JPS58177444A - Heat treatment of ni-cr alloy - Google Patents

Abstract

PURPOSE:To improve the stress corrosion cracking resistance of an Ni-Cr alloy having a specified composition, by regulating the C and N contents of the alloy and by prescribing conditions during final annealing and conditions during isothermal heat treatment to be carried out after the annealing. CONSTITUTION:An Ni-Cr alloy consisting of 0.015-0.050% C, <=1.0% Si, <=1.0% Mn, <=0.030% P, <=0.030% S, 25-35% Cr, <=0.020% N, 0.01-1.0% Ti, 6.0-10.0% Fe and the balance Ni with inevitable impurities or further contg. 0.01-1.0% Al is annealed under conditions within the range bounded by points A, B, C, D in the upper figure. The annealed alloy is held at 600-850 deg.C for 0.5-100hr under conditions within the range bounded by points E, F, G, H in the lower figure. By the treatment, Cr carbide is precipitated in the grains, and the precipitation of Cr carbide on the grain boundaries is reduced to prevent the deterioration of the stress corrosion cracking resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a heat treatment method for improving the stress corrosion cracking resistance of Ni-Cr alloys, particularly Ni-based alloys containing 25 to 35 Cr.

Ni-based alloys containing Cr are originally materials with excellent stress corrosion cracking resistance. Therefore, parts #I are extremely susceptible to stress corrosion cracking, such as heat exchanger tubes used in chemical grounds or steam generator tubes in pressurized wooden nuclear reactors.
C is A11oy 600 (759GNi.

1551Cr, 8%Fe) Godaka Cr N1-based alloy is used. However, even with the above-mentioned A1107600, when used in a steam generator tube of a nuclear reactor, stress corrosion cracking may occur under certain usage conditions.

This stress corrosion cracking is caused by approximately 3'g of the presence of tensile stress, a factor derived from the environmental conditions used, and a factor of the material itself.
! This cracking occurs when the elements are aligned, and this cracking can be prevented if even the l element is completely removed.

However, when used in the above-mentioned steam generator, for example, surface polishing and residual stress, as well as tension stress due to thermal stress during nuclear reactor operation, etc., are unavoidable. Furthermore, regarding factors caused by the usage environment, although extremely strict water quality control is carried out on the water used, it is nevertheless difficult to completely eliminate such factors.

Therefore, the best way to prevent stress corrosion cracking is to improve the material properties themselves and reduce the susceptibility to stress corrosion cracking.

However, in high-Ni alloys containing 25 to 35 - of Cr, the solid solution amount of C is extremely small, so even if the C content is reduced as much as possible by over-refining, 101! ! Since Cr carbides mainly precipitate at grain boundaries during processes such as No. 11 construction, a Cr-deficient layer is formed at the grain boundaries and the corrosion resistance of that portion deteriorates.

For example, products such as the steam generator mentioned above usually have a
Final annealing is performed at a temperature of 00 to 1150°C, so when cooling, welding, or using tit
°C) Precipitation of KCr carbides at grain boundaries is observed, and a Cr-depleted layer is formed to resist stress corrosion.The cause of this grain-boundary type stress corrosion cracking is the formation of a Cr-depleted layer, so the precipitation of Cr carbides is prevented. By preventing stress corrosion cracking, stress corrosion cracking can be prevented by recovering the Cr-depleted layer once formed.

Therefore, the first method to prevent the precipitation of Cr carbides is to increase the cooling rate, but it is difficult to completely prevent the precipitation of Cr carbides at a cooling rate that can be achieved in a knee-jerk manner. In addition, a method of repairing the Cr-depleted layer by heating already precipitated Or carbides at a predetermined temperature for a long time to diffuse CrO from the surrounding area into the Cr-depleted layer is effective in reducing stress corrosion cracking susceptibility. However, a Cr-depleted layer may be formed again during cooling after heat treatment.

Here,##! Annealing Iorisha Sore K11l < constant temperature treatment s
Focusing on the fact that Cr carbides precipitate at grain boundaries during cooling of iI, forming a Cr-depleted layer and deteriorating stress corrosion cracking resistance,
As a result of various studies conducted by the present inventors, 25-3511Cr
Even if there is a high Cr Nt-based alloy, the C content and i#
Because the relationship with the final annealing temperature is not appropriate, a Cr-depleted layer is formed again during the cooling of the final annealing or during cooling of the hot part Wwk for the purpose of Cr carbide precipitation, which is performed after the final annealing, and the susceptibility to stress corrosion cracking increases. After further experiments, we determined that the final annealing temperature was controlled by the C content in the alloy, the KN content 1, and the final annealing temperature. The present invention was completed by discovering that by repairing the Cr-depleted layer through heat treatment, it is possible to prevent the Cr-depleted layer from forming again during cooling.

Therefore, the gist of the present invention is that C: 0.015
~0.050%, St: 1,011 or less.

Mn: 1-0- or less, P: 0.03 (lt or less).

S: 0.030- or less tCr: 25-35
S tN: 0.020- or less, Ti: 0.01-
1.0 fluent.

Fe: 6.0~10.0'lk Further, if desired, ht: 0.01~1.091° A Ni-Cr alloy consisting of Ni containing unavoidable impurities is added to A (0,01S, 950)
.

B (0,05,1000), C(0,05
, 900) and D C0,015,850).
E (0,5,850) and F (100,850) in the figure.

At each point of G(100,600) and H(1,600), W! This is a heat treatment method for a Ni--Cr alloy for improving stress corrosion cracking resistance, which is characterized by holding the temperature in a temperature range of 600 to 850°C for 15 to 100 hours under conditions within a range of 600 to 850°C.

That is, in the present invention, firstly, the formation of Cr carbides is suppressed as much as possible by limiting the C content and N content in the alloy, and the annealing conditions and subsequent heat treatment conditions are also regulated. ? Rather, by doing so, the intragranular K
The purpose is to precipitate Cr carbide to reduce the amount of carbide precipitated at grain boundaries, thereby preventing deterioration of stress corrosion cracking resistance.

The reason why the composition range of the alloy is limited as described above in the present invention is as follows.

C: C#i Since C#i is harmful to stress corrosion cracking resistance, the amount of C is limited to 0.015% to 0.050% in the present invention. If it is less than o, o is s, it is not sufficient to improve stress corrosion cracking resistance by final annealing and constant temperature heat treatment according to the present invention.

Si, Mn: These are all deoxidizing elements, ht:
ht is also a deoxidizing element, but in the lifting platform of the present invention, it is a desired component and when added, tlO,o1~Cr: Cr
#'i is an essential element for improving corrosion resistance; if it is less than 25, the corrosion resistance required in the present invention cannot be ensured. On the other hand, when the number of cycles exceeds 35, hot workability deteriorates significantly. Therefore, in the present invention, the Cr content is limited to 25 to 35%.

p, s: These elements generally impair hot workability, but
If the amount is less than 0.030 g1, it will not have any substantial effect on hot working, so in the present invention, each amount is 0.030 g1 or less.
Limited to 1 or less.

Ti: Hot workability can be improved by adding 0.01m or more of Ti, but on the other hand, the effect is saturated even if it is added in excess of 1.0%, so if Ti is added from 0.01% to
1.0%.

N: An element that deteriorates NFi corrosion resistance, and C
In the present invention, it, o, 020 is used to promote grain boundary precipitation of r.
Please limit it to below.

Fs: F@ requires addition of 6.0 g or more to ensure hot workability, but if it exceeds 10%, a decrease in cage resistance is seen, so F.6. θ~10LsK is limited.

Nl:Nit:l is an effective element for improving corrosion resistance, especially N.
Ni≧50- is required to improve stress corrosion cracking resistance in high @ high pressure water containing aOH (under alkaline environment).

The alloy having such a composition is then subjected to a final annealing and isothermal or stabilization heat treatment. That is, in the graph of FIG. 1 where the X axis is the C content (%) and the Y axis is the final annealing temperature (℃), the
), B(0,05,1000).

C (0,05,900) Oh! and D (0,815,85
Heat treatment is performed at a final annealing temperature determined by the C content within the range surrounded by the four points 0).

In Figure 1, C1 has a lower C content than extension DA.
O,01S- As explained below, the stress corrosion cracking resistance in the circle K is low and not substantial in the base material, and the stress corrosion cracking resistance is higher than that of the straight line BC, that is, stress corrosion cracking resistance is higher than the C content of 0.05- or more. C111tO,
A range of 0.015 to 0.05 is good. Further, when direct #AB is annealed at a high temperature, the stress corrosion cracking susceptibility increases in the base metal, while when annealed at a temperature lower than IIIDC, recrystallization is not sufficiently performed. Therefore, direct 11AB, BC, C
The appropriate range is within the range surrounded by 4[11] of D and D.

Next, the isothermal heat treatment to sufficiently precipitate Cr carbides in the grains and repair the Cr-depleted layer is limited to a temperature range of 600 to 850 degrees Celsius. This is because it is disadvantageous in terms of operational economy as it requires a large amount of carbide, and if it exceeds 850u, the precipitation of this carbide is small and the above effect is saturated, and rather, solid solution C precipitates at the grain boundaries as carbide during use. This is because a Cr-depleted layer is formed and stress corrosion cracking resistance deteriorates. Next, the constant temperature heat treatment time was determined to be 0.5 to 100 hours because the treatment time varies depending on the C content of the TiNi-based alloy or the annealing conditions. Allow sufficient time for the layer to heal. In any case, the isothermal heat treatment is carried out as long as necessary to sufficiently precipitate the intragranular and partially intergranular kCr longitudinals. In addition, the KFi heat treatment chamber I, which determines the completion of repair of the Cr-deficient layer! up to 100
Boiling material cooled at a rate as fast as ℃/min to 65%
Immersed in HNOI solution for 48 hours and the corrosion rate was 0.
．． If it is less than 5f/m''hr, it is considered that sufficient repair has been made.

Furthermore, the reason for the limitation and the effects thereof will be further clarified with examples of the heat treatment method according to the present invention.The following Example-1 is merely shown to explain the present invention, and the present invention does not apply thereto. It will be clear to those skilled in the art that the invention is not limited to the following.

! 1! Example 'Various materials whose alloy compositions are shown in Table 1 were melted in a 17kt vacuum furnace7, and the resulting ingots were forged, hot rolled, and heat treated according to conventional methods, and then cold cut for 301 seconds. Final annealing is performed at 875-1000°C, and further annealing is performed at 850°C-6.
A constant temperature heat treatment at 00°C was performed. A stress corrosion IJ test piece measuring 2-(thickness) x 1O (width) x 75 (length) was taken from each test material.

501NaOH (caustic soda) #! at 318°C using a tocrepe (high i! high pressure scale). liquid in liquid 2
It was subjected to a immersion test for 7,000 hours. During the test, the depth of stress corrosion cracking was measured using a microscope.

The results thus obtained are summarized in Table 2K.

Further, when these data are organized based on annealing conditions and constant temperature heat treatment conditions, the graphs shown in FIGS. 1 and 2 are obtained.

[Brief explanation of the drawing]

Figure 1 is a graph plotting the maximum crack depth in relation to annealing temperature and cm; Figure 2 is a graph plotting maximum crack depth in relation to isothermal heat treatment temperature and holding time. Applicant's agent Patent attorney Akira Hirose - Makul Figure 452
qch Fuichi 2;

Claims (1)

[Claims] C: 0. OI5' ~0.0501G, Si:1.01
below. Mn: 1-0% or less, P: 0.03011
below. s: 0.030% or less TCr: 25-35
1G +N: 0.02 OL11 or less, Ti:
0.01~1.0 S #Fe: 6.0~10
．． 0'Ik Further, if desired, At: 0.01 to 1.
O%. A (0,015,95Q) in FIG. 1 of the accompanying drawings shows a Ni-Cr alloy consisting of Ni containing the remaining unavoidable impurities. B (0,05,1000), C (0,05,90
0) and D (0,015,850) under conditions within the circumference of the trunk, and further
(0,5,850), F(100,850). Stress corrosion cracking resistant, characterized by being maintained in the temperature range KO of 600 to 850°C for 5 to 100 hours under conditions within the trunk area surrounded by each point of G (100,600) and H (1,600) Heat treatment method for Ni-Cr alloy to improve properties.