JPS5856747B2 - Heat treatment method for improving intergranular stress corrosion cracking resistance of gamma prime precipitation-strengthened Ni-based alloy used in hot water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for heat treating a gamma prime precipitation strengthened Ni-based alloy used in hot water such as in a nuclear reactor.

0, known as a gamma prime precipitation-strengthened Ni-based alloy.
, 0.5%C-15,5%Cr-0,7%A7-2
．． 5%Ti-1%Nb-7%Fe-Bait,
Ni-based alloys with Ni as the basic composition are heated at 982°C or 10°C.
It is heat treated to form a solid solution at 66°C and then aged at 704°C, and is used as a spring material for supporting fuel rods in boiling water reactors, but this heat treatment has the disadvantage of poor intergranular stress corrosion cracking resistance. .

The present invention is the discovery of a new heat treatment method for the purpose of improving the intergranular stress corrosion cracking resistance of gamma prime precipitation-strengthened Ni-based alloys.

In the present invention, a gamma prime precipitation-strengthened Ni-based alloy used in hot water is subjected to solid solution treatment in which it is held at 950 to 1100°C for 0.1 to 5 hours and then cooled to room temperature.
From a heat treatment method characterized by performing a stabilization treatment of holding room temperature at 00 to 900°C for 1 to 50 hours, followed by an aging treatment of holding room temperature cooling at 650 to 750°C for 1 to 50 hours. Become.

Comma prime precipitation-strengthened Ni-based alloys require solid solution treatment in which they are heated to a high temperature and then cooled at a cooling rate that does not cause precipitation in order to dissolve carbides or gamma prime phase into a solid solution. If the solid solution treatment temperature is lower than 950°C, the solid solution will not be sufficient;
If the temperature is higher than 0.degree. C., the crystal grain size becomes excessively coarse, which is not preferable.

Therefore, the solid solution treatment temperature is limited to 950 to 1100°C.

The important point of the present invention is that stabilization treatment is performed prior to aging treatment.

In order to give the alloy sufficient strength, it is necessary to uniformly precipitate the fine gamma prime phase, but to do so, 6
It is necessary to carry out an aging treatment at 50 to 750°C for one hour or more.

However, in this case, M2a06 type carbides preferentially precipitate at grain boundaries and form a layer with a low Cr concentration near the grain boundaries, thereby increasing the susceptibility to intergranular stress corrosion cracking.

Therefore, before aging treatment at 650-750℃,
If stabilization treatment is performed by holding room temperature at 900°C for 1 to 50 hours, M23C6, which precipitates at such high temperatures, will have less tendency to preferentially precipitate at grain boundaries (rather, it will precipitate uniformly within the grains). , a layer with a low Cr concentration is not formed near the grain boundaries.

After precipitating M23C6 uniformly within the grains in this way, 6
If aging treatment is performed at 50 to 750°C, most of the M23C6 has already precipitated, so the amount of M23C6 that newly precipitates at the grain boundaries will be significantly reduced, increasing the susceptibility to intergranular stress corrosion cracking due to aging. It is possible to reduce the degree.

Stabilization treatment is undesirable at temperatures below 800°C, as M23C6 tends to preferentially precipitate at grain boundaries.
℃ or higher, the amount of M23C6 precipitated is not sufficient.

Further, although at least 1 hour is required to cause the precipitation of M23C6 to occur sufficiently uniformly, a long time exceeding 50 hours is not necessary.

For the above reasons, the stabilization treatment in the present invention is 800 to 9
Limited to those that can be maintained at 00°C for 1 to 50 hours.

Aging treatment is not practical at temperatures below 650°C, as precipitation of the gamma prime phase slows down, while at temperatures above 750°C, the grain size of the precipitated gamma prime becomes coarse, making it impossible to obtain sufficient strength.

Further, although at least 1 hour is required to cause the gamma prime phase to precipitate sufficiently uniformly, a long time exceeding 50 hours is not necessary.

For the above reasons, the aging treatment in the present invention is 650 to 75%.
Limited to those that can be maintained at 0°C for 1 to 50 hours.

After the solid solution treatment, the effects of the present invention will not be impaired even if an optional aging treatment is added before and after the stabilization treatment on the high temperature side and between the aging treatment on the low temperature side.

Next, embodiments of the present invention will be described.

Gamma prime precipitation strengthened Ni-based alloy samples with the compositions shown in Table 1 were subjected to the heat treatment shown in Table 2 to obtain ASTM 28-
When the intergranular corrosion resistance test specified in No. 72 (24-hour corrosion) was conducted, the corrosion weight loss results were shown in Table 3.

From this table, it can be seen that the heat treatment method of the present invention has much better intergranular corrosion resistance than the conventional heat treatment method.

As explained above, the heat treatment method of the present invention has the advantage of significantly improving intergranular corrosion resistance, so it can be applied to gamma prime precipitation-strengthened Ni-based alloys used in hot water that are prone to intergranular stress corrosion cracking. If so, the service life can be significantly improved.

Claims (1)

[Claims] 1% by weight: C0.08% or less, Si0.50% or less,
Mul, 00% or less, 80.01% or less, Ni+Co7
0.00% or more, Co1.00% or less, Cr14.00
~17.00%, CuO, 5% or less, Ti2.25~2
．． 75%, NbO, 70~1.20%, A10.40~
950-11 gamma prime precipitation strengthened Ni-based alloy with a composition of 1.00% and 5.00-9.00% Fe.
800 after solid solution treatment held at 00℃ for 0.1 hour or more
Resistance of gamma prime precipitation-strengthened Ni-based alloy used in hot water characterized by stabilization treatment held at ~900°C for 1 hour or more, followed by aging treatment held at 650-750°C for 1 hour or more. A heat treatment method that improves intergranular stress corrosion cracking.