Method of nitriding steel

Abstract: A heat treatment furnace for fluorinating a nitriding steel comprises: a liftable inner cover 2 to contain steel material 3; a liftable bell shaped outer cover 1 which covers the inner cover to define a space between the inner and the outer covers, the interior of the inner cover serving as a fluorinating or nitriding chamber, the said chamber communicating with a feed pipe 9 for fluorine- or fluoride-containing gas or for nitriding gas and with an exhaust pipe 12a; and the space between the inner cover and the outer cover serving as a heating chamber; and a heater 4 for heating the fluorinating or nitriding chamber, disposed with the heating chamber.

Description: This invention relates to a method of nitriding steel and to a heat treatment furnace used in the method. Methods of nitriding steel articles or works to form a nitrided layer on their surface have been employed for the purpose of improving the mechanical properties such as wear resistance, corrosion resistance and fatigue strength among others, of the articles or works. Examples of prior art nitriding methods are gas nitriding and gas soft nitriding and methods using ammonia gas or a mixture of ammonia gas and a gas containing a carbon source (RX gas). However, these methods are unsatisfactory in that uneven nitriding occurs when alloy steel or works of a complicated shape are treated. Generally, steel material is nitrided at a temperature not lower than 500°C. For absorption and diffusion of nitrogen into the steel surface layer, it is necessary that the surface should be free not only of organic and inorganic contaminants but also of any oxide layer. It is also necessary that the steel surface layer itself should be highly activated. However, it is impossible in prior art nitriding methods to prevent oxide layer formation and to activate the steel surface layer completely. For example, in a typical case of cold-working austinitic stainless steel works, passive surface coating layers are removed by cleaning with a hydrofluoric acid-nitric acid mixture prior to charging the stainless steel into a treatment furnace. It is difficult to remove the coating layers completely and to activate the steel surface layers completely, so that formation of a satisfactory nitrided layer is almost impossible. In order to remove organic and inorganic contaminants from the steel surface, degreasing with an alkaline cleaning solution or washing with an organic solvent such as trichloroethylene and the like is carried out. However, the organic solvents having the best cleaning effect are no longer favoured because of environmental considerations. The inventors found that when steel works are fluorinated by holding them in a heated condition in a fluorine- or fluoride- containing gas atmosphere prior to nitriding, followed by nitriding cleaning (removal of organic or inorganic contaminants, oxide layer and the like) and activation of the steel surface can be achieved to give a superior nitrided layer. Such a method is the subject of Japanese Patent Application No. 177660/1989 and United States Serial No. 479,013. In this method, the steel articles are heated in a furnace in contact with fluorine- or fluoride-containing gas, such as NF₃, as a pretreatment. As a result, organic and inorganic contaminants adhering to the steel surface are destroyed by activated fluorine atoms and any passive coating layer such as an oxide layer on the steel surface is converted to a fluorinated layer, which covers and protects the steel surface. The works are then nitrided. The fluorinated layer is destroyed and removed during nitriding by introducing a mixture of nitriding gas containing a nitrogen source (e.g. NH₃ gas) and H₂ gas into the heated furnace. In more detail, a clean and activated steel surface can be obtained by destruction and removal of the fluorinated layer, so that nitrogen atoms in the nitriding gas rapidly penetrate and diffuse inside the bare, clean and activated steel to form a deep, uniform nitrided layer. The inventors also developed a furnace having two chambers for nitriding and fluorinating for carrying out this method, which is the subject of European Patent Application No. 90 308 460. As a result of experiment with this apparatus, they found out that there was a great difference in the time required for fluorinating the steel with the fluorine- and fluoride-containing gas and that required for nitriding it. Thus, it is not possible with prior art methods and apparatus to conduct the process of pretreatment and nitriding of steel works continuously and effectively. According to the invention there is provided a method of nitriding steel comprising: holding steel material in a first heat treatment furnace in a heated state in a fluorine- or fluoride-containing atmosphere to fluorinate the material; and holding the fluorinated steel material in a second heat treatment furnace in a heated state in an atmosphere of nitriding gas, characterized in that pieces of fluorinated steel material from the first furnace are introduced successively into ones of a plurality of second heat treatment furnaces to nitride them and in that the total time in the first and second furnaces is divided between the furnaces such that the amount of material treated in the second furnaces per unit time is the same as that treated in the first furnace per unit time. A heat treatment furnace suitable for fluorinating or nitriding steel material comprises: a liftable inner cover to contain the steel material; a liftable bell shaped outer cover which covers the inner cover to define a space between the inner and the outer covers; the interior of the inner cover serving as a fluorinating or nitriding chamber, the said chamber communicating with a feed pipe for fluorine- or fluoride-containing gas or for nitriding gas and with an exhaust pipe; and the space between the inner cover and the outer cover serving as a heating chamber; and a heater for heating the fluorinating or nitriding chamber, disposed with the heating chamber. In methods of nitriding steel according to the present invention, the steel works are pretreated using fluorine- or fluoride-containing gas prior to nitriding, to enable formation of a deep and uniform nitrided layer. In addition, amount of nitrided steel produced per unit time can be increased greatly over prior art methods since the pretreatment and the nitriding are conducted not in the same furnace but in separate furnaces. The establishment ratio of the two furnaces is decided rationally on the basis of the amount of treated steel produced per unit time by a fluorinating heat treatment furnace and the amount of treated steel produced per unit time in a nitriding heat treatment furnace. In heat treatment furnaces suitable for use in carrying out the present invention, since liftable inner and outer covers are provided, the inside of the furnace can be repaired easily and quickly by lifting the covers, when, for example, the inner surface of the inner cover, fan other component is corroded by fluorine or fluoride containing gas, nitriding gas or the like. Also, wear may be easily checked. The term "fluorine- or fluoride-containing gas" as used herein means a dilution of one or more fluorine sources such as NF₃, BF₃, CF₄, HF, SF₆ or F₂ in an inert gas such as N₂. NF₃, BF₃, CF₄, and F₂ are gaseous and SF₆ is liquid at ambient temperature. NF₃ is best suited for practical purposes, since it is superior in safety, reactivity, controllability, handling and other properties. F₂ is not preferred since it has high reactivity and toxicity. It is difficult to handle and to use in the furnace. BF₃, SF₆ similar compounds are effective in the formation of the fluorinated layer but are not preferred because of the formation of noxious boron and sulphur compounds. Gases such as FC1₃ are not preferred because of the formation of chlorides such as FeC1₃ which readily sublime. Generally, the fluorine- or fluoride-containing gas is used at a high temperature. From the view point of effectiveness, the concentration of the fluorine source component, such as NF₃, is preferably between 0.05 and 20% (percentages are by weight throughout this specification) in the fluorine or fluoride containing gas, preferably between 2 and 7%, and more preferably between 3 and 5%. The invention finds application in the treatment of various types of steel, including carbon steel and stainless steel. The shape of the steel work is not significant. Shapes such as plate, coil, and worked screw may be used. The invention contemplates treatment of not only a single material, but also alloys. In methods according to the invention, steel works are fluorinated, for example as below mentioned. The steel works are charged into a first heated furnace for fluorination and heated to raise the temperature of the works to between 150°C and 600°C, preferably between 250°C and 380°C. Fluorine- or fluoride-containing gas such as NF₃ is then fed into the heated furnace. The steel articles are held at the above-mentioned temperature in the fluorine or fluoride containing atmosphere for from 10 to 120 minutes, preferably from 20 to 90 minutes, more preferably from 30 to 60 minutes. As a result, the passive coating layer (comprising mainly an oxide layer) on the steel surface is converted to a fluorinated layer. This reaction is believed to proceed as follows: Fe0 + 2F → FeF₂ + 1/2 0₂; or Cr₂0₃ + 4F → 2CrF₂ + 3/2 0₂ The invention will be further described, by way of example, with reference to the drawings, in which: Figure 1 shows an apparatus suitable for use in carrying out the method of the invention; and Figure 2 shows, schematically, the operation of an apparatus in a method according to the invention. The method of the invention may be carried out using the heat treatment furnace shown in Fig. 1. Reference numeral 1 indicates a bell shape of outer cover, and 2 a cylindrical inner cover covered by the outer cover 1. A frame 10 including an attachment point 10a is disposed at the top of the outer cover 1 for attachment of a hook of a crane of the like. A lid body 11 having an attachment point 11a for attachment of a hook of a crane or the like is disposed at the top of the inner cover 2. The interior of the inner cover 2 is a fluorinating or nitriding chamber and the space between the outer and inner covers 1 and 2 is a heating chamber. Reference numeral 3 is steel material which can be charged into and removed from the inner cover 2. The steel material 3 is placed on a frame 15 having a central hole 14 and piled in a space between a first cylindrical mesh body 16 extending from the central hole 14 upwardly and a second cylindrical mesh body 17a extending from the periphery of the frame 15 upwardly, which is divided into a plurality of stages by porous dividing plates 17b each having a central hole. A hole 4 for insertion of a burner is made in a wall surrounding the lower part of the outer cover 1 and an exhaust port 4a is made in a wall surrounding the upper part of the outer cover 1. Reference numeral 5 indicates a base, 6 a fan for circulating air in the furnace. The fan 6 faces the central hole 14 of the frame 15 and air in the furnace is circulated by the fan through the central hole 14 and the first cylindrical mesh body 16. A heat exchanger 7 is disposed in a pipe 7a extending downwardly from the base of the inner cover 2. A circulating blower 8 for forced cooling is also disposed in the pipe 7a. The reference numeral 9 indicates a pipe for introducing fluorine- or fluoride-containing gas such as NF₃ or a nitriding gas into the inner cover 2. An exhaust pipe 12a for taking out exhaust gas from the inner cover 2 is divided into two pipes over its middle section. One of the divided pipes 17 has a valve 18, and the other divided pipe 19 has a valve 20 and a vacuum pump 21. When the exhaust gas pressure in the inner cover 2 is high, the gas is led through the divided pipe 17, and when low, the gas is led through the other divided pipe 19, the exhaust gas being evacuated by the vacuum pump 21. A noxious substance eliminator 12 is connected to the end of the exhaust gas pipe 12a. The eliminator 12 comprises left and right activated charcoal cylinders 22, a heater coil 23 around the cylinders 22, and a fin tube heat exchanger 24. The exhaust gas is introduced into the activated charcoal cylinders 22 and residual NF₃ and the like in the exhaust gas is heat reacted with the activated charcoal to convert it to harmless CF₄. The exhaust gas is led to the fin tube exchanger 24 and cooled therein. A scrubber 13 is provided with a pipe 25 extending from the heat exchanger 24. The scrubber 13 is water filled and the exhaust gas from the heat exchanger 24 is bubbled through the water to dissolve HF (a by-product of the reaction of NF₃) and to remove H₂0 and H₂ in the exhaust gas from the inner cover 2 in the water. Thereby, the exhaust gas is rendered completely harmless and is released to the atmosphere. The method of nitriding steel using the furnace described above is as follows. The outer cover 1 and the inner cover 2 of a first heat treatment furnace are lifted up by a crane (not shown) or the like by hooking the hooks thereof, separately, to the attachment points 10a, 11a of the outer cover 1 and the inner cover 2. After the steel material 3 has been placed on the frame 15, the outer cover 1 and the inner cover 2 are replaced (as in Fig. 1). Then, a burner (not shown), is inserted through the hole 4 into the heating chamber formed by the space between the outer cover 1 and the inner cover 2, and the chamber is heated. Thereby, the steel material 3 within the inner cover 2 is heated. Then, fluorine- or fluoride-containing gas such as NF₃ is introduced into the inner cover 2 from below through the pipe 9, to fluorinate the steel material. In this way, it generally takes about 30 to 60 minutes, as mentioned before, for fluorination to be completed. Since the steel material 3 is now covered with a fluorinated film on the surface, the surface is protected without being oxidized even if it is in contact with external air. In this state it is stored or immediately nitrided in a second heat treatment furnace. The second heat treatment furnace is of the same structure as the first heat treatment furnace and operates in a similar manner. That is, the inner cover 2 and the outer cover 1 thereof are lifted up, the fluorinated steel material 3, fluorinated in the first furnace A, is charged into the heat treatment furnace A', and the inner cover 2 and the outer cover 1 are replaced (see Fig. 2). Then, flame from the burner heats the space between the inner cover 2 and the outer cover 1 to heat the steel material 3 inside the inner cover 2 to the nitriding temperature of between 400°C and 700°C. Then, NH₃ gas or a mixture of NH₃ and a carbon source gas is introduced from below into the heat treatment furnace through the pipe 9 and this state is maintained for 120 minutes. During this process, the fluorinated layer is reduced or decomposed, for example according to the following formulae, by H₂ or a small amount of moisture (a by-product of the nitriding reaction), to form an activated steel surface. CrF₄ + 2H₂ → Cr + 4HF 2FeF₃ + 3H₂ → 2Fe + 6HF It is possible to decompose the fluorinated layer by blowing a mixture of N₂ gas and H₂ gas, or H₂ gas alone, prior to introducing N₂ gas. This is preferred because of a by-product of the reaction with NH₃ gas, ammonium fluoride. Active nitrogen atoms in the nitriding gas origin act on the activated steel surface to penetrate and diffuse into the steel. As a result, an ultra-hard, uniform and deep compound layer (a nitrided layer) containing a nitrided substance such as CrN, Fe₂N, Fe₃N, Fe₄N and subsequently a hard nitrogen atom diffused layer is formed to provide a complete nitrided layer by addition of the diffused layer to the compound layer. Thus, by the present invention, the steel surface appearing on decomposition of the fluorinated layer is activated and nitrogen atoms act thereon and penetrate the surface to form a uniform and deep ultra-hard nitrided layer. EXAMPLE 1 As shown in Fig. 2, two second heat treatment furnaces A' were used for nitriding compared with one first heat treatment furnace A for fluorinating. FLUORINATING After manufacturing a plurality of austinitic SUS screws (samples), they were cleaned with steam and trichloroethylene. The cleaned samples were charged into a first heat treatment furnace A to heat them at 300°C, and twice the volume of the inner cover 2 of a gas containing 1% NF₃ and 99% N₂ was introduced into the inner cover and held for 10 minutes. Then, a portion of the samples was taken out and checked. It was confirmed that a fluorinated layer had formed over the whole surface. NITRIDING The fluorinated samples were transferred into one of the two second heat treatment furnaces A' and a mixed gas containing 25% NH₃, 10% C0₂, 40% H₂ ad 25% N₂ was introduced into the second heat treatment furnace A', and nitriding was conducted at between 400°C and 600°C over six hours. Then, the samples were air-cooled and removed. A nitrided layer was uniformly formed over the surface of samples. During nitriding, after further samples had been fluorinated in the first heat treatment furnace A, these fluorinated samples were charged into another heat treatment furnace A' to nitride them. Thus, two nitriding furnaces A' were used with one fluorinating furnace A and operated continuously to cut the waiting time of the first heat treatment furnace A and to achieve effective nitriding. EXAMPLE 2 Fluorinating and nitriding were carried out by combining two second heat treatment furnaces A' with one first heat treatment furnace A. FLUORINATING The furnace A is evacuated to 1.3 x 10⁴ Pa (100 torr) and using a mixed gas containing 0.1% NF₃ and 99.9% N₂, fluorinating was conducted by holding samples at 350°C for 30 minutes. Otherwise, the treatment was carried out as in Example 1. NITRIDING The nitriding temperature was 570°C and a gas containing 25% NH₃, 5% CO, 10% H₂ and 60% N₂ was used. The treatment time was 5 hours. Otherwise, the treatment was carried out as in Example 1. In this example, the same result as in Example 1 was obtained. EXAMPLE 3 Fluorinating and nitriding were carried out by combining three second heat treatment furnaces A' with one first heat treatment furnace A. FLUORINATING The furnace A was evacuated to 1.3 x 10³ Pa (10 torr) and using a mixed gas containing 2% NF₃ and 98% N₂, fluorinating was conducted by holding samples at 330°C for 40 minutes. Otherwise, the treatment was carried out as in Example 1. NITRIDING Nitriding was conducted at 570°C for 7 hours using a mixed gas containing 25% NH₃, 10% C0₂, 25% H₂ and 40% N₂, and holding the samples for 40 minutes at 330°C. Otherwise, the treatment was carried out as in Example 1. In this example, the same result as in Example 1 was obtained. EXAMPLE 4 The temperature of the first heat furnace A in the fluorinating of Example 1 was 200°C and a mixed gas of 1% F₂ and 99% N₂ was used. The amount of the fluorine gas used and the holding time were changed to three times the volume of the inner cover 2 and 20 minutes respectively. Otherwise, the fluorinating and nitriding were carried out as in Example 1. The nitrided layer of the thus obtained samples was superior to that of Example 1 in which NF₃ was used in the fluorinating stage. In these examples, an outer cover and an inner cover are disposed within each furnace. However, it is possible to use a pit type furnace as one of these furnaces. However, with a pit type furnace, there are some disadvantages in that it takes more labour to charge and discharge the works and in that it is troublesome to replace and repair the furnace material when it is worn out by fluorinating and nitriding. As mentioned above, by holding the steel works in a heated state in a fluorine- or fluoride-containing atmosphere, organic or inorganic foreign matter is removed, and at the same time any passive coating layer such as an oxide layer on the steel surface is converted into a fluorinated layer, and then the steel works are nitrided. In this way, the passive coating layer on the steel surface is converted to a fluorinated layer which protects the steel surface and keeps it in good condition. Therefore, even if some time passes between formation of the fluorinated layer and nitriding, the fluorinated layer formed on the steel surface keeps the steel surface in a good condition. As a result, an oxide layer will never be formed on the steel surface again. The fluorinated layer is decomposed and removed during subsequent nitriding and thereby the bare steel surface is uncovered. The bare surface is activated, so that nitrogen atoms penetrate easily into the steel during nitriding. Thereby, nitrogen atoms penetrate deeply and uniformly into the steel from the steel surface to form a superior nitrided layer. In the method of nitriding according to the present invention, fluorinating and nitriding are not conducted in the same furnace, but in separate furnaces. A plurality of furnaces for nitriding, which needs a considerably longer time than fluorinating, are used with one furnace for fluorinating, so that there is no downtime for the fluorinating furnace. Thereby, continuity and a highly efficienct nitriding can be achieved.

Claims: A method of nitriding steel comprising: holding steel material in a first heat treatment furnace in a heated state in an atmosphere of fluorine- or fluoride-containing gas to fluorinate the material; and holding the fluorinated steel material in a second heat treatment furnace in a heated state in an atmosphere of nitriding gas, characterized in that pieces of fluorinated steel material from the first furnace are introduced successively into ones of a plurality of second heat treatment furnaces to nitride them and in that the total time in the first and second furnaces is divided between the furnaces such that the amount of material treated in the second furnaces per unit time is the same as that treated in the first furnace per unit time. A method according to claim 1, in which the fluorine or fluoride containing gas is at least one of NF₃, BF₃, CF₄, HF, SF₆ and F₂ in an inert gas. A method according to claim 1 or 2, in which the fluorine source component is present in the fluorine or fluoride containing gas in an amount of from 0.05 to 20% by weight. A method according to claim 1, 2 or 3 in which the fluorinating is conducted at temperatures of from 150°C to 600°C. A method according to any of claims 1 to 4, in which the nitriding gas is ammonia or a mixture of ammonia and a carbon source component, and in which a mixture of nitrogen and hydrogen, or hydrogen, is introduced into the second heat treatment furnace prior to introducing the nitriding gas thereinto.

Inventors: KITANO KENZO, MINATO TERUO, SENBOKUYA JARUO, TAHARA MASAKI, KITANO, KENZO, MINATO, TERUO, SENBOKUYA, JARUO, TAHARA, MASAKI
Applicants: DAIDO OXYGEN, DAIDOUSANSO CO., LTD.
Application year: 1990
Main classification: C23C8
IPC codes: C23C8