US2201425A - Alloy steel - Google Patents

Description

. bides can exist within the range of concentration Patented May '21, 1940 UNITED STATES PATENT OFFICE ALLOY STEEL Karl Torkel Berglund, Sandviken, Sweden, as-

signor to Sandvikens Jcrnverks Aktiebolag, Sandviken, Sweden, a company of Sweden No Drawing. Application April 28, 1938, Serial No. 204,754. In Sweden May 3, 1937 2 Claims.

loy base the composition of which lies within the following approximate limits:

' Per cent Carbon 0. 20- 2. 50 Chromium 8 -22 Iron, maximum about 90 Having regard to the resistancy to corrosion the lower limit of the chromium content of all of these steels maybe placed at about 2%- With less exacting requirements as to resistancy'to corrosion, somewhat lower chromium contents may also be used. The upper limit of the chromium content is at about 20%, having regard to the hardenability of the steels.

The carbon content that is most suitable for difierent fields of use is determined, in the main, by the maximum hardness that it is sought to attain by hardening, and also by the desired qualities of the edge of the tool. The maximum hardness that it is possible to obtain by hardening, increases with increasing carbon content, but simultaneously, the difficulties connected with shaping and working the steel in hot and cold state also increase.

If the steel is to be used for articles having a thin cutting edge of a suificient durability for practical purposes, a carbon content of at least 0.50%, and preferably not lower than 0.70%, is required. An increased carbon content gives as a rule a better edge, but having regard to the possibility of working the steel by cold rolling and cold drawing and to the difficulties of rendering the carbides in a sufficiently finely divided state, a higher carbon content than about 1.35% can hardly be considered for this purpose. The improved edge obtained through an increased carbon content is explained by the increased quantity of hard and wear-resistant carbides. An increased carbide content, however, impairs as a rule the resistancy to corrosionof the steel, due to the chromium content of the carbides being so high that the chromium content of the ground mass is reduced.

According to existing equilibrium diagrams of the alloy system iron-chromium-carbon, two carhere contemplated, namely, a trigonal carbide (Cr,Fe)1Cs and a cubic carbide (Cr,Fe) 4C or, probably more correctly, (Cr,Fe)2aCs. At temperatures above about 800 C. only the trigonal carbide can exist when equilibrium is attained, whereas at temperatures below about 800 C. also the cubic carbide appears together with the trigonal one.

Generically speaking the conclusion may be drawn that for one and. the same carbon content the amount by weight of carbides when in the cubic phase should be 1.5 to 1.8 times the amount obtained if the carbides are in the trigonal phase.

On the other hand, owing to its greater quantity the cubic carbide may occasion that, for a certain chromium content of the steel, the ground mass becomes poorer in chromium, with the resultant risk of inferior resistancy to corrosion.

According to the present invention the carbides or at least the greater portion of the same may be obtained in the cubic phase by incorporating with a chromium steel containing 0.20 to 1.35% carbon and 8 to 22% chromium, a further alloy component of molybdenum, cobalt and copper. The steel according to the present invention is thus mainly characterized by that, in addition to an alloy base of the composition above set forth, and normal amounts of manganese, silicon, phosphorus, and sulphur present in commercial iron and steel, it also contains the combined alloy component molybdenum-cobalt-copper, the content of these elements being each at least 0.20% and together at least 0.60% and at the most of the weight of the steel thus alloyed. Molybdenum may be wholly or partially replaced by tungsten.

It has been proved by tests that the resistancy to corrosion of an alloy steel according to'this invention is not impaired but, on the contrary, is improved both in the annealed state and, above all, in the hardened state.

This twofold effect of a combined alloy component of molybdenum, cobalt and copper was not to be expected in view of the fact that a moderate addition of one of these elements alone does not result in any noticeable deviation from the equilibrium diagram for iron-chromium-carbon alone.

In addition to the gain of an improved resistancy to corrosion due to the said combined addition of alloy components, the advantage of a larger quantity of carbides for a. given carbon content is also gained, and thus greater hardness and increased wearing strength in the hardened state, and it has also been found that the complex cubic carbide that is formed under these conditions, can more easily be obtained in a finely dispersed state in connection with the shapmosphere of carbon dioxide so as to eliminate the risk of the carbides being oxidized by the oxygen of the air. A great number of tests have ven the result that in the case of a chromium steel containing about 1% carbon and from 13 to 14% chromium but having no other intentional alloy components, the quantity of the carbides amount to from 7 to 8 per cent ofthe weight of the steel.

A further addition of moderate quantities of manganese, cobalt or copper does not noticeably change the quantity of the carbides. On the other hand, a certain increase of the quantity of carbides is obtained by an addition of tungsten or molybdenum. If, however, in accordance with the present invention, a composite addition of molybdenum, cobalt and copper is made, for instance, about 1% of each substance, the carbide content of the steel is increased to 18 or 19%. This increase of the carbide content is quite surprising in view of the fact that cobalt and copper each by itself does notincrease the carbide content.

A roentgenographic examination of carbide residues has given the following results: In a chromium steel of this type without extra alloy com- .ponents the carbides consisted of approximately equal parts trigonal carbide and cubic carbide, and thus conformed with the requirements of the equilibrium diagram. On the other hand, the carbide residue of a chromium steel to which the composite component molybdenum-cobalt-copper had been added, proved to consist practically completely of cubic carbide, in spite of the fact that the said steel had been heated during a long period of time at about 860 C. that is to say, at a temperature range at which, accordinging to available equilibrium diagrams, the trigonal carbide only should be present.

The great resistancy to corrosion shown by a steel of the said type containing molybdenumcobalt-copper will be explained in part, it is believed, by the following analyses shown by the carbide residues:

Percent Percent Percent Fe Cr Cal-hides of a chromium steel with Mo-Co-Gu-addition. 5. 64 40. 7 47. 3 carbides of a chromium steel without extra alloy elements 8. 20 26. 63. 4

Per cent Carbon 1.02 Manganese 0.41 Silicon 0.19 Chromium 12.9 Molybdenum 0.95 Cobalt 1.49 Copper 1.00

The analysis of the carbide residue of this steel corresponds to the formula (Cr, Fe, Mn, Mo, C0)22.44Cs.oo

a value which, having regard to the possible accuracy of analysis, corresponds exceedingly well to the theoretical formula (Cr, Fe) 2308, and whichis also in keeping with the results of the roentgenographie examination.

It may be pointed out that the addition of copper appears to be of essential importance to the marked increase in resistancy to corrosion as well as to the formation of the desired cubic carbide.

As regards the manganese content it may be mentioned that in a steel according to the present invention the manganese content should be less than 0.65%, since with the composite alloy component here contemplated a greater optimum hardness can be obtained below this limit than at a higher manganese content.

An alloy steel according to the present invention which is particularly suitable for the manufacture of fine hardened edge tools, such as razor blades and surgical instruments, contains:

For special fields of use it may be desirable slightly to modify the mechanical and other properties of the alloy steels by an addition of further alloy elements, over and above the composite alloy component molybdenum-cobalt-copper. Thus, the steel according to the present invention may contain percentages exceeding 0.05% of one or more of the alloy elements: Beryllium, boron, aluminium, titanium, vanadium, arsenic, selenium, zirconium, niobium, tin, antimony, tantalum, uranium.

Since, as previously stated, molybdenum may be replaced wholly or partially by tungsten, the term molybdenum in the following claims should be construed as covering tungsten also.

I claim:

1. An alloy steel of the martensitic type, which is stainless in the hardened condition, characterized by containing when in hardened condition,

substantially all of the carbides in the cubic phase and possessing great hardness and wearing strength, comprising as its principal alloying ingredients carbon in the proportion of 0.20 to 1.35%, chromium in the proportion of to 22%, nickel less than 0.20%, phosphorus less than 0.05%, molybdenum in the proportion of 0.20 to 5%, cobalt in the proportion'of 0.20 to 5%, copper in the proportion of 0.20 to 5%, the remainder being iron with impurities in an amount which will not alter the properties of the alloy.

2. An alloy steel of the martensitic type, which is stainless in the hardened condition, charac-- terized by containing when in hardened condition substantially all of the carbides in the cubic phase and possessing great hardness and wearing strength, comprising as its principal alloying ingredients carbon in the proportion of 0.70 to 1.35%, chromium in the proportion of 10 to 18%, nickel less than 0.20%, phosphorus less than 0.05%, molybdenum in the proportion of 0.20 to 2%, cobalt in the proportion of 0.20 to 2%, copper in the proportion of 0.20 to 2%, the remainder being iron with impurities in an amount which will not alter the properties of the alloy.

KARL TORKEL BERGLUND.