The uniaxial creep rupture strength of Mod. 9Cr-1Mo steel welded joints is lower than that of the base metal, due to the existence of the heat affected zone (HAZ) softening region which is formed by welding heat. In this paper, the multiaxial creep damage of welded joints is evaluated analytically using the Finite Element Method (FEM), so as to study the width of HAZ softening region that affects creep damage.To begin with, creep strain equations were established based on the result of uniaxial creep tests. The uniaxial creep damage was evaluated analytically using these creep strain equations, and the result agreed with the experimental result. It was found from the uniaxial creep damage analysis that the effect of HAZ softening region on creep rupture strength is negligible, in the case of relatively narrow HAZ softening region compared with the diameter of welded joints.The analytical result of internal pressure creep tests using the FEM was correlated well with the experimental result. Therefore, the creep damage of welded joints including HAZ softening region can be evaluated analytically.Key words: Heat resistant steel, Mod. 9Cr-1Mo steel, 9Cr steel, Creep, Creep rupture strength, Softening, HAZ, FEM, Welded joint, Boiler tube, Simulation Table I. Chemical composition of Mod. 9Cr-1Mo steel.

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This paper describes a methodology for systematizing knowledge concerning natural laws for engineering design. It is assumed that natural law can be represented by constraint and behavior, and that relations among natural laws can be described by using a scheme of constraint and behavior. The methodology of this study is a systematization focusing on the constraint of natural laws and relations among natural laws. After some discussion, it was found that natural laws could be systematized into a natural law description model, and a relational knowledge of natural laws could be systematized into a natural law relational model, a dictionary and an operation algorithm of natural laws. As a result of this study, it is confirmed, through some examples, that the assumptions made in this study are valid and that this approach is feasible for engineering design.

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Some discussions about the mechanism of the unexpected severe impingement corrosion of condenser tubes were made in relation to hydrotalcite. It was elucidated experimentally that hydrotalcite is the initial corrosion product of Aluminum Brass in the natural sea water. Even if iron rich protective films are formed on hydrotalcite, they will be broken quite easily by the collision of foreign bodies with smaller energy in the sea water. This will increase the possibility of corrosion. The most important countermeasure is to supress the initial corrosion of condenser tubes by the effective ferrous ion dosing and to prevent hydrotalcite formation. It was pointed out that more attension should be paid to hydrotalcite from the standpoint of corrosion engineering of condenser tubes.