JP6287390B2 - Gas soft nitriding method of low alloy steel - Google Patents

Description

  The present invention relates to a gas soft nitriding method for low alloy steel.

Steel parts used in automobiles and various industrial machines, such as carburizing quenching, induction quenching, nitriding, soft nitriding, etc. to improve mechanical properties such as fatigue strength, wear resistance, and seizure resistance A surface hardening heat treatment is applied. In the nitriding treatment and soft nitriding treatment, heat treatment is performed in a ferrite region having a heating temperature of A ₁ or less and no phase transformation is used, so that heat treatment strain can be reduced. Therefore, nitriding treatment and soft nitriding treatment are often used for parts having high dimensional accuracy and large parts, and are applied to gears used for transmission parts of automobiles and cranks used for engines, for example. In particular, the soft nitriding treatment is effective in suppressing surface decarburization because C also penetrates into the steel as compared with the nitriding treatment.

The soft nitriding treatment includes gas soft nitriding treatment, salt bath soft nitriding treatment, and the like, and gas soft nitriding treatment having excellent productivity is mainly used for automobile parts and the like. By the gas soft nitriding treatment, a compound layer having a thickness of 10 μm or more containing carbonitrides such as Fe _2-3 (N, C) and Fe ₄ (N, C) is formed on the steel surface. The compound layer is extremely harder than steel, and improves wear resistance and surface fatigue strength at the initial stage of use.

  However, since the compound layer has a low deformability, peeling and cracking are likely to occur during use, and it is difficult to use the compound layer for a part to which an impact stress or a large bending stress is applied. Further, although the gas soft nitrocarburizing treatment has a small heat treatment strain, it may be necessary to correct long parts such as a shaft and a crank. Depending on the thickness of the compound layer, cracks may occur during correction, and the fatigue strength of the part may decrease.

Therefore, in the gas soft nitriding treatment, it is required to reduce the thickness of the compound layer and to eliminate the compound layer. By the way, it is known that the compound layer can be controlled by the nitriding potential (Kn) obtained by the following (Formula 1) from the treatment temperature of the soft nitriding treatment, the NH ₃ partial pressure and the H ₂ partial pressure.

Kn = (NH ₃ partial pressure) / [(H ₂ partial pressure) ^3/2 ] (Formula 1)

  If the nitriding potential Kn is lowered, the compound layer can be made thinner and further eliminated. However, simply lowering Kn makes it difficult for nitrogen to penetrate into the steel, and the hardness of the hardened layer called nitrogen diffusion layer is low and the depth is shallow, resulting in fatigue strength, wear resistance, seizure resistance, etc. Does not improve. Further, there is a method of removing the compound layer by mechanical polishing or shot blasting after the gas soft nitriding treatment, but the cost becomes high.

For such a problem, the atmosphere of the gas soft nitriding treatment is controlled by a nitriding parameter Kn ′ = (NH ₃ partial pressure) / [(H ₂ partial pressure) ^1/2 ] different from the above nitriding potential, A method of making the hardened layer depth uniform has been proposed (see, for example, Patent Document 1). Further, in nitriding treatment, a method of using a jig whose surface is made of a non-nitriding material when placing a nitriding product in a processing furnace has been proposed (see, for example, Patent Document 2). .

  When the nitriding parameter proposed by Patent Document 1 is used, the compound layer generated on the outermost surface can be suppressed in a short time. However, the hardened layer depth may not be sufficiently secured depending on the required characteristics. Further, as proposed in Patent Document 2, when a non-nitriding jig is prepared and fluorination treatment is performed, new problems such as selection of the jig and an increase in work man-hours arise.

JP 2006-28588 A JP 2007-31759 A

An object of the present invention is to provide a gas soft nitriding method for low alloy steel that can secure a certain hardened layer depth and suppress the formation of a compound layer having low toughness. Specifically, the average thickness of the compound layer on the steel surface after gas soft nitriding performed in a gas atmosphere containing NH ₃ , H ₂ and N ₂ should be 5 μm or less, and the hardened layer depth should be secured. The present invention provides a method for gas soft nitriding of low alloy steel that can be used.

  The present inventors have studied in order to obtain a deep hardened layer by thinning the compound layer formed on the surface of the low alloy steel by gas soft nitriding. Although the gas soft nitriding atmosphere contains carburizing gas, it was found that the nitriding potential has a great influence on the formation of the compound layer. Therefore, in the first half of the gas soft nitriding treatment, a nitriding potential (hereinafter also referred to as “Kn value”) is increased to form a compound layer, and in the latter half, the Kn value is lowered to decompose the compound layer, and nitrogen penetrates into the steel. The method of spreading was directed and studied repeatedly. Furthermore, when the Kn value was increased, studies were also conducted so that a layer (hereinafter also referred to as a porous layer) in which voids were generated by gasification of nitrogen was not formed in the vicinity of the surface of the low alloy steel. As a result, the following findings (a) to (c) were obtained.

(A) Kn value in gas soft nitriding treatment Generally, the Kn value is the NH ₃ partial pressure of the atmosphere in the furnace in which the gas soft nitriding treatment is carried out (soft nitriding treatment atmosphere or simply the atmosphere), H _{Using 2} partial pressures, it is defined by (Equation 1) below.
Kn = (NH ₃ partial pressure) / [(H ₂ partial pressure) ^3/2 ] (Formula 1)
The Kn value can be controlled by the gas flow rate, but it takes a certain amount of time after the flow rate is set until the Kn value of the gas soft nitriding atmosphere reaches the equilibrium state. Is changing from moment to moment. Further, when the Kn value is changed during the gas soft nitriding treatment, the Kn value fluctuates until the equilibrium state is reached. Since the fluctuation of the Kn value affects the compound layer and the depth of the hardened layer, the present inventors consider not only the average value of the Kn value but also the range of variation, and the NH ₃ during gas soft nitriding treatment is important. It was found that it is necessary to measure the partial pressure and H ₂ partial pressure, calculate the Kn value, and control with high accuracy.

(B) About coexistence of suppression of formation of compound layer and ensuring of cured layer depth In order to form a cured layer, it is more efficient to use the compound layer as a nitrogen supply source. That is, in order to suppress the formation of the compound layer and ensure the depth of the hardened layer, the compound layer is formed in the first half of the gas soft nitriding treatment, the compound layer is decomposed in the second half, and at the end of the gas soft nitriding treatment, It is preferable to control the Kn value so that the compound layer is almost eliminated.

(C) Suppression of porous layer formation When the compound layer is generated in the first half of the gas soft nitriding treatment, when the porous layer is formed, the carbonitride is decomposed and the carbon-nitrogen diffusion layer (cured layer) is formed. However, the voids remain as they are, and the fatigue strength and the bending straightness (the presence or absence of cracks in the hardened layer by bending straightening) are reduced. Therefore, when the compound layer is generated in the first half of the gas soft nitriding treatment, it is necessary to limit the upper limit of the Kn value so that the porous layer is not generated as much as possible.

  This invention is made | formed based on such knowledge, The summary is as follows.

(1) When performing a gas soft nitriding treatment in which the low alloy steel is heated to 550 to 620 ° C. and the entire treatment time A is 1.5 to 10 hours,
The nitriding potential Kn _X obtained by the following (Equation 1) is in the range of _0.10 to 1.00, the average value Kn _Xave of the nitriding potential Kn _X is _0.20 to 0.55, and the processing time is After high Kn value processing with X time,
The nitriding potential Kn _Y obtained by the following (Equation 1) is in the range of 0.01 to 0.20, the average value Kn _Yave of the nitriding potential Kn _Y is 0.02 to 0.15, and the processing time is Perform low Kn value processing for Y time,
The average value Kn _Xave of the nitriding potential of the high Kn value treatment,
The average value Kn _Yave of the nitriding potential of the low Kn value treatment,
The processing time A;
A processing time X of the high Kn value processing;
Wherein the processing time Y low Kn value processing, the following average value Kn _ave nitriding potential soft-nitriding obtained by (Equation 2) is a low alloy steel, which is a 0.05 to 0.20 Gas soft nitriding method.
Kn = (NH ₃ partial pressure) / [(H ₂ partial pressure) ^3/2 ] (Formula 1)
_{Kn ave = (X × Kn Xave} + Y × Kn Yave) / A ··· ( Equation 2)

  According to the gas soft nitriding method of the present invention, it is possible to thin the compound layer formed on the surface of the low alloy steel, suppress the formation of the porous layer, and obtain a deep hardened layer. And according to the present invention, industrial contributions such as improvement of mechanical properties such as fatigue strength, wear resistance, and seizure resistance of low alloy steel parts, and improvement of bend straightening can be achieved. Extremely prominent.

It is a figure explaining the relationship between surface hardness and a compound layer thickness with respect to the average value of the nitriding potential of a high Kn value process. It is a figure explaining the relationship between surface hardness and a compound layer thickness with respect to the average value of the nitriding potential of a low Kn value process. It is a figure explaining the relationship between surface hardness and a compound layer thickness with respect to an average nitriding potential.

The present inventors performed gas soft nitriding treatment in an atmosphere composed of NH ₃ , H ₂ , N ₂ , CO ₂ , CO, and H ₂ O using SCr420, which is an alloy steel material for mechanical structure of JIS G 4053. It was. Gas nitrocarburizing treatment is to control heat treatable furnace atmosphere was heated to a predetermined temperature, NH _3, N _2, performed by introducing the gases H ₂ and CO _2, NH ₃ and nitrocarburizing treatment atmosphere While measuring the partial pressure of H _2, the gas flow rate was adjusted to control the nitriding potential Kn value. Note that CO and H ₂ O contained in the atmosphere are generated by a chemical reaction between CO ₂ and H ₂ . Kn values were determined from the following equation (1) by NH ₃ partial pressure and the H ₂ partial pressure.
Kn = (NH ₃ partial pressure) / [(H ₂ partial pressure) ^3/2 ] (Formula 1)

The H ₂ partial pressure was measured by converting the difference in thermal conductivity between the standard gas and the measurement gas into a gas concentration using a heat conduction type H ₂ sensor directly attached to the nitrocarburizing furnace body. The H ₂ partial pressure was continuously measured during the gas soft nitriding process. The NH ₃ partial pressure was measured by attaching a manual glass tube NH ₃ analyzer outside the furnace and calculating the partial pressure of residual NH ₃ every 15 minutes. The nitriding potential Kn value was calculated every 15 minutes when the NH ₃ partial pressure was measured, and the NH ₃ flow rate and the N ₂ flow rate were adjusted so as to converge to the target value. The CO ₂ addition amount, and the display unit the CO ₂ addition rate [%] with respect to the total gas input amount, so that CO ₂ loading in the gas nitrocarburizing treatment is constant, to adjust the flow of the gas CO ₂ .

After the gas soft nitriding treatment, the cross section of the sample was polished, etched and observed with an optical microscope. Etching was performed with a 3% nital solution for 20-30 seconds. The compound layer exists in the surface layer of the low alloy steel and is observed as a white uncorroded layer. Measure the thickness of 4 compound layers every 30 μm from 5 visual fields (field area: 2.2 × 10 ⁴ μm ² ) taken at 500 times with an optical microscope, and take the average of 20 points. Was defined as the compound thickness. Since peeling and cracking are greatly suppressed when the compound layer thickness is 5 μm or less, in the present invention, the compound layer thickness is set to 5 μm or less.

Furthermore, the voids were evaluated by observation with an optical microscope. Five visual fields were measured at a magnification of 1000 (visual field area: 5.6 × 10 ³ μm ² ), and the ratio of voids in an area of 25 μm ² in the range of 5 μm depth from the outermost surface was calculated for each visual field. When the voids are 10% or more, the surface roughness becomes rough, the compound layer becomes brittle, and the fatigue strength is lowered. Therefore, the object of the present invention is to make the voids less than 10%.

  Next, the Vickers hardness in the depth direction from the surface of the sample was measured with a test force of 1.96 N in accordance with JIS Z 2244. Moreover, the average value of 3 points | pieces of the Vickers hardness in a 50 micrometer position from the surface was defined as surface hardness. In the case of a general gas soft nitriding process in which a compound layer exceeding 5 μm remains, the surface hardness is 270 to 310 HV for S45C and 550 to 590 HV for SCr420. Therefore, the surface hardness in the present invention was targeted to be 290 HV or higher for S45C and 570 or higher for SCr420. Further, the effective hardened layer depth is defined as the depth from the surface where the Vickers hardness satisfies 250 HV for S45C and 300 HV for SCr420.

In the case of a general gas soft nitriding treatment in which a compound layer is generated at 10 μm or more at a treatment temperature of 570 to 590 ° C., the effective hardened layer depth is a value ± 20 μm determined by the formula (A).
Effective hardened layer depth (μm) = 130 × {treatment time (hour)} ^1/2 Formula (A)
Therefore, the effective hardened layer depth in the present invention is targeted for the formula (B).
Effective hardened layer depth (μm) ≧ 130 × {treatment time (hour)} ^1/2 Formula (B)

  In the gas soft nitriding treatment, the nitriding potential Kn value in the first half was increased to form a compound layer, and then the nitriding potential Kn value in the second half was lowered to decompose the compound layer, and nitrogen and carbon were permeated and diffused into the steel. . Gas soft nitriding with a relatively high nitriding potential Kn value is referred to as high Kn value processing, and gas soft nitriding with a relatively low nitriding potential Kn value is referred to as low Kn value processing.

When the gas flow rate is adjusted to reduce the Kn value, a certain amount of time is required until the partial pressure of NH ₃ and H ₂ in the furnace is stabilized. Adjustment of the gas flow rate for changing the Kn value may be performed once, or may be performed twice or more as necessary. After the high Kn value processing and before the low Kn value processing, the Kn value can be once lowered and then increased. The boundary between the high Kn value process and the low Kn value process was the point where Kn last fell below 0.15.

The total of the processing time X of high Kn value processing (hereinafter may be referred to as processing time X) and the processing time Y of low Kn value processing (hereinafter may be referred to as processing time Y) is preferably The overall processing time A of the gas soft nitriding process. The nitriding potential for high Kn value processing is referred to as Kn _X , the average value thereof is referred to as Kn _Xave , the nitriding potential for low Kn value processing is referred to as Kn _Y , and the average value is referred to as Kn _Yave .

First, the present inventors examined the average values Kn _Xave and Kn _Yave of nitriding potential using _SCr420 as a test material. In the gas soft nitriding treatment, the temperature of the atmosphere is 590 ° C., the CO ₂ addition amount is 5%, the treatment time X is 1.0 hour, the treatment time Y is 2.0 hours, Kn _Yave is 0.03, and Kn _Xave is The change was made from 0.10 to 0.70. The total processing time A is 3.0 hours. It was found that by setting Kn _Xave to 0.15 or more, the effective hardened layer depth becomes a target of ₂₂₅ μm or more.

FIG. 1 shows the relationship between Kn _Xave , surface hardness, and compound layer thickness. When Kn _Xave is less than 0.20, the surface hardness does not reach the target of 570 HV. When Kn _Xave exceeds 0.55, the compound layer thickness becomes larger than 5 μm. Therefore, the average value Kn _Xave of the nitriding potential in the high Kn value processing needs to be in the range of _0.20 to 0.55. Furthermore, the lower limit of Kn _Xave is preferably 0.25 or more and the upper limit is preferably 0.50 or less.

Next, the temperature of the soft nitriding atmosphere is 590 ° C., the CO ₂ addition amount is 5%, the processing time X is 1.0 hour, the processing time Y is 2.0 hours, Kn _Xave is 0.30, and Kn _Yave is Gas soft nitriding was performed by changing the pressure from 0.01 to 0.20. The total processing time A is 3.0 hours. It was found that by setting Kn _Yave to 0.02 or more, the effective hardened layer depth becomes 225 μm or more as a target.

FIG. 2 shows the relationship between Kn _Yave , surface hardness, and compound layer thickness. When Kn _Yave is less than 0.02, the surface hardness does not reach 570 HV. On the other hand, when Kn _Yave exceeds 0.15, the compound layer thickness becomes larger than 5 μm. Therefore, the average value Kn _Yave of the nitriding potential in the low Kn value processing needs to be in the range of 0.02 to 0.15. Furthermore, the lower limit of Kn _Yave is preferably 0.02 or more, and the upper limit is preferably 0.13 or less.

Next, the present inventors examined the range of the nitriding potentials Kn _X and Kn _Y. In the gas soft nitriding treatment, the atmosphere temperature is 590 ° C., the CO ₂ addition amount is 5%, the treatment time X is 1.0 hour, the treatment time Y is 2.0 hours, Kn _Xave is 0.30, and Kn _Yave is 0. As shown in Table 1, the minimum values Kn _Xmin and Kn _Ymin and the maximum values Kn _Xmax and Kn _Ymax of Kn _X and Kn _Y were changed as shown in Table 1. The total processing time A is 3.0 hours. Table 1 shows the compound layer thickness, voids, effective hardened layer depth and surface hardness.

As shown in Table 1, when Kn _Xmin is less than 0.09, the compound layer is not sufficiently formed during X hours, and the effective cured layer depth after the treatment does not reach the target of 225 μm. If Kn _Xmin is less than 0.10, the surface hardness does not reach 570 HV. On the other hand, when Kn _Xmax exceeded 1.10, the compound layer thickness was larger than 5 μm, and when Kn _Xmax exceeded 1.00, the voids became 10% or more. Accordingly, the range of the nitriding potential Kn _X for the high Kn value processing needs to be in the range of 0.10 to 1.00. Furthermore, the lower limit of Kn _X is preferably 0.20 or more and the upper limit is preferably 0.80 or less.

Moreover, as shown in Table 1, when Kn _Ymin is less than 0.01, not only the compound layer disappears by the low Kn value treatment, but also denitrification and decarburization occur from the surface layer, and the surface hardness. Does not reach 570HV. On the other hand, if Kn _Ymax exceeds 0.20, the compound layer does not sufficiently decompose, and the thickness of the compound layer exceeds 5 μm. Therefore, the range of the nitriding potential Kn _Y for the low Kn value processing needs to be in the range of 0.01 to 0.20. Further, the lower limit of Kn _Y is preferably 0.02 or more and the upper limit is preferably 0.18 or less.

Next, we examined the nitride potential Kn _ave gas nitrocarburizing process. Gas soft nitriding treatment was performed by changing the _{temperature of the nitrocarburizing} atmosphere to 590 ° C. and changing the treatment time X, the treatment time Y, the range of the nitriding potential and the average value (Kn _X , Kn _Y, Kn _Xave , Kn _Yave ). . Then, the average value Kn _ave nitride potential by the following (Equation 2), the compound layer thickness was investigated the relationship between the effective case depth and surface hardness.
_{Kn ave = (X × Kn Xave} + Y × Kn Yave) / A ··· ( Equation 2)
If Kn _ave is 0.04 or more, effective case depth was found to reach the target, which is calculated by the following formula (A).
Effective hardened layer depth (μm) = 130 × {treatment time (hour)} ^1/2 Formula (A)

FIG. 3 shows the relationship between _Knave , surface hardness, and compound layer thickness. If Kn _ave is less than 0.05, the surface hardness does not reach 570HV. On the other hand, if Kn _ave exceeds 0.20, compound layer is greater than 5 [mu] m. Therefore, Kn _ave should be in the range of 0.05 to 0.20. Further, the lower limit of Kn _ave is 0.06 or more, and the upper limit is preferably 0.18 or less.

Hereinafter, the present invention will be described in detail.
The metal material which is the object of the gas soft nitriding method of the present invention is low alloy steel. The low alloy steel in the present invention is defined as steel in which Fe accounts for 93% or more, preferably 95% or more. For example, it is suitable for carbon steel materials for machine structures specified in JIS G 4051, structural steel materials guaranteed hardenability specified in JIS G 4052, alloy steel materials for machine structures specified in JIS G 4053, etc. Can be applied. The content of the alloy element may deviate from the provisions of these standards, and further, Ti, V, Al, Nb, etc. effective for improving the hardness of the surface layer portion by gas soft nitriding, and other elements Can also be contained as appropriate.

(A) Gas soft nitriding temperature The temperature of soft nitriding (soft nitriding temperature) is mainly correlated with the diffusion rate of nitrogen and affects the depth of the hardened layer. If the treatment temperature is too low, the diffusion rate of nitrogen is slow and it becomes difficult to increase the depth of the hardened layer. Further, when the gas nitrocarburizing treatment at a temperature exceeding _C1 point A, that the ferrite phase (alpha phase) the diffusion rate of nitrogen than smaller austenite phase (gamma-phase) is generated, to deepen the hardening depth It becomes difficult. In the present invention, the gas soft nitriding treatment temperature is set to 550 to 620 ° C.

(B) Processing time A for the entire soft nitriding process
The time from the start to the end of the gas soft nitriding treatment performed in a gas atmosphere containing NH ₃ , H ₂ , N ₂ , CO ₂ , CO and H ₂ O (soft nitriding treatment time) is the formation and decomposition of the compound layer and nitrogen. This affects the depth of the hardened layer. If the treatment time is too short, the depth of the hardened layer cannot be increased. If it is too long, the surface hardness may decrease due to denitrification and decarburization. In addition, an increase in processing time causes an increase in manufacturing cost. Therefore, the processing time (overall processing time A) is 1.5 to 10 hours. Note that the atmosphere for performing the gas soft nitriding treatment may inevitably contain impurities such as oxygen, and the total amount of NH ₃ , H ₂ , N ₂ , CO ₂ , CO, and H ₂ O is 99.5% or more ( % By volume) is preferable.

(C) Carbon supply method For the carbon supply source, carbon monoxide, carbon dioxide or hydrocarbons are used. Although not particularly limited in the present invention, it is preferred to add by CO _2. Further, the addition amount in the case of adding by CO ₂ is not particularly limited, but if the CO ₂ addition amount is too large, carbides are formed on the steel surface and nitrogen penetration into the steel is hindered. Therefore, the CO ₂ addition amount is preferably 8% or less. More preferably, it is 5% or less.

(D) High Kn Value Treatment The high Kn value treatment is a step of forming a compound layer containing carbonitride on the surface of the low alloy steel, and the range of the nitriding potential Kn _X is 0.10 to 1.00, the average value Kn _Xave is set to _0.20 to 0.55. Further, the lower limit of Kn _X is preferably 0.20 or more, the upper limit is 0.80 or less, the lower limit of Kn _Xave is preferably 0.25 or more, and the upper limit is preferably 0.50 or less. The treatment time X is preferably 0.50 hours or longer. If the minimum value Kn _Xmin of the nitriding potential is less than 0.10 or the average value Kn _Xave is less than 0.20, the formation of the compound layer is insufficient, the depth of the cured layer cannot be obtained, and the surface hardness Decreases. On the other hand, when the maximum value Kn _Xmax of the nitriding potential exceeds 1.00 or the average value Kn _Xave exceeds 0.55, a thick compound layer is formed. The porous layer may be larger than 5 μm and contain 10% or more voids.

(E) Low Kn Value Treatment The low Kn value treatment is a step of decomposing the compound layer formed on the surface of the low alloy steel. The range of the nitriding potential Kn _Y is 0.01 to 0.20, the average value Kn _Yave. Is set to 0.02 to 0.15. Further, the lower limit of Kn _Y is preferably 0.01 or more, the upper limit is 0.18 or less, the lower limit of Kn _Yave is 0.02 or more, and the upper limit is preferably 0.13 or less. The treatment time Y is preferably 0.50 hours or longer. When the minimum value Kn _Ymin of the nitriding potential is less than 0.01 or the average value Kn _Yave is less than 0.02, denitrification occurs from the surface layer and the surface hardness decreases. On the other hand, when the maximum value Kn _Ymax of the nitriding potential exceeds 0.20 or the average value Kn _Yave exceeds 0.15, decomposition of the compound layer becomes insufficient, and the depth of the hardened layer and the surface hardness decrease. .

(F) mean value Kn _ave of nitriding potential of the gas nitrocarburizing treatment
The gas soft nitriding treatment of the present invention is a step of heating a low alloy steel in a gas atmosphere containing NH ₃ , H ₂ , N ₂ , CO ₂ , CO and H ₂ O to perform gas soft nitriding treatment, and has a high Kn value. After the processing, low Kn value processing is performed. After the low Kn value process, the process is terminated without increasing the nitriding potential. The average value Kn _ave of the nitriding potential of the gas soft nitriding is obtained by the following (formula 2) from the processing time X, the processing time Y, the range of the nitriding potential and the average value (Kn _X , Kn _Y Kn _Xave , Kn _Yave ). .
_{Kn ave = (X × Kn Xave} + Y × Kn Yave) / A ··· ( Equation 2)

Mean value Kn _ave of nitriding potential of the gas nitrocarburizing treatment is performed at a 0.05 to 0.20. Further, the lower limit of Kn _ave is 0.06 or more, and the upper limit is preferably 0.18 or less. The Kn _ave is less than 0.05, case depth can not be obtained, surface hardness is lowered. On the other hand, if Kn _ave exceeds 0.20, compound layer is thicker than 5 [mu] m.

  JIS standard SCr420 (JIS G 4053 alloy steel for machine structure) and S45C (JIS G 4051 carbon steel for machine structure) were melted in a 50kg vacuum melting furnace, then cast into an ingot and hot forged. Thus, a steel bar having a diameter of 20 mm was manufactured. For SCr420, in order to homogenize the structure, after heating to 920 ° C. and holding for 30 minutes, air cooling (normalizing), further heating to 900 ° C., holding for 30 minutes and water cooling (quenching), Then, it hold | maintained at 600 degreeC for 1 hour (tempering process). About S45C, after heating to 870 degreeC and hold | maintaining for 30 minutes, it air-cooled.

A test piece of 15 mm × 80 mm × 5 mm was collected from the obtained steel bar by machining, and each gas of NH ₃ , H ₂ , N ₂ , CO ₂ was introduced into the furnace of the gas nitrocarburizing furnace. After performing the soft nitriding treatment under the conditions shown in No. 2, oil cooling was performed using 80 ° C. oil. After the gas soft nitriding treatment, the cross section in the direction perpendicular to the length direction of the test piece was mirror-polished, etched, and the thickness of the compound layer and the void were measured with an optical microscope. Etching was performed with a 3% nital solution for 20-30 seconds.

The compound layer is observed as a white, uncorroded layer existing on the surface layer from 5 visual fields (field area: 2.2 × 10 ⁴ μm ² ) taken at 500 times, and 4 compound layers for each 30 μm. The average value of 20 points was determined as the compound thickness. Furthermore, five fields of view were observed at 1000 times, and the ratio of voids in an area of 25 μm ² in a range of 5 μm depth from the outermost surface was calculated.

  Further, in accordance with JIS Z 2244, the Vickers hardness was measured at a test force of 1.96 N from the surface to 50 μm, 100 μm, and thereafter to a depth of 1000 μm every 50 μm. Vickers hardness (HV) was measured at three points each, and the average value was obtained. The surface hardness is an average value of three points at a position of 50 μm from the surface. The effective hardened layer depth is the depth from the surface of 300 HV for SCr420 and 250 HV for S45C, and was obtained by plotting the HV hardness against the distance from the surface.

The thickness of the compound layer is 5 μm or less, the void ratio is less than 10%, the surface hardness is 290 HV or more for S45C, 570 HV or more for SCr420, the effective hardened layer depth is Judged.
Effective hardened layer depth (μm) ≧ 130 × {treatment time (hour)} ^1/2 Formula (B)

  The results are shown in Table 2. From Table 2, it is clear that the test numbers 21 to 23 and the test numbers 26 to 28 that satisfy all the conditions defined in the present invention have good characteristics. On the other hand, test number 24, test number 25, test number 29, and test number 30 that deviate from the conditions defined in the present invention are comparative examples. In test number 24, the maximum value of the nitriding potential of the high Kn value treatment is too high, and voids are generated. In Test No. 25, the minimum value and average value of the nitriding potential of the high Kn value treatment are low, the effective hardened layer depth is shallow, and the surface hardness is lowered. In test number 29, the average value of the nitriding potential of the low Kn value treatment is high, and a thick compound layer remains. In Test No. 30, the minimum value and average value of the nitriding potential of the low Kn value treatment are low, and the surface hardness is lowered.

Claims (1)

When carrying out gas soft nitriding treatment in which the low alloy steel is heated to 550 to 620 ° C. and the total treatment time A is 1.5 to 10 hours,
The nitriding potential Kn _X obtained by the following (Equation 1) is in the range of _0.10 to 1.00, the average value Kn _Xave of the nitriding potential Kn _X is _0.20 to 0.55, and the processing time is After high Kn value processing with X time,
The nitriding potential Kn _Y obtained by the following (Equation 1) is in the range of 0.01 to 0.20, the average value Kn _Yave of the nitriding potential Kn _Y is 0.02 to 0.15, and the processing time is Perform low Kn value processing for Y time,
The average value Kn _Xave of the nitriding potential of the high Kn value treatment,
The average value Kn _Yave of the nitriding potential of the low Kn value treatment,
The processing time A;
A processing time X of the high Kn value processing;
Wherein the processing time Y low Kn value processing, the following average value Kn _ave nitriding potential soft-nitriding obtained by (Equation 2) is a low alloy steel, which is a 0.05 to 0.20 Gas soft nitriding method.
Kn = (NH ₃ partial pressure) / [(H ₂ partial pressure) ^3/2 ] (Formula 1)
_{Kn ave = (X × Kn Xave} + Y × Kn Yave) / A ··· ( Equation 2)

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