WO1997014820A1

WO1997014820A1 - Method of treating titanium parts

Info

Publication number: WO1997014820A1
Application number: PCT/US1996/016714
Authority: WO
Inventors: Douglas W. Fay, Sr.
Original assignee: Sturm, Ruger & Company, Inc.
Priority date: 1995-10-18
Filing date: 1996-10-17
Publication date: 1997-04-24

Abstract

A method of treating titanium-containing parts and the parts so-treated, which treatment includes placing the parts in a furnace chamber (12) having a gaseous atmosphere of 15-25 % carbon monoxide, 30-50 % nitrogen and 30-50 % hydrogen, are disclosed, The atmosphere is at a temperature of between 1450-1850 °F and the treatment lasts for 5-300 minutes. Parts experience improved hardness, strength and wear resistance.

Description

METHOD OF TREATING TITANIUM PARTS

Background Of The Invention

It has been proposed to heat treat metals in hydrogen and nitrogen atmospheres (U.S. Patent No. 3,211,590) or in an ammonia and methane atmosphere (U.S. Patent No. 3,891,473). The treatment of titanium in a neutral atmosphere of helium has also been suggested (U.S. Patent No. 4,098,623).

No prior method has included titanium parts treatment in an atmosphere of nitrogen, hydrogen and carbon monoxide.

Low carbon steels and iron have been treated in atmospheres of 20% carbon monoxide, 40% hydrogen, and 40% nitrogen and gas. Treatment of titanium alloy specimens using propane-argon mixtures or methane-argon mixtures has been reported for improving resistance to galling fTransactions of the ASM Vol. 46, pp. 257, 267-70 (1953).

Summary Of The Invention

Broadly, the present invention comprises a method of treating titanium-containing parts having the steps of creating in a furnace at atmospheric pressure mixture of nitrogen, methanol and optionally natural gas or propane, causing such mixture to obtain an equilibrium; then introducing the titanium parts into the furnace and finally reducing the furnace temperature to a range of 1450°F. to 1850F° for a time period of between five (5) and three hundred (300) minutes. The temperature range selected should yield a two phase structure balanced between alpha and beta phases.

It is a feature of the method that the surface area of the titanium parts so treated are very hard and gall resistant. It is a feature that a coating caused to form on and at the alloy surface during treatment consists predominantly of titanium oxides with regions of oxynitrides and at times some carbonitrides.

It is a feature that the surface treatment results in a complex coating which imparts a smooth and relatively hard gall-resistant surface with an appealing blue-back coloration. This coating consists of a variety of oxides, oxynitrides and carbonitrides not to exclude other possible forms of carbon based surface modification constituents with graphite and/or a reduced frictional surface polymer material such as Teflon-like hydrocarbon polymer.

Related Applications

This application is a continuation-in-part of U. S. Patent Application Serial No. 08/436,686 filed May 8, 1995 which in turn was a continuation application of U.S. Patent Application Serial No. 08/232,890 filed April 25, 1994.

Brief Description Of The Drawings

Fig. 1 is a partially sectioned elevational view of the apparatus used to carry out the present inventive method.

Description Of The Preferred Embodiment

In Fig. 1, apparatus 10 includes furnace 11, furnace chamber box 12 and titanium parts basket 14 supported by blocks 15. Chamber 12 has three gaseous conduit inlets 16, 17, and 18. Conduit 16 supplies methanol from methanol tank 21 to sparger 22 with shaft. Conduit 17 supplies natural gas from gas tank 23 and conduit 18 supplies nitrogen from nitrogen tank 24. Fan 27 with shaft provides for mixing and circulation of gases in chamber 12 and exhaust chimney 29 (not shown) conducts exhaust gases from chamber 12. Pressure in furnace 11 is atmospheric or slightly above atmospheric. Sensor 37 measures the temperature, oxygen and gas components in chamber 12. Process monitoring unit 31 controls valves 33, 34, 35 in conduits 16, 17, and 18 respectively to control the mixture in chamber 12.

The combination of the gases in chamber box 12 creates a hydrocarbon inert gas mixture which is varied to achieve the metallurgical layer characteristics of the parts desired and is composed of an atmosphere of carbon monoxide between 15- 25%, hydrogen between 30-50% and nitrogen between 30-50% using, as supply, gaseous nitrogen, methanol, and propane or natural gas. Gaseous percentages are by volume. Process control equipment 'employed may include but is not limited to oxygen measuring and control. Atmosphere characterization equipment may be used but is not necessary.

After completion of the treatment in furnace 11, the titanium parts are transferred to a vacuum furnace at a temperature in the range of 1200°F-1450°F for 30 to 300 minutes. This subsequent step reduces the hydrogen level in the parts to improve yield strength. The vacuum furnace is at a vacuum of between 10 to 500 micrometers of pressure. One atmosphere of pressure equals 760,000 micrometers.

Parts which are treatable using the present invention are 100% titanium or alloys of titanium including:

Alloy A Component Percent

Titanium 90

Aluminum 6

Vanadium 4

Alloy B

Component Percent Titanium 94.5 Aluminum 3 Vanadium 2.5 Alloy C

Component Percent

Titanium 82

Zirconium 4

Tin 2

Molybdenum 6

Aluminum 6

Percentages of the alloys are by weight. Tests run on cast Ti-6AL-4V alloys treated by the present invention and untreated produced the following results:

1. Mechanical Properties

Surface Tensile Strength Condition Hardness . Hk25 UTS KSI YS,KSl %EL

Untreated 350 - 380 132.7 114.3 10.2

Treated 980 - 1300 137.2 121.7 12.2

The surface hardness of treated samples was significantly higher than that of the untreated samples with the balance of properties essentially similar. Tensile ductility is measured by the percent of elongation or the percent change in cross-sectional area of the tensile specimen when the specimen is tested to failure in a conventional tensile testing situation.

2. Wear Resistance

Abrasive testing using a #10 grit aluminum oxide media indicated the treated samples wore less than untreated samples.

Condition % Wear

Treated .3774

Untreated .4533 When 100% titanium or titanium alloys are used, it is preferred that the titanium portion be an alpha-beta composition in which a significant percentage of both alpha and beta crystalline structure phases coexist. The alpha plus beta region of the phase diagram is a solid phase. The reaction taking place in the furnace is solid state diffusion of the furnace atmosphere including nitrogen.

Example A batch of firearm triggers made of Alloy A were placed in a furnace in which nitrogen, methanol and natural gas are introduced in proportions of nitrogen: 40% (gas) ; methanol: 58-60% (liquid) ; and propane: 0-2% (gas) to create in the furnace a gaseous atmosphere of:

Constituent Percent Carbon monoxide 15-25 Hydrogen 30-50

Nitrogen 30-50

The gaseous atmosphere was at or near atmospheric pressure. The treatment atmosphere is a complex mixture of: nitrogen

C0₂ CO hydrogen oxygen methane and water. This mixture should exist in equilibrium in the furnace. Methane dissociates to form hydrogen and carbon dioxide and carbon monoxide to accomplish the parts treatment including carburizing of the parts surfaces. Equilibrium is obtained upon completion of reactions of the mixture in the furnace. An equilibrium reaction equation is 2N₂+CH₃OH *=► 2N₂+2H₂+CO. Equilibrium determination is accomplished by indirect means such as the analysis of the implanted or diffused carbon level in the material surface treated. Other methods include the measurement of the carbon potential by the oxygen probe located in the furnace which indicates the carbon potential by an algorithm assuming a consistent level of CO gas in the furnace atmosphere. Carbon potential is largely equivalent to a function of the oxygen content assuming a constant CO content. If the CO content changes, the same oxygen % will yield a different carbon potential. Again, an algorithm is used to put all the variables in perspective. The control of carbon by maintaining a consistent atmosphere and then using oxygen % as a control parameter is very important.

A principle of control of the process is controlling the proportions of gases flowing into the furnace.

Claims

I CLAIM :

1. A method for treating titanium-containing parts comprising 1) introducing into the furnace box a mixture of component gases in turn comprising a) 30-50% nitrogen; b) 30-50% hydrogen; and c) 15-25% carbon oxygen compound 2) introducing titanium parts into a furnace box having air therein;

3) sealing the box from the outside atmosphere; and

4) monitoring and controlling the temperature in the box at between 1450F⁰ and 1850F⁰.

2. The method of claim 1 in which the carbon oxygen compound is carbon monoxide.

3. The method of claim 1 in which the titanium- containing parts include a significant percentage of both alpha and beta crystalline structure phases.

4. The method of claim 1 in which the introduction of component gases is controlled so that the furnace atmosphere comprises by volume: i) 30-50% nitrogen; ii) 30-50% hydrogen; and iii) 15-25% carbon monoxide.

5. The method of claim 1 in which the titanium parts are treated in a vacuum chamber following treatment in the furnace at temperatures of 1200°F-1450°F for thirty (30) to three hundred (300) minutes.

6. A titanium-containing part treated by the method of claim 1.