US2326838A - Deep-hardening silicon steel - Google Patents

Description

Patented Aug. 17, 1943 2,326,838 DEEP-HARDENING SILICON STEEL Walter Crafts, Niagara Falls, N. Y., assignor to Electro Metallurgical Company, a corporation of West Virginia No Drawing. Application March 2, 1940, Serial No. 321,915

Claims. (01. 14831) This invention relates to steels, and more specifically to steels the hardness of which may be increased by rapid cooling from temperatures within or above the critical range. For the sake of simplicity of presentation, the application of the invention will be described herein principally as it may be applied to carbon steels containing between 0.2% and 1% carbon, up to about 2% manganese, and up to about 2% sili con; but as the description proceeds it will be evident that the invention may be applied to a wide variety of hardenable steels.

When hardening steels by rapid cooling from high temperatures, it is often desired to produce a deep or thick zone of hardened material rather than a thin or shallow hardened case. The depth to which a piece of steel will harden, at a given rate of heat extraction, is difierent in difierent steels. The property of the steel which involves this relative susceptibility to mass efiect in hardening seems to be inherent, and for convenience it will be termed herein deep-hardenability.

An accepted, convenient measure of deephardenability is afforded by the Jominy" test, described in detail in A hardenability test for carburizing steel by W. E. Jominy and A. L. Boegehold, Trans. Am. Soc. for Metals, vol. 26, p. 5'74 (1938). To summarize briefly, the test is made on a small bar of steel of standardized shape and dimensions, and comprises heating the entire bar to the desired hardening temperature, quickly extracting heat through one end face of the bar, grinding ofi superficial scale and decarburized skin and producing a flat surface suitable for a hardness test, and measuring the Rockwell C hardness along the length of the bar. The distance from the hardened end at which the hardness becomes less than Rockwell C 50 is referred to herein as the Jominy depth. If the same hardening temperature and cooling conditions be used for a series of steels, the relative depths of the hardened zones indicate the relative deep-hardenability of the steels of that series, each to the others.

An object of this invention is to improve the deep-hardenability of hardenable steels generally, and of plain carbon and low alloy steels (less than 5% in the aggregate of steel-strengthening elements for example chromium, tungsten, molybdenum and nickel) especially.

Further objects -are to provide deep-hardenable steels of novel compositions, and deephardened articles made therefrom.

' The deep-hardenability of steel may be enhanced in some degree by adding in suitable percentages any one of the elements which can be taken into solid solution in steel at high temperatures, for instance, boron, silicon, molybdenum, tungsten, chromium, or nickel. These hardening elements diifer widely in the minimum percentage required to impart a useful degree of deep-hardenability, and in the maximum degree of deep-hardenability obtainable while maintaining other necessary properties of the steel.

1 have observed that combinations of certain elements may be used to impart a greatly enhanced deep-hardenability without substantial sacrifice of other desirable properties of the steel. The present invention is based on these observations.

More specifically, the invention comprises the addition to a hardenable silicon steel of more than two of the special elements, aluminum, magnesium, calcium, barium, strontium, titanium, zirconium, and vanadium, in a suitable percentage greater than that required for grain refinement.

The silicon content of the steel may suitably be between 0.1% and 2%. If the silicon content is above 0.35%, the total proportion of said special elements may be any proportion substantially greater than that required for grain refinement, up to say 1%, or somewhat higher. The percentage required for grain refinement will depend on the kind of steel, the steelmaking conditions, and the kind and number of deoxidizing and grainrefining elements used, and may be determined empirically by the methods now in use by metallurgists and steelmakers. 'In most instances, at least 0.03%, and preferably at least 0.1%, of said special elements are added, and it will not ordinarily be necessary to add more than 0.25%.

If the silicon content does not exceed 0.35%, somewhat more of the special elements must be added to achieve a useful enhancement of hardenability. In general, the aggregate percentage of residual silicon plus added special elements should exceed 0.35%, and preferably exceeds 0.65%.

When the above-described percentages of silicon and special elements are employed, a medium manganese oil hardening type of steel such as the S. A. E. 1345 type may be hardened to 50 Rockwell C or higher throughout a circular section oneinch, or even more, in diameter.

For each element and each combination of elements there appears to be an optimum percentage which imparts a maximum depth of hardenability and a frequent result of an increasein percentage beyond the optimum is a decrease; of deep-hardenability below that imparted by the optimum. For reasons of economy, or to obtain a steel having a certain desired combination of physical properties, it will often be desired to add either less or more of the elements than will impart a maximum depth of hardening. Hence, the invention is not limited to the use of the optimum percentages.

In most cases it will be preferred to use alkaline earth metals only in combination with other elements of those listed above, inasmuch as their beneficial effects are considerably greater when they are used in such combinations than when they are used alone.

Typical effects of representative combinations used according to this invention are indicated in Table A, which contains figures derived from actual test data. The relative Jominy hardenability depths are given, in hundredths of an inch, of steels containing besides iron the perand manganese are by analysis, the percentage of silicon is by analysis except where indicated to be nominal, and the percentages of the remaining element are the percentages added to the steel just before casting. In Table 0, the yield point (Y. P.) and tensile strength (T. S.) of the steels of Table B are given in thousands of pounds per square inch; percent El. designates percentage elongation in a two inch initial gage length, and percent R. A. designates percentage reduction in area, upon fracture of the 0.505 inch diameter standard (A. S. T. M.) tensile test specimens. Under Izod are given the Izod impact test results, in foot-pounds, using a standard specimen one-centimeter square with standard V notch one mm. deep. The Jominy hardness test depth is that actually measured, in hundredths of an inch, to Rockwell C 50, the same hardening temperature and cooling conditions having been used in all hardness tests.

Table B centage of silicon and special elements indicated. 0 The same hardening temperature and cooling steels (rest 1mm) conditions were used in all tests.

Steel Pert P61; P61; Per; Per Pert cen cen cen cen cent cen Table A 0 Mn I Si Al 2: v

Percentage addition to Jominy hardness test, depth in 4 0.51 1.68 0.6002) 0.535 0.035 0.035 steels containing 0.25% hundredths of an inch to Rock- 5 0.61 1.61 0.50m) 0.035 0. 535 0. 035 to 1% Si, about 1.0% Well 0 50, corrected to 0.45% C 8 0.48 1.70 0. 75(1t) 0.105 0.105 0.105 Mn, 0.45% to 0.55% C, by factor: rest Fe :l=0.0l% Q==l=0.012 inch (n)=Nominal. Percent Percent Percent 2 Table C Zr V steel steel steel steel Properties of steels of Table B 0. 035 0. 032 0. 23 $7 0.105 0.1 0. 4 6 l N 0.535 0. 035 0. 035 e1 swel 0 P Per 15:33:51 0.035 0.535 0.035 67 i (I) Y. P T. S cent cent Izod test El. ILA. depth 1 Not determined.

207.0 7.0 25.4 7.5 67 The fact that considerable deep-hardenability 213.3 0.5 32.1 7.5 74 is imparted by any of a large number of combinations of elements is an important one, because the latitude in choice of elements affords an opportunity to control the cleanness, grain size, tensile strength, and toughness of the steel through selection of appropriate combinations. In general, the deep-hardening steel of best quality for most purposes will be obtained by the addition of more than two of the special elements described above, within the range of percentages suggested herein. This generalization holds true not only for the steels of the S. A. E., 1345 type (medium manganese oil hardening) chosen for purposes of exemplification in Table A, but for low-alloy steels of all types, including those containing one or more of the elements chromium, tungsten, molybdenum, nickel, phosphorus, sulfur, etc. However, it is not to be understood that all properties necessarily improve in a consistent procession as the number and percentage of added elements increases. It must be 'borne in mind that the relative effectiveness of different elements is not the same, and also that certain combinations of elements are better in some respects than other combinations comprising the same number of elements The high quality attainable in steels according to this invention is indicated in Tables B and C by test data of the physical properties of several representative steels after forging, quenching from 850 C., and drawing at 400 C. for one hour. In Table B, the indicated percentages of carbon The complex deoxidizing agents described in the patents numbered 2,221,781; 2,221,782; 2,221,783; 2,221,784; and 2,269,407, granted on joint applications by James H. Critchett and Walter Crafts, are admirably suited for use in accordance with the present invention, provided they are used in a proportion substantially greater than that required for grain refinement.

I am aware that Some slight increase in deephardenability has been observed to accompany the grain coarsening brought about by adding, to a low silicon steel, aluminum in excess of that required for grain refinement. Such effect should not be confused with the considerable increase in deep-hardenability obtained according to this invention in the high silicon alloys.

The advantages of the invention may be exploited in any of several ways. For instance, the cheaper steels among those described above may be used instead of more expensive, more highly alloyed steels heretofore used to obtain the desired strength. Or, present high strength steels may be even further strengthened by applying the principles of the invention, either by deeper hardening to a lower average hardness or by deeper hardening to the same or even higher hardness. Among the various steels described, there is a wide range of choice in respect to such factors as cost, grain size control, ductility, strength, toughness, and types of inclusions. Thus, the invention is capable of a Wide field of asaaase application, which will be apparent to metallurgists and steelmakers. Therefore, although numerous specific examples have been given herein to illustrate the principles of the invention, it will be understood that such examples ar merely illustrative and do not restrict the invention beyond the requirements f the claims and the state of the art.

I claim: 1. A quench-hardened article composed.of a deep-hardenable steel comprising 0.1% to 1% silicon; between 0.2% and 1% carbon; manganese in a percentage up to about 2%; vanadium in a percentage between 0.03% and 1%; and between 0.03% and 0.25% of each of the grain refining elements aluminum, zirconium, and titanium; the

aggregate percentage of silicon and said grain refining elements being at least 0.65%; remainder iron, such steel having the property of being quench-hardenable to upwards of 50 Rockwell C to a depth at least 0.3 inch below the surface throughout a one inch diameter circular section.

2. A quench-hardened article composed of a deep-hardenable steel comprising 0.1% to 2% silicon; between 0.2% and 1% carbon; manganese in a substantial percentage up to about 2%; 0.03% to 0.5% vanadium; 0.03% to 0.5% aluminum; 0.03% to 0.5% zirconium; 0.03% to 0.5% titanium; remainder iron and incidental impurities; the aggregate percentage of silicon, vanadium, aluminum, zirconium and titanium being at least 0.35% and the aggregate percentage of aluminum, vanadium, zirconium and titanium being not over 1%; such steel having the property of being quench-hardenable to upwards of 50 Rock-' well C to a depth at least 0.3 inch below the surface throughout a one inch diameter circular section.

3. A quench-hardened. article, other than a nitride case-hardened article, composed of a deep-hardenable steel comprising 0.1% to 2% silicon; 0.2% to 1% carbon; manganese in a substantial percentage up to about 2%; at least 0.03% but less than 0.5% aluminum; at least 0.03% of each of at least two of the grain refining elements titanium, vanadium and zirconium, the aggregate remainder iron and incidental impurities; the eggreg'ate percentage of silicon, aluminum, and

' said grain refining elements being at least 0.65%;

such steel having the property of being quench hardenable to upwards of Rockwell C to a depth at least 0.3 inch below the surface throughout a one inch diameter circular section.

4. An article as claimed in claim 3, further characterized in that it contains at least one of the alkaline earth metals magnesium, calcium, barium and strontium, such metals being added in an aggregate amount of between 0.03% and 0.5%.

5. A quench-hardened article as claimed in claim 3, other than a nitride case-hardened article, composed of a deep-hardenable steel having the property of being quench hardenable to upwards of 50 Rockwell C to a depth at least 0.3 inch below the surface throughout a one inch diameter circular section, and further characterized in that said steel comprises 0.1% to 2% silicon, 0.2% .to 1% carbon, manganese in a substantial percentage up to about 2%, at least 0.03% but less than 0.5% aluminum, 0.03% to 0.97% zirconium, and 0.03% to 0.97% vanadium, the remainder iron and incidental impurities; the aggregate percentage of zirconium and vanadium being not over 1% and the aggregate percentage of silicon, aluminum, zirconium, and vanadium being at least 0.65%,

WALTER CRAFTS.