US2155349A - Alloy steels - Google Patents

Description

j uses in the arts.

Patented Apr. 18, 1939 UNITED. STATES? PATENT OFFICE Chicago, 111.; said United States Steel Grossmann assignor to Corporation, New York,

N. Y., a corporation of New Jersey No Drawing. Original application September 14, 1935, Serial No. 40,655. Divided and this application November 2, 1936, Serial'No. 108,899

2 Claims.

The present invention relates to alloy steels.

This application is a division of the application for. United States Letters Patent filed on September 14, 1935, by the present applicants, for improvements in Alloy steels, which application bears Serial No. 40,655.

In the strict sense, steel may be said to be an alloy of iron and carbon. However, in a metallurgical sense, in which the term is used in this specification, an alloy steel is steel (that is-iron and carbon) alloyed with another chemical element or other chemical elements.

An object of the present invention is to produce an alloy steel which is well adapted to resist failure by fatigue, especially for use in highly stressed parts, such as springs.

A further object is to produce an alloy steel for general purposes which involves no special equipment in its manufacture and which has improved characteristics over steel as heretofore known and used. I

A further object is to provide a steel well adapted for use in cushioning springs, which steel will effectually resist deterioration in service.

A further object is to provide an alloy steel in which the characteristics of vanadium are utilized to advantage in the manufacture of alloy steels.

A further object is to provide an improved steel well adapted to meet the needs of commercial operation.

Further objects will appear as the description proceeds.

The present invention contemplates the accomplishment of the objects above referred to by providing a shallow hardening alloy steel. To the skilled steel manufacturer the term shallow hardening alloy steel has a definite significance, meaning a steel which, when heated to above its critical temperature and then cooled, has a relatively pronounced tendency to change rapidly from the austenitic to the pearlitic type of internal structure. Steel of fine grain structure tends to be shallow hardening, though of .course certain alloy steels, even though they be fine grained, may not be shallow hardening in comparison with the steels for which invention is claimed herein. Of course it is possible to pro duce shallow hardening steel by using a very restricted amount of carbon, and such steel has its The present invention is limited to -steels in which the carbon content is equal to or greater than 45% by weight.

In defining the degree of shallow hardening contemplated in the present invention it may be observed that certain alloy steels, as for example nickel chromium steel, have characteristics differing from steels contemplated in the present invention.- As is well known, alloy steels as ordinarily known to metallurgists will, when heated to a temperature of say 1600 deg. F., form austenite, which is understood by metallurgists to be a solution of carbon in iron. As the steel is cooled from the temperature referred to, the structure changes, and when the steel reaches the neighborhood of about 900 deg. F. there is a tendency to form pearlite, which is a tough substance of low hardness. In the case of most alloy steels as now known and used, as for example the nickel chromium steel above referred to, the length of time in the temperature zone in the neighborhood of 900 deg. F. required for the formation of pearlite is in the neighborhood of 20 seconds. The temperature zone referred to may range from 1050 to 850 deg. F. In the case of the alloy steels forming the subject matter of the present invention, the period of time required in the 900 deg. zone referred to is in the neighborhood of 8 seconds. Considering a body of steel which has been heated to say about 1600 deg. F. and quenched in water, it will be readily understood that in the case of nickel chromium steel or the like the period of dwell in the 900 deg. zone referred to may be too short to form pearlite, but said period of dwell may be ample for the formation of pearlite in the composition of shallow hardening steels. In other words, using the example above referred to, the period of time in which the center of the member under test is in the temperature zone of 900 deg. F. above referred to may be less than 20 seconds but more than 8 seconds. In the event that pearlite has not formed in the quenching operation, the alloy steel, upon further cooling, will form martensite, which is a hard material. In the event that pearlite has formed in the center of the member under test in the quenching operation, this pearlite will retain its identity when the member under test has cooled to atmospheric temperature. The above example illustrates a measure which may be applied to define the degree of hardenability contemplated in the present invention. For convenience, the measure of hardenability may be expressed in connection with an alloy steel bar. The present invention contemplates a composition of alloy steel such that when a cylindrical bar of 1 inches or more in diameter of homogeneous composition is heated to a temperature of approximately 1600 deg. F. and then quenched in still water at room temperature, the material at the axis of said bar will, upon cooling to room temperature,

have a Rockwell hardness of not more than,

' 1 inch or more in diameter of homogeneous composition is heated to a temperature of approximately 1600 deg. F. and then quenched instill oil at room temperature, the material at the axis of said bar will have a Rockwell hardness of not more'than 50C.

It will be understood, of course, that the references immediately above made to bars of specified diameters are merely for convenience of definition of one property of these steels. The steels, of course, are useful in members of all dimensions including small sections in which the steels harden through to their axes or midsec' tions.

In carrying out the present invention it is contemplated to use in the manufacture of the improved alloy steel a predominating amount of vanadium either with or without other shallow hardening elements. correspondingly, the present invention contemplates a relatively low amount of the deep hardening elements (such for example as manganese, chromium and/or nickel). The relative terms referred to in the preceding sentence have reference to the functions of the alloying elements referred to as commonly used in alloy steels in common use today.

Proceeding to a more specific definition of alloying elements contemplated in the present invention, the following observations may be made:

Since vanadium reaches its full effectiveness at approximately .15%-.2% by weight, a predominating amount of vanadium in the alloy steel according to the present invention would be approximately .075%-.2% by weight.

On the other hand, chromium may be said to have its greatest effectiveness at about 1.5% by weight, so that a predominating amount of chromium would be about .'75%-1.5%.

Since manganese reaches its maximum effectiveness at about 2% by weight, a predominating amount of manganese would be about l.25%-2% by weight 7 Since nickel reaches its full effectiveness at about 3% to 4%,, a predominating amount of nickel would be about 1%-4% by weight.

Therefore, expressing the purpose of the pres ent invention in other language, it may be said that the present invention contemplates the use of vanadium in the amount of .075%.2% by weight, said element being used without chromium or manganese if preferred; or, if chromium be used, then with less than .75%'-by weight of chromium; or, if manganese be used,then;w'ith less than 1.25% of manganese. Expressed in still other language, certain advantages of the present invention will be had if the shallow hardening element (vanadium) is used to a percentage'equal 'to at least 50% of its fully effective amount in is by reference to the "critical cooling rate of the materials. Critical cooling rate'of steels may be defined as that rate above which a steel will harden in the cooling process and below which a steel will not harden. When the term harden is used in this connection it is intended to convey the meaning that the material of the steel becomes the hard material martensite. The present invention contemplates steels which when quenched in a quenching medium at atmospheric temperatures will just harden if it passes 1325 deg. F. at a rate above 50 deg. per second but below 150 deg. per second.

Those skilled in the art will understand that manganese is usually present in the manufacture of steel to the extent of about .2 %-.5% by weight, since manganese is commonly used as a deoxidizer in steel making; and for the same reason silicon is usually present in amounts of about .2%.

' Proceeding now to a still more specific recitation of analysis of steel falling within the scope of the present invention, the following example may be recited:

Per cent Carbon from .85 to Manganese from .20 to .50 Silicon from .10 to .35 Chromium from .20 to .50 Vanadium from .075 to .20

Balance largely iron with traces of impurities.

It will be understood, of course, that the analysis above noted is illustrative only, and the objects of the present invention may be realized by proper balancing of shallow hardening elements against deep hardening elements. For example, the carbon may vary from .30% to 1%;

the manganese may vary from .40% to 1.25%; the

chromium may vary from 0 to .75%; the vanadium may vary from .075% to .20%. A choice of the elements referred to to provide a shallow hardening steel meeting the tests recited in this specification will fall within the scope of this invention.

Anyone familiar with the manufacture of alloy steels has readily available to him information as to the constituents and the amounts thereof which must be used in producing the steels havgrain size as used in this specification means 1 from #5 to #8 on the ASTM chart of grain sizes. This may be further explained as meaning more than12 grains per square inch when examined at r a magnification of diameters. The fact is well known that fine grain size tends to shallow hardening; It is also well known that very fine grains, particularly #7 and #8, according to the ASTM chart, exhibit structural features known as abnormality, meaning thereby a particular distribution of the iron carbide (that iscementite) observed in the carburizing test (McQuaid-Ehn carburizing test), which test is used in determining grain size.

As is well known, the usual method of discover ing the austenite grain size of the steel is to carburize said steel at about 1700 deg. F. for about eight hours. The carbon absorbed in this carburizing operation will upon cooling arrange itself at the boundaries of the austenite grains and so outline them, thus making it possible to determine the size of these austenite grains. In coarsegrained steels this carbon (which is usually present in the form of iron carbide or 'cementite) arranges itself in continuous layers or envelopes which completely and clearly outline the austenite grains. In very fine-grained steels, however, the iron carbide coalesces into separate particles, so that the grain boundaries are not outlined continuously. This latter condition of the iron carbide (cementite), associated with a small amount of ferrite in its immediate neighborhood, is often termed divorcement of the iron carbide or cementite and is commonly known as abnormality. Divorcement of the cementite or iron carbide has been referred to by some as dispersion of the cementite.

In order to arrive at fine grain size (including abnormality) it has .been common to add, in the furnace ladle or in the molds, about one pound of aluminum per ton of steel. It has been discovered that if vanadium be used as one of the shallow hardening elements, the amount of aluminum may be materially reduced. Vanadium tends to produce fine grain structure and shallow hardening in steel over a relatively wide temperature range in the hardening operation.

The alloy steels forming the subject matter of this invention are characterized by a high degree of ductility, toughness, resiliency and resistance to shock and fatigue.

As is well known, silicon is used in the manufacture of allow quality steels for the purpose of combining with the oxygen and thereby removing said oxygen. An excess of silicon always appears in the finished steel of this type and may properly be considered to be a trace of impurity. According to the appended claims such silicon will be included under the term traces of impurities."

Though certain preferred analyses and steps have been recited in the foregoing specification, many modifications will occur to those skilled in the art. It is intended to cover all such modifications that fall within the scope of the appended claims. 1

What is claimed isl. A shallow hardening alloy steel which is composed of the following ingredients substantially in the amounts specified:

Per cent Carbon from .85 to .95 Manganese from .20 to .50 Chromium from .20 to .50 Vanadium from .075 to .20

Per cent Carbon from .45 to 1 Manganese from .40 to 1.25 Chromium from--- an appreciable amount to .75 Vanadium from .075 to .20

remainder iron with traces of impurities.

WALTHER MATHESIUS. MARCUS A. GROSSMANN.

CERTIFICATE OF CORRECTION.

April 1 19 9.

. WALTHER MAIHEsIUs, ET AL. It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line 26, for "95" read .95; page 5, first column, line 57, for the Word "allow" read alloyyend that the said Letters Patent should be read Patent No. 2,155,5149.

with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 15th day of June, .A. D. 1939.

(Seal) Acting Commissionerof, Patents.