US3615370A

US3615370A - Heat-resisting chromium-molybdenum-vanadium steel

Info

Publication number: US3615370A
Application number: US738103A
Authority: US
Inventors: Kenneth Arnold Ridal; John Mccann
Original assignee: English Steel Corp Ltd
Current assignee: English Steel Corp Ltd
Priority date: 1967-06-29
Filing date: 1968-06-19
Publication date: 1971-10-26
Anticipated expiration: 1988-10-26
Also published as: JPS4634306B1; FR1570294A; AT295569B; GB1218927A

Abstract

A heat-resisting alloy steel which is basically a 1 percent chromium-molybdenum-vanadium steel, with the addition of at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.03 to 0.15 total percentage by weight, from 0.002 to 0.010 percent by weight boron and from 0.5 to 3.0 percent by weight cobalt. High-creep strength, rupture ductility and tensile strength properties are developed by austenitizing the steel in the range 950* C. to 1,060* C., hardening by cooling, and tempering in the range 600* C. to 700* C. for from 3 to 60 hours.

Description

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i 1 we Stts ate Inventors Kenneth Arnold lRitlal;

John McCann, both of Yorkshire, England Appl. No. 738,103 Filed June 19, 1968 Patented Oct. 26, 1971 Assignee English Steel Corporation Limited Yorkshire, England Priority June 29, 1967 Great Britain lllllEAT-RESlSTING CHROMIUM-MOLYB1DENUM- VANADIUM STEEL 6 Claims, No Drawings U.S. Cl 75/128 B, 75/128 F, 75/128 V, 75/128 W lint. Cl C22c 39/20 Field of Search 75/l28.4,

[56] Relierences Cited UNITED STATES PATENTS 2,880,085 3/1959 Kirkby 75/128.6 2,968,549 1/1961 Brady 75/128.6 3,008,820 11/1961 Hurley 75/1 28.6

Primary ExaminerHyland Bizot Attorney-Stevens, Davis, Miller& Mosher ABSTRACT: A heat-resisting alloy steel which is basically a 1 percent chromium-molybdenurn-vanadium steel, with the addition of at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.03 to 0.15 total percentage by weight, from 0.002 to 0.010 percent by weight boron and from 0.5 to 3.0 percent by weight cobalt. High-creep strength, rupture ductility and tensile strength properties are developed by austenitizing the steel in the range 950 C. to l,060 C., hardening by cooling, and tempering in the range 600C. to 700C. for from 3 to 60 hours.

BACKGROUND OF THE INVENTION Conventional 1% chromium-molybdenum-vanadium steels when used for aeroengine turbine shafts although having reasonable tensile strength of the order of 40 to 50 tons per square inch, exhibit poor high-temperature creep resistance leading to failure of the shaft from creep. Attempts to increase the creep resistance of aeroengine turbine shafts by manufacturing them from 3% chromium-molybdenum-vanadium steels, although increasing the tensile strength of the shafts to up to 85 tons per square inch, have failed to increase the creep resistance to a satisfactory degree. Moreover the aforesaid conventional low-alloy steels have exhibited inferior creep properties when used for steam and gas turbine components subject to a combination of high temperature and stress, and when used for other rotating components such as discs, and components subject to stress, such as bolts, fasteners and tubes. 1

Furthermore, the aforesaid conventional low-alloy steels have restricted ranges of tensile properties, may not retain their tensile properties over a wide tempering range and are not usually suitable for the manufacture of large sections due to variation of tensile properties throughout the section coupled with low through hardenability.

It is accordingly an object of this invention to provide a new and improved heat-resisting alloy steel having an optimum combination of high-temperature creep strength, rupture ductility and tensile strength, superior to that previously attainable with low-alloy heat-resistin g steels.

It is another object of this invention to provide a new and improved heat-resisting alloy steel which can be heattreated to a wide range of tensile properties.

It is a further object of this invention to provide a new and improved heat-resisting alloy steel which is exceptionally resistant to tempering, and thereby maintains good tensile properties over a wide range of tempering.

It is still another object of this invention to provide a new and improved heat-resisting alloy steel which attains good tensile properties equally well in small or large sections and has high through hardenability.

SUMMARY OF THE DESCRIPTION The foregoing objects are accomplished by providing a heat-resisting alloy steel containing by weight from 0.15% to 3.5% carbon, not more than 0.35% silicon, from 0.4% to 1.0% manganese, from 0.4% to 1.0% nickel, from 0.7% to 1.4% chromium, from 0.5% to 1.5% molybdenum, and from 0.20% to 0.60% vanadium, and further containing at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.03% to 0.15% total by weight, from 0.002% to 0.010% by weight boron, and from 0.5% to 3.0% by weight cobalt to improve the creep strength, rapture ductility and tensile strength of the steel, the balance, except for impurities and incidental constituents which include from to 0.040% by weight sulfur and from 0% to 0.040% by weight phosphorus, being iron. The steel of the invention is heat treatable by austenitizing in a temperature range 950 C. to 1060 C., hardening by cooling, and tempering in a temperature range 600 C. to 700 C. for from 3% to 60 hours.

DESCRIPTION OF PREFERRED EMBODIMENTS Other objects, features and advantages of the invention will become apparent on reading the following detailed description.

By way of example, steels produced in accordance with the invention and having constituents in a preferred quantity range and in specific quantities within the range are set out in the following as Type A and Type B respectively:

Constituent Type A of Type B Z ol'totul total weight weight (vacuum at:

remelted) Carbon from 0.22 to 0.28 0.23 Silicon 0.30 mnx. 0.30 Manganese from 0.5 to 0.7 0.39 Nickel from 0.5 to 0.7 0.68 Chromium from 0.9 to 1.1 1.07 Molybdenum from 0.65 to 0.85 0.85 Vanadium from 0.40 to 0.50 0.50 Cobalt from 1.5 to 2.5 2.18 Boron from 0.004 to 0.008 0.005 Titanium,

Niobium and/or from 0.08 to 0.10 0.06 Titanium Tantalum In each of the foregoing types of steel the balance of the constituents is iron except for impurities and incidental constituents, such as sulfur which is present in an amount not exceeding 0.015% by weight in Type A and of 0.009% by weight in Type B, and phosphorus which is also present in an amount not exceeding 0.015% by weight in Type A and of 0.009% by weight in Type B.

The addition of the elements cobalt, titanium, niobium or tantalum, and boron in combination to what is basically a 1% Cr-Mo-V type of alloy steel, jointly stabilizes the structure, in particular the vanadium-carbide-hardening phase, and renders the alloy steel suitable for high-temperature applications. Moreover, in steels of the present invention, it is preferable to keep the carbon content in the range 0.22% to 0.28% by weight to ensure optimum creep strength and creep ductility properties. Also the manganese and nickel contents should preferably to controlled within the stated ranges so that the combined manganese and nickel content in the steel is not greater than 1.5% by weight, to prevent the steel reverting to an austenitic structure on tempering. Furthermore, the chromiumand molybdenum-hardening elements are kept within the stated ranges to prevent chromium and molybdenum carbides detrimentally replacing vanadium carbide, which is a creep-strength-improving phase produced by the vanadium in the steel.

It is to be noted that the steel contains at least one of the group of strong carbide-forming elements comprising titanium, tantalum and niobium within the range 0.03% to 0.15% by weight, and in fact it is possible to have these three alloying constituents either individually or in combination within the stated range. Indeed the combined use of cobalt, a strong carbide former (titanium, tantalum and niobium) and boron considerably improves the creep properties of the steel over the conventional 1% chromium-molybdenum-vanadium and 3% chromium-molybdenum-vanadium steels in common usage. In fact, the combination of creep strength, rupture ductility and tensile strength attainable with an alloy steel of the invention within the stated ranges is superior to that previously developed in low-alloy heawesistant steels.

Steel produced with the invention is heat-treated by austenitizing in the temperature range 950 C.l060 C., and is then hardened by either cooling in air, steam, water mist, or oil. The desired mechanical properties are then attained by tempering the steel in the range 600-700 C. for 3 to 6 hours. In this way the steel can be heattreated to a wide range of tensile properties, from 50 to tons/square inch ultimate tensile strength. Furthermore, a steel produced in accordance with the invention is exceptionally resistant to tempering, in that the tensile properties are maintained over a wide range of tempering, has good hot-strength properties up to 600 C., and is capable of being surface hardened by nitriding to give a good case.

The properties of the steel can be attained equally well in small and large sections, due to the high through hardenability of the steel. lnparticular the tensile properties may be attained with a range of martensitic and/or bainitic structures. The optimum structures for resistance to creep strain are upper bainitic structures. When heattreated to an 85 tons per square inch condition, the alloy steel of the invention is capable of withstanding a stress of 20 tons per square inch for hours at 550 C. while exhibiting a total plastic strain of less than 0.1%.

For test purposes, l-inch diameter bars were prepared from steel within the aforesaid ranges, hardened for 1 hour at 1050 C. and air cooled. The effect of tempering treatment on mechanical properties is shown by the following test results, which illustrate the wide range of useful properties and great intrinsic resistance to softening of a steel produced according to the invention.

EFFECT OF TEMPERING TREATMENT ON MECHANICAL PROPERTIES Proof stress Ultimate (tons/ tensile sq. in.) strength Percentage Tempering (tons/ Percentage reduction treatment 10% 20% sq. in.) elongation of area 4 hours 600 C- 75.0 70.1 87. 3 19. 02. 5 8 hours 600 C 74. 6 80.8 86.0 17. 5 59. 0 20 hours 600 C 73.4 78.0 85. 7 15.0 51.0 4 hours 650 C 70.1 72.0 76. 9 19.0 50. 6 8 hours 650O 67.2 68.4 73.5 10.0 50.2 20 hours 650 C 62. 2 63.7 70.4 17.5 47. 0 4 hours 700C 61.4 63.1 70.0 19.0 57.5 8 hours 700 C 58. 3 50. 2 65. 9 21.0 61. 7 20 hours 700 C 51. 6 52. 3 00. 3 20. 0 60. 2 60 hours 700 C 48.6 40. 3 57.3 19. 0 (10.2

As aforesaid, steels produced in accordance with the invention have good hot-strength properties up to 600 C. For test purposes, samples of the steel were austenitized at 1050 C. for 1 hour, air cooled, tempered at 625C. for 8 hours, and air cooled. The effect of test temperature upon mechanical properties is shown by the following rest results:

1n steels produced in accordance with the invention, lowtemperature transformation products such as martensite are preferred, if the best impact strength and creep ductility is required. This may be demonstrated by the following test results obtained from l-inch diameter bars of steel of the invention, which were oil-hardened from 975 C. and tempered for 8 hours at 625C.

MECHANICAL PROPERTIES AT ROOM TEMPERATURE P r c Ult' t t '1 i i roo s ress ima e ensi e e onga ion (tons/sq. in.) strength at g gg ia 0.2% (tons/sq. 111.) 4 Area area CHARPY VNOTOH IMPACT TEST N otch-tensile strength (tons/sq. in.) after 300 hrs. at-

Notch-tensile Impact value strength (foot-pounds) (tons/sq. in.) 400 C. 500 0. 550 C.

CREEP PROPERTIES AT 550 C.AN1) 20 TONS/SQ. IN.

These properties exhibited by the steel produced according to the invention are superior to those previously attained on low-alloy steels, and in some respects may be compared with those commonly obtained on the high-chromium steels (12% chromium, molybdenum, vanadium, niobium). However, the steel of the present invention is more resistant to tempering, is capable of being nitrided, and is more economic than the highalloy steels.

The above steels should be particularly useful for the manufacture of aeroengine turbine shafts, but are also suitable for a variety of applications. In particular the aforesaid ste'els are suitable for use in steam and gas turbines, where the component fabricated therefrom is subject to a combination of high temperature and stress. Particular examples are rotating components, shafts and discs, and components subject to stress such as bolts and fasteners, and tubes.

It is to be understood throughout this specification that weights given in tons, refer to long tons.

While preferred embodiments have been described, it is to be understood that various modifications and changes may be made without departing from the spirit and scope of the invention. What is claimed is: v

1. Heat-resisting alloy steel consisting essentially of by weight from 0.15% to 0.35% carbon, not more than 0.35% silicon, from 0.4% to 1.0% manganese, from 0.4% to 1.0% nickel, from 0.7% to 1.4% chromium, from 0.5% to 1.5% molybdenum, and from 0.20% to 0.60% vanadium, and further containing at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.03% to 0.15% total by weight, from 0.002% to 0.010% by weight boron, and from 0.5% to 3.0% by weight cobalt to improve the creep strength, rupture ductility and tensile strength of the steel, the balance, except for impurities and incidental constituents which include from 0% to 0.040% by weight ,sulfur and from 0 to 0.040% by weight phosphorus, being iron.

2. Heat-resisting alloy steel according to claim 1 containing at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.08% to 0.10% by weight.

3. Heat-resisting alloy steel according to claim 1 containing by weight from 0.004% to 0.008% boron.

4. Heat-resisting alloy steel according to claim 1 containing by weight from 1.5% to 2.5% cobalt.

5. Heat-resisting alloy steel according to claim 1 in which the combined manganese and nickel content is not greater than 1.5% by weight.

6. Heat-resisting alloy steel according to claim 1, containing by weight, from 0.22% to 0.28% carbon.

Claims

2. Heat-resisting alloy steel according to claim 1 containing at least one element selected from the group comprising titanium, tantalum and niobium in the range from 0.08% to 0.10% by weight.
3. Heat-resisting alloy steel according to claim 1 containing by weight from 0.004% to 0.008% boron.
4. Heat-resisting alloy steel according to claim 1 containing by weight from 1.5% to 2.5% cobalt.
5. Heat-resisting alloy steel according to claim 1 in which the combined manganese and nickel content is not greater than 1.5% by weight.
6. Heat-resisting alloy steel according to claim 1, containing by weight, from 0.22% to 0.28% carbon.