DE69113789T2 - Process of nitriding steel. - Google Patents

Abstract

This invention relates to a method for forming a uniform, deep nitride layer on and in steel works at low cost, wherein a steel work is fluorided in heated condition in an atmosphere of a mixed gas composed of fluorine gas and inert gas with or without nitrogen trifluoride gas and, then, nitrided in heated condition in an atmosphere of nitriding gas.

Description

The present invention relates to a method for nitriding steel, for nitriding steel, which comprises subjecting a steel workpiece to a special pretreatment which promotes a deep and uniform nitride layer or shell.

For improving the wear resistance, corrosion resistance and mechanical properties such as fatigue strength of steel, it is common practice to form a nitride layer or shell on the surface of steel. Typical of this technique is the nitriding process (gas nitriding, gas soft nitriding) using ammonia gas alone or a mixed gas consisting of ammonia and a carbon source-containing gas (RX gas). Processes of this type have problems with process stability, which is that when treating an alloy steel workpiece or a steel workpiece with a convoluted shape, the resulting nitride shell tends to be uneven.

While steel workpieces are generally nitrided at temperatures not lower than 500ºC, the adsorption and diffusion of nitrogen on and into the surface layer of steel requires not only the absence of organic and inorganic impurities but also the absence of an oxide film. In addition, the steel surface itself must also be of high activity. In fact, however, in such nitriding processes it is impossible to prevent the formation of an oxide film or to achieve complete activation. For example, taking an austenitic stainless steel workpiece, it is generally cleaned with hydrofluoric acid-nitric acid to remove the passivation film before placing it in the nitriding furnace, but it is difficult to completely remove the passivation film and impossible to completely activate the surface layer of the steel. Therefore, it is almost impossible to form a satisfactory nitride shell. Furthermore, the removal of organic and inorganic impurities Prior to nitriding, degreasing is generally carried out by alkali degreasing or organic cleaning with, for example, trichloroethylene, but the latest anti-pollution regulations (control against the destruction of the ozone layer) prevent the practice of organic cleaning, which is the most effective cleaning method available to date, and this factor is a major obstacle to the formation of a satisfactory nitride shell.

Under these circumstances, the present inventors have previously found that if a steel workpiece is first fluorinated in a heated state under a fluoride-containing gas blanket such as NF₃ prior to nitriding and then nitrided, both cleaning (removal of organic and inorganic impurities and removal of oxide film) and activation of the steel surface can be achieved to give a satisfactory nitride shell, and a patent application for this technology has been filed (Japanese Patent Application No. 1-177,660 and European Patent Application EP 0 408 168). In this method, the steel workpiece is first heated in a furnace for pretreatment and contacted with a gas such as NF₃. As a result, the organic and inorganic impurity components adhering to the steel surface are destroyed by the activated fluorine atoms to leave a pure steel surface, and at the same time, the passivation film including the oxide film on the steel surface is converted into a fluoride film to cover and protect the steel surface. The steel workpiece is then nitrided. In this nitriding process, the above-mentioned fluoride film is destroyed and removed by introducing a mixed gas consisting of a nitriding gas (i.e., NH3 gas) containing a carbon source and H2 gas into the furnace under heating. More specifically, the destruction and removal of the above-mentioned fluoride film leaves a pure and activated steel surface, and the N atoms in the nitriding gas rapidly penetrate and diffuse into this purified and activated steel surface. steel to form a uniform and deep nitride shell. Despite the desirable performance characteristics of NF3 gas mentioned above, it has the disadvantage of high cost. In addition, a fairly high temperature (280 to 500ºC) is required for sufficient fluorination and this means a significant thermal energy consumption which adds to the treatment costs.

European patent application EP 0 481 136, which was not published before the filing date of the present application, describes a nitriding process in which a fluoride layer is formed on the surface of the steel using a mixed gas of 1% F₂ and 99% N₂.

The present invention, developed under the above circumstances, has as its object to provide a method for nitriding steel which is capable of forming a uniform and deep nitride shell at low cost.

In order to achieve the above object, the present invention is directed to a method for nitriding steel, in which a steel workpiece is fluorinated by heating it to a temperature between 200 and 250°C under a blanket of a fluorine gas-nitrogen gas mixture and then nitriding the same workpiece in the heated state under a blanket of nitriding gas.

The inventors of the present invention have conducted a series of investigations on cost reductions of a nitriding process using NF₃ as a fluorinating gas and found that fluorine gas (F₂), which was previously considered unsuitable for fluorination at the time of development of the above-mentioned basic invention using NF₃ as a fluorinating gas, actually has excellent fluorination activity and the fluorine gas effects fluorination at a much lower temperature than NF₃. The present invention is based on the above-mentioned finding.

This means that the present invention is directed to a fluorination process which uses a mixture of F₂ and an inert gas N₂. By this technique strong fluorination can be achieved at a comparatively low temperature in the range of 200 to 250°C.

It was thus found that there is a temperature difference of as much as 100 to 150°C between the fluorination temperature in the case of using F₂ alone (F₂ + N₂) and the fluorination temperature in the case of using NF₃ alone (NF₃ + N₂).

As the F2 gas (fluorine gas), there can be used not only a general F12 gas which is produced by a melt electrolysis method and the like, but also F2 gas which is produced by thermal cracking by introducing a fluorine-containing compound such as BF3, CF4, HF, SF6, CF6, WF6, CHF3, SiF4 into a thermal cracking apparatus. F2 as used in this invention includes F2 produced by thermal cracking.

The present invention will now be described in more detail.

The concentration of F₂ is set at 0.05 to 20% (volume percent, the same applies hereinafter). The disadvantage of F₂ is that since it is highly reactive, controlling fluorination at high concentration is difficult. Although F₂ is fairly easy to control at a concentration not exceeding 1%, prolonged treatment is required to achieve sufficient nitriding of steel. Therefore, the preferred F₂ concentration is 3 to 10%.

The substrate steel for the present invention includes a variety of steels such as carbon steel, stainless steel, etc. The steels are not limited in shape or the like, and may be in the form of a plate or a coil, or in the processed form of a screw or the like. The substrate steel for the present invention is also not limited to the aforementioned steels, but also includes alloys of the aforementioned steels and alloys based on the aforementioned steels and supplemented with other metals.

Preferably, according to the present invention, the substrate steel is treated using either (A) a first heat treatment furnace for fluorination and a second treatment furnace for nitriding or (B) a single heat treatment apparatus having both a fluorination chamber and a nitriding chamber.

In the case of treating the substrate steel using (A) a heat treatment furnace for fluorination and a heat treatment furnace for nitriding, the process may, for example, comprise the following steps. First, fluorination is carried out in said heat treatment furnace for fluorination in the manner described below. Accordingly, the steel workpiece to be nitrided is placed in the first heat treatment furnace for fluorination and heated to a temperature of 200 to 250°C. Then, in the same state, fluorine gas (F₂ + N₂) is introduced into the heating furnace, and the steel tool is kept at the same temperature as above in an atmosphere of this fluorine gas for about 10 to 120 minutes, preferably for about 20 to 90 minutes, and for still better results for about 30 to 60 minutes. In the case of using F₂ which is obtained by decomposition of a compound such as BF₂, the process may comprise the following steps: was formed, a cracking apparatus is placed in front of the heat treatment furnace or in the vicinity of it. After thermal cracking of the above compound, the F₂ formed is mixed with N₂ and the mixture is introduced into the heating furnace. This process converts the passivation film (mainly consisting of oxide) on the steel surface into a fluoride film. This reaction proceeds according to the following reaction formulas.

FeO + F2 TFeF2; + 1/2O2

Cr₂O₃ + 2F2 T2CrF2; + 3/2O2

The above-mentioned treatments are each carried out using a heat treatment furnace, such as the one shown in Figure 1.

Referring to the accompanying drawings, reference numeral 1 denotes a bell-shaped outer cover and 2 denotes a cylindrical inner cover enclosed by the outer cover 1. On the upper part of the outer cover 1 is integrally arranged a frame structure 10 having engagement means 10a for engagement with a crane hook or the like. On the upper part of the inner cover 2 is integrally arranged a cover structure 11 having engagement means for engagement with a crane hook or the like. Inside the inner cover 2 a fluorination chamber is formed and the space between the two covers 1 and 2 forms a heating chamber. Reference numeral 3 denotes steel workpieces which are inserted into and removed from the inner cover 2. The steel workpieces 3 are arranged on a platform 15 having a central opening 14 and stacked in the space between a first cylindrical grid part 16 extending upward from said central opening 14 and a second cylindrical grid part 17a extending upward from the periphery of the platform 15 through interposed perforated partition walls 17b each having a central opening. Reference numeral 4 denotes an opening formed in the peripheral wall of the lower part of the outer cover 1 for installing a burner, and 4a denotes an exhaust opening formed in the upper wall of the outer cover 1. Reference numeral 5 denotes a base and 6 denotes a fan for circulating the furnace atmosphere. The fan 6 is arranged opposite the central opening 14 of the platform 15 and circulates the furnace atmosphere through the central opening 14 and the cylindrical grille member 16 extending upward therefrom. Reference numeral 7 denotes a heat exchanger arranged in a pipe 7a extending downward from the base of the inner cover 2. Reference numeral 8 denotes a circulation fan for forced cooling arranged in the pipe 7a downstream of the heat exchanger 7, while a pipe for introducing fluorine gas into the inner cover 2 is indicated at 9. Indicated at 12a is an exhaust piping for discharging exhaust gas from the inner cover 2, which is bifurcated at an intermediate portion, one branch piping 17 is equipped with a valve 18 and the other branch piping 19 is equipped with a valve 20 and a vacuum pump 21. When the gas pressure of the exhaust gas in the inner cover 2 is high, the path via the branch piping 17 is used, while when the pressure of the exhaust gas is low, the path via the branch piping 19 is used for vacuum evacuation by the suction force of the vacuum pump 21. Reference numeral 12 denotes a pollution prevention device connected to the outlet end of the exhaust piping 12a. This pollution prevention device 12 comprises a pair of activated carbon columns 22, a heater coil 23 wound around the periphery of each column, and a fin-type heat exchanger 24, and functions in such a manner that the spent gas introduced into the activated carbon column 22 is converted into harmless CF4 by thermal reaction of the residual F2 etc. with the activated carbon and is supplied to the fin-type heat exchanger 24 for cooling. Indicated at 13 is a wet scrubber which is arranged in a pipe 25 extending from the heat exchanger 24. This wet scrubber contains water and has the function of carefully treating the spent gas to make it harmless for release into the atmosphere by bubbling the spent gas from the pipe 25 to dissolve the HF fraction of the spent gas (which was produced by the reaction of F₂ with H₂O and H₂ incidentally in the inner cover 2) in water.

Using this heat treatment furnace, the fluorination is carried out as follows. In other words, the hook of a crane or the like (not shown) is connected to the engaging means 10a and 11a of the outer cover 1 and the inner cover 2 to hang the outer cover 1 and the inner cover 2 on the crane. In this state, the base steel 3 is placed on the platform 15 as shown, and the outer cover 1 and the inner cover 2 are lowered to their original positions (this state is shown in Fig. 1). Then, the heat from a burner (not shown) inserted into the opening 4 is released into the heating chamber formed between the outer cover 1 and the inner cover 2, thereby heating the steel workpiece 3 in the inner cover 2. Then, the fluorination gas is introduced into the inner cover 2 from its bottom through the pipe 9 for fluorination. The duration of this fluorination is, as mentioned above, generally about 30 to 60 minutes.

Then, nitriding is carried out as follows. Since the steel workpiece is now covered with a fluoride film after the fluorination treatment described above, it remains inactive without surface oxidation even when exposed to the atmosphere such as air. The steel workpiece is either stored in this state or immediately subjected to nitriding in a second heating furnace for nitriding. This second heating furnace for nitriding is similar in structure to the first heating furnace described above. Accordingly, the internal Cover 2 and the outer cover 1 are pulled up from this second heating furnace A', the steel workpiece 3 is then stacked, and the inner cover 2 and the outer cover 1 are lowered to their original positions. Then, the heat of a flame from a burner is emitted into the space between the inner cover 2 and the outer cover 1 to heat the steel workpiece in the inner cover 2 to a nitriding temperature of 480 to 700 °C. In this state, NH₃ gas or a mixed gas consisting of NH₃ and a gas containing a carbon source is introduced into the furnace from the bottom of the heating furnace through a pipe 9, and the steel workpiece is kept in this state for about 120 minutes or more. In this process, the fluoride film is destroyed by H₂ or a small amount of water (by-product in the course of the nitriding reaction) to give way to an active steel surface, e.g. B. according to the following reaction formulas.

CrF4; + 2H2 T Cr + 4HF

2FeF3; + 3H2 T2Fe + 6HF

Referring to the above-mentioned removal of the fluoride film, the film can be destroyed by introducing a mixed gas of N₂ and H₂ or H₂ before introducing the nitriding gas. This practice is actually preferred because problems due to the by-production of ammonium fluoride can be avoided.

On the active steel surface thus formed, the active nitrogen originating from the nitriding gas acts to penetrate and diffuse into the steel workpiece. As a result, an ultra-hard compound layer (nitride layer) containing nitrides such as CrN, Fe₂N, Fe₃N and Fe₄N is formed uniformly and of sufficient depth toward the inside of the steel workpiece from its surface, followed by the formation of a hard diffusion layer of N atoms, and the above compound layer and the diffusion layer form the entire nitride shell.

If both fluorination and nitriding are carried out in a single heat treatment furnace (B), a furnace having, for example, the structure shown in Figure 2 is used. In this illustration, 1' designates a furnace and 2' a metal basket loaded with steel workpieces (not shown). Reference numeral 3' designates a heater, 5' an exhaust pipe, 6' an adiabatic wall, 7' a door, 8' a fan, 10' a post, 12' a vacuum pump and 13' an exhaust treatment unit. At 21 is indicated the furnace body, which has an adiabatic wall divided into two spaces 23' and 24' by a partition wall or a sliding panel that can be freely opened and closed. The sliding board 22' is designed to keep the two spaces 23', 24' gas-tight and heat-insulated and to be freely opened and closed by vertical displacement. The reference numeral 23' denotes a fluorination chamber, while a nitriding chamber is denoted by 24'. Both the fluorination chamber 23' and the nitriding chamber 24' are equipped with a base 25' which accommodates the metal basket 2'. The base 25' consists of a pair of rails and is arranged so that the metal basket can be slid on the rails selectively into the fluorination chamber 23' or the nitriding chamber 24'. The reference numeral 26' denotes a gas inlet pipe for introducing fluorination gas into the fluorination chamber 23', while a temperature sensing probe is denoted by 27'. The front opening of the fluorination chamber 23' is detachably covered with a laterally driven cover 7'. The reference number 28' designates a nitriding gas line for introducing nitriding gas into the nitriding chamber 24'.

In the above heating furnace, nitriding is carried out as follows. First, the basket 2' containing the steel workpiece is placed in the fluorination chamber 23' and in this state, the internal temperature of the fluorination chamber 23' to heat the steel workpiece to 200 to 250ºC. Then, in this state, the fluorine gas (F₂ + N₂) is introduced into the chamber for fluorination for 30 to 60 minutes. After completion of fluorination, the fluorination chamber 23' is ventilated to discharge the gas.

Then, nitriding is carried out as follows. The above-mentioned slide table 22' is opened to transfer the steel workpieces and the metal basket 2' as a unit into the nitriding chamber 24', and the slide table 22' is then closed. In this state, the internal temperature of the nitriding chamber 24' is raised to heat the steel workpiece to 480 to 600°C, and H₂ gas is introduced into the nitriding chamber 24' to maintain this state for one hour, thereby destroying the fluoride film covering the steel surface to expose the substrate surface of the workpiece. Then, nitriding is carried out at this temperature for 4 to 5 hours while introducing nitriding gas, i.e., a mixed gas of NH₃, N₂, NH₂, CO and CO₂, into the nitriding chamber 24'. Thereafter, the internal temperature is lowered to 350 to 450°C, and in this state, cleaning is carried out by introducing a mixed gas consisting of H₂ and N₂ or a mixed gas consisting of N₂, H₂ and CO₂ for one hour. Thereafter, the gas consumed in the nitriding chamber 24 is discharged and the sliding table 22' is opened. Then, the steel workpiece and the metal basket 2' are brought into the fluorination chamber 23' as a unit and the sliding table 22' is closed, followed by cooling in this state. This cooling is carried out by introducing nitrogen gas from the gas inlet pipe 26' into the fluorination chamber 23'. The steel workpiece thus treated has a deep and uniform nitride shell. In this connection, heating of the steel workpiece for fluorination in the nitriding chamber 24' can be carried out by heating the same. This means that the steel workpiece is placed directly in the nitriding chamber 24' and heated there. Then the sliding table 22' is opened and the workpiece is Fluorination is transferred to the fluorination chamber. The steel workpiece is then brought back to the nitriding chamber 24' for nitriding. In this case, the nitriding chamber 24' can be preheated by utilizing the heat required to fluorinate the workpiece.

According to the present invention, the steel surface exposed after destruction of the fluoride film has been highly activated and the nitrogen atoms act on this activated steel surface to form an ultra-hard nitride layer of great depth and uniformity. In addition, the gas used for fluorination is F2 and compared to the use of NF3, it is not only inexpensive but also allows the use of a lower fluorination temperature, thus helping to reduce the treatment costs to a considerable extent.

Short description of the drawings

Figure 1 is a cross-sectional view showing an embodiment of a heat treatment furnace used in the present invention, and

Figure 2 is a basic cross-section of another heat treatment furnace.

Examples of the invention are given below.

Examples

First, an example using a pair of heating furnaces is described.

example 1 Fluorination

A variety of austenitic stainless steel bolts (sample) were prepared and cleaned with trichloroethylene vapor. The bolts were placed in a primary heating furnace (Figure 1) in which they were sufficiently heated to 200ºC as described above.

In this state, a mixed gas consisting of 10% F2 and the balance N2 was introduced into the furnace in an amount equal to 5 times the internal volume of the furnace per unit time, and the workpiece was held for 60 minutes. After that, some of the samples were taken out and the surface layer of each sample was examined. It was confirmed that a fluoride film was formed over the entire surface.

nitration

The samples subjected to this fluorination treatment were placed in a second heating furnace A' and NH3 + 50% RX gas was introduced into the furnace at 530ºC for 6 hours for nitriding. After completion of this treatment, the samples were air cooled and taken out of the furnace. The above treatment yielded nitrided austernitic stainless steel screws.

Comparison example 1

The treatment described in Example 1 was repeated except that the fluorination gas was replaced by a mixed gas of NF₂ and NF₃ (concentration 1%) and the fluorination temperature was replaced by 410°C to obtain nitrided stainless steel screws.

The hardness, texture and thickness of the nitride shell of the product according to Example 1 were compared with those of the product of Comparative Example 1. As a result, both products were found to be equivalent in quality. The cost of the product according to Example 1, however, was 1/3 of the cost of the product according to Comparative Example 1.

Some examples using a single heat treatment furnace (B) are given below.

Example 3

Fluorination and nitriding were carried out using a heat treatment furnace comprising a fluorination chamber and a Nitriding chamber was carried out as shown in Figure 2. The respective treatments were carried out as described in the text of this specification above, and the conditions in each treatment were the same as in Example 1. The same result was obtained as in Example 1.

As described above, the method of the present invention, which uses inexpensive fluorine gas for fluorination, allows a drastic reduction in the treatment cost. Furthermore, since the fluorination can be carried out at a temperature lower by 100 to 150°C than that of the fluorination with NF₃, the thermal energy requirement is significantly reduced, which also contributes remarkably to the cost reduction. In particular, since the fluorination can be carried out at such a relatively low temperature, the cooling time following the fluorination can also be shortened, so that the whole process is accelerated. Furthermore, since the fluorine gas has an intense odor, it is more amenable to leak detection than NF₃, and the pollution problem associated with harmful F₂ can be prevented with greater certainty. Furthermore, this lower temperature for fluorination brings further design advantages in the case of a heat treatment furnace (continuous furnace) having both a fluorination chamber and a nitriding chamber. For example, there is the advantage of extending the life of the seal of the sliding plate between the nitriding chamber and the fluoridation chamber. Since the gas used for fluorination is highly corrosive, the aging characteristics of the seal are less evident when the temperature of the fluoridation chamber is lower, so that a longer life can be realized. Among other advantages are the simplification and longer life of reinforcements and other parts of the structure.

Claims (1)

A method for nitriding steel, in which a steel workpiece is fluorinated while heated to a temperature of 200 - 250ºC in a mixed gas atmosphere consisting of fluorine gas and N₂ gas, then nitrided while hot in a nitriding gas atmosphere, characterized in that the concentration of fluorine gas in the fluorinating atmosphere is from 3 to 10 vol%.