US2516125A - Alloy steel - Google Patents

Description

FIGURE FE- MN-Nl AL LOYS ALLOY STEEL l. R. KRAMER ET AL Filed April 11, 1949 3.0 EQUIVALENT MANGANESE 260 HARDNESS IRVIN BRINELL Patented July 25, 1950 UNITED STATES PATENT OFFICE- ALLOY STEEL Irvin R. Kramer and Stewart L. Toleman, Washington, D. 0., and. Walter T. Haswell, Ligonier,

9 Claims.

and. the martensite subsequently tempered. In

other Words, the bestcombination of strength and ductility is found in steels which have been tempered aiterfirst being heat treated to produce a martensitic structure. Martensite as used throughout this specification, is defined as the phase whose formation from austenite is dependent only on the lowering of temperature below a point (fixed by composition) called the MS point and is thus independent of time (cooling rate).

The formation of martensite from a given quantity of austenite is predicated upon cooling said austenite to a low temperature within a certain critical period of time which is primarily also a function of composition. This critical cooling period, or reciprocally, the critical cooling rate,

is that necessary to cool the steel to a relatively low temperature before the pearlite and pearlitetype reactions (producting soft products) have had time to begin. Thus, since the maximum cooling rate attainable in the center of a piece of steel is inherently limited by heat transfer factors, it is necessary to employ material having a proper alloy content if the soft-product reactions are to be inhibited for a sufliciently long period, and thus a large section of steel obtained in the 'fullyhardened (martensitic) condition.

'Even though the cooling rate in the interior of a large section can, to a limited extent, be increased by rapid cooling (quenching) of the surface, this is not generally feasible due to the induced residual stresses attendant'upon large differences in cooling rate between surface and center. Thus, from' both the viewpoint of the limited maximum cooling rate attainable in the center of .a large section and the viewpoint of minimizing differences in cooling rate between surface and center, it is necessary to use a composition having. a very low critical cooling rate if a fully hardenedlarge-section product is to be obtained. For example, since the center of a 6-inch plate cooling in air cools froma temp ture around 1600 F. at an average rate of about 100 F./-hr.,. and that of an 18-inch plate at an average rate of about 50 -F./hr., compositions amended April 30, 1928; 370 0. G.757)

for fully hardenable plates of these dimensions must have, respectively, not greater critical cooling rates.

After hardening, it is necessary to temper the martensitic material to make it sufliciently ductile for practical use. As will appear below, phenomena associated with the tempering process may critically affect the properties of the finished product.

An air-hardening steel containing 2-6% manganese, together with a suitable tempering treatment therefor, is disclosed in a French patent issued in 1934 to the Societe Industrielle & Commerciale Des Aciers, and that patent represents the furtherest prior advance in the art of airhardening manganese steels of which we are aware.

The invention herein described also comprises an air-hardening steel containing 26% manganese and may be considered an improvement on the invention of the French patent.

It is the general object of-this invention to provide a series of air-hardening manganese steels of improved mechanical properties.

It is another object to provide a series of manganese steels which may be fully hardened in large sections.

It is theparticular object of the invention to provide a series of air-hardening manganese steels'which willhave, in large sections, excellent impact and ballistic properties after being fullyhardened and tempered, and will retain the properties for an indefinite period of time.

Other objects and advantages will in part be obvious and in part appear hereinafter.

This invention comprises a series of steel compositions, and includes particular embodiments, such asarticles having one of these compositions, wherein the utility of the composition becomes operative. The steels of this invention contain, among other components, manganese in quantitles from 2-6 percent. It should be pointed out that these limitation are not arbitrary, but are related to the recognized classification of "manganese steels. Manganese steels containingless than 2% Mn are classified as pearlitic steels and constitute a well established group of structural materials, eg the S. A. E. 1300 seriescontaining 1.60 to 1.90% Mn. On the other hand, if more than6% Mn is present the material begins to approach the Hadfield austenitic steels in composition. Substantial quantities of retained austenite willexist at room temperature and further cooling will be necessary to attain the fullest possible formation of martensite. Of course the 6% figure isnot absolute, and due to factors such as stabilization, awsmall quantity (about 1-5%) of retained austenite may exist at room temperature in our steels, but nevertheless they will be ifully hardened in the sense that no more martensite is formed even on cooling to liquid air temperatures.

In steels containing other elements in alloying proportions the limits set for the quantity of manganese alone may not be controlling of the classification of the steel, i. e. the nature of the austenite transformation product. Therefore, if a fully hardened material is to be obtained in large sections upon air cooling it is necessary'to specify composition in terms of a factor based upon all of the alloying components present. We call this factor the manganese equivalen and it will be quantitatively defined hereinafter. By alloying component is meant a component which, by virtue of its inherent nature and of the quantity of it present, relative to the overall composition, substantially aifects the austenite transformation. It is to be distinguished from components which merely affect properties of the transformation product, or merely facilitate fabrication, or are present unintentionally. Thus, while in some steels molybdenum, silicon and chromium substantially effect austenite trans- 5 formation, here whatever effect they exert is masked by the more powerful influence of manganese and nickel, and they are not alloying components in the steels of this invention.

It should be pointed out that neither a 2% Mn content in a straight manganese steel, nor a 2% manganese equivalent in a complex alloy steel, will produce a fully hardened structure on slow (air) cooling, but rather a partially pearlitic structure. Note that insofar as the French patent teaches equivalency in this respect throughout the 26% range of manganese content, its teaching is contradicted. We have found a way to define an alloy composition which permits manganese to be as low as 2% and still produces a fully hardened material in large sections upon slow cooling, and which at the same time (and this is even more important) eliminates a peculiar detrimental effect of tempering said material.

It has been found that upon tempering the martensitic structure formed with a straight manganese steel suificiently to induce reasonable ductility, viz. at from about 550 C. (1022 F.) to about 675 C. (1245 F.) for from about 2 hours to about 4 hours, a substantial quantity (roughly -30%) of austenite is formed; and that upon cooling to room temperature after tempering, this austenite decomposes to form a product which has a very detrimental effect upon the impact properties of the steel.

The X-ray data establish that this product is an alpha phase. It can be either bainite or martensite. It is, of course, obvious that the chemical composition of the temper-formed austenite will be different from that of the original matrix austenite from which the martensite was first formed. Accordingly, the'respective austenitemartensite reactions will be different. The formation of austenite on tampering, and its decomposition to an alpha'constituent on cooling is shown by the X-ray data in Table I, the water quenched W. Q. specimens showing the phases present at the tempering temperature and the furnace cooled F. C. those present after (slow) cooling from the tempering temperature. The specimens are a 3.66% Mn, 0.11% C steel (note the similarity to one of the examples given in the French patent) austenitized at 900 C. (1650 F.) furnace-cooled at a rate corresponding to that at the center of a 6-inch plate, and tempered at 650 4 C. to (1200 F.) and taken to room temperature as indicated:

The gamma line, of course, proves the presence of austenite and the broad alpha line, the presence of the decomposition product. The impact properties of the furnace-cooled specimens (in which the austenite has had time to decompose) are much lower than those of the water quenched specimens. Thus, it might appear that simply by quenching, the unfavorable effect of decomposition of this temper-formed austenite could be overcome. However, this is not the case because of the fact that the austenite retained upon quenching the tempered straight manganese steels also decomposes in time (beginning in about '7 to 14 days) to produce an equally brittle product. And further, in a section larger than about 2 inches, it is not possible by quenching to even temporarily forestall the decomposition of the austenite forming on tempering. The effect of standing time on temper-formed austenite was determined for three manganese steels and is shown in Table II. The steels were slowly cooled from austenitizing temperature, tempered at about 600 C. (1110 F.) and water-quenched from tempering temperature. X-ray data were taken within 24 hours of heat treatment and again after approximately 8 months.

Table II Gamma line Alloy Mn 0 Mo Si 24hrs. 8months 0.99 0.14 fairly strong--. none.

0.20 do very faint. 0.04 0.03 do trace..

.to the formation of a special precipitate such as carbide, nitride or oxide. See I-Iollomon, "Temper Brittleness 36 Trans. A. S. M. 473, (1946). It will be apparent to those skilled in the art that the austenite formation, proven here by X-ray data, could only occur in a steel having an A1 critical temperature low enough (alloy content high enough) to fall into the tempering temperature region.

It has been found, and herein lies the most important aspect of the invention, that the addition of nickel to the manganese steels, in requisite amounts, forestalls indefinitely the breakdown of the temper-formed austenite, with either slow or rapid cooling (and thus in any section size) after tempering, and makes possible the production of a tempered manganese steel having high impact toughness.

Further, it has been found that nickel (in any quantity) has an effect 'on the critical cooling rate about half as stron as i that3ofmanganese, and thus, by use' of appropriate am'ountsgcan be substituted for manganese, simply from the point of view of obtainingsa fully hardened structure in large sections. 1 Thus we define manganese equivalent as the 430031: manganese ncontentrplus one-half of the nick'elcontent; w

With regard to the relation'ofanickel content to impactproperties'ofi the temperedmaterial it has been found, as will-beseen from TablelILthat nickel has a disproportionately (weak efiect' t- .ward stabilizing rthe temper-formed ;austenite in quantities less than 1%. 1

.From about 1% to about 2.5%.nicke1, there is a continuous improvement in impactproperties with nickel content. IAbovethat limit it would appear that the .austenite is completely stabilized and no further improvementin impact propcities is observed. The nickel above 2.5%, however, is still operative to increase the hardenability of the steel inaccordance with the definition of the manganese equivalent, and thus the overall composition is defined to permit nickel as high as 8%, this limit being fixed .by the manganese equivalent and minimum manganese content limitations. i

It may be of interest to point out thatasecond tempering treatment, usually performed at a 1 temperature somewhat'below that of the first, brings about some improvement in impact properties inthemanganese-nickelsteels.

Carbon hasa weakefiect'on the propertiesof these steels within the rangefrom about 0.05% to 'about 0.30-%.. If 1ess-than..0.05% Cis present "the steel will not 'form hard'martensitei :iIcf more -than 0.30% ispresent," some carbides will 'form on cooling which have a 'detrim'ental eflection impact as well as other properties: The efiect of carbon on hardenabilityi his appreciable. only 'vention. It is found x'toraise the tensilestrength of "the final product without detracting from other properties. This finding is believed merely to confirm the teaching of the French patent since the molybdenum here apparently performs the same function as therein disclosed, Viz. simply improvement in mechanical properties. Its effect is more or less continuous up to about 1 0.505035%. It is interesting to point out that While molybdenum known to have a beneficial effect in overcoming temper-brittleness, it has no such'effect on the tempering henomenon ob- .served here. As a matter of preferred'practice, molybdenum is added to most of the steels made in accordance with this invention, but it is not an alloying component as that term is hereinbefore defined. Similarly, silicon is present in these alloys simply because it is inherentin good steel-making practice. Its use in the quantities shown is determined simply by practice well known in the art and not related to the inven- H tion. In Table IV below are given general mechanical properties for a few of the Ni-Mn steels of this invention. In addition to the analysis ;given, each steel contains quantitiesof molybdenum and silicon as set out above. The steels were all fully hardened by slow cooling from an .austenitizing temperature of 900 C. (1650 F.) .and tempered at 600 C. (1112 F.) for 4 hours.

I Table IV Per Cent Per Cent Alloy TVS Y. S Elong. Bed. of

. 2 inch Area P. s. z P. 8.11. I MKP 3.42 M V 1.55 Ni. 22 63 0.11 C MKR 3.35 Mn p 1.70 Ni- 23 66 0.17 G. MGK 3.47 Mn- 1.47 Ni- 25. 2 62.9

0.20 (3-- MKS 3.52 M

r 1.53 Ni 29.3 0.08 C :MCT 2.73 MJ1 1.93 Ni. 1 27 58 0.22 C I MDO 2.72 Mn. 2.24 Ni. 28 62 0.19 O 'MDV 2.85 M11.

2.35 Ni- 29 61 0.17 o H v The teaching of the French patent inregard .to improvement in mechanical propertiesby additions of vanadium also applies tothe improved material of thisinvention, but in such aremark- -able way that it seems desirable to give specific examples. The effect is clearly seenby comparison 0f the steels MML (3.20 Mn, Ni, 060 M0,

. 0.17 Si and 0.13 C) and MMK 1.05 Mn, 1.74 Ni,

x in sections havingia relatively low' hardenability 0 '(low manganese equivalent) as seen from the accompanying drawing, Fig. 1.- .As.-will also be seen fromFig. 1 themanganese equivalent 'must be at least 35 '1 to get a martensiticristructure in alloy;toiorma;preferredxembodiment 10f. theline.

0.60 Mo, 0.17 Si, 0.13 C and 0.19 V). after austenitizing at 900 C. (1650 F.), cooling, and tempering for 8 hours at:(a')"600 C. (1112"F.) (0) 550 C. (1030 F.) and (0) 500 C. (930 F.) the properties given in Table V were obtained.

Table V l ercent Percent Tensile Yield Elong. of Red Strength 2in'ch of Area a '31 65 21 58 (b) 20 57 19.5 53 (c) 1 19.5 56.5 a I 21.7 67.2

1 "In' Table VI below are given compositionsuof some other Mn-Ni-V steels which have been prepared;

8f Contrary to 'the effect of this/element in alloy steels generally, and contrary to the suggestion of the French patent, it has been found that Table VI chromium has a distinctly detrimental effect on 5 5 the impact properties of these steels, and at the same time contributes only slightly to the ten- Alloy Mn N1 M of s1 0 V sile properties of the material. Chromium has been found to accelerate the breakdown of'the g3 24 aforementioned temper-formed austenite. Thus, 1174 25 :1: 10 I25 I18 10 if pure ferrous material (ingot iron) is available, 66 its use is recommended and chromium should be 1.67 .83 .14 .27 .18 1. 59 .50 .05 .13 .18 entirely avoided. In industrial practice, of course, 49 04 it is necessary to use scrap as'a partial source 1. 74 00 17 .13 .10 2.22 53 .20 40 .20 .22 of II'OIl. and thus 1t is practically impossible to 15 make a heat of commercial steel without from i about 0.1% to about 0.3% residual chromium. Table VII shows the ordmary p operties of some Fortunately, however, it has been found that by ofthese alloys with indicated tempering treatthe simple expedient of raising the minimum merit after austenitizing at 900C. (1650 F.) and nickel content in rough proportion to the amount slowly cooled. 20 of chromium present the detrimental effect of the Table VII 0 7 Percent Percent Charpy Alloy Tempered T S Y s Elong. R. Ts/Ys V-notch Ft.-lbs.

930F.8hrs.W.Q 151,000 120,750 13.0 53.0 85 31 1110 F.'8 hrs. w. Q 131,000 107, 750 21. 0 01. 0 02 07 030 F.-8 hrs. w. Q 147, 750 122, 000 22. 0 04. 0 s3 27 1110 F.-8 hrs. w Q. 135, 750 110,730 23.0 00.0 87 100 030 F.8 hrs. 151, 000 150,000 13. 0 33. 0 s 10 1110 F.-8 hrs.w Q 102,000 130,000 10.3 42.2 84 09 0a F-8 hrs.W 102,000 103,000 15.0 43.0 85 13 1100* F-8 hrs. w Q- 174,000 150,000 15.3 34:0 00 40 030 F8 hrs. w o 200,200 170,000 15 30.0 00 17 1110* F.8 hrs. w. 01.... 172,000 147,000 13. 5 40. 5 s0 20 Table VIII shows the mechanical properties of latter element will be overcome. Of course the two other Mn-Ni-V steels tested at an elevated amount of residual chromium which will occur temperature, viz 500 C. (930 F.). Note that in the finished product must be estimated in adthese specimens received a double tempering Vance, or determined from analysis of the melt, treatment before testing. in order'that the minimum nickel content neces- Table VIII Allo Tempering Treatment T S Y S Percent Percent y long. R. A.

MMD 1200 F.8 hrs. F. o.+1022 F.s hrs. F. 0 80,000 75,000 35 70 MMD. 1112 F.8 hrs. F. 0.+1022 F.8 hrs. F. 0-- 118,000 110, 000 20 MMD 032 F.8 hrs. F. o.+032 F.-8 hrs. F. 0.. 125,000 118,000 32 71 1112 F.8 hrs. F. o.+1022 F.-s hrs. F. 0 100,000 83,000 35. 7s 1112" F.8 hrs. F. o.+1022 F.8 hrs. F. 0.- 110,000 103,000 22 35 1111s.... 032 F.8 hrs. F. c.+032 F.s hrs. F. o 130,000 115,000 30 72 1 Two other Mn-Ni-V steels were given stressrupture tests, the stresses required to cause rupture in 1000 hours at the indicated temperatures being determined as shown in Table IX. Since it is suggested that these steelscould be used for applications such as turbine rotors, comparative data are given for two rotor steels. AAC

and ZR, nowused for turbine rotors.

An important element which bears critical relation to this invention and must be given special (:bnSiderationin this specification is chromium.

sary to insure against decomposition of the temper-formed austenite will be known. It has been found that some increase above the minimum nickel content of 1% should be specified if anything but ingot iron (or equivalent) is used as a base metal for the alloys, and that if as much as 0.1% chromium occurs this increase should amount to about 0.4 to about 0.5% nickel. Chromium up to 0.30%, which should be about the maximum residual from scrap pick up, may be tolerated if at least 2.25% nickel is present (1.25% above the minimum for non-chromium steels). Thus, it is necessary with chromiumbearing steels to add. nickel over and above the 1% lower limit in quantities roughly about four times the totalchromium content. In such case, of course, the manganese content must be adjusted to provide a given desired manganese equivalent. v'Ihe data in Table X demonstrate the detrimental effect of chromium and the mannerin which it may be overcome by increasing the nickel: content as set'forth above. The data in Table X are from steels austenitized at 900 C. (1650 F.) and slowly cooled; then tempered at 600 C. (1110 F.) for 4 hours and water quenched. Noteithat water' quenching didinot forestall the breakdown of austenite in the non-nickel bearing steels of section size necessary for these specimens (described below) long enough for the test to be made.

steel being further limited in that it is at least 1% plus about four times the residual chromium content.

5. An alloy steel consisting essentially of from Table X Oharpy Limit Mn N1 M0 Cr S1 0 Fracture V-notch Veloclty Impact Ft.-lbs. 3. 32 1. 53 0. 47 0. 25 0. l4 3, 600 145 3. 33 l. 70 0. 27 0.11 0.17 3, 600 140 3. 42 1. 55 0. 28 0. 35 0. 11 3, 600 105 3. 63 1. 42 0. 45 0. 18 0. 32 0. 12 3, 420 67 3. 65 1. 50 0. 26 0.25 0. 08 0. 17 3, 300 3. 58 1. 50 0. 23 0. 41 0. 16 0. 17 2, 800 45 2. 79 2. 28 0. 52 0. 30 0. 14 0. 11 3, 500 3. 17 2. 29 0. 50 0. 29 0. 14 0. 11 3, 460 124 3.17 2. 42 0. 29 0. 27 0. 28 O. 23 3, 450 3. 50 2. 41 0. 50 0. 23 0. 28 0. 23 3, 440

Figure 2 shows comparative limit-velocity values at a varying Brinell hardness for a typical alloy (nominal composition: 3.5% Mn, 2.5% Ni, 0.12% C) of this invention and for an acceptable armor plate. Thus, the dashed line is a plot of limit-velocity versus hardness for our steel, and the solid line (A) is such a plot for an acceptable armor plate 1% inches in thickness. Varying hardnesses were developed by different tempering treatments. The points, B and C, represent, respectively, single tests in the as-received condition of specimens cut from the center of a 12-inch and an 18-inch plate of acceptable armor plate. Limit-velocity is used here as meaning the velocity of a .50 caliber bullet required to just "shoot oil a finger 2 inches long and 1 x 1 /2" square, notched at its base.

It will be understood that the examples herein described are illustrative only and that the invention is not to be limited except as defined by the herewith appended claims.

What is claimed is:

1. An alloy steel comprising essentially from about 0.05% to about 0.30% carbon, from about 2% to about 6% manganese, from about 1% to about 8% nickel, balance iron and incidental impurities, and having a manganese equivalent between 3.5 and 6, the nickel content of said alloy steel being further limited in that it is at least 1% plus about four times the residual chromium content.

2. An alloy steel comprising essentially from about 0.05% to about 0.30% carbon, from about 2% to about 6% manganese, from about 2.25% to about 8% nickel, balance iron and incidental impurities, and having a manganese equivalent between 3.5 and 6, said alloy steel containing from traces to about 0.3% chromium.

3. An alloy steel consisting essentially of from about 0.05% to about 0.30% carbon, from about 2% to about 6% manganese, from about 1% to about 8% nickel, from about 0.01% to about 0.3% silicon, from about 0.01 to about 0.7% molybdenum, balance substantially all iron plus residual quantities of elements commonly present in steel but containing no measurable quantity of chromium, and having a manganese equivalent between 3.5 and 6.

4. An alloy steel consisting essentially of from about 0.05% to about 0.30% carbon, from about 2% to about 6% manganese, from about 1% to about 8% nickel, from about 0.01% to about 0.3% silicon, from about 0.01% to about 0.7% molybdenum, balance substantially all iron plus residual quantities of elements commonly present in steel, and having a. manganese equivalent between 3.5 and 6, the nickel content of said alloy about 0.05% to about 0.30% carbon, from about 2% to about 6% manganese, from about 1.5% to about 8% nickel, from about 0.01% to about 0.3% silicon, from about 0.01% to about 0.7% molybdenum, balance substantially all iron plus residual quantities of elements commonly present in steel, and having a manganese equivalent between 3.5 and 6, and containing from traces to about 0.1% residual chromium.

6. An alloy steel consisting essentially of from about 0.05% to about 0.30% carbon, from about 2% to about 6% manganese, from about 2.25% to about 8% nickel, from about 0.01% to about 0.3% silicon, from about 0.01% to about 0.7% molybdenum, balance substantially all iron plus residual quantities of elements commonly present in steel, and having a manganese equivalent between 3.5 and 6, and containing from about 0.1% to about 0.3% residual chromium.

'7. A slow-cooled, fully hardened steel article from about 2 inches to about 18 inches in maximum section size having a chemical composition at defined in claim 1 and further limited chemically in that the manganese equivalent within the limits recited is approximately proportional to the section size within the limits recited.

8. A slow-cooled, fully hardened, tempered steel article from about 2 inches to about 18 inches in maximum section size having a chemical composition as defined in claim 1 and further limited chemically in that the manganese equivalent within the limits recited is approximately proportional to the section size within the limits recited, said steel article being characterized by high impact strength.

9. A slow-cooled, fully hardened, tempered steel article from about 2' inches to about 18 inches in maximum section size having a chemical composition as defined in claim 1 and further limited chemically in that the manganese equivalent within the limits recited is approximately proportional to the section size within the limits recited, said steel article being characterized by the absence of decomposition products of temper formed austenite.

IRVIN R. KRAMER. STEWART L. TOLEMAN. WALTER T. HASWELL.

REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PATENTS Number Country Date 768,468 France Aug. '7, 1934 440,894 Great Britain Jan. 8, 1936

Claims (1)

1. AN ALLOY STEEL COMPRISING ESSENTIALLY FROM ABOUT 0.5% TO ABOUT 0.30% CARBON, FROM ABOUT 2% TO ABOUT 6% MANGANESE, FROM ABOUT 1% TO ABOUT 8% NICKEL, BALANCE IRON AND INCIDENTAL IMPURITIES, AND HAVING A MANGANESE EQUIVALENT BETWEEN 3.5 AND 6, THE NICKEL CONTENT OF SAID ALLOY STEEL BEING FURTHER LIMITED IN THAT IT IS AT LEAST 1% PLUS ABOUT FOUR TIMES THE RESIDUAL CHROMIUM CONTENT.

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