Surface hardening of Ti alloys by gas-phase nitridation: kinetic control of the nitrogen activity
a technology of nitrogen activity and surface hardening, which is applied in the direction of solid-state diffusion coating, metal material coating process, fatigue resistance, etc., can solve the problems of invariably precipitating carbides and nitrides, reducing the concentration of the desired interstitial solute, and reducing the potential effect of formation
Inactive Publication Date: 2006-03-09
ERNST FR +2
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Benefits of technology
[0042] maintaining the work piece in the nitrogen gas for a period of time so that nitrogen is absorbed onto the surface of the work piece and diffused into the work piece for a desired distance while preventing the formation of nitrides on the surface of the work piece and within the work piece.
[0048] maintaining the work piece in the nitrogen gas for a period of time so that nitrogen is absorbed onto the surface of the work piece and diffused into the work piece for a desired distance while preventing the formation of nitrides on the surface of the work piece and within the work piece.
Problems solved by technology
For many such advanced applications, however, these alloys lack sufficient surface hardness, wear resistance, fatigue resistance, and corrosion resistance.
References 1,15-18 However, those approaches invariably cause precipitation of carbides and nitrides, predominantly at the surface and at grain boundaries.
This decreases the concentration of the desired interstitial solute, and the presence of titanium carbides and nitrides compromises the fatigue resistance.
Unfortunately, removal of the layer containing the precipitates generally introduces substantial surface roughness or cracks, negatively impacting the mechanical and corrosion behavior in a similar fashion as the presence of carbide or nitride particles.
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[0073] The present invention, relates to an a process for improving the surface properties of titanium alloys through inward diffusion of interstitial solutes without the formation of any second phases.
[0074] The Ti—N equilibrium phase diagram in FIG. 1 indicates considerable solubility of nitrogen in pure titanium. At 1050° C., the solubility reaches its maximum of 23 atomic % (at %) in titanium (Ti). Correspondingly high solubilities exist in Ti-based alloys. However, because unalloyed titanium undergoes an allotropic phase transformation from α-Ti to β-Ti at 882° C., which is usually undesired in engineering materials, the highest possible nitridation temperature for two-phase alloys, such as Ti-6Al-4V, is probably lower. At 860° C., for example, the nitrogen solubility in a-titanium is still about 14 at %.
[0075] According to published data (Reference 19), as shown in FIG. 2, a nitrogen content of 14 at % increases the hardness of α-titanium from about 375 HV to about 900 HV. M...
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Abstract
The present invention relates to surface hardening of a metal surface of a work piece formed of titanium and titanium alloys. The method comprises the steps of heating the work piece; exposing the metal surface to a nitrogen gas having a partial pressure lower than about 10−2 Pa; and maintaining the work piece in the nitrogen gas so that nitrogen is absorbed onto the surface of the work piece and diffused into the work piece for a desired distance while preventing the formation of nitrides on the surface of the work piece and within the work piece.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of U.S. Provisional Patent Application No. 60 / 576,871 filed on Jun. 3, 2004, which is incorporated herein by reference.FIELD OF THE INVENTION [0002] The present invention relates to surface hardening of metals, and specifically to surface hardening of titanium and titanium alloys by the method of gas-phase nitridation with kinetic control of nitrogen activity. BACKGROUND [0003] Lightweight structural alloys based on titanium (Ti) have important aerospace and other industrial applications. For many such advanced applications, however, these alloys lack sufficient surface hardness, wear resistance, fatigue resistance, and corrosion resistance. A powerful approach to enhance their utility is to harden a thin layer below the surface of the material by inward diffusion of interstitial solutes, particularly carbon and nitrogen. This has long been recognized, and a number of treatments have been proposed ove...
Claims
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Login to View More IPC IPC(8): C23C8/24
CPCC23C8/24
Inventor ERNST, FRANKHEUER, ARTHUR H.MICHAL, GARY M.
Owner ERNST FR