Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
2002-10-28
2003-08-19
Turner, Archene (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C059S901000, C075S235000, C075S239000, C148S319000, C248S309200, C428S217000, C428S472000, C428S699000
Reexamination Certificate
active
06607850
ABSTRACT:
TECHNICAL FIELD
Chemical deposition effected by a tumbling action in a sealed retort is used to form hard, wear-resistant vanadium or niobium carbide (or both) coatings on steel articles such as chain pins. Carbide coated articles are claimed as well as a novel substrate steel and the method of forming the coatings.
BACKGROUND OF THE INVENTION
This invention is concerned most particularly with two types of chains—traditional roller chains and so-called silent chains. Both roller chains and silent chains use pins as important components.
In roller chains, the pins are free to rotate in hardened bushings; in silent chains, the cylindrical pins bear directly against the inside link apertures and are press fitted into outside guide links. The joint of the silent chain consists of the pins that rotate relative to the non-guide row links of the chain. The silent chain uses a series of links that are interlaced—that is, the inside links are press fitted onto the pins; inside links in the same row of the chain, and outside links in the adjacent or non-guide row, are interlaced with the links in the links in the guide row. See the description, for example, in
FIG. 4
of Kozakura et al U.S. Pat. No. 6,068,568. Generally, the contact stress between the pin and the bushing in a roller chain will be lower than that of the contact stress between the pin and the link apertures of a silent chain, leading to the general observation that roller chains tend to have better wear characteristics than silent chains. This invention is therefore aimed primarily at improving the wear characteristics of silent chains, but is applicable to both types of chains, and to any steel parts, in chains or otherwise, subject to wear.
An example of pretreatment of a substrate to improve the application of a hard coating may be found in Hale's U.S. Pat. No. 4,608,098. A chemical vapor deposition process is described in Sarin et al U.S. Pat. No. 4,957,780. FeV is employed as a carbide-forming material by Arai et al U.S. Pat. No. 4,400,224. Vanadium carbide coatings have been placed on small steel parts in the past, but generally by a salt bath procedure such as is disclosed in the U.S. Pat. No. 4,400,224 patent and/or Arai et al U.S. Pat. No. 4,778,540. See also Arai et al U.S. Pat. Nos. 4,686,117, 4,786,526, 4,844,949, and 4,892,759, proposing a fluidized bed. A fluidized bed is also proposed by Lennartz in U.S. Pat. No. 5,498,442. Pins having a hard chromium carbide layer can be made by depositing the chromium from FeCr powder surrounding the pin surface, at 970 degrees C. The chromium diffuses from the powder and deposits on the pin surface, where it draws carbon from the substrate to form the carbide. Substrate steels having low carbon contents are not useful for this purpose, and accordingly it is necessary to carburize the pins, adding to the expense of the procedure. Nevertheless, such pins operate satisfactorily in roller chains, where the pins do not experience as much stress as those used in silent chains.
Chromium carbide coatings applied by chemical deposition have been tried on silent chain pins, but, under the higher surface stress of the silent chain, microbits of the hard coating spalled from the surface can add to and accelerate wear of the exposed substrate, which has a significantly lower hardness than the bits. In roller chains, the pin can wear completely through the bushing with the aid of loose chromium carbide particles. The importance of the adhesion layer, which bonds the coating to the substrate, is thus illustrated. A general observation may be made also that good adherence of the hard coating is considerably more difficult to achieve for vanadium carbide coatings than for chromium carbide coatings.
Chromium improves the adhesion of vanadium carbide coating to the substrate steel by forming a diffusion bond. This effect can be achieved by using ferro-chromium powder or elemental chromium powder in a chromium deposition process. But the use of ferro-chromium and elemental chromium powders is frequently foreclosed or inhibited by environmental regulation.
The composition of the pin substrate steel has significant effects on vanadium coated steel pins. We have found that appropriate carbon content of the substrate steel can ensure the thickness of the coating and impart strength and hardness, and appropriate chromium content in the substrate steel is important for good adhesion of the coating to the substrate steel pins. Various steels having moderate chromium levels have been disclosed in the patent literature for various purposes. See, for example, the U.S. Patents to Sattler U.S. Pat. No. 1,773,793, Corning U.S. Pat. No. 1,496,979, Nagumo et al U.S. Pat. No. 3,907,553, Philip et al U.S. Pat. No. 3,901,690, DeSouza U.S. Pat. No. 4,224,060, Kato et al U.S. Pat. No. 4,842,818, Arata et al U.S. Pat. No. 4,902,473, Hamada et al U.S. Pat. No. 5,013,525, and Fukushima U.S. Pat. No. 5,944,920. But no commercially available steel has been found to meet the preferred carbon and chromium specifications. Commercially available steels having the desired chromium contents have a very low carbon content, requiring that the pins and other parts or articles to be coated have to be carburized to a higher carbon level prior to the coating process, which increases the cost of the final products and reduces statistical quality performance because of the variations introduced by the carburizing process. Also the commercially available candidate steels tend to include far more molybdenum than necessary for our purposes, which unnecessarily increases the cost of the substrate steel and the desired articles made from it such as chain pins. The overall cost of the coated pin can be further reduced by lowering or eliminating this costly element in the steel. Our invention therefore includes a novel steel composition.
SUMMARY OF THE INVENTION
We have invented a process for the formation of a hard surface on steel articles, particularly on small parts such as pins used in chains. We employ a rotary, or tumbling, contact diffusion process in a sealed vessel to form a vanadium carbide, niobium carbide, or mixed vanadium
iobium carbide coating on the steel article. Our rotary contact diffusion process is a chemical deposition process carried out with the aid of a powder mixture (pack), in which steel articles such as chain pins are immersed or mixed, containing a particulate vanadium, niobium or mixed vanadium
iobium source, preferably in the form of FeV and/or FeNb, and a halide catalyst, preferably iron chloride. When the mixture in a sealed rotating or tumbling retort or other vessel is heated to the temperature range of 1600-2000° F., preferably 1700-1900° F., the catalyst reacts with the vanadium and/or niobium to produce vanadium and/or niobium chloride (halide) that diffuses in the pack and is transferred, assisted by the tumbling action imparted by the rotation or rocking of the vessel, to the surface of the steel article. The vanadium and/or niobium, as strong carbide-forming elements, draws carbon from the substrate steel to form a carbide layer with a hardness of over HV2000. The reactions that take place in the retort can be expressed as:
FeV
+
Cl
2
⇔
VCl
2
+
Fe
VCl
2
+
C
⇔
VC
↓
+
Cl
2
and/or
FeNb
+
Cl
2
←
→
NbCl
2
+
Nb
NbCl
2
+
C
←
→
NbC
+
Cl
2
Succinctly, our invention involves depositing vanadium and/or niobium on the surface of the steel article, in the form of a halide, by chemical deposition effected by tumbling. The halide is transformed to vanadium and/or niobium carbide on the surface of the steel article, the carbon for displacing the halide and combining with the vanadium and/or niobium being diffused from the steel substrate. Accordingly, we prefer the carbon content of the steel be in the range of 0.7% to 1.2% by weight.
We also prefer a chromium content in the subs
Fornell Doug
Hanayama Yoshito
Mishima Kunihiko
Tada Naosumi
Wang Yumin
Borg-Warner Inc.
Krayer William L.
Turner Archene
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