JPH06184628A - Vacuum heat treatment method - Google Patents

Abstract

PURPOSE:To secure strength and ductility by introducing a pressurized cooling gas exceeding the atmospheric pressure to make a quenching speed larger after a member to be heat treated is placed in a vacuum container and heated in vacuum. CONSTITUTION:At introducing a compressed gas into a high temp. vacuum furnace, a gas temp. is lowered due to adiabatic expansion, producing the cooling effect of gas. Because a temp. range of 400-750 deg.C makes the heat transfer due to heat conduction and heat convection rather than heat radiation the main stream, in order to improve heat transfer, the gas pressure is elevated, increasing the gas density. The way of gas cooling is different in a cooling speed depending on material and thickness, accordingly, it is preferable to control a pressure applying method and gas introducing speed.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a novel vacuum heat treatment method.

[0002]

2. Description of the Related Art Many steels and non-ferrous alloys exhibit their material characteristics by heat treatment. In order to prevent oxidation, precision shaped parts are generally subjected to solution treatment, quenching or tempering under vacuum. Such parts include low alloy steel machine parts, high chrome heat resistant steel steam turbine parts,
18-8 stainless machine parts, 17-4PH stainless machine parts and aluminum bronze pumps, precision casting, precision forging and machining parts such as marine parts.

Since water and oil cannot be used for cooling in the vacuum heat treatment furnace, it is usually filled with argon or nitrogen gas for cooling. However, a material or a thick product having poor hardenability has a drawback that a sufficient quenching and cooling rate cannot be obtained, and required material properties cannot be obtained. For this reason, a precision shaped product is subjected to a finishing process after heat-treating a material having a finishing margin in the air or an inert atmosphere. The heat treatment in the atmosphere causes a large finishing margin due to oxidation, and surface oxidation cannot be completely prevented in an inert atmosphere.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to increase the cooling rate in a vacuum furnace during vacuum heat treatment, to facilitate solution treatment or quenching that requires a high cooling rate, and to obtain required material properties. It is to provide a vacuum heat treatment method by which the above can be obtained.

[0005]

According to the present invention, a member to be heat-treated is placed in a vacuum vessel, heated and held under a vacuum, and then an inert gas such as nitrogen or argon is increased in a cooling process in a vacuum furnace. The cooling rate is increased by introducing it at a pressure exceeding 1 atm, which is higher than the atmospheric pressure. Pressurized pressure is 2-10kg /
cm ² is preferable, and 3 to 7 kg / cm ² is particularly preferable.

[0006]

[Function] In solution treatment of precision castings, processed products, forged products, etc. of Cu alloys, Ni-based alloys, ferritic and austenitic stainless steels, mechanical properties deteriorate when the cooling rate is low, and when remarkable, crystallization occurs. Carbide precipitates at the grain boundaries and deteriorates corrosion resistance. In addition, materials exhibiting martensite and bainite transformation do not have these transformation structures when the quenching and cooling rate is low, so that sufficient hardness cannot be obtained. Especially 30
The temperature range from 0 to 750 ° C is important.

When a compressed gas is introduced into a high temperature vacuum furnace, the gas temperature is lowered by adiabatic expansion, and a gas cooling effect is produced. The gas pressure decreases as the temperature decreases, so the gas is replenished, but since the conventional vacuum heat treatment furnace has a pressure resistance of 1 atm, it can only be raised to a gas pressure of 1 atm or less. For this reason, in the temperature range of 400 to 750 ° C., heat conduction and heat transfer due to convection become the mainstream rather than radiation, so that the cooling effect does not become large below 1 atm because the gas density is low. In order to improve the heat conduction in this temperature range, it is important to increase the gas pressure and increase the gas density in a vacuum furnace having a high pressure resistance in order to increase the cooling effect. For cooling during this period, it is preferable to adjust the cooling rate by gradually increasing the gas pressure. Since the cooling rate of the gas is different depending on the material and the thickness thereof, it is preferable to adjust the pressure application method and the gas introduction rate according to them. For rapid cooling, rapid introduction and pressurization should be performed. It is preferable that the cooling rate for quenching and solution treatment is usually 8000 ° C./h or more.

Heating in a vacuum is carried out under a high vacuum of 10 ^-3 torr or more for a material containing an easily oxidizable element such as Al or Ti depending on the material, or for a material subjected to a bright heat treatment, and a vacuum of less than that otherwise. It is preferable to do it below. Particularly, the former is preferably 10 ^-3 to 10 ^-5 torr, and the latter is preferably 0.1 to 10 torr.

When introducing the cooling gas, it is preferable to stir the cooling gas with a fan.

As an example of a product to which the present invention is applied, 12%
First stage nozzle of high pressure turbine of steam turbine made of Cr steel, neck for electron microscope made of 18Cr-8Ni steel,
There are precision castings such as reactor internal members, closures for city gas flow meters made of 17-4PH stainless steel, and pump blades made of aluminum bronze.

[0011]

【Example】

(Example 1) Austenitic stainless steel precision castings were heat-treated under the conditions shown in Table 1.

Table 1 shows the chemical composition (% by weight) of the materials used.

No. 1 component of Table 1 and No. 1 A and B of Table 2
The heat treatment conditions were compared. The results are shown in Table 3. Table 3
As can be seen from No. 1A and No. 1B, the mechanical properties of No. 1B had a low elongation and did not satisfy the standard values.

All products No. ¹ to 3 are 10 ^-1 to
After heating and holding in a vacuum of rr or less, the cooling gas was introduced and pressurized in three stages in order to adjust the cooling rate of the product to be equal in each department of the product, and then cooled to the final pressure. The same heating was performed for tempering, and the cooling was performed without pressurizing gas.

(Example 2) Heat treatment of an 11% Cr heat-resistant steel precision casting was carried out under the conditions shown in Table 2. No. 2 of Table 1
The components were compared under the heat treatment conditions of A and B. As a result, as can be seen from the mechanical properties of Nos. 2A and 2B in Table 3, the mechanical properties of 2B did not satisfy the standard value of tensile strength.

(Example 3) A 17-4PH steel precision casting product was heat-treated under the conditions shown in Table 2. The No. 3 components in Table 1 were compared under the heat treatment conditions of A and B. as a result,
As can be seen from the mechanical properties of No. 3A and 3B in Table 3,
Regarding the mechanical properties of 3B, the specified value of tensile strength was not satisfied.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

(Example 4) A1BC3 precision castings shown in Table 4 were heat-treated under the conditions shown in Table 5.

Heating at 950 ° C. and 550 ° C. is 10 ⁻³ torr
Under vacuum. The No. 4 components of Table 4 were compared under the heat treatment conditions of A and B shown in Table 5. As a result, N in Table 6
o As can be seen from the mechanical properties of 4A and 4B, the mechanical properties of 4B did not satisfy the specified tensile strength. The method of introducing the gas in the cooling is the same as in the first embodiment.

[0022]

[Table 4]

[0023]

[Table 5]

[0024]

[Table 6]

[0025]

EFFECT OF THE INVENTION In the vacuum heat treatment, high pressure cooling gas is applied to increase the quenching cooling rate to secure strength and ductility, and the required standard value can be satisfied.

Claims (5)

[Claims] 1. A member to be heat treated is placed in a vacuum container, heated under vacuum, and held at a predetermined temperature, and then a cooling gas is introduced into the vacuum container, and the cooling gas is kept at atmospheric pressure. A vacuum heat treatment method characterized by introducing pressure over. 2. A precision casting product is placed in a vacuum vessel, heated under vacuum, and held at a predetermined temperature, and then a cooling gas is introduced into the vacuum vessel and the cooling gas is kept at 1 atm. A method for vacuum heat treatment of a precision casting product, which comprises introducing pressure to exceed and cooling. 3. A precision forged product is placed in a vacuum container, heated under vacuum, and held at a predetermined temperature, and then a cooling gas is introduced into the vacuum container, and the cooling gas is kept at 1 atm. A vacuum heat treatment method for a precision forged product, which is characterized by introducing pressure over and cooling. 4. A precision processed product is placed in a vacuum container, heated under vacuum, and held at a predetermined temperature, and then a cooling gas is introduced into the vacuum container, and the cooling gas is kept at 1 atm. A vacuum heat treatment method for a precision processed product, which comprises introducing pressure over and cooling. 5. A member to be heat treated is placed in a vacuum container, heated under vacuum, and held at a predetermined temperature, and then a cooling gas is introduced into the vacuum container, and nitrogen or argon and its cooling gas are used as the cooling gas. A vacuum heat treatment method comprising introducing a mixed gas under a pressure of more than 1 atm and cooling the mixture.