JP2013007065A - Nitriding-quenching method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve following problems: a flow rate of an ammonia gas that is flown during nitriding-quenching becomes unstable, and which causes an increase in variation in surface hardness of a steel article after being subjected to the nitriding-quenching.SOLUTION: In a nitriding-quenching method, a reduction step of reducing oxide films overlying on surfaces of an inconel (R) heater 5 and the other metal materials in a heat-treating furnace 1 by introducing an ammonia gas and a nitrogen gas at a flow ratio of an ammonia gas to a nitrogen gas to a ratio of 2:1 into the heat-treating furnace 1, is introduced before a nitriding-quenching step of the steel article, thus, the flow rate of the ammonia gas is stabilized in the nitriding-quenching step preceding the reduction step, thereby suppressing the variation in surface hardness of the steel article subjected to the nitriding-quenching.

Description

  The present invention relates to a nitrogen quenching method for hardening the surface of a steel product.

  Conventionally, as disclosed in Patent Document 1, a steel work such as iron or an iron alloy in an atmosphere of ammonia gas or nitrogen gas (abbreviated as “steel product” in this specification) in a heat treatment furnace. Nitrogen quenching method that diffuses and permeates nitrogen into the surface of the steel to harden the surface of the steel product.

JP 2009-270155 A

  By the way, at the start of the nitriding and quenching method, the surfaces of Inconel (registered trademark) and other metal materials used as heater materials during the nitriding and quenching process are covered with an oxide film. When such an oxide film is present, the oxide film acts as a reducing agent on the steel product in the nitrocarburizing and quenching process of the steel product, and as a result, it greatly affects the decomposition of ammonia gas essential for the nitriding quenching. Due to this influence, the ammonia concentration in the heat treatment furnace becomes unstable, and as a result, the surface hardness of the steel product to be nitrogen-quenched is varied (including both within and between lots, the same applies hereinafter).

  Therefore, the present invention introduces a new process for stabilizing the ammonia concentration in the heat treatment furnace immediately before the nitriding and quenching process to prevent variations in the surface hardness of the steel product after nitrogen quenching. An object of the present invention is to provide a method for nitriding and quenching.

(Aspect of the Invention)
In the following, aspects of the invention that is recognized as being capable of being claimed in the present application (hereinafter referred to as claimable invention) will be exemplified, and each exemplified aspect will be described. Here, as in the claims, each aspect is divided into paragraphs, numbers are assigned to the respective paragraphs, and the descriptions of other paragraphs are cited as necessary. This is for the purpose of facilitating the understanding of the claimable invention, and is not intended to limit the combination of the constituent elements constituting the claimable invention to those described in the following sections. In other words, the claimable invention should be construed in consideration of the description accompanying each section, the description of the embodiment, etc., and as long as the interpretation is followed, another aspect is added to the aspect of each section. Moreover, the aspect which deleted the component from the aspect of each term can also be one aspect of the claimable invention.
In the following items, each of the items (1) to (3) corresponds to each of claims 1 to 3.

(1) A nitriding and quenching method comprising a reduction step of reducing an oxide film present on the surface of a metal material in a heat treatment furnace before a nitriding and quenching step of a steel product.

  “Iron and steel products” are iron or iron alloy workpieces or bulk products, for example, automotive parts, bearing holders, clutch plates, pressed parts, and other steel parts. Target what is required.

  “Nitrogen quenching” is an austenite region of the iron-nitrogen phase diagram, where nitrogen is diffused and permeated into steel products for a predetermined time at a predetermined temperature, and then immersed in oil and rapidly cooled to hard nitrogen martensite. It is a method for hardening the surface of a steel product, characterized in that the structure layer is formed to a depth of about 10 to 20 μm of the surface layer of the steel product.

  The “heat treatment furnace” includes a vacuum chamber capable of shutting off the atmosphere and the interior space of the heat treatment furnace in an air-sealed state before and during heat treatment, and in the vacuum chamber, a heat-treated product (main article) In the specification, a heater capable of heating steel products) is provided. Such a heater is preferably made of a heat-resistant metal in which a nichrome wire or the like capable of supplying an electric current from the outside is embedded, and Inconel (registered trademark), which is a nickel-based superalloy, is generally used in a heat treatment furnace. Therefore, in the present specification, the heater material is described as being made of Inconel (registered trademark).

  The heating temperature by the above heater is measured at various times in the heat treatment furnace at any time by thermocouples attached to several places inside the heat treatment furnace, and the temperature at each place in the heat treatment furnace is used as a control means such as a sequencer. It is preferable to control with a predetermined temperature pattern by a control means such as a sequencer while feeding back.

  The vacuum chamber is preferably provided with a vacuum door provided with a shutter mechanism that can be opened and closed at the time of carrying-in / out of a steel product to be subjected to nitrogen quenching and that is driven by a driving means such as a pneumatic cylinder. In addition, it is preferable that a pipe-shaped conduit is disposed at an appropriate location in the vacuum chamber, a check valve is provided in the conduit, and the vacuum chamber and the vacuum pump are air-sealed and connected via the check valve. .

  The vacuum pump is used to evacuate the inside of the heat treatment furnace once with the vacuum pump before introducing ammonia gas or nitrogen gas (before gas replacement), or to reduce the heater surface to a constant degree of internal atmosphere. Used to discharge the oxygen gas generated when it is released or to stabilize the atmosphere in the vacuum chamber appropriately so that the gas concentration of ammonia gas and nitrogen gas at a certain ratio is maintained. Is done.

  A spare chamber connected to the vacuum chamber via a check valve and having an internal space partitioned by a vacuum door that can be opened and closed in a vacuum atmosphere is provided adjacent to the vacuum chamber. It is preferable to put the steel product in the spare chamber before. The preliminary chamber can also be made into a vacuum atmosphere by a vacuum pump, and when the evacuation is completed and the reduction process is completed, the vacuum door is opened, and the steel product can be transported and arranged in the heat treatment furnace. In this way, it is possible to prevent air, particularly oxygen and moisture from entering the vacuum chamber during the transportation and placement of the steel product in the heat treatment furnace, and during the transportation and placement of the steel product to the heat treatment furnace. The surface state of the heater made of Inconel (registered trademark) from which the oxide film on the surface has been removed in the reduction step and other metal materials can be maintained. The steel product may be preheated to a certain temperature before nitriding and quenching in the preliminary chamber. This is to prevent heat shock when a high temperature suddenly acts on steel products during heat treatment.

  In the reduction process, after evacuation is completed with a vacuum pump, a certain amount of nitrogen gas and ammonia gas are in a certain ratio, preferably in the concentration of both gases, in the heat treatment furnace without transferring and placing the steel product. By flowing for a certain time so that the ratio is preferably 1: 2, the inside of the vacuum chamber is maintained at a constant temperature while replacing the vacuum atmosphere with a mixed gas of nitrogen gas and ammonia gas. The oxide layer on the surface of Inconel (registered trademark) is removed. In this reduction step, when a metal material other than the surface of Inconel (registered trademark), for example, the inner wall of the vacuum chamber or other parts is oxidized, these portions can be reduced at the same time.

  According to this section, before nitrogen-quenching of steel products, ammonia gas and nitrogen gas at a predetermined ratio are introduced into the heat treatment furnace, and the heat treatment furnace is maintained in a reducing atmosphere for a certain period of time. The surface of the substrate can be reduced. As a result, the oxide film present on the surface of Inconel (registered trademark) heaters and other metal materials can be removed in advance before nitrogen quenching, and the ammonia decomposition reaction in the heat treatment furnace becomes constant. Residual ammonia concentration for diffusing and permeating into the surface layer during nitrogen quenching is stabilized, and finally, variation in surface hardness of the steel product subjected to nitrogen quenching can be suppressed to a small level.

In other words, according to this section, before nitriding and quenching of steel products, the surface state of Inconel (registered trademark) heaters and other metal materials generally used in heat treatment furnaces is sufficiently reduced to remove the oxide film. Immediately thereafter, the nitriding quenching can be started. As a result, when nitriding and quenching steel products, there is no oxide film on the heater and other metal materials made of Inconel (registered trademark), so there is no reaction with oxygen from the oxygen film and ammonia. Gas (ammonia concentration) does not cause hunting, and variation in the decomposition reaction of ammonia gas in the heat treatment furnace is suppressed.
As a result, the ammonia concentration can be made constant, and the ratio with the nitrogen concentration introduced into the heat treatment furnace, that is, nitrogen concentration: ammonia concentration = 1: 2 (stoichiometric ratio) can be kept constant, so Variations can be suppressed.

(2) The nitriding and quenching step is performed after the ammonia concentration in the heat treatment has been reduced to a predetermined value in the reduction step.
This section illustrates the timing of the start of the nitriding and quenching process.
(3) In the reduction step, after introducing ammonia gas and nitrogen gas into the heat treatment furnace at a constant ratio, the temperature in the heat treatment furnace is set to be approximately the same as a predetermined temperature maintained during the nitrogen quenching step. The nitriding quenching method according to (1) or (2), wherein the inside of the heat treatment furnace is reduced by doing so.

  The “predetermined temperature” is preferably about the same as the temperature maintained in the subsequent nitriding and quenching process, and is preferably 760 to 810 ° C., for example. This is because if the temperature is different from the temperature maintained in the subsequent nitriding and quenching process, the degree of vacuum is changed in the nitriding and quenching process, which is preferable. The “predetermined temperature” is preferably 1000 ° C. If the temperature exceeds 1000 ° C., the crystal grains of Inconel (registered trademark) become coarse, which is not preferable.

(4) Item (3) is characterized in that the reduction step removes an oxide film present on the surface of an Inconel (registered trademark) heater and other metal materials before nitriding and quenching of steel products. Nitrogen quenching method as described.
This section specifies the reduction step by an active description.

(5) A method of performing nitriding and quenching of steel products in a heat treatment furnace including a vacuum chamber and an Inconel (registered trademark) heater, wherein the vacuum chamber is evacuated, and the vacuum step In order to replace the vacuum atmosphere formed in step 1 with ammonia gas and nitrogen gas, a gas introduction step of introducing ammonia gas and nitrogen gas at a constant ratio, and a heater is heated to a predetermined temperature and introduced in the gas introduction step. Reduction process of reducing the surface of Inconel (registered trademark) heater and other metal materials in the vacuum chamber with the ammonia gas and nitrogen gas, and after the reduction process is completed, the steel product is removed from the vacuum chamber of the heat treatment furnace. A steel product carrying-in process to be carried in, and a nitrogen-quenching process in which the steel product is subjected to nitrogen quenching at a predetermined temperature in an atmosphere of a mixed gas of ammonia gas and nitrogen gas A steel product unloading step for carrying out the steel product for which the nitrogen quenching step has been completed, and a cooling step for immersing the steel product for which the nitrocarburization quenching has been completed is immersed in oil for cooling. Nitrogen quenching method for steel products.

  This section defines all the main processes corresponding to the embodiment described below for the nitrocarburizing method of steel products. The description of this section will be described in the following embodiment.

  ADVANTAGE OF THE INVENTION According to this invention, the nitriding quenching method of the steel product which can prevent the variation in the surface hardness of the steel product nitrocarburized and quenching can be provided.

FIG. 1 is a schematic cross-sectional view of a heat treatment furnace used in the present embodiment. FIG. 2 plots the ammonia flow rate with a measuring instrument along the treatment time in the nitriding and quenching process with the abscissa indicating the nitriding quenching treatment time (min) and the ordinate indicating the ammonia flow rate (m ³ / hr). It is a graph drawn. An example is shown by the plot graph of a dotted pattern, and a comparative example is shown by the graph of the black square plot. FIG. 3 shows the measurement of the depth of the martensite layer corresponding to the depth of the nitrocarburizing and quenching of the nitrous-quenched steel product after the nitriding and quenching step, the steel product unloading step, and the cooling step are completed. It is the bar graph which computed about 3σ which is a parameter | index which shows the dispersion | variation in statistics, and took the depth (mm) of nitriding quenching on the vertical axis | shaft. The calculation result of an Example is shown on the right, and the bar graph which plotted the calculation result of the comparative example on the left is shown.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments according to the present invention will be described with reference to the accompanying drawings. The device described below and its components, components, parts, and materials are examples of the embodiments of the present invention. However, it is not limited to this. Moreover, in the figure, the part which attached | subjected the same code | symbol represents the same thing and the same member, and each dimension and each ratio of an apparatus or a member are not what reflected the actual thing but shown schematically.

Hereinafter, this embodiment according to the present invention will be described with reference to FIG.
FIG. 1 is a schematic cross-sectional view of a heat treatment furnace 1 used in the present embodiment.
As shown in FIG. 1, a heat treatment furnace 1 includes a vacuum chamber 2, a base 3, a steel product placement chamber 4, a heater 5, a gas conduit 6, a steel product placement stage 7, a vacuum suction conduit 8, and a vacuum pump. 9 and gas flow control means 10.

  The vacuum chamber 2 is made of stainless steel having a predetermined plate thickness and has a substantially cylindrical shape, and includes a vacuum door (not shown; the same applies hereinafter). The size of the vacuum chamber 2 is, for example, an internal diameter of 1400 to 1600 mm and a height of 1800 to 2300 mm in a batch type. This size is appropriate for the convenience of the operator of the batch work and for the convenience of assembly, repair, and inspection of the heat treatment furnace, but is not limited to this depending on the amount of steel products to be processed. .

  From the vacuum door of the vacuum chamber 2, a steel product (not shown; the same applies hereinafter) to be subjected to nitriding treatment is carried in and out. The vacuum door has a shutter structure that can be opened and closed by a pneumatic cylinder with an O-ring. A preparation chamber (not shown; the same applies hereinafter) for steel products is provided so as to be connected to the vacuum chamber 2 in an air-sealed state. By providing such a preparation chamber, a steel product to be subjected to nitriding treatment is placed in a preparation chamber held in a vacuum state during a reduction process described later, and a vacuum door with a shutter structure is provided after the reduction process is completed. When the steel product is loaded into the steel product placement stage 7 of the steel product placement chamber 4 and opened, it is preferable that oxygen does not enter the vacuum chamber 2 once reduced and does not form an oxide layer. The preparation chamber is also provided with a heater as necessary, and the steel product is preheated at a certain temperature. After the steel product is loaded into the steel product placement stage 7 in the steel product placement chamber 4, the vacuum door is closed.

The base 3 is made of heat-resistant bricks, heat-resistant fiber bodies, and the like, and is spread on the inner bottom surface of the vacuum chamber 2 as a base of the steel product placement chamber 4. In order to prevent heat fatigue and metal fatigue due to heat of the vacuum chamber wall, the heat-resistant bricks, heat-resistant fiber bodies, etc., which will be described below, are used as appropriate. You may make it construct for this.
The base 3 is preferably in the shape of a bowl as shown by the dotted line in FIG.

  The steel product arrangement chamber 4 is arranged on the base 3 and is preferably arranged at the approximate center of the vacuum chamber 2. This is because the temperature distribution inside the steel product placement chamber 4 tends to be uniform when placed at such a position. The steel product placement chamber 4 is provided with a steel product placement stage 7. The steel product to be processed is placed directly on the steel product placement stage 7 or is housed in a heat-resistant ceramic container such as a crucible or box and placed in the steel product placement chamber 4.

  As described above, the heater 5 is preferably a rod-shaped metal body made of Inconel (registered trademark). And it is preferable to provide a plurality of heaters 5 in the vertical direction and the horizontal direction around the steel product placement chamber 4 so as to uniformly heat the steel product placement chamber 4. If the vacuum chamber 2 has a size such as the above-described diameter of 1400 to 1600 mm and a height of 1800 to 2300 mm, for example, the heater 5 has a diameter of about 200 mm in the vertical direction and a length of about 1300 mm in the vacuum chamber 2. 10 to 18 inconel (registered trademark) products, four inconel (registered trademark) products having a diameter of about 200 mm in the horizontal direction and a length of about 600 to 700 mm are installed. In this way, several heaters 5 made of Inconel (registered trademark) are installed in order to equalize the heat of the steel product, but since a large number of heaters 5 are installed, the surfaces of these heaters 5 are installed. The reduction step of reducing the oxide layer that is initially present in advance is important. Note that the above-mentioned preferable number of heaters 5 can be appropriately changed by those skilled in the art depending on the size of the vacuum chamber 2 of the heat treatment furnace 1 and the processing amount of steel products.

  Inconel (registered trademark) is a trade name of Special Metals, and is based on nickel. It is divided into things. Inconel has heat resistance, corrosion resistance, oxidation resistance, creep resistance, etc. and excellent high temperature characteristics, so the surface treatment of the metal surface layer by heat such as carburizing, other carburizing, carburizing, carburizing, etc. It is a metal material suitable for a heater of a heat treatment furnace to be applied.

  Further, the heater 5 is heated by supplying a current from an external power source (not shown). In particular, the heat treatment furnace 1 is heated by heating the heater 5, and then performs temperature pattern control such as holding the predetermined temperature for a predetermined time and stopping the heating. It is preferable to provide sequencer control means (not shown) capable of measuring the pair (not shown) and feeding back the temperature of each part so that the current supplied to the heater 5 can be controlled.

  The gas conduit 6 is a stainless steel cylindrical tube that is provided in an air-sealed state by vacuum welding so as to penetrate a predetermined portion (upper part in FIG. 1) of the wall of the vacuum chamber 2. Ammonia gas and nitrogen gas are introduced into the vacuum chamber 2 through the gas conduit 6 in a reduction process described later. Since ammonia gas and nitrogen gas need to be introduced at a constant ratio, the gas flow control means 10 is disposed between the gas conduit 6, the ammonia gas tank (not shown), and the nitrogen gas tank (not shown). It is preferable. In order to prevent the backflow of ammonia gas and nitrogen gas, the gas conduit 6 and the ammonia gas tank (not shown), the nitrogen gas tank (not shown), or more preferably the gas conduit 6 and the gas. It is preferable to provide a check valve (not shown) between the flow control means 10.

Since the steel product placement stage 7 has been described above, the description thereof will be omitted.
The vacuum evacuation conduit 8 is a stainless steel cylindrical tube that passes through a predetermined portion (lower left portion in FIG. 1) of the wall of the vacuum chamber 2 and is attached in an air-sealed state by vacuum welding. The evacuation conduit 8 is connected to a vacuum pump 9 via a check valve 11.

  The vacuum pump 9 generally uses a rotary pump. In such a heat treatment furnace 1, the degree of vacuum in the internal atmosphere is not required unlike vapor deposition and ion plating, so a rotary pump is sufficient. However, the rotary pump is provided with an oil filter (not shown).

  The vacuum pump 9 is operated before the ammonia gas and nitrogen gas are introduced into the vacuum chamber 2 in a reduction process described later, and the inside of the vacuum chamber 2 is evacuated. In particular, oxygen and moisture are pumped to prepare for a suitable reduction treatment and nitriding treatment. After completion of the evacuation, it is preferable to output a command for introducing ammonia gas and nitrogen gas into the vacuum chamber 2 to the gas flow control means 10 via the signal line 12.

Using the heat treatment furnace 1 having the above-described configuration, a manufacturing process related to nitriding and quenching for steel products according to the present embodiment will be described below.
The manufacturing process includes a vacuum drawing step, a gas introduction step, a reduction step, a steel product carry-in step, a nitrous quenching step, a steel product carry-out step, and a cooling step.

<Vacuum drawing process>
In the evacuation step, a vacuum door (not shown) of the vacuum chamber 2 is air-sealed, and the vacuum pump 9 is operated in a state where the outside atmosphere and the inside of the vacuum chamber 2 are shut off, and the atmosphere inside the vacuum chamber 2 is changed. In this step, the vacuum state is set to a vacuum degree of 65 Pa or less. If the degree of vacuum is higher than 65 Pa, the inside of the furnace is oxidized, which is not preferable.
<Gas introduction process>
In the gas introduction step, ammonia gas and nitrogen gas are introduced into the vacuum chamber 2 in the vacuum state formed in the vacuuming step, and the vacuum atmosphere is replaced with a mixed gas of ammonia gas and nitrogen gas. It is a process. The flow rate ratio of ammonia gas and nitrogen gas is preferably 2: 1 from the stoichiometric ratio based on the decomposition reaction formula of ammonia gas. For this purpose, the gas flow control means 10 controls the flow rate ratio.

<Reduction process>
The reduction step is a step of removing the oxide film on the surface of the inside of the heat treatment furnace 1, particularly the surface of the heater 5 made of Inconel (registered trademark). Therefore, heat treatment is performed at a high temperature of, for example, 760 to 810 ° C. while supplying current to the heater 5 and controlling the heating temperature of the heater 5 in a reducing gas atmosphere of ammonia gas and nitrogen gas defined by the above flow rate ratio. It is held until the surface of the heater or the like in the furnace 1 is sufficiently reduced. The holding time for sufficient reduction can be the time until the residual ammonia concentration in the furnace reaches a predetermined value. In this embodiment, for example, 90 to 160 min is maintained until the residual ammonia concentration reaches 0.14 vol%. The reason why the high temperature is set to 760 to 810 ° C. is to make it the same level as the temperature maintained in the subsequent nitriding and quenching step. This is because if the temperature is different from the temperature maintained in the subsequent nitriding and quenching process, the degree of vacuum changes in the nitriding and quenching process. The reason why the holding time is 90 to 160 min is not preferable because the reduction is insufficient if it is shorter than 90 min. On the other hand, if it is longer than 160 min, the reaction rate is lowered and the effect is reduced, which is not preferable.

  In the above temperature control, the internal temperature of the heat treatment furnace 1 is measured with a thermocouple (not shown) installed in the vacuum chamber 2 of the heat treatment furnace 1, and the measured internal temperature is provided with temperature control means (not shown). It is preferable to maintain the internal temperature of the heat treatment furnace 1 at a constant temperature for a certain time while feeding back to a control means such as a sequencer (not shown).

By introducing this reduction step, the conventional problems can be solved for the following reasons.
First, the nitriding potential P (N), which indicates the ease of nitrogen diffusion / penetration into the surface of the steel product to be subjected to nitrogen quenching, using ammonia gas and nitrogen gas,
P (N) = P (NH ₃ ) / {P (H ₂ ) ^3/2 } Formula (1)
It can be expressed as.

In ideal nitriding quenching, it is preferable to flow ammonia gas and nitrogen gas so as to satisfy the formula (1).
More specifically, the flow rate ratio of ammonia (NH ₃ ) and nitrogen (N ₂ ) gas is set to 2: 1 (stoichiometric ratio), and the nitriding potential P (N) is managed by the ammonia gas concentration. This flow ratio is
NH ₃ (ammonia) → 1 / 2N ₂ (nitrogen) + 3 / 2H ₂ (hydrogen) (2)
And derived from disassembling.

On the other hand, the undecomposed residual ammonia gas (NH ₃ ) is fed back to the gas flow control means 10 and reused while the gas flow control means 10 adjusts the gas flow rate ratio to 2: 1. That is, the fed-back residual ammonia gas (NH ₃ ) increases or decreases the flow rates of ammonia gas and nitrogen gas to be reflowed to make the ammonia gas concentration constant.

  However, in reality, such ideal processing has not been performed. This is because the ammonia gas concentration cannot be made constant because the hunting of the ammonia gas flow rate is large. Therefore, the increase and decrease of the flow rate of the introduced ammonia gas is increased, the variation of the ammonia gas concentration inside the heat treatment furnace 1 is increased, and under such a condition, the variation of the surface hardness of the nitrocarburized steel product is increased. growing.

Therefore, in the present invention, an oxide film of Inconel (registered trademark) which is a material of many heaters 5 in the inside of the heat treatment furnace 1 works as a reducing agent, and an oxide film on the surface of Inconel (registered trademark). In view of the fact that the occurrence of this has a great influence on the decomposition of ammonia gas, a reduction process for removing the oxide film on the surface of Inconel (registered trademark) is introduced immediately before the nitriding quenching process.
As a result, variation in the decomposition reaction of ammonia (ammonia gas) in the heat treatment furnace 1, that is, in the vacuum chamber 2, can be suppressed, and the concentration of ammonia (ammonia gas) can be made constant. Quenching can be carried out suitably, and the variation in surface hardness of the nitrocarburized steel product is stabilized.

<Steel product import process / Nitrogen quenching process>
After completion of the above reduction process, the steel product to be nitrocarburized is placed on the steel product placement stage 7 in the steel product placement chamber 4 in the heat treatment furnace 1. At this time, it is preferable that the steel product to be nitrogen-quenched is carried out from a vacuum atmosphere preparation chamber (not shown) as described above and carried into the steel product placement chamber 4 in the heat treatment furnace 1. .
And the electric current supplied to the heater 5 is adjusted, the temperature in the steel product arrangement | positioning chamber 4 is heated so that it may become suitably 760-810 degreeC, and the flow rate ratio of ammonia gas and nitrogen gas is set. The steel product is nitrogen-quenched and held for a predetermined time so that it is preferably 2 to 1.

This is because if the temperature in the steel product placement chamber 4 is less than 760 ° C., the temperature of the steel product does not rise to the austenite region, and if it is higher than 810 ° C., voids are generated near the surface of the steel product, which is not preferred.
The reason why the flow rate ratio of ammonia gas to nitrogen gas is preferably 2 to 1 is from the stoichiometric ratio derived from the above-described equation (2).

  The holding time is appropriately adjusted depending on the volume of the steel product to be nitrocarburized, the internal volume of the heat treatment furnace 1 (the internal volume of the vacuum chamber 2), and the steel product placement chamber 4.

  According to this nitriding and quenching process, since the Inconel (registered trademark) material of the heater 5 has been oxidized to have an oxide film on the surface, the flow rate of ammonia gas is hunted, and the nitriding and quenching varies. Was big. However, as described above, as a result of introducing the reduction process immediately before the nitriding and quenching process in this embodiment, the problem is solved, and the depth of the surface hardened layer of the steel product, that is, the variation in the surface hardness of the steel product is suppressed. It is possible to achieve a suitable nitriding quenching.

<Steel product unloading process, cooling process>
In the cooling process after completion of the above nitriding and quenching process, the supply of current to the heater 5 is stopped, and in the steel product carrying-out process, the vacuum door (not shown) of the vacuum chamber 7 is opened, and the nitrogenized and quenched steel Unload the goods. After that, in the cooling process, the steel product that has been nitrogen-quenched is immersed in oil while being put in a mesh basket, etc., rapidly cooled and hardened, pulled up from the oil, and degreased and properly degreased and surface-hardened. To obtain a steel product that is uniformly enhanced.

  As described above, according to the present embodiment, the flow rate of ammonia gas flowing during the nitriding and quenching is reduced as a result of the reduction of the surface of Inconel (registered trademark), which is a heater material, in the heat treatment furnace before the nitriding and quenching step. It is possible to prevent variations in the surface hardness of steel products that are stable and nitrogen-quenched.

Hereinafter, processing conditions of an example corresponding to the present invention and a comparative example substantially corresponding to the prior art will be shown. Further, in FIG. 2 by showing the graph of the ammonia flow to the treatment time ^{(min) (m 3 / hr} ), it shows a significant difference in the embodiment for Comparative Example concerning the stability of the ammonia flow ^(m 3 / hr). And the significant difference of the Example with respect to the comparative example regarding the dispersion | variation in the hardened layer of the surface by nitriding quenching is shown in FIG.

[Processing conditions of the reduction process in the example and the comparative example (common conditions in the example and the comparative example)]
Steel products: Cylindrical steel products having an outer diameter 30 and an inner diameter 20 of SCM20 material.
・ Heat treatment furnace: Diameter 1456mm x Height 2080mm (substantially cylindrical)
-Heater: Inconel (registered trademark) 14 pieces of diameter 210 mm x length 1300 mm are arranged in an average around the steel product processing chamber 4 in the vertical direction, and Inconel (diameter 210 mm x length 650 mm) Four (registered trademark) products are arranged around the steel processing chamber 4 and in the horizontal direction on average.
・ Holding temperature during nitriding and quenching: 800 ℃
Introduction gas flow rate: flow rate of ammonia gas (NH ₃ gas); 1.2 m ³ / hr, flow rate of nitrogen gas (N ₂ gas); 0.6 m ³ / hr

[Retention time of the reduction process of the example and the comparative example (conditions different between the example and the comparative example)]
-Example: 100 min
・ Comparative example: 15 min

<Evaluation of stability of ammonia flow rate (m ³ / hr)>
FIG. 2 is a graph in which the horizontal axis represents the nitriding quenching treatment time (min) and the vertical axis represents the ammonia flow rate (m ³ / hr) in the nitriding quenching step. A plot line graph is shown, and a comparative example is shown as a black square plot line graph.

As can be seen from the graph shown in FIG. 2, when the reduction process has a long time of 100 min, the fluctuation width in the vertical direction of the plot is longer when compared with the comparative example with a short time of 15 min. It can be seen that the ammonia flow rate (m ³ / hr) is smaller in the example than in the comparative example.

<Evaluation of variation in surface hardened layer by nitrogen quenching>
In Fig. 3, after the nitriding and quenching process, the steel product unloading process, and the cooling process are completed, the nitrous-quenched steel product is filled with resin, and a cross section from the surface toward the depth direction is photographed with a metal microscope, and a photographic image The depth of the martensite layer corresponding to the depth of nitrocarburizing and quenching was measured, and one statistical index, 3σ, was calculated for statistical purposes. mm), the statistical calculation results of the example with a long reduction process time of 100 min are plotted on the right, and the statistical calculation results of the comparative example with a short reduction process time of 15 min are plotted on the left.

  As can be seen from the graph shown in FIG. 3, the length of the reduction process is 100 min and the length of 3σ is shorter in the example in which the time is 15 min than in the comparative example, and the example is longer in the example than the comparative example. It was also found that there was little variation in the nitrocarburized layer.

<Comprehensive evaluation>
The embodiment of the conditions corresponding to the present invention has a more stable ammonia gas flow rate (m ³ / hr) during the nitriding quenching than the comparative example of the conditions close to the prior art, and after the nitriding quenching There was less variation in the depth of the surface-cured layer. This is because the time for the reduction process introduced in the example immediately before the nitriding quenching process is longer than that in the comparative example, that is, the time for the reduction process is set to 100 min in the example, whereas the time in the comparative example is set to 15 min. It is determined that the time has been set.

Claims (3)

  A nitriding and quenching method comprising a reduction step of reducing an oxide film present on the surface of a metal material in a heat treatment furnace before a nitriding and quenching step of a steel product.   The nitriding and quenching step is performed after the ammonia concentration in the heat treatment is reduced to a predetermined value in the reducing step.   In the reduction step, ammonia gas and nitrogen gas are introduced into the heat treatment furnace at a constant ratio, and then the temperature in the heat treatment furnace is set to be approximately the same as a predetermined temperature maintained during the nitrous quenching step. The nitriding quenching method according to claim 1, wherein the inside of the heat treatment furnace is reduced.