Metal treatment – Process of modifying or maintaining internal physical... – Carburizing or nitriding using externally supplied carbon or...
Reexamination Certificate
1998-11-30
2001-12-11
King, Roy (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Carburizing or nitriding using externally supplied carbon or...
C148S209000, C148S230000
Reexamination Certificate
active
06328818
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for treating the surface of a ferrous material which stably forms a surface hardened layer such as a compound layer (hereinafter, simply referred to as “chromium carbide nitride layer”) of chromium nitride or chromium carbide nitride on the surface of a ferrous material in order to improve mechanical properties such as resistance to wear, resistance to heat, resistance to oxidation, resistance to fatigue and the like, as well as to a salt bath furnace used therefor.
PRIOR APT
It is widely known that mechanical properties such as resistance to wear, resistance to heat, resistance to oxidation, and resistance to fatigue and the like can be improved by forming a chromium carbide nitride layer on the surface of a ferrous material. As such a method for forming a chromium carbide nitride layer on the surface on a ferrous material, there have been proposed various methods, for example, a plating-diffusing method and a chromizing method (JP-B 42-24967, U.S. Pat. No. 4,242,151 ) as well as a salt bath method (JP-B 3-65435, JP-B 4-24422, JP-B 4-24423, JP-B 4-47028, JP-B 4-47029, JP-A 2-159361, JP-A 3-202460).
Among the above methods, for example, a method shown in JP-B 3-65435 is a method using a salt bath or the like, in which a chromium carbide nitride layer is formed on the surface of an iron alloy material by nitriding-treating the surface of the iron alloy material to form a nitrided layer, and heat-treating the iron alloy material in the presence of a treating agent comprising {circle around (1)} a chromium material such as pure chromium, chromium alloy, chromium compound and the like, {circle around (2)} chloride, fluoride, borofluoride, oxide, bromide, iodide, carbonate, nitrate or borate or ammonium halide or metal halide of an alkali metal or an alkali earth metal to diffuse chromium.
The above methods exemplify many salts such as chloride, fluoride, borofluoride, oxide, bromide, iodide, carbonate, nitrate or borate of an alkali metal or an alkali earth metal and indicate that a chromium carbide nitride layer is formed by using them alone or in a combination thereof. However, among the respective salts, the salts other than chloride are salt bath agents which are not practically suitable at all for use in view of effects on oxidativeness of a salt bath and thermodynamic viewpoints. In addition, these salts have minus action such as conversely causing erosion of articles to be treated and the like and reversely form a chromium carbide nitride layer with difficulty.
In addition, the above methods exemplify chromium compounds such as ferrochromium as well as chloride, fluoride, oxide and the like such as CrCl
3
, CrF
4
, Cr
2
O
3
, K
2
CrO
3
and the like. However, since chlorides of chromium includes many hydrates, they are disadvantageous in that they raise dew point in a salt bath. In addition, regarding fluoride and oxide, there is a problem that necessary chemical equilibrium for producing a chromium carbide nitride layer is not attained from a thermodynamic point of view. Therefore, these methods are not suitable as a treating agent for producing a chromium carbide nitride layer, being significantly problematic.
Further, in the above methods, there is described that oxides such as Al
2
O
3
, ZrO
2
and the like and cyanides such as NaCN and the like are added in order to adjust the viscosity of a salt bath. However, studies and experiments by the present inventors confirmed that the addition of Al
2
O
3
and ZrO
2
has little effects on adjustment of a salt bath and the addition of them reversely inhibits production of a chromium carbide nitride layer. In addition, the addition of cyanide promotes nitridation and complex formation of molten chromium and an iron alloy material to produce no chromium carbide nitride layer and, additionally, since the produced complex salt tend to cause explosive burning, it is very dangerous. Thus, these methods were found not to be suitable for use.
Like this, in the prior art salt bath methods, the salt bath properties were fundamentally elucidated insufficiently. Even if a chromium carbide nitride layer could be formed on the surface of a ferrous material in laboratories, the formed layer is scattered and a salt bath life is short and, thus, the prior art methods have many problems on quality stability and economy. Therefore, a chromium carbide nitride layer can not be formed with stable quality and, thus, industrial production is not currently performed.
SUMMARY OF THE INVENTION
The present invention was done in view of such the circumstances and the object thereof is to provide a method for treating the surface of a ferrous material which can stabilize quality, prolong a salt bath life and considerably reduce a treating time, and makes industrial mass production possible, as well as a salt bath furnace used therefor.
In order to attain the above object, the present method for treating the surface of a ferrous material firstly features nitriding-treating a ferrous material to form a nitrided layer composed of at lease one of iron nitride and iron carbide nitride on the surface thereof, and heating to maintain the ferrous material at a temperature of 500 to 700° C. in an treating agent (A) below and, thereby, chromium is diffused into the nitrided layer to form at least one compound layer of chromium nitride and chromium carbide nitride.
(A) A treating agent containing the following (a) as a main component and containing the following (b) and (C):
(a) at least one of alkali metal chloride and alkaline earth metal chloride;
(b) glass having silicone oxide as a main component; and
(c) chromium.
The present method for treating the surfaces of a ferrous material secondly features nitriding-treating a ferrous material to form a nitrided layer composed of a nitrogen-diffused layer on the surface thereof, and heating to maintain the ferrous material at a temperature of 500 to 700° C. in the above treating agent (A) and, thereby, chromium is diffused into the nitrided layer to form a chromium-concentrated layer.
Also, in order to attain the above object, the present salt bath furnace features a salt bath furnace for conducting molten salt bath treatment which comprises a treating cell in which the treating agent is introduced, a heating means for heating to melt the treating agent in the treating cell, and a stirring means for stirring the treating agent which has been melted in the treating cell, wherein the bottom of the treating cell has a slant surface, and the stirring means is provided in a deep portion lower than the slant surface of the bottom of the treating cell.
In order to produce a surface hardened layer such as a chromium carbide nitride layer and the like having industrially stable and high quality, the present inventors, in process of a series of studies, paid an attention to that it is important to appropriately control basicity of a salt bath and maintain thermodynamic activity and equilibrium of chromium ions in a salt bath in order to produce a stable chromium carbide nitride layer by salt bath treatment. Subsequently, we found that basicity of a salt bath can be appropriately maintained by inclusion of glass powders having silicon oxide as a main component in a salt bath containing as a main component at least one of alkali metal chloride and alkaline earth metal chloride as a main component and containing chromium, which resulted in completion of the present invention.
Then, mode for carrying out the present invention will be explained in detail.
In the present invention, a ferrous material is nitriding-treated to form a nitrided layer composed of at lease one of iron nitride and iron carbide nitride on the surface thereof, and the ferrous material is heated to maintain at a temperature of 500 to 700° C. in an treating agent containing as a main component at least one of alkali metal chloride and alkaline earth metal chloride and containing silicon oxide as a main component and chromium and, thereby, chromium is diffused into the nitrided layer to form at least
Miyagi Isao
Taniguchi Wataru
Armstrong Westerman Hattori McLeland & Naughton LLP
Coy Nicole
King Roy
Maizuru Corporation
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