JPS59110723A - Preparation of 12% cr type cast steel product - Google Patents

Abstract

PURPOSE:To enhance the high temp. creep strength of a welded part and to delay a creep crack propagation speed, by a method wherein a 12% Cr type cast steel product deteriorated by the repairing welding of a defect part is subjected to austenitizing treatment in a predetermined temp. range and the treated product is hardened and succeedingly annealed. CONSTITUTION:A 12% Cr type cast steel product (containing various components, for example, shown by the table) deteriorated in the material characteristics of the welded part and the heat affected part thereof caused by the repearing welding of the casing for a steam turbine or valve casing is subjected to heat treatment as mentioned hereinbelow. That is, the 12% Cr type cast steel is subjected to austenitizing treatment at 1,020-1,080 deg.C and the heated steel is hardened and succeedingly annealed to enable the enhancement of the high temp. creep strength of the welded part and to make it possible to delay the creep crack propagation speed thereof. Therefore, the rupture life of the 12% Cr type cast steel product is prolonged to a large extent, said steel product can be used safely for a long time and the operation efficiency and reliability thereof is enhanced to a large extent.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention is a 12% Cr-based cast steel used for steam turbine casings, pulp casings, nozzle boxes, etc., which recovers deteriorated material properties by repairing defective parts and welding them. The present invention relates to a method for manufacturing Cr-based cast steel products.

[Technical background of the invention and its problems]

Currently, steam turbine parts such as casings, valve casings, and nozzle boxes contain I Cr -IMo -0,
25V cast steel is used, but if a high-temperature, high-pressure turbine is built in the future to improve thermal efficiency, conventional low-alloy cast steel will lack high-temperature strength.

Therefore, the use of 12% Cr cast steel, which has even better high-temperature properties, is being considered. However, materials that exhibit excellent creep strength at high temperatures have the contradictory property of accelerating the creep crack propagation rate, and this acceleration of the creep crack propagation rate significantly reduces the fracture life of the casing material. let

Therefore, a casing material that can cope with high temperatures is required to have not only excellent creep strength but also a slow creep crack propagation speed.

It is used after being subjected to strain relief annealing at a high temperature. The casing that has undergone this repair welding will suffer from deterioration of the material properties of the repair weld and heat affected zone due to the quenching and annealing effects of the weld and heat affected zone due to the heating associated with repair welding, such as deterioration of the material properties of the repair weld and heat affected zone. This causes a decrease in high-temperature creep strength and an acceleration of creep crack propagation speed. If there are pinholes or nonmetallic inclusions in this part, cracks will occur in the repair welds or heat affected zone of the casing during operation of the steam turbine, and these cracks will propagate over time. This can lead to large cracks, or even destruction in extreme cases.

This deterioration of the material properties of repair welds and heat-affected zones is no exception even in 12%'Cr cast steel, which has deteriorated high-temperature properties, such as high-temperature creep strength and thermal fatigue strength.

For example, the weld and the heat-affected zone very close to the weld are quenched during the cooling process after repair welding, increasing the high-temperature creep strength but increasing the creep crack propagation speed.

On the other hand, the heat-affected zone far from the weld is annealed by the heating associated with repair welding, and its high-temperature creep strength is significantly reduced.

[Purpose of the invention]

The present invention has been made in consideration of the above points, and is intended to restore deteriorated material properties by repairing defective parts of 12% Cr cast steel used for steam turbine casings, pulp casings, nozzle boxes, etc. An object of the present invention is to provide a method for producing a 12% Cr cast steel product that has both excellent creep strength and slow creep crack propagation speed.

[Summary of the invention]

The method for manufacturing a 12% Cr-based cast steel product according to the present invention includes
In the process of manufacturing Cr-based cast steel products, after repair welding of defective parts, austenitizing treatment is performed at 1020°C to 1080°C, followed by quenching, followed by tempering. Of course, in the case of large cast steel products, stress relief annealing or annealing is normally performed after repair welding.

The method for manufacturing 12% Cr-based cast steel products according to the present invention increases the high-temperature creep strength and accelerates the creep crack propagation rate of the welded zone created by defect repair welding and the heat-affected zone very close to the welded zone. The decrease in creep strength in the heat-affected zone away from the weld can be improved by performing quenching treatment within a limited heat treatment temperature range after repair welding, thereby preventing creep crack propagation without decreasing creep strength at high temperatures. This was done after researching how to slow down the speed.

Here, to explain the limited heat treatment temperature range of the 12% Cr cast steel according to the present invention, the heat treatment after repair welding is performed at a temperature to completely remove the austenitic structure of the weld zone and heat-affected zone. 10 in the temperature range necessary to fully exhibit solid solution strengthening and precipitation strengthening, and to have both excellent creep strength and slow creep crack propagation speed at high temperatures.
When the temperature exceeds 80℃, Cr, Mo, y
When used as a steam turbine casing, the amount of precipitated carbonitrides, which are a combination of Nb, etc., and C and N, increases and increases the creep strength, but on the other hand, the crystal grains become coarser and this decreases the creep ductility. , it makes stress relaxation and creep deformation at the tip of the crack that occurred in the repair weld difficult, accelerates the creep crack propagation speed, and
At a heat treatment temperature of less than 20°C, it is not possible to create an austenite structure in which carbonitrides are sufficiently dissolved, so that alloying elements that are not dissolved in solid solution may remain as lumps in the heat treated alloy after repair welding. Since solid solution strengthening and precipitation strengthening cannot be fully exerted, the creep strength at high temperatures is significantly reduced, so this range is set.

〔Effect of the invention〕

As explained above, according to the present invention, 12% Cr cast steel is limited to 12% Cr cast steel, which has deteriorated material properties in welded parts and heat affected zones that accompany repair welding of steam turbine casings, valve casings, nozzle boxes, etc. By applying heat treatment in the specified temperature range, the high-temperature creep strength of the weld is increased,
In addition, the creep crack propagation speed can be slowed down.

Therefore, the fracture life of 12% Cr-based cast steel products can be greatly extended, and they can be used safely for long periods of time, and their operating efficiency and reliability can be greatly improved.

Furthermore, by using the 12% Cr-based cast pot product according to the present invention, it is possible to increase the steam temperature and improve the turbine thermal efficiency, which is of similar industrial utility.

[Embodiments of the invention]

A 12% Cr steel body having the chemical composition shown in Table 1 was melted and cast using a high frequency vacuum induction melting furnace. Cast 12%C
rThe cast steel body was subjected to a quenching and tempering process that is generally performed at 1050°C for 4 hours, then quenching, then heated at 725°C for 4 hours, and then tempered.

Table 1 Next, two test materials were cut out from the 12% Cr cast steel body that had undergone the above-mentioned quenching and tempering treatment, and each test material was heated to 1150°C as a representative temperature corresponding to the heating temperature of the heat-affected zone during repair welding. After heating at two temperatures of 1000°C and 1000°C, quenching was performed, followed by strain relief annealing at 600°C for 4 hours. Each of the test materials subjected to this heat treatment was 115
0'O-A treated material and 1000°C-A treated material.

In addition, for the other one, the representative temperatures corresponding to the heating temperature of the heat affected zone accompanying repair welding were 1150℃ and 10℃.
After heating at two temperatures of 00°C for 1 hour, quenching was continued, followed by tempering at a temperature of 725°C for 4 hours. The test materials subjected to this heat treatment are referred to as a 1150°C-B treated material and a 1000'' O-B treated material, respectively.

Next, each of these prepared B-treated materials was divided into four parts, and the four B-treated materials were heated at 1150°C and 1100°01105.
After heating at 0°C and 1000°C for 4 hours, quenching was performed, followed by heating at 725°C for about 4 hours and tempering. The test materials subjected to this heat treatment are referred to as a 1150°C-C treated material, a 1100°C-C treated material, a 1050°C-C treated material, and a 1000°C-C treated material.

Creep test pieces and CT test pieces for creep crack propagation tests were prepared from the above A and C treated materials, and tests were conducted on each.

The results of these tests are shown in Table 2.

- 7: G R As is clear from Table 2, the 12% Cr cast steel products according to the present invention exhibit high creep rupture strength, such as those with a high austenitizing temperature in C treatment. but,
The higher the austenitization temperature in the C treatment, the faster the creep crack propagation rate, and the 1050"Q-C treatment according to the present invention showed a faster creep crack propagation rate.
Cr cast steel was Comparative Example 3, which was subjected to 1100°C-C treatment.
Or about 8.5 to 4 times slower than Comparative Example 7, and 11
The creep crack propagation velocity was about 8 times slower than that of Comparative Example 2 or Comparative Example 6, which were treated at 50° C.-C, and the fracture life was about 8.5 to 4 times or about 8 times longer, respectively. ing.

In addition, in Comparative Example 1, which was subjected to the 1150°C heating treatment that was performed on conventional lCr-lMo-0,25V cast steel products, the high temperature creep strength was high, but the creep crack propagation rate was about 5.5 times faster. In Comparative Example 5, which was subjected to the heating treatment at 1000°C, the high temperature creep strength was approximately 1/2.

In this way, in the process of manufacturing the 12% Cr-based cast steel product according to the present invention, after repair welding of defective parts, austenitizing treatment was performed in a limited temperature range, followed by quenching, followed by tempering treatment (inventive example 1.2) The alloy has far superior creep rupture strength compared to alloys that have undergone other heat treatments (Comparative Examples 1 to 8), and has a slow creep crack propagation rate without reducing the creep rupture strength. I was able to confirm that I have it.

Agent Patent Attorney Kensuke Chika (1 person) 112
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Claims (1)

[Claims] A 12% Cr cast steel product characterized in that, in the process of manufacturing a 12% Cr cast steel product, defective parts are repaired and welded, then austenitized at 1020°C to 1080″C, then quenched, and then tempered. manufacturing method.