Patent Application
20070267105
The nitriding-oxidizing method for a metal member, of the present invention, comprises the steps of: providing a powdery nitriding agent comprised of 10 to 90% by volume of a powdery nitride compound and 90 to 10% by volume of an inorganic powder, wherein the powdery nitride compound has an average particle size of 1 to 10 μm and can decompose below a nitriding-oxidizing temperature to generate nitriding gas, and the inorganic powder has an average particle size of 20 to 100 μm and does not react; and embedding an essential part of the metal member to be nitrided and oxidized into the powdery nitriding agent, wherein the metal member is composed of ferrous alloys or non-ferrous alloys; and proceeding nitridization-oxidization at a temperature of 400 to 900° C. for 0.5 to 20 hours, while an oxygen-containing gas is always present in the powdery nitriding agent.
The powdery nitriding agent composed of 10 to 90% by volume of a powdery nitride compound and 90 to 10% by volume of an inorganic powder is preferred. Moreover, a powdery nitride compound with an average particle size of 1 to 10 μm and an inorganic powder with an average particle size of 20 to 100 μm are preferred. In the present invention, the average particle size is determined by scanning electronic microscopy (SEM). If the amount of the powdery nitride compound in the powdery nitriding agent is less than 10% by volume (i.e., the amount of the inorganic powder is more than 90% by volume), nitridization may be incomplete. To the contrary, if the amount of the powdery nitride compound in the powdery nitriding agent is more than 90% by volume (i.e., the amount of the inorganic powder is less than 10% by volume), the oxygen-containing gas existing in the powdery nitriding agent is insufficient. Thus, oxides cannot be formed sufficiently. In these cases, the effect of the nitridization-oxidization will be reduced.
When a metal member undergoes nitridization-oxidization or the nitridization-oxidization and reoxidization according to the present invention, it is preferred that the metal member consisting of ferrous alloys or non-ferrous alloys (for example, nickel alloy, cobalt alloy, and titanium alloy) contain chromium, molybdenum, manganese, tungsten, vanadium or aluminum as nitrided elements. As the metal member consisting of ferrous alloys or non-ferrous alloys undergoes nitridization-oxidization, the essential part of the metal member to be nitrided and oxidized is embedded in the powdery nitriding agent, and is then heated at a temperature of 400 to 900° C., while an oxygen-containing gas (air or oxygen-rich air) is always present in the powdery nitriding agent. Preferably, the heating time (the nitriding-oxidizing time) is about 0.5 to 20 hours. If, for example, an electrical furnace is used in nitridization-oxidization, the electric furnace can be an open type, a hermetic type, or an oxygen controllable type electric furnace.
In nitridization-oxidization, oxygen-containing gas always exists in a powdery nitriding agent (if necessary, the oxygen-containing gas can be supplied into the powdery nitriding agent, simultaneously), so that oxygen can diffuse from the surface of the treated metal member into the inner part thereof to react with Cr contained inside to form Cr2O3 precipitate. This inner oxidation retards nitrogen diffusion and allows hardness gradient to decrease, so that the toughness of the substrate can be maintained. However, oxide (Cr2O3) precipitate is not formed in the diffusion layer according to conventional nitriding processes.
Applying a temperature program described below can eliminate moisture generated in the early steps of nitridization-oxidization. Furthermore, by controlling the temperature and elapsing time for generating nitrogen, as well as heating the treated objects, nitridization-oxidization can be applied to big and batch objects.
3. After the heating profile is increasing from room temperature to 200±20° C., it is held for a specific period of time, and then it is increased to the predetermined temperature and held. The purpose of this temperature program is to allow moisture to be evaporated, powdery nitriding agent to be preheated, and the temperature of the treated objects to be controlled.
The powdery nitriding agent is preheated to 180±20° C. and held. The objects to be treated are heated to the predetermined temperature. Thereafter, these objects are placed in the preheated powdery nitriding agent, or the preheated powdery nitriding agent is placed around these objects. The purpose of this temperature program is to allow generated nitriding gas to be used sufficiently by heating the objects to the nitriding temperature, so that big objects can be treated by this method. Moreover, the bigger the objects to be treated, the bigger the difference among temperature increasing rate of the objects to be treated, hot air in the furnace, and the powdery nitriding agent. Such a difference can be eliminated when nitriding agent and treated objects are heated separately.
The thickness and composition of the surface compound layers and the diffusion layer produced in the nitriding-oxidizing method of the present invention, for example, is shown as followed:
Oxides layer: 1 to 3 μm in thickness, including Fe2O3, Fe3O4, FeCr2O4 and Cr2O3.
Nitrides layer: 1 to 2 μm in thickness, including Cr2N and CrN.
Diffusion layer: 10 to 150 μm in thickness, including a nitrogen diffusion layer and Cr2O3 precipitate.
When nitridization-oxidization is carried out on tool steels, alloy steels, or metal members having inert coating by using powdery nitriding agent, suitable treatment conditions are exemplified as below:
1. In the case that high surface hardness and high wear resistance are required, the amount of the powdery nitride compound to be used in the powdery nitriding agent is less than 40% by volume, and the nitriding-oxidizing treatment temperature is set at 500 to 540° C., preferably. This process is suitable for, for example, SKD61 of high-carbon cold work tool steels, dies and parts of high-speed tool steels, and the casting dies and parts of SKD61 of hot work tool steels tending to be highly worn. That is, to improve nitrogen diffusion in high-carbon steels, it is preferred to decrease the amount of the powdery nitride compound in the powdery nitriding agent to increase the amount of oxygen.
2. In the case that high thermal fatigue resistance and the diffusion layer with small hardness gradient are required, the amount of the powdery nitride compound to be used in the powdery nitriding agent is 20 to 60% by volume, and the nitriding-oxidizing temperature is set at 520 to 560° C., preferably. This process is suitable for, for example, casting dies and hot forging dies of SKD61 of hot work tool steels having severe thermal fatigue.
3. In the case that the wear due to impact and softening of hardness due to operation at high temperatures, the amount of the powdery nitride compound to be used in the powdery nitriding agent is more than 60% by volume, and the nitriding-oxidizing temperature is set at 540 to 580° C., preferably. This process is suitable for, for example, hot forging dies.
4. In the case that high surface strength and wear resistance are required for cold work tool steels and a small hardness gradient of the diffusion layer is needed, preferably, the amount of the powdery nitride compound to be used in the powdery nitriding agent is 20 to 60% by volume, and the nitriding-oxidizing temperature is set at 480 to 520° C. This process is suitable for, for example, cold forging dies and punch dies.
When nitridization-oxidization is carried out on high-chromium steels by using powdery nitriding agent, suitable treatment conditions are exemplified as below:
2. Because high chromium steels are mostly applied to dies and parts having the requirement of dimension precision, the nitriding-oxidizing temperature is preferably set at 500 to 540° C. Nevertheless, in the case that the dimension precision is in the micrometer unit, the nitriding-oxidizing temperature is preferably set at 480 to 500° C., so that the holding time of nitridization-oxidization can be increased.
As for metal members having inert coatings, for example, metal member consisting of titanium alloys, the preferred nitriding-oxidizing temperatures are higher than 700° C. (due to hard inert coating).
The method for nitriding-oxidizing and reoxidizing for a metal member, of the present invention, comprises the steps of: providing a powdery nitriding agent comprised of 10 to 90% by volume of a powdery nitride compound and 90 to 10% by volume of an inorganic powder, wherein the powdery nitride compound has an average particle size of from 1 to 10 μm and decomposes below a nitriding-oxidizing temperature to generate a nitriding gas, and the inorganic powder has an average particle size of from 20 to 100 μm and does not react under the nitriding-oxidizing conditions; embedding an essential part of the metal member to be nitrided and oxidized into the powdery nitriding agent, wherein the metal member is composed of ferrous alloys or non-ferrous alloys, and then carrying out nitridization-oxidization at temperatures of 400 to 900° C. for 0.5 to 20 hours while an oxygen-containing gas is always present in the powdery nitriding agent; and allowing the metal member to carry out an oxidization in an oxygen-containing atmosphere (air or oxygen-rich air) at a temperature of 400 to 900° C. for 0.25 to 8 hours after the nitridization-oxidization. The nitriding-oxidizing conditions in early steps are the same as above-mentioned conditions. In reoxidization, for example, if an electric furnace is used, the electric furnace can be an open type, an oxygen controllable type, or a steam-introduced type electric furnace.
In the reoxidization, the formation of red ferric oxide (Fe2O3) rust layer on the top surface can be inhibited, and many dark black ferrosoferric oxide (Fe3O4) layers are formed by a temperature program as followed:
2. Heating profile is increased from room temperature to 360±20° C. and is held for a specific period of time, and then is increased to the predetermined temperature and held. The purpose of this temperature program is to allow moisture in the early steps to be evaporated, the formation of Fe2O3 to be inhibited, and Fe3O4 layer to be formed at high temperatures.
3. Heating profile is increased from room temperature to 360±20° C. and is held for a specific period of time, and then is increased to the predetermined temperature and steam is introduced. The introducing period of steam can vary with the requirement.
In reoxidization, nitrogen in the diffusion layer formed in the previous nitridization-oxidization can be dispersed again, so that a small hardness gradient formed.
Despite that oxide precipitate is produced in the diffusion layer by nitridization-oxidization under the condition that the oxygen-containing gas is present in the powdery nitriding agent, the amount of Cr2O3 precipitate in the diffusion layer will be increased due to a tight Cr2O3 layer formed on the surface. Consequently, the treated metal member has an excellent melting loss resistance to the melting loss (corrosion) resulting from electrochemical reactions between ferrous alloys and non-ferrous alloys (corrosion) and the melting loss resulting from abrasion of flowing liquid.
After nitridization-oxidization and reoxidization, the ferrous oxide layer, the mixing layer of chromium oxide with chromium nitrides are formed from the top surface inwards. The diffusion layer is a mixture of nitrogen diffusion layer with Cr2O3 precipitate.
The thickness and composition of the surface compound layers and the diffusion layer produced in nitridization-oxidization and the reoxidization, for example, is shown as followed:
Oxide layer: 2 to 20 μm in thickness, including Fe2O3, Fe3O4, FeCr2O4 and Cr2O3.
Nitride layer: 1 to 4 μm in thickness, including Cr2N and CrN.
Diffusion layer: 10 to 200 μm in thickness, including a nitrogen diffusion layer and Cr2O3 precipitate.
When reoxidization is carried out on tool steels and alloy steels (such as improved SKD61 material) underwent nitridization-oxidization, suitable treatment conditions are exemplified as below:
1. The problems resulting from low temperature operation or high temperature operation can be overcome by oxidized coatings produced in reoxidization. Low temperature operation includes lead free tin alloys soldering, and high temperature operation includes hot forging, hot battering, non-ferrous alloys casting, and the like. In this case, the temperature for reoxidization is preferably set at 500 to 600° C.
2. Tool steels and alloy steels are used after a “quenching-annealing” heat treatment, thus subsequent treatments at high temperatures become very important. Precision error or softening of hardness of dies and parts caused by inadequate temperature program must also be considered. In this case, reoxidizing temperature is preferably set at 520 to 560° C.
When the reoxidization is carried out on high-chromium steels, non-ferrous alloys undergone the nitridization-oxidization, suitable treatment conditions are exemplified as below:
The purpose of the reoxidization is to form an oxidized coating of Fe3O4 and Cr2O3 on the surface. Ferrous alloy steels with an oxidized coating can have a relatively better melting loss resistance comparing with aluminum alloys, lead (II) alloys, magnesium alloy, lead free tin alloy for soldering, and the like. Furthermore, high chromium steels are used for applications at room temperature, low temperatures and high temperatures.
1. In the case of applications at room temperature, wear resistance is considered. Dimension precision is also very important. The purpose of reoxidization is to form compound layers of Fe3O4 and Cr2O3. The temperature for reoxidization is preferably set at 480 to 520° C.
2. In the case of applications at low temperatures (150 to 400° C.), solder bath has a greater tolerance, and contacts with tin alloy liquid for a long time. Therefore, to terminate electrochemical reaction becomes very important, and the formation of a thicker, oxidized coating is required. The reoxidizing temperature is preferably set at 540 to 580° C.
3. In the case of applications at high temperatures, the reoxidizing temperature is preferably set at 520 to 580° C. to solve problems of seizing and melting loss (resulting from casting dies) of equipment parts for casting which contact with molten melt.
Ferrous alloys suitable for the nitridization-oxidization and reoxidization by using a powdery nitriding agent include high-speed tool steels, alloy tool steels, ultra-high strength steels, structural alloy steels containing chromium, molybdenum, manganese, tungsten, vanadium, or aluminum. Because the Cr2O3 oxidized layer formed on the top surface and Cr2O3 precipitate formed in the diffusion layer, ferrous alloys preferably contains 1% by weight of chromium.
Ferrous alloys that can be treated by the treatment of the present invention are exemplified in the following:
Even in the case of metal members with inert oxidized coatings, the nitriding-oxidizing method also can be applied without pretreatment for removing the oxidized coating, by degrading ammonia into hydrogen ions at high temperatures and reducing such hydrogen ions with oxygen in the inert oxidized coatings.
A metal member consisting of SKD61 was embedded in a powdery nitriding agent consisting of 20% by volume of dicyandiamide with an average particle size of 6 μm and 80% by volume of Al2O3 with an average particle size of 70 μm. Nitridization-oxidization was carried out on the metal member in an open type electric furnace at a temperature of 460, 480, 500, 520, 540, 560 or 580° C. for 15 hours while an oxygen-containing gas was always present in the powdery nitriding agent. The hardness of the metal member treated at each of the above-mentioned temperatures was measured. The results are shown in
A metal member consisting of SKD61 was embedded in a powdery nitriding agent consisting of 40% by volume of dicyandiamide with an average particle size of 6 μm and 60% by volume of Al2O3 with an average particle size of 70 μm. Nitridization-oxidization was carried out on the metal member in an open type electric furnace at a temperature of 460, 480, 500, 520, 540, 560 or 580° C. for 15 hours while an oxygen-containing gas was always present in the powdery nitriding agent. The hardness of the metal member treated at each of the above-mentioned temperature was measured. The results are shown in
A metal member consisting of SKD61 was embedded in a powdery nitriding agent consisting of 70% by volume of dicyandiamide with an average particle size of 6 μm and 30% by volume of Al2O3 with an average particle size of 70 μm. Nitridization-oxidizing was carried out on the metal member in an open type electric furnace at a temperature of 460, 480, 500, 520, 540, 560 or 580° C. for 15 hours while an oxygen-containing gas was always present in the powdery nitriding agent. The hardness of the metal member treated at each of the above-mentioned temperature was measured. The results are shown in
A metal member consisting of SKD61 was embedded in a powdery nitriding agent consisting of 10, 20, 30, 40, 60, 70 or 90% by volume of dicyandiamide with an average particle size of 6 μm and a corresponding residual amount of Al2O3 with an average particle size of 70 μm. Nitridization-oxidization was carried out on the metal member in an open type electric furnace at 520° C. for 15 hours while an oxygen-containing gas was always present in the powdery nitriding agent. Thereafter, the metal member underwent reoxidization under atmosphere at 520° C. for 6 hours. The hardness of the metal member treated as above-mentioned was measured. The results are shown in
A metal member consisting of SKD61 was embedded in a powdery nitriding agent consisting of 20% by volume of dicyandiamide with an average particle size of 6 μm and 80% by volume of Al2O3 with an average particle size of 70 μm. Nitridization-oxidization was carried out on the metal member in an open type or hermetic type electric furnace at 540° C. for 10 or 20 hours while an oxygen-containing gas was always present in the powdery nitriding agent. The hardness of the metal member treated as above-mentioned was measured. The results are shown in
Additionally, a metal member consisting of SKD61 was embedded in a powdery nitriding agent consisting of 20% by volume of dicyandiamide with an average particle size of 6 μm and 80% by volume of Al2O3 with an average particle size of 70μm. Nitridization-oxidization was carried out on the metal member in an open type or hermetic type electric furnace at 540° C. for 10 hours while an oxygen-containing gas was always present in the powdery nitriding agent. Thereafter, the metal members underwent reoxidization in an open type or hermetic type electric furnace at 540° C. for 10 hours. The hardness of the metal member treated as above-mentioned was measured. The results are shown in
The invention has been described using exemplary preferred embodiments. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
| Number | Date | Country | Kind |
|---|---|---|---|
| JP2006-142021 | May 2006 | JP | national |
