JPS5867854A - Preparation of nickel base high chromium alloy excellent in stress, corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To obtain the titled alloy in which excellent stress corrosion cracking resistance is not lowered by welding or SR treatment after welding, by a method wherein the cold working ratio of a 30% Cr-60% Ni type alloy after hot working is increased and low temp. long-term annealing within a specific range is carried out as final heat treatment. CONSTITUTION:An alloy containing, on the basis of %, 0.04 or less C, 1.0 or less Si, 1.0 or less Mn, 0.03 or less P, 0.005 or less S, 50-80 Ni, 15-35 Cr, 0.50 or less Al, in addition, 0.5-2.0 Mo and/or 0.5-2.0 W and, according to necessity, 0.2-1.0 Ti and comprises the remainder of substantially Fe is subjected to hot rolling and, after cold rolling is carried out at a working ratio of 38%, final annealing is successively carried out under the heating temp. and the holding time conditions within a range encircled by straight lines connecting points A, B, C, D and E in the drawing. In this case, when a cold working ratio is increased, an extremely large amount of slip bands are generatd in the alloy, and therefore, if annealing is carried out at 850 deg.C or less remarkably lower than a conventional temp., a large amount of carbide in the alloy can be finely dispersed and precipitated in grain within a short time in an opposite manner as compared with a conventional method.

Description

Detailed Description of the Invention This invention has excellent stress corrosion cracking resistance, and the excellent stress corrosion cracking resistance can be applied to welding and subsequent SR (Stress Corrosion).
The present invention relates to a method for producing a nickel-based high chromium alloy that does not deteriorate even after heat treatment.

In recent years, three
A 0%Cr-60%Ni alloy has attracted attention, and its practical use is currently underway. This is because the 30% Cr-60% Ni system is characterized by superior resistance to stress corrosion cracking compared to other steels and alloy materials. Although the advantages of this material are certain, even when this material is used, stress corrosion cracking (hereinafter abbreviated as SCC) occurs in the weld heat affected zone and even in the base metal during use under the above environment. ) is unavoidable. This is done during the manufacturing process of finished products or during welding during equipment assembly and subsequent SR treatment (55
After heating and holding at 0°C for about 20 hours, Cr carbide precipitates at the grain boundaries, creating a Cr-depleted layer near the grain boundaries, and SC
This is considered to be because the original performance of the 30'1bcr-60ZNi system is impaired as a result of the generation of C.

The present invention uses 30%Cr-60%N at the finished product stage immediately after manufacture.
In addition to having the excellent SCC resistance inherent to the i series,
The present invention aims to provide a method for manufacturing a Ni-based alloy product that does not become sensitized to SCC even after subsequent welding and SR treatment.

That is, the present invention provides (i) C0.04 or less, Si 1.0% or less, Mn
1.0% or less, Po, 03% or less, SO, 005% or less, ('Ji 50-80%, Cr15-35%, Ag
An alloy containing 0.50% or less and 0.2 to 1.0% of Ti as necessary, with the remainder essentially consisting of Fe, ■C0,04
% or less, Si 1.0% or less, Mn 1.0% or less, Po,
03% or less, SO, 005% or less, N150-80%
, Cr15-35%, AJ 0.50% or more F, Mo
0.5 to 2.0'%, Wo, 5 to 2.0%, or both of them, further containing 0.2 to 1.0% of Ti as necessary, and the remainder substantially consisting of Fe. Alloy, After hot working, the above alloy is cold rolled at a working rate of 38% or more, and then A(0,
5,850), B (0,5,750), C (1st generation 67
5) Excellent SCC resistance characterized by final annealing at a heating temperature and holding time within the range surrounded by the straight line connecting the five points D (100,675) and E (100,850). The gist of this paper is a method for producing a nickel-based high chromium alloy.

In the case of 30%Cr-60%Ni alloy, finished products such as plates and tubes are generally cold-rolled to 110% or less after hot working.
It is manufactured by performing short-time annealing at 0° C. and a holding temperature of about 152 to 300. In order to avoid sensitization in the S=R treatment for such steel types, it is generally accepted that precipitation of carbides after annealing must be suppressed as much as possible. That is, when the amount of carbide precipitation is large, Cr carbide precipitation due to SR treatment is accelerated, which tends to cause sensitization due to Cr deficiency. Precipitation of undissolved carbides decreases because the higher the annealing temperature, the higher the solid solubility of C.

Furthermore, regarding cold working before annealing, the smaller the degree of working, the smaller the slip band that becomes the core of carbide precipitation, so carbide precipitation tends to be suppressed. For this reason, the manufacturing method described above has been applied to the 30%Cr-60%Ni alloy, but since this method requires a high annealing temperature, on the other hand, the cooling process of annealing is Furthermore, products that have undergone high-temperature annealing have a large amount of carbide precipitated at grain boundaries due to the SR treatment performed at a low temperature of C solid solubility, so if Cr carbide precipitates However, based on the method of the present invention, cold working before annealing is performed at a higher degree of working than before. This results in significantly more slip bands occurring in the alloy. Here, 8
If annealing is carried out at a temperature lower than 50°C, which is lower than conventional II), a large amount of carbides will be finely dispersed and precipitated within the grains in the alloy within a short time, completely contrary to the conventional case. I will do it.

As the solid solubility of C decreases due to the lower annealing temperature, the amount of carbides to be precipitated increases, and since a large amount of slip bands, which become the nucleus of precipitation, have already been generated, This is a result of effectively promoting precipitation. Moreover, in this case, regarding the precipitation of zero carbide, which causes sensitization, as the precipitation is accelerated by a large amount of slip bands, the recovery of the Cr-depleted layer caused by the precipitation is also effectively accelerated. During annealing, the recovery of the 0-depleted layer is completed within a relatively short time, just enough to cross the line ABC in FIG. A product in which the Cr-depleted layer has been recovered is not sensitized, at least at that stage, and has the high SCC resistance inherent to the alloy.

When the product obtained from the above is subjected to SR treatment, carbides are further precipitated, but the amount can be kept extremely small. In other words, the amount of carbide precipitated by the SR treatment is basically proportional to the difference in C solid solubility between the treatment temperature and the 11th annealing temperature, and therefore, the lower the annealing temperature, the greater the amount of carbide in the SR treatment. The amount of precipitation decreases. At the same time, if the precipitation of Cr carbide is already sufficient during the annealing stage, the amount of precipitation is further reduced. In this way, if the amount of carbide precipitated by the SR treatment is small, and if the carbide is finely dispersed in the alloy in advance as described above, even if Cr carbide is precipitated by the SR treatment, the Cr
The depletion layer only occurs around the carbides dispersed within the grains. As is well known, sensitization is a phenomenon that occurs only when Cr-depleted layers are continuously generated along grain boundaries, and there is no concern about sensitization in the above-mentioned dispersed state.

In addition, even when subjected to welding and SR treatment after the annealing,
When the amount of C in the alloy is 0.04% or less as in the present invention, sensitization can be avoided. When heated to a high temperature by welding, precipitated carbides re-incorporate according to the C solid solubility corresponding to the heating temperature, and follow a trajectory where they precipitate again in the next SR treatment, but during this SR treatment, the amount of C increases. 0.04
%, even if appropriate processing and heat treatment are performed, a Cr-depleted layer will be formed at the grain boundaries, causing cracks to occur.

The manufacturing conditions and reasons for limiting the alloy components used in the present invention will be explained below.

Figure 1 shows the heating temperature and holding time in the final annealing.
It is a chart showing the influence on SCC resistance after treatment. This was obtained through experiments, and the experiments were basically conducted in accordance with the method shown in Examples below. The alloy used in the experiment had the components shown in Table 1 below, and the degree of cold working was 38%. In the figure, O: crack depth 0.05a
less than, X: also indicates 0.05 times or more, A,
The scope of the present invention is surrounded by connecting points B, C, D, and E.

When the heating temperature exceeds 850°C, the C solid solubility is too high and precipitation of carbides by annealing is insufficient, resulting in poor SR
The amount of broken blood during treatment increases and SCC resistance deteriorates through sensitization due to continuous precipitation at grain boundaries. On the other hand, if the temperature is lower than 675°C, recrystallization, which is the original purpose of annealing, cannot be sufficiently achieved no matter how long the temperature is maintained. When holding 11"J, 01
If the sugaring temperature is less than 5 hours, recrystallization cannot proceed sufficiently, and recovery of the Cr-deficient layer due to Cr carbide precipitation is insufficient, resulting in a decrease in SCC resistance after the SR treatment.

If it exceeds 100 hours, there is a big disadvantage in terms of economy.

f, d. Heating temperature 675-850'c1 Holding time 0.
Even if 5 to 100 hours are satisfied, point B in the figure (025 hours,
Straight line B connecting 750c) and 0 point (10 o'clock quadruple 675℃)
In the region under C, recrystallization and recovery of the Cr-depleted layer occur together with p
There is a shortage of this.

FIG. 2 is a diagram showing the influence of the cold working rate before annealing and the amount of C in the alloy on the SCC property after welding + SRSR treatment. The experiment that yielded this result was basically conducted using the same method as the examples described below, and the final annealing was performed at 800°C.
The alloy used in X10hx is the same as in FIG. In the figure, the area surrounded by a solid line indicates the scope of the present invention. The meaning of OlX is the same as in FIG. The cold working rate is 3
If it is less than 8 qb, a sufficient slip band of IA cannot be secured, but a precipitation state in which carbides are finely dispersed within the grains during annealing cannot be obtained, and the effect of promoting recovery of the zero-depleted layer is insufficient, resulting in the above-mentioned problems. Since the Cr-depleted layer cannot be expected to be recovered by annealing at a low temperature and for a relatively short time, there is a concern that sensitization may occur already at the stage after the annealing, which is not as good as after welding + SRSR treatment. Even if the cold working rate is 38% or more, if the amount of C in the alloy is too large,
Good SCC resistance cannot be expected after welding + SR treatment. In general, the more c-shaped, the easier it is to obtain a large amount of carbide through annealing, but on the other hand, the carbide precipitated by the cold working + annealing melts again during welding, and the amount of carbide redecipitated by the SR treatment increases. , it becomes difficult to avoid 0 deficiency near grain boundaries due to precipitation of 0 carbide ◇In order to prevent this 0 deficiency near grain boundaries, which causes sensitization, the amount of C must be 0.04
% or less is required.

Next, the ingredient limitations (excluding C) of the alloy targeted by the present invention will be described.

Si, Mn, Ae: All of them are deoxidizing elements, and below their respective lower limits, they are ineffective, and when the upper limits are exceeded, either the effects become saturated or the cleanliness of the alloy deteriorates.

Ni: It is a key element that has a remarkable effect of improving corrosion resistance and improves resistance to SCC in high-temperature water containing C#- and in an alkaline solution (NaOH) environment, and extremely high corrosion resistance can be expected when the Ni content is 50 or more.

On the other hand, if it exceeds 80%, the effect is saturated and the amount of C that can be added is
Since the r amount is limited, it was set to 80 inches or less.

Cr: Like Ni, this is an essential element for improving corrosion resistance.

If the thickness is less than 15 inches, the effect will be insufficient, while if it exceeds 35 inches, the hot workability will deteriorate significantly.

Ti: A carbide-forming element, and because C is fixed as TiC through sensitization treatment, it is effective in suppressing the precipitation of harmful zero carbide. If it is less than 0.2%, the effect cannot be fully expected, and if it exceeds 1.0%, problems will arise in terms of alloy cleanliness. In the case of an alloy in which C and Cr are relatively low in the metal-containing components, it is not necessary to add Ti.

M to W: These are components effective in strengthening the passive film, and their addition improves corrosion resistance, especially concentrated C6.
- is effective in delaying the occurrence of SCC caused by -, but if neither is added at least 0.5 g, the cleanliness of the alloy will deteriorate.

Steam S: Both are impurity components, and if they exceed 0.030%, they impair thermal processability.

Next, examples of the present invention will be described.

Alloys having the components (2) to 0 shown in Table 1 were vacuum melted for 30 minutes, forged and stretched, and softened, then cold worked and final annealed under the conditions shown in Table 2. After that, %550”
After 120 hours of low-temperature heat treatment (equivalent to SR treatment) or TIG welding (with tongue, 60A1 without fuel), low-temperature heat treatment similar to the above is performed as SR treatment, and from these materials two parts XIQm width x 75M are made. Two long test pieces were taken. These two test pieces were overlapped and bent into a U shape, and this was further restrained by 5III+.
The so-called double U-shaped bending test piece is made of h5, 3Jα
- It was immersed for 100 hours in non-degassed high temperature water at 300°C containing 300 pr'm Cp which had been stored in a tococlave. After the test, the crack depth in the cross section of the U-shaped inner test piece was investigated. The results are summarized in the second table and ■.

Table 1 (wt%) Table 2 -D Table 2 -〇In Table 2 I and The depth is 0.0 for both low temperature heat treated material and welded + SR treated material.
It showed an extremely small value of 0.03fl or less. On the other hand, in the comparative example, the annealing conditions for (2,'ll~G2) are outside the range of the present invention, so the SCC of the low temperature heat-treated material is much higher than that of the present invention example, and the SCC for (O'3)~OD is Since the degree of stretching is below the range of the present invention, the SCC resistance of the welded + SR treated material is significantly inferior. Furthermore, C ~ (4υ is C in the alloy
The amount is too high, and the Scc of the welded + SR treated material becomes significant. By the way, conventional examples (4 to
(From 411i1, it can be seen that even if the amount of C in the alloy is lowered, the SCC resistance of the low temperature heat treated material and the welded + SR treated material is hardly improved.

As is clear from the above explanation, according to the manufacturing method of the present invention, 30%Cr-60%N, which has excellent stress corrosion cracking resistance,
It is possible to obtain an alloy product which has the original properties of the i-series alloy and whose properties remain good not only at the finished product stage but also when subjected to subsequent welding or further SR treatment, and thus The present invention can be said to be an extremely useful invention in the sense that it makes it possible to take advantage of the excellent corrosion resistance of the 60% Ni-30% Cr alloy in corrosion-resistant equipment assembled by welding.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the influence of final annealing heating temperature and holding time on 1Tlt SCC property after SR treatment of a 60%Ni-30%Cr alloy, and Figure 2 is also a diagram showing the effects of cold rolling workability and C? FIG. 3 is a diagram showing the influence on SCC resistance after welding element SR treatment. Fig. 1. A dumbfounded hair (h) Fig. 2. C hair in the car (%)

Claims (2)

[Claims] (1) CO, 04% or less, Si 1.0 or less, M
ll 1.0% or less, Po, 03chi or less, 50.005
% or less, Ni 50% to 80%, Cr15 to 35%, A
After hot working, an alloy containing 0.50% or less of TiO and 1.0% of TiO, with the remainder substantially consisting of Fe, is then cold-rolled at a working rate of 38% or more, and then the alloy is shown in the attached drawings. A (0,5,850), 13 (0,5,
750), C(10,675), r)(100,675
), E(100,850), the final annealing is carried out at a heating temperature and holding time within the range surrounded by the straight line connecting the five points. Method for manufacturing high chromium alloys. (2) C0.04qb or less, Si 1.0% or less,
Mn 1.0% lead, Po, 03 chi or less, 50.0
05% or less, Ni 50-80%, Cr15-35%,
A 1.50% or less, Mo 0.5 to 2.0%, Wo,
Contains 5 to 2.0% of one or both of Ti and optionally 0.2 to 1.0% of Ti, and the remainder is substantially F.
After hot working, the alloy consisting of A (0,5
,850), B(0,5,750), C(10,675
), D (100,675), E (100,850) 5
A method for producing a nickel-based high chromium alloy with excellent stress corrosion cracking resistance, characterized by performing final annealing at a heating temperature and holding time within a range surrounded by straight lines connecting points.