US3038797A - Alloy steel for forgings - Google Patents

Description

3,038,797 ALLOY STEEL FOR FORGINGS Samuel J. Manganelio, Wilkins Township, Allegheny nited States atent met in a nickel, molybdenum, vanadium steel containing less than .40% manganese, between 3.50 and 4.25% nickel, and between .18 and .40% vanadium. Not only does such steel have an 0.02% offset yield strength in County and Steiner Churchill B 5 the range of 94,000 to 98,000 p.s.i. but such increase in 2:25: 3; gg g iiig s Steel Corporation a corpoyield strength is accompanied by a decreekllse irsiofrapcture 7 a earance transition tem erature to lesst an N0 Drawmg' 4%igg g k w g fl 2 s? No. 852,596 The fracture appearanc transition temperature is de- I termined by using V-notch Charpy specimens. The frac- This invention relates to improvements in magnetic 10 mm pp f tYPPSiiiOYI @Q p difiel's from the alloy steels for forgings and more particularly to alloy more familiar ductility transition temperature. The fracsteels providing high yield strength combined with good epp f transmqn temperature detelmlfled y notch toughness f large generator rotor fol-gings, breaking specimens at different temperatures and deter- In the electric power industry, there is a need for a s from the pp a e of the fraetures at w strong, tough, and magnetic steel suitable for large gentsmpefatllre Steel fflllfid 111 3 50704116016, 50%43T1tt1e erator rotor iorgings that serve as the rotating field com- 0 Dllcille ffacturfis pp y and fibrous, bl1 t ponents of large steam-turbine generators. These large 011mg ffictllfes PP y f granular; P) f pf rotor forgings range in diameter from 12 to 70 inches mg th lfactllfed 5136911116113 'Wlth a Chan, lelatlve and in length from 15 to 35 feet. The steel rotors should ductlllty blltfleiless of the frafitjlfe 10 3! be be free of internal defects so they can withstand high tarmlPed- I11 Fofiimst, the ductllliy ifansltion p operating stresses. They must be tough (nonbrittle) to mm 15 fieiermmefi usually y measllrmg i energy resist the propagation of any internal or external cracks Sorbed 111 fi f speclmens at dlfierem temperatures or defects present or incurred in service. Propagation of and determmmg at What temperature h amount of these cracks or defects results in sudden failure in servenergy absorbed sudden1y {ncreases or decmases; The ice. In addition, these field rotor forgings should exhibit fracture FP 2111510011 l f filmlshes a a high magnetic permeability (B/H or slope of the mag more reliable and reproducible indication of the notch netization curve) to minimize the current necessary to tqjghness of i .i g i fii; tran' generate the required magnetic field. i gs fgg gg i g i g fii m ractum It Is accorihngly an obiectpf this f i ip prelude Our investigations have demonstrated that the desired alloy generaor rotor forgmgs ccmlimmg h1gh yleld properties may be obtained with the following compostrength good notch toughness and hlgh magnenc nents within the following ranges (percent by weight). m y- It is preferable that rotor-forging ingots of the steel be It is another object to provide a readily forgeable alloy vacuum cast steel providing the foregoing properties. f

The type of steel commonly used for rotor orgings v for high speed generator units is a nickel, molybdenum, 0 Mn Ni Gr M0 V vanadium steel containing 0.18/ 0.33% carbon, 0.40/ 0.70% manganese, 0.15/0.40% silicon, 2.50% minimum 350 U nickel, 0.20/0.70% molybdenum, 0.03/0.12% vanadium, 40 040 (1-50 and sometimes up to 0.75% chromium if higher strength I is desired. Such steel usually exhibits an 0.02% offset Fjff fg f f usuallmpmmesm Common amountsyield strength of about 65,000 to 90,000 p.s.i., a fracture The preferred range of componcnts is as follows appearance transition temperature (based on shear fracture appearance) of to 200 F., and a magnetizing 45 force (H) of 600 to 1300 ampere-turns per inch to pro- 0 Mn i N1 Cr M0 V Al duce a flux density (B) of 130,000 lines per square inch. This steel falls far short of properties desired by the 0 8;- steam-turbine generator manufacturers for generator 50 0.24 0.35 0. 40 4.00 0.30 0.30 0. 30 rotors, i.e. 90,000 p.s.i. yield strength (0.02% offset), 50" F. maximum fracture appearance transition temu Peramre and 725 ampere-011115 P inch, Atypical composition of our new steel is:

0 Mn P s Si Ni Cr M0 V Al Fe 0. 22 0. 30 0.012 0.012 0.25 3.75 0.25 0.30 0. 22 0. 010 Remainder and Impurities 1 Acid-soluble.

maximum, to produce a flux density of kilolines per square inch.

We have discovered that the desired objectives can be Test steels of the following compositions are illustrative of the unusual combination of properties resulting from the composition of our invention:

1 Acid-soluble.

Steel A is similar to the rotor forging steel mentioned hereinabove as being in common use in the industry; it differs from that rotor forging steel only in that it contains less manganese. Steel B is similar to Steel A except for a higher nickel content; Steel C is similar to Steel A with higher vanadium content. The results with these steels show that the desired properties cannot be obtained without increasing both nickel and vanadium. When both nickel and vanadium are increased the required amount and the manganese is low (Steel D), the desired properties are obtained.

Forged bars of these four steels were homogenized 2 hours at 1700 F., air cooled at room temperature, austenitized 4 hours at 1500 F., cooled at about 120 F. per hour to 600 F. (approximately the cooling rate that prevails at a location a few inches below the surface of a normalized 45-inch-diameter rotor forging), and freefurnace cooled to room temperature. This constitutes the normalizing treatment, to which all specimens were subjected. With air-cast steels, it is necessary to hold between 450 and 625 F. to prevent cracking or flaking. However our steel, if vacuum cast, may be air cooled to room temperature without harmful effects and in fact it has a beneficial effect on notch toughness without lowering the yield strength.

The tensile specimen blanks were tempered 24 hours at either 1140 or 1070 F. and air cooled. Tempering in the range of 1060 to 1080 F. is preferred. Tensile specimens (0.505-inch-diameter) were machined from The followand tested at room temperature. The following results were obtained:

Ampere-Turns per Inch for 130 Kilolines per Square Inch After normalizing and tempering, the steels of our invention have a microstructure consisting essentially of ferrite and bainite.

The results given above show that the desired 0.02% offset yield strength (90,000 p.s.i.) is not obtained in Steel A but is obtained in Steel B tempered at 1070 F. Furthermore, the magnetic properties of Steel B are very good (magnetizing force of 626 ampere-turns per inch). However, the transition temperature of Steel B, though good, is still rather high (60 to 75 F.). By increasing the vanadium content from 0.10 to 0.22%, however, a marked improvement in the transition temperature was obtained (Steels C and D). Steel C, which contains only 2.97% nickel, exhibits a low transition temperature F.) but also exhibits a low yield strength (79,000 p.s.i.) like Steel A. This indicates that 2.97% nickel the blanks and tested at room temperature. mg results were obtained: or 0.22% vanadium, by itself, will not give an 0.02%

Table l Yield Point, Yield Strength, Temperp.s.i. p.s.i. Tensile Elongation Reduction Steel ing Strength, in 2", of Area, Temp p.s.i. Percent Percent F. Upper Lower 0.02% 0.2%

Ofiset Offset After normalizing, the impact-specimen blanks were offset yield strength of 90,000 p.s.i. By increasing the tempered 24 hours at either 1140 or 1070 F., then either brine-quenched or slowly cooled (about 7 F. per hour to 750 F., then free-furnace cooled to room temperature). Charpy V-notch impact specimens were then machined from the blanks and tested at temperatures in the range 100 F. to +80 F. The following results were obtained:

After normalizing, some of the magnetic-test specimen blanks were tempered for 24 hours at 1160 R, freefurnace cooled, tempered 24 hours at 1120" F., and again free-furnace cooled (both coolings at about 150 F. per hour). Other magnetic-test specimen blanks were tempered at 1070 F. and were similarly cooled. Magnetictest specimens (1 71 inches wide by A inch thick by 12 inches long) were machined from the tempered blanks nickel content from 2.97% to above 3.50% and the vanadium content to above .'18%, however, the desired combination of high strength, good notch toughness, and high magnetic permeability (relatively low magnetizing force value) was obtained (Steel D). Moreover, Steel D was only slightly susceptible to temper embrittlement, as shown by the increase of only 20 F. in the transition temperature of the specimens slowly cooled from temporing over the transition temperature of those brinequenched from tempering. It is believed that the low manganese content of Steel D contributed greatly toward its relative insensitivity to temper embrittlement.

The mechanisms by which increases in nickel and, to a lesser degree, vanadium improve the strength of the nickel, molybdenum, vanadium steel are not fully understood, but it is believed that nickel strengthens the ferrite and that vanadium forms carbides that also serve to strengthen the steel. Vanadium, above about 0.18%, improves the notch toughness of nickel, molybdenum, vanadium steel by one or more of the following mechamsms:

(1) Refining the austenitic and ferritic grain size. (The austenite grain size of the steels containing 0.22% vanadium was ASTM No. 7 or finer, and the ferrite grain size was 8 /2 or finer. An austenite grain size of ASTM No. 4 or finer is desirable, as such fineness of grain is necessary for satisfactory ultrasonic testing.)

(2) Combining with and hence lowering the amount of carbon in solution.

spar-3,797

(3) Decreasing, perhaps, the susceptibility of nickel, molybdenum, vanadium steel to temper embrittlement.

The low manganese content is believed to make the steel less susceptible to temper embrittlement.

This application is a continuation-in-part of our copending application Serial No. 803,829, filed April 3, 1959, now abandoned.

While we have shown and described several specific embodiments of our 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 our invention, as defined in the appended claims.

We claim:

1. A steel characterized by an 0.02% oifset yield strength in excess of 90,000 p.s.i., a transition temperature below 50 F., a high magnetic permeability, as indicated by a magnetizing force of below 725 ampereturns per inch to produce a flux density of 130 kilolines per square inch and substantial freedom from temper embrittlement, said steel containing by weight .17 to .26% carbon .20 to .40% manganese .15 to .40% silicon 3.50 to 4.25% nickel .20 to .50% molybdenum .18 to .40% vanadium Up to 50% chromium .10% max. acid soluble aluminum balance iron and residual impurities.

2. A generator rotor forging characterized by an 0.02% offset yield strength in excess of 90,000 p.s.i., a transition temperature below 50 F., a high magnetic permeability, as indicated by a magnetizing force of below 725 ampere-turns per inch to produce a flux density of 130 kilolines per square inch, a ferritic-bainitic microstructure and substantial freedom from temper embrittlement, said steel containing by weight .17 to .26% carbon .20 to .40% manganese .15 to .40% silicon 3.50 to 4.25 nickel .20 to .50% molybdenum .18 to .40% vanadium Up to 50% chromium .10% max. acid soluble aluminum balance iron and residual impurities.

3. A steel characterized by an 0.02% offset yield strength in excess of 90,000 p.s.i., a transition temperature below F., a high magnetic permeability, as indicated by a magnetizing force of below 725 ampere-turns per inch to produce a flux density of kilolines per square inch and substantial freedom from temper embrittlement, said steel containing by weight .18 to .24% carbon .20 to .35 manganese .15 to .40% silicon 3.70 to 4.00% nickel Up to 30% chromium .20 to .30% molybdenum .18 to 30% vanadium .05 max. acid soluble aluminum .18 to .24% carbon .20 to .35 manganese .15 to .40% silicon 3.70 to 4.00% nickel Up to .30% chromium .20 to .30% molybdenum .18 to 30% vanadium .05% max. acid soluble aluminum balance iron and residual impurities.

References Cited in the file of this patent UNITED STATES PATENTS 934,697 Schneider Sept, 21, 1909 1,261,742 Churchward Apr. 2, 1918 2,206,370 Scherer July 2, 1940 2,845,345 Bauscher et a1. July 29, 1958 OTHER REFERENCES Hall: Nickel in Iron and Steel, 1954, page 168. Published by John Wiley and Sons, Inc., New York, NY.

Claims (1)

1. A STEEL CHARACTERIZED BY AN 0.02% OFFSET YIELD STRENGTH IN EXCESS OF 90,000 P.S.I., A TRANSITION TEMPERATURE BELOW 50* F., A HIGH MAGNETIC PERMEABILITY, AS INDICATED BY THE MAGNETIZING FORCE OF BELOW 725 AMPERETURNS PER INCH TO PRODUCE A FLUX DENSITY OF 130 KILOLINES PER SQUARE INCH AND SUBSTANTIAL FREEDOM FROM TEMPER EMBRITTLEMENT, SAID STEEL CONTAINING BY WEIGHT