Embrittlement and fracture toughness of highly irradiated austenitic steels for vessel internals of WWER type reactors. Part 2. Relation between irradiation swelling and irradiation embrittlement. Physical and mechanical behavior

Abstract

The mechanical test results and fractographic observations reported in Part 1 are discussed from the standpoint of possible fracture mechanisms in austenitic steels subjected to intensive neutron irradiation. We put forward the mechanisms that relate the γ α → -transformation to the occurrence of a ductile-brittle transition in the irradiated austenitic steels and presents a criterion that defines the irradiation conditions whereby the γ α → -transformation leads to the ductile-brittle transition.Some possible reasons for the deterioration of the material ultimate strength at a very high (above 20%) level of swelling are discussed.Introduction. Part 1 [1] presented the results of experimental investigations of mechanical properties and irradiation swelling of austenitic steel 08Kh18N10T and its weld metal upon intensive neutron irradiation, which were performed in order to clarify the nature of the effect of the swelling on mechanical properties of austenitic steels.For this study some specimens were irradiated at a "low" temperature (T irr = 330-340°C) with a dose of 46 dpa (hereinafter referred to as the LTI material) and the other at a "high" temperature (T irr = 400-450°C) with a dose of 49 dpa (called the HTI material). These irradiation conditions were chosen for the purpose of producing different levels of swelling in the specimens. Specifically, the irradiation swelling is close to zero in the LTI material and ranges between 3 and 13% in the HTI material.The experimental investigations performed demonstrate that for the base metal the temperature functions σ 0 2 . ( ) T test and σ u test T ( ) are almost equidistant for the unirradiated and LTI material. For the HTI irradiated base metal which has a significant irradiation swelling, these functions are not equidistant to those for unirradiated and LTI specimens. However, for the weld metal they are equidistant for all three material states studied. We noted a significant increase in plasticity of the HTI material in comparison to that of the unirradiated material and the HTI material. At T test ≤°200 C the plasticity of the base metal upon HTI is close to zero.It was suggested in [1] that the base metal upon HTI undergoes an γ α → phase transformations.

Optimization of Automatic TIG Welding Parameters of AISI 304L ASS Welds Using Response Surface Methodology

“…These steels have also a good weldability. However, a phenomenon of embrittlement at high temperatures (600-900 °C) following the precipitation of intermetallic compounds and chromium carbides, and at low temperatures (300-550 °C) following the precipitation of martensite, residual austenite and ferrite phase (σ, α) causes a dramatic fall in mechanical and chemical properties [5][6]. This is mainly due to the instability of the ferrite phase due to thermal excitation, hypertrempe treatments or welding thermal cycles.…”

Embrittlement and fracture toughness of highly irradiated austenitic steels for vessel internals of WWER type reactors. Part 3. Analysis of crack propagation conditions

“…Based on the earlier findings [6] we can infer that for a material which has experienced the g a ® -transformation the temperature T tr »°400 C. This transition temperature corresponds to the tensile test temperature…”

“…It was demonstrated earlier in Part 2 [6] that an g a ® -transformation, which triggers a ductile-brittle transition, may occur in an austenitic steel upon some neutron dose depending on the irradiation temperature.…”

“…Special Features of Failure in Intensively Irradiated Austenitic Steels. In Part 2 [6] and in [7] it was demonstrated that in the course of neutron irradiation, at some value of the damaging dose, which depends on the irradiation temperature, the phase composition of austenitic steels may undergo a change through the g a ® -transformation. Figure 1 illustrates the neutron irradiation conditions whereby the g a ® -transformation takes place leading to a ductile-brittle transition in steel 08Kh18N10T.…”