US3476556A - Austenitic heat-resisting steel containing niobium and vanadium - Google Patents

Description

United States Patent 3,476,556 AUSTENITIC HEAT-RESISTING STEEL CONTAIN- ING NIOBIUM AND VANADIUM Ryoichi Sasaki and Humio Hataya, Hitachi-shi, Japan, as-

signors to Hitachi, Ltd., Tokyo, Japan, a corporation of Japan Filed Mar. 23, 1966, Ser. No. 536,862 Int. Cl. C22c 39/26, 39/20 US. Cl. 75--128 17 Claims ABSTRACT OF THE DISCLOSURE An austenitic steel having excellent high temperature mechanical strength, workability and weldability, and consisting essentially of 0.07 to 0.2% by weight carbon, 0.2 to 1.5% by weight silicon, 0.5 to 3% by weight manganese, 14 to 20% by weight chromium, 8.5 to 13% by weight nickel, 0.7 to 2% by weight molybdenum, 0.1 to 0.5% by weight niobium, 0.1 to 0.5% by weight vanadium, and the balance being iron and small amount of accompanying impurities, said steel being particularly useful for fabricating heating equipment, such as boiler tubes, which is normally subjected to temperatures in excess of about 600 C.

This invention relates to austenitic heat-resisting steels satisfactorily usable in a high temperature region at temperatures more than 600C. and more particularly to those which show an excellent mechanical strength at a temperature more than 650 C. and up to about 700 C. More specifically, the present invention contemplates the provision of a heat-resisting steel which is especially suitable for use as a material of heating equipments such as boiler snperheater tubes.

This invention relates to tube materials for use in heating equipment satisfactorily usable in a high temperature region at temperatures more than 600 C., and more particularly to those which show an excellent mechanical strength at a temperature more than 650 C., and up to about 700 C., and show excellent workability and weldability.

With the recent remarkable development of heavy industries in the field such as of thermal power plants, heating apparatus employed in such thermal power plants are being made successively larger in size and capacity. Further as these heating apparatus becomes more and more large-sized, the temperature region for use with these heating apparatus has been elevated to a value of from 650 C. to 700 C. from the previous value of the order of 550 C. to 600 C. However, heat-resisting steel satisfactorily usable at these high temperatures is quite limited since steel material generally has a defect that its mechanical strength, especially its creep rupture strength is lowered at such high temperatures.

A stainless steel containing 18% by weight chromium and 8% by weight nickel has been typical of heat-resisting steels usable at a temperature of the order of 600 C. to 650 C., but this steel shows an insuflicient mechanical strength at 650 C. Accordingly it is the present status that a heat-resisting steel containing 17% by weight chromium, 12% by weight nickel and 2.5% by weight molybdenum is mainly used for heat-resisting applications. Due to the fact, however, that the above-described heat-resisting steel fails to show a suificient strength at a high temperature of the order of 700 C., an austenitic steel comprising a further additive element such as niobium, tungsten or titanium in the above-described heatresisting steel comes to a wide use.

While these materials have relatively excellent mechanical strength in their respective operating temperature regions, they are defective in their workability and Weld- 3,476,556 Patented Nov. 4, 1969 ability owing to the fact that special elements are contained in large amounts therein. Further since, these heatresisting steels contain a large amount of nickel therein, the cost of apparatus including large-sized members made from these steel materials will become quite high.

If therefore it is possible to obtain a steel material which contains less amount of nickel and yet has a high strength at those high temperatures, operational characteristics of apparatus made from such steel material can remarkably be improved and the cost involved therein can be reduced. Moreover if a steel material having excellent workability and weldability could be obtained, a remarkable advantage is derivable therefrom because manufacture of apparatus from such steel material is so much facilitated. In an apparatus such as a boiler which is adapted to operate by burning heavy oil, steel material thereof is liable to be corroded by a vanadium compound included in the heavy oil with the result that its resistance to corrosion is degraded. In such a case, operational characteristics of the boiler will be remarkably improved if the steel material is sufficiently resistant to corrosion.

The present invention intends to accomplish the abovedescribed requirements and has for its first object to provide a novel composition of chrominm-nickel-molybdenum heat-resisting steel containing niobium and vanadium therein.

The second object of the present invention is to provide a heat-resisting steel containing niobium and vanadium therein to which either copper or boron or both of them are additionally added, and to provide a heat-resisting steel of the above composition to which tungsten is further added.

The third object of the present invention is to provide a heat-resisting steel containing niobium and vanadium therein to which either calcium or magnesium or both of them are additionally added.

Another object of the present invention is to provide an improved heat-resisting steel having high mechanical strength in a high temperature region, especially in the vicinity of 600 C. to 700 C.

A further object of the present invention is to provide a heat-resisting steel having an excellent workability and a high resistance to corrosion.

A special object of the present invention is to provide a heat-resisting steel which contains niobium, vanadium and nickel in relatively small amounts, which is therefore inexpensive and which is especially suitable for use as a material for heating equipments such as boiler superheater tubes.

The above and other objects, advantages and features of the present invention will become more apparent from the following description when read in conjunction with the accompanying drawings.

In the drawings:

FIG. 1 is a graphic illustration of results of creep rupture tests for 10 hours and 10 hours at 650 C. and 700 C. taken on the austenitic heat-resisting steel embodying the present invention; and

FIGS. 2 through 4 are graphic illustrations of the relation between creep rupture time and stress at 650 C. and 700 C. measured on the austenitic heat-resisting steel according to the invention and prior heat-resisting steels.

Basically the present invention is accomplished by adding 0.1 to 0.5% by weight niobium and 0.1 to 0.5% by weight vanadium to a chromium-mickel-molybdenum steel.

The present invention is also accompanied by adding either 1 to 4% by weight copper or 0.002 to 0.2% by weight boron or both of them to a chromium-nickelmolybdenum steel containing niobium and vanadium in the above-described amounts.

1 Several kinds of chromium-nickel-molybdenum heat- 0 resisting steels containing therein niobium and vanadium are already known, and it is also commonly known in the art that niobium and vanadium are combined with carbon to form their carbides which contribute to improvements in high temperature strength of the steels. However these heat-resisting steels contain niobium and vanadium in large amounts, and a large amount of nickel must be added thereto in order to obtain the austenite structure in a stable form. These heat-resisting steels are therefore defective in that a lot of cost is involved in the manufacture and they have poor workability and weldability.

With a view to eliminate these and other defects of the prior heat-resisting steels of this type, the inventors discovered a novel heat-resisting steel in which amounts of nickel, molybdenum, niobium and vanadium are set at suitable ranges and which has a high temperature strength sufficiently endurable for use at a high temperature region in the vicinity of 600 C. to 700 C. in addition to its excellent resistance to corrosion. The heat-resisting steel according to the invention has an improved workability and weldability by virtue of the fact that the amounts of nickel, niobium and vanadium are set at proper ranges which are less than the amounts contained in the prior heat-resisting steels of this type.

The present invention relates further to a novel heatresisting steel obtained by adding copper, boron and tungsten, and in some instances, calcium and magnesium to the alloy of the above-described kind.

According to the present invention, niobium, vanadium, boron, copper and other elements are not merely arbitrarily added to a chromium-nickel-molybdenum steel, but proper ranges of these elements are determined on the basis of the viewpoint as described above so that the heat-resisting steel is most suitable for use as a material for components of heating equipment working at a high temperature in the vicinity of 600 C. to 700 C.

More precisely the present invention provides an aus-' tenitic heat-resisting steel consisting essentially of 0.07 to 0.2% by weight carbon, 0.2 to 1.5% by weight silicon, 0.5 to 3 by weight manganese, 14 to by weight chromium, 8.5 to 13% by weight nickel, 0.7 to 2% by weight molybdenum, 0.1 to 0.5% by weight niobium, 0.1 to 0.5% by weight vanadium, and the balance being iron and accompanying impurities.

The present invention also provides an austenitic heatresisting steel obtained by adding 1 to 4% by weight copper and 0.002 to 0.2% by weight boron either singly or compositely to the above-described steel composition.

The present invention also provides an austenitic heatresisting steel obtained by adding 0.2 to 1% by weight tungsten to the above-described steel composition including copper and boron in the above-specified amounts.

its strength can be increased in proportion to a larger content of carbon as far as carbon is contained up to a certain limit. A portion of carbon combines with such elements as molybdenum, niobium and vanadium to form carbides of the latter which carbides enhance the effect of improvement in the steel strength. However, a carbon content of less than 0.07% by weight is undesirable because strength suitable for use at a high temperature of the order of 650 C. to 700 C. cannot be obtained, while a carbon content of more than 0.2% by weight is also objectionable because toughness of the steel is impaired and its creep rupture strength at elevated temperatures for a long period of time is thereby lowered. Carbon is therefore added in an amount of 0.07 to 0.2% by weight, and its content should be limited to a range of 0.1 to 0.15% by weight in order that the steel exhibits especially excellent properties.

Chromium is added to give the steel a suflicient resistance to oxidation and to form the austenite structure in cooperation with nickel. For this purpose, addition of chromium in an amount more than 14% by weight is necessary because suflicient resistance to oxidation cannot be exhibited with chromium addition of less than the above limit. A chromium content of more than 20% by weight is undesirable because ferrite is liable to form in the steel of the invention containing such a low amount of nickel and formation of ferrite results in lowered workability and strength. Moreover, addition of such a large amount of chromium will result in formation of the sigma phase and the steel will become brittle when the steel is subjected to elevated temperatures for a long period of time. Accordingly, the objects of the present invention cannot be fully satisfied unless chromium is added in an amount of 14 to 20% by weight. In order that the addition of chromium can exhibit an especially good effect, chromium must be added in an amount of 16 to 18% by weight.

When coexistent with chromium, nickel cooperates therewith to form the austenite structure and contributes to an increase in the high temperature strengh. Since however nickel is an expensive element, addition of nickel in too much an amount is undesirable in view of the costs involved in the manufacture of heating units. With a nickel amount of less than 8.5% by weight, however, a full austenite structure cannot be obtained and ferrite partly existing in the steel structure lowers the strength. Nickel addition of more than 13% by weight is also objectionable due to the fact that manufacture of heating units requires a high cost which is contradictory to the objects of the present invention and creep rupture strength is conversely lowered. Accordingly, nickel must be added in an amount of 8.5 to 13% by weight and an amount of of 9.5 to 11.5% by weight is especially preferred so that the steel can have especially excellent properties by the addition of nickel.

Molybdenum of a small amount is effective to remarkably improve the high temperature strength of the steel because molybdenum combines with carbon to form a carbide thereof. With molybdenum addition of less than 0.7% by weight, however, its effect of addition cannot effectively be exhibited, while molybdenum addition of more than 2% by weight is undesirable because the effect of improvement is saturated and the strength would be rather lowered. In view of the above fact, molybdenum is added in an amount of 0.7 to 2% by weight and a further effective range is from 1 to 1.5% by weight.

The steel according to the invention including the above-described elements may further contain a suitable amount of desulfurizer or deoxidizer such as silicon and manganese. Further, as described above, the steel contains accompanying impurities such as phosphorous and sulfur usually found in iron and other additive elements and nitrogen absorbed during the melting operation. These impurity elements such as phosphorous and sulfur would not adversely affect the properties of the steel if each element is contained in an amount of less than 0.03% by weight. Usually nitrogen in an amount of less than 0.03% by weight is contained in steel.

Silicon exhibits an excellent efiect when it is contained as a deoxidizer. With silicon addition of less than 0.2% by weight, however, it is impossible to obtain sound steel due to insufficient deoxidization, while with silicon addition of more than 1.5% by weight, weldability of the steel is degraded. It is therefore necessary to limit the range from 0.2 to 1.5% by weight. In order that the effect of addition of silicon can especially efiectively be exhibited, its amount should be limited to a range of 0.3 to 0.6% by weight.

Manganese is added as a deoxidizer as in the case of silicon and also acts as a desulfurizer. With manganese addition of less than 0.5% by weight, deoxidization and desulfurization are insufiicient and forgeability of the steel is also unsatisfactory, while with manganese addition of more than 3% by weight, its effect of addition is saturated and the steel strength would rather be lowered. Accordingly the manganese content is limited from 0.5 to 3% by weight, and the most excellent effect can be exhibited by manganese addition in an amount of 1 to 2% by weight.

As will be apparent from the foregoing description, chromium-nickel-molybdenum austenitic steel exhibits an excellent elfect as a basic steel composition for the heatresisting steel according to the present invention. Addition of niobium and vanadium, and in addition to these elements, addition of such additive elements as copper, boron and tungsten to the above basic steel composition are effective to further improve the high temperature strength of the steel. The range and efiect of addition of these additive elements will become apparent from the following description with regard to various examples of the steel according to the present invention.

Table 1 shows chemical compositions in percent by weight of various heat-resisting steels employed in the experiments by the inventors.

The Specimens Nos. 1 to 15 shown in Table 1 were subjected to a creep rupture test at 650 C. and 700 C. As a result of the test, these heat-resisting steels show creep rupture strength as shown in FIG. 1.

From FIG. 1 it will be apparent that the creep rupture strength at 650 C. and 700 C. of Specimen No. 1 which do not contain niobium and vanadium is extremely low compared with other specimens. It will however be seen that the creep rupture strength can abruptly be increased by adding niobium and vanadium in small amounts as in Specimens Nos. 2 to 15. Niobium and vanadium when added in proper amounts to chromium-nickel-molybdenum steel exhibit an excellent effect as well be seen on Specimen N0. 4. The creep rupture strength of Specimen No. 4 is remarkably higher than those of Specimens Nos. 2 and 3 to which niobium and vanadium are also added.

Further addition of a suitable amount of copper to the chromium-nickel-molybdenum steel already including niobium and vanadium can increase the strength at elevated temperature, especially in the vicinity of 700 C. When, on the other hand, boron instead of copper is singly added to the steel, the strength in the vicinity of 650 C. becomes higher than in the case of single addition of copper, but the steel to which copper is singly added shows a higher strength in the vicinity of 700 C. than in the case of the steel to which boron is singly added.

The specimens to which both of copper and boron are added show marked increase in their strengths and their creep rupture strengths at 650 C. and 700 C. It is noteworthy that the creep rupture strength in this case makes an abrupt increase. For example, Specimen No. 14 will be compared with Specimen No. 1. The 10 hours creep rupture strength of Specimen No. 14 at 700 C. is 20 kg./mm. whereas that of Specimen No. 1 at the same temperature and the same time is about 9 kg./mm. Likewise the 10 hours creep rupture strength of Specimen No. 14 at 700 C. 18 kg/mm. whereas that of Specimen No. l at the same temperature and the same time is about 6 kg./mm. It will be seen that in both cases the differ- TABLE 1.-CHEMICAL COMPOSITION IN PERCENT BY WEIGHT Manga- Chro- Molyb- Nio- Vana- Tung- Cal- Magne- Carbon Silicon nese Nickel mium denum bium dium Copper sten Boron cium sium Specimen No.2

0. l5 0. 46 1. 20 9. 93 17. 20 1. 70 0. 18 0. 50 0. 13 0. 2. 07 11. 00 16. 30 1. 21 0. 21 0. 41 0. l8 0. 96 1. 88 8. 90 18. 10 1. 84 0. 32 0. 20 0. 10 0. 58 1. 14 14. 30 16. 80 l. 49 0. 36 0. 38 0. 14 0. 47 2. 06 10. 72 16. 1. 53 0. 39 0.39 0. l1 0. 42 2. 03 10. 72 15. 59 l. 92 0. 21 0. 43 0. 12 0. 42 1. 80 12. 16 15. 41 1. 65 0. 12 0.37 0. 10 0. 57 1. 25 8. 52 15. 00 1. 84 0. 47 0. 24 0. 12 0. 60 1. 22 10. 89 16. 03 1. 55 0. 18 O. 22 0. 10 0. 47 1. 35 10. 43 16. 80 1. 93 0. 29 0. 31 0. 07 0. 49 1. 41 9. 92 19.05 1. 81 0. 14 0. 34 0. l2 0. 34 1. 88 8. 88 17. 1. 76 0. 12 0. 44 0. 07 0. 40 1. 94 10. 30 18. 43 1. 63 0. 08 0. 28 0. 11 0. 88 0. 92 14. 06 15. 70 1. 56 0. 89 0. 0. 07 0. 46 1. 73 11. 90 16. 77 1. 94 0. 10 0. 68 1. 22 9. 48 14. 45 1. 08 0. 10 0. 68 1. 14 10. 00 15. 80 1. 19 0. 11 0. 68 1. 26 10. 00 16. 50 1. l0 0. l1 0. 61 0. 98 9. 45 15. 15 1. l3 0. 11 0. 65 1. 12 9. 10 14. 80 1. 08 0. 11 0. 83 1. 14 10. 38 16. 58 1. 22 0. 12 1. 05 2. 68 10. 25 17. 05 1. 93

In Table 1, Specimen Nos. 2 to 15 and 17 to 23 fall under the chemical compositions of the heat-resisting steel according to the invention. Since the amounts of the additive elements including niobium, vanadium, copper, boron and tungsten in these alloys are determined by chemical analysis of the alloys actually manufactured, these elements must be added in amounts somewhat more than the desired amounts in adding these elements to steel melts. This is because these elements, when added to steel melts, are consumed as by oxidization and the amounts of these elements actually remaining in the alloy steels are less than the amounts which have been added. Loss due to oxidization is especially marked for calcium and magnesium.

ence therebetween is more than 10 kg./mm. This difference in the strength is more marked in the test at 650 C. More precisely, the 10 hours creep rupture strength of Specimen No. 14 at 650 C. is 28 kg./mm. whereas that of Specimen No. 1 at the same temperature and the same time is 13 kg./mm. while the 10 hours creep rupture strength, of Specimen No. 14 at 650 C. is 24 kg./ mm. whereas that of Specimen No. 1. at the same temperature and the same time is about 10 kg./mm. It will be seen that in both cases the creep rupture strength of Specimen No. 14 is more than twice that of Specimen No. 1.

Still further addition of tungsten to the steel already including both of copper and boron is eifective to increase the creep rupture strength under test duration of hours, but the 10 hours creep rupture strength does not make any increase.

From the foregoing description, it has been made clear that addition of niobium and vanadium and further addition of such elements as copper and boron to chromiumnickel-molybdenum steel exhibit a marked effect in the improvement of high temperature strength of the steel and each element has its own peculiar feature for the improvement of strength in a temperature region in the vicinity of 650 C. or 700 C. and under a testing time of 10 or more hours. In order to more clarify the effect of these elements, the relation between creep rupture time and stress as 650 C. and 700 C. of the specimens of the steel according to the invention is compared with that of Specimen No. 1 and results at respective temperatures of 650 C. and 700 C. are shown in FIGS, 2 and- 3'.

As will be apparent from FIGS. 2 and 3, the creep rupture resistance of Specimen No. 1 is extremely poorer than those of other specimens and the 10 hours rupture strength is about 10 kg./mm. at 650 C. In contrast thereto, the specimens including niobium and vanadium show considerably high strength. Higher creep rupture strength is exhibited by those specimens to which either copper or boron is singly added in addition to the above-described niobium and vanadium. At 650 C., that specimen to which boron is singly added shows a higher strength than those to which copper is singly added. On the contrary, it is apparent that at 700 C. the effect of copper appears more powerfully as the creep rupture testing time becomes longer. Further it will be noted that composite addition of copper and boron gives a remarkable effect which is far superior to the effect obtained by the single addition of one of these elements. Still further, addition of tungsten in addition to copper and boron is effective to remarkably improve the rupture stress when the test duration is less than 10 hours, but the effect of tungsten addition is reduced as the duration is further extended to an extent that those including no tungsten therein show better resistance to creep rupture with test duration longer than 2x10 hours.

Specimens Nos. 16 to 23 are prepared for the purpose of testing the effect of calcium and magnesium and do not include any amount of niobium and vanadium therein.

In this test, Specimens Nos. 17 to 23 containing calcium 1 and magnesium therein are compared with Specimen No. 16 not containing any of calcium and magnesium in order to find out how these elements contribute to improvements in the high temperature strength.

FIG. 4 shows the relation between creep rupture time at 650 C. and stress of some of these specimens. Creep rupture strength under test duration of 10 hours of Specimens Nos. 18, 20 and 23 to which calcium and magnesium are singly or compositely added is 16.5 to 18.5 kg./mm. and is higher than the value 14 kg./mm. of Specimen No. 16 which does not contain any of calcium and magnesium.

The addition of calcium and magnesium is effective to prevent steel material from being corroded by vanadium compounds contained in heavy oil when the steel material is used to make heating equipment for heavy oil burning apparatus and is also effective to improve the high temperature strength as described above. In order to compare weight loss due to corrosion, 100 milligrams of vanadium pentoxide (V 0 was dispersed on each of Specimens Nos. 16 to 23 of size 8 x 5 x mm. and the specimens were heated at 800 C. for 100 hours. The weight loss in grams due to corrosion was 1.7 grams for Specimen No. 16, 0.9 gram for Specimen No. 17, 0.5 gram for Specimen No. 18, 0.4 gram for Specimen No. 19, 0.6 gram for Specimen No. 20, 0.5 gram for Specimen No. 21, 0.5 gram for Specimen No. 22 and 0.9 gram for Specimen No. 23. From the test results it was confirmed that the weight loss in each of Specimens Nos. 17 to '23 was less than half that of Specimen No. 16 and thus it was clarified that steel material containing calcium and magnesium can withstand corrosion even when exposed to heavy oil ash for a period more than twice the period in which conventional steel materials can withstand.

From the above test results, it will be apparent that, when calcium and magnesium are added to the chromiumnickel-rnolybdenum steel containing niobium and vanadium, this steel also possesses an increased resistance to corrosion against the action of vanadium pentoxide.

From the foregoing detailed description with regard to the specific examples, it has been made clearthat addition of niobium and vanadium and further addition of copper, boron and other elements described above to chromiumnickel-molybdenum steel are remarkably effectivefor improvements in the properties of heat-resisting steel, especially the mechanical strength at a temperature of 650 C. and up to about 700 C. It is considered that such remarkable effect is obtained owing to the fact that niobium and vanadium form their carbides in the steel and these carbides mainly contribute to the maintenance of high strength at elevated temperatures. With the content of each of niobium and vanadium of less than 0.1% by weight, suflicient strength cannot be obtained, while with the content of more than 0.5% by weight, workability and weldability are lowered and creep rupture strength is also lowered.

A larger amount of vanadium results in lowering of resistance to oxidation. Accordingly it is necessary to limit the amount of each of niobium and vanadium to a range of 0.1 to 0.5% by weight. When both of these elements are contained in the above range, sufficient creep rupture strength can be obtained Without lowering the workability and weldability. An advantage is also derivable in that nickel may be added in less amount and thus the cost can be reduced. An especially preferred range in the above-described range is 0.15 to 0.4% by weight for each of niobium and vanadium.

Copper is an element which is effective to raise the creep rupture strength. However, less than 1% by weight copper is undesirable because such a small amount does not contribute to improvements in the strength, while more than 4% by weight is also objectionable because workability and weldability are lowered. Therefore a range of 1 to 4% by weight is preferred and an especially preferred range is 1.5 to 2.5% by weight. Copper in an amount of less than 0.2% by weight is usually contained in steel even when copper is not especially added, but copper in such a small amount is unobjectionable.

Several examples will be described hereunder in order to verify the fact that niobium, vanadium and copper of the above-specified ranges exhibit an excellent effect for improvements in workability and weldability.

TABLE 2.-CHEMICAL COMPOSITION IN PERCENT BY WEIGHT Manga- Chro- Molyb- N io- Van a- Carbon Silicon nese N iekel mium denum Copper bium dium Specimen N 0.:

0. 12 0. 48 1. 62 10. 4O 17. 18 1. 40 0. 07 0. 30 O. 26 O. 12 0. 43 1. 92 10. 23 16. 81 1. 33 1. 93 0.25 0. 25 0. 14 0. 46 2. 16 9. 38 l7. l0 1. 45 0. 20 0. 40 0. 15 0. 24 2. 42 10. 30 16. 00 1. 40 1. 95 0. 44 0. 36

Specimens Nos. 24 and 25 were prepared in the form of billets. These billets were rolled by a Mannesmann mill and then subjected to cold working by a Pilger mill and subsequent cold drawing. By the above process, it was possible to obtain steel tubes having a diameter of 50.8 mm. and a thickness of 7.5 mm. From the above fact, it was ascertained that the steel materials have excellent properties of being easily hot and cold worked. Specimens Nos. 26 and 27 were prepared in the form of welding rods, and a Fisco cracking test was carried out by use of these welding rods. Test results showed that the rate of cracking occurrence was 13.2% for Specimen No. 26 and 17.2% for Specimen No. 27. It was thus ascertained that the rate of cracking occurrence is considerably low as a welding rod of austenitic heat-resisting steel and the steel materials are satisfactorily practically usable for welding applications. Further, a welding test on steel pipes having the compositions of Specimens Nos. 24 and 25 by use of these welding rods proved that welding operation can be carried out without developing any defect.

Boron must not be added in too much an amount since it imparts an undesirable effect to forgeability and weldability although it improves the high temperature strength and toughness to resist creep rupture. It is therefore desirable to limit its percentage to 0.002 to 0.2% by weight, and an especially effective range preferred in the invention is 0.002 to 0.06% by weight.

The effect of addition of tungsten is effectively exhibited when it is coexistent with copper and boron, but it is hard to expect a marked elfect by the addition of this specific element. However, addition of 0.2 to 1% by weight tungsten is effective although not so marked.

Calcium and magnesium added in small amounts are effective to increase the resistance to corrosion and to improve the high temperature strength. However, addition of too much amounts of these elements are objectionable due to the fact that melting operation is relatively diflicultly carried out. Therefore, each of these elements is desirably added in an amount of 0.002 to 0.2% by weight, and an especially preferred range for each element is 0.005 to 0.05% by weight.

The present invention is accomplished by a suitable combination of the elements described above and, when these elements are suitably added in the above-specified ranges, the effect so attained is so excellent which could not have been attained heretofore. The heat-resisting steel according to the invention can remarkably extend the service life of components of apparatus designed to operate at a temperature in the vicinity of 600 C. to 700 C. when used to make such components.

What is claimed is:

1. An austenitic heat-resisting steel consisting essentially of 0.07 to 0.2% by weight carbon, 0.2 to 1.5% by weight silicon, 0.5 to 3% by weight manganese, 14 to 20% by weight chromium, 8.5 to 13% by weight nickel, 0.7 to 2% by weight molybdenum, 0.1 to 0.5% by weight niobium, 0.1 to 0.5% by weight vanadium, and the balance being iron and accompanying impurities.

2. An austenitic heat-resisting steel according to claim 1, further containing 1 to 4% by weight copper.

3. An austenitic heat-resisting steel according to claim 1, further containing at least one metal selected from the group consisting of 0.002 to 0.2% by weight boron, 0.002 to 0.2% by weight calcium, and 0.002 to 0.2% by weight magnesium.

4. An austenitic heat-resisting steel according to claim 2, further containing at least one metal selected from the group consisting of 0.002 to 0.2% by weight boron, 0.002 to 0.2% by weight calcium, and 0.002 to 0.2% by weight magnesium.

5. An austenitic heat-resisting steel according to claim 3, containing 0.002 to 0.2% by weight boron, and 0.2 to 1% by weight tungsten.

6. An austenitic heat-resisting steel according to claim 10 4, containing 0.002 to 0.2% by weight boron, and 0.2 to 1% by weight tungsten.

7. An austenitic heat-resisting steel consisting essentially of 0.1 to 0.15% by weight carbon, 16 to 18% by Weight chromium, 9.5 to 11.5% by weight nickel, 1 to 1.5% by weight molybdenum,0.3 to 0.6% by weight silicon, 1 to 2% by weight manganese, 0.15 to 0.4% by weight niobium, 0.15 to 0.4% by weight vanadium, and the balance being iron and accompanying impurities.

8. An austenitic heat-resisting steel according to claim 7, further containing 1.5 to 2.5% by weight copper.

9. An austenitice heat-resisting steel according to claim 7, further containing at least one metal selected from the group consisting of 0.002 to 0.2% by weight boron, 0.002 to 0.2% by weight calcium, and 0.002 to 0.2% by weight magnesium.

10. An austenitic heat-resisting steel according to claim 8, further containing at least one metal selected from the grOup consisting of 0.002 to 0.2% by weight boron, 0.002 to 0.2% by weight calcium, and 0.002 to 0.2% by weight magnesium.

11. An austenitic heat-resisting steel according to claim 1, containing at least one metal selected from the group consisting of 1.5 to 2.5% by weight copper, 0.002 to 0.06% by weight boron, 0.005 to 0.05% by weight calciurn, and 0.005 to 0.05% by weight magnesium.

12. An austenitic heat-resisting steel according to claim 1, further containing at least one element selected from the group consisting of 1 to 4% by weight copper and 0.002 to 0.2% by weight boron.

13. An austenitic heat-resisting steel consisting essentially of 0.07 to 0.2% by weight carbon, 0.2 to 1.5% by weight silicon, 0.5 to 3% by weight manganese, 14 to 20% by weight chromium, 8.5 to 13% by weight nickel, 0.7.to 2% by weight molybdenum, 0.1 to 0.5% by weight niobium, 0.1 to 0.5% by weight vanadium, at least one element selected from the group consisting of 0.002 to 0.2% by weight calcium, and 0.002 to 0.2% by weight magnesium, and the balance being iron and small amount of accompanying impurities.

14. An austenitic heat-resisting steel consisting essentially of 0.07 to 0.2% by weight carbon, 0.2 to 1.5% by weight silicon, 0.5 to 3% by weight manganese, 14 to 20% by weight chromium, 8.5 to 13% by weight nickel, 0.7 to 2% by weight malybdenum, 0.1 to 0.5% by weight niobium, 0.1 to 0.5% by weight vanadium, at least one element selected from the group consisting of 1 to 4% by weight copper and 0.002 to 0.2% by weight boron, and at least one element selected from the group consisting of 0.002 to 0.2% by weight calcium, and 0.002 to 0.2% by weight magnesium, and the balance being iron and small amount of accompanying impurities.

15. An austenitic heat-resistant steel according to claim 7, further containing at least one element selected from the group consisting of 1.5 to 2.5% by weight copper and 0.002 to 0.2% by weight boron.

16. An austenitic heat-resisting steel according to claim 7, further containing at least one element selected from the group consisting of 0.0002 to 0.2% by Weight calcium and 0.0002 to 0.2% by weight magnesium.

17. An austenitic heat-resisting steel. according to claim 15, further containing at least one element selected from the group consisting of 0.002 to 0.2% by weight calcium and 0.002 to 0.2% by weight magnesium.

References Cited UNITED STATES PATENTS 3,154,412 10/1964 Kasak 75128.85 3,303,023 2/1967 Dulis :75-128.6 2,190,486 2/1940 Schafmeister 75-428 2,405,666 8/ 1946 Norwood 75--128 X 3,152,934 10/1964 Lulg.

FOREIGN PATENTS 647,701 12/ 1950 Great Britain.

HYLAND BIZOT, Primary Examiner

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