US2354147A - Steel - Google Patents

Description

July 18, 1944. H. SCOTT 2,354,147

STEEL Filed Jan. 14. 1942 Fi i 1.

legend DPH Re 700 59-5 750 62.0 800 64.0 8 5 65-0 850 66.0

BENDING STRENGTH IN PSI x10 s:

a v 4 700- S: 0.704 Mo) BENDING STRENGTH 11v Psi x10 v 9 00 l l I l I I n I l I 960 85' 800 750 700 650 600 DP]! WITNESSES: HARDNES 5 INVENTOR W Howard Scott Patented July 18, 1944 2,254,141 sruar.

Howard Scott, Forest Hills, Pa" assignor to Westinghouse Electric & Manufacturing Company, mEastPittsburgh, 2a., a corporation oi Pennsyi- Application January 14, 1942, Serial No. 426,686

4 Claims.

This invention relates to steel.

Tool steel has its mai zimum hardness as quenched, but in this condition it is found that the steel has a minimum resistance to shock and bending stresses. Therefore, the steel cannot be used at its highest hardness where the steel has the greatest resistance to wear, but instead it is necessary with the tool steels utilized heretofore to temper the steel to a lower hardness to increase its bending strength. As a result, the resistance to wear and penetration is decreased in order to obtain adequate toughness for use in the industry.

In classifying tool steels as tortoughness, the bending strength of the steel may be taken as a criterion because the tools are usually loaded in service by bending, and the bending strength is a measure of toughness, there being little plastic deformation in hardened tool steels at the breaking load. A bending strength test which can easily be duplicated is to quench a rectangular bar A x x 3 inches as machined to size after heating in a protective atmosphere without change in the surface carbon content, after which the steel may or may not be tempered to several lower hardness values. A load is then applied at a point on the flat side of the bar midway between roller supports two inches apart in a Brinell hardness testing machine. The break-' ing load recorded is a measure of the maximum fiber stress at the time of fracture, assuming purely elastic deformation, and the bending strength is, therefore, in this test identical with the modulus of rupture.

Among the tool steels employed heretofore, the plain carbon tool steel having a carbon content of about 1.1% has the highest bending strength excluding the high speed steels. The high speed steels, however, are quite expensive and their use is not justified unless the characteristic of heat resistance is required in addition to strength. The plain carbon steel cannot be hardened without high residual stress, and where the steel is employed in tools or the like having a diameter of over about inch, it is impossible to completely harden the steel to the center of the article to minimize the residual stresses. Different attempts have been heretofore-.made to produce a steel which will have a high bending strength at high hardness, but these attempts have not been successful.

It is an object of this invention to provide a steel having a high bending-strength and high hardness.

Another object of this invention is to provide a carbon tool steel having as essential alloying elements thereof silicon and molybdenum in predetermined amounts for increasing the bending strength of the steel at high hardne. values.

Other objects of this invention will become apgrant from the following description when taken in conjunction with the accompanying drawing, in which:

Figure 1 is a graph, the curves of which illustrate the effect of combined silicon and molybdenum contents in different amounts on the bending strength of steels at different hardness values; and

Fig. 2 is a graph, the curves of which illustrate the improvement in the bending strength at high hardness values where silicon and molybdenum are employed as essential alloying elements of the steel.

The steel of this invention is formed by following the usual metallurgical practices. In accordance with this invention, the following listed alloying elements are present in the steel in amounts by weight within the ranges given, it being understood that the balance is iron with not over 20% of impurities:

Per cent Carbon .80 to 1.20 Manganese .15 to 1.00 Nickel Not over .15 Chromium .05 to 1.50 Aluminum Vanadium} f Molybdenum .50 to 5.00 Silicon .50 to 3.50

Chromium, nickel, and manganese are usually employed as alloying elements of steel in order to confer deep hardening to the steel. With the steel of this invention, however, the amount of such elements is limited for it is found that such alloying elements if employed in substantial amounts greatly reduce the bending strength of the resulting steel. For this reason, the chromium content is maintained at not more than 1.50%, itbeing found that with up to this amount of chromium the other alloying elements, such as silicon and molybdenum, counteract the detrimental eifect of the chromium on the bending strength.

' to completely eliminate nickel and to maintain the manganese content at less than .50% in the steel, although up to .15% of nickel and up to 1.00% of manganese can be tolerated.

Aluminum and vanadium, either by themselves or combined, function in the steel as grain refining elements, it being found that where from .01 to 0.5% of either aluminum or vanadium or both is employed that an extremely fine grain is secured in the resulting steel. No more than .5% of aluminum or vanadium should be used, howev r, because of the detrimental effect of these constituents on other characteristics of the steel.

Chromium, nickel, manganese, and carbon within the ranges given will not of themselves impart the required degree of hardenability to the steel. The silicon and molybdenum contents of the steel when present in a preferred amount are effective for simultaneously increasing the hardenabiiity and the bending strength of the steel. In practicing this invention, it is preferred to maintain the silicon and molybdenum contents in a preferred relation in which Si+0.'l Mo) =3 to 5.5%

it being found that this relation of the silicon and molybdenum gives unusual bending strength to the steel at any given hardness above 700 DPH when the remaining alloying elements fall within the range given hereinbefore.

'In the following table, there is listed the analysis of a number of steels comprising this invention as well as a 1.1% carbon tool steel and others included for comparison purposes. In the steels listed. it is of course understood that the remainder of each of the steels constitutes iron with not more than 0.20% of impurities present therein.

These steels are preferably quenched from a temperature between 1550" F. and 1650 F. to produce maximum hardness.

When tested in the manner referred to hereinbefore, it is found that the steels listed in the table given hereinbefore have different bending strengths at different hardness levels. By referring to the following table, the bending strength of the steels listed in the foregoing table having difl'erent hardness values is given:

Bending strength in p. s. i/i000 st- Steel N0.

By referring to the table, the eflect of the alloying constituents of the steel on the bending strength may be ascertained. If the carbon content of the steel is below the lower limit of the preferred range of the carbon, then it is found 76 that the steel does not have the required bending strength at high hardness values. This is evident from the steel identified as R880 in the table, the carbon content of that steel being only .73%. Thus, even though the silicon and molybdenum contents are present in accordance with the formula given hereinbefore, if the carbon content is low, the steel will not have the required bending strength. Likewise, if the steel has a high manganese content, such as is present in the steel identified a M131, the resulting steel will not have required bending strength.

As can be seen from the foregoing tables, the bending strength of the steel tends to rise with an increase in the silicon and molybdenum content of the steel it being found that the carbon, manganese, chromium and vanadium has substantially no effect on the bending strength of the steel when maintained within the ranges given. The effect of the combined silicon and molybdenum content on the bending strength of the steel at the high hardness values is more clearly illustrated by reference to Fig. 1 of the drawing. In this figure, each of the curves is illustrative of the bending strength of the steel at a particular hardness, as shown, the silicon and molybdenum content varying in accordance with the formula s1+0.7' Mo) =.2% to 5.5%

As is clearly illustrated where the Si+0.7 Mo) =3% to 5.5%

and the alloying elementsare present in the ranges given hereinbefore, unusual and unexpected bending strength is obtained in the steel at hardness values ranging from 700 to 850 DPH.

In the steel the silicon is somewhat more effective than molybdenum in increasing the bending strength at the high hardness values. An appreciable molybdenum content is desired, however, as it appears to be more effective than silicon in conferring hardenability. Therefore, at least 0.50% Of molybdenum is required in the alloy, the upper limit being not more than 5%.

Referring to Fig. 2 of the drawing, the curves illustrate the eflect of the silicon and molybdenum content on the bending strength of the steel at high hardness values, curve ll being representative of the bending strength of a No. 3 tool steel an unalloyed steel having a carbon content of 1.1%, curve I! being representative of the bending strength of steel No. 681 listed in the foregoing table, and curve It being representative of the steel No. 4268 listed in the foregoing table. As illustrated by these curves, an increase in the combined silicon and molybdenum content in accordance with the formula Si+0.7 Mo) =3 to 5.5% eil'ects an increase in the bending strength of the steel at high hardness values.

In considering the bending strength of the alloys and the eifect of the alloying elements, silicon and molybdenum thereon, it is essential to recognize that their effect is entirely different at the hardness levels considered herein than at the lower hardness levels. Again referring to Fig. 2, it is seen that the bending strength of the 1.1% carbon tool steel represented by curve Ill increases to a high maximum at about 650 DPH as the hardness is reduced by tempering. At lower hardness values, the controlling factor determining rupture strength is shearing strength, and the fracture has a shearing component at the edges which is absent in the fractures produced in the steel at higher hardness values. The bending strength obtained at high hardness levels, as referred to herein is, therefore, a measure of cohesive strength of the steel. The improvement in the bending strength at the high hardness levels where the shearing component is missing obtained by utilizing silicon and molybdenum as essential alloying elements of the steel of this invention is quite unexpected. 1111s is especially true when it is considered that heretofore it was thought that the addition of alloying components had very little eifect on the mechanical properties of steel so long as the steel was completely and uniformly hardened as is the case where the steel is hardened at hardness values where a shear component is in evidence on tensile fracture tests. Contrary to the belief prevalent among those skilled in the art prior to this invention, the silicon and molybdenum content of the steel of this invention when present in the relation given hereinbefore is effective for producing a steel having materially higher bending strength at hardness values in the range of 700 to 850 DPH than is available in carbon tool steel or any other steel heretofore known outside of the high speed steel class.

Thesteel identified in the table given herein before as No. 4268 has an exceptionally high bending strength at the high hardness levels of 800 to 850 DPH and is especially useful as tools,

such as chisels, and armor piercing projectiles or as other articles where high bending stresses must be resisted before-penetration is accomplished. The steel identified as R681 is also particularly useful for these applications, it being found that both steels 4268 and R681 have unusual high bending strength at the high hardness levels.

For these particular applications, it is preferred to employ a steel containing, by weight, from 0.90% to 1% of carbon, from 0.30% to 0.70% of manganese, from 0.3% to 0.5% of chromium, from 1.2% to 2.5% of silicon, from 3% to 4% of molybdenum, a grain refining amount of vanadium and/or aluminum which is preferably below .4%, and the balance iron and incidental impurities. Such steels are particularly effective where the silicon and molybdenum content are present in accordance with the formula Although this invention has been described with reference to a particular embodiment thereof, it is, of course, not to be limited thereto except insofar as is necessitated by the scope of the appended claims.

I claim as my invention:

1. A steel comprising from 0.80% to 1.20% of carbon, from 0.15% to 1.00% of manganese, not more than 0.15% of nickel, from 0.05% to 1.50% of chromium, from 0.01% to 0.50% of metal selected from the group consisting of aluminum and vanadium, from 0.50% to 5.00% of molybdenum, from 0.50% to 3.50% of silicon and the balance iron, the silicon and molybdenum being present in accordance with the formula:

% silicon+0. 7 molybdenum) =3.o% to 5.5%

2'. A steel comprising about 0.93% of carbon, about 0.67% of manganese, about 0.37% of chromium, about 0.39% of vanadium, about 2.50% of silicon, about 3.0% of molybdenum, and the balance iron.

3. A steel comprising from 0.90% to 1.00% of carbon, from 0.30% to 0.70% of manganese, from 0.3% to 0.50% of chromium, from 1.2% to 2.5% of silicon, from 3.0% to 4.00% of molybdenum, a grain refining amount but less than .4% of vanadium, and th balance iron.

4. A steel comprising from .90% to 1.00% of carbon, from 0.30% to 0.70% of manganese, from 0.3% to 0.50% of chromium, from 1.2% to 2.5% of silicon, from 3.0% to 4% of molybdenum, from .20% to .40% of vanadium, and the balance iron, the silicon and molybdenum being present in accordance with the formula:

% silicon-+0.7 molybdenum) =4.5%

HOWARD SCOTT.

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