US2177454A - Alloy steel for internal combustion valves or valve elements - Google Patents

Description

' Patented Oct. 24, 1939 UNITED STATES PATENT OFFICE Barry L. Frevert and Francis E. Foley, Philadelphia,'Pa., assignors to The Midvale Company,

Philadelphia,

Pa., a corporation of Delaware No Drawing. Application February 23, 1938, Serial No. 191,966

8 Claims.

The problem of producing a steel suitable for use in the exhaust valves of internal combustion engines is one which has long engaged the atten tion of metallurgists because of the constantly increasing severity of the" conditions of service. Ethyl lead in gasoline, higher compression ratios, increased temperatures, have each presented in turn a fresh problem for solution. Material satisfactory at one stage of development has been made obsolete by the next advance in combustion engineering.

Many patents have been granted for compositions suitable for the valve requirements of their corresponding dates, but today automotive engineers are demanding a new steel reasonable in cost, free from objectionable hot oxidation, free from pick-up, and strong enough at operating temperatures to withstand the stresses of service, especially the tendency of valves to umbrella or flatten, with loss of sealing power. A tensile strength of about 6000 pounds per square inch at 1600 F. has been in the past considered quite satisfactory, and approximately this hot strength characterizes one of the valve steels in widest use today, but valve steels of such limitedhot strength, even though possessing good resistance to scaling, do not satisfy even present day requirements and their abandonment as a valve steel in the near future awaits only the development of a valve steel possessing a greatly superior hot strength without sacrifice of scaling resist ance.

Alloys of the essentially Cr-Mn type have heretofore been used almost exclusively for their physical properties and their resistance to staining, at atmospheric temperatures; but their resistance to scaling has been little investigated and their strength when exposed to high temperatures not at all, so far as we are aware. It is in this field that our invention lies.

An alloy steel of high, low and medium carbon.

content and containing chromium within a broad range of 10% to 45%, manganese within a broad range of 3% to 25%, with or without silicon up to 3%, with or without a small percentage of nickel, is disclosed in an expired patent. Certain alloys containing constituents whose proportions would respond to'the exceedingly broad ranges of the patent may be, as claimed for them, re-

sistant to oxidation at high temperatures, or to stain, to be readily forgeable and machinable and to have a high tensile strength relative to cast iron; but we are aware of no disclosure of any specific compositions within such broad ranges that would have, in addition to these qualities, the vital requirement of high strength at the elevated operating temperatures to which valves for internal combustion engines are subjected. I

As an example of a chromium-manganese that 5 would respond to these broad ranges, there has been developed more recently an alloy steel adapted more particularly to the manufacture of articles made of sheet metal and containing carbon less than 3%, chromium 16-22% and manganese 6% to 14%, with or without nickel. This steel alloy is subjected to a heat treatment to produce a material having the great strength and toughness essentialfor the deep drawing, cold,

of articles made of sheet steel. But it is wholly 15 deficient in the combination of qualities required ,for valve steel.

While the resistance to scaling of alloys of the class above discussed is superior to that of less highly alloyed steel, they do not possess sufiicient strength for satisfactory continuous service as internal combustion exhaust valves at the high temperatures of operation. Knowledge ,of such alloys does not teach how to secure a material suitable in both respects for use as internal combustion engine valves, and none of them discloses such proportioning of the alloying ingredients as would produce a steel having the characteristics now deemed necessary for such valves.

Annealing boxes have been made of a manganese-chromium-copper-aluminum alloy-a composition patented by us November 7, 1933, No. 1,933,900-having sufiicient strength when hot, and sufiicient toughness when cold, for the pur-- 86 pose intended, besides a high resistance to oxidation. For valves of internal combustion engines this composition is not satisfactory, having too low hot strength combined with a tendency towards red-shortness. This tendency towards 4o red-shortness can be corrected by substituting nickel for much or all of the copper, and we find that a reduction in the amount of chromium present increases the hot strength, although at the loss of part of the scale resistance unless means are taken to overcome it.

For various purposes, including valves and valve seats for internal combustion engines, it is known to use a chrome-nickel or manganesemolybdenum or tungsten alloy and a chrome- 60 nickel or manganese-silicon-molybdenum or tungsten alloy, with proportions of these elements that shall produce a steel initially ferritic, as cast, rolled, forged or annealed. These alloys may be called high chromium alloys, since they contemplate the use of a minimum of 18% chromium and a maximum as high as 35%. They are hardened by the simple operation of heating without subsequent accelerated cooling. The proportions of the constituent alloys have, however, been so adjusted as to respond to these two conditions. The type of hardening to which the materials respond is that well-known as dispersion or precipitation hardening; In the case of steels, it is accompanied by a serious loss of toughness which may limit or prevent uses for which the material would otherwise be well suited.

There is also known a nickel-chromium-silicon steel for exhaust valves, with carbon .20-2.00, nickel over .65 and less than 4, chromium 10-25, and silicon from more than 1-6. This is also a ferritic or alpha steel with a high critical temperature.

On the other hand, ferrous alloys wholly or predominantly austenitic by nature have critical temperatures below atmospheric and generally harden but little by simple heating; conversely, they tend to soften under the influence of heat much more slowly than do the usual ferritic alloys and they maintain a very high degree of toughness. It is in this field of alloys wholly or predominantly austenitic that our invention lies. A simple way of differentiation between our alloys and the two types of alloys last mentioned is that our alloys attract a magnet not at all or but feebly, whereas the other alloys are all strongly magnetic.

We have found that sufficient hot strength can be obtained in an austenitic steel containing medium chromium by the use of an appropriate amount of nickel or a less amount of nickel in connection with a considerably greater amount of manganese, nickel being a much more effective hardener than manganese. Both elements suffer in this connection from the same defectthat of causing a loss in scale resistance of the alloy at high heat.

While it would seem that appropriate additions of silicon or chromium, or both, should correct this condition, our researches proved that effective amounts of them, alone, seriously reduce the hot tensile strength.

We have found that a small addition of molybdenum to a heat containing about 15% chromium will maintain the cold tensile strength almost unimpaired, with but a slightly decreased resistance to scaling. If the chromium be of the order of 20%, the effect of molybdenum tends to be reversed and to reduce the hot strength.

Aluminum in suitable amount has been proved by us both to reduce the amount of scaling at heat and to maintain the hot strength, the latter effect being unexpected.

We have developed different compositions containing chromium, manganese, silicon, and nickel, with or without addition of aluminum and/or molybdenum, in proportions within comparatively limited ranges, each of which produces an alloy steel which combines in high degree the major requirements hereinbefore specified. None ofthem, however, possesses all of these major requirements in the highest degree. The proportions of the several alloying constituents necessarily vary in accordance with the qualities which,

in any particular case, it is sought to secure in' the highest degree, The present disclosure is therefore not intended to include all the compositions which we have! developed. Theydo include, however, alloy steels which possess the quality of tensile strength-resistance to dis tortion-when exposed to high temperature, many times that characterizing commonly used ferritic exhaust valve steels and a resistance to scaling which is in excess of that of valve steels which heretofore have been accepted as satisfactory; and they also include alloy steels which possess a maximum degree of resistance to scaling and a hot strength which, although not of maximum degree, substantially exceeds commercial requirements. All of them are readily machinable and hardenable.

The following is a preferred example of an alloy steel having maximum resistance to scaling with quite high hot strength:

(a) Carbon .48, chromium 14.5, manganese 3.5, nickel 2.2, silicon 2.6, aluminum 8, molybdenum .5.

If the proportions of chromium, manganese, silicon and nickel do not vary greatly from those above specified, either molybdenum or aluminum or both molybdenum and aluminum may be excluded from the composition and still produce a valve steel quite equal to the other in scale resistant qualities and nearly its equal in hot strength. If only aluminum be excluded, the hot strength will not bereduced at all andthe scale resistantv qualities will not be reduced to a serious extent. I

If only molybdenum be excluded, scale resistance will not be reduced at all and while the hot strength will be somewhat decreased, it will still remain quite high. If still greater hot strength be desired, it can be secured by increasing the proportion of molybdenum up to (say) 1.5, although in that case it. is advisable to retain the aluminum in a proportion of about one per cent.

' (d) Carbon .52, chromium 19.6, manganese 9.5,

silicon 3, nickel 2.2, molybdenum .5.

This composition gives a hot strength comparable to that characterizing the preferred compositions, but with a reduced, yet still good, resistance to scaling.

For certain uses, and even for certain valve steels, it may be desirable to secure the highest possible hot strength when that quality, rather than the highest degree of scaling resistance, is the desideratum. The following are example compositions that fully meet this requirement:

(f) Carbon .43, chromium 10.44, manganese 9.3,

silicon 2.65, nickel 1.8, molybdenum 1.81.

(9) Carbon .5, chromium 18, manganese 10, silicon 3, nickel 3.

is even superior in tensile strength at high temperatures to composition (at) but with somewhat reduced scaling resistance. A reduction in the percentage of nickel to about 2% and in the percentage of chromium to about 15% gives a steel j equal to (g) in resistance to scaling and somewhat superior to it in hot strength and which, being cheaper, is preferable.

Generally speaking: If the chromium in the alloy be increased, the other elements being held substantially constant, scaling resistance is improved, but the hot strength falls. The hot strength, however, may still be maintained high and quite sufficient where resistance to scaling is the prime requirement rather than strength to resist the deformation occurring in valves of large diameter. The addition of aluminum improves resistance to scaling without reducing the hot strength. Molybdenum materially increases the hot strength. Silicon is necessary in some substantial proportion if good scaling resistance is desired, although less silicon is required if aluminum be added. Nickel cannot replace manganese, nor manganese nickel, but both are required and the manganese should be in higher proportion than the nickel. In some compositions the proportion of nickel is permissibly very low. If a relatively high percentage of chromium be used,

it should be associated with a-higher percentage of manganese or nickel, but less silicon is permissible. If a relatively low percentage of chromium be used, lower percentages of manganese and nickel suffice, but a higher percentage of silicon is required. With low chromium and high silicon, the percentage of aluminum may be small and sometimes may be omitted. If it be desired to secure, not merely a very good, but an exceptionally high, resistance to heat, molybdenum cannot be omitted, but with increase in the percentage of chromium the percentage of molybdenum must usually be reduced.

It is to be understood from the foregoing that the compositions we have developed range from those having great resistance to scaling and moderate hot strength to those having great hot strength with a lessened degree of scale resistance. Within this range one skilled in the art can select that combination which will best resist the conditions to be met.

A certain degree ofhardening, presumably by precipitation of carbides in the austenite, can be obtained in those alloys essentially without aluminum, and with manganese in the lower range, by reheating to about 1600 R; if the manganese be in the higher range, hardening takes place at about 1400 F. with the production of a certain amount of magnetic attraction.

With aluminum in appreciable quantities, say of the order of 1.5% or more, there is a pronounced hardening at about 1000 F., presumably due to the precipitation of such a compound as NiAl in a certain proportion of the grains rendered ferritic by local enrichment of aluminum. Such hardening forms part of the subject-matter of Patent No. 1,943,595, applied for in 1931 by Foley, one of the present applicants, and granted in 1934.

While in certain claims we have specified certain ranges of proportions of the alloy constituents, it will be understood that we do not claim all compositions in which the proportions of the several constituents respond to the claim, but that such proportions must be so adjusted that the percentage of iron will be at least a'ndnot over 85%, which also definitely excludes the use in any one composition of minimum or maximum proportions of all the other elements specified; and (which is of primary importance) the proportions must be so adjusted as to produce a predominantly austenitic steel. Merely varying the proportion of iron will not sufllce. It is possible so tovary the proportions within the ranges specified as to produce an initially predominantly ferritic steel or a steel which cannot be said to be predominantly ferritic or austenitic; but such steels do not have the characteristic qualities of our improved steel. With the aid of the examples given herein, and with the knowledge of those skilled in the art, the production of a predominantly austenitic steel can be assured.

In our preferred composition, the proportion of chromium should not materially exceed 17% if the best combination of scaling resistance and high hot strength are sought. While the efiiciency of silicon to greatly promote resistance to scaling is well known, its tendency to produce the ferritic state is such that it should not be used in high proportion. In our preferred composition the silicon should not exceed 3.5% and in some compositions may be kept as low as 1.5% to 2.5% and yet sufficiently promote resistance to scaling. The amount of silicon may be reduced as that of aluminum is increased but it should not be below 1.25. Nickel increases the hot strength, assists the manganese in making the steel austenitic, improves the crystalline structure and tends to keep the scale adherent; but its use except in small proportion so decreases resistance to oxidation that it should constitute only a minor proportion, preferably not exceeding 3%, and a much smaller proportion often suflices. Manganese is quite essential to lower the critical temperature and render the steel austenitic. The proportion required varies with the proportion of the other ingredients. and should rarely be less than 3%. Molybdenum, which increases the hot strength, is preferably not in greater proportion than about 2.5% and a smaller proportion, from .5% to 2% is usually adequate in those compositions which include it. Like that of silicon, the tendency of aluminum to produce the ferritic state is undesirable, but its contribution to oxidation resistance issuch that its presence in small proportion, not over about 2%, preferably from .5% to l is desirable.

The percentage of carbon is not of substant al importance. It may vary from .10 to 1.25. The

illustrative compositions given herein and other austenitic steels embodying our invention that we have tested indicate that the ranges of the other constituents must be as follows: Chromium 10 to 18%, preferably not over 17% if a very high hot strength is desired; manganese 3 to 11%, with manganese from 7 to 11% where maximum hot strength is desired; nickel 1.5 to 4%; and silicon 1.25 to 4%.

It is necessary, in most compositions, to limit the proportion of silicon to within 4% and often to within 3% to insure the production of a dominantly austenitic steel.

Molybdenum and aluminum, either or both, are permissive additions in an amount not exceeding 2.5% of either. The percentage of silicon should be between 2.5 and 4% if no aluminum be added and may be reduced to the specified minimum of 1.25% only if aluminum be added in such proportion that the two metals of the group are between 2.5 and 5%.

An important factor in the production of our improved alloy is that the proportion of chromium and nickel and (when used) of molybdenum and aluminum may be maintained so low that valves and valve seats for internal combustion may beproduced at minimum expense and still meet present day exacting requirements.

Patent 182 1. An alloy steel having exceptional strength at high temperature and resistance to scaling which contains carbon .10 to 1.0%, chromium over 10 and less than 20%, manganese and nickel 5 to 13%, the manganese being over 3% and less than 10.25% and the nickel being over 1.75% and not over 3.5%, the percentage of manganese in any composition substantially exceeding the percentage of nickel therein, and 2.5 to 4.5% of metal of the group consisting of silicon and aluminum, the silicon being over 1.25%; the balance of the composition being substantially iron. v

.2. An alloy steel having exceptional strength at high temperature and resistance to scaling which contains carbon .10 to 1.0%, chromium over and less than 20%, manganese and nickel '5 to 13%, the manganese being over 3% and less than 10.25% and the nickel being over 1.75% and not over 3.5%, the percentage of manganese in any composition substantially exceeding the percentage of nickel therein, and silicon 2.5 to 4%; the balance of the composition being substantially iron.

3.'An alloy steel having high strength at high temperature and maximum resistance to scaling comprising carbon .10 to 1.0%, chronium not varying more than 2% from 14.75%, nickel not varying over 1% from 2.5%, manganese varying less than 0.5% from 3.5%, the percentage of manganese' in any composition substantially exceeding the percentage of nickel thereimand 2.5 to 4% of metal of the, group consisting of silicon and aluminum, the silicon being over 1.25%; the balance of the composition being substantially iron.

4. An alloy steel having high strength at high temperature and maximum resistance to scaling comprising carbon .10 to 1.0%, chromium not varying more than 2% from 14.75%, nickel not varying over 1% from 2.75%, manganese varying less than 0.5% from 3.5%, the percentage of manganese in any composition substantially exceeding the percentage of nickel therein, and silicon not varying more than 375% from 3.25%; the balance of the composition being substantially iron.

5. A valve or valve element for internal combustion engines having the composition and characteristics set forth in claim 1.

6. A valve or valve element for internal combustion engines having the composition and characteristics set forth in claim 3.

7. The alloy steel defined in claim 1 in which the steel is dominatingly austenitic.

8. The alloy steel defined in claim 3, in which the steel is dominatingly austenitic.

HARRY L. FREVERT. FRANCIS B. FOLEY.