US3223562A - Heat treating process for martensitic transformation alloys - Google Patents

Description

United States Patent O M 3 223,562 HEAT TREATING PROCESS FOR MAR'I'ENSTIC TRANSFORMATEON ALLOYS William I. Bassett lll, Gardena, Calif., assigner, by mesne assignments, to Union Carbide Corporation, a corporation of New York Filed May 1, 1961, Ser. No. 106,865 16 Claims. (Cl. 148-125) This application is fa continuation in part of my applications S. N. 842,427, filed September 25, 1959, now abandoned, and S. N. 65,858, led October 28, 1960, for Method and Apparatus for Heat Treating Metals.

This invention relates to the heat treatment of alloys having an austenitic to martensitic type transformation, and has as its basic object to improve the tensile and uniformity characteristics of Such alloys in relation to the plasticity thereof. The invention is especially applicable to the conditioning of ferrous all-cys having the austeniticmartensitic type phase change. The term plasticity is used in a broad sense to include all of the desirable properties of ductility `and formability, such as the ability to withstand bending and deformation as in forming operations; machinability, i.e. the adaptability to cutting and abrading in shaping and finishing operations, and in particular the ability to undergo the removal of metal smoothly with a minimum of tearing action; and malleability, Le., the ability to undergo a high degree of deformation, as in coining operations, various impact extrusion operations and miscellaneous forming operations wherein considerable proportions of the alloy are moved from one area to another, without being fractured or weakened in the process.

More specifically, the invention is directed to the heat treatment of austenitic-martensitic transforming type alloys as contrasted to precipitation-hardening alloys not having substantial austenitic-martensitic transformation properties.

An object is to provide an improved heat-treating process whereby improved tensile strength may be imparted to alloys without incurring the extent of loss of ductility, malleability, etc. which heretofore has been regarded as an inevitable consequence of processing to increase tensile properties. Conversely, the invention aims to provide a heat-treated Ialloy article having improved plasticity `as defined above, together with improved tensile and allied properties. Stated more specifically, the invention provides for attaining such plastic properties as are normally attained only by a considerable sacrifice in tensile properties, while simultaneously attaining the high tensile properties which can be attained, in conventional hardening processes, only through an extreme sacrifice in plasticity and which, under normal processing methods, are partially restored to the alloy by a process wherein the hardness and tensile properties are tempered in order to improve the plasticity.

A further object is to provide an improved process in which stress-relieving is `accomplished -at much lower temperatures than are utilized in normal tempering or stressrelieving operations.

Other objects and advantages will become apparent in the ensuing specifications and appended drawing, in which:

FIG. 1 is la chart of my process, illustrating in particu- 3,223,562 Patented Dec. 14, 1965 ICC lar the change in temperature level between the several stages of the process; and

FIG. 2 is a chart of my process, compared to processes of the prior art.

GENERAL DESCRIPTION OF THE PROCESS The basic process, in its optimum embodiment, includes the following steps:

(l) Austenitizing, i.e. heating the alloy to a temperature .above the upper transformation temperatures Iand maintaining it at that temperature until transformation to austenite is complete.

(2) Primary quench through the transformation range.

(3) Secondary or cool quench to approximately room temperature.

(4) Refrigerative quench.

(5) Restoration to room temperature.

(6) Stress relief.

Terminology of the arzt-In considering the differences between my process and known processes, it is helpful to review certain terminology utilized in discussing austenitic-martensitic transformations. The term austenitizing may be utilized to designate the initial heating of the alloy to `a point within a temperature range wherein its structure becomes austenite, namely; a solid solution of carbon in iron. The recognized temperature range for austenitizing most alloy steels extends from about 1380 F. to about 1800 F.

In quench-hardening an austenitized alloy, in the higher portion of the quenching range, if the quenching rate or cooling proceeds sutiiciently slowly, there will be formed one of the decomposition products including ferrite and cementite, which most hardening processes `attempt to avoid by quenching rapidly down to a lower temperature zone, where begins the transformation from .austenite to the desired hard martensitic structure. This transformation takes place through a range of decreasing temperatures down to a completion level referred to in the following paragraph, which range is referred to herein as the critical martensitic transformation range, or more briefly, as the transformation range.

The term Ms is commonly used to designate the upper temperature level of ythis critical transformation range, for the alloy being worked. The term Mf is used to designate what is commonly regarded as the lower level of this range. Actually, this is a level where approximately 90% transformation to martensite has taken place, and this level is now more commonly referred to as the M90 level rather than the Mf level. This is the meaning applied to these terms as used in these specifications and claims. It is generally recognized that some residual transformation of austenite to martensite takes place below the Mf level.

It is common practice to depict the transformation characteristics of an alloy by fa chart (such as is shown in the drawing) wherein temperature levels are charted on a vertical Y axis and elapsed quenching time is charted (usually logarithmically) on a horizontal X axis. In plotting the formation of decomposition and transformation products, there results an area (such as the area designated A-l-F-I-C in the drawing) lying to the right of an S-curve line (TTT curve, or time, temperature, transformation curve) which area is herein referred to generally as the decomposition products area. The invention, in common with most hardening processes, attempts to ravoid all of the decomposition structures in this area. Intermediate the upper and lower limits of the temperature range from austenitic down to the Ms beginning of martensite transformation range, the TTT curve at the margin of the decomposition products :area :has a bulge toward the Y axis which bulge is commonly referred to as the nose of the TTT curve (indicated at n in the drawing).

My process is distinguished from pri-or known heattreating processes, principally in departing from accepted relationships between the various steps of austenitizing, quenching, tempering and stress-relieving steps which are utilized in most heat treating processes. In this respect, it differs from the better known heat treating processes such as Martempering and Austempering, which are currently regarded as improvements over processes such as the well known oil-quench wherein temperature is dropped on a straight line curve through the martensitic transformation range.

The prior urn-In order to compare my process with the better known conventional processes, the latter are here reviewed briefly.

Ol-quench.-In the conventional `oil quench, which is a relatively quick quench, the temperature is dropped rapidly on a substantially straight line curve, avoiding the nose of the decomposition products area, down to arnbient temperature (approximately room temperature) beginning at a conventional austenitizing temperature and dropping entirely through the martensite-transformation range without waiting for internal and external temperatures to equalize.

Martemperng-ln this more recently developed process, the alloy is rst austenitized at a conventional austenitizing temperature, is then quenched rapidly on a quench curve avoiding the nose of the decomposition products area, to a temperature somewhat above the conventional Ms level, is held at this level until external and internal temperatures have equalize/d, and is then quenched more slowly (e.g. in air) through the martensite transformation range down to room temperature.

Austempering.-In this process, based upon isothermal transformation of austenite to a non-martensitic structure, after an initial heating step in which the alloy is austenitized at a conventional aus-tenitizing temperature, the alloy is cooled to a temperature level somewhat above the Ms level, and is held at this level for an extended period of time going all the way through the decomposition products area above the martensite formation range (with a resultant formation of Bainite), for a sufficient time to complete such transformation. Then the alloy is cooled down to room temperature.

The invention-My invention is distinguished over all such prior heat treating processes in combination of departures ,from the characteristics utilized in the several steps of the known processes. More specifically, my process utilizes, in some of 4the basic heat treatment steps, departures in temperature level-s at certain stages of the process, time (duration of various aspects of transformation) and rate (of change between various stages and levels of transformation). For example, -the invention in general permits, if desired, the use of a higher austenitizing temperature, for any selected alloy, than that conventionally used for such alloy.

At vthe outset, consideration may be given to the well known fact that during a quenching operation, if carried beyond a maximum permissible time interval, there arrives a point where separation between carbon and iron occurs, with a resultant softening effect (separation of ferritic-pearlitic complex). Accordingly, time is a most important factor in a quenching operation, and in order to attain satisfactory results the invention utilizes a quenching step wherein the alloy is quenched at the most rapid practicalrate for the alloy, until temperatures has dropped past the nose of the TTT curve, so that the martensitic structure may develop without diffusion of carbon. It will of course be understood that for various different alloys, the permissible time interval will vary.

My improved heat-treating process is especially useful when it incorporates therein the magnetic quenching phase disclosed in my two co-pending applications identiiied above and best results are obtained by utilizing the magnetic treatment in the quenching step. However, I have found that significantly improved results, though of more moderate degree, over prior heat-treating processes, are obtained in my heat-treating process without the assistance of the magnetic treatment during quenching, and the present invention involves the use of the heattreating process per se, apart from the magnetic treatment.

In -my improved process, after `an initial heating step in which the alloy is austenitized, it is quenched in a hot medium such as an agitated salt bath or mineral oil, without interruption through the critical martensite transformation range without waiting for temperature equalization in the work piece, but instead of continuing this quench down to room temperature, as in the standard oil quench process, it is arrested at a temperature near the M level, and the work is held at that level beyond the point where equalization of internal and external temperatures occurs. In holding beyond the point where temperature equalization occurs (and where it is commonly understood that expansion and release of thermal energy has occurred) some initial stress-relieving of the martensite already formed, takes place, thus avoiding such undesirable effects. vFor most alloys, I iind it best to hold the temperature of the primary quench bath at a level somewhat below the M90 level, although for other alloys, the temperature equalization level may coincide with or be somewhat above the M90 level, although below the Ms level (for example, where the alloy has a high austenitizing temperature, and elevation of the primary quench temperature is beneficial in minimizing damaging or undesirable effects of quenching stresses).

Following the completion of the primary quench, the alloy is transferred to a cooler medium (such as water) and is cooled therein down to room temperature; is then transferred to a refrigerated medium, maintained at a very low temperature (which may be lower than F.) and is further quenched therein down to the temperature of that medium; and then is subjected to a prolonged stress-relieving operation at a temperature below conventional stress-relieving temperature for that alloy.

DETAILED DESCRIPTION Austenz'tizng.-In the initial step (hardening or austenitizing) performed in a furnace, the part is heated to a temperature above the upper critical temperature of austenitic transformation, to an austenitizing temperature. More specifically, the temperature of the part is raised to a temperature between about l500 F. and 2500 F. in the heat-treating step, depending on the specific composition of the alloy.

Primary quench- At the end of the austenitizing step, the part is transferred to a liquid quench medium in which the temperature is maintained at an equalization level slightly lower than the M90 temperature which for most steels is in the range between 200 F. and 750 F. For example, for nickel-chrome-molybdenum steels, the quench bath will be maintained at a temperature in the range of 200 F. to 400 F. and for chrome-molybdenum steels, in the range of 200 F. to 500 F.

While other quenching mediums having comparable characteristics of heat-absorption rate and adaptability for maintaining an elevated quenching temperature within the range stated above, can be used, I prefer to use a quench comprising a fused salt, for optimum results.

The part is retained in the salt bath (either in continuous, non-,interrupted quench -or in a cyclic isothermal quench as hereinafter referred to) for a period of time depending upon the alloy being treated, during which period martensitic transformation progresses to the point where at least 90 percent transformation is effected. This time interval for nickel-chrome-molybdenum type 4340 alloy steel may range between ten minutes and twenty minutes depending upon the section size of the work being quenched. For type 431 stainless steel, the duration may range between twenty minutes and one hour for comparable section sizes.

In actual practice of the invention, I have obtained good results by utilizing a cyclic quench wherein the part is subjected to two or more periods of treatment in the salt bath, interrupted by a rapid quench down to approximately room temperature. For example, good results have been obtained by initially quenching the part isothermally in the salt bath for a period of about ten minutes, then dropping the temperature to room temperature as rapidly as possible (e.g. in a water quench) then placing the part back in the salt bath and holding it therein for a second period of approximately ten minutes. The time interval at room temperature between the two salt bath treatments is quite short, the procedure preferably being one in which the part is placed back in the salt bath for the second period of isothermal treatment immediately upon reaching room temperature in the water quench. Actually, the second treatment in the salt bath involves some stress-relieving operation to relieve quench stresses and resultant cracking or other undesirable results thereof.

The end of the quenching operation described above completes the primary stage of quench processing for alloy compositions which undergo the desired phase transformation at relatively high temperatures. The part is then subjected to the secondary quench.

Secondary or cool quench- In the secondary quench, which may endure for a period as short as one minute, the temperature of the parts is quickly reduced from the temperature of the primary quench bath down to room temperature. The secondary quench may utilize a conventional water bath maintained at about room temperature (eg. 70 F.). In this quench, the transformation from austenitic to martensitic phase is substantially completed, although some latent austenite (e.g. a minor percentage remaining after the completion of the martensitic transform-ation stage described above) may remain.

Refrigeratve qhench.-The parts are then immediately transferred from the secondary quench to the refrigerative quench wherein any latent austenite is transformed to the martensitic state. In the refrigerative quench, the parts are subjected to a refrigerated medium (e.g. Dry Ice and alcohol) at an extremely low temperature, in the range of 100 F, to 150 F., for an extended period of time which may extend to three hours.

Restoration to room temperatura-At the end of the refrigerative step, the parts are allowed to return to room temperature.

Stress relief.--Internal stresses induced in the metal by the treatment received in the three stages of quenching, are relieved, subsequent to the refrigerative quench by a stress-relieving operation to which the part is subjected for a prolonged period of time which may range all the way from one hour to 120 hours or more. The temperature as applied to this stress-relieving operation may be in the range of 200 F. to 300 F. for lowalloy steels and martensitic stainless steels considerably lower than the temperatures utilized in stress-relieving such alloys in conventional processes up to a range of 700 F. to 1000 F. for tool steels including ch-rome steels.

The drawing-Referring now to the drawing, and to FIG. l specifically, the chart shown therein designates quenching time upon the X axis, reading from left to right, and designates temperature in degrees Fahrenheit, on the Y axis. The TTT curve is so designated in the chart. At the upper left hand corner of the chart, the broken line t indicates outside temperature of a part being quenched and the broken line t' designates internal temperature. The point of intersection of the two lines t and indicated at indicates the stage in a quenching operation where these temperatures are equalized. For the particular alloy being charted, the area labeled A-l-F-i-C (for austenite plus ferrite plus cementite) designates the area of formation of such decomposition products in a conventional quenching operation, occurring at the maximum time interval ending at l. In this area, the resultant structure will not have the properties sought in the heat treating process, because of diffusion of carbon. The horizontal line Ms designates the temperature at which martensitic transformation begins as cooling progresses, and the line M00 designates the temperature level at which martensitic transformation is nearly (e.g. percent) complete. This line is commonly designated the Mf line. The area embraced between the Ms and M00 lines represents the martensitic transformation range.

The vertical lines t, t', progressing from top to bottom, indicate the cooling occurring in the quench step (eg. cooling from austenitizing temperature to a temperature below the M00 temperature). As in most conventional hardening processes, the cooling curve t, t' descends sufciently vertically (rapidly) to avoid the nose n of the TTT curve. The broken line w of the chart indicates a period of time during which a part is subjected to the prima-ry isothermal quenching step of my process, and its level indicates the temperature (near the M00 level) at which this step is sustained after the initial, quick temperature drop indicated at t, t. The descending line d designates the further cooling occurring during the water quench or secondary quench. The broken line v indicates a further time interval in which the secondary quench takes place, and its level indicates the equilibrium temperature level of this secondary quench step.

The descending line d indicates cooling from room temperature to the range of F. or lower which occurs in the refrigerative quench and the broken horizontal line z indicates the time interval of such step. The level of this line z indicates the temperature equilibrium attained in this step. The diagonally ascending broken line r indicates the final stress relieving step wherein the temperature is raised to a level which remains below the tempering level of normal processes and is held at that level for a prolonged time period which in most cases is not less than 20 hours.

In the refrigerating step, the extremely low temperature to which the work is subjected results in latent transformation into substantially 100 percent martensite.

In the further and nal step of stress relief, in which latent stresses are removed, I find that it is possible to attain substantially complete stress relief within a considerably lower temperature range than is utilized in conventional processes, the maximum temperature to which the work is elevated in my process being in the order of 200 F. lower than those considered necessary in conventional processes. I attribute this to the fact that the austenitic-martensitic transformation stresses developed in my process are -only a small fraction of the extent of such stresses developed in conventional processes.

Referring now to FIG. 2, the disclosure of my process therein corresponds generally to that shown in FIG. l (with the exception that it does not include the steps following the secondary quench). FIG. 2 further discloses in contrast to my process, the better known prior art processes, namely, austempering, martempering and oil quench, represented by respective broken lines (dot dash line for austempering, dash line for martempering and dotted line for oil quench) and respectively labeled by these names. FIG. 2 illustrates graphically how, in the austempering process, temperature equalization is performed at a temperature level above the martensite transformation range, going through the decomposition products'area, with a resultant formation of bainite. The curve for martempering is shown at e as effecting equalization at a level above the Ms level and then proceeding slowly through the martensite range as indicated by the diagonal portion of the curve). The oil quench curve indicates the continuous uninterrupted quenching downwardly through the martensite transformation range and down at q to -room temperature Without interruption.

I find that the grain structure as fixed in my process, does not undergo any noticeable further transformation even over many months of observation, and accordingly, I have concluded that the combination of processing steps in my process results in a stable, stress-relieved condition in the meal which remains essentially permanent.

As the final result of my process, I achieve greatly increased tensile properties for any given degree of plasticity.

Examples of use of process-The invention may be more specically identified by the following specific examples of the process as applied to representative ferromagnetic alloys, compared to a conventional oil-quench.

Example 1.--Type 4130 chromium molybdenum alloy steel-non-magnetiC--non-cyclc quench Analysis: (percent by weight) carbon 0.28/033, manganese, OAD/0.60; phosphorus, 0.04 max.; sulphur, 0.04 max.; silicon, O/0.35; nickel, none; chromium, 0.80/1.10 molybdenum 0.15/1.10.

Austenitize to l625 F.-l800 F. range.

Primary quench, non-cyclic, non-magnetic, in salt bath at 300 F. Duration, 10 min. Terminal temperature about 300 F.

Cool quench in water at room temperature, beginning immediately aftercompletion of primary quench. Duration, 1 min. Terminal temperature, room temperature.

Refrigerative quench in refrigerated brine solution at about 100 F., beginning directly after secondary quench. Duration about 3 hours.

Restoration to room temperature, following refrigerative quench.

Stress relief-reheat to 212 F. Soak at that temperature for 20 hours and air cool.

Comparative results-Example I as compared to conventional heat treatment of type 4130 alloy steel quenched in oil from 1550 F. austenitizing temperature-tempered by drawing at 800, on a 1" round specimen.

Convention- Processed by the ally Processed Invention 280,600 max. Tensile Strength, p.s.1 210,000 265,100 average.

251,500 min. 12.0% max. Elongation 11% 10.3% average.

7.0% min. 39.8% max. Reduction o Area 44% 33.5% average.

27.8% min.

Example Il.-Type 4340 nickel-chromiam-molybdenum alloy steel-non-magnetic, non cyclic quench Restoration to room temperature, following refrigerative quench.

Stress relief-reheat to about 212 F. and hold at that temperature for about 20 hours. Then air cool.

Comparative results-Example II as compared to conventional heat-treatment of type 4340 alloy steel quenched in oil from 1500 F. and tempered at 400 F.

GONVENTIONALLY PROCESSED Yield, Tensile, Elongation, Reduction of p.S.. p.s.i. percent Area, percent PROCESSED BY INVENTION Test 712K 333, 000 348, 000 10.0 287 297, 000 320, 000 s. o 3e. 1 340,200 10.0 27.0 M 341,200 11.0 2s. 4

Example IIL-Type 4340 chromium nickel-molybdenum alloy steel-non magnetic, cyclic quench Analysis: Carbon 0.38/ 0.43, manganese 0.60/ 0.80; phosphorus 0.025 max., sulphur 0.025 max.; silicon 0.20/ 0.35 nickel 1.65/ 2.00; chromium 0.70/ 0.90; molybdenum O20/0.30.

Austenitize to 1700 F.

Primary quench-Four cycles of 5 minutes each in 300 F. salt bath, following one another Without delay between cycles. Beginning immediately after austenitizing completed. Terminal temperature about 300 F.

Cool quench-in water at room temperature, beginning immediately after primary quench. Duration, about 1 minute.

Refrigerative quench-in refrigerated brine solution at about --112 F., beginning directly after secondary quench. Duration about 3 hours.

Restoration to room temperature, following refrigerative quench.

Stress relief-reheat to about 212 F. and hold at that temperature for about 18 hours, then air cool.

Comparative results-Example III as compared to conventional heat treatment of type 4340 alloy steel quenched in oil from 1500 F. and tempered at 40G-500 F.

CONVENTIONAL PRO CESS Tensile, Elongation, Reduction Yield, p.s.1. p.s.i. percent of Area, percent PROCESSED BY INVENTION Test JBX762 Code No. II-

NM40 340, 000 l0 27 Test No. 2 g 341, 200 11 28. 4

Example I V.-T ype 4130 chrome-moly alloy steel--nonmagnetic cyclic quench CONVENTIONALLY PRO CESSED Tensile, Elongation, Reduction Yield, p.s.. p.s.i. percent of Area, percent PROCESSED BY INVENTION Test .TB X720 274, 200 11 31 'Test N0. 2 266, 300 10 34. 3

While the examples given above are based upon data derived from tests using the cyclical form of the primary quench operation, I find, from comparative test using the non-cyclic or single stage primary quench, that substantially the same results are obtained in the single stage quench.

I claim:

I. An improved process of effecting an austenitic to martensitic type transformation in an alloy subject to such transformation, including the following steps: heating the article to and maintaining it at an austenitizing temperature until a homogeneous solid `solution of austenite is obtained; then rapidly quenching the article in a primary quench through its critical martensitic transformation range in which about 90% of martensite is formed; arresting said quench at an equalization temperature approximately equal to the temperature at which said 90% of martinsite is formed; subjecting said alloy to said equalization temperature while external and internal temperatures of the article are equalized; and then further quenching the article from said temperature level down to room temperature.

2. The process defined in claim 1, wherein said primary quench is performed in a liquid quenching medium maintained at a temperature near said lower limit of said transform-ation range.

3. An improved process of effecting an austenitic to martensitic type transformation in an alloy of the group including chromium molybdenum alloy steel and nickel chromium molybdenum alloy steel subject to such transformation, including the following steps: heating the article to and maintaining it at an austenitizing temperature until a homogeneous solid solution of austenite is obtained; then quenching the article in a primary quench, at a rate sufficiently rapid to avoid formation of transformation products other than martensite, through the critical martensitic transformation range in which about 90% of martensite is formed; arresting said primary quench at an equalization slightly below the temperature at which said 90% of martensite is formed; holding the article of said equalization temperature slightly 'below the temperature at which said 90% of martensite is formed; holding the article at sai-d equalization temperature until external and internal temperatures are substantially equalized; and then quenching in a secondary quench from said temperature level down to room temperature.

4. The process defined in claim 3, wherein said primary quench is performed in a hot liquid quenching medium maintained at said temperature level just below said lower limit of the critical transformation range; and wherein said second stage quench is performed at a slower rate than said first stage quench.

5. The process defined in claim 4, wherein said primary quench is performed in a salt bath and said secondary quench is a water quench.

6. An improved process of effecting an austenitic to martensitic type transformation in an alloy of the group including chromium molybdenum alloy steel and nickel chromium molybdenum alloy steel subject to such transformation, including the following steps: heating the article to and maintaining it at an austenitizing temperature until a homogeneous solid solution of austenite is obtained; then quenching the article in a primary quench through its critical martensitic transformation range in which about of martensite is formed without waiting for equalization of internal and external temperature, so as to avoid formation of decomposition products, to a temperature level substantially at the M-90 temperature of said alloy; arresting said quench at said temperature level in an equalization step until internal and external temperatures are substantially equalized; then quenching the article in a secondary quench from said temperature level down to room temperature.

7. The process defined in claim 6, wherein said equalization step is continued for a period of time extending beyond the point where the temperature equalization is substantially completed, so as to effect initial stress relief in martensite formed up to that point, thus avoiding undesirable effects of quenching stresses.

8. An improved process of effecting an austenitic to martensitic type transformation in an alloy selected from the group including chromium molybdenum alloy steel and nickel chromium molybdenum alloy steel and having austenitic to martensitic transformation properties comparable to those of 4130 chrome moly steel and 4340 nickel chrome moly steel, including the following steps: heating the article to and maintaining it in an austenitizing temperature until a homogeneous solid solution of austenite is obtained; then quenching the article in a primary quench, rapidly through its critical martensitic transformation range in which about 90% of martensite is formed, down to an equalization temperature level approximately at the M-90 temperature of said alloy, in a heated liquid quenching medium maintained at said equalization level; holding the article in said liquid quenching medium at said equalization level until external and internal temperatures of the article are substantially equalized; then removing the article from said liquid quenching medium and quenching it in a cooler medium down to room ternperature.

9. The process dened in claim 8, including the subsequent step of subjecting the article to refrigeration at a sub-zero F. temperature to stabilize its transformed structure.

10. The process defined in claim 9, including the further subsequent step of tempering the stabilized alloy at a temperature lower than the recognized tempering level for that alloy.

11. The process defined in claim 10, wherein, in said tempering step, the alloy is held at the tempering level for a period of time considerably longer than the recognized tempering period for that alloy.

12. An improved process of effecting an austenitic to martensitic type transformation in an alloy selected from the group including chromium molybdenum alloy steel and nickel chromium molybdenum alloy steel and having austenitic to martensitic transformation properties comparable to those of 4130 chrome .moly steel and of 4340 nickel chrome moly steel, including the following steps: heating the article to and maintaining it at an austenitizing temperature until a homogeneous solid solution of austenite is obtained; cyclically quenching the austenitized article in a primary quench in a heated liquid quenching medium maintained at an equalizing temperature level just below the lower limit of the critical martensitic transformation range in which about 90% of martensite is formed for that alloy, in a series of stages including a rst stage in which 'i l the -article is cooled from the austenitizing range through said critical transformation range to said equalizing temperature level, avoiding vformation of decomposition products and without awaiting equalization of external and internal temperatures in the article, a stage of arrested cooling in which the article is held at said equalization temperature level in said heated liquid quenching medium for `an extended period of time until its internal and external temperatures are substantially equalized, followed by a second stage of quenching in a cooler quenching medium from said equalizing temperature level down to room temperature, followed by a reheating of the article in said heated quenching medium back to said equalizing ternperature level; and then further quenching the part in a secondary quench in `a cooler quenching medium from said equalizing temperature down to room temperature.

13. The process defined in claim 12, wherein said reheating step is performed by holding said article in said heated liquid quenching medium for an extended period of time approximately las long as that of said temperatureequalizing step.

14. The process defined in claim 12, including the further step, following said cyclic primary quench, of subjecting the article to refrigeration at a sub-zero F. temperature to stabilize the transformed alloy structure.

15.1'1he process defined in claim 12, including the further steps, following said cyclic primary quench, of rst `subjecting the article to refrigeration at a sub-zero F. temperature to complete the transformation; and thereafter tempering the alloy over a prolonged period of time at a temperature lower thanr the recognized tempering range for that alloy.

|16. The method deiined in claim 12., wherein, in sai stage of arrested cooling, the article is held at the equalization temperature level for a period of time extending beyond the point where temperature equalization is substantially completed, so as to effect initial stress relief in martensite formed up to that point, thus avoiding undesirable effects of quenching stresses.

Ratei-ences Cited by the Examiner UNITED STATES PATENTS 1,924,099 8/1933 Bain@ et a1. 14s-143 12,350,532 6/1944 Richardson 14s-144 2,441,628 5/1948 Gritmhs et a1. 14s- 143 OTHER REFERENCES' Metals Handbook: 1948 edition, published by the A.S.M., page 668, Fig. 2; and table on page 307 relied upon.

Atlas of Isothermal Transformation Diagrams: by U.S. Steel Corp. page 38 relied on.

A.I.M.E. Trans: 1930, Transformation `of Austenite at Constant Subcritical Temperatures, by Davenport and Bain, page 12 relied on.

The Making, Shaping and Treating of Steel: by U.S. Steel Corp., page S11 relied on.

Steel and lts Heat Treatment: vol. 1 by D. K. Bullens, page 459 relied on.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, ROGER L. CAMPBELL,

Examiners.

Claims (1)

1. AN IMPROVED PROCESS OF EFFECTING AN AUSTENITIC TO MARTENSITIC TYPE TRANSFORMATION IN AN ALLOY SUBJECT TO SUCH TRANSFORMATION, INCLUDING THE FOLLOWING STEPS: HEATING THE ARTICLE TO AND MAINTAINING IT AT AN AUSTENITIZING TEMPERATURE UNTIL A HOMOGENEOUS SOLID SOLUTION OF AUSTENITE IS OBTAINED; THEN RAPIDLY QUENCHING THE ARTICLE IN A PRIMARY QUENCH THROUGH ITS CRITICAL MARTENSITIC TRANSFORMATION RANGE IN WHICH ABOUT 90% OF MARTENSITE IS FORMED; ARRESTING SAID QUENCH AT AN EQUALIZATION TEMPERATURE APPROXIMATELY EQUAL TO THE TEMPERATURE AT WHICH SAID 90% OF MARTINSITE IS FORMED; SUBJECTING SAID ALLOY TO SAID EQUALIZATION TEMPERATURE WHILE EXTERNAL AND INTERNAL TEMPERATURE OF THE ARTICLE ARE EQUALIZED; AND THEN FURTHER QUENCHING THE ARTICLE FROM SAID TEMPERATURE LEVEL DOWN TO ROOM TEMPERATURE.

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