US2103848A - Heat treatment of steels - Google Patents

Description

HARDNESS llOCKWELLC" 01 Dec. 28, .1937. J. w. HARSCH 2,103,843

HEAT TREATMENT OF STEELS 7 Filed May 12, 1954 DEPTH BELOW SURFACE- I'NCHES T ji.%%m0&

ATTORNEY.

Patented ea. 28, 1937 aioaaie naa'r 'rnna'rivinn'r or s'ranns Application May i2, 193%, Seriai No. 725,2hii

i oiaim.

My invention relates to methods of heat treating steels or alloy steels.

In accordance with my invention, in the heattreatment 'of metals containing carbon, such as steels, alloy steels or the like, decarburization or oxidization, or both, during the heat-treatment are precluded by utilizing as or in an atmosphere, in which the metal is heated, products resulting from cracking of an organic compound, or mixture of organic compounds, comprising carbon,

hydrogen and oxygen in. fixed or definite proporions.

In accordance with my invention, during heating of articles of hardenable steel to quenching 1.? temperature, substantially to prevent loss of carloan at or near the surface of the articles by reaction with such components of the furnace atmosphere as carbon dioxide, water vapor, oxygen, etc., I introduce a hydrocarbon gas or vapor, to

90 provide a component of the furnace atmosphere from the heat insulating material of the furnace whose carburizing power is preferably in equilibrium with or slightly greater than the tendency for the articles to lose carbon by reaction with said one or more other components of the furnace 3:, atmosphere.

My invention also resides in methods hereinafter described and claimed.

Fig. 1 of the drawing illustrates an arrangement for hardening in accordance with my invention;

Fig. 2 shows explanatory curves; Fig. 3 is a detail view in section and on enlarged scale of-parts appearing in Fig. 1.

In general. dies, tools, gears and the like, are :15 hardened by heating them to suitable extent beyond a critical point of the particular steel or alloy of which they are composed, and then quenching them, usually in oil, water or liquid. Heretofore, it has been difllcult to avoid defective 40 pieces due to decarburization and formation of scale during the heating period. 'If the atmosphere in which a work is heated is oxidizing, the surface of the work after being heated to quenching temperature contains less carbon than the core and is too low in carbon content to obtain the desired surface hardness during quenching. The usual result is the formation of soft spots or soft skin on the surface rendering the work unfit 5 for use, or at least shortening its working life. Scale also is formed by oxidizing atmospheres and causes losses because of the difiiculty of its removal and the eifect upon quenching.

The development of tool and alloy steels requir- 55 ing high quenching temperatures have accentu- (Ci. Him-=46) ated the dimculties of decarburization and scaling.

Many. expedients have been resorted to in attempts to procure a non-oxidizing or neutral atmosphere; for example, combustibles, as oily ii waste, sawdust, paper and. the like have been placed in the furnace to deplete the oxygen; in some cases combusted gases have been used; in oil or gas fired furnaces, the ratio of fuel to air has been controlled to prevent excess of oxygen, etc. Consistent results have not been obtained with any of these methods. Even though, the oxygen was exhausted from the heat-treating chamber, the surface of the work was, nevertheless, decarburized to greater or less extent. As a 15 result of investigation, it was found that the decarburization was due to the presence of carbon dioxide, or water vapor, or both. Rapid decarburizing occurred even when the water vapor was as low as .05%. In fact, traces of water vapor m were sufiicient to cause decarburization. The furnace atmosphere usually also contains some water vapor, or carbon dioxide, because of infiltration.

Attempts to conserve or replace the surface carbon by dipping the hot pieces in cyanide before quenching, or by covering or packing the pieces with bone black during heating, have not been satisfactory because of the practical impos- 3o sibility of obtaining consistently good results.

I have found that the efi'ects of water vapor, oxygen, and carbon dioxide, etc, can be offset by heating the work in a slightly carburizing atmosphere; i. e., instead of trying to remove these decarbonizing elements from the heating atmosphere, or subsequently to supply carbon to the work, I add to the atmosphere surrounding the work during heating suficient carburizing agent to insure that the rate of carburization is equal 40 to or preferably slightly" higher than the rate of decarbonization. Theoretically, during the heating, it would suffice to efiect equilibrium between the rate at which carbon of the iron is reacting with the decarburizing constituents of the atmosphere and the rate at which carburiz ing is being effected; but, in practice, the carburizing agent, as a gaseous hydrocarbon, should be supplied slightly in excess, as the resultant formation of a faint soot deposit is visible evidence that the work is in proper condition for quenching; i. e., that the concentration of carbon at or near the surface is the same as, or perhaps slightly higher than the concentration more remote from the Su face. so

By practice of my method, consistent results are obtained, and actually higher surface hard nesses are produced than were previously obtainable; for example, with oil-quenching steels 4 points (Rockwell C) increase of hardness has been obtained, and with water-quenching steels an increase of 1 points has resulted. The surfaceof the work so heated has no scale, ensuring faster and more uniform quenching action. Improved quenching results are obtained because the surface is kept clean, afiording an increased depth of hardeningfor any given quenching temperature, therefore allowing lower quenching temperatures than before for the same hardness, with consequent less likelihood of distortion.

By way of example, with silverware dies, the

production life of dies hardened by my method.

compared to the production life of dies hardened by prior methods was five or more times longer. Moreover, several hours of stoning were necessary, after hardening by the prior methods, before the dies could be used, whereas, the original finish of the dies, hardened by my method, was restored by a brief light rubbing with crocus cloth. v Curves A, C and D, Fig. 2, are hardness curves of three specimens, identical before hardening,

and hardened respectively in accordance with my method, and in accordance with the prior method in an electric and a gas furnace whose atmos pheres were not controlled. In all threecases the pieces were heated to the same temperature above critical and quenched.

Not only is the surface hardness of specimen A significantly higher than the surface hardness of the other specimens, but its hardness to the center is higher than the greatest hardness of any point of the other specimens, and at the center the hardness of specimen A is tremendously greater than the center hardness of specimens D and C. The superior surface hardness of specimen A resuited from prevention of loss of carbon at and near the surface, and because of rapid quenching action, due to the clean surface of this specimen, a

' the hardness extends far below the surface. Both of the other specimens C and D are of inferior surface hardness due to loss of carbon near the surface; curve C shows this very clearly as the piece at the surface is softer than it is from below the surface to about and much softer than at a depth of from about to, say, For both specimens C and D, the hardness toward and near the center is poor because the quenching action was impeded by the fouled surface of the pieces.

Moreover, the variation in surface hardness (not shown by Fig. 2) of specimen A is only about a half point Rockwell C, whereas, the surface hardness variation of the others is as much as six times. as great.

My method of hardening should not be confused with carburizing in the sense that the term carburizing is used in the art. In carburizing; the steels are of lowcarbon content, c. g., 0.2%

or less, and the object is to add carbon to form a high carbon case on the low carbon core. The

temperature used in carburizing is chosen to effect penetration into the metal of carbon at a desired rate, and is not functionally related to critical perature for a long period of time, for example; four hours or usually more. Hardening in gen:

quenching them.

Depending upon the type and capacity of the furnace and the nature and size of the load, the time for hardening may vary considerably, as from a few minutes to perhaps not more than two hours; and, as above stated, the time, in any event, is that required to heat the work to the desired supercritical temperature and is not related to duration of an essential reaction, as in carburizing.

Curves A and B, Fig. 2, illustrate the difference between hardening and carburizing. Curve A represents the hardness curve of a specimen of' oil-hardening tool steel hardened in accordance with my method; the steel, before hardening, is of high and uniform carbon content from surface to center, and after hardening, the core, as well as the surface, is hard because of the heat -treatment. Curve B represents the hardness curve of a carburized specimen which, before carburization, is of low carbon content and which, after carburizing and hardening, is still of low carbon content (except at and closely adjacent the surface) so that it cannot be materially hardened by heat treatment except at or near the surface.

The inferior sub-surface hardness shown in curve B exists because the core is not capable of being hardened, whereas, the inferior hardness near the center of specimens having curves C and D is due to poor quenching action characteristic of prior hardening methods. As above pointed out, the steel of specimens C and D, if treated in accordance with my method, has the characteristic A, whereas, the specimen of curve B cannot have such a hardness characteristic.

The two processes exemplified by curves A and B are for different types of steels, involve different considerations of time and temperature, and are for obtaining different ends; the steels represented by curves A and B have different fields of use.

In general, my method is contrasted in that it is concerned with the prevention of loss of carbon from the work during heating to a supercritical temperature, and prevention of the surface fouling or formation of scale during that time.

The atmosphere for effecting my method may be prepared by cracking a liquid hydrocarbon in a chamber separate from the hardening chamber. Clean work and freedom from surface decarburization areobtainable by introducing hydrocarbon oils, vapors, or gases into the hardening furnace. However, it was found necessary to take into consideration the fact that a number of equilibrium reactions exist in the furnace between carbon dioxide, carbon monoxide, carbon, water vapor and other breakdown components of the oils. These caused formation of water vapor at certain temperatures and resulted in oxidization and decarburization the effect of which could not always be overcome at the higher temperatures where the equilibrium of the reactions shifted so that water vapor was largely eliminated. Also, the direct introduction of the agent in some types 01, equipment is apt to cause an excess deposit of soft spots due to quenching difllculties.

In any event, the supply of carburizlng agent should continue during heating, as it has been found that an atmosphere containing hydrocarbon, and carbon monoxide, hydrogen, and/or water vapor decreases in carburizing power with increasing time of exposure in the hardening chamber.

It is preferable to use a pre-cracking chamber for preparing the gas or vapor and establishing the desired condition insofar as equilibrium reactions are concerned. The soot, or free carbon, if any, resulting from the cracking, is deposited in the crackingchamber and not in the hardening chamber into which the gas, after preparation, is introduced.

A satisfactory system for performing my method is shown in the drawing, to which reference is now made.

A suitable hydrocarbon as ethyleneglycolmonoethylether, fusel oil or the like, is disposed in a container I having a gauge 2 for showing the level. The flow of the liquid through the pipe 3 to the pre-cracking furnace 4 is controlled by a. needle valve 5 provided with a sight glass 6. The pre-cracking furnace comprises an inner shell I defining a heating chamber and an outer shell 8, the space between which is filled with a suitable heat-insulating material 9. The cylindrical space defined by the inner shell 1 is heated in any suitable manner, preferably by electrical heaters In, as in the furnace shown in Fig. 1 of Wrighton Patent #1,165,055. Within the shell I is disposed the retort ll having at its upper end a peripheral groove I! which receives the flange of a removable cover 13. The groove is filled with aluminum oxide or other sealing material to form a sealed cracking chamber. Within the chamber is disposed tube ll whose lower end opens to the interior of chamber II and whose upper and secured to cap l3 and thereby closed with respect to container II is in communication with feed pipe 3'. The liquid hydrocarbon falling through pipe 3' which extends through the cover l5 of the furnace, strikes the upper disc Ilia. of the target assembly l6 within tube I which, together with the other structure, is heated to cracking temperatures. The lower disc lob and the apertured plates I50, lid also are part of the target assembly which is supported by a few wires lie extending across tube ll. The gas formed by the pyrolytic breakdown of the oil passes. around the lower end of the tube I up around the outer shell H to the outlet pipe II which extends downwardly through the base ll of the furnace. :The inlet pipe I! of the hardening furnace 20 which projects through the base 2| thereof into the hardening chamber H is connected to the outlet ll of the pre-cracker by pipe 22.

The cracking of fusel oil tion comprising hydrogen, carbon and oxygen, for a given cracking temperature results in the productlon, by reaction endothermic in character, of methane, carbon monoxide, hydrogen and carbon in fixed proportions. The relative proportions of methane and carbon monoxide, the significant components of the furnace atmosphere of definite composition so produced, is controllable by varying the cracking temperature and/or the rate of flow or feed of the fusel oil or equivalent.

It has heretofore been proposed in heattreating methods, such as annealing, to prepare the furnace atmosphere by cracking of various hydrocarbon compounds such as naphtha, mixtures of methane and hydrogen, fuel gas, benzol or tar, but these materials and methods are unsuited to my purposes because, inter alia, these substances or mixtures lack oxygen, either alone or in proper combination, and so fail to provide a carbon-containing atmosphere of proper character.

It has also been proposed with substances or mixtures of this latter namedcharacter to crack them by a combustion reaction, exothermic in character, by which oxygen, as that of the air or otherwise not present in the substance or mixture, in uncontrollable and widely varying proportions enters into the composition of the resulting products, and in consequence the furnace atmosphere is very indefinite and not suited to my purposes, not only because of the practical impossibility of controlling the amount of oxygen which enters into the reaction, but also because the cracking temperature is the temperature of the reaction itself or dependent upon the reaction and so not to practical extent subject to control or controllable.

For quenching temperatures with which my invention is concerned, to wit, ranging from about 1300 F. to about 2000 F., steels or alloy steels of high carbon content are to prohibitive extent decarburized if their heating to proper quenching temperature within the aforesaid range be efi'ected in the presence of an atmosphere consisting of products of combustion, which at higher temperatures, as about 2300 F., do not I effect material decarburization.

Furthermore, organic compounds of indefinite or inconsistent chemical composition, as tars, bone oil, blast furnace and coke oven gases, are unsuited to my purposes and are'not comprehended within my invention, for they do not afiord or permit consistently reproducible results because use of them prevents the necessary stability of composition of the furnace atmosphere.

Soot and/or tarry deposits resulting from the breakdown of the hydrocarbon fluids, whether gaseous or liquid, are deposited in the pre-cracking unit and are not introducedinto the work chamber of the hardening furnace. The construction of the pre-cracking unit, as indicated,

ethyleneglycolmonoethyiether rr r-r HH H H and like organic compounds of definite composithe pipe I]. Similarly, removal of plugs 25 and 26 of the pipe crosses 24 and 2'1 permits the connecting pipe 22 between the units to be freed of any deposits. The target assembly may be removed for cleaning by withdrawing the supporting wires We.

The hardening furnace specifically illustrated comprises the inner shell or lining 28 between which and the outer shell 29 is disposed a suitable heat-insulating material 30. The retort which defines the hardening chamber, in which the work is disposed, is heated as by electric resistors 28a, although other sources of heat may be employed, if desired. The top of the furnace is closed by a cover 32 which is provided with an auxiliary cover 33 for the retort, the latter having a flange 34 which is received by the sealing groove 35 of the retort. Aluminum oxide or other sealing material is disposed in the groove to form an air-tight joint. The carburizing gas entering the chamber from pipe 19, flushes out the air and spent gases which are forced out through the pipe 36 which extends through the lid 32. The exhaust gas issuing from pipe 36 is ignited,- as by the continuously acting spark-plug S, as a safeguard against asphyxiation.

The metal used" for the hardening chamber, the pre-cracking chamber, and the covers thereof, etc., is a nickel chromium alloy such as 25% nickel, 18% chrome, remainder iron, since this material has sufficient resistance to carburization at temperatures within the heat-treating range which is from about 1300 F. to 2000 F. Both the cracking unit and the furnace are fabricated from sheet-metal, rather than made of castings, because of the decreased weight.

The pyrometers 31, 38 permit the temperatures of the pre-cracker and hardening furnace to be determined and controlled, preferably automatically. The temperture of the atmosphere in the hardening furnace will be within the range of from about 1300-2000 F. depending upon the nature of the steel. In general, the temperature within the aforesaid range, is lower for nickel alloy steels and higher for chromium alloy steels. In all cases, the work is heated to a predetermined temperature above the critical point of its steel and then removed for quenching. The atmosphere within the furnace is maintained in a slightly carburizing state by controlling the feed of oil to the pre-cracker.

With the system described, it is possible to heat pieces of work in the hardening chamber to the desired extent above a critical point and, by control of the furnace atmosphere, to maintain a slight carburizing action, loss of carbon from the surface of the work and scaling can be avoided. The furnace temperature and state of the furnace atmosphere beingunder control, the pieces of successive runs or batches are uniform in character, yielding uniform results upon quenching.

The time at which the work arrives at its critical temperature may be observed by noting the abrupt change or hump in the rate of change of the work temperature curve in accordance with the method of Wrighton Patent #1,188,128, which change may be amplified by the arrangement shown in Martin #1543582 or, as in Harsch et al. #1,911,l91, the determination can be accurately made by observance of the humps" in a curve indicative of the differences between the work temperature and the furnace temperature existing as the work is being heated, the source of heat being controlled to maintain a constant difference between the work and the furnace temperature. The hump methodis not suited for high-speed tool steels, as tungstensteels, because their critical temperatures are far too low compared to their necessary quenching temperatures; for example, their critical temperatures are about 1600 F., whereas the proper quenching temperatures are about 2300 F.

In general, the work is heated'as rapidly as possible without causing distortion to a desired temperature above the critical point duringwhich time the furnace atmosphere is maintained slightly carburizing to prevent loss of carbon from the work, and then the work is removed and quenched. By rapidly stirring or circulating the gas, the concentration of the gas may be maintained highly constant even for dense loads. This can be effected by disposing a fan in the furnace; that is, the hardening furnace may be similar in construction to that disclosed in my application Serial No. 511,694, filed January 28, 1931, Patent No. 2,032,209, February 25, 1936.

It has been found in practice that the rate of quenching is substantially higher than previously obtainable, affording materially greater sub-surface hardness, as shown by curve A, Fig. 2. This is due to the freedom of the surfaces of the pieces from scale, which materially retards the quenching action whether the quenching medium be oil, water, air or other suitable quenching fluid. Because of the higher quenching rate, the quenching temperatures may be lowered, particularly on oil-hardening steel, which is of advantage since it reduces warpage difiiculties encountered at higher temperatures.

The rate of heating, the quenching temperature, and the condition of the surface of the work, the three vital factors of hardening, are by my method brought within control so that enhanced surface and subsurface hardness can be obtained consistently.

It is also of advantage to maintain the furnace atmosphere slightly carburizing during heating of previously hardened pieces to suitably high drawing temperature for tempering or toughening, after which the pieces are withdrawn and quenched. Loss of carbon from the surface of the work is prevented, and the surface is kept clean, affording better quenching action. Forcible circulation or agitation of the furnace atmosphere is desirable, as it ensures uniformity of temperature through the load and also ensures that the atmosphere in contact with all parts of the work is slightly carburizing.

I disclaim herefrom the production of the heattreating atmosphere by cracking naphtha, tar, benzol, fuel gas, or mixtures of methane and hydrogen, and disclaim herefrom methods of heat treatment which involve production of the heattreating atmosphere by cracking of organic compounds which do not contain oxygen, and methods which involve production of the heat-treating atmosphere by combustion of organic compounds or by cracking them at temperatures of or imposed by combustion.

Nor is there comprehended by my invention any method of hardening, tempering or toughening of steels and alloy steels, of high carbon content, in accordance with which the steel is heated to a quenching temperature in the presence of products of cracking of an organic compound or compounds comprising carbon, oxygen and hydrogen in definite proportions, and the effect of either substantially decarburizing or substantially carburizing the steel and so to have caused material change in the carbon content of the steel at its surface at the time of quenching; or any method in accordance with which products of combustion are utilized for protective atmosphere, which products of combustion, at the temperatures utilized by me, do not in practice prevent material decarburization of high carbon steels which are quenched at temperatures of about or below 2000 F.

What I claim is:

In the art of hardening, tempering or toughening steels or alloy. steels of high carbon content, the method of maintaining, during heating to quenching temperature within a range of from about 1300 F. to 2000" F., the carbon content of the steel substantially constant and its surface in condition ensuring high rate of and uniform quenching, which comprises bringing the steel to quenching temperature, simultaneously providing about the steel an atmosphere comprising products of endothermic cracking of an organic compound, or mixture of organic compounds, of chemically definite composition of carbon, hydrogen and oxygen, and maintaining in said atmosphere a concentration of carbon preventive [of oxidation and decarburization of and case-formation upon the steel.

. JOHN W. HARSCH.

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