JPS62218515A - Heat treatment of fe base heat resisting alloy superior in creep rupture property - Google Patents

Abstract

PURPOSE:To obtain Fe base heat resisting alloy superior in smooth creep strength, etc., by hot forging ingot of Fe base heat resisting alloy having a prescribed component compsn., then applying solid soln. heat treatment by homogenization holding, next the first step ageing treatment, then the second step ageing treatment from temp. higher than previously described temp. CONSTITUTION:Steel ingot of Fe base heat resisting alloy composed of, by weight % 0.002-0.12 C, <=1.0 Si, <=2 Mn, 12-18 Cr, 20-35 Ni, 1-2 Mo, 1.2-3.0 Ti, <=1 Al, 0.001-0.015 B, 0.1-1 V and the balance Fe with inevitable impurity is hot forged. Next, the steel is soaked at 890-1,100 deg.C and solid soln. heat treated, then soaked at 650-720 deg.C for 5-35hr. Next, the steel is subjected to the second step ageing treatment in which it is heated to temp. higher than the soaking temp. by 20 deg.C, soaked for 5-35hr, then cooled, to obtain Fe base heat resisting forging alloy having previously described characteristics and superior in notch creep strength and rupture elongation.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a heat treatment method for Fe-based heat-resistant alloys having excellent creep rupture properties.

(Prior Art) Forged structures made of Fe-based heat-resistant alloys have conventionally been manufactured using, for example,
It is used for rotating bodies such as gas turbine disks, etc., and has recently attracted attention as a rotor material for ultra-high temperature and high pressure steam turbines. ``These devices tend to be operated at higher temperatures and pressures for the purpose of energy conservation, and because they require frequent startup and shutdown, the high-temperature toughness of the devices and forged structures is becoming more demanding than ever before. There is a demand for materials with excellent reliability.

Conventionally, such Fe-based heat-resistant alloys have been subjected to solution treatment by heating and holding at a temperature of around 980°C for several hours, and then
A heat treatment method is employed in which aging treatment is performed at a temperature of 705 to 760°C for more than ten hours.

However, although the forged structure obtained by such a heat treatment method has high smooth creep strength, it has very low elongation at break and notch creep strength, and has a significant tendency to weaken notch. This tendency is particularly noticeable for large forged products. Therefore, when a forged product heat-treated according to the conventional method is used as a high-temperature structure such as a rotating body that is driven under harsh conditions, cracks may occur from stress concentration areas due to the shape, and serious consequences may occur. be.

(Objective of the Invention) The present invention was made in order to solve the above-mentioned problems in a heat treatment method for a forged structure made of a Fe-based heat-resistant alloy.
The object of the present invention is to provide a heat treatment method that provides a Fe-based heat-resistant forged alloy with excellent notch creep strength and elongation at break.

(Structure of the Invention) First method of heat treatment of Fe-based heat-resistant alloy with excellent creep rupture properties according to the present invention (hereinafter referred to as the method 1 of the present invention).
is C0.01-0.12%, Si 1.0% or less, Mn 2% or less, Cr 12-18%, Ni 20-35%, Mo 1-2%, Ti 1.2-3.0% in weight%. , AJ 1% or less, B 0.001-0.015%, V 0.
After hot forging a steel ingot of an Fe-based heat-resistant alloy consisting of 1% to 1%, the balance being iron and unavoidable impurities, it is soaked and maintained at a temperature of 890 to 1100°C to undergo solid solution treatment, and then the ingot is heated to 650 to 720°C. ℃
A first stage aging treatment is carried out at a temperature of 720 to 790°C and 20°C or higher than the temperature of the first stage aging treatment. It is characterized by a second aging treatment in which the temperature is raised to a high temperature, soaked for 5 to 35 hours, and then cooled.

The second method of heat treatment of a Fe-based heat-resistant alloy with excellent creep rupture properties according to the present invention (hereinafter referred to as the present invention method 2) is a method for hot-forging a steel ingot of a Fe-based heat-resistant alloy having the same chemical composition as above. After that, a solution treatment is performed by soaking and holding at a temperature of 890 to 1100°C, and then a first stage aging treatment is performed by soaking and holding at a temperature of 720 to 790°C for 5 to 35 hours, and then, A second stage in which the temperature is lowered to a temperature in the range of 630 to 720 °C and 20 °C or more lower than the aging treatment temperature in the first stage, and after soaking and holding for 5 to 35 hours, the second stage is cooled.
It is characterized by performing aging treatment.

Furthermore, in the third method of heat treatment of Fe-based heat-resistant alloys having excellent creep rupture properties according to the present invention (hereinafter referred to as the present invention method 3), a steel ingot of Fe-based heat-resistant alloys having the same chemical composition as above is hot-treated. After forging, it is soaked and maintained at a temperature of 890 to 1100°C and subjected to solid solution treatment, then 650 to 720°C.
A first stage aging treatment is performed by soaking and holding at a temperature of 5 to 35 hours at a temperature of 720 to 790 °C, and 20 °C or more than the above first stage aging treatment temperature. After performing the second stage aging treatment in which the temperature is raised to a high temperature and soaked for 5 to 35 hours, the second stage aging treatment is further carried out at a temperature in the range of 630 to 720°C, and as described above. It is characterized in that the temperature is lowered to a temperature 20°C or more lower than the original temperature, and after soaking and holding for 5 to 35 hours, a third stage aging treatment is performed in which the temperature is cooled.

First, the reason for limiting the chemical composition of the Fe-based heat-resistant alloy in the present invention will be explained.

C is an important element not only for ensuring alloy strength and stabilizing austenite, but also for steel manufacturing, but when the amount added is less than 0.01%, strength cannot be ensured. However, when the amount added exceeds 0.12%, the alloy becomes brittle due to precipitation of carbides.

St is an important element in steelmaking, but when added in excess, hot workability deteriorates significantly, so the amount added is 1.
．． 0% or less.

Like St, Mn is an important element in steel manufacturing, but when added in excess, high temperature oxidation resistance and strength decrease, so the amount added is limited to 2% or less.

Ni is an essential element for ensuring stable nonmagnetic properties and for precipitation strengthening. In order to effectively obtain such hardening, it is necessary to add at least 20%. However, Ni is an expensive element, and when added in excess, it causes a decrease in strength. Therefore, it is preferable to add a small amount of Ni as long as it satisfies the required quality. The first limit for the amount of addition is 35%.

Cr is added to provide oxidation resistance and strength. In order to effectively obtain such an effect, at least 12%
Requires the addition of From the viewpoint of the above effects, the higher the amount, the better, but on the other hand, if the amount added is too large, it will cause a decrease in the stability and toughness of the structure, so the upper limit of the amount added is 1.
It shall be 8%.

MO has the effect of increasing strength through solid solution in austenite and precipitation of carbides, and in order to effectively obtain this effect, it is necessary to add 1% or more, but if it is added in excess of 2% or more. When added, it causes a decrease in hot workability and creep rupture ductility.

Ti is an important element that determines creep strength in the present invention. If the amount added is less than 1.2%, the age hardening properties will be significantly reduced and the strength will be reduced. On the other hand, 3.
If it is added in excess of 0%, the strength will be high, but the toughness, structural stability, and hot workability will deteriorate.

AI is important as a deoxidizing agent in steelmaking, and it also
Similarly, it has the effect of increasing strength.

However, when added in excess of 1%, the strength is rather reduced.

B has the effect of improving creep rupture properties as a grain boundary strengthening element. In order to effectively obtain this effect, it is necessary to add at least 0.001%;
．． When added in excess of 0.015%, hot workability is reduced.

■ is 0.1% to improve creep rupture elongation.
It is necessary to add the above. However, addition of an excessive amount of more than 1% causes a decrease in elongation at break.

Unlike the conventional heat treatment method, which performs a single-stage aging treatment after solution treatment, the method of the present invention performs a multi-stage aging treatment after solution treatment, thus achieving excellent notch creep strength and smooth creep elongation at rupture. A Fe-based heat-resistant alloy forged structure is obtained.

The reason why the creep rupture properties of Fe-based heat-resistant alloys are improved by such multi-stage aging treatment is not yet clear. However, according to the research conducted by the present inventors, the creep fracture phenomenon indicates time-dependent grain boundary fracture, and therefore appears to be closely related to grain boundary precipitates during aging holding or creep testing. That is, according to the research of the present inventors, film-like TiC precipitates relatively quickly at grain boundaries in the early stage of aging, and creep cracks are generated from the interface between this film-like TiC and the matrix and propagate. This appears to have led to its destruction.

Therefore, the multi-stage aging treatment according to the present invention increases crack propagation resistance by changing the shape of the film-like precipitates, and at the same time increases the aging time as a whole.
It is thought that the creep rupture properties are improved by coarsening of γ゛ (Ni3Ti) precipitated within the grains and relieving stress concentration on the grain boundaries against deformation.

Of course, the above two factors are not the entirety of creep strength improvement, and the present invention is not limited by any theory.

The method of the present invention involves heating an Fe-based heat-resistant alloy having the above-mentioned chemical components to a temperature of 890 to 1100°C, soaking at this temperature for 0.5 to 10 hours, and then cooling with oil, air, or It is water-cooled, subjected to solution treatment, and then subjected to multi-stage aging treatment.

(Example) The present invention will be described below in detail and specifically based on Examples.

Example 1 A 100 kg ingot of a Fe-based heat-resistant alloy having the chemical composition shown as Inventive Steel 1 in Table 1 was hot forged to form a 50 Tsuru square material, which was heated at 1010° C. for 2 hours and then water-cooled to form a solution. After the chemical treatment, multi-stage aging according to the present invention and single-stage aging according to the conventional method were performed.

FIG. 1 shows the results of creep rupture tests of Fe-based heat-resistant alloys heat-treated by the conventional method and methods 1.2 and 3 of the present invention.

The aging treatment conditions are as follows.

dish rice rebellion After heating at 740°C for 16 hours, cool.

After heating at 700℃ for 20 hours, 740℃ at 50℃/hour
The temperature was raised to , maintained at this temperature for 20 hours, and then cooled.

After heating at 740℃ for 20 hours, 700℃ at 50℃/hour
The temperature was lowered to , and after being maintained at this temperature for 20 hours, it was cooled.

After heating for 20 hours at 00℃, 740℃ at 50℃/hour.
℃, held at this temperature for 20 hours, then heated to 50℃.
The temperature was lowered to 700°C at a rate of °C/hour, maintained at this temperature for 20 hours, and then cooled.

In addition, the creep test conditions were as follows: 46 kgf/mm2 stress at 650°C for smooth creep strength, and 60 kgf/n at 650°C for notched creep rupture strength.
+m'' stress. The same applies to the following.

As is clear from the results shown in FIG. 1, the alloy produced by the conventional method has excellent smooth creep rupture strength, but is significantly inferior in elongation at break and notched creep rupture strength. Compared to this conventional method, both the alloys according to methods 1 and 2 of the present invention have
Significantly improved notch creep strength and elongation at break.

Example 2 In this example, appropriate values of the aging treatment temperature and aging treatment time in the multi-stage aging treatment according to the method of the present invention were determined.

(1) Raw plate 1 An alloy having the same chemical composition as above was subjected to two-stage aging treatment under various temperature and time conditions, and its creep rupture strength was investigated. The results are shown in Table 2 and Figure 2. Especially the second
As shown in the figure, the aging treatment temperature (a) of the first stage was 650
When the temperature is lower than 0.degree. C., aging is difficult to occur, and both the smooth and notched creep rupture strengths are low. but,
When the temperature is in the range of 650 to 720°C, both smooth and notched creep rupture strength is improved. At high temperatures exceeding 720°C, creep rupture strength rapidly decreases due to overaging. These results indicate that the optimum temperature for the first stage aging treatment is in the range of 650 to 720°C.

In addition, as shown in Figure 3, the second stage aging treatment temperature (C) is optimally in the range of 720 to 790°C, as overaging occurs at 800°C and the creep rupture strength rapidly decreases. It is shown that something is true.

Next, the optimum aging temperature was investigated within the above temperature range. The results are shown in Figure 4. First stage aging treatment time (b
), when the time is less than 5 hours, a decrease in notch creep strength is observed, while on the other hand, even after 40 hours, there is no significant decrease in the leap rupture strength. Therefore, when considering the upper limit time from the viewpoint of energy saving, the first stage aging treatment time (b) is preferably 5 to 35 hours.

Similarly, as a result of examining the second stage aging treatment time (d), as shown in FIG. 4, 5 to 35 hours is suitable.

(2) Uncooked or plated 1 An alloy having the same chemical composition as above is subjected to solution treatment in which it is soaked and held at 1010°C, and then heated at various temperatures (a).
The specimens were soaked for 20 hours at a temperature of 100 mL, then lowered to various temperatures (b), and kept soaked at this temperature for 20 hours to conduct a creep rupture test.

The results are shown in Table 3. The conditions for the creep test were the same as described above. From this result, the aging treatment temperature in the first stage is 720.
~790℃, second stage aging treatment temperature is 630~720℃
is shown to be suitable.

Furthermore, as for the aging treatment time in each stage, it was confirmed from the results of a test similar to that of Method 1 of the present invention that 5 to 35 hours is suitable.

(3) An alloy having the same chemical composition as above was subjected to solution treatment in which it was soaked and held at 1010°C, and then heated at various temperatures (a).
After soaking for 20 hours at the same temperature, the temperature was raised to various temperatures (b), and after soaking and holding at that temperature for 20 hours, the temperature was further lowered to various temperatures (c), and at that temperature it was heated for 20 hours. A three-stage aging treatment was performed in which the specimen was soaked for a period of time and then cooled. The results of the creep rupture test are shown in Table 4. The conditions for the creep test were the same as described above. From this result, the aging treatment temperature is 650-720℃ for the first stage and 720-790℃ for the second stage.
It is shown that 630-720°C is suitable for the third stage.

Furthermore, as for the aging treatment time in each stage, it was confirmed from the results of a test similar to that of Method 1 of the present invention that 5 to 35 hours is suitable.

Example 3 Using the inventive mB and C and the comparative mA and D having the chemical components shown in Table 1, the same inventive method 1.2 as shown in Example 1 was carried out after the same solution treatment as in Example 1. Aging treatment according to 3.

For comparison, aging treatment was also performed using a conventional method. Table 5 shows the results of a creep rupture test performed on these.

Comparative steel A has an extremely low notch rupture strength because the Ti content is less than that specified in the present invention. On the other hand, the comparative steel has an excessive amount of Ti than specified in the present invention, and similarly has a significantly low notch rupture strength.

Compared to these comparative steels, it is clear that according to the method of the present invention, an Fe-based heat-resistant forged alloy having not only excellent smooth creep strength at rupture but also excellent notched creep strength and elongation at fracture can be obtained.

(Effects of the Invention) As described above, the method of the present invention significantly improves the notch creep strength and fracture ductility, so the forged product made of the Fe-based heat-resistant alloy produced by the method of the present invention has strict stability requirements. Suitable for rotating structures under high temperature conditions.

[Brief explanation of drawings]

Figure 1 is a graph showing the creep rupture strength of Fe-based heat-resistant alloys subjected to aging treatment according to the conventional method and the method of the present invention. Figure 4 is a graph showing the relationship between the first stage aging temperature and creep rupture strength when the second stage aging treatment is performed.
Figure 3 is a graph showing the relationship between the second-stage aging temperature and the creep rupture strength when the temperature is raised as it is after the first-stage aging treatment and the second-stage aging treatment is performed. It is a graph showing the relationship between the aging treatment time in the first stage and the second stage and the creep rupture strength when the second stage aging treatment is performed by raising the temperature as it is after the aging treatment. Fig. 1 ■ Fig. 2 4b oblique a (ce)

Claims (3)

[Claims] (1) Weight%: C 0.01-0.12%, Si 1.0% or less, Mn 2% or less, Cr 12-18%, Ni 20-35%, Mo 1-2%, Ti 1.2 ~3.0%, Al 1% or less, B 0.001-0.015%, V 0.1-1%, after hot forging a Fe-based heat-resistant alloy steel ingot consisting of the balance iron and unavoidable impurities. , subjected to solid solution treatment by soaking and holding at a temperature of 890 to 1100°C, and then heated to 650 to 720°C.
The first aging treatment is carried out by soaking and holding at a temperature of 5 to 35 hours, and the temperature is in the range of 720 to 790 °C,
In addition, the creep rupture property is characterized in that a second stage aging treatment is performed in which the temperature is raised to a temperature 20° C. or more higher than the first stage aging treatment temperature, and the temperature is soaked for 5 to 35 hours, and then the second stage is cooled. A heat treatment method for Fe-based heat-resistant alloys with excellent properties. (2) Weight%: C 0.01-0.12%, Si 1.0% or less, Mn 2% or less, Cr 12-18%, Ni 20-35%, Mo 1-2%, Ti 1.2 ~3.0%, Al 1% or less, B 0.001-0.015%, V 0.1-1%, after hot forging a Fe-based heat-resistant alloy steel ingot consisting of the balance iron and unavoidable impurities. , subjected to solid solution treatment by soaking and holding at a temperature of 890 to 1100 °C, then 720 to 790 °C
The first aging treatment is carried out by soaking for 5 to 35 hours at a temperature of 630 to 720 °C,
In addition, the creep rupture property is characterized in that the temperature is lowered to a temperature 20°C or more lower than the first stage aging treatment temperature, and after soaking and holding for 5 to 35 hours, a second stage aging treatment is performed. An excellent heat treatment method for Fe-based heat-resistant alloys. (3) Weight%: C 0.01-0.12%, Si 1.0% or less, Mn 2% or less, Cr 12-18%, Ni 20-35%, Mo 1-2%, Ti 1.2 ~3.0%, Al 1% or less, B 0.001-0.015%, V 0.1-1%, after hot forging a Fe-based heat-resistant alloy steel ingot consisting of the balance iron and unavoidable impurities. , subjected to solid solution treatment by soaking and holding at a temperature of 890 to 1100°C, and then heated to 650 to 720°C.
A first stage aging treatment is carried out by soaking and holding at a temperature of 720 to 790 °C for 5 to 35 hours, and then a temperature of 720 to 790 °C,
In addition, after performing a second aging treatment in which the temperature is raised to a temperature higher than the first aging treatment temperature by 20°C or more and soaked for 5 to 35 hours, the temperature is 630 to 720°C. Creep characterized in that the temperature is lowered to a temperature 20° C. or more lower than the aging treatment temperature in the second stage, and after soaking and holding for 5 to 35 hours, a third stage aging treatment is performed in which the temperature is cooled. A heat treatment method for a Fe-based heat-resistant alloy with excellent fracture properties.