US3316084A - Forging steel for elevated temperature service - Google Patents

Description

United States Patent 3,316,084 FORGING STEEL FOR ELEVATED TEMPERATURE SERVICE Samuel J. Manganello, Penn Hills Township, Allegheny County, Pa., assignor to United States Steel Corporation, a corporation of Delaware No Drawing. Filed May 18, 1964, Ser. No. 368,404 1 Claim. (Cl. 75-128) This invention relates to improvements in forging steels for elevated temperature service and more particularly to steels suitable for heavy forgings such as steam-turbine rotors.

Steam rotor forgings are heavy forgings ranging up to several feet in diameter and which must withstand temperatures of the order of 1100 F. in service. Also these forgings must meet exacting specifications including a minimum room temperature yield strength of about 85,000 psi, a minimum elongation of about 14 percent, a minimum reduction of area of about 35 percent, a maximum Charpy V-notch 50-percent shear fracture transition temperature of 250 F., and good stress-rupture life in the range of 950 to 1200 F. combined with notch insensitivity during stress-rupture testing. In addition, steam-turbine rotors must be dimensionally stable at the operating temperatures of the intended use.

It is accordingly an object of this invention to provide a steel for heavy forgings for elevated temperature use having the above mentioned properties.

It is a further object of this invention to provide steamturbine rotors having the above mentioned properties.

I have discovered that a steel containing carbon, nickel, chromium, vanadium, molybdenum and manganese with- Chromium 0.85/ 1.45 Vanadium 020/065 Manganese -0.30/1.10 Molybdenum 0.90/225 Nitrogen max 0.015 Silicon max .40 Aluminum (acid soluble) max 0.05

balance iron and residual impurities. Within such compositional ranges the manganese and molybdenum should be balanced so that with less than 0.75 percent manganese the molybdenum should be at least 1.6 percent and with .75 percent or more manganese, the molybdenum should not exceed 1.6 percent. As indicated the steel may contain deoxidizers such as silicon and aluminum in amounts up to 0.40 and 0.05 percent respectively.

Thus within the foregoing ranges of my invention, there are two preferred analyses in weight percent:

Carbon 0. /0. 0. 25/0. 3 Nickel. 0. 70/1. 25 0. 70/1. 25 Chromium 0. 85/1. 0. 85/1. 45

Vauadiuin. 0. 20/0. 0. 20/0. 65 Manganese. 0. 30/0. 0. 75/1. 10 Molybdenum 1. 6/2. 25 0. /1. 6 Nitrogen, maximuin 0. 015 0. 015 Silicon, maximum .40 40 Aluminum, maximum 05 05 with the balance iron and residual impurities.

The benefits of this invention are illustrated by steels of the compositions in weight percent listed in the following Table I:

TABLE I Steel C Mn P S Si I Ni Cr Mo V Al* N 0. 28 0.31 0. 012 0. 011 0. 28 l. 03 1. 18 2. 04 0.25 0. 005 0. 006 0. 28 0. 30 0. 014 0. 011 0. 28 l. 07 1.18 1. 67 0. 47 O. 006 (J. 008 0. 29 0. 87 0.012 0. 014 0. 29 0. 78 1.15 1. 09 0. G4 0. 006 0. 008 0. 29 0. 30 0. 012 0. 011 0. 27 0. 76 1.15 1. 97 t]. 28 0. 004 0. 008

*Acid soluble.

in the hereinafter set forth restricted ranges and wherein the manganese and molybdenum are critically proportioned will provide the aforementioned required properties. The composition of my invention has the desirable qualities of not forming pearlite or proeutectoid ferrite after normalizing, good notch toughness, room temperature yield strength and elevated temperature stress-rupture properties. The compositional ranges (percent by weight) of my improved steel are:

Forged longitudinal specimen blanks of the foregoing steels were austenitized for four hours at 1775 C., cooled to 600 F. at about F. per hour (approximately the cooling rate at a location 3 inches from the surface of an air-cooled 45-inch diameter rotor forging), furnace cooled to room temperature, tempered for 24 hours at the temperature indicated in the following Table II and thereafter furnace cooled to room temperature. The room temperature tension test values and the Charpy Carbon 0.25 /-0.35 V-notch impact values for these steels so treated are given Nickel 0.70/ 1.25 in Table II:

TABLE II Charpy Yield Reduction Energy V-notch Steel Tampering Strength, Elong. in 2, of area, Absorbed 50% Shear Temp, F. p.s.i., 0.2% Percent Percent at 75 F., Fracture- Ottset tt.-1b. Transition Temp, F.

The stress-rupture test results (obtained with combination smooth-notched creep rupture specimens) on two of the steels are listed in the following Table III:

itizing, tempered, and furnace cooled to room temperature.

A review of the data indicates that the steel composition of this invention produces the desired properties for TABLE III 1,000 F., 60,000 p.s.i. 1,000 F., 50,000 p.s.i. 1,100 F., 40,000 p.s.i. Steel TIRJ El. RA. T.R. El. R.A. T.R. El. RA.

1,000 F., 30,000 p.s.i. 1,200 F., 20,000 p.s.i. 1.250 F., 15,000 p.s.i. Steel T.R. El. R.A. T.R. El. RA. I.R. El. RA.

1 Time torupture in hours. 1 Elongation 111 1 inch. 3 Reduction of area.

*Speeimen blanks were austenitized for 4 hours at 1,850 F. and furnace cooled to room temperature before being subjected to the above heat treatment.

The normalized steels had a uniform microstructure consisting of an intimate mixture of ferrite plus bainite. The steel may be single or double-normalized (homogenized then normalized) and still exhibit the good combination of microstructure and properties. Thus the steel has sufficient hardenability to permit heavy forgings to cool during normalizing without forming proeutectoid ferrite and pearlite which have an undesirable effect on strength and impact properties.

I have found that even with the specified nickel content, manganese and molybdenum must be critically balanced to achieve the desired improved mechanical properties. In evaluating the various test results, it was observed that when the manganese content dropped below a certain level, the steel was not satisfactory unless the molybdenum content was above a certain level. Further investigation revealed that the converse was also true. Thus, when the manganese is in the range 0.30 to 0.75 percent the molybdenum should be 1.6 to 2.25 percent, and when the manganese is in the range 0.75 to 1.10 percent the molybdenum should be less than 1.6 percent. The silicon content should not be much higher than 0.40 percent, because high amounts of silicon are deleterious to stress-rupture properties. Higher amounts of nickel and vanadium are beneficial to notch toughness (impact properties). In fact, increasing the nickel content appears to be also beneficial to the tensile and stressrupture properties of this steel. Higher amounts of molybdenum (and to a certain extent, vanadium) are beneficial to the stress-rupture properties. The carbon content must be kept within a rather narrow range (0.26 to 0.35 percent) to maintain high yield strength and reasonably good notch toughness.

The heat treatment to which these chromium-molybdenum-vanadium steels are subjected is critical. The use of final austenitizing temperatures higher than about 1800 F. is detrimental to notch toughness; the impairment to notch toughness is caused chiefly by austenite grain coareening. However, the time at austenitizing temperature will vary within the conventional range of to hours depending upon the cross-sectional area of the forging under treatment. The tempering temperature is maintained within the range of 1225 to 1280 F. for times of 20 to 40 hours at temperature. As is conventional, the Steel is air cooled (in still or agitated air) after austensteam-turbine rotors and other heavy forgings for elevated temperature service. Thus the steel has a room temperature 0.2 percent offset yield strength in excess of 85,000 p.s.i., a fracture transition temperature less than 250 F., good resistance to temper embrittlement, very good stress-rupture properties at temperatures up to 1200 F., and is notch insensitive. As previously stated, the steel has a microstructure consisting of a mixture of ferrite and bainite.

While I have shown and described several specific embodiments of my invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of my invention, as defined in the appended claim.

I claim:

A heavy forging steel for elevated temperature service characterized by a minimum room temperature yield strength of 85,000 p.s.i., a maximum Charpy V-notch 50 percent shear fracture transition temperature of 250 F. and good stress-rupture properties in the range of 950 to 1200 B, said steel consisting essentially of Carbon 025/035 with the balance iron and residual impurities.

References Cited by the Examiner UNITED STATES PATENTS 1,660,790 2/1928 Herman l28 X 2,798,805 7/1957 Hodge 75--12 8 X 2,921,849 1/ 1960 Furgason 75-128 HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

P. WEINSTEIN, Assistant Examiner.