Ultra-fast boriding of metal surfaces for improved properties

Abstract

A method of ultra-fast boriding of a metal surface. The method includes the step of providing a metal component, providing a molten electrolyte having boron components therein, providing an electrochemical boriding system including an induction furnace, operating the induction furnace to establish a high temperature for the molten electrolyte, and boriding the metal surface to achieve a boride layer on the metal surface.

Description

CROSS REFERENCE TO RELATED PATENT APPLICATIONS[0001]The present application claims priority to U.S. Provisional Patent Application No. 61 / 059,177, filed Jun. 5, 2008 incorporated herein by reference in its entirety.STATEMENT OF GOVERNMENT INTEREST[0002]The United States Government has certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the United States Government and The University of Chicago and / or pursuant to Contract No. DE-AC02-06CH11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory.FIELD OF THE INVENTION[0003]This invention is directed to an ultra-fast surface treatment method that results in hard, wear, corrosion and erosion resistant, and low-friction surface layers on metallic substrates. More particularly, the present invention relates to an ultra fast electrochemical boriding technique which can lead to dramatic improvements in the mechanical and tribological properties of treated meta...

AI Technical Summary

Benefits of technology

[0013]The present invention provides a method for producing metallic products with hard boride layers for a variety of mechanical and erosion resistant applications. The preferred method involves preparation of a molten electrolyte consisting of about 90 wt. % borates of alkaline and alkaline earth elements (such as borax) and about 10 wt. % carbonates of alkaline and alkaline-earth elements (such as sodium and / or calcium carbonate) or sodium chloride. Addition of small amounts (0.1 to 5 wt. %) of other halides (chlorides, fluorides, and iodides, etc.) of alkaline and / or alkaline-earth elements (like, LiCl, NaCl, CaCl2) can have positive effects as electrolyte enhancers. Oxides, hydroxides, and carbonates of such elements may also be used to control the viscosity and melting point of the electrolyte. Furthermore, using at least one of a high frequency induction furnace, external agitation, mixing of electrolyte or vibrating / shaking of the work piece holder can help overcome diffusion barriers in the electrochemical process and thus help achieve fast boriding and thick boride layers (about 100 micrometers or more in the case of low carbon steels) with desirable mechanical properties in short processing times (for example, less than an hour). Such a procedure can also result in a more uniform boride layer thickness on the surfaces of odd-shaped or intricate work pieces.

[0014]In electrochemical boriding, graphite is often used as the crucible material. The same graphite crucible can also serve as the anode of the electrochemical cell. Due to the high temperature nature of the boriding process, the graphite crucible or anode may undergo oxidation and hence thin down or wear out after repeated uses. As an alternative approach, in our process, we can also use the metallic and / or borided forms of titanium, aluminum, zirconium, hafnium, vanadium, niobium, tantalum, nickel, molybdenum, chromium, tungsten, cobalt, iron and their alloys as anodes and / or crucible materials. Specifically, we can form a thin boride layer on the surface of these metals by reverse polarization (i.e., by making the crucible a cathode) and then switch back to the regular boriding practice by changing the polarity, switching the cathode with the anode again. In particular, the boride layers that form on titanium (and its alloys) have excellent resistance to high temperature corrosion and oxidation. They are also electrically conductive, hence they can be an ideal choice for the industrial-scale boriding operations. Alternatively, iron and its alloys can also be borided first and then used as crucibles and / or anodes. Iron borides are also electrically conductive (this is why they form thick boride layers during our boriding process). In fact, reverse polarization of anodes and / or crucibles can be done as needed if the boride layer thickness on the crucible or the anode surface is reduced or there is a need for repair of a thinned down or worn area. Such a practice will ensure long durability and hence low cost.

[0015]The thickness and composition (e.g., type of boride, such as FeB or Fe2B, or Fe3B, diffusion layer) of borided surface layers can be controlled to achieve performance and durability requirements of a given application. For certain applications, Fe2B could be a preferred phase due to its superior strength and toughness. During the boriding process, the boriding temperature and / or current density may be maintained low to achieve only this phase over the other. Alternatively, one can also keep the boriding duration short but leave the work pieces in the molten electrolyte for a longer duration to allow excess boron to diffuse or distribute evenly within the structure and hence stabilize the Fe2B phase over the FeB phase. Nano-to-micro scale boride phases can also be produced in a given surface region by selectively reacting diffusing boron atoms with secondary phases and / or alloying elements within that region. This allows achieving multiple objectives, such as improved mechanical properties without degrading thermal and / or electrical properties of the base material. It is also possible to partially or selectively boride the surface or a region of a work piece by various masking methods as will be discussed in Examples.

Problems solved by technology

Due to the high temperature nature of the boriding process, the graphite crucible or anode may undergo oxidation and hence thin down or wear out after repeated uses.

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