Nobuhiko Nishimura

Phase Transformation and Properties of Gr.91 at Around Critical Temperature

During the fabrication processes of boiler components, heating, forming and welding processes are commonly applied to the materials used. The effect of heating and strain induced by the these fabrication processes are very important in maintaining the integrity of creep strength enhanced steels, because it is well known that the mechanical properties of normalized and tempered high strength steels are relatively sensitive to thermal and work history. In particular, while Gr.91 steel has been used extensively for boiler and pressure components over the last two decade, the heat treatment and forming procedures for this steel are not fully established based on fundamental transformation and strain behavior. This paper deals with the phase transformation behavior of Gr.91 during tempering at around the Ac1 temperature, and the hardness/creep properties of Gr.91 steel heated at temperatures from just below Ac1 to above Ac3 combined with strain.Copyright

Irradiation-induced spinodal decomposition and its process in FeMo alloys

Unexpected phase transformations induced during irradiation are major problems in developing fusion reactor materials. Induced spinodal decomposition under 1 MeV electron irradiation is studied in Fe-Mo alloys with respect to its confirmation and the physical background of the phenomenon. The coherent spinodal temperature under irradiation increases with increasing electron flux and reaches a saturation value, which corresponds to the chemical spinodal temperature in the absence of irradiation. The results are analyzed in terms of the heterogeneous segregation of solutes, the constraint spinodal decomposition under inducement of point defects, and the relaxation of coherent strain associated with composition modulation. The conclusion is that an excess of interstitials and vacancies relaxes the coherent strain, resulting in the inducement of decomposition up to the chemical limit.

Effect of Repair Welding on the Creep Rupture Strength of an Aged Weldment

Weldments in high-energy piping of fossil power plants are known to suffer extensive creep damage over the course of long-term operations. Repair welding after the removal of the deteriorated areas is considered as a potential method for improving the life of such weldments. Very few papers, however, are reported concerning the influence on creep rupture behaviour of repair welding performed at the end of the creep life of the weldments. In this paper, creep test of large weld specimens was carried out to create substantial numbers of creep voids, and these specimens were then repair-welded using various methods. On the basis of the results of subsequent creep rupture tests and stress analyses of these test specimens, the influence of repair welding on the creep rupture strength of deteriorated weldments was discussed.

A New Heat Treatment Technology for On-Site Life Extension of High Energy Pipe Weldments Degraded by Creep Damage

This report presents the development of technology enabling temporary countermeasures for the on-site application of life-extending induction heating to high energy pipe weldments having experienced degradation due to creep. The results of an investigation into the cross-sectional structure thus suggested that material life can be extended by the application of regenerative heat treatment. The life-extending effect of regenerative heat treatment is greatest toward the short-life vicinity of the pipes outer surface, thus providing an efficient life extension. The remaining life converted for an actual installation from the result of real scale creep burst test increased from 63,000hrs before treatment to 200,000hrs after treatment, confirming that life can be extended by approximately 130,000hrs.Copyright

Creep Properties of Gr.91 Castings and Forgings With High Residual Elements

Gr.91 castings are used in high temperature valves for main steam lines and elsewhere, while Gr.91 forgings are essential products for power boilers and in turbine construction. However, scrap steel is very often used as a raw material in the manufacturing of Gr.91 castings and forgings, suggesting the inclusion of high levels of residual tramp elements in the products when the quality of the raw material and process is out of control. The influence of residual elements such as P, S, Cu, Sn, and Sb on creep properties is a very important issue from the standpoint of using Gr.91 castings and forgings in high temperature components. This study focuses on the effect of the amount of tramp elements on the creep properties of Gr.91, investigating the creep mechanisms of tempered martensite lath structures with the above-noted elements. As a result, it was found that creep strength increased by either increasing the content of P or S, or by decreasing the amount of Sn or Sb. The Cu content did not affect creep strength at 600°C. At 650°C, the creep rate was unaffected by the tramp elements. Whereas the carbide distribution was hardly influenced by the proportion of the tramp elements, the martensitic lath spacing was observed to be dependent on the content, and was also related to the minimum creep rate obtained in creep testing at 600°C.Copyright