US3331682A - Low alloyed heat resisting steel - Google Patents

Description

United States Patent M 3,331,682 LOW ALLOYED HEAT RESISTING STEEL Ryoichi Sasaki and Mizuaki Kurosawa, both of Hitachishi, Japan, assignors to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan No Drawing. Filed Jan. 14, 1965, Ser. No. 425,441 6 Claims. (Cl. 75--126) The present invention relates to heat resisting steel and more particularly to a new and improved kind of low alloyed heat resisting steel especially suitable for material of steel forgings and castings such as rotor shafts and casings of steam turbines in a heat power plant.

With the recent tendency towards the larger capacity of heat power plants, working temperature and pressure of steam turbines in the plant have become remarkably higher, and as a result of the large capacity assigned to the steam turbine, steel forgings used as turbine rotor shafts have become more and more large-sized. Some of rotor shafts for such large capacity steam turbines are being manufactured to have a diameter in excess of 1,000 millimeters and among them there are rotor shafts having such a large diameter of 1,600 millimeters. Recent design trend towards more powerful steam turbines is more and more demanding a rotor shaft of a further greater size.

Hitherto, 1 chrome-1% molybdenum- A vanadium steel having an excellent creep strength has been used as the one which meets the demand as described above. This material 'has a considerably high degree of hardenability, but it is difficult to sufiiciently exert the effect of heat treatment deep into a central portion of a rotor shaft of large diameter as described above since the cooling rate at the central portion of the rotor shaft in quenching is considerably slow. Therefore, the central portion of such rotor shaft generally has poorer characteristics compared with those of the outer peripheral portion and this tendency is more marked as the diameter of the rotor shaft becomes greater. As a result of such difference in the characteristics, there is a great difference between impact values of a V-notch Charpy impact test taken on the central portion and the outer peripheral portion of the rotor shaft. Such heat treatment as water quenching, oil quenching or the like may be considered in order that the central portion can also sufiiciently be hardened, but this manner of treatment is not readily applicable to large-sized articles because there is a danger that quenching cracks may form therein. In the case of a rotor shaft having a diameter of 1,600 millimeters, the cooling rate at its central portion may be quite slow even if it is water cooled. Material of a turbine rotor shaft must have a certain degree of toughness in order to provide safety against brittleness rupture and it is desirable that the material has sufficient toughness even at its central portion. Further, a large-sized turbine casing as it is cast may sometimes have a weight in excess of 20 tons and has a complicated shape. It is therefore difficult to exert a sufiicient heat treatment effect deep into the interior thereof.

If a Charpy impact value satisfies the requirement for a rotor shaft of a low pressure turbine, it is possible to design a turbine plant in a manner that a unitary rotor shaft can carry thereon rotors of a high pressure and a low pressure turbine on a unitary rotor shaft can carry thereon rotors of an intermediate and a low pressure turbine, thus offering a remarkable advantage in the manufacture of steam turbines. From the foregoing description, it will be readily known that, if a steel material can be offered, which has :a high-temperature strength comparable to that of presently used 1 chrome-1% molybdenum- A vanadium steel, in which the effect of heat treatment extends deep into the central portion of a large- 3,331,682 Patented July 18, 1967 sized rotor shaft and which has a Charpy impact value higher than heretofore, such steel material can extremely easily be handled for quenching treatment to thereby obtain a large-sized rotor shaft of excellent mechanical properties.

Therefore, itis the primary object of the present invention to provide a new and improved kind of chromemolybdenum-vanadium heat resisting steel to which yttrium is added.

Another object of the present invention is to provide a novel type of heat resisting steel having a high tensile strength and a suflicient resistance against brittleness rupture.

A further object of the present invention is to provide a new type of heat resisting steel which has a remarkably excellent heat treatment effect and on which a suflicient heat treatment effect can be exerted even at a. slow cooling rate.

A still further object of the present invention is to provide a low alloyed heat resisting steel which is especially suitable for material of rotor shafts and casings of steam turbines for use in a heat power plant.

According to the present invention, there is provided a low alloyed heat resisting steel consisting of a steel in cluding therein from 0.05 to 0.5% by weight carbon, from 0.5 to 3.0% by weight chromium, from 0.3 to 2.0% by weight molybdenum, and accompanying impurities, and yttrium added to said steel in an amount of from 0.01 to 0.6% by weight.

There are other objects and particularities of the present invention which will become obvious from the following detailed description. The accompanying drawings show the prominent effect of the present invention as compared with a prior steel, in which:

Essentially, the present invention is attained by adding from 0.01 to 0.6% by weight yttrium to a chrome molybdenum-vanadium steel.

It is known that a steel material in which a large amount of yttrium or up to about 3% by weight yttrium is added to the so-called high chrome steel containing therein about 20% by weight chromium is used as a heat resisting alloy for use of electric heating. The purpose of addition of yttrium in this alloy is to obtain an anti-oxidation property at a high temperature of the order of 1,350 C., but it is not yet disclosed that any other characteristics than the above-described anti-oxidation property are exhibited by the addition of yttrium. It is true that the addition of yttrium to such high chrome steel offers an advantage in respect of improvement in toughness due to fine-grained crystals, but it is also true that its creep strength is reduced to a remarkable extent. According to experiments by the inventors, it was found that a creep rupture time of a high chrome steel contain ing about 9% by weight chromium and an additive of yttrium is reduced to less than one tenth of a creep rupture time of the same material but without any yttrium additive therein.

The present invention has been devised not only on the basis of the fact that yttrium acts to improve the heat resisting property but also on the basis of an entirely novel discovery that addition of yttrium to a low alloyed steel, especially to a. low chrome steel such as 1 chrome-1% molybdenum- A vanadium steel can uniformly and remarkably improve the characteristics such as creep strength, tensile strength and Charpy impact value.

More specifically, the present invention relates to a low alloyed heat resisting steel in which from 0.01 to 0.6% by weight yttrium is added to a steel containing therein from 0.05 to 0.5% by weight carbon, from 0.5 to 3.0% by weight chromium and from 0.3 to 2.0% by weight molybdenum. Yttrium referred to in the present invention of 90% by weight yttrium, 1.0% by weight lutetium, 1.0%

7 may be the one which is chemically pure, or the one which separable from yttrium. However, it is to be understood that in any of the above cases yttrium must be a principal component thereof. For example, metallic yttrium described in a pamphlet published by Santoku Metal Industry Qo., Ltd. in Japan may be employed, which consists by weight ytterhium, 1.0% by weight erbium, 2.0% by weight holmium and 4.0% by weight dysprosium.

Since yttrium is a metal of relatively-light weight, it is advantageous that yttrium is added in the form of an alloy with iron to the above-described low alloyed steel. For example, it may be advantageous to employ the socalled yttrium-iron mother alloy or ferroyttrium which consists of more than 20% yttrium group elements, less than 80% iron and less than 0.05 titanium.

In the present invention, the amount of yttrium to be added is limited to a maximum of 0.6% by weight because an amount in excess of the above maximum brings forth a remarkable disadvantage in respect of economy and because, in an amount in excess of the above maximum, the effect of yttrium is saturated with the result that any appreciable improvements in the hardenability and mechanical properties are not exhibited. Further, the minimum value of 0.01% is an amount which is absolutely necessary for yttrium to exhibit its effect. An amount of yttrium addition of from 0.1 to 0.4% is preferred to obtain the desired improvements in the characteristics and economy. According to experiments by the inventors, it was found that an amount of the order of 0.3%'is most advantageous for obtaining a best result.

In the low alloyed steel of the present invention, a carbon percentage of less than 0.05% will result in poor hardenability and unsatisfactory creep strength. A carbon percentage of more than 0.5% will result in reduced toughness which in turn causes a lowered Charpy impact value. Therefore, the carbon percentage must lie within a range of from 0.1 to 0.4% in order that the inventive low alloyed steel canexhibit sufficiently satisfactory characteristics.

' Chromium is added in an amount of from 0.5 to 3.0% for the purpose of insuring a high hardenability and antioxidation property and preventing graphitization of carbon. However, with a chromium percentage of less than 0.5%, satisfactory hardenability can not be obtained and sufiicient prevention of graphitization can not also be effected, while with a chromium percentage in excess of 3.0%, creep strength is lowered. In order that the inventive low alloyed steel can exhibit the optimum characteristics, the chromium percentage should lie within a range of from 0.7 to 1.6%.

Molybdenum is an element which improves the creep strength and toughness and is added in an amount of from 0.3 to 2.0% With a molybdenum percentage of less than 0.3% its effect is insufficieut, While with a percentage of more than 2.0%, its effect is saturated and the antioxidation property of the steel is somewhat lowered. A molybdenum percentage of from 0.85 to 1.65% is most preferred in order to obtain a heat resisting steel which is quite satisfactory in respect of mechanical properties;

Besides carbon, chromium and molybdenum, the low alloyed steel includes a suitable amount of a desulfurizing agent and a deoxidizing agent such as manganese, silicon and the like, and impurities coexisting with iron and other elements. Such impurities include, for example, phosphorus, sulfur, nitrogen and the like, which, however, will not adversely affect the characteristics of the steel provided that each of these impurities is contained in an amount of less than 0.03%. A manganese percentage of from 0.5 to 1.0% will be sufiicient to exhibit a satisfactory action and a silicon percentage of from 0.1 to 0.5% is especially preferred. However, a silicon percentage up to about 1.0% is unobjectionable.

From the foregoing description, it will be apparent that 1 chrome-1 A molybdenum- A vanadium steel is most advantageously used to provide the low alloyed heat resisting steel according to the present invention whichis most suitable for material of rotor shafts, casings and like parts of steam turbines.

The present invention will be described in more detail with regard to a preferred embodiment thereof. Table 1 the inventors.

TABLE 1.CHEMICAL COMPOSITION (PERCENT) Sample 0 Si Mn Cr M0 V Fe Amount N o. I of Y added 0. 25 0. 1. 25 1. 25 0. 25 Balance 0 0.41 0. 1. 34 1. 35 0. 26 Balance 0 0. 25 0.70 1. 25 l. 25 0. 25 Balance 0. 03 0. 25 0. 70 1. 25 1. 25 0.25 Balance 0. 03 0.25 0.70 1. 25 1. 25 0.25 Balance 0. 06 0.37 0. 96 1. 28 1. 31 0. 30 Balance 0.10 0. 34 1. 06 1. 34 1. 31 0. 26 Balance 0.30 0. 24 1. 21 1. 41 1. 26 0. 31 Balance 0. 60

In connection with the above table, special attention must be given to the amount of yttrium added to the steel. Or more precisely, the percent by weight yttrium shown in the above table does not represent the amount of yttrium which may finally remain in the alloyed steel. It .was found from the result of spectroscopic analysis and the like that yttrium is consumed by oxidation, etc. when it is added to a steel melt and the actual' amount of yttrium remaining in alloyed steel is less than the amount added to the molten steel. According to our experiment, the remaining amount was in many cases aboutone tenth the amount actually added. An amount of loss of yttrium,

due to oxidation, etc. may progressively increase with the increase in an amount of yttrium added. However, such amount of loss of yttrium is relatively small even with a small amount of yttrium addition such as of the order' of 0.01%, and the object of the present invention can fully be attained. 7

Experiment I TABLE 2.EFFECT OF COOLING RATE ON HARDNESS (H5) OF STEEL 7 Sample No. Water Oil Air 600 0.] 300 0.] 100 0.] 50 0.} 25C.!

cooling cooling cooling hr. hr. 7 hr. hr. hr.

In the Table 2, the Samples 1 and 2 represent 1 chrome- IM; molybdenum- A vanadium steel to which yttrium is not added. It will be apparent from the above table that the heat resisting steel according to the present invention has substantially a greater degree of hardness than the Samples 1 and 2 and its hardness generally increases with the increase in the amount of yttrium addition. The effect of hardness increase is especially marked at a slow cooling rate. For example, the Sample 2 shows a hardness of size of 9.2. Thus, it is apparent that the addition of yttrium is effective to provide a considerably fine-grained structure.

Experiment 11 The Samples 2, 4, 5, 6, 7 and 8 shown in Table l were heated to a temperature of 955 C., then quenched and tempered for 60 hours at a temperature of 640 C. to find out the effect of yttrium addition on the mechanical properties thereof. The results were as shown in Table 3.

TABLE 3.MEOHANICAL PROPERTIES 236 H at a cooling rate of C. per hour, whereas the Samples 4 through 8 show a hardness of more than 310 H at the same cooling rate. This fact shows a peculiar feature of the present invention that the inventive steel is most suitable for large-sized forgings and castings. Because, for example, in a turbine rotor shaft of large diameter made of the inventive steel, there will not be any appreciable difference between -a hardness at a point adjacent its surface and a hardness at its central portion which cools at a slow rate in quenching. Thus, the present invention can most effectively attain the object which has not been attained with prior materials. At a cooling rate below 100 C. per hour, a marked improvement in the hardness was observed and at the same time improved hardenability could be obtained. These facts verify the high utility of the present invention. The Samples 1 and 3 in Table 2 show a lower Brinell hardness than the other samples owing to the fact that they have a lower carbon content. However, it will be seen that the Sample 3 having 0.03% yttrium added thereto shows a higher hardness than the Sample 1. It will also be seen that the Sample 4, which has a composition approximately similar to the Sample 2 and is different from the Sample 3 only in the carbon content, shows a hardness far higher than those of the Samples 2 and 3 and therefore has an improved hardenability.

Samples 1 and 7 in Table l were heated to a temperature of 955 C., then quenched to cool at different cooling rates and tempered for 60 hours at a temperature of 640 C. Sample 7, to which 0.3% by weight yttrium is added has no ferrite therein and shows a sufliciently improved hardenability compared with the Sample 1, to which no yttrium is added. Further, the Sample 1 has a grain size of 6.7 whereas the Sample 7 has a finer grain Cooling V-notch Creep rupture Sample No. qti ri lii ng s ii ia ii g isli ih e ii t red ii e ion ii r iilzi s r s i C./hr. (p.s.i.) (p.s.i.) (Percent) value (ft.- 42,600 p.s.i.

lb.) (hr.)

It will be apparent from the above table that mechanical properties such as the tensile strength, yield point, elongation, and area reduction of the steel are not affected at all by the increase in the yttrium amount and by different rates of cooling as at 600 C. per hour and C. per hour, while a marked increase is observed in the impact value required for a material of turbine rotor shafts and casings. It will further be seen that the impact value at the cooling rate of 100 C. per hour is substantially equal to that at the cooling rate of 600 C. per hour. It will also be seen that, in the creep rupture strength at 550 C. required for a material of rotor shafts for high and intermediate pressure turbines, no lowering of creep rupture time is observed and, on the contrary, the creep rupture time shows an increase.

It will be quite apparent from the foregoing description that the heat resisting steel according to the present invention shows an improved hardenability compared with the prior 1 chrome-1% molybdenum- A vanadium steel and, even when quenched at a slow cooling rate, still possesses a toughness necessary for insuring safety against brittleness rupture.

What is claimed is:

1. A low alloyed heat resisting steel consisting essentially of from 0.05 to 0.5% by weight carbon, from 0.5 to 3% by weight chromium, from 0.3 to 2% by weight molybdenum, from about 0.01 to 0.6% by weight yttrium added to the molten alloy, and the balance iron and impurities.

2. A low alloyed heat resisting steel consisting essentially of from 0.10 to 0.40% by weight carbon, from 0.70 to 1.60% by weight chromium, from 0.85 to 1.65% by weight molybdenum, from 0.10 to 0.40% by weight vanadium, less than 1.0% by weight silicon, less than 7 1.0% by weight manganese, 0.5% by weight nickel, from about 0.01 to 0.6% by weight yttrium added to the molten alloy, and the balance iron and impurities.

3. A low alloyed heat resisting steel according to claim 1, wherein the amount of addition of yttrium is from about 0.1 to 0.4% by Weight. 7

4. A low alloyed heat resisting steel according to claim 2, wherein the amount of addition of yttrium is from about 0.1 to 0.4% by weight.

5. In a steam turbine, a rotor shaft made from the low alloyed heat resisting steel according to claim 2.

6. In a steam turbine, a turbine casing made from the low alloyed heat resisting steel' consisting essentially of 0.05 to 0.5 by weight carbon, 0.5 to 3.0% by weight chromium, 0.3 to 2.0% by weight molybdenum, from 8 about 0.01 to 0.6% by weight yttrium added to the molten alloy, and the balance iron and impurities.

References Cited UNITED STATES PATENTS 2,190,486 2/1940 Schafmeister 75128 2,994,604 8/1961 McGurty 75126 3,002,833 10/1961 McGurty 75126 X 3,017,265 1/l962. McGurty 75--126 3,031,297 4/1962 Baranow 75l26 3,060,016 10/1962 Melloy 75126 3,092,491 6/1963 Payson 75-126 DAVID L. RECK, Primary Examiner.

P. WEINSTEIN, Assistant Examiner. I

Claims (1)

1. A LOW ALLOYED HEAT RESISTING STEEL CONSISTING ESSENTIALLY OF FROM 0.05 TO 0.5% BY WEIGHT CARBON, FROM 0.5 TO 3% BY WEIGHT CHROMIUM, FROM 0.3 TO 2% BY WEIGHT MOLYBDENUM, FROM ABOUT 0.01 TO 0.6% BY WEIGHT YTTRIUM ADDED TO THE MOLTEN ALLOY, AND THE BALANCE IRON AND IMPURITIES.