US1697083A - Ferrous base article of manufacture - Google Patents

Description

Jan. 1, 1929.

W. H. PHILLIPS FERROUS BASE ARTICLE OF MANUFACTURE;

Original Filed April 14, 1927 2 Sheets-Sheet WITNESS E5 Jan. 1, 19290 1,697,083

W. H. PHILLIPS FERROUS BASE ARTICLE OF MANUFACTURE Original Filed Ap gil 14, 192'? n 2 Sheets-Sheet '2 h \D N 'C) n I\ E, 8 m U s \o g I gt: 2 Ex Y w s f m: F

wrmasszs I! mime F 2 2 9M. v 5' WM Patented Jan. 1, 1929.

WILLIAM H. PHILLIPS, OF SHAIJISR" TOWN-SHIP; ALLEGHENY" COUNT-Y, PEENNSYL- VANIA, ASSIGNOR TO MO PENNSYLVANIA, A CORP LYBDENUM CORPORATION ORATIO'NOF DELAWARE.

OF AMERICA, OF PITTSBURGH,

FERROUS BASE ARTICLE OF MANUFACTURE Continuation of application Serial N This application is a continuation of:v my

pending application April 14, 1927.

In the accompanying (l SerialNo, 183 910, filed rawings, to which reference will later be made,Fig. 1 is a photonncrograph of a tough nitrogen-ated case; Fig. 2 a photomicrograph of a brittle nitrogenated case; and Fig.

tain steel.

' The phenomena of surface nitrogenation whi ch of iron and steel, to

8 curves plotted from data obtained in the nitrogenatlon of a oer this invention pertains were extensivel investigated b *Fr as shown in his article in is, 1271 1923),

found, among other thing according s, thatby subject Stahl und Eisen,

to which he ing plain carbon steel to ammonia-at an elevated ten'iperature there is formed on the surfaceot the steel a very hard and extremely brittle iron nitride layer of what he termed nitrides, Fe N and Fe N.

composed largely the higher and lower iron He also found that in some instances thicker andharder iron nitride layers are similarly formed on the surface of low carbon alloy steels containing chromium, titanium, aluminum, manganese,

vanadium and silicon.

In research and commercial development work heretofore done on steel, the best results, practical standpoints, the use of relatively containing aluminum and ever, the nitrogenation of tended by serious disadvan spect to procedure and uit. As to procedure, it

the nitrogenation of from commercial and have been obtained by low carbon alloy steels chromium. Howthese steels is attages both with re.-

to the resulting prodrequires the exposure of an article formed of aluminum-chromium alloy steel to an atmosphere containing nitrogen, derived from ie introduction of ammonia into a furnace containing the arti ole, at a temperature o f-about 540 C. for a period of fifty or more hours to form on the steelacase about in thickness. As to the one-thirty-sccond of an inch product resulting from the nirtogenation of aluminum-chromium steels, the layers y y have Satisfartory h termed iron nitride? ardness and thickness, but they are deficient in toughness and cohesiveness. These cedural, economic and ph disadvantages, proysical, incident to o. 183,910,,h1ed-Apri1' 14, 1927. This application filed July 13,

1928. S.eria1No. 292;f107I ofthe article, ivhichina'ddit'ion to being-hard are also tough, cohesive, and resist-ant to corrosion, andwhichartieles may be produced at less cost than prevailedprior to my inven illOll'. i

Previous disclosuresin this art appear to have drawn distinctions between various types of ferrous materials adapted for'nitrogcnation; My invention is, on the contrary, applicable to alltypes of ferrous material, whether iron or' steel, and 'whether cast, wrought or heat treatcd'in any manner used intheqproductionzofferrous articles- In the specification and claims the term ferrous base is used for purposes of description as comprehending all these materials in their various conditions, g s

The invention ispredicated primarily upon my: discovery thatits stated'objects may be attained by alloying molybdenum in substantial amount with the ferrous base used in the manufacture of articles which are to be surface hardened by nitrogenation. processes. I have dis'coveredithat an, article formed of steel containing molybdenum as an essential alloyingv element may be provided with a hard, adherent, ton gli cohesiveand corrosionness by subjecting it to nitrogenation atan elevated temperature for a period of time less thanhalf, and in some, cases less than a fifth offthat heretofore considered necessary for nitrogen atiou of Asjust used, and as further used herein and in the appended claims with referenceto nitrogenated cases the term, toughness defines that physical characteristic of the materi al otithe case. which enables it to undergo nation whichfirmly adhere to the main body resistant layeror case of'satisfactory thick articles made of the steels heretofore consideredlbest: suited forthe pur graphingthe specimen and the shape of the;

plastictlow with little or no spalling in the immediate;vicinity of an indentation in the case formed by the Vickers"diamond indenter hardness tester. Similarly, the term brittleness is used herein to definethat physical characteristic of-the case which prevents it from undergoing plastic deformation and instead causes it to spall in the immediate vicin: ity of an indentation of the type'j'u'st referred to. Also, as applied to the cases produced in accordance with the invention, the-term cohesiveness describes the ability of each difterential physical unit of the case material to cling to neighboring differential physical units. p I

These terms may be further understood and the benefits of the invention shown in. part by refcrencc'to Figs. 1 and 2. Fig. l illustrates a tough case formed by nitrogen'ation. of steel containing molybdenum in accordance with the invention, but not containing chromium. 2 illustrates a brittle nitrogenated case on a chromiunrsteel containing no molybdenum.

After nitrogenation, one surface of these samples was ground on a taper of one to twenty. The grinding was carried out slowly under a heavy air blast to retain the structure of the metal in an unchanged condition. Hardness measurements were then made on 'these. ground faces, beginning at the outer surface of the case, using a Vickers diamond indenter hardness tester.

The surface surrounding Fig. 2 was located about 0.00s inch belowthe outer riitrog-- enated surface. Itwill be observed that the material of the case was not capable of undergoing plastic flow as thus indented, but

instead spalled severely. The'outer fourthousandths of the case was all found to be brittle, and the first tough material was found just below the region of Fig. 2. The hardness at this latter point, as measured by the Vichers machine, was about 1000 oil.

The surface surrounding the in'dei shown in l, and characteristic of the tough cases produced by the invention, was located about 0.0005 .inch below the outer surface of the specimen as nitrogenated, and

the harcness was about 1000 Brinell, will be noted that the indentation is clear and sharp, and that the material of the case has been plastically deformed without spalling.

This specimen was illuminated'in "the direction of the arrow 1. The material in the region 2 shows plasticfiow. The'apparent lack of plastic flow in regions 3,4- and 5 is due to 'the'type of illumination used in'photd material as piled up by plastic flow. Examination under tlemicroscope showed that" plastic tlow iad actually takenplace on all tne indentation; The curved,

on I

not 'craclrs, but are actually" nesagosa rial. This was proved by rotation of the specimen under the microscope upon which the lines disappeared. I

The hardness tester referred to is pro terred because of its ability to distinguish between tough-and brittle nitrogenated cases. The Herbert pendulum hardness tester reveals no material ditt'erence in hardness between these two specimens at the locations of Figs. 1 and 2. The difference in quality between these two cases, however is vital from the standpoint of abrasive wear; and the differences shown bears out the proven principle that resistance to abrasive wear is best secured by a combination of hardness and toughness.

Isis my belief that the nitrogenation of ferrous materials heretofore nitrogenizedl have discovered that the addition of molybdenum to the ferrous base so accelerates or activates the nitrogenation process as to give more satisfactory results than heretofore obtained, and in less time than was 'fOIlHEI'lY COHSlilGTBd necessary. My observation of these nitrogenated articles leads me to believe that the beneficial action of molybdenumin the nitrogenation process is due to itsunique property of readily forming nitrides which dissociate readily according to a reversible reaction, whereby, in the terrous base, molybdenum becomes an internal transterrer of active nitrogen. lhus the molybdenum actsas an energetic nitrogen collector, transferringnitrogen in a form in which it can. be distributed at a rate and in a condition so. ii that it is readily assimilable by the metal, perhaps as an easily diilusible nitride, or as nitrogen in a particularly reactive or dififusible state; Molybdenum may,

and probably does, contribute to the hardness f in other ways, as by increasing the hardness of the iron by forming additional solid solutions which go hand in hand with increased hardness, I I

in addition to containing molybdenum as an essential alloying element, the ferrous base oifwhich the articles are formed may ducing hardness, strength or other desired characteristics in the product; and it may,

and usually does, contain the usual elements present as impurities and as incidental to crunn ercial processes or" manufacture of iron or steel. in articles formed of steel, which embody the preferrediform of this invention,

the steel press contains aluminum or also contain other alloying elements for pro 4 are normal.

reevgesa Molybdenum-from about 0.75 to about 1.00%. Siliconfrom about 0.20 to about 0.25%. Phosphorus not over about 0.04%.

sulphur -not over about 0.04%. lhe manganese and silicon contents of this typical analysis arethose customarily found in steel as incidental toitS manufacture, and

the percentages of phosphorus and sulphur its a specific example of the inventioman article formedof steel containing:

lluminumuhl 2.2 arbon 0. 20% hlianganese in; M 0.55% lglolybdenum 0.89% "ilicon 0.18% Phosphor-11s-; below 0.044% 801 0110; M below 0.04%

carbon and 0.60 per cent of molybdenum after being subjected. to nitrogen at a tem perature of 540 C. for a period of twenty hours, was found to have a satisfactorily hardened surface layer. 7

The foregoing examples of nitrogenation according to the invention illustrate the benefits to be obtained by the addition of molyb denum as an alloying element. ln order to obtain a case of the same depth without the use of molybdenum, the steel would have to be subjected to nitrogenation for a much. longer period. My experiments indicate that as the nitrogenation of any'steel by heating in. ammonia is prolonged, the surface starts on transition from a tough toward a brittle condition. Thus, processes requiring long exposure are not only expensive, but tend to produce .an inferior product. I

As stated above, the alloy heretofore considered best for this purpose is a chromiumaluminum steel. In order to produce a sat isfactory case upon chromhim-aluminum steels, they are, in practice, subj ected to nitrogenation for very long periods of time. As further illustrative of the advantage attendant upon the use of molybdenum, the following examples show the results obtained with sucha steel as compared with those obtained with a steel of comparable analysis containing molybdenum, but substantially no chromium: I lhe composition of the steels was as follows, the amounts being given in percentages Steel 1n M0 or Mn s1 r s Steel 1%. is typical of that referred to as heretofore considered the best for nitrogenation. Steel 'B is similar to steel A, but contains molybdenum instead of chromium.

Similar samples of each were nitrogenated under similar conditions of ammonia supply, and time, at 540 C. After nitrogenation, the a samples were cooled in the furnace and one surface of each sam le was ground to a. one to. twenty taper. a-rdness' measurements were made on these groundfaces, using a Vickers diamond indenter hardness. tester. The depth of the case was also measured with a taper gauge. a it was found. that after new. genation for 10 hours, the case formed on steel A was about 0.0055 inch thick, and its hardness expressed in Brinell number, was about 700 at'0.001 inch belowthe surface. Steel B, after nitrogenation for the same length of time developed a case about 0.01 inch thick, the hardness of which 0.001 inch below the surface was about 825. Samples of each of the above types of steel nitrogenatedv for. hours showed the following results.

lln still another test the nitrogenated case formed on a steel containing about 0.? 5 per cent of aluminum, and about 0.2; per cent of -molybdenum was tested for hardness in the manner described above. At 0.001 inch below the surface the Brinell hardness was about 050, and at 0.01 ineh'it was about 54.0. As compared with the cases produced when larger amounts of molybdenum are used, this, case was not quite as hard toward the surface, but the transition from the case to the core was more gradual. Such a case is for many applications.

The examples just given clearly show one of the differences between the action of molybdenum and that of chromium, in that steel B develops a deeper and harder case than steel A under the same nitrogenation.conditions. Another difference betweenthe action of these two metals is shown in the type of case produced, as shown in F l and 2 and described in connection therewith.

The examples cited show that nitrogenated cases, equal in hardness and depth to the best previously obtainable, can be produced in materially less time. Likewise, the economy of time results in substantial saving in costs. The coatingsproduced when molybdenum is used are tougher and more cohesive than those previously known in the art, and they adhere satisfactorily to the unnitrogenated core.

The invention is applicable to various forms of articles which require, or which it is desired to provide with, hard, tough, cohesive and corrosion resistant surface layers. lVhatever the article may be, it is first formed, and before being subjected to the nitrogenation process may be heat-treated to give its main body the desired physical characteristics. The character of the case formed by the nitrogenation process may vary depending upon the heat treatment previously given the article. The article is then subjected to any suitable nitrogenation process, as for example by being heated in contact with any substance capable of providing nitrogen in a form capable of causing nitrogenation.

of value In the examples cited, the articles were nitrogenated at or about 540 C. The heating temperature may, however, vary according to the composition of the article, its shape, etc. I t has been stated by Fry that nitrogenation above about 580 C. causes the production of brittle cases, and that nitrogenation must be carried out below this temperature in order to produce satisfactory nitrogenated cases. My experiments, however, have shownthat both the rate of ammonia dissociation and of absorption of nitrogen by the article may be at a maximum at higher temperatures, for example at or near 700 C. This is shown in Fig. 3 in which the curves represent data obtained in the nitrogenation of a steel having the following composition:

- Per cent. Aluminum l 2:50 Carbon 0.19 Manganese 0.64 Molybdenum 0.83- Phosphorus 0.010 Silicon 4 0.29 Sulfur 0.023

Curve 1 shows the effect of nitrogenation temperature upon the rate of change of the ammonia concentration in the efiluent gas from the furnace. From 150 to 550 C. the change is gradual, from about 550 to 700 C. the rate of dissociation increases rapidly, and above 700 C. it decreases quickly. Curve 2 represents the effect of temperature upon the gain in weight, plotted as milligrams per square centimeter of exposed surface. This curve follows the same general outline as curve 1, reaching a, peak at 700 C.

The curves of Fig. 3 show that with this steel, which falls within the typical analysis referred to hercinabove, the rate of ammonia dissociation and of nitrogen absorption are at amaximum at. or about 700 C. The ferrous base articles-containing molybdenum according to the invention may be treated at temperatures above 580 C. to produce hard, tough, cohesive, and adherent cases, contrary to the prevailing belief. In many instances it may be desirable to operate at temperatures near the peak of the curve typical of the alloy being treated, because of the increased rate of nitrogenation, and the consequent economies.

Nitrogenated cases are also resistant to corrosion, offering protection against the rusting action of the atmosphere. Ordinary carbon steel appears to become sufficiently corrosion-resistant for most purposes after nitrogenation, but the case is very brittle and spalls or chips off easily. I have found that the tough and coherent nitrogenated case produced on the ferrous base articles according to the invention is satisfactorily resistant to a variety of types of corrosion.

For corrosion resistance, hardness is not usually of prime. importance, and I have found that cases possessing satisfactory corrosion resistance and good physical properties may be obtained by nitrogenation at elevated temperatures where the nitrogen absorption is at a maximum, such as at 700 C. Thus nitrogenation for short periods, three hours for example, at 700 C. will give suflicient nitrogen absorption to provide a case which is satisfactorily corrosion-resistant for a wide variety'of applications. This decreased time of treatment for such purposes is. of course of great commercial importance.

The mechanism of hardening by nitrogenation is obscure, but it is probable that the nitrogen acts by forming a hard nitride or nitrides, and that this or these, being hard, confer that property to the case. The

nitrogen hardening may also be due to diffusion of such nitrides to form solid solutions, which are generally of increased hardness, as is well known; and also nitrogen itself may diffuse and form solid solutions. Any, or all, of these phenomena may occur, and others may contribute wholly or in part.

Although molybdenum, the use of which characterizes this invention, may increase the hardness by forming solid solutions, its

chief functionis to accelerate nitrogen fixation and transference 1n a'manner peculiar to this metalitself'. My observations make it seem probable that the mcehanism of hardening is about as follows. Iron, when in a massive state, combines with or dissolves nitrogen slowly, so that the hardening process is slow. at best, and as previously mentioned, the cases formed'on iron or low carbon steel are very brittle. While the physical characteristics of nitrogenated cases are claimed to be improved somewhat by. the

- use of chromiumwhich characterized the alloys previously considered best suited for hardening action.

this purpose, yet even when chromium is used the nltrogenatlon period has been long,

because the chromium nitrogenation prod-.

ucts form slowly and are relatively stable. The stability of, such products materially delays the progress of .nitrogenation, because they do not readily give up nitrogen to the metal, or otherwise assist in hastening the Molybdenum, however, take up nitrogen in the form liberated in the dissociation of ammonia, and furthermore,

' the nitrogen thus taken up is apparently readily liberated in a form in which it diffuses and passing progressively inward, the molyb-.

nitrogen-and liberates it, and" I may,

most readily through the solid metallic matI'lXk'. Some of the nitrogen thustaken up is 7 set free, and diffuses inwardly into the surrounding metal to form solid solutions and nitrides, It appears also that when the concentration analogous in character and hardening action,

has become suflicient, a compound to iron carbideis deposited. This compound cementite, because of its appearance under the microscope, At the same time the molybdenum'below'the surface. has been receiving nitrogen by this diffusion from that nearer the surface, and it in turn liberates nitrogen for rediffusion. Thus beginningatthe surface,

'd'enum takes u by virtue of t much more rapi where the sluggish direct diffusion of nitrogen itself was probably the chief or only factor; In addition the faster or more ready is action the nitrogenation is diffusion of the nitrogen or nitrides'caused. by molybdenum, produces superior results y appears to readily for lack of a better term be called mtride d than was heretofore known .rosion. v IIItQStImOIIywhereof, IY- sign my name-ff because more uniform distribution is secured.

From the foregoing it sufliciently appears that according to this invention there are provided at reduced cost of manufacture ferrous base articles which have improved surfacelayers produced by nitrogenation. As has been explained, these advantages are attained by the addition of molybdenum to iron combined with carbon, and particularly by the addition of molybdenum to steel which preferably, also contains aluminum.

' No claim is made herein for the method of forming a nitrogenated case on-the surface of a ferrous basealloy, the method herein disclosed being claimed in a divisional application.

According to the provisions of the patent statutes, Ihave explained the principle of my invention, and have specifically described What I now consider to represent its best embodiments and the manner in which its advantages may be attained. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specificallydescribed. Furthermore, I desire to have. it imderstood that the foregoing explanations of about 4 per cent of molybdenum as a characterizing'element, and having a hard, tough,

coherent, and adherent nitrogenated surface. 1 3; An article of manufacture composed of a steel containing from a substantial amount up to about 4.0 per cent of'molybdenum as a characterizing element,'and about 0.1,to 4 per cent of aluminum, and having'ahard,.

tough, coherent and adherent nitrogenated is resistantto atmospheric cor- I surface which wninmnni rnninirs

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