JP2012246558A - Nitriding treatment apparatus and cross sectional hardness distribution prediction system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cross sectional hardness distribution prediction system after nitriding a workpiece made of a high alloy tool steel, and a nitriding treatment apparatus incorporating the prediction system.SOLUTION: The prediction system includes: a unit (a correction calculation unit S1) which receives the external input of the alloy composition of the workpiece, and obtains a difference by mass% in the alloy composition between the workpiece and a standard workpiece for each of the predetermined nitride generation elements, corrects the standard sectional nitrogen distribution as the nitrogen distribution of the section after the standard workpiece is subjected to the nitriding treatment under the predetermined nitriding condition to the corrected sectional nitrogen distribution, and calculates the nitride amount distribution; and a unit (a hardness conversion unit S2) which obtains the sectional hardness distribution from the nitride amount distribution for the corrected sectional nitrogen distribution.

Description

  The present invention relates to a system for predicting the distribution of cross-sectional hardness after nitriding a workpiece made of high alloy tool steel, and a nitriding apparatus incorporating the system.

  Nitriding is widely performed for the purpose of improving the durability of machine members made of tool steel such as molds and machine parts. Nitriding is a surface hardening treatment method in which nitrogen is diffused from the surface of a material to be treated into the inside thereof to strengthen the solution, and at the same time, the vicinity of the treatment surface is modified and hardened by precipitation of fine nitrides. Compared to a surface hardening treatment method in which a hardened film such as plating is applied to the treated surface, a discontinuous cross section is not generated, so that not only wear resistance but also impact resistance is excellent. In general, since the treatment can be performed at a relatively low temperature of A1 or less on the equilibrium diagram of steel, it is possible to prevent a large strain from being applied to the material to be processed. Even if there is, processing can be applied.

  Typical nitriding methods include salt bath nitriding, gas nitriding, plasma nitriding, and the like. The salt bath nitriding method is a method in which a material to be treated is immersed in a high temperature salt bath to diffuse active nitrogen into the inside. The gas nitriding method is a method of diffusing nitrogen generated by decomposing ammonia gas from the surface of the material to be treated. The plasma nitriding method introduces gas ions activated in the plasma atmosphere to the surface of the material to be treated. This is a method of diffusing nitrogen. In any nitriding method, the cross-sectional hardness distribution in the vicinity of the processing surface varies depending on nitriding conditions such as temperature and nitriding environment (salt bath concentration and atmosphere concentration). That is, although the mechanical characteristics change, the relationship between the nitriding conditions and the mechanical characteristics has been experimentally and empirically obtained so far. That is, nitriding conditions for obtaining desired mechanical characteristics have been determined from data obtained by repeatedly performing nitriding with a sample test piece and actually measuring the cross-sectional hardness distribution and the like.

  On the other hand, for example, Patent Document 1 discloses that the life prediction of a mold made of alloy tool steel subjected to nitriding is performed by an X-ray diffraction method. According to the technology that can measure the mechanical properties of the material to be treated that has been nitrided by such non-destructive method, the measured value is fed back to the nitriding treatment conditions, so that the nitriding treatment that matches the desired mechanical properties is performed. A system for determining conditions could be given.

  Furthermore, it is also considered to determine the nitriding conditions that can obtain the desired mechanical characteristics by simulation calculation. For example, Non-Patent Document 1 describes a method for calculating the concentration distribution of nitrogen diffused into the steel when plasma nitriding is performed. The nitrogen concentration distribution has a direct relationship with the mechanical properties of the material to be treated. Specifically, after determining the relational expression between the ammonia partial pressure in the processing unit and the nitrogen intrusion amount per unit time from the surface of the steel to the inside from the measured value, the intruded nitrogen is changed into the alloy element and the alloy nitriding. The concentration distribution formed by diffusion while forming an object is calculated according to Fick's second law. Here, precipitation of alloy nitride is calculated by a reaction rate constant obtained by actual measurement for each alloy element.

JP 2005-331477 A

Hiraoka, Inoue; Technical paper “Nitrogen concentration distribution prediction technology in plasma nitriding of steel”; Daido Special Steel Co., Ltd. “Electrical Steelmaking”, Vol. 81, No. 1, 2010, pp. 15-24

  As described above, regarding the relationship between the nitriding conditions and the obtained mechanical characteristics, in particular, in the case of a material to be processed containing a relatively large amount of nitride-forming elements such as high alloy tool steel, a machine by precipitation of fine nitrides. The effect on the physical characteristics is increased. That is, it is inefficient because it is necessary to repeatedly perform the test using the sample test piece for each alloy composition of the material to be processed. Therefore, a system that can determine nitriding conditions from desired mechanical characteristics and that can predict mechanical characteristics from nitriding conditions has been demanded.

  The present invention has been made in view of the situation as described above, and an object of the present invention is to provide a system for predicting the cross-sectional hardness distribution after nitriding a workpiece made of high alloy tool steel, and An object is to provide an integrated nitriding apparatus.

  A cross-sectional hardness distribution prediction system according to the present invention is a cross-sectional hardness distribution prediction system obtained by nitriding a material to be processed made of high alloy tool steel in a nitriding atmosphere, and the alloy composition of the material to be processed In response to the external input, Δ [Si], Δ [Cr], Δ, which are mass% differences from the reference alloy composition of the standard workpiece for at least the nitride-forming elements of Si, Cr, Mo, V, W [Mo], Δ [V], Δ [W] are obtained, and the standard cross-sectional nitrogen distribution, which is the nitrogen distribution of the cross-section after nitriding the standard workpiece under the predetermined nitriding conditions, is corrected to the corrected cross-sectional nitrogen distribution. A correction calculation unit for calculating the nitride amount distribution, and a hardness conversion unit for obtaining the cross-sectional hardness distribution from the nitride amount distribution for the corrected cross-sectional nitrogen distribution, and the processing target under the predetermined nitriding conditions Cross-sectional hardness distribution when nitriding the material It is characterized by prediction.

  According to this invention, the nitride amount distribution in the corrected cross-section nitrogen distribution based on the alloy composition can be obtained for the material to be treated that has been nitrided under the predetermined nitriding conditions, and the cross-sectional hardness distribution can be obtained. In other words, the cross-sectional hardness distribution can be predicted for each high alloy tool steel having a different alloy composition that gives different cross-sectional hardness distribution even under the same nitriding conditions, and the nitriding process can be performed more efficiently. . On the other hand, since the cross-sectional hardness distribution can be obtained from the nitriding treatment conditions, an example of the nitriding treatment conditions required to obtain a desired cross-sectional hardness distribution can be obtained.

  In the above-described invention, the standard cross-sectional nitrogen distribution may be an analytical solution of Fick's second law. According to this invention, the nitride amount distribution for the corrected cross-sectional nitrogen distribution based on the alloy composition of the material to be processed can be obtained more easily.

In the above-described invention, the correction calculation unit includes an intruding nitrogen amount calculating unit that calculates an amount of nitrogen entering the processing material from the nitriding atmosphere in the processing surface of the processing material, and the processing surface. The second diffusion is performed by subtracting the amount of nitride-forming nitrogen for each of the nitride-forming elements forming the nitride from the amount of first diffusion nitrogen that is sequentially diffused from the first to the depth direction. And a sequential calculation unit for calculating the amount of nitrogen, wherein the amount of nitride forming nitrogen is ρ _MeN is the density of nitride, V _MeN is the volume of nitride, t is time, K is a reaction rate constant, and C solute nitrogen content of the alpha in the iron _Me, when an alloy element content that does not form a nitride of the C _N alpha in the iron,
It is good also as a characteristic to calculate by the formula represented by these. According to this invention, the nitride amount distribution for the corrected cross-sectional nitrogen distribution based on the alloy composition of the material to be processed can be easily obtained by calculation.

  In the above-described invention, the intrusion nitrogen amount calculation unit assumes that the amount of nitrogen entering the material to be processed from the nitriding atmosphere does not change over time, and this is the nitrogen concentration in the vicinity of the surface layer of the standard cross-sectional nitrogen distribution It is good also as a characteristic to calculate from. According to this invention, the nitride amount distribution for the corrected cross-sectional nitrogen distribution based on the alloy composition of the material to be processed can be obtained more easily by calculation.

  One of the prediction systems described above is incorporated, and a display unit that displays the cross-sectional hardness distribution obtained by the hardness conversion unit may be provided. According to this invention, the cross-sectional hardness distribution after nitriding can be predicted and displayed on the nitriding apparatus for each workpiece having a different alloy composition, so that the nitriding process can be performed more efficiently.

It is a flowchart of the prediction system by this invention. It is a figure which shows (a) cross-sectional nitrogen distribution and nitride amount distribution, and (b) solid solution nitrogen amount and nitride amount. It is a figure which shows the prediction system by this invention. It is a flowchart of the principal part of the prediction system by this invention. It is a figure which shows the calculation method of cross-sectional nitrogen distribution and nitride amount distribution in the prediction system by this invention. It is a figure which shows the example of the cross-sectional nitrogen distribution and nitride amount distribution which were calculated in FIG. It is a graph which shows the cross-sectional hardness computed with the prediction system by this invention, and its measured value. It is a graph which shows the cross-sectional hardness computed with the prediction system by this invention, and its measured value. It is a list of the composition of steel used in the demonstration test and the hardness after nitriding. It is a plot figure which shows the relationship between the surface nitrogen concentration calculated by the prediction system by this invention, and hardness. It is a plot figure which shows the relationship between the precipitate formation nitrogen concentration calculated by the prediction system by this invention, and hardness.

  This inventor considered the prediction system which estimates the cross-sectional hardness distribution after nitriding the to-be-processed material which consists of high alloy tool steel which is a process target. That is, as shown in FIG. 1, in the prediction system, a standard processing material having a known component composition is used for the cross-sectional nitrogen distribution, which is a nitrogen distribution of a cross-section after the processing material is nitrided under predetermined nitriding conditions. The standard cross-sectional nitrogen distribution, which is the nitrogen distribution in the cross section after nitriding under the same nitriding conditions, is corrected (corrected) to the cross-sectional nitrogen distribution to calculate the nitride amount distribution of the material to be processed (correction calculating step S1). The physical quantity distribution is converted into a cross-sectional hardness distribution (hardness conversion step S2). Here, the “cross-sectional hardness distribution” is a distribution of hardness when the hardness is measured sequentially from the planar shape surface of the material to be processed in the depth direction (center direction of the material to be processed) and in the vertical direction.

  Specifically, in the correction calculation step S1, as shown in FIG. 2 (a), the standard cross-section nitrogen distribution 1 is corrected to the cross-section nitrogen distribution 2 from the difference in the content of nitride-forming elements in the standard process material and the process material. To do. Here, as shown in FIG. 2 (b), the same amount of nitrogen penetrates the standard material to be treated and the material to be treated from the nitriding atmosphere in the same nitriding time and generates nitrides while entering the inside. However, the amount of nitride precipitation increases as the amount of nitride-forming elements in the component composition increases. The standard cross-sectional nitrogen distribution 1 can be corrected to the cross-sectional nitrogen distribution 2 from the relationship between the nitride precipitation amount and the nitride-forming element amount, and at least the nitride amount distribution 4 of the material to be processed can be calculated. The nitride amount distribution 3 is a distribution of the nitride amount of the standard workpiece. In general, in the case of tool steel, nitride-forming elements are elements such as Si, Cr, Mo, V, and W.

  Incidentally, FIG. 2 illustrates the case where the content of the nitride generating element in the material to be processed is higher than that of the standard material to be processed. However, when the standard material to be processed does not include the nitride generating element, nitrogen is removed from the nitriding atmosphere. Intrudes into the interior without forming nitrides and dissolves, for example, according to Fick's second law. That is, the standard cross-sectional nitrogen distribution 1 follows Fick's second law. The same amount of nitrogen enters the standard material and the material to be treated from the nitriding atmosphere in the same nitriding time, and the standard cross-sectional nitrogen distribution 1 according to Fick's second law can be easily obtained. Therefore, at least the nitride amount distribution 4 can be calculated.

  In the hardness conversion step S2, the hardness is calculated from the nitride amount distribution 4 obtained in the correction calculation step S1, using a predetermined empirical formula obtained by collecting actual measurements of nitride amount and hardness and performing regression calculation. Get the height distribution. Here, an empirical formula is obtained by weighting the contribution ratio to the hardness for each nitride by regression calculation, and the hardness distribution is obtained by using the empirical formula as the nitride amount distribution 4 for each nitride. Is more preferable.

  Next, one example of the prediction system according to the present invention will be described in detail with reference to FIGS.

<System configuration>
As shown in FIG. 3, the prediction system 10 includes a CPU 11 for performing arithmetic processing, a ROM 12 and a RAM 13 associated therewith, an input unit 14 for appropriately inputting data, a storage unit 15, and the like. The storage unit 15 includes a storage area 20 in which various calculation data and the like are stored, and stores a correction calculation unit 18 and a hardness conversion unit 19 as programs. Further, the correction calculation unit 18 includes an intrusion nitrogen amount calculation unit 18a and a sequential calculation unit 18b as programs constituting the correction calculation unit 18. Each component is the same as a general computer system. Furthermore, a controlled device 22 is connected to the CPU 11 via the interface unit 21, and control is performed using data obtained by the prediction system 10. For example, the controlled device 22 is a nitriding furnace.

<System processing>
Next, system processing in the prediction system 10 described above will be described. Here, an example in which the standard material to be treated is steel having an alloy composition (reference alloy composition) that does not contain a nitride-forming element as described above will be described.

  In the correction calculation step S1, processing as shown in FIG. 4 is sequentially performed according to the program of the correction calculation unit 18.

  First, the nitride generation element amount difference between the standard processing material and the processing material is obtained (S11). That is, an external input of the alloy composition of the material to be processed is obtained via the input unit 14 (see FIG. 3) of the prediction system 10, and the mass% difference from the alloy composition (reference alloy composition) with the standard material to be processed is at least Δ [Si], Δ [Cr], Δ [Mo], Δ [V], Δ [W], which are mass% differences in nitride-forming elements of Si, Cr, Mo, V, and W, are obtained. Here, since these nitride generating elements are not included in the standard processed material, the mass% of the nitride generating elements of the alloy composition of the externally input processed material is directly expressed as Δ [Si], Δ [Cr], Δ [ The values are Mo], Δ [V], Δ [W]. These values are stored in the storage area 20 of the storage unit 15.

  Next, the standard cross-sectional nitrogen distribution of the standard material to be processed according to Fick's second law is corrected to the cross-sectional nitrogen distribution of the material to be processed (S12).

  Prior to this correction calculation, when the material to be treated is nitrided under a predetermined nitriding condition W1, an intrusion nitrogen amount F that enters from the treatment surface of the material to be treated to the inside from the nitriding atmosphere is obtained (S121). The nitriding treatment condition W1 includes a nitriding treatment method, a nitriding treatment atmosphere, temperature, pressure, treatment time, and the like.

  The intrusion nitrogen amount F is obtained from, for example, an actual measurement value of a standard material to be nitrided under the nitriding condition W1 or an actual measurement value such as a standard cross-sectional nitrogen distribution, and this is stored in the storage area 20 in advance. Can be used. That is, for the purpose of the present invention, the amount of nitrogen entering the steel per hour does not change with time unless the nitriding conditions are changed, and also does not change with respect to the alloy composition of the steel. In the case where the surface of the standard material to be processed has a Fe nitride layer with a thickness that cannot be ignored, the intrusion nitrogen amount F is the amount of nitrogen that penetrates the Fe nitride layer and enters the inside.

  By the way, the intrusion nitrogen amount F can be obtained by calculation without using the actual measurement value in the standard material to be processed as described above. That is, it can be obtained by calculation from an empirical formula obtained by, for example, performing regression calculation on the result of the actual measurement value of the intrusion nitrogen amount F for several nitriding conditions.

For example, as an example of such an empirical formula, Formula 1 is a radical nitridation process in which the temperature is 500 ° C., the pressure is 130 Pa, the discharge voltage is 420 V, and the gas composition ratio of ammonia and hydrogen gas is 1: 4 to 4: 1. 4 is a relational expression between the ammonia partial pressure P _NH3 obtained by changing the amount of nitrogen and the amount of intrusion nitrogen per hour dC / dt.

Furthermore, as another example of the above-described empirical formula, Formula 2 is a radical nitriding process in which the temperature is 500 ° C., the pressure is 130 Pa, and the gas composition ratio of ammonia and hydrogen gas is 1: 4 (ammonia partial pressure P _NH3 = 26 Pa). Is a relational expression between the discharge voltage V obtained by changing the discharge voltage from 420 V to 520 V and the intrusion nitrogen amount dC / dt per time.

Furthermore, as another example of the above empirical formula, Formula 3 is a radical nitridation treatment in which the pressure is 130 Pa, the discharge voltage is 420 V, and the gas composition ratio of ammonia and hydrogen gas is 2: 3 (ammonia partial pressure P _NH3 = 52 Pa). Is a relational expression of the temperature T obtained by changing the temperature from 450 ° C. to 550 ° C. and the amount of intrusion nitrogen per hour dC / dt.

  Nitrogen with an intrusion amount of nitrogen F enters the surface of the material to be treated from the nitriding atmosphere within a predetermined time, and this is used as a boundary condition while forming a nitride for nitrogen diffusing into the material to be treated. Sequential calculation modeling the state of diffusion is performed according to the program of the sequential calculation unit 18b to obtain the cross-sectional nitrogen distribution 2 (S122 to S124).

Consider the diffusion of nitrogen at time t = t ₁ as shown in FIG. Here, C on the vertical axis is the amount of nitrogen.

As shown in FIG. 5 (a), assuming that the intrusion nitrogen amount F is given from the treated surface X = 0, a part of the remaining nitrogen on which the alloy nitride is precipitated is solidified in a minute section in the depth direction position. A part of the molten metal tends to diffuse further deeper. In detail, in the minute section U _n−1 of the depth direction position X = x−Δx and X = x, there is the remaining nitrogen 41 on which alloy nitride is precipitated, and the depth direction from the minute section U _n−1. according to a second law of Fick towards small sections _{U n} position X = x and X = x + Δx, as shown by the arrow _{J 1,} the diffusion of nitrogen 41a is a portion of the nitrogen 41 diffuses (S122). That is, only the nitrogen 41b is dissolved in the minute section Un _-1 .

Figure 5 Referring also to (b), the small section U _n, a plurality of types of alloy nitride 32 portion of the diffusion nitrogen 41a reacts with each of the nitride forming elements a (M1, M2, M3 ...) Precipitate (S123). Diffusing nitrogen 42a is a portion of the remaining nitrogen 42, in accordance with the second law of Fick, as indicated by the arrow _{J 2,} further toward the small sections _{U n + 1} of the depth direction position X = x + [Delta] x and X = x + 2Δx diffusion (S124). That is, only nitrogen 42b is dissolved in small sections _{U n.}

Under the boundary condition that the intrusion nitrogen amount F is given from the processing surface X = 0, the sequential calculation (S122 to S124) according to the model of FIG. 5 is performed from the processing surface X = 0 to a predetermined depth position. Is performed until a cross-sectional nitrogen distribution 2 (see FIG. 6) at time t = t ₁ is obtained. Incidentally, when the amount of nitrogen 42b than the amount of nitrogen that can be dissolved in small sections U _n is large, the nitrogen 42b is dissolved to such solubility limit, the remainder is possible to perform sequential calculation in addition to the diffusion of nitrogen 42a preferable.

Meanwhile, in the sequential calculation (S122~S124), alloy nitride 32 is nitrogen indicated by the arrow J ₁ that diffuse into small sections U _n as described above is produced by the reaction with the respective nitride forming elements The amount of each nitride can be obtained by the following equation 4 using the reaction rate constant of the nitride-generating element.
Here, the density of n = 1, [rho _{MeN nitride,} the volume of nitrides _{V MeN,} the time t, the reaction rate constant K, and solid solution nitrogen content of α in iron _{C N,} a _{C Me} The amount of alloying element that does not form nitrides in α-iron.

In Equation 4, a known value may be used as the reaction rate constant K, but the following reaction rate constant K obtained experimentally may be used.

1Cr: K _Cr = 1.23 × 10 ⁷ exp (−18006 / T) [1 / mass% · sec] (Formula 5)
3Cr: K _Cr = 4.85 × 10 ¹⁰ exp (−23172 / T) [1 / mass% · sec] (Formula 6)
6Cr: K _Cr = 1.95 × 10 ¹¹ exp (−23537 / T) [1 / mass% · sec] (Formula 7)
_{Mo: K Mo = 3.19 × 10} 4 exp (-13213 / T) [1 / mass% · sec] (Equation 8)
_{V: K V = 1.85 × 10} 5 exp (-14144 / T) [1 / mass% · sec] (Equation 9)
W: K _W = 4.01 × 10 ⁷ exp (−13477 / T) [1 / mass% · sec] (Formula 10)

Here, since Cr largely depends on the alloy composition, it is preferable to appropriately use a reaction rate constant K depending on the alloy composition. For example, Formula 5 is used when the Cr content is about 1% by mass, Formula 6 is used when it is about 3% by mass, and Formula 7 is used when it is about 6% by mass. In general, the nitriding treatment is often performed at a temperature of 723 K or less, and at such a temperature, the Si reaction rate constant K is very small. For the purpose of the present invention, the precipitation of Si nitride is ignored. It is good to do. These equations 5 to 10 (if necessary, Si reaction rate constant K) are stored in the storage area 20 of the storage unit 15 of the prediction system 10 and are appropriately called and used.

  By using Expression 4 for the sequential calculation (S122 to S124) according to the model of FIG. 5, the nitride amount distribution 4 of the nitrides M1, M2, M3... For each nitride generating element as shown in FIG. Can be obtained (S13).

  In the subsequent hardness conversion step S2, the nitride amount distribution 4 obtained in the correction calculation step S1 is converted into a hardness distribution by a predetermined hardness conversion formula according to the program of the hardness conversion unit 19 of the storage unit 15.

For example, the hardness conversion formula is the following empirical formula obtained from regression calculation based on some measured hardness values, and is stored in advance in the storage area 20 of the storage unit 15. Note that weighting is given to the contribution of each nitride to the hardness. That is, [Si (x)], [Cr (x)], [Mo (x)], [V (x)], and [W (x)], which are mass% of each nitride-forming element, have respective coefficients. Is given.

[N (x)] = [Si (x)] + 3.8 [Cr (x)] + 2.5 [Mo (x)] + 10.7 [V (x)] + [W (x)] (Formula 11)

[Si (x)], [Cr (x)], [Mo (x)], [V (x)], [W (x)] are Δ [Si], Δ [ It is obtained from the values of Cr], Δ [Mo], Δ [V], Δ [W].

  As described above, in the prediction system 10, the nitride amount distribution 4 for the cross-sectional nitrogen distribution 2 based on the alloy composition of the material to be processed can be obtained, and the cross-sectional hardness distribution can be obtained. In other words, the cross-sectional hardness distribution can be predicted for each high alloy tool steel having a different alloy composition that gives different cross-sectional hardness distribution even under the same nitriding conditions, and the nitriding process can be performed more efficiently. . On the other hand, since the cross-sectional hardness distribution can be obtained from the nitriding treatment conditions, an example of the nitriding treatment conditions required to obtain a desired cross-sectional hardness distribution can be obtained.

  Next, a verification test comparing the cross-sectional hardness distribution obtained by the prediction system 10 and the actual measurement value will be described.

<Verification test 1>
FIG. 7 shows the measured value of the cross-sectional hardness distribution of the sample nitrided under the following conditions and the cross-sectional hardness distribution predicted by the prediction system 10 described above.
Nitriding method: Radical nitriding Temperature: 500 ℃
Pressure: 130Pa
Ammonia amount: 2 (l / min)
Hydrogen amount: 0.5 (l / min)
Discharge voltage: 420V
Composition: 0.38C-1Si-5.3Cr-1.2Mo-0.85V-bal.Fe
(Including other unintended impurity elements)
Heart hardness: 450Hv

Similarly, FIG. 8 shows the cross-sectional hardness distribution of a sample nitrided under the following conditions and the cross-sectional hardness distribution predicted by the prediction system 10 described above.
Nitriding method: Salt bath soft nitriding Temperature: 550 ℃
OCN concentration: 31wt%
Composition: 0.38C-1Si-5.3Cr-1.2Mo-0.85V-bal.Fe
(Including other unintended impurity elements)
Heart hardness: 500Hv
Two types were shown when nitriding was performed for 10 hours and 20 hours.

  Here, radical nitridation hardly produces a compound layer on the surface of nitriding treatment, but salt bath soft nitriding treatment produces a compound layer. However, as shown in FIGS. 7 and 8, it can be seen that the prediction by the prediction system 10 reflects the actual measurement values well.

<Verification test 2>
Next, a predicted value and an actual measurement value of hardness by the prediction system 10 when a 50 mm long square bar having a typical alloy composition as an alloy tool steel as shown in FIG. 9 is nitrided under the following conditions: Was measured.

  Specifically, a 30 kg steel ingot having an alloy composition shown in FIG. 9 obtained in a vacuum high-frequency melting furnace was forged into a square bar having a square cross-sectional shape with a side of 42 mm and annealed at 700 ° C. for 3 hours. This was cut into a square bar with a length of 50 mm having a square cross-sectional shape with a side of 10.5 mm and roughly processed. The square bar was kept at 1030 ° C. for 0.5 hour and then quenched in an oil bath, and further kept at 600 ° C. for 1 hour and tempered by air cooling. Furthermore, the same tempering was performed. From this square bar, a 50 mm long square bar having a square cross-sectional shape of 10 mm on a side was cut out as a sample and subjected to radical nitriding treatment.

Here, the nitriding treatment was performed by changing the bias voltage and the gas composition ratio. The furnace temperature is 500 ° C., the furnace pressure is 130 Pa, the bias voltage is 420 to 520 V, the flow ratio of NH ₃ gas to H ₂ gas is 1: 4 to 4: 1, and the mixing ratio is 0. After holding for 5 to 20 hours, the furnace was cooled.

  The test specimen after nitriding was cut out and polished at the central cross section, and the nitrogen concentration of this 10 mm square polished surface was measured by EPMA (Electron Probe Micro Analyzer) analysis. Furthermore, the nitriding surface was measured with a Vickers hardness tester. The hardness at a position of 0.02 mm was measured.

  FIG. 10 shows measured values of hardness with respect to the surface nitrogen concentration, and FIG. 11 shows measured values of precipitate-forming nitrogen concentration and hardness weighted to the surface nitrogen concentration by the above equation 11. As can be seen from FIG. 10, the plotted points in FIG. 11 have less variation than in FIG. 10, and the precipitate-forming nitrogen concentration weighted more than the surface nitrogen concentration reflects the actually measured hardness. .

  In this demonstration test, it was confirmed that the amount of nitrogen entering the steel per hour did not change with time unless the nitriding conditions were changed, and hardly changed with respect to the alloy composition of the steel.

<Application of prediction system>
According to the prediction system 10 described above, the cross-sectional hardness distribution can be predicted from the nitriding conditions and the alloy composition. That is, as shown in FIG. 1, the cross-sectional hardness distribution can be displayed by combining the prediction system 10 with the controlled device 22 and providing the controlled device 22 with a display unit (not shown). The operator of the controlled device 22 can know the cross-sectional hardness distribution prior to the nitriding process. In addition, since the prediction system 10 can obtain the cross-sectional hardness distribution from the nitriding treatment conditions, the prediction system 10 performs calculation by repeatedly changing the nitriding treatment conditions as appropriate so as to obtain the same cross-sectional hardness distribution as the desired cross-sectional hardness distribution. In this way, the nitriding conditions that can give the desired cross-sectional hardness distribution can be determined. As described above, according to the controlled device 22 combined with the prediction system 10, nitriding can be performed more efficiently.

DESCRIPTION OF SYMBOLS 1 Standard cross-section nitrogen distribution 2 Cross-section nitrogen distribution 4 Nitride amount distribution 10 Prediction system 18 Correction calculation part 19 Hardness conversion part 22 Controlled equipment

Claims (5)

A system for predicting the cross-sectional hardness distribution obtained by nitriding a workpiece made of high alloy tool steel in a nitriding atmosphere,
In response to an external input of the alloy composition of the material to be treated, Δ [Si] which is a mass% difference from the reference alloy composition of the standard material to be processed for at least the nitride-forming elements of Si, Cr, Mo, V, and W , Δ [Cr], Δ [Mo], Δ [V], Δ [W], and a standard cross-sectional nitrogen distribution that is a nitrogen distribution of a cross-section after nitriding the standard workpiece under predetermined nitriding conditions A correction calculation unit for calculating the nitride amount distribution by correcting the corrected cross-sectional nitrogen distribution,
A hardness converter for obtaining the cross-sectional hardness distribution from the nitride amount distribution for the corrected cross-sectional nitrogen distribution;
And predicting a cross-sectional hardness distribution when the material to be treated is nitrided under the predetermined nitriding conditions.   The prediction system according to claim 1, wherein the standard cross-sectional nitrogen distribution is an analytical solution of Fick's second law. The correction calculation unit is an intrusion nitrogen amount calculation unit that calculates an amount of nitrogen entering the processing material from the nitriding atmosphere on the processing surface of the processing material;
Diffusion is performed by reducing the amount of nitride-forming nitrogen for each of the nitride-forming elements that form nitrides from the amount of first diffusion nitrogen that diffuses sequentially from the surface to be processed in the depth direction. A sequential calculation unit for calculating a second diffusion nitrogen amount,
The amount of nitride forming nitrogen is as follows: ρ _MeN is the density of the nitride, V _MeN is the volume of the nitride, t is the time, K is the reaction rate constant, and C _Me is the amount of solid solution nitrogen in α iron, C _N Is the amount of alloying elements that do not form nitrides in α-iron,
The prediction system according to claim 2, wherein the prediction system is calculated by an expression represented by:   The intrusion nitrogen amount calculation unit calculates the nitrogen amount penetrating from the nitriding atmosphere into the material to be processed without being time-dependent, and calculates this from the nitrogen concentration in the vicinity of the surface layer of the standard cross-sectional nitrogen distribution. The prediction system according to claim 3, wherein: 5. A nitriding apparatus incorporating a prediction system according to claim 1 and comprising a display unit for displaying a cross-sectional hardness distribution obtained by the hardness conversion unit.