US2435511A - Method of making metal bodies - Google Patents

Description

Feb, 3, 194s. R, E, RICE 2,435,511

METHOD 0F MAKING METAL BODIES Filed May 15, 1945 /0 #aa/es 111 1.1m mmlm 66 UNITED STATES PATENT OFFICE METHOD OF MAKING METAL BODIES Richard E. Rice, Winthrop, Mass., assignor, by mesne assignments, to Isthmian Metals, Inc., Boston, Mass., a corporation of Massachusetts Application May 15, 1945, Serial No. 593,977

(Cl. 'l5-22) 8 Claims. 1

This invention deals with the making of metal bodies by powder pressing, and more particularly with a method for precision shaping articles of hard high carbon and alloy steels, and to the articles made by that method.

A main object of the invention is to make undistorted accurately dimensioned articles having a strength approaching that of the best and hardest steels, without going to the expense of successively machining, hardening, and precision-grinding.

Other objects of the invention are to shorten the time consuming sintering operations heretofore needed in connection with powder pressing; to press steel or steel alloy compacts to heretofore unattainable densities and strengths by rendering the compacts peculiarly susceptible to plastic deformation, while the dies and punches performing-the work remain strong and rigid; and to press precisely shaped steel articles which can then be hardened to martensite and bainite, or to either martensite or bainite, without substantial distortion even when considerable variations of density are unavoidable due to the irregularity of the shapes to which the powder is pressed.

These objects are attained by using two pressings and an intervening heat treatment, taking advantage of the known fact that certain steels in the soft austenitic (gamma) state characteristic of temperatures above the critical, can be quenched, not indeed to room temperature but to a temperature below that of rapid pearlite formation, and above the Ar" range (generally between 200 F. and 1000 F.) without immediately reverting to the harder (alpha) forms that are stable at these temperatures. A description of the Ar" range is found in Chapter VI of Epstein, Alloys of Iron and Carbon. By seizing promptly after such intermediate-temperature quenching, the opportunity of pressing the material to final dimensions before it has time to transform to the stable form, I gain the advantages of a soft, yieldable pressure and without heating the dies or punches to a degree which would seriously weaken them. Moreover, this makes unnecessary the sintering which would otherwise be needed to develop strength and toughness after pressing, and which would cause distortion and loss of the great accuracy of dimensions secured in the pressing operation.

Detail procedures utilizing the general concepts of the invention will vary greatly with the composition of the steel used, which may include such alloying elements as nickel, silicon, chro- 65 mium, manganese, molybdenum, tungsten, vana dium and tantalum, as well as their carbides with the exception of nickel carbide which may be only partially diffused in the steel, when they are also partially present as particles, conferring great hardness and resistance to wear.

In general, alloying elements other than cobalt delay the transformation and lower the Ar" range. Excessive manganese, chromium, or nickel or any combination thereof may even postpone transformation indefinitely. This invention is therefore limited to steels having an Ar" range whose upper limit is above room temperature.

In general, I prefer to carry out the process according to the invention in these steps:

(a) Cold press the powder mixed with a 1ubricant such as stearic acid, to a density of about 7 (porosity for example 10%) The die and punch surfaces are also lubricated. As an alternative, the loose powder may be packed into a mold.

(b) Heat the compact to 7001100 1T'. to drive olf the lubricant, then heat to above the critical temperature of the particular steel, transforming it to austenite. If undiffused alloy constituents are pr`e's`t, it may be well to hold this heat long enough to allow diffusion. But I prefer to use a powder in which the alloying constituents are already so uniformly diffused that a brief heating suffices. The heating is done in a nonoxidizing atmosphere, for which a convenient test is non-discoloration of bright 18-8 stainless steel.

(c) Quench the compact to a temperature below its critical temperature, but above its Ar" range. The quenching 4is, done rapidly enough so that the steel remains wholly or largely austenltic in a temporarily stable state.

(d) Before much (if any) of the austenite has had time to transform, re-press or coin the compact quickly at about the same temperature, with a suitably heated die and punch, to its nal dimensions, preferably using a high pressure such as to 90 tons per square inch. If the pressing was prompt enough, the compact may thus attain densities as high as 7.5 to 7.7 or higher which results in very high strength. The shape may be much more complex than is allowable with ordinary cold-pressing for two reasons: (1) The material ows better; and (2) even if densitydierences remain, there is no final high-temperature sintering to give substantial distortion. In certain cases it is possible to combine the quenching with the pressing, by placing the heated austenitic compact quickly in a massive die somewhat below coining temperature and applying immediate pressure, thus rapidly abstracting heat from the relatively small compact and bringing it down to the coining temperature, preferably before coining is completed. While I prefer the structure to be substantially austenitic when the compact is coined, it should be at least 50% austenitic.

(e) Transform the austenite to a stable form. This occurs spontaneously without distortion, either by forming bainite if the temperature (1,) is somewhat increased or4 lowered or (2) remains fairly constant, or by (3) forming martensite if the compact is cooled promptly to room temperature` This cooling should not be quicker than is needed to prevent the change to bainite, lest the strains set up by high temperature-gradients, and the expansion on changing to martensite at different rates from one part of the piece to another should cause distortion or weakening or leave residual strains; al1 these precautions can be easily observed in ther/course of my process. By thus transforming the austenite to the final stable structure at a low temperature and with low temperature gradients, the dimensions of the final piece can be held to within .0005 inch per inch of the dimensions imparted to it by the die during the final pressing operation. Cooling in air or non-oxidizing gas is suitable as a rule. No sintering is needed.

(f) Drawing, tempering, etc., in known ways is optional.

The above described procedure according to the invention is graphically recapitulated in the accompanying gure which is the isothermal transformation diagram (compare for example Atlas of Isothermal Transformation Dia-grams, United States Steel, 1943) of a typical 0.8% C, 0.8% Mn steel, austenized at 1650 F. and of grain size 6. The left hand vertical axis indicates temperature levels and the right hand vertical axis final hardness in Rockwell C hardness degrees, over time plotted logarithmically along the horizontal axis. The curves I and II indicate transition between austenite and austenite-ferrite on the one hand and austenite-ferrite and bainite on the other hand. The above referred to Ar' and Ar" ranges are likewise indicated.

Letters (a) to (e) (corresponding to the above outlined and similarly numbered steps) indicate the phases of the process according to my invention. as follows:

The co-ld pressed compact is held during a suitable period (a)- b) at a temperature. ta at which it is transformed to austenite. During (c) it is quenched to a temperature to below its critical temperature, but above the Ar" range. During (d) the compact is quickly repressed at substantially constant temperature tc, The repressed, still austenitic compact is then transformed to a stable form in one of several ways, as follows: It can be reheated to a certain degree as indicated at (el) and then transformed, at constant temperature, to bainite, to attain a predetermined hardness characteristic of the transformation temperature; or it can be maintained approximately at repressing temperature and transformed to bainite, attaining the hardness corresponding to that temperature, as indicated at (e2); or the compact can be cooled to room temperature, as indicated at (e3), to form martensite.

During all of these operations, it is preferred to protect the pieces from oxidation by retaining them in inert or non-oxidizing atmospheres.

My process differs sharply from ordinary hotpressing, which presses at much higher temperatures in the effort to soften the material by temperature alone. This of course also tends to soften the dies and punches. ,In ordinary hot pressing the softness of the material to be hot pressed does not change with time so long as the temperature is kept constant. I use, on the contrary, material quenched to a temporarily-stable state of unnatural" softness at the working temperature, and coin within a limited time, before that softness vanishes.

I prefer using a coining temperature of 400 F. to 800 F. although the invention is not restricted to this range, in handling all steel compositions. Hitherto this has been considered as wholly unsuitable for working iron and steel, either in hot pressing or in shaping an ingot. This tempera-,

ture has been designated as the blue-brittle range (see Z. Jeffries and R. S. Archer, The

Science of Metals? page 182), because working iron or steel in stable state in this temperature range results in an unusually brittle condition when the iron or steel is cooled to room temperature.

In the blue-brittle range iron and steel are stronger and harder and less ductile, and therefore less suitable for working. Therefore, the dies and punches are not soft when heated up to the temperature of the material for coining the same while in its metastable condition, and in fact are likely to be in the blue-brittle temperature range notable for u nyielding hardness.

When iron powder mixed with powdered alloying elements is used as the starting material I prefer to use iron powder of high purity and free from hydrogen embrittlement and work hardening. The purity of the powder should be such that the total dissolved impurities are less than 0.3%-0.5% and that the loss in weight when the powder is heated in dry hydrogen for 2 hours at 1800 F. should be less than Ofi-0.7% and the grain size should be No. 9 or larger on the A. S. M. scale of grain sizes for various metals.

The invention will be further illustrated but is not intended to be limited bythe following examples:

Example 1 Electrolytic iron is well mixed (as by ball milling) with 1% metallic manganese, .9% carbon, and 1% stearic acid lubricant. This mixture is pressed at 25 tons per square inch, and sintered at 2000 F. for three hours (to allow diffusion, particularly of the manganese) in hydrogen containing 0.32% by volume of natural gas. The sintered and substantially homogeneous compact is then quenched in a bath of molten solder at 500 F., pressed (coined) within two minutes at tons per square inch ina die also at about 500 F., ejected from the die, and rapidly cooled, transferring the austenitic piece substantially wholly to martensite. Subsequent tempering by known methods to increase toughness is optional.

Since the transformation of austenite-having the above composition does not begin until 2 minutes after the steel is quenched below the critical temperature, the compact is austenitic when coined. In an actual sample compact, the resulting density was 7.54 corresponding to about 2% porosity and its hardness was Rockwell C-58.

After drawing at 1100 F. its tensile strength u. was ove" 150,000 pounds perM square' inch and its hardness Rockwell B-102.

' A similar compact was made from the same starting material, pressed at the same pressure, and sntered at the same temperature for the same' length" of time followed by slow cooling (rather than quenching), in order to produce a pearlitic structure, The sintered compact was heated to 500 F. and coined with punches and dies also heated to 500 F. at 90 tons per square inch. The density of this' pieceV was then 7.11 corresponding to about 9% porosity, and had very much inferior physical properties, due to the greater resistance' of pearlite to co'ining as compared with temporarily-stable austenite. In hardening such a piece, heating above the critical temperature followed by quenching produces substantial distortions', and therefore such a piece cannot be made t the close tolerances which are possible by the pressing of temporarily-stable austenite according to the invention, which processrequires no subsequent high temperature heat treatment to producev high hardness.

Example 2 The starting material is a steel powder which may have approximately the composition of S. A. E. 1080 steel, namely: 0.75-0.88% carbon, 0.60-0.90% manganese, 0.04% maximum phosphorus and 0.05% maximum sulphur, the remainder being iron. Such a steel powder is too hard to give a high density, of the order of 7.3-7.7, and high physical properties, when cold-pressed and sintered by conventional methods. According to my method, this powder may be mixed with 1% of stearic acid lubricant, in a ball-mill or other suitable mixer. It is then cold-pressed in a suitable die at a pressure of 27-90 tons per square inch, for example into the form of a rod approximately 2 x 1A. x 1/4 inches, which is ejected from the cold-pressing die. The cold-pressed piece is then sintered in a non-oxidizing, non-carburizing, and non-decarburizing atmosphere, which may consist of hydrogen and methane, The sintering temperature may be between 1500 F. and 2400" F., but it is preferably 2000 F.

Since the starting material is a pre-formed steel powder in which the carbon and alloying materials are dissolved and substantially uni- -formly distributed in and throughout the powder particles, a shorter sintering period may be used than in the previous example, where homogenization and the production of the steel must take place, The sintering period should be long enough to render the steel material wholly austenitic. The top of the Ar" range 0f this steel is about 430 F. ,At the end of the sintering period, the sintered austenitic steel piece is quenched in a bath of mercury of about 520 F.

After the austenitic steel piece has substantially reached this temperature, and during the interval before substantially any bainite has formed in the piece, so that the steel piece remains austenitic, it is placed in a suitable die which is at 520 F., and pressed by means of similarly heated punches under a coining pressure of approximately 90 tons per square inch. This coining pressure may preferably be maintained for about seconds. The piece is then austenitic. The coined austenitic steel piece is then ejected from the die, and may then be cooled in any of the ways previously described, to transform the austenite to martensite or bainite, or to a mixture of the martensite and bainite in any desired proportions.

I claim:

U 1. A method of making steel articles requireing great accuracy of final dimensions and having a high strength and hardness comprising the steps of subjecting a porous powder compact predominantly of ironadof to a temperature above its critical temperature for a su'icient length of time substantially completely to convert it to the austenitc state, quenching the compact to a subcritical temperature below 1,000 F. and above the top limit of the range within which austenite transforms instantaneously from austenite to martensite, subjecting the compact to the action of accurately dimenoned dies to impart to the compact the dimensions of the dies while the metal of the compact is still predominantly austenitic; said subcritical temperature being selected so that when the dies are heated up from room temperature to this temperature they will be in a condition close to their maximum strength and hardness, and thereafter removing the shaped compact from the dies and cooling it to'room temperature.

2. A method of making a steel body of predetermined nal dimensions whose composition is such that the upper limit of its Ar range is above room temperature, comprising the steps of subjecting a powder compact of said composition to a temperature above its critical temperature to convert it to the austenitic state, cooling the compact from above its critical temperature to a subcritical temperature below its Ar' range but above said Ar range, in which the austenite is at a temporarily stable condition, subjecting the compact to compression in a die to cause the compact to conform to the dimensions of the die while the metal of the compact is still predominantly austenite, and thereafter removing the compact from the die and cooling it to a temperature at which the compact will have transformed to a stable state.

3. A method of making a steel body of predetermined final dimensions whose composition is such that the upper limit of its Ar" range is above room temperature, comprising the steps of subjecting a powder compact of said composition to a temperature above its critical temperature to convert it to the austenitic state, cooling the compact from above its critical temperature to a subcritical temperature below the Ar' range but above said Ar" range at which the austenite is in a temporarily stable condition, subjecting the compact to compression in a die at said subcritical temperature to cause the compact to conform to the dimensions of the die while the metal of the compact is still predominantly austenite, and thereafter cooling the compact to a temperature at which the compact will have transformed to a stable structure which consists substantially of martensite and will have said predetermined dimensions.

4. A method of making a steel body of predetermined nal dimensions whose composition is such that the upper limit of its Ar" range is above room temperature, comprising the steps of subjecting a powder compact of said composition to a temperature above its critical temperature to convert it to the austenitic state, cooling the compact from above its critical temperature to a subcritical temperature below the Ar' range but above said Ar" range at which the austenite is in a temporarily stable condition, subjecting the compact to compression in a die at said subcritical temperature to cause the compact to conform to the dimensions of the die while the metal of the compact is still predominantly aus- I tenite, and thereafter allowing the compact to transform to a stable structure which consists substantially of bainite, said transformed compact having said predetermined dimensions.

5. A method of making a steel body of ypredetermined nal dimensions whose composition is such that the upper limit of its Ar range is above room temperature, comprising the steps of subjecting a powder compact of said composition to a temperature above its critical temperature to convert it to the austenitic state, cooling the compact from above its critical temperature to a subcritical temperature below the Ar' range but above said Ar range at which the austenite is in a temporarily stable condition, subjecting the compact to compression in a die at said subcritical temperature to cause the compact t conform to the dimensions of the die while the metal of the compact is still predominantly austenite, and thereafter cooling the compact to a temperature at which it will have transformed to a stable structure which consists of martensite and bainite and will have said predetermined dimensions.

6. A method of making an article requiring great accuracy of final dimensions, comprising the steps of subjecting a porous powder compact predominantly of iron and carbon to a temperature above its critical temperature to convert it substantially to a stable austeniticvv state, cooling the compact to a temporarily stable austenitic state at a subcritical temperature below the Ar range and above the temperature at which austenite transforms instantaneously to martensite, subjecting the compact to the yaction of a master die to impart to it the dimensions of the die while the metal of the compact is still predominantly austenitic, and thereafter cooling the shaped compact from Vsaidnsubcritioal temperature to room temperature.

7. A method of making a steel compact of accurate nal dimensions, and high hardness characterized by the steps of subjecting a porous compact of steel powder while in a temporarily stable austenitio state at a subcritical temperature below the Ar range to molding and compacting pressure with a master die of accurate dimensions which is at said subcritical temperature to impart to the compact the dimensions of les'. Sr

U the die while the metal of the compact is still predominantly austenitic, said subcritical temperature being such that the master die is in a stable crystalline state at the molding and compacting temperature, and cooling the shaped compact from lsaid subcritical temperature to room temperature.

8. A method of making a steel compact of accurate nal dimensions, and high hardness characterized by the steps of subjecting a porous compact of steel powder while in a temporarily stable austenitic state at a subcritical temperature to molding and compacting pressure with a master die of accurate dimensions which has been raised from room temperature to said subcritical temperature, to impart to the compact REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,500,571 Brandenburg July 8, 1924 1,924,099 Bain et al Aug. 29, 1933 2,183,358 Six Dec. 12, 1939 2,333,573 KalischerY Nov. 2, 1943 Re. 22,452 Clements Mar. 4, 1944 OTHER REFERENCES Powder Metallurgy, by Jones, published by Edward Arnold and Co., London, 1937, page 37.

Alloys of Iron and Carbon, vol. 1, by Epstein, published by :McGraw-Hill Book Co., New York, 1936. page 177.

Powder Metallurgy, Vby Wulff, published by American Society for Metals, Cleveland, 1942, pages 392-544.

Certificate of lCorrection Patent No. 2,435,511. February 3, 1948.

RICHARD E. RICE It is hereby certied that errors appear in the printed specification of the above numbered patent requiring correction as follows: Column 1, line 4, after the word steels strike out the comma and insert instead a period; lines 4 and 5, strike out and to the articles made by that method.; line 44, after yieldable insert mass 'which can be pressed to very low porosity 'without excessive; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the'Patent Oce.

Signed and sealed this 27th day of April, A. D. 1948.

THOMAS F. MURPHY,

Hamam Qomam'ssonervf Patents.

Certificate of lCorrection Patent No. 2,435,511. February 3, 1948. RICHARD E. RICE It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows: Column 1, line 4, after the word steels strike out the comma, and insert instead a period; lines 4 and 5, strike out and to the articles made by that method.; line 44, after yieldable insert mass which can be pressed to very low porosity without excessive; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 27th day of April, A. D. 1948.

THOMAS F. MURPHY,

Saisie/n# laf/minime' 0f Pregunte.

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