plant environment. In this study the effects of NaOH concentration on the crack growth properties of Alloy 600 were identified by crack growth tests under a high stress ratio. The crack growth curves of mill-annealed Alloy 600 and thermally treated Alloy 600 were determined in various environments between All-Volatile Treatment water and a 40% NaOH solution. The crack growth rates and fracture surfaces are discussed in terms of the effects of environment and heat treatment.

High Temperature Strength. Prediction of Crack Initiation Life of Converter Shell Based on Stress Analysis.

In order to clarify effective measures to extend a life of converter shell in a steel production plant, the thermal fatigue crack initiation life was predicted by the strain range partitioning method.A three-dimensional FEM model of the converter which is composed of a shell and bricks was established. An inelastic strain of the shell caused by a thermal cycle during operation was evaluated by an elastic-plastic-creep analysis. Since the thermal expansion of bricks is partly absorbed by a gap between bricks, the inelastic strain of the shell is affected by the gap size. Therefore, in the FEM model the hyperelastic element was used in order to describe the deformation behavior of bricks and gaps, and the stress analysis was conducted taking the gap size into consideration.Based on the analytical results and creep-fatigue properties of a shell material, the crack initiation life of the shell was predicted. The relationship between the shell life and cooling conditions at the outer surface of the shell and the relationship between the shell life and the gap size were examined. The following results were obtained.(1) The predicted shell life corresponded well with that in the actual converter.(2) It is possible to extend the shell life by decreasing the maximum shell temperature which can be attained by increasing the film coefficient of cooling at the outer suface of the shell.(3) It is also possible to extend the shell life by increasing the gap size. Futhermore, the larger the gap size is, the decrease of shell temperature becomes more effective for the shell life extention.

QUANTITATIVE EVALUATION OF CORROSION FATIGUE CRACK INITIATION PROCESS FOR HIGH TENSILE STRENGTH STEEL

Materials for construction tend to be used in more harsh environments nowadays. Much effort has been spent already in order to understand the effect of environment on fatigue strength. Especially, the environmental effect on fatigue crack propagation behavior has been clarified in many combinations of materials and environmental conditions. However, each step of the corrosion fatigue process has not been evaluated in detail. In this study, each period from initiation and growth of corrosion pits to initiation and propagation of corrosion fatigue cracks is estimated. Material used was a carbon steel HT60 of 630MPa class tensile strength. A micro-drilled notch was introduced at the surface of test specimen by machining. The notch of 80μm diameter, which imitated corrosion pits was expected to fill the role of concentrated stress sites. Test environments were 3.5 weight percent NaCl solution and 1.33×10-4Pa air in a chamber of scanning electron microscope (SEM). Fatgue tests were conducted under a load ratio of 0.1 at a frequency of 1.5Hz in NaCl solution and at 20Hz in a vacuum. While under continuous observation by SEM, a crack which reached 20 to 30μm in length was defined as fatigue crack initiation. A regression equation, with an accuracy within a factor of 5, for fatigue crack initiation life was derived for both environments. It became clear that the effect of stress and notch shape on initiation life was smaller in NaCl solution than that in a vacuum. In addition a relationship between stress intensity factor, calculated from shadow figure of notch, and crack initiation life was deduced. In NaCl solution, the initiation life of corrosion pits was estimated to be quite short, and a ratio of fatigue crack initiation life to total failure life was about 35% regardless of stress intensity factors.