US3689329A

US3689329A - Carbon steel spring elements

Info

Publication number: US3689329A
Application number: US852157A
Authority: US
Inventors: Roy F Kern
Original assignee: Caterpillar Tractor Co
Current assignee: Caterpillar Inc
Priority date: 1969-08-20
Filing date: 1969-08-20
Publication date: 1972-09-05
Anticipated expiration: 1989-09-05
Also published as: CA934993A; BE754940A; GB1319291A; FR2058914A5

Abstract

A CARBON STEEL COMPOSITION FOR SPRING ELEMENTS IS DISCLOSED WHEREIN THE STEEL INCLUDES CARBON, MANGANESE, FILICON AND BORON. IN ACCORDANCE WITH ITS CLASSIFICATION AS A "CARBON STEEL," IT DOES NOT INCLUDE NORMAL ALLOYING ELEMENTS, PARTICULARLY CHROMIUM. THE PRESENT STEEL IS PARTICULARLY CHARACTERIZED BY EXCELLENT DURABILITY AND FATIGUE LIFE AS WELL AS BY LOW COST. THE CARBON STEEL CONSISTS ESSENTIALLY OF ABOUT .50-.80 PERCENT CARBON, ABOUT 0.50-1.65 PERCENT MANGANESE, ABOUT 0.02-0.60 PERCENT SILICON, ABOUT 0.050 MAXIMUM PERCENT SULFUR, ABOUT 0.040 MAXIMUM PERCENT PHOSPHORUS, ABOUT 0.0005-0.007 PERCNET BORON, BALANCE MAINLY IRON.

Description

United States Patent 3,689,329 CARBON STEEL SPRING ELEMENTS Roy F. Kern, Peoria, Ill., assignor to Caterpillar Tractor Co., Peoria, Ill. No Drawing. Filed Aug. 20, 1969, Ser. No. 852,157 Int. Cl. CZId 9/02 U.S. Cl. 148-36 3 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to a novel'carbon steel composition which has been found suitable for use in spring elements.

Spring elements such as coil springs, leaf springs, torsion bars, etc. have conventionally been made from alloy steels within the prior art. This preference appears to have arisen out of the general belief that carbon steel springs could not be quenched and tempered to high strength levels in sizes much greater than 0.250 inch in diameter or thickness. Accordingly, carbon steel has not been considered for use in spring elements since it was believed that the weight and size of the spring elements would have to be inordinately great to achieve adequate strength standards.

As a basis for the present invention, it has been found that carbon steels may be employed to produce very satisfactory spring elements through the addition of boron and the control of various other elements within the steel composition.

It is important to note that the present invention is concerned only with carbon steels and not with alloy steels of the type which has been used for spring elements in the prior art. It therefore appears useful to include herein the standard definitions for alloy steels and carbon steels, as defined for example by the American Iron and Steel Institute.

An alloy steel is defined by the American Iron and Steel Institute as follows:

When the maximum range given for the content of alloying elements exceeds one or more of the following limits: manganese 1.65 percent, silicon 0.60 percent, copper 0.60 percent or in which a definite range or a definite minimum quantity of the following elements is specified or required Within the limits of the recognized field of constructional steels: aluminum, chromium up to 3.99 percent, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium-or any other alloying element added to obtain a desired alloying effec A carbon steel is defined by the American Iron and Steel Institute as follows:

No minimum limit for aluminum, chromium, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, zirconium-or any element to obtain a desired alloying effect. Specific minimum copper shall not exceed 0.40 percent. Maximum content for any of the following elements shall not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

To provide adequate strength in most spring elements,

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which are subject to very high fatigue loading, it appears that at least approximately 0.50 percent carbon is necessary to provide optimum life. Prior to the present invention, it was generally held that boron could not contribute significantly to hardenability as would chromium, for example, because the boron effect was thought to decrease sharply with increasing percentages of carbon.

Contrary to this general belief, it has been found as a basis for the present invention that boron is very effective in increasing hardenability of medium to high carbon spring steels of the type set forth herein.

A very basic advantage for the present carbon steel in the manufacture of spring elements lies in the reduction of costs which it makes possible. This cost reduction is made possible through the elimination of various alloying agents of which chromium is a particularly important example. In addition, manufacture of spring elements from the present carbon steel compositions is made independent of elements such as the above alloying agents which tend to be relatively scarce. On the other hand, the elements present within the steel composition of the present invention tend to be available in plentiful supply.

Still further, spring elements manufactured from the present carbon steel exhibit at least equivalent static strength and fatigue strength as compared to spring elements manufactured from conventional alloy steels. It further appears that spring elements manufactured according to the present invention exhibit certain advantages over alloy steels. One outstanding advantage for carbon steels lies in its shell hardening characteristics to provide endurance limit fatigue strength. A second advantage is the fracture toughness of carbon steels according to the present invention. Because of these advantages, spring elements formed from the present carbon steel composition may have increased durability and fatigue life as compared to spring elements formed from conventional alloy steels.

Additional important considerations for the manufacture of such spring elements lies in the ease with which carbon steels may be cold formed, forged, machined, welded, and heat treated under proper parameters. These characteristics further contribute to the facility with which the spring elements may be manufactured and to their finished physical characteristics.

It is accordingly an object of the present invention to provide carbon steel compositions suitable for manufac turing spring elements.

It is a further object to provide such compositions through the addition of boron to medium to high carbon steels.

It is a further object of the present invention to provide carbon steel spring elements with high durability and fatigue life.

It is still another object to provide spring members formed from carbon steel comprised of carbon, manganese, silicon and boron.

Carbon steel compositions of the present invention are comprised of iron in large part. Other elements which are particularly considered within the composition include carbon, manganese, silicon, and boron as well as maximum percentages for sulfur and phosphorus.

The useful approximate range of the above noted elements in the steel composition are as follows:

Element: Percent Carbon 0.5-0.8. Manganese 0.50-1.65 Silicon 0.020.60. Sulfur 0.50 max. Phosphorus 0.040 max. Boron 00005-0007. Iron Balance.

The above example generally characterizes the broad range of composition for the present invention. For example, any increase of carbon beyond 0.80 percent appears to detract from the advantage of the boron, namely, hardenability. Likewise, as the boron content exceeds the approximate ratio set forth above, a resulting decrease in notch toughness tends to appear.

To further characterize the above composition of the present carbon steel, in view of the definition of carbon steel, it may be further specified that the steel should include no more than 0.40 minimum percent copper and no more than 0.60 maximum percent copper.

Preferred approximate limits for a carbon-manganeseboron steel according to the present invention are as follows:

Elements: Percent Carbon 0.55-0.65. Manganese 1.10-1.40. Silicon 0.02-0.30. Sulfur 0.050 max.

Phosphorus 0.040 max. Boron 00005-0003. Iron Balance.

Steels of this composition may be produced, for example, in flat bars, wire, cold finished or hot finished bars fabricated by generally customary techniques. For example, hot-coiled springs may be heated to 1600 F., coiled, oil-quenched and tempered immediately thereafter. They may also be coiled, cooled, reheated to 1,550 F. and then oil-quenched followed by a tempering operation at 700 F. Cold-coiled springs may be heated to approximately 1550 F., quenched and then tempered also at 700 F.

Typical properties for recoil spring stock formed with the above composition are as follows:

Hardness: Rockwell-C42-48 Tensile strength: 200,000-250,000 p.s.i. Yield strength: 180,000-220,000 p.s.i. Grain size (-8 McQuaid-Ehn): Fine The subject composition also possesses improved notched tensile strength, notch toughness and fatigue properties as compared with conventional spring steel stock.

When either the broad composition range or the preferred composition as set forth above is to be employed for heat-treated parts, the silicon content is preferably held to approximately .02-0.30 percent with the steel being preferably characterized as having a fine grain.

To form wire spring stock up to approximately 1.5 inches in diameter and leaf spring stock up to approximately 1.0 inch in thickness, the manganese composition of about 1.10-1.40 in the preferred composition may be maintained. However, for wire spring stock over 1.5 inches in diameter up to approximately 1.875 inches in diameter, as well as for leaf spring stock in the approximate thickness range of 1.0 to 1.25 inches, the manganese content of the preferred composition is held to the approximate range of 1.20-1.40 percent.

What is claimed is:

1. Carbon steel spring stock in wire form of approximately 1.500-1.875 inch diameter and in leaf form of approximately 1.00-1.250 inch thickness, consisting essentially of carbon about 0.55-0.65 percent, manganese about l.20-1.40 percent, sulfur about .05 percent maximum, phosphorus about .04 percent maximum, copper about 0.60 percent maximum, silicon about .25-.45 percent, boron about .0005-.003 percent, balance essentially iron.

2. Carbon steel spring stock of claim 1 which includes from about .4 to .6 percent copper.

3. The carbon steel spring stock of claim 1 wherein the spring stock is oil quenched from a temperature range of from about 1550 F. to 1600 F. and tempered at a temperature on the order of about 700 F.

References Cited UNITED STATES PATENTS 1,992,905 2/ 1935 Wills -123 2,280,283 4/1942 Crafts 75-123 2,527,731 10/1950 Ilacqua 267-1 2,542,220 2/1951 Urban 75-123 2,861,908 11/1958 Mickelson 148-31 OTHER REFERENCES D. K. Bullens, Steel and Its Heat Treatment, vol. III,

John Wiley and Sons, =Inc., New York, 1949, pp. 64-71.

HYLAND BIZOT, Primary Examiner J. E. LEG RU, Assistant Examiner US. Cl. X.R.