US3158515A

US3158515A - Metal treatment in molten alkali-barium-boro-silicate glass and composition

Info

Publication number: US3158515A
Application number: US196616A
Authority: US
Inventors: Harold J Michael
Original assignee: North American Aviation Corp
Current assignee: North American Aviation Corp
Priority date: 1962-05-22
Filing date: 1962-05-22
Publication date: 1964-11-24
Anticipated expiration: 1981-11-24

Description

- 3,158,515 METAL TREATB/IENT IN MOLTEN ALKALI-BARI- UM-BORO-SILICATE GLASS AND COMPOSITION Harold J. Michael, Columhus,ihio, assignor to North American Aviation, Inc. K Na Drawing. Filed May 22, 1962, Ser. No. 196,616

' 12 Claims. (Cl. 14820) This invention relates generally to the treatment of metals to develop improved properties therein, and specifically concerns a method of heat treating metals and compositions for use in connection with such heat-treating.

Basically, the method of this invention employs the manipulative steps of immersing to-be-treatedmetal in a bath of molten glass having particular characteristics, maintaining the metal in such molten glass bath a sufiicient length of time to achieve adesired elevated temperature in the metal, and afterwards removing the heated metal from the bath of molten glass to allow the metal to cool to ambient temperature. No step of coating the metal with a protective composition prior to heating is required. Fur ther, no subsequent step of removing a portion of the heattreating composition employed in the operative steps "of this invention from the heat-treated metal is *required. The heat-treating composition employed with' the method is essentially a glass that is completely and automatically spalled from the heat-treated metal by cooling alone. The metallurgical objectives of heat-treating metals to develop-';improved properties therein are well-understood and it is recognized that many different methods have been developed for attaining such objectives. One basic c0n ventional heat-treating method involves heating the metal to and maintaining the metal at an elevated temperature, and afterwards cooling the 'metal from the elevated em, perature to an ambient tempera ture, all in an air atmosphere. In some instances, an atmosphere comprised of an inert gas is substituted for" the air atmosphere. Sometimes the metal is more rapidly cooled asby quenching in water after heating. Another common practice is to heat-' treat metal in a neutral molten bath essentially comprised of a mixture of salts. It is also conventional toprovide a special protective coating on to-be-treated metals prior to their immersion in a heat-treating composition or atmosphere which otherwise would cause undesirable metallurgical end results in the heat-treated metal. The various disadvantages associated with such methods and known heat-treating medium compositions may' be avoided through the practice of my invention. 5 An important object of this invention is to provide a method of heat-treating metals which completely prevents oxidation, decarburization, corrosion, and the like'at surfaces of the metal during heating to a desired elevated temperature in a molten bath,duringmaintenance of the metal at such elevated temperature in the molten bath, and during cooling of the metal to an ambient temperature after removal from the bath.

Another object of this invention is to provide a complete method of heat-treating metals which'essentially involves or requires only the manipulative. steps of immersing the metal in a molten bath, heating the metal to a desired elevated temperature during'immersion in the molten bath, maintaining the to-be-treated metal at the desired elevated temperature a sutficient time' 'to effect metallurgical changes in the metal, removing the metal from the molten bath to afterwards cool the metal from United State Pa c tQ 3,158,515 Patent ecl No v 24, 1964 2 thedesired elevated temperature to an. ambient temperature"; 4 Another object of this inventionis toprovide' a method of heat treating metals which does not require the application of a protective coating .to the metal prior to or during heating to an elevated temperatu're'and forthe prevention of oxidation, decarburization, corrosio'n,or;the like. 1 7 Another object of this invention is to provide a method of heat-treating metals which does not'require an operative step of removing a protective. coating orretained heattreating composition from surfaces of the metal subsequent to cooling from a desired elevated temperature.

A still further object of this invention is to provide a heat-treating composition which does not cause undesirable chemical or metallurgicalchanges to metals immersed in a molten bath comprised of such heat-treating composition. 4

Another object of this invention is to provide a heattreating composition which does not decompose, vaporize, ,or give off corrosive or toxic fumesor vapors when maintained ina molten bath state at an elevated heat-treating temperature. 1 I v Q j v Another object of this invention is tov provide a, heattreating composition'which does not become contami ists in'a molten'state. I I

Another object of this invention is to provide a" heattreating composition which is usedfin' molten bath form and which requires a minimum effort to maintainer reetify the molten bathin or to a preferred operatingcondition. 1 A still further object of this invention is to provide a heat-treating composition which is automatically and completely spalled from metal immersed and heat-treated in a'molten b-ath' of the composition after the metal is removed from the molten bath composition to cool to'an ambienttemperature. i

Another object of this invention is to provide a heat: treating composition which may be used in molten bath form and which automatically and completely spalls from a metal heat-treated'therein when the metal is afterwards removed and cooled to an ambient temperature, the temperature at which such heat-treating composition spalls from the metal being below the temperature at which the metal readily oxidizes, corrodes, or becomes deca'rburized by exposure to air or the like.

A still further object of this invention is to provide" a heat-treating composition'which produces a spalled-otf cullet that maybe reused for heat-treating purposes. 7

A still further object of this invention is to provide a heat-treating composition which has adequate viscosity at a desired heat-treating elevated temperature 'whereby' draining of the composition from metalremoved from a molten bath comprisedof the composition does not cause a loss of protection againstoxidation, decarburization, corrosion, and the like at corner and edge regions of the; metal. I Another object of this invention is to provide a he'at treating composition which is used in molten bath form and which can be readily adjusted to provide different viscosity characteristics at elevated temperatures whereby agiven-desired viscosity may be developed for molten .3 treating composition which has a formulation permitting use of the composition for heat-treating metals of any size or shape compatible with a molten bath of the composition thereby avoiding the necessity of adjusting formulations for metal size.

Another object of this invention is to provide a heattreating composition which is elfective to remove unwanted surface oxides and the like from to-be-treated metals immersed in a molten bath comprised of the composition.

Another object of this invention is to provide a heattreating composition which does not plate or deposit composition metal elements on metals immersed in" a molten bath of the composition for heat-treating purposes.

A still further object of this invention is to provide a heat-treating composition which may be used in molten bath form without limitation as to configuration characteristics, such as holes and the like, of metals immersed therein.

Another object of my invention is to provide a heattreating composition which may be used in molten bath form in a manner that does not require removal of the molten composition from the bath container if heating.

and cooling of the bath over a wide temperature range or shut-down of bath operations is required.

Another object of this invention is to provide a heattreating composition which employs readily available and readily combinable ingredients to comprise the composition.

Other advantages and results obtained by practice of this invention will become apparent during consideration of the following description. I

This description, for purposes of orderly development and presentation, is separated into' two parts. The first part concerns the heat-treating composition used in this invention, its characteristics, and its formulation. The second part relates to the method or operative steps aspect of my invention and details use of the heat-treating composition in the heat-treating of various metals to obtain changed physical properties.

HEAT-TREATING COMPOSITION Throughout this description and in the claims repeated reference will be made to the use of glass as the heattreating composition of this invention. Not all glasses are satisfactory or operative as to the practice of my invention and the term glass or molten bath of glass does not include all known vitreous compositions. It is necessary therefore to describe the general characteristics of the composition employed as a part of this invention and to detail specific examples thereof.

The heat-treating composition of this invention has a controlled viscosity characteristic which is established and measured with respect to molten state temperature at which the composition is employed for heat-treating purposes. The heat-treating composition of this invention is employed for heat-treating purposes generally at a temperature which is in the range that extends from approximately 1400 F. to 2500 F. As to common metals which have been heat-treated using this invention, the heat-treating composition is formulated for operative temperatures which are in the range of from approximately 1700 F. to 2150 F. Such common metals have included alloy steels (Type H-ll, for instance), corrosion-resistant alloys (Types PH -4 Mo, 17-7 PH, and 17-4 PH, for instance), titanium alloys (Type 7 A1-4 M0, for instance), exotic metals (Ren 41, for instance), and the like. Other metalssuch as aluminum alloys, copper alloys, and the like have comparatively low melting temperatures and generally the use of heattreating temperatures in the range of 1700 .F. to 2150 F. is not desired. Formulation of the heat-treating composition is controlled to produce a viscosity at the operating temperatures of this .inVeIlfiQ l wh is in h range of approximately two (2) to six (6) poises. The two (2) poise value is essentially considered as a minimum viscosity; the six (6) poise value is a preferred upper limit even though greater viscosities at temperature can be tolerated. The viscosity information given herein is established by observation and is based on a comparison basis. True viscosity measurements taken at the elevated operating temperature of this invention are extremely difiicult to obtain. However, I have been able to make apparent-type viscosity determinations by comparison with Gardner bubble viscosity standards at room (or ambient) temperature (25 C.). Thus, at each operating temperature of this invention an apparent composition viscosity of from approximately two (2) poises to approximately six (6) poises is obtained, with an apparent viscosity of four (4) poises being preferred.

It is a purpose of this invention to obtain the preferred viscosity (viscosity range) at the temperature which the molten glass is actually used. The Example I composition given hereinafter has a formulation which develops the proper viscosity when the composition is heated to an operating temperature in the range of 1800 F. to 1900 F. as for heat-treating an alloy steel such as Type H-ll. The Example II compositions given hereinafter has a viscosity in the preferred range when heated to an operating temperature of 1950 F. for heat-treating an exotic alloy metal such as Ren 41. The heat-treating composition of this invention given as Example 111 develops the preferred molten state viscosity when maintained at a temperature of approximately 1750 F. and may be used in the heat-treatment of certain titanium alloys. The control of viscosity in this invention is developed largely through composition formulation adjustments related to the amount of such ingredients as the aluminum oxide and the barium oxide in the theoretical oxide composition, and also related to the amount of batch ingredients having lithium, potassium, and sodium elements.

The heat-treating composition of this invention also has a preferred thermal contraction characteristic. This characteristic is measured largely with reference to a temperature range immediately below the softening point temperature at which the composition develops a loss of pyro-plasticity upon cooling. Such characteristic may also be equated to the compositions rate or coefficient of thermal contraction; it is also related directly to the coefiicient of thermal expansion for the material. In all cases it is preferred that the heat-treating composition have a rate of thermal contraction which is appreciably less than the rate of thermal contraction of the metal which has been or is to be heat-treated therein. The treated metal, when removed from a molten 1 7 bath of the heat-treating glass, retains a thin continuous film of the heat-treating composition over the entire exterior surface. Such continuousfilm provides protection for the metal against oxidation, decarburization, corrosion, and the like as the metal is cooled in an air atmosphere from the heat-treating temperature to ambient temperature.

point temperature for the compositiomthe thin continuous protective film begins to solidify. -However, in the cooling temperature zone immediately below the composition softening point temperature the solid or nearly prefer that the rate of thermal contraction for the heat- V treating composition in the temperature range immediately below the glass softening point be characterized as not causing separation of retained composition from the metal until the metal has cooled to below thetem- As the metal and the adhering heat-treating com-. position are cooled and passed through the softening perat'ure at which the metal readily oxidizes. At this temperature significant amounts of oxide are immediately formed at the metal surface when the partially heated metal -is exposed to an oxygen-containing environment. In the case of Type H-ll alloy steel the temperature is in the range of 600 F. to 800 F.; the oxidized surface is sometimes characterized by a blue or blued color. In the case of the hereinafter-described Example I composition used in connection with the heat-treating of Type H.1 1' alloy. steel, the heat-treating composition begins to solidify upon' the metal during cooling and at approximately 900 F. However, the solidified composition does not spallsfrom the metal until the metal temperature has been lowered to a value in the range of from 500 ,F. to 600 F. At such temperatures the H-11 metal does not readily oxidize after loss of the protective film.

The heat-treating composition of this invention is also characterized as being substantially free of metal elements lead, zinc, copper, andthe like and compounds, especially oxides, which contain such metal elements. In the practice of this invention I prefer not to use metalcontaining ingredients which would be readily reduced when" the composition is maintained in a molten state with the heat-treated metal'immersed therein. In this manner I avoid the possible depositing of unwanted metals on the metal being processed. It is also preferred that the heat-treating composition not contain any significant amount of nucleating agents such at titanium dioxide. 4 The presence of a nucleating agent tends to cause glass system re-crystallization thus acting as a deterrent to spalling.

" The following specific formulations are given as examplesof heat-treating compositions which may be employed-"in the practice of this invention. In each instance the composition is first given as a theoretical oxide formulation and-afterwards is given in terms of an actual batch ingredient formulation which may be employed to obtain the theoretical oxide formulation. Generally,

each of the following example glasses may be referred to as having an alkali-barium-boro-silicate' glasssystem. The total alkali content is kept comparatively high in order that the molten composition might readily absorb unwanted oxides present on the' surface of metals immersed therein for heat-treating purposes. The subsequent presence of the removed unwanted oxide in the molten heat-treating composition does not affect performance of the heat-treating composition.

Example I.Heat-Treating Composition [Parts by Weight] Theoretical Oxide: Amount Silicon dioxide (SiO 36.9 .Boric oxide (B 0 20.0

. Barium oxide (BaO) 10.8 Aluminum oxide (A120 i 4.6

Sodium oxide-l-potassium oxide+lithium ox- The heat-treating composition based on the above formulation has been used for heat-treating materials which require an operating heat-treatment temperature of approximately 1850 F. ,-The composition may be used ;in .the operating temperature range of approximately 5 Example II.Heat-Txeating Composition [Parts by Weight] Theoretical Oxide:

Amount Silicon dioxide (SiO 37.5 .Boric oxide (B 0 18.0 Barium oxide (BaO) 10.0

Sodium oxide-i-potassium oxide+lithium oxide Na' o+K 0+Li o 21.5

Fluorine (F 43 Calcium oxide-l-ferric oxide (CaO+Fe O 0.7 Total 100.0

Batch ingredient:

Felspar 44.5 Anhydrous borax 24.0 Lithium carbonate 11.5 Sodium-silico-fluoride 7.0 Soda ash- 1.0 Barium carbonate 12.0 Total 100.0

:The' Example II c omposition may beused for heattrea'ting metals at an operating temperature of approximately 1950 F. The composition has a viscosity of approximately four (4) poises at that temperature. The

composition appears entirely suitable for heat-treating an exotic metal. such as Rene v41. .Uponcooling of the metal after heat-treating the composition retained on exterior. surfacesspallstromthe part at temperatures below whichthe metal isreadily oxidized. Spalling occur's at atemperature of approximately 600 -F. to 700 F. The softening .point temperature of this composition is approximately 950 F. p Example IIl.'Heat-Treating Composition i [Parts by Weight] 'laheoretical-- Oxide:- v I Amount Silicon dioxide(SiO 38.2 'B oric" oxide (B 0 21.0 Barium oxide (BaO) 9.4 Aluminum oxide (A1 0 2.0

Sodium oxide+potassium oxide+lithium ox- The formulation of the Example III composition develops a heat-treating medium which may be used-at a nominal operating-temperature of 1750 F. as for the heat-treatment of; certain titanium alloys (7 A l-4 Mo alloy). -The compo sition is consideredsuitable-for most temperaturesinthe range of approximately 1700 F. to 1800 F. The material has a. softening point temperature of approximately 850 It will completely spall from metal treated therein and serving as the composition host when the metal has been cooled'to a temperature of from approximately 450 F. to 550 F.

The above formulations detail three different heattreating compositions which may be used in the practice of this invention. However, such formulations are not considered all-inclusive as to the heat-treating composition of this invention. From experience a quantity range for each ingredient may be developed to provide for formulations which may be used advantageously in connection with the heat-treating of metals. The range of oxide ingredients in the theoretical oxide composition for suitable materials, on a parts by weight basis, appears to be as follows:

Silicon dioxide (Slog) 25-50 Boric oxide (B 10-30 Barium oxide (BaO) -15 Aluminum oxide (A1 0 2-8 Sodium oxide-l-potassium oxide+lithium oxide (Na O+K O+Li O) -34 Fluorine (F 0-8 Calcium oxide-l-ferric oxide (CaO+Fe O 0-1 Felspar 5-47 Anhydrous borax 12-40 Silica 0-28 Lithium carbonate 0-15 Sodium-silico-fiuoride 0-13 Soda ash 1-7 Barium carbonate 8-16 In the Example I formulation the 23 parts of alkali metal oxides were comprised of 15.4 parts sodium oxide (Na O), 5.5 parts lithium oxide (U 0), and 2.1 parts potassium oxide (K 0).

The above compositions are preferably prepared in frit form for us in the practice of this invention by conventional smelting, quenching, grinding, and drying practices. In each case the prepared fritis charged into a heat-treating furnace for melting and subsequent use. In some cases, however, the frit may be charged into a molten bath to maintain the existing molten bath at a prescribed operating level. The furnace equipment provided for containing the molten bath must be adequate in size for the particular metals (parts) to be heat-treated therein. The furnace interior is preferably provided with a refractory lining comparable to the linings provided in known glass smelting furnaces. The frit charge should be heated to temperature by resistance elements rather than by electrode action. The resistance elements are preferably of a molybdenum-type alloy composition.

HEAT-TREATING METHOD STEPS temperature to a desired elevated temperature which effects metallurgical changes in the metal, such heating taking place while the metal is immersed in a molten bath of the preferred heat-treating composition, and

' (2) Cooling the heated metal from the obtained ele' vated temperature to an ambient temperature to further effect metallurgical changes in the metal, such cooling taking place while the metal is protected by a thin continuous protective film of retained heat-treating composition.

From a manipulative standpoint, practice of the inven tion described and claimed herein involves the following operations: i

(l) Immersing the to-be-treated metal in a molten bath of the preferred heat-treating composition,

2); Maintaining the metal in such molten bath a suflicient length of time to achieve a desired elevated temperature in the metal, and

(3') Afterwards removing the heated metal from such molten bath to another environment to achieve cooling of the heated metal to an ambient temperature.

Further details of the method aspects of this invention are provided in the following examples.

Example IV Type H-ll alloy steel was immersed in a molten bath of heat-treating composition which had a formulation in accordance with Example I and which was at an operating temperature of approximately 1850" F. The metal was maintained in such-molten bath a suflicient time whereby the temperature of the metal reached approximately 1850 F. throughout its entire structure. Afterwards, the metal was removed from the molten bath and allowed to cool in an air environment to room temperature.

No special preparation of the metal prior to immersion in the molten bath was required. A thin continuous film of heat-treating composition was retained over the entire surface of the metal after the metal was removed from the bath. Excess heat-treating composition was permitted to drain from the removed metal into: the molten bath in heat-treating'furnace. When the metal surface had cooled to a temperature of approximately 900 F. the heat-treating composition retained on the surface of the metal began 'to solidify. Further cooling of the metal resulted in automatic and complete spalling of the retained heat-treating composition from the metal when a temperature of between 500 F. and 600 F. had been reached. The heat-treated metal was afterwards carefully examined and found to be completely free of oxide surface material, completely free of decarburized surface metal, and free of contamination caused by the molten heat-treating bath. Traces of oxide present at the metal surface prior to immersion in the molten bath were completely removed. No heat-treating composition was retained on the metal after spalling had been completed. Thus,.no subsequent step of removing heat-treating material by mechanical action or chemical action was required. The operative and manipulative steps described above vvith respect to the Type H-ll metal were part of an austenizing treatment wherein carbon and other alloying elements are taken into solution and wherein air cooling is performed to form martensite from austenite. The length of time during which the metal is retained in the molten bath varies with configuration and size. molten bath for as little as approximately twenty (20) minutes. Shapes having maximum dimensions up to approximately three inches (3") may require immersion in the molten bath for as long as approximately two (2) hours.

Example V Rene 41 metal can also be heat-treated m accordance with this invention. Suchheat-treatment, which may be a for annealing purposes, employs the heat-treating com- Sheet metal may require immersion in the heat-treating composition a .sufiicient time to bring the metal temperature to approximately 1950 F. After Wards, the metal is removed from the bath and cooled in an air environment to room temperature. The results obtained by the practice of the steps of this invention are identical to those recited above in connection with Exam-- Example VI Metal such as titanium alloy having the designation 7 Al-4 Mo may also be heat-treated using my invention. A molten bath of heat-treating composition having the formulation of Example 111 is provided and is maintained at approximately l750 F. The titanium alloy met-a1 is immersed in the molten bath and maintained therein a suflicient time to achieve a temperature of approximately 1750 F. throughout. Afterwards, the metal is removed from the molten bath and excess heat-treating composition allowed to drain from the metal into the heat-treating furnace. The metal, however, is cooled by water quenching rather than in an air environment. The objectives of this invention are achieved in connection with such heat-treatment. Accordingly, it is not necessary to cool the heat-treated metal in an air environment from the desired elevated temperature to an ambient temperature in order to obtain all the advantages of this invention.

Retained heat-treating composition spalled from the metal upon cooling may be referred to as cullet and is preferably returned to the heat-treating bath for further use. The molten bath described above in connection with Examples I through VI does not decompose or vaporize. Therefore, it does not give ofl. corrosive or toxic fumes such as those associated with molten baths of neutral salts. Further, the bath does not become contaminated by undesired oxide inclusions received from the metals processed therein. The only observed change is one of color tinting. No rectification or chemical balancing of the molten bath is required. However, it is necessary to add composition frit to maintain a prescribed bath operating level to the extent that cullet is not returned for reuse.

The composition and method steps of this invention are also considered to have application to brazing operations. In the aircraft industry it is common practice to manufacture honeycomb core panel assemblies by brazing component parts with a suitable brazing alloy while such parts and alloy are contained in a sealed metal retort. A sealed retort manufactured of metal and having properly placed component parts and brazing alloy therein may be immersed in a molten bath of the composition of this invention to effect the required brazing. Upon completion of the brazing and removal from the bath the retained composition adhering to the exterior surface of the retort will spall. Thus, the retort will be protected against deterioration due to oxidation while at all temperatures above that temperature at which its metal readily oxidizes.

I claim:

1. A method of heat-treating a metal part by heating and cooling to and from a temperature substantially greater than 600 F. to 700 F., comprising thesteps of:

(a) Covering said metal part with molten alkalibarium-boro-silicate glass while the temperature of said metal part is less than 600 F. to 700 F., (1)) Heating said molten glass-covered metal part from a temperature of less than 600 F. to 700 F. to a temperature substantially greater than 600 F. to 700 F.,

10 (c) Cooling said heated; molten glass-covered metal part fromsaid temperature substantially greaterthan 600 to 700 F. to a temperature whereat the molten glass covering said heated metal part becomes solidified in adhering relation to the surface of said heated metal part, and p (d) Completely spalling said adhered solidified. glass from said cooled metal part by further cooling said metal part to a temperature below 600 F. to 700 F.

2. The method defined by claim 1, wherein said molten alkali-barium-boro-silicate glass consists of approximately 25% to 50% SiO 10% to 30% B 0 5% to 15% BaO, 2% to 8% A1 0 and 10% to 34% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

3. The method defined by claim 1, wherein said molten alkali-barium-boro-silicate glass consists of approximately 37% SiO 20% B 0 11% BaO, 5% A1 0 and 23% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

4. A method of heat-treating metal by heating and cool-' ing to and from an elevated temperature substantially greater than 600 F. to 700 F. comprising the steps of:

(a) Immersing said metal part in a bath of molten 'alkali-barium-boro-silicate glass while the surface temperature of said part is at a temperature less than 600 F. to 700 F.,

(b) Maintaining said immersed metal part in said molten glass bath a sufficient time for said metal part to reach said elevated temperature,

(0) Removing said immersed metal part from said molten glass bath after said metal part has reached said elevated temperature and with a thin continuous protective film of said molten glass in adhering relation to the entire surface of said metal part, and

(d) Cooling said removed metal part from said elevated temperature to a temperature below 600 F. to 700 F. to completely spall said retained molten glass film from the entire surface of said metal part after solidification and at a temperature below 600 F. to 700 F.

5. The method defined by claim 4, wherein said alkalibariurn-boro-silicate glass consists of approximately 25 to 50% SiO 10% to 30% B 0 5% to 15% BaO, 2% to 8% A1 0 and 10% to 34% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

6. The method defined by claim 5, wherein said alkalibarium-boro-silicate glass consists of approximately 37% SiO 20% B 0 11% BaO, 5% A1 0 and 23% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

7. The method defined by claim 5, wherein said alkalibarium-boro-silicate glass consists of approximately 38% SiO 21% B 0 9% BaO, 2% A1 0 and 25% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

8. The method defined by claim 5, wherein said alkalibarium-boro-silicate glass consists of approximately 37.5% SiO 18% B 0 10% BaO, 8% Al O and 21.5% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

9. A heat-treating alkali-barium-boro-silicate glass composition which is free of the oxides of lead, zinc, or copper and which consist of approximately 25 to 50% SiO- 10% to 30% B 0 5% to 15% BaO, 2% to 8% A1 0 and 18% to 25% alkali metal oxide, each on a theoretical oxide content and percentage weight basis.

10. The heat-treating composition defined by claim 9, which consists of approximately 37% SiO 20% B 0 11% BaO, 5% Al O and 23% alkali metal oxide.

11. The heat-treating composition defined by claim 9, which consists of approximately 37.5% SiO 18% B 0 10% BaO, 8% A1 0 and 2 1.5% alkali metal oxide.

12; The heat-treatingv composition defined by claim 9, 2,846,325 Bennett et a1. Aug. 5,1958 which consists of approximately 38% S10 21% B 0 2,889,238 Long et a1 l June 2, 1959 9% B210, 2% A1 0 and 25% alkali metal oxide. 2,898,253 Schneider et a1 Aug. 4, 1959 2,906,631 Rindone Sept. 29, 1959 References Cited in the file of this patent 5 2,959,503 Lindson Nov; 8, 1960 UNITED STATES PATENTS h OTI-IER REFERENCES 2,020,467 Heinz Nov. 12, 1937 American Gas Association Monthly, April 1959, pages 2,420Q644 Athy et a1 May 20, 1947 26-28.

Claims

1. A METHOD OF HEAT-TREATING A METAL PART BY HEATING AND COOLING TO AND FROM A TEMPERATURE SUBSTANTIALLY GREATER THAN 600*F. TO 700*F., COMPRISING THE STEPS OF: (A) COVERING SAID METAL PART WITH MOLTEN ALKALIBARIUM-BORO-SILICATE GLASS WHILE THE TEMPERATURE OF SAID METAL PART IS LESS THAN 600*F. TO 700*F., (B) HEATING SAID MOLTEN GLASS-COVERED METAL PART FROM A TEMPERATURE OF LESS THAN 600*F. TO 700*F. TO A TEMPERATURE SUBSTANTIALLY GREATER THAN 600*F. TO 700*F., (C) COOLING SAID HEATED, MOLTEN GLASS-COVERED METAL PART FROM SAID TEMPERATURE SUBSTANTIALLY GREATER THAN 600*F. TO 700*F. TO A TEMPERATURE WHEREAT THE MOLTEN GLASS COVERING SAID HEATED METAL PART BECOMES SOLIDIFIED IN ADHERING RELATION TO THE SURFACE OF SAID HEATED METAL PART, AND (D) COMPLETELY SPALLING SAID ADHERED SOLIDIFIED GLASS FROM SAID COOLED METAL PART BY FURTHER COOLING SAID METAL PART TO A TEMPERATURE BELOW 600*F. TO 700*F.