US3139337A - Alloys - Google Patents

Description

United States Patent 3,139,337 ALLOYS Clarence J. Boyle and David L. Newhouse, Schenectady,

N.Y., assignors to General Electric Company, a corporation of New York No Drawing. Filed May 31, 1962, Ser. No. 198,714

5 Claims. (Cl. 75126) This invention relates to new and useful alloys. More particularly, it relates to steel alloys which are particularly useful for producing forgings for use at high temperature, particularly those of large size, which at the same time have suitable room temperature properties.

The trend toward larger and larger capacities in steam turbines operating at inlet steam temperatures of the order of 1000 F. and above requires increased strength in the metals of construction at these temperatures including the rotors for such turbines. While low chromium, molybdenum and vanadium steel alloys can be strengthened somewhat by certain modifications in heat treatment, such heat treatment also tends to reduce high temperature ductility and to reduce the capacity of the material to withstand rupture cracking. at points of stress concentration. Therefore, only a small increase in high temperature strength can be safely attained in such steels by altering their heat treatment.

It has been known for some time to use a class of alloys containing, by weight, about to 13 percent chromium strengthened with various amounts of molybdenum, tungsten, vanadium and columbium (niobium) for small components such as bolts and turbine buckets which must withstand high temperatures. These alloys make possible the production of such parts having good high temperature ductility in combination with high temperature strength and low temperature properties which are superior to those obtained with low alloy steels. Because of the success of such alloys for high temperature materials of the types described, it seemed only logical to attempt to construct large forgings, such as turbine rotors, and the like, of such compositions which hold such promise for both high temperature and low temperature operation. However, it was found that when large forgings of the order of 20 inches in diameter and upward and added as strengtheners to segregate, resulting in the for-.

mation of ferrite and/or massive carbides. The high temperature rupture strength and ductility, however, were satisfactory. It has also been known that when the columbium content of small components, such as high temperature bolts and buckets, is lowered, the strength characteristics of such relatively small parts are lowered.

An object of this invention is to provide steel alloy compositions for large forgings which are characterized by suitable high temperature and room temperature physical characteristics.

Briefly, the invention relates to steel alloy compositions for large forgings in which the columbium content normally used as a strengthening addition is drastically reduced and the alloying constituents so chemically balanced that there is essentially no ferrite in the final microstructure. A typical steel alloy of this invention contains, by weight, carbon 0.10 to 0.30%, manganese 0.40 to 1.20%, silicon 0.60% max., nickel 1.20% max, chromium 8.0 to 13.0%, molybdenum 0.50 to 1.50%, vanadium 0.15 to 0.30%, columbium 0.03 to 0.12%, nitrogen 0.04 to 0.10% and as maximum residuals not to be added, phosphorus 0.025%, sulfur 0.025%, cobalt 0.25 aluminum 0.05%, titanium 0.05%, tin 0.04%, and tungsten 0.25%, with the remainder iron. The preferred range of materials is carbon 0.15 to 0.23%, manganese 0.50 to 1.00%, silicon 0.40% max., nickel 1.0% max, chromium 10.0 to 12.0%, molybdenum 0.80 to 1.20%, vanadium 0.15 to 0.25%, columbium 0.03 to 0.12%, nitrogen 0.04 to 0.08%, and as maximum residuals not to be added, phosphorus, 0.025%, sulfur 0.025 cobalt 0.25%, aluminum 0.05%, titanium 0.05%, tin 0.04%, and tungsten 0.25%, with the remainder iron. These compositions are, as pointed out above, so balanced that there is essentially no ferrite in the final structure. A specific example of such a useful alloy desiginat'ed Example 1 is one containing carbon 0.18%, manganese 0.60%, silicon 0.33%, nickel 0.19%, chromium 10.97%, molybdenunr 1.06%, vanadium 0.21%, columbium 0.075%, and nitrogen 0.06%, with the residuals not to exceed those above. Another specific example designated Example 2 contains carbon 0.18%, manganese 0.75%, silicon 0.25 nickel 0.70%, chromium 10.50%, molybdenum 1.0%, vanadium 0.20% columbium 0.06% and nitrogen 0.06% with the residuals as above.

Those features of the invention which are believed to be novel are set forth with particularity in the claims appended hereto. The invention will, however, be better understood and further objects and advantages thereof appreciated from a consideration of the following descrip tion.

It was found, as pointed out above, that when alloys of the above general type were used for making large forgings, including relatively large amounts of columbium, such as had heretofore been used, and without particular regard to the presence of ferrite, the room temperature transverse tensile ductility was low because of the segregation of excess columbium carbides and ferrite. Alloys of the type described having about 0.57% columbium and 0.32% columbium were prepared, but while certain of their characteristics were superior even at the lower preceding columbium level, excess segregated columbium carbides were present along with ferrite which detracted from the room temperature tensile ductility.

Although lowering the columbium content would be expected to result in improved ductility, the experience with small forgings and bar stock would indicate that a substantial reduction in columbium would also result in a lowering of high temperature strength. It was found that in large forgings a reduction of columbium content to 0.03 to 0.12% resulted in essentially the same strength as rotors containing the larger amounts of columbium and that the ductility was substantially improved. While there is no intent herein to be bound by any partticular theory as to the operation of the invention, it is believed thatthe relatively long, high temperature hot working and heattreating cycles required for large forgings more effectively place the columbium carbides formed in solution so that even small amounts of columbium are very elfective in large forgings as compared to Small forgings and bars which are not normally so treated.

Not only is it important that the columbium be critically controlled as pointed out above, but it is also necessary that the various other alloying elements be. included in such proportionate amounts as to obtain a precisely and chemically balanced material which will provide homogeneous and clean products which at the same time are characterized by good high and low temperature physical characteristics.

It has been found, for example, that each of the constituents should be precisely controlled, as set forth herein,

and that essentially no ferrite be present. The absence of ferrite is arrived at by carefully balancing the composition using a method of chromium equivalents. By this method each alloying constituent is given a numerical value as an austenite promoter or ferrite promoter, it having been found that when the numerical value of each alloying constituent is multiplied by the weight percent ofthe constituent presentand algebraically added and the sum is less than ten, the structure obtained is essentially free from ferrite. The values of each of the chromium equivalents as austenite promoters and ferrite promoters are set forth in the table below, and it will be understood that any reference to chromium equivalents herein refers to the chromium equivalent calculated using the values in the table.

Chromium Equivalents Austenite promoters:

Ni 4 N 3 O Ferrite promoters:

Si n +6 Cr l- 1 M +4 W +1.5 V 1 1 Cb 5 Referring to a'specific alloy composition (Example 2) used in connection with the present invention is one containing, by weight, 0.18% carbon, 0.75% manganese,

0.25% silicon, 0.70% nickel,-10.50% chromium, 1.00%

molybdenum, 0.20% vanadium, 0.06% columbium and 0.06%nitrogen, with the remainder essentially iron except for ordinary residuals, as pointed out above, and impurities. When the chromium equivalent is calculated for the above specific alloy, it is found that it is equal to about 5.2. For Example 1 the chromium equivalent' The quantities of the various constituents in the alloy ranges and examples set forth herein are critical. If larger amounts than the carbon specified are used, there tend to be formed excess segregated carbides with attendant poor low temperature characteristics. On the other hand, lower amounts of carbon in the balanced composition tend to promote the occurrence of ferrite which is to be avoided. Manganese is usually present in such alloys as a common constituent. Less'than the amounts of manganese specified tends to promote the production of ferrite, whereas larger amounts of manganese will lower the lower high critical temperature and tend to reduce the high temperature strength. Silicon in excessive amounts tends to promote ferrite and this is to be avoided. Nickel in too small amount tends to increase the ferrite but excessive amounts will reduce the lower critical temperature and tend to reduce the high temperature strength. Amounts of chromium in excess of those specified promote the presence of ferrite whereas lesser amounts are believed to detract from the strength of the material. Molybdenum is a strength producer and lower'amounts result in a loss of high temperature strength. On the other hand, amounts of molybdenum over those specified tend to produce excessive ferrite. Vanadium in amounts less than those specified again reduces the high temperature strength and amounts over those specified in the balanced composition tend to promote ferrite. Columbium also is a strength producer and amounts less than those specified substantially weaken the strength of the final structure. On the other hand, amounts over those specified, as pointed out above, promote the occurrence of ferrite and the production of excessive undissolved or segregated columbium carbide which detracts from the physical characteristics, particularly at room temperature. Nitrogen also is an essential constituent of the present alloys as a strengthening agent in addition to being an austenite former and is important in control of ferrite. Amounts of nitrogen in excess of those specified, while it produces less ferrite, may tend to cause undesirable porosity. The rest of the constituents mentioned, such as cobalt, aluminum, titanium, tin and tungsten, are not over the maximum specified.

In order to promote the desired high temperature and room temperature characteristics, the present materials after forging are subjected to a carefully controlled heat treatment. The forgings of the present invention which are typically about 45 inches in maximum diameter and weigh about 40,000# are first uniformly heated to a minimum of 1900 to 1950 F. and a maximum of 1950 to 2000 F. and held at temperature for a time sufficieut to complete transformation to austenite. The forgings are then quenched in a suitable medium, such as oil or water, to a temperature of about 200 to 500 F. at which temperature transformation to martensite is substantially complete. The forgings are then equalized within this temperature range for a period of from about 12 to 36 hours. The forgings are then tempered to obtain desired uniform properties throughout the forging. This may he arrived at by one or more tempering steps. Typically, the forging is first tempered at a temperature of from about 1000 to 1100 F. to avoid isothermal transformation of any austenite remaining untransformed during the quench. and equalizing treatment. This first temper is usually carried out for a period of from about 12 to 48 hours. After the first temper the forgings are cooled to a temperature below about 200 F. and held at this temperature for a minimum of about 12 hours to complete the transformation of any such retained austenite to martensite. The second temper is carried out at a temperature of about 1100 F. minimum for a minimum of about 12 hours to obtain uniformly desired properties throughout the forging.

Referring specifically to Example 1, a forging of this composition having a maximum diameter of 45 inches and Weighing about 40,000# was heated for 24 hours at 1925 F., quenched in oil to a temperature of 400 F. and equalized for 24 hours at 300 F. The forging was first tempered for 36 hours at a temperature of 1050 F., cooled to F., and held atthis temperature for 24 hours. The forging was then tempered at 1150" F. for 24 hours. 1

Shown in the table below are the room temperature properties of Example 1 above as compared with a prior art alloy containing, by Weight, carbon 0.18%, manganese 0.66%, silicon 0.05%, nickel 0.38%, chromium 11.08%, molybdenum 1.04%, vanadium 0.22%, columbium 0.57%, and nitrogen 0.051%, with 'theresiduals not exceeding those mentioned above and having a chromium equivalent of 8.91. In the table the term MRR signifies that the sample was taken from a radially directed piece midway between the bore and the circumference of the forging. The term MRT signifies that the sample Was taken from a piece cut from the forging at a piece midway between the bore and circumference, the piece being positioned mutually perpendicular to both the radius and longitudinal aXis of the forging. The term SR signifies that the sample was taken from a piece on the surface of the forging which Was radially directed. The term BR signifies that the sample was taken from a piece at the bore of the forging which was radially directed. The term .BT signifies that the sample was taken from a piece at the bore of the forging again mutually perpendicular to the radius and the longitudinal axis of the forging.

Room Temperature Mechanzcal PIOPEIIIGS Tensile 0.02% Percent Charpy Loca- Strength, Yield Percent of Re- Impact Ex. tion of 1,000 Strength, Elonduct-ion Energy, Sample p.s.l. 1,00i gation of Area Ft.-Ll

1 MRR 143 106 16 41.6 MRT 141. 5 105 16 36 SR 138 100 11. 5 27. 7 12 Pr1orArt MRR 122 94 9 11.9 SR 124 95 10 18.4 4 BR 122 94 7.5 7.7 3

10 Hr. Rupture Strength (p.s.i.) at- Ex. Location of Sample 900 F. 1,000 F. 1,050 F. 1,100 F.

1 SR 56,000 37,000 28,000 18, 000 BT 57,000 36, 000 25, 000 Prior Art SR 29,000 21,000 15,000

The 10,000 hour percent elongation at 1000 F. of an SR sample of the prior art material was 5.5, whereas this value for an SR sample of Example 1 Was 6. A BT sample of Example 1 had a corresponding elongation of 7% From the above, it will at once be apparent that the present materials are to be preferred both for their high temperature and low temperature mechanical properties. For example, the present materials have a much better reduction of area at higher tensile strength ranges. They also have a much higher rupture strength and better rupture ductility.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An alloy consisting essentially of by weight percent, carbon 0.10 to 0.30, manganese 0.40 to 1.20, silicon 0.60 maximum, nickel 1.20 maximum, chromium 8.0 to

13.0, molybdenum 0.50 to 1.50, vanadium 0.15 to 0.30, columbium 0.03 to 0.12, nitrogen 0.04 to 0.10, and as maximum residuals phosphorus 0.025, sulfur 0.025, cobalt 0.25, aluminum 0.05, titanium 0.05, tin 0.04, and tungsten 0.25, with the remainder iron, said alloy having a chromium equivalent of less than 10'.

2. An alloy consisting essentially of by weight percent, carbon 0.15 to 0.23, manganese 0.50 to 1.00, silicon 0.40 maximum, nickel 1.0 maximum, chromium 10.0 to 12.0, molybdenum 0.8-0 to 1.20, vanadium 0.15 to 0.25, columbium 0.03 to 0.12, nitrogen 0.04 to 0.08, and as maximum residuals phosphorus 0.025, sulfur 0.025, cobalt 0.25, aluminum 0.05, titanium 0.05, tin 0.04, and tungsten 0125, with the remainder iron, said alloy having a chromium equivalent of less than 10.

3. An alloy consisting essentially of by Weight percent, carbon 0.18, manganese 0.60, silicon 0.33, nickel 0.19, chromium 10.97, molybdenum 1.06, vanadium 0.21, columbium 0.075, nitrogen 0.06, and as maximum residuals phosphorus 0.025, sulfur 0.025, cobalt 0.25, aluminum 0.05, titanium 0.05, tin 0.04, and tungsten 0.25, with the remainder iron, said alloy having a chromium equivalent of about 9.

4. An alloy consisting essentially of by weight percent, carbon O.l8, manganese 0.75, silicon 0.25, nickel 0.70, chromium 10.50, molybdenum 1.0, vanadium 0.20, columbium 0.06, nitrogen 0.06, and as maximum residuals phosphorus 0.025, sulfur 0.025, cobalt 0.25, aluminum 0.05, titanium 0.05, tin 0.04, and tungsten 0.25, with the remainder iron, said alloy having a chromium equivalent of about 5.2.

5. An alloy for large forgings consisting essentially of by weight percent, carbon 0.10 to 0.30, manganese 0.40 to 1.20, silicon 0.60 maximum, nickel 1.20 maximum, chromium 8.0 to 13.0, molybdenum 0.50 to 1.50, vanadium 0.15 to 0.30, columbium 0.03 to 0.12, nitrogen 0.04 to 0.10, and as maximum residuals phosphorus 0.025, sulfur 0.025, cobalt 0.25, aluminum 0.05, titanium 0.05, tin 0.04, and tungsten 0.25, with the remainder iron, said alloy having a chromium equivalent of less than 10 and being uniformly heated to a minimum of about 1900" F. and a maximum of about 2000 F. for a time suflicient to substantially complete transformation to austenite, quenched until cooled to a uniform temperature at which transformation to martensite is substantially complete and tempered.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. AN ALLOY CONSISTING ESSENTIALLY OF BY WEIGHT PERCENT, CARBON 0.10 TO 0.30 MANGANESE 0.40 TO 1.20, SILICON 0.60 MAXIMUM, NICKEL 1.20 MAXIMUM, CHROMIUM 8.0 TO 13.0, MOLYBDENUM 0.50 TO 1.50, VANADIUM 0.15 TO 0.30, COLUMBIUM 0.03 TO 0.12, NITROGEN 0.04 TO 0.10, AND AS MAXIMUM RESIDUALS PHOSPHORUS 0.025, SULFUR 0.025 COBALT 0.25, ALUMINUM 0.05, TITANIUM 0.05, TIN 0.04, AND TUNGSTEN 0.25, WITH THE REMAINDER IRON, SAID ALLOY HAVING A CHROMIUM EQUIVALENT OF LESS THAN 10.