Nickel alloy products

Abstract: Nickel alloy is formed with a nitrided surface. The alloy of the invention has a much better corrosion resistance than iron and so anti-corrosion treatments, such as plating, are not required. Nickel alloy products having such a surface show good lubricity and tightening properties, without seizure and scuffing problems.

Description: This invention relates to nickel alloy products formed by pressure casting with their surfaces nitrided and hardened, which have both mechanical strength and high corrosion resistance. Generally used various kinds of tapping screws, bolts, nuts, washers, rivets, plugs, screws, and screw parts are usually formed out of structural carbon steel. They are used after applying neutral-or carbo-quenching, thermal refining, and then anticorrosion treatment. Also, there are stainless steel products other than the above-mentioned carbon steel ones from a viewpoint of corrosion resistance. Although a market of the stainless steel products is small because their cost performance and strength are inferior to those of carbon steel, the market is expanding steadily. Under such circumstances,both the corrosion resistance and mechanical strength tend to be demanded at the same time. For this reason, as for screws, for example, products wherein 18-8stainless steel material is nitrided and hardened aiming at improvements of deficient strength and scuffing resistance that are conventional weak points, and the like have appeared. However, said nitrided stainless steel products have some disadvantages such as the appearance of rusting on the nitrided surface in a short period. On the other hand, products such as bolts made of nickel alloy are generally used as high corrosion resistant material, which is better than SUS in a field such as petrochemical plants. However, just as SUS, the nickel alloy material has smaller heat conductivity which is one third of that of iron, and has problems that seizure, and scuffing phenomena (a state that seizure caused combination of parts) tend to occur. It also has a problem of inferior tightening property because the coefficient of skin friction is high and improvement of hardness by quenching is impossible. And, nickel alloy material is generally employed as material which is hard to be carburized and nitrided, and cementation-hardening by elements such as carbon and nitrogen is difficult which is easy with iron material. EP 0 408 168 discloses the pre-nitriding fluorination of stainless steel SUS 305, which contains 10.5-13.0% Ni, at a temperature of 350°C. EP 0 551 702, unpublished at the priority date of the present invention, discloses the pre-nitriding fluorinating of nickel alloy plates, formed of an alloy comprising more than 25% Ni. Accordingly, the present invention can provide nickel alloy products with their surfaces nitrided and hardened which have both high mechanical strength and scuffing resistance. The present invention provides a method for nitriding nickel alloy products selected from bolts, rivets, nuts, pipe thread plugs, washers, pins, inserts, turnbuckles, shackles, pipe swage blocks, screws and screw parts, comprising more than 25% by weight nickel, comprising heating the said nickel alloy product in a fluorine- or fluoride-containing gas atmosphere at a temperature of 550-600°C to fluorinate the nickel alloy product and heating the fluorinated nickel alloy product in a nitriding atmosphere to form a nitrided layer in the surface layer of the nickel alloy product. As nickel alloy material of the above-mentioned nickel alloy products, nickel alloys containing more than 25 weight % (it will be abbreviated to "%" hereafter) nickel, for example, Ni-Cr, Ni-Cr-Mo, Ni-Cr-Fe, Ni-Cr-Co and the like are used in the method of the invention. Examples of such alloys with a high nickel content are inconel, hastelloy, and incolloy. Alloy containing more than 25% nickel and not more than 50% iron is more suitable. Bolts, rivets, nuts, pipe thread plugs, washers, pins, inserts, turnbuckles, shackles, pipe swage blocks, screws and screw parts are included in nickel alloy products. The fluorine- or fluoride-containing gas for a fluorine- or fluoride-containing gas atmosphere, in which the above-mentioned nickel alloy products react, is a fluorine compound gas, such as NF₁ , BF₃, CF₄, HF, SF₆, C₂F₆, WF₆, CHF₃, or SiF₄. They are used independently or in combination. Besides, fluorine compound gas with F in its molecular structure can be used as the above-mentioned fluorine- or fluoride-containing gas. Also F₂ gas formed by cracking fluorine compound gas in a heat decomposition device and preliminarily formed F₁ gas are employed as the above-mentioned fluorine- or fluoride-containing gas. Such fluorine compound gas and F₂ gas are mixed for use according to the case. The above-mentioned fluorine- or fluoride-containing gas such as fluorine compound gas and F₂ gas can be used independently, but are generally diluted by inert gas such as N₂ gas for the treatment. The concentration of the fluorine- or fluoride-containing gas itself in such diluted gas should amount to, for example, 10,000 to 100,000ppm, preferably 20,000 to 70,000ppm, more preferably 30,000 to 50,000ppm. The nickel alloy products with their surfaces nitrided and hardened by the method of the invention are provided by heating the non-nitrided nickel alloy products in a fluorine- or fluoride-containing gas atmosphere of such concentration and fluoriding. In this case, the nickel alloy products are heated at a temperature of 550 to 600°C. The heating time of the above-mentioned nickel alloy products in a fluorine- or fluoride-containing gas atmosphere may be appropriately selected depending on the nickel alloy species, geometry and dimension of the alloy, the heating temperature and the like, generally several minutes to several hours. The treatment of nickel alloy products in such fluorine- or fluoride-containing gas atmosphere allows "N" atoms to penetrate into the nickel alloy, which was impossible in the past. Though the mechanism of the penetration has not yet been proven, it can be understood as follows. An oxidized layer of NiO formed on the nickel alloy surface inhibits penetration of "N" atoms for nitrization. Upon heating the nickel alloy with the oxidized layer in a fluorine- or fluoride-containing gas atmosphere as mentioned above, the oxidized layer of NiO is converted to a fluorinated layer of NiF₂. "N" atoms for nitrization penetrate more readily into the fluorinated layer of NiF₂ than into the oxidized layer of NiO, that is, a nickel alloy surface is formed which is in a suitable condition for the penetration of "N" atoms by the above-mentioned fluorination. Thus, it is considered that "N" atoms in the nitriding gas penetrate uniformly into the nickel alloy to a certain depth when the nickel alloy is held in a nitriding atmosphere with such a suitable surface condition to absorb "N" atoms as follows, resulting in the formation of a deep uniform nitriding layer. Then, as mentioned above, nickel alloy products with a suitable surface condition to absorb "N" atoms by fluorination are heated in a nitriding atmosphere to nitride. In this case, the nitriding gas forming the nitriding atmosphere is a simple gas composed of NH₃ only, or a mixed gas composed of NH₃ and a carbon source gas (for example, RX gas), for example, a mixed gas composed of NH₃, CO, and CO₂. Mixtures of both gases can be also used. Generally, the above-mentioned simple gas or gas mixture mixed with an inert gas such as N₂ is used. According to the case, H₂ gas is added to those gases. In such a nitriding atmosphere, the above-mentioned fluorinated nickel alloy products are heated, generally at a temperature of 500 to 700°C , and treatment time of 3 to 6 hours. By this nitriding treatment, a close nitriding layer (consisting of entirely single layer) is formed uniformly on each surface of the above-mentioned nickel alloy, whereby the surface hardness of the nickel alloy products reaches more than Hv=600, generally Hv=800 to 1100, while thatof the base material thereof is Hv=280 to 380 in Vickers hardness. The thickness of the nitrided hard layer basically depends on the nitriding temperature and time, generally 2 to 50 µ m. A temperature below 500 °C causes difficulty in forming a nitrided hard layer, and at a temperature over 700°C , a fluorinated layer is damaged and Ni is easily oxidized thereby resulting in a tendency of uneven nitrided layer formation. Moreover, the profile roughness of the nitrided and hardened layer surface decreases, which is a defect in the products. A fluoriding temperature over 600°C is not appropriate for an industrial process becausethe furnace materials of a muffle furnace are worn out due toan extreme fluoriding reaction. From a viewpoint of forming a nitrided hard layer, it is also preferable that the difference between the fluoriding temperature and the nitriding temperature is as small as possible. For example, a proper nitriding layer is not formed by nitriding given after fluoriding and cooling once. The above-mentioned fluoriding and nitriding steps are, for example, taken in a metallic muffle furnace as shown in Fig. 4, that is, the fluoriding treatment is carried out first, and then the nitriding treatment is carried out inside the muffle furnace. In Fig. 4, the reference numeral 1 is a muffle furnace, 2 an outer shell of the muffle furnace, 3 a heater, 4 an inner vessel, 5 a gas inlet pipe, 6 an exhaust pipe, 7 a motor, 8 a fan, 11 a metallic container, 13 a vacuum pump, 14 a noxious substance eliminator, 15, 16, 30, and 31 cylinders, 17 flow meters, and 18 a valve. Nickel alloy products 10 are put in the furnace 1. Then the cylinder 16 is connected to a passage and the products are fluorided by introducing a fluorine- or fluoride-containing gas atmosphere such as NF₁ through the cylinder 16 with heating. The gas is lead through the exhaust pipe 6 by the action of the vacuum pump 13 and detoxicated in the noxious substance eliminator 14 before being discharged. Then the cylinders 15, 30, and 31 are connected with a duct for introducing nitriding gas into the furnace 1 in order to carry out nitriding . After nitriding, the gas is spouted out via the exhaust pipe 6 and the noxious substance eliminator 14. Through the series of these operations, fluoriding and nitriding treatments are carried out. A device as shown in Fig. 5 can be employed instead of the one shown in Fig. 4. This comprises a fluoriding chamber on the left side and a nitriding chamber on the right side. In the figure, the reference numeral 2' are metallic containers, 3.' a heater, 5' an exhaust gas pipe, 6'and 7' open-close covers, 11' a base, 21 a furnace body with adiabatic walls, and 22 a barrier movable up and down. The barrier 22 divides the inner space of the furnace body 21 into two chambers, 23 and 24. The chamber 23 is designed for a fluoriding chamber and 24 is for a nitriding chamber. The reference numeral 25 is a rack comprising two rails on which a metallic container 2' having nickel alloy articles therein can slide back and forth between chamber 23 and 24. The reference numeral 10' are legs of the rack 25. The reference numeral 26 is a gas introducing pipe which leads a fluorine- or fluoride-containing gas into the fluoriding chamber 23, 27 a temperature sensor, and 28 a nitriding gas introducing pipe. High-nickel based heat resistance alloy is desirable as the material for the above-mentioned metallic muffle furnace 1 instead of stainless steel material. That is, stainless steel is more easily fluorinated than nickel material with high nickel content, and needs large quantities of expensive fluorine source because of its high temperature for fluoriding. The device of Fig 5 is a continuous treatment system in which the inner temperature of a fluoriding chamber 23 is raised by heating on nitriding in the nitriding chamber 24, nickel alloy articles are introduced into the fluoriding chamber 23 under that condition to be fluorided. After exhausting the gas in fluoriding chamber 23, the nickel alloy articles together with the metallic container are transferred to the nitriding chamber 24 by opening and shutting the barrier 22. And then, nitriding is carried out under that condition thereby conducting fluoriding and nitriding continuously. The adoption of NF₃ as the fluorine- or fluoride-containing gas is suitable in particular for the above-mentioned fluoriding. That is, NF₃ is a useful gaseous substance that has no reactivity at the room temperature allowing operations and detoxication of exhaust gas to be easy. The following modes for carrying out the invention illustrate the invention. Fig. 1 is a front view of a hexagon headed bolt as an example of nickel alloy products nitrided according to the method of the present invention, Fig. 2 is a front view of a tapping screw as an example of nickel alloy products nitrided according to the method of the present invention, Fig. 3 is a front view of a tapered pin as an example of nickel alloy products nitrided according to the method of the present invention, Fig. 4 schematically shows a construction of a treatment furnace for carrying out nitriding according to the present invention, and Fig. 5 schematically shows a construction of another furnace. Example 1 Nickel alloy products such as hexagon headed bolts (M8) shown in Fig. 1, tapping screws shown in Fig. 2, and tapered pins shown in Fig. 3 were prepared by cold casting 61Ni-22Cr-9Mo nickel alloy material, and they were charged into a treatment furnace 1 as shown in Fig. 4. After vacuum purging the inside of the furnace 1 fully, it was heated to 550°C Then, in that state, fluorine- or fluoride-containing gas ( NF₃ 10 Vol% + N₂ 90 Vol%) was charged into the furnace and an atmospheric pressure was formed in it and the condition was maintained for 40 minutes. Then after exhausting the above-mentioned fluorine- or fluoride-containing gas out of the furnace, nitriding gas ( NH₃ 50 Vol% + N₂ 35 Vol% + Co 10 Vol% + Co₂ 5 Vol% ) was introduced into the furnace and the inside of the furnace was heated to 550 °C. Nickel alloy products were nitrided by being held in this condition for 3 hours and then taken out. Through this nitriding process, the surface hardness of the above-mentioned nickel alloy products reached Hv=850 ∼ 900 in Vickers hardness. Uniformly nitrided and hardened layers were formed across the whole surface of the nickel alloy product and the thickness of nitrided and hardened layers was 25µm. Also those samples were submitted to salt spray tests according to JIS, and no rust occured even after 720 hours. Moreover, for tapping screws, screw tests using SPCC iron plates, which have 2.3 mm thickness, were carried according to JIS, and it was found that internal threads were formed in the SPCC plates showing good tapping property without breaking the screw threads. Example 2 Drilling screws and cap screws formed by pressure casting 61Ni-23Cr-14Fe nickel alloy material were charged into a treatment furnace 1 as shown in Fig. 4. After vacuum purging the inside of the furnace 1 fully, it was heated to 550 °C. Then, in that state, fluorine- or fluoride-containing gas ( NF₃ 10 Vol% + N₂ 90 Vol% ) was charged into the furnace and an atmospheric pressure was formed in it and the condition was maintained for 40 minutes. Then after exhausting the above-mentioned fluorine- or fluoride-containing gas out of the furnace, nitriding gas ( NH₃ 50 Vol% + N₁ 35 Vol% + Co 10 Vol% + Co₂ 5 Vol% ) was introduced into the furnace and the inside of the furnace was heated to 600 °C. Nitriding treatment was carried out in this condition for 7 hours. Through this nitriding process, the surface hardness of the above-mentioned nickel alloy products reached Hv=950 ~ 1000 in Vickers hardness : while the core hardness is Hv=310 ∼ 320. Uniform nitrided and hardened layers were formed across the whole surface of the nickel alloy product and the thickness of nitrided and hardened layers was 35 µm. Also, for drilling screws among the above-mentioned nitrided nickel alloy products, drilling tests were carried out using SPCC plates with 1.6 mm thickness, Ti plates with 1.2 mm thickness, and SUS plates with 1.0 mm thickness. As a result, the time taken for screwing the SPCC plates with a load of 15 kg was 2.4 seconds, which is about the same drilling time as that of iron products. For the Ti plates and SUS plates, about the same level of drilling property was obtained as that of SPCC plates. As mentioned hereinbefore, nickel alloy products with their surfaces nitrided and hardened according to the method of the invention have surface layers formed as nitrided and hardened layers. The invention comprises converting oxidised films of the surfaces of the nickel alloy products to fluorided layers, and forming the surface layers into nitrided and hardened layers by conducting nitriding treatment. Generally, nickel alloy material includes elements such as Cr and Mo that can easily form hard intermetallic compounds such as CrNi and MoNi by reacting with "N" atoms. Since "N" atoms can penetrate the previously formed fluorided layers even at the time of nitriding, "N" atoms penetrate uniformly into the surface layers of the nickel alloy products to a certain depth when nitriding. As a result, it become possible to form the close uniformly nitrided and hardened layers only in their surface layers at the certain depth resulting drastic improvement of surface hardness without raising the base material stiffness of the nickel alloy products. Therefore, since the nitrided and hardened surfaces of the nickel alloy products produced according to the present invention have much better corrosion resistance than iron products have in corrosion surroundings, anticorrosion treatment such as plating is not required, and rich lubricity and excellent tightening property can be obtained without seizure and scuffing phenomena.

Claims: A method for nitriding nickel alloy products selected from bolts, rivets, nuts, pipe thread plugs, washers, pins, inserts, turnbuckles, shackles, pipe swage blocks, screws and screw parts, comprising more than 25% by weight nickel, comprising heating the said nickel alloy product in a fluorine- or fluoride-containing gas atmosphere at a temperature of 550-600°C to fluorinate the nickel alloy product and heating the fluorinated nickel alloy product in a nitriding atmosphere to form a nitrided layer in the surface layer of the nickel alloy product. A method according to claim 1 in which the fluorine- or fluoride-containing gas atmosphere is composed of at least one of: (a) a fluorine compound gas containing at least one component selected from the group: NF₃, BF₃, CF₄, HF, SF₆, C₂F₆, WF₆, CHF₃ and SiF₄; (b) F₂ gas formed by cracking the above-mentioned fluorine compound gas (a) ; and (c) pre-formed F₂ gas; optionally together with an inert gas. A method according to claim 1 or 2 in which the nitriding atmosphere is NH₃ gas, a mixture of NH₃ gas and RX gas, an inert gas mixture of an inert gas and NH₃ gas or NH₃ gas RX mixture, or a gas mixture formed by mixing H₂ gas with the inert gas mixture. A method according to any preceding claim in which the nickel alloy product is heated in a nitriding atmosphere at a temperature between 500°C and 700°C.

Inventors: HAYASHIDA TADASHI, KITANO KENZO, MINATO TERUO, SENBOKUYA HARUO, TAHARA MASAAKI, HAYASHIDA, TADASHI, KITANO, KENZO, MINATO, TERUO, SENBOKUYA, HARUO, TAHARA, MASAAKI
Applicants: DAIDO HOXAN INC, DAIDO HOXAN INC.
Application year: 1992
Main classification: C23C8
IPC codes: C23C8