Takanori Nakazawa

Creep rupture strength of heat affected zone for 9Cr ferritic heat resisting steels

Abstract This paper describes creep rupture characteristics of weld heat affected zone, HAZ for 9Cr ferritic steels that are promising materials for nuclear energy uses. In general, creep rupture strength in the heat affected zone of peak temperature between 900 and 1000°C is lower than that in the base metal for ferritic steels. Grain refinement and coagulation of carbides for 9Cr–1Mo steels cause decrease in creep rupture strength of the HAZ. The hardness in the simulated HAZ heated to around 1000°C decreases during creep. This seems to be related to weakening of the HAZ at 1000°C. However, substitution of W for Mo is very effective in enhancement of creep rupture strength of the HAZ due to higher stability of carbides and increase in quantity of precipitated carbides during creep rupture test.

Study on Metallography of Low Cycle Creep Fatigue Fracture of Type 316 Stainless Steels

The metallography of creep fatigue fracture of conventional type 316 (SUS316) and low carbon medium nitrogen type 316 (316MN) steels has been studied. The creep fatigue lives depend on the grainboundary precipitation during creep fatigue. Coarse carbides in SUS316 would cause grainboundary embrittlement, while very fine Laves phase in 316MN may have no influence. Since carbide precipitation by aging results in loss of matrix strength of SUS316, creep fatigue life increases. On the other hand, the grainboundary precipitation of 316MN is so small and stable that creep fatigue life does not change by aging. The decrease in remaining ductility by loading cycles depends on grainboundary embrittlement by precipitation and work hardening.

Study on Fracture Mechanism and a Life Estimation Method for Low Cycle Creep-Fatigue Fracture of Type 316 Stainless Steels

In order to improve the accuracy of creep-fatigue life estimation method, experimental investigations were conducted focusing to the ductility exhaustion rule. At first, it was found by the tensile test for type 316 stainless steels that the ductility decreased and fracture mode changed with the decrease of strain rate. The tensile test was also performed using the creep-fatigue interruption test specimen and it was found that there was a good correlation between the decrease of residual ductility and the creep damage. From these points of view, the authors proposed the improved life estimation method based on the ductility exhaustion concept and confirmed its effectiveness.

Mechanical Properties and Microstructure of SUS304 Brazed Joint with Ni-Based Filler Metals Added Cr Powder

In the brazed joint of stainless steel with BNi-2 filler, brittle Cr-B compounds form in the vicinity of the centerline of the brazed joint. These compounds cause a decrease in joint strength. In this study, BNi-2 filler supplemented with Cr powder has been used in brazing stainless steel in orde r to disperse brittle Cr-B compounds uniformly in the brazed joint and improve joint strength. The mechanical properties and microstructures of the brazed joints were investigated. Moreover, a comp arison of the brazed joint with that using the BNi-2 filler was conducted.

High Temperature Fatigue Properties of the 316 FR Steel

Type 316 FR stainless steel has been developed as a candidate material for fast breeder reactor of next century. For the structural integrity design of high temperature components including reactor vessel, long-term data and analysis method are investigated for the new 316 FR steel especially to evaluate its time-dependent low-cycle fatigue behavior. The present paper reports dependencies of fatigue life on the strain rate from 10-2 to 10-5S-1, and on the temperature dependencies from 500°C to 600°C. Data are analized by a parametric method formerly proposed by the authors. It is shown that the method has a good predictability of the fatigue life up to very low strain rate of 10-6S-1.

Creep Properties of Low-Carbon Nitrogen-Alloyed Modified 316 Stainless Steel.

Effects of carbon, nitrogen and tungsten on the tensile and creep properties of a composition-controlled modified 316 stainless steel for high-temperature use, have been investigated. This steel has a lower carbon and higher nitrogen content than conventional 316 stainless steel (SUS316). The results obtained are as follows.(1) Modified 316 exhibits high creep ductility, which changes little with increasing rupture time. It also has a generally higher creep strength than SUS316, and this advantage increases with longer rupture time.(2) The improved creep strength in the modified 316 is attributable mainly to a lower creep rate in the tertiary creep range, and this reduced creep rate is thought to be explicable in terms of stable solution hardening by nitrogen and suppressed grain boundary embrittlement resulting from the low carbon content.(3) Addition of tungsten increases the creep strength of the modified 316 with little change in creep ductility. A high tungsten content, however, is observed to enhance Fe2(Mo, W) formation. It is thought that the resultant reduction in Mo and W contents in the grain matrix lowers the creep strength as the rupture life increases.