EP0188995B1

EP0188995B1 - High chromium cast steel for high-temperature pressure container and method for the thermal treatment thereof

Info

Publication number: EP0188995B1
Application number: EP19850730139
Authority: EP
Inventors: Makoto Nagasaki Technical Institute Nakamura; Yorimasa Nagasaki Technical Institute Takeda; Akitsugu Nagasaki Technical Institute Fujita; Yusaku Nagasaki Technical Institute Takano; Kazunari Japan Cast.& Forg.Corp. Takebayashi; Mitsuo Japan Cast.& Forg.Corp. Minami; Yasuhiro Japan Cast.& Forg.Corp. Tashiro
Original assignee: Japan Casting and Forging Corp; Mitsubishi Heavy Industries Ltd
Current assignee: Japan Casting and Forging Corp; Mitsubishi Heavy Industries Ltd
Priority date: 1984-10-17
Filing date: 1985-10-11
Publication date: 1991-01-23
Also published as: DE3581527D1; EP0188995A1

Description

Background of the invention

Field of the invention

This invention relates to a method for the thermal treating of a cast high temperature pressure vessel with a composition as given in the first part of the claim.

Description of the prior art

The cylinder or valve chest of steam turbines usually suffers high temperature and high pressure and should have high hot strengths such as, for example, creep rupture strength and load bearing capacity. These parts have such a complicated, thick structure that when hot steam is flown therein at the time of starting, they are locally heated, where a compression strain takes place. When these portions are cooled, a great tensile stress may be left in the portions. In these portion, cracks are apt to initiate due to the fatigue by the heat. The crack initiation or the presence of the defects may lead to serious crackings by brittle failure.
To avoid this, the steel for these parts should have high ductility and toughness. If the parts are made by casting, the repair by welding is necessary. For the assembling and work, welding is necessary. In this sense, good weldability is essentially required, which, in turn, requires good ductility and toughness.
Where the parts are made by casting, it is the usual practice to use low alloy cast steels such as 1% Cr-0.5% Mo cast steels, 2 1/4% Cr-1 % Mo cast steels, and Cr-Mo-V cast steels. However, these steels do not necessarily have satisfactory hot strength, coupled with another disadvantage that they are rather poor in oxidation resistance at high temperatures.
In recent years, there is a trend of operating power plants in high efficiency or under high temperature and high pressure conditions for the purpose of energy saving. However, the known steels do not meet the requirements with regard to the hot strength and oxidation resistance. Accordingly, there is a demand for novel steel materials which have higher hot strength and better oxidation resistance than the known counterparts.
Typical steels which have relatively high hot strength and good oxidation resistance, are so-called austenite steels. Although austenite steels have high creep rupture strength at high temperatures, the load bearing capacity at low temperatures or normal temperatures is poor. In addition, there is the high tendency toward greater thermal stress because of the small heat transfer efficiency. When the austenite steels are used to make cylinders or valve chests of steam turbines, they are liable to deform by the thermal stress at the time of starting or load variation. Thus, this will place an additional burden when the parts are designed. Since the austenite steels do not undergo the so-called transformation, it is not possible to control the structure by heat treatment. If these steels are made by casting, the cast structure remains as it is. The fabrication by forging will need the adjustment of crystal grains by properly controlling the forging history prior to the heat treatment. With the cylinder or valve chest of steam turbines which has a complicated shape, it is difficult to make an austenite steel with uniform quality by forging, thus impeding the inherent properties of the steel material.
For the above-stated reasons, there is a high demand of the development of pressure containers such as a cylinder or valve chest of a steam turbine made from more convenient ferrite steels.
Attention has now be paid, as meeting the demand, to so-called 12 chromium steels comprising about 8-15% of chromium. The 12 chromium steels have such a high chromium content that they have better oxidation resistance than the afore-described low alloy steels, and have practical utility as a good hot strength steel as obtained by rolling or forging on a relatively small scale. This type of steel has been expected as having good hot strength even when obtained by casting.
A great number of 12 chromium cast steels are widely known in the art and summarized in Table 1 below.
However, these known steel materials have not necessarily satisfactory characteristic properties because of some compositional defects. For instance, when these steels are used to make large-sized cast products such as a cylinder or valve chest of a steam turbine, delta-ferrite may be produced because of the segregation, or good ductility and toughness cannot be imparted because of the precipitation of carbide or the insufficiency of quenching.
Even though good ductility and toughness are obtained, hot strength cannot be obtained, making it difficult to use these steels as high-temperature, pressure container materials.
With 12 chromium cast steels, even if the formation of delta-ferrite and the precipitation of carbide are suppressed and a high degree of hardenability is ensured, there is the problem that high ductility and toughness cannot be obtained when the steels are made for use as large-sized or thick articles.
GB-A-10 15 043 discloses a large size forged steel essentially for a turbine rotor material with a composition and a thermal treating which are comparable to those mentioned in the claim but differs partly in respect to the range of some of the components.
To obtain high ductility and toughness for a cast high temperature pressure vessel the GB-A-10 15 043 gives however no example.

Summary of the invention

In accordance with the present invention, there is provided a method for thermal treating of a cast high temperature pressure vessel which consists of, on the weight basis, 0.08-0.12 of carbon, not larger than 0.7% of silicon, not larger than 0.8% of manganese, 0.4-0.7% of nickel, 9-11 % of ch romium, 0.65-1.00% of molybdenum, 0.13-0.20% of vanadium, 0.03-0.07% of niobium, 0.03-0.07% of nitrogen, and the balance of iron and inevitable impurities. The cast steel may further comprise not larger than 0.7% of cobalt.
The method comprises heating the vessel to a temperature of 1000-1100°C for normalization, cooling the steel down to a temperature not higher than 250°C, tempering the steel once in a temperature range of 550-750°C, cooling down to not higher than 150°C and tempering the resulting steel at 680-750°C.

Brief description of the drawings

Fig. 1 is a graphical representation of the drawability of various cast steels used with the invention and for comparison in relation to tensile strength;
Fig. 2 is a graphical representation of an impact strength of various cast steels used with the invention and for comparison in relation to tensile strength;
Fig. 3 is a graphical representation of the Larson-Miller index of various cast steels used with the invention and for comparison in relation to tensile strength;
Fig. 4 is a schematic view of showing a test steel used in Example;
Fig. 5 is a graphical representation of a tensile strength, 0.2% proof strength, drawing rate and elongation in relation to test temperature for different points of a test steel;
Fig. 6 is a graphical representation of a stress and elongation in relation to a function determined by absolute temperature, creep rupture time and creep rate; and
Fig. 7 is a graphical representation of vibration amplitudes in relation to 25% load-lowered fatigue life cycle.

Detailed description and preferred embodiments of the invention

The present invention is accomplished based on the following test results. We found that in order to obtain desirable properties of high chromium cast steels, the respective components contained in the steels had to be defined in specified ranges.
A number of tests were conducted. In Table 2, there are shown tested steels used in the tests. Each test steel was prepared by melting 50 kg of a steel material in a 50 kg vacuum high frequency blast furnace and casting the melt in a sand mold. The resultant test steels were divided into two groups, which were each heated to 1030°C for 10 hours, cooled down to 300°C at a rate of 73°C/hr. After cooling in air, the respective groups were tempered at 650 and 700°C for 10 hours, respectively, and then subjected to tensile strength, impact strength, high-temperature tensile and creep rupture tests. The reason why the test steels were kept at the respective temperatures for 10 hours and the cooling rate from 1030°C to 300°C was determined at 73°C/hour is that large-sized parts such as a steam turbine cylinder are assumed for the application of these steel materials.
The test results of the tensile strength at normal and high temperatures and the impact strength are shown in Table 3-1 where the tempering was carried out at 700°C and Table 3-2 where the tempering was carried out at 650°C.
Fig. 1 shows the drawability rate, which is considered to typically represent ductility, in relation to tensile strength, on the basis of the results of tensile strength at normal temperatures. Fig. 2 shows the results of an impact strength at normal temperatures, which typically represents toughness, in relation to tensile strength, in which the impact strength shown is a 2 mm V-notched Charpy impact strength. Fig. 3 shows the results of a creep rupture strength, which is typical of high-temperature strength, in relation to tensile strength.
From Fig. 1, it will be seen that the relation between the drawing rate, which typically associates with ductility, and the tensile strength is considered to change mainly based on the content of carbon. Smaller contents of carbon have the higher tendency toward the improvement of the ductility.
The steel Nos. 1 to 8 were tested in order to determine components of the cast steel of the present invention and careful attention was paid to the suppression of delta-ferrite and the hardenability. In other words, the test results were obtained under conditions where formation of delta-ferrite was suppressed and the hardenability was sufficiently ensured. In general, so-called 12 Cr cast steels show such a tendency as described above when small-sized articles were made at a large cooling rate. In contrast, when large-sized articles are made from these cast steels, various problems are presented due to the difference between the cases of the small large-sized articles. In order to avoid such problems, the test was conducted while taking the heat treatment into due consideration, from which the above fact was found.
Fig. 2 reveals that the impact strength also tends to decrease with an increase in content of carbon though not prominent as compared with the drawing rate. In n Fig. 2, the impact strengths of steel Nos. 8 and 12 are low. This is because of the formation of delta-ferrite.
Fig. 3 reveals that at the same tensile strength, the steels tested tend to have increasing creep rupture strengths with a decrease in content of carbon.
This tendency is contrary to the common knowledge for ordinary small-sized articles, but is first confirmed by the above test in which the heat treatment of the tested materials is simulated to the treatment for large-sized articles.
The reasons why the respective components of the cast steel according to the invention are determined as defined before are described.
The carbon content should be low from the standpoint of the ductility, toughness and high-temperature strength and is determined to be in the range of 0.08 to 0.12% according to the above test results. Less amounts are unfavorable because delta-ferrite is liable to produce with the lack of hardenability, making it difficult to ensure toughness. This is why the lower limit is determined as 0.08%. The upper limit of 0.12% is as follows: the test results reveal that larger amounts of carbon still keep good ductility and toughness, but are obtained merely from the test where relatively small-sized test materials, which are obtained under good ingotting conditions, are used; with large-sized actual materials, there is the possibility that the lowerings of ductility and toughness with an increase of the carbon content become more pronounced; and when the carbon content is large at a tensile strength ranging from 700-800 N/ mm² (70 to 80 kgf/mm²) which is a practical range for the cast steel, there is the tendency of lowering the creep rupture strength.
The reason why the silicon content is determined to be not larger than 0.7% is as follows: when this type of steel is used as a cast steel, it is the usual to use a rather high content of silicon so as to ensure good forgeability and the silicon content is determined to be within the allowable range according to the usual practice. Higher silicon contents have effects of ensuring good fluidity of molten metal and killing molten metal and may be effective in preventing so-called cast defectives, but will tend to cause micro and macrosegregations, making it difficult to obtain steels of stable properties. The upper limit of 0.7% is a range where the above problem does not appear pronouncedly.
The reason why the manganese content is not larger than 0.8% is that with this type of cast steel, manganese serves to mitigate the adverse influence of sulfur, prevents formation of delta-ferrite and improves the hardenability. Thus, the content is allowed to such an extent. Although higher contents may be used, there may be the fear that the properties of the cast steel may vary. Thus, 0.8% is determined as the upper limit.
Nickel is used in a content of from 0.4 to 0.7%. With this type of cast steel, a smaller amount of nickel is preferred in order to improve the creep rupture strength. Too small amounts will tend to form delta-ferrite and precipitate pro-eutectoid ferrite, so that the toughness lowers to such an extent as not to be used as a cast steel material. Accordingly, the range of from 0.4 to 0.7% is determined around 0.5% which is ordinarily used for these purposes.
Chromium is determined in the range of 9 to 11 %. This is because with this type of cast steel, higher contents of chromium result in more improved creep rupture strength. However, too higher contents tend to form delta-ferrite and precipitate pro-eutectoid ferrite, which makes it difficult to ensure high toughness. This is why the above range is used.
The reason why the molybdenum content is determined in the range of from 0.65% to 1.00% is that molybdenum gives a well-balanced effect with regard to an improvement of the creep rupture strength when added in an amount of approximately 1 %. However, it too large contents are used, the resultant steel becomes embrittled on heating at high temperatures for a long time and there are tendencies toward the formation of delta-ferrite and precipitation of pro-eutectoid. Taking into consideration segregation as will occur in the case of large-sized cast steel materials, the content is determined in a slightly smaller range.
Vanadium is determined to range from 0.13% to 0.20%. In general, it is accepted that the creep rupture strength is improved when vanadium is added in an amount of about 0.25%. However, according to our experiment, it was confirmed that the creep rupture strength tended to be improved when the amount of vanadium was smaller similar to the case of carbon. Therefore, the content of vanadium was lowered within a range not impeding hardenability. Taking a controllable range into consideration, the above- defined range is determined. When vanadium is added to the type of cast steel, to which the present invention is directed, in such an amount as recited above, it is considered necessary to determine the content as having a range by 0.06%.
The content of niobium is determined in the range from 0.03 to 0.07%. Niobium prevents the growth of crystal grains by interaction with nitrogen and serves to improve ductility and toughness. When used in combination with vanadium, it acts to improve the creep rupture strength. Similar to vanadium, too large amounts result in a lowering of the creep rupture strength with the case of large-sized materials. It was also experimentally confirmed that too large amounts caused carbon nitride to precipitate in segregated portions. The above range is determined while taking in view a controllable range about 0.055% at which its effect was experimentally confirmed.
Nitrogen is determined in a range of from 0.03 to 0.07%. As described before, the ductility and toughness lower at larger amounts of carbon when large-sized materials are made. Nitrogen has not such undesirable effects, but an effect of improving the creep rupture strength by increasing precipitation of carbon nitride similar in quality to the carbide when coexisting with carbon. In coexistence with vanadium and niobium, nitrogen has advantages of suppressing the growth of crystal grains and preventing the formation of delta-ferrite and the precipitation of pro-eutectoid ferrite. Nitrogen has also the effect of improving the hardenability with the attendant effect of imparting good ductility and toughness. However, too large a content of nitrogen has the tendency toward a lowering of the creep rupture strength similar to carbon. The above range is determined as a controllable range around 0.05% at which the effect nitrogen was confirmed.
Aside from the above components, cobalt may be added in order to prevent the formation of delta-ferrite and precipitation of pro-eutectoid and increase the hardenability. Cobalt has similar effects as nickel but is different from nickel in that it gives little adverse influence on the creep rupture strength. If the nickel content is limited for some reasons by which there arise problems in the formation of delta-ferrite, the prevention of pro-eutectoid and the hardenability, it is favorable to add cobalt. However, if cobalt is added in too much an amount, the balance of the steel properties may be lost. The allowable range of cobalt is up to 0.7%, which is controllable range around 0.5% at which the effect was confirmed.
The steels having the above components correspond to steel Nos. 1, 2 and 3 in Table 2. The results of Figs. 1 to 3 demonstrate that these steels are better in ductility, toughness and creep rupture strength than similar steel Nos. 4 to 7. This is mainly due to the effect of carbon. Although steel No. 8 has a better creep rupture strength, ductility and toughness are not so high. This is attributed to the influence of nitrogen.
Steel Nos. 9 to 12, which are close or similar to existing steels, have problems in ductility, toughness and creep rupture strength.
As will be seen from the foregoing, the high chromium cast steel used with the invention has been made according to test results and the knowledge obtained therefrom and can solve the problems of lowering ductility and toughness involved in large-sized materials, which impedes practical utility of this type of cast steel. In addition, the creep rupture strength is also improved. The cast steel used with the invention has high utility as a high chromium cast steel for high-temperature pressure containers such as a cylinder or valve chest of a steam turbine. Thus, it has a very high industrial value.
The cast steel of the type described above has to be used after proper thermal treatment in order to attain the purposes of the invention.
The thermal treating method according to the invention is described in detail.
As will be seen from Figs. 1 and 2, the ductility and toughness of the cast steel material used with the invention tend to decrease with an increase of the tensile strength. In order to keep high ductility and toughness, a certain limitation should be placed on the tensile strength.
The materials used in the test are of the small size and ingotted under well-balanced conditions. In practical applications as large-sized articles, there is the high possibility that the lowerings of the ductility and toughness by improving the tensile strength becomes much more pronounced. Moreover, when the tensile strength is lowered to an extent, the creep rupture strength lowers. In view of these facts, the tensile strength was set in the range of from 700-800 N/mm² (70 kgf/mm²to 80 kgf/mm²). In the thermal treatment of the invention, consideration was first given to the tensile strength.
Next, it must be taken into consideration that the steel of the invention is used to make large-sized thick pressure containers having complicated constructions.
With a large-sized, thick, complicated construction, when normalization is effected, great residual stresses remain in various portions. In general, high chromium cast steels have low ductility and toughness in a normalized condition and when it is cooled to low temperatures near normal temperatures, there is the danger of producing cracks therein. The occurrence of the cracks is also taken into consideration in the practice of the invention.
In this connection, the cooling after the normalization is stopped on the way and martensite produced at that time is tempered to impart ductility and toughness, after which the cast steel is completely cooled.
However, because of the good hardenability of the steel used with the invention, non-transformed austenite remaining during the normalization is left, as it is, at the time of tempering and is transformed into martensite when cooled after the tempering at temperatures below the cooling temperature after the normalization. In this condition, hardened martensite remains.
When high-temperature steels are used at high temperatures at which hardened martensite exists, it is general that notch sensitivity of the creep rupture increases or the life lowers considerably owing to the high temperature low cycle fatigue or heat fatigue when the retention time under the action of tensile stress at high temperatures is prolonged. This is also overcome in the practice of the invention.
The method of the thermal treatment is described along with the reasons for definition of several parameters.
The steel used with the invention has to be normalized in orderto impart desired properties thereto. In order to make a satisfactory solid solution of reinforcing elements and ensure a good creep rupture strength, the normalizing temperature should be 1000°C or higher. Too high temperatures may cause coarse crystal grains, presenting a problem with regard to ductility and toughness. The upper limit of the temperature should be approximately 1100°C. Accordingly, the normalizing temperature is determined within the above range.
After the normalization, the steel is cooled. When it is cooled down to normal temperatures, there is the fear of causing cracks as mentioned above. On the contrary, if the cooling is insufficient, residual austenite remains in large amounts. This austenite remains, as it is, even when the steel is subsequently tempered. After the tempering, hardened martensite is formed, with the tendency that cracking will occur as well. Accordingly, the steel should be cooled down to a temperature at which martensite transformation in adequate amounts takes place and non-transformed austenite is reduced in amount. With the steel of the invention, the steel is cooled to 250°C or below.
Subsequent tempering should be effected at temperatures not lower than 550°C, at which not cracks produce. However, if the temperature increases to high a level, the tensile strength cannot be maintained at 700-800 N/mm² (70 to 80 kgf/mm²). The upper limit for this is about 750°C. Accordingly, the temperature ranges from 550 to 750°C.
As described before, even when the tempering is carried out, hardened martensite remains in this condition, producing such problems as mentioned before. To avoid this, after the first tempering, the steel is again tempered.
In this connection, if the cooling temperature after the first tempering is not low enough the austenite transformation, non-transformed austenite is left. This austenite remains as it is after the second tempering and is changed into hardened martensite upon subsequent cooling. In order to overcome this disadvantage, the cooling should be carried to a satisfactory extent before the second tempering or after the first tempering. This cooling temperature should be 150°C or below in the practice of the invention.
Next, the second tempering temperature should be determined to control the tensile strength in the range of 700-800 N/mm² (70 to 80 kgf/mm²). For this purpose, the temperature should be in the range of from 680 to 750°C.

Example

In order to confirm the effects of the invention, a one ton ingot material was made as shown in Fig. 4 and tested. The dimension in the figure is expressed in terms of mm.
In Table 4, chemical components of the steel material are indicated. The steel was melted in an electric furnace and cast in a sand mold. Thereafter, it was heated to 1030 to 1040°C for 10 hours, cooled in air to a temperature of 150 to 250°C, subjected to first tempering at a temperature of 690 to 705°C for 10 hours and cooled in air down to 60 to 80°C. Subsequently, the steel was again tempered as 710°C for 10 hours and cooled in air. The resultant steel material was used for the test.
Table 5 shows the results of tensile strength at normal temperatures and an impact test using a 2 mm V-notched test specimen. In Figs. 5, 6 and 7, there are shown the results of high-temperature tensile strength, creep and creep rupture strengths, and high-temperature low cycle fatigue.
As will be seen from the results of the table and the figures, the method of the invention is effective in imparting good properties to the high-temperature pressure containers.
Thus, the present invention has industrially great merits.

Claims

Method forthermal treating of a cast high temperature pressure vessel consisting of (on weight basis) 0.08-0.12 of carbon, not larger than 0.7% of silicon, not larger than 0.8% of manganese, 0.4-0.7% of nickel, 9―11% of chromium, 0.65―1.00% of molybdenum, 0.13-0.20% of vanadium, 0.03-0.07% of niobium, 0.03-0.07% of nitrogen, the balance of iron and inevitable impurities and optionally not more than 0.7% of cobalt characterized in heating the vessel to a temperature of 1000-1100°C for normalization, cooling down to a temperature not higher than 250°C, tempering it once in a temperature range of 550-750°C, cooling down to not higher than 150°C and tempering again at 680-750°C.