JPS607025B2 - Heat treatment method for Ni-based alloy containing Cr - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for improving the stress corrosion cracking resistance of a Ni-based alloy containing Cr.

Ni-based alloys are originally materials with excellent stress corrosion cracking resistance. (Product name, 75%Ni-15%C
Ni-based alloys such as r-8% Fe) are used. Stress corrosion cracking occurs due to the coexistence of three factors: the presence of tensile stress, the environment in which it is used, and the characteristics of the material itself.The first two are related to the conditions in which the material is used, and they cannot be completely eliminated. is often impossible.

For example, in the steam generator tubes of the above-mentioned nuclear reactors, in addition to residual stress due to bending, tensile stress is inevitably present due to thermal stress during operation, and extremely strict water management is carried out. However, it cannot be said that this is a usage environment in which stress corrosion cracking will never occur. In the end, the most basic and effective measure to prevent stress corrosion cracking is to improve the properties of the material itself and reduce the susceptibility to stress corrosion cracking as much as possible. As described above, Ni-based alloys are preferably used as materials with low stress corrosion cracking susceptibility.

However, in a material such as Inconel 600, which contains Cr, intergranular cracking may occur depending on the usage conditions.

That is, in a high-Ni alloy containing Cr, the solid solution amount of C is small, so even if the C content is reduced as much as possible during the refining process, Cr carbides will precipitate during the subsequent product heat treatment process.

Since this Cr carbide mainly precipitates at grain boundaries, a Cr-depleted layer is formed at the grain boundaries, and the corrosion resistance of that portion deteriorates. The cause of intergranular stress corrosion cracking is thought to be the formation of the above-mentioned Cr-depleted layer, so the preventive measures are to prevent the precipitation of Cr carbides or to erase the Cr-depleted layer once formed. There are two possible methods. ．． In products such as steam generator tubes,
A softening heat treatment at a temperature of 900° C. or higher is usually performed.

The only way to prevent the precipitation of Cr carbides during cooling from such a temperature range is to increase the cooling rate, but it is necessary to completely prevent the precipitation of Cr carbides at an industrially achievable cooling rate. It is difficult. On the other hand, the method of repairing the Cr-depleted layer by heating the material in which Cr carbide has already precipitated at a predetermined temperature for a long time to promote the diffusion of Cr from the surrounding area into the Cr-depleted layer that has been formed is a method for repairing the Cr-depleted layer. Although this is an effective method for improving stress corrosion cracking resistance, it has the drawback that it requires a long processing time, such as requiring 15 hours or more at 70,000, resulting in poor productivity.

This invention is basically a method of improving the stress corrosion cracking resistance by restoring the Cr-depleted layer once formed, which significantly shortens the processing time and improves production efficiency. This paper proposes a method that enables a significant improvement in performance.

The feature of this invention method is that the Ni-based alloy containing Cr is
Heating at a temperature of 0,000 to 875 CO, then 600
The purpose is to perform heating for a predetermined period of time at a temperature of ℃ to 7500℃.

Here, the Ni-based alloy is one that contains about 60% or more of Nj, and contains Cr as one of the contained elements.

That is, since the above-mentioned grain boundary type stress corrosion cracking becomes a problem in high-Ni alloys having a low C hardness and also containing Cr, it is natural that such alloys are the object of the present invention. A typical example of such an alloy is Inconel 600, which is currently widely used in nuclear reactor steam generators, etc., and will be explained below using Inconel 600 as an example. It goes without saying that the method of this invention can be applied to Ni-based alloys.

The heat treatment of the present invention is carried out after the softening heat treatment at 900 oo or more, which is normally applied to products made of Ni-based alloys such as Inconel 60 clams.

In other words, normally Ni-based alloy products are melted, hot worked,
Manufactured through the steps of softening heat treatment - lining processing - softening heat treatment. The conditions for softening heat treatment after hot working and softening heat treatment after cold working are usually 1100 qo and held for about 30 minutes for the former.
The latter is maintained at 900oo or more for more than 2 minutes. The softening phosphorus dulling after cold working is carried out by appropriately selecting the above heating temperature and time depending on the target strength of the product. The method in this invention is the same as a conventional method up to the softening heat treatment after the cold working in the above steps, so the conditions are not particularly limited. 0
After the heat treatment after this cold working, the
Heat treatment at 5qo (hereinafter referred to as first stage heat treatment) and 6
0,000 to 75,000° C. (hereinafter referred to as second stage heat treatment) may be performed, or the once cooled alloy may be reheated at an arbitrary cooling rate after the softening heat treatment to perform the first stage and the second stage heat treatment. Two-stage heat treatment may be performed.

When processing following softening heat treatment, during cooling,
The same effect as the Lo stage heat treatment can be obtained by slowing down the cooling time in the temperature range of 80,000 to 875 pm for 10 minutes or more.

The time of the second stage heat treatment largely depends on the heating temperature.

That is, it takes a short time on the high temperature side and a long time on the low temperature side. FIG. 1 is a chart showing the relationship between temperature and time of this second stage heat treatment. ta Point A (2. separate r, 75000),
B (160hr, 600qo), C (1 song r, 600o
The shaded area surrounded by the line connecting D (0.2 rule r, 750oo) is the appropriate processing condition.

The main purpose of the first stage heat treatment in the method of this invention is to improve the rate of repair of the Cr-depleted layer carried out in the second stage heat treatment.

FIGS. 2 and 3 are examples showing the effects of this first stage heat treatment. The amount of Cr-depleted layer can be determined by an intergranular corrosion test, but here, the degree of corrosion is measured by the weight loss (hours/hour) of the specimen after immersion in boiling 40% HN03 for 24 hours. The amount of deficient layer was determined. The test piece is 75
．． 05%Ni-15.86%Cr-8.10%Fe-0
．． 024% Inconel 600. Figure 2 shows the first
It is a graph showing the degree of corrosion according to the above test when a test piece was subjected to step heat treatment (isothermal heating at 850 oo for 15 minutes) and a test piece which was not subjected to second step heat treatment at 700 oo.

As is clear from this, in the case where the first stage heat treatment was not performed, the degree of corrosion did not decrease, that is, the Cr-depleted layer did not completely repair even if the second stage heat treatment was performed for one heating time.

On the other hand, in the case where the first stage nose heat treatment was performed, complete restoration of the Cr-deficient layer was observed after only one hour of the second stage heat treatment. Furthermore, in the same way, the first stage heat treatment (1 at 85,000
650 with and without isothermal heating (5 minutes of isothermal heating)
FIG. 3 shows the respective degrees of corrosion when the second stage heat treatment of heating at q0 was performed.

Even with the first stage heat treatment, an increase in the amount of corrosion is observed at the initial stage of heating at 650o0, but this is due to Cr.
This figure shows the precipitation of carbides and the formation of a Cr-depleted layer that occurs along with the precipitation. However, if the heat treatment is carried out at 650° C. for one hour or more, the Cr-depleted layer will be repaired and the amount of corrosion will rapidly decrease. That is, complete repair of the specimens subjected to the first stage heat treatment was observed by performing the second stage heat treatment for 4 hours.

On the other hand, in those that were not subjected to the first stage heat treatment, the Cr-deficient layer did not recover even if the second stage heat treatment of 3 feeding hours was performed. From the viewpoint of thermal efficiency, it is desirable that the second stage heat treatment is performed successively after the first stage heat treatment.

However, even if the material that has been cooled after the first stage heat treatment is reheated to the above temperature range and then subjected to the second stage heat treatment, the effect will not change. The reason why the first stage heat treatment is set to 800qo to 875o0 is because at 875°C or higher, the effect of shortening the recovery time of the Cr-depleted layer during the second stage heat treatment as shown above is small; This is because, in order to obtain the same effect, a long heat treatment is required, which is uneconomical.

The time of the 18th differential heat treatment is 10 minutes or more and less than 1 hour.

Even if the heat treatment is carried out for one hour or more, the effect of long-time heating is not observed and it is economically disadvantageous.

If the heating time is less than 10 minutes, the amount of carbide precipitated at the grain boundaries is small, so that the next second stage heat treatment will take a long time. This first stage heat treatment can be replaced by adjusting the cooling rate after the softening treatment from 900° C. or higher. That is, the same effect as described above can be achieved by spending 10 minutes or more but less than 1 hour at a temperature of 800 to 875 degrees during cooling.
Next, the reason why the second stage heat treatment is performed at a temperature within the conditions surrounded by points A, B, C, and D as shown in FIG. 1 of the attached drawings is as follows.

In other words, if heat treatment is performed at a temperature of 75,000 qC or higher, there is a high risk that a Cr-depleted layer will form again during cooling after the heat treatment, resulting in deterioration of corrosion resistance.
Even if the heat treatment is carried out at a temperature of 0000 or less, the effect of restoring the Cr-deficient layer can be obtained, but the production efficiency is significantly reduced and it is uneconomical, so the temperature is set at 600000 or more. Furthermore, point C (1 shoe r,
60000) and point D (0.2 law r, 7500C), the Cr-depleted layer cannot be repaired in a shorter time range. If the time is longer than the straight line connecting point A (2. ball r, 75000) and point B (160 hr, 60000), there will be no effect of long-term heat treatment and it is not economically preferable. Therefore, for these reasons, the temperature surrounded by points A, B, C, and D in the graph of FIG.
The 288th heat treatment shall be performed within a limited time range.

Next, embodiments according to the present invention will be described.

Inconel 600 (the composition of which is the same as the examples in Figures 2 and 3) was selected as a representative example of a Ni-based alloy, and was melted by vacuum melting to 17
After melting K9 steel ingot, forging and hot rolling at 120,000, after softening heat treatment at 110,000 and holding 3 powder, cold rolling at 30% reduction, and then holding at 950℃ for 30 minutes. After the softening heat treatment, the heat treatment method according to the present invention was applied.For comparison, the heat treatment was performed under conditions other than the conditions according to the present invention, and the heat treatment was performed as it was after the softening heat treatment (conventional method). Each of these materials was tested for intergranular corrosion and stress corrosion cracking.
The temperature and time of the first-stage heat treatment and second-stage heat treatment of each test material are shown in Tables 1 and 2. Table 2 shows the case where slow cooling treatment was used instead of the first stage heat treatment. The test materials according to the subordinate method in Table 1 were softened at a temperature of 95,000, then the cooling rate was reduced to 50,000, and material number 16 was
50oC'min, material number 17 is 400℃/min
Cooling was performed.

Note that the intergranular corrosion test was evaluated using the method described above.

In the stress corrosion cracking test, test pieces were immersed in a non-degassed 30,000 aqueous solution containing blood chloride ions of 1,000 for 1,000 hours.

The test piece is a so-called double U-shaped bending test piece in which two layers of willow, 10 ribs wide, 75 ribs long, and 2 layers thick, are bent into a U-shape with a radius of 8 legs and further restrained with 5 willow nuts and bolts. was used.

In this case, tensile stress is applied to the outer side of the bend of the inner test piece among the two pieces, and there is a gap, which is the condition where stress corrosion cracking is most likely to occur. After immersion in the aqueous solution for 100 hours, the maximum crack depth at the center of the inner test piece was measured using a microscope.

Therefore, it can be determined that the smaller the maximum crack depth, the higher the stress corrosion cracking resistance. The results of each of these tests are also shown in Tables 1 and 2.

Table 1 Table 2 As is clear from the test results in Tables 1 and 2, all of the materials of the present invention that have been heat treated according to the present invention have a corrosion degree in the intergranular corrosion test that is lower than that of the conventional material and the comparative material. It is significantly lower than that of other types, and fewer cracks occur in stress corrosion cracking tests, and even when cracks do occur, the maximum crack depth is smaller than other types.

As described above, this invention is an invention of high industrial value that can significantly improve the stress corrosion cracking resistance of Cr-containing Ni-based alloys.

[Brief explanation of drawings]

Fig. 1 is a chart showing the limited range of temperature and time of the 28th hair heat treatment of this invention, and Fig. 2 is a chart showing the limited range of the temperature and time of the hair heat treatment of this invention.
Second stage heat treatment (with and without powder)
70000), and FIG. 3 is a chart showing the heating time and corrosion degree when the second foundation heat treatment is 650 qo under the conditions of FIG. 2. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. A Ni-based alloy containing Cr is subjected to hot working, softening heat treatment, and cold working, followed by softening heat treatment, and then
A (2.5 hr, 750° C.) in the chart in which the horizontal axis in FIG. B(
160hr, 600℃), C (16hr, 600℃),
Cr characterized in that heat treatment is performed under arbitrary conditions within the temperature and time range surrounded by D (0.25 hr, 750 ° C.)
A method for heat treating a Ni-based alloy containing. 2. A patent claim characterized in that the heat treatment at 800°C to 875°C is a slow cooling process in which the cooling time within this temperature range during cooling after the heat treatment at 900°C or higher is 10 minutes or more and less than 1 hour. The heat treatment method according to item 1.