JP2004183099A - Production method of valve spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for setting the optimum nitrogen potential in nitriding treatment of a valve spring. <P>SOLUTION: After carrying out nitriding treatment in a nitrogen gas atmosphere within a first concentration range, another nitriding treatment is carried out in a nitrogen gas atmosphere within a second concentration range which is lower than the first. A high nitrogen gas concentration increases the nitrogen concentration at the surface of the valve spring and forms an Fe<SB>2</SB>-<SB>3</SB>N layer. After forming the Fe<SB>2</SB>-<SB>3</SB>N layer on the surface, the nitrogen gas concentration in the atmosphere is lowered, which inhibits the formation of a new Fe<SB>2</SB>-<SB>3</SB>N layer on the surface. Meanwhile, the existing Fe<SB>2</SB>-<SB>3</SB>N layer is dissolved, and the resulting nitrogen N diffuses into the interior and dissolves in ferrite to perform reinforcement intended inherently by nitriding. The 2-stage shifting (from high to low concentration) of the concentration enables deeper nitriding treatment within a shorter time. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a valve spring for an internal combustion engine of an automobile and a method of manufacturing the same, and particularly to a surface nitriding treatment.

  In order to increase the fatigue strength of the valve spring, it is effective to increase the surface hardness. Conventionally, nitriding has been performed as a surface treatment for the purpose of improving fatigue strength. Since the nitriding treatment requires a long time, shortening the nitriding time is a major issue in reducing the cost. As a method of shortening the nitriding time, there are a method of increasing the concentration of the processing gas (in many cases, an ammonia gas is used) during the nitriding treatment to increase the nitrogen potential, and a method of increasing the processing temperature.

When the nitrogen potential is increased, a compound layer of iron nitride (Fe ₂ N, Fe ₃ N, etc .; hereinafter, referred to as Fe _2-3 N) is formed on the surface. Since Fe _2-3 N is very hard, when it is used for a member that is subjected to repeated loads, such as a valve spring, cracks occur early and the entire valve spring breaks. Patent Literature 1 describes a surface white layer that is not spring steel but is generated in nitriding of a die steel.

  On the other hand, nitriding must be performed after heat treatment (quenching / tempering) for the entire valve spring, but when the nitriding temperature is increased, the hardness of the material adjusted by tempering decreases and the internal strength decreases. There's a problem. Patent Literature 2 describes a high-temperature nitriding treatment at 510 to 620 ° C. Therefore, nitriding of a valve spring is currently performed at a low temperature of 400 to 500 ° C.

JP-A-8-104973 JP-A-7-286256 Nomura, "Materials and Processes" (1996), 500 Yard Kogan, Metallurgy and Heat Treatment of Metals (1984), 10 (Я.Д.КОГАН: Металловедение и термическая обработка металло) Fujikoshi Surface Enhancement Study Group "Surface Enhancement to Know" (1988), 101

The present invention has been made in order to solve a problem related to a nitriding treatment in a manufacturing process of a valve spring, and an object of the present invention is to firstly form an Fe _2-3 N layer on a surface without forming an Fe _2-3 N layer. Moreover, it is an object of the present invention to provide a method capable of performing a deep nitriding treatment in as short a time as possible.

  A second object of the present invention is to provide a method for easily calculating the optimum nitriding conditions (temperature and time) according to the chemical composition of the steel material of the valve spring. By using this method, it is possible to perform a nitriding treatment while maximizing the effect of surface strengthening without lowering the internal hardness of the material, and to manufacture a valve spring with a long life. In the calculation, the target was to set the surface hardness to Hv700 or more, the internal hardness to Hv550 or more, and the nitriding depth to 30 μm.

  A method of manufacturing a valve spring according to a first aspect of the present invention, which has been made to solve the above-described problem, includes performing a nitriding treatment in a nitriding gas atmosphere having a first concentration range, and then performing a second treatment with a lower concentration than the second concentration. The nitriding treatment is performed in a nitriding gas atmosphere in a concentration range.

  Ammonia gas can be used as the nitriding gas. In this case, it is desirable that the first concentration range is 50 to 100% and the second concentration range is 0 to 50%.

  Such a change in the concentration range can be performed only once when the processing time needs to be shortened as much as possible, and can be repeated two or more times when it is desired to increase the nitriding depth.

Next, a method for manufacturing a valve spring according to a second invention is to provide a coil spring made of steel containing C: 0.50 to 1.00%, Si: 1.20 to 2.50%, and Mn: 1.0% or less by weight. After quenching and tempering, nitriding is performed at a temperature T (absolute temperature K) and time t (s) that satisfy all of the following expressions (1) to (3).
(1) When (-1.60Si + 2.14)> = 0, T> {700- (1205Si-867)} / (-1.60Si + 2.14)
If (-1.60Si + 2.14) <0, T <{700- (1205Si-867)} / (-1.60Si + 2.14)
(2) T <{550- (236Si + 735)} / (-0.23Si-0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-21.5)-(254C-96.4) * 1000 / R / T)} ²
Here, R is a gas constant (8.31 J / mol · K).

In the case of steel to which 0.5% or less of Ni and 0.40 to 1.50% of Cr are added, the following formula is used.
(1) When (-1.60Si + 0.20Cr + 2.14)> = 0, T> {700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
When (-1.60Si + 0.20Cr + 2.14) <0, T <{700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
(2) T <{550- (236Si + 735)} / (-0.23Si-0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-7.9Ni-21.5)-(254C-40.7Ni-96.4) * 1000 / R / T)} ²

Further, in the case of steel to which 0.5% or less of Mo and 0.60% or less of V are added, the following equation is used.
(1) When (-1.60Si + 0.20Cr + 2.14)> = 0, T> {700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
When (-1.60Si + 0.20Cr + 2.14) <0, T <{700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
(2) When (-0.23Si + 1.85Mo-0.42)> = 0, T> {550- (236Si-1054Mo + 735)} / (-0.23Si + 1.85Mo-0.42)
When (-0.23Si + 1.85Mo-0.42) <0, T <{550- (236Si-1054Mo + 735)} / (-0.23Si + 1.85Mo-0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-15.5V + 17.2Mo-7.9Ni-21.5)-(254C-105V + 127Mo-40.7Ni-96.4) * 1000 / R / T)} ²

First, the first invention will be described. As described above, when the concentration of the nitriding gas is high (the nitrogen potential is high), the nitrogen concentration on the valve spring surface increases, and an Fe _2-3 N layer is formed. Therefore, once the Fe _2-3 N layer is formed on the surface as described above, the nitrogen gas in the atmosphere is reduced in concentration. Then, the formation of a new Fe _2-3 N layer on the surface is suppressed, and the existing Fe _2-3 N layer dissolves and nitrogen N diffuses inward, forming a solid solution in ferrite. It performs the strengthening action by the original nitriding. Thus, by changing the concentration in two steps (from high concentration to low concentration), the nitriding treatment can be performed in a shorter time and deeper.

  This phenomenon was analyzed by simulation. In addition, as an example of the simulation of nitriding, Nomura performs a calculation in consideration of the precipitation of nitride (Non-Patent Document 1), but the simulation in consideration of the change in the nitrogen atmosphere as described above has not been performed yet. .

  Next, with regard to the second invention, calculation is performed by giving a different diffusion coefficient for each nitride phase when Kogan is pure iron (Non-Patent Document 2). However, there is no example in which a simulation is performed on the hardness distribution when the influence of the material components is taken into consideration.

  Therefore, using the similarity between the nitrogen concentration distribution and the hardness distribution in the diffusion layer, the hardness distribution is expressed by three parameters: (1) surface hardness, (2) internal hardness, and (3) nitriding depth. Then, the effect of each material component on each parameter was determined by multiple regression analysis from the measured values, and a prediction formula for hardness distribution was created.

  Hereinafter, the methods and results of these simulations will be described together.

(1) 1. Nitriding model
1.1 Nitriding The reaction equation for gas nitriding is expressed as equation (1).

(1)

(2)
ここで、Cは窒素濃度、Dは窒素の拡散係数である。 1.2 Diffusion equation The concentration change due to diffusion can be expressed by the diffusion equation of equation (2).

(2)
Here, C is the nitrogen concentration, and D is the diffusion coefficient of nitrogen.

1.3 Shift of phase boundary In steel, there are three nitride phases, α phase (solid solution of nitrogen), γ 'phase (Fe ₄ N), and ε phase (Fe _2-3 N), depending on the nitrogen concentration contained in the steel. . When the potential of the nitrogen gas is low, only the α phase needs to be considered. However, when the nitrogen potential increases, two or more different phases are generated, and a boundary surface is generated. (Fig. 1)

(3)
(3)式のξは相境界の位置を示し、添字の1,2は相境界を挟む2つの領域を示す。x>ξの領域を1、x<ξの領域を2として区別した。 The phase boundary moves according to the exchange of nitrogen atoms at the phase boundary, and the moving speed can be expressed as in equation (3).

(3)
In Equation (3), ξ indicates the position of the phase boundary, and suffixes 1 and 2 indicate two regions sandwiching the phase boundary. The region where x> ξ was identified as 1 and the region where x <ξ was identified as 2.

1.4 Diffusion Coefficient of Nitrogen in Pure Iron Generally, the diffusion coefficient is expressed by an equation of the form D = D ₀ · exp (-Q / RT) · f (C). Here, the frequency term D ₀ and the activation energy Q are coefficients determined by the material, and f (C) is a coefficient when there is concentration dependency. These values are experimental values as shown in FIG. 2 for pure iron. In this calculation, the calculation was performed using the diffusion coefficient of nitrogen in pure iron in FIG.

1.5 Solid solubility limit of each nitrided phase (Solute nitrogen concentration)
In the calculation, the solid solution limit value of nitrogen (upper limit value Max, lower limit value Min) of each nitride phase was the value in the case of pure iron (FIG. 3).

1.6 Surface nitrogen concentration At the start of nitriding, it was assumed that a thin nitrided layer having a nitrogen concentration equilibrium with the atmospheric gas already existed on the steel surface.

2. Calculation results and discussion
2.1 Two-step nitridation (540 ° C x 2 hours)
The gas composition of the NH ₃ + H ₂ mixed gas was changed in pattern 1 (a) and pattern 2 (b) of FIG. FIG. 5 shows the calculation results of the compound layer depth and the diffusion layer depth in that case.

  It is considered that the reason why there is no difference in the diffusion layer depth between the pattern 1 and the pattern 2 is that nitrogen is continuously supplied from the compound layer to the diffusion layer as long as the compound layer is present on the surface regardless of the gas atmosphere. That is, it is considered that the growth of the diffusion layer is determined only by the temperature and the time, and is not affected by the atmospheric gas.

On the other hand, there was a difference in the growth of the compound layer depending on the pattern. The pattern 2 had a compound layer depth about twice that of the pattern 1. Since the ratio of the compound layer depth of patterns 1 and 2 is almost the same as the time ratio of the gas composition NH ₃ 100%, the compound layer may grow in proportion to the time when the NH _{3 is} 100%. It is speculated that there is not.

3. Experimental verification
3.1 Relationship between Nitrogen Concentration Distribution and Hardness Distribution As far as the diffusion layer is concerned, the hardening of steel by nitriding is caused by nitrogen atoms penetrating the steel lattice and distorting the lattice. That is, the higher the nitrogen concentration, the harder the relationship. FIG. 6 shows the relationship between the nitrogen concentration and the hardness. EPMA was used for measuring the nitrogen concentration, and a micro-Vickers hardness tester was used for measuring the hardness. Judging from FIG. 6, it seems that there is a linear correlation between the nitrogen concentration and the hardness of the diffusion layer.

(4)
窒素濃度分布と硬さの相関は拡散層のみを考慮したものである。ここで、aは表面硬さ、bは内部硬さにそれぞれ対応し、[a-b]は表面硬さと内部硬さとの差を表す。 Assuming that there is a linear correlation between the nitrogen concentration distribution and the hardness distribution, the hardness distribution should be able to be approximated by an error function like the concentration distribution. Therefore, the hardness distribution is approximated by equation (4).

(Four)
The correlation between the nitrogen concentration distribution and the hardness considers only the diffusion layer. Here, a corresponds to the surface hardness, b corresponds to the internal hardness, and [ab] represents the difference between the surface hardness and the internal hardness.

  The definition of the nitriding depth here is a depth obtained by adding the depth of the compound layer and the depth of the diffusion layer. In the method of reading the nitriding depth from the graph by determining the criterion such as hardness and the like, an error is likely to be included. Therefore, from the viewpoint of removing the influence of the error, the depth is determined from the entire hardness distribution.

3.2 Verification of calculation (concentration two-step nitriding)
At the same temperature and the same concentration pattern as the nitriding conditions used in the simulation, nitriding was performed using a steel material added with vanadium equivalent to SAE9254, and the thickness of the compound layer and the thickness of the diffusion layer were measured.

  First, the hardness distribution of the sample cross section was measured with a micro Vickers hardness tester at a load of 100 g, and 2√ (Dt) when the measured hardness distribution was approximated by equation (5) was defined as the nitriding depth (Fig. 7). Next, the depth of the compound layer was measured from the surface texture photograph of FIG. Then, the depth of the compound layer was subtracted from the nitriding depth to obtain an actual measured value of the diffusion layer depth (FIG. 9).

  Similar to the simulation, the measured values did not show much difference in the diffusion layer depth, and the difference was about twice as large in the compound layer.

  There is a difference of about 5 times between the simulation result and the experimental result in both the compound depth and the diffusion layer depth. This is because the calculation was made using the diffusion coefficient of nitrogen in pure iron. In fact, it seems necessary to estimate the effect of the added element.

4. Summary of concentration two-step nitridation-As a result of comparison between the simulation and the experiment, similar tendency was observed in both the growth of the diffusion layer and the growth of the compound layer, and the effectiveness of the simulation was confirmed.
-By the two-step concentration nitriding, it is possible to suppress the growth of the compound layer while growing the diffusion layer.
-As long as there is a compound layer, the diffusion layer grows regardless of the gas composition of the atmosphere.
The compound layer grows in proportion to time, and the diffusion layer grows in proportion to 1/2 power of time.

5. Effect of spring material components (multiple regression analysis)
In the nitriding of springs, nitriding is often performed at 400 ° C to 500 ° C. Therefore, various spring materials are nitrided at a nitriding temperature of 400 ° C to 500 ° C and a gas composition of 100% ammonia, and parameters a and b characterizing the hardness distribution , 2√ (Dt) was obtained from the approximation by the nonlinear least squares method, and the influence of the material components was calculated by multiple regression analysis.

  The materials used for nitriding are 10 steel grades. FIG. 10 shows the component ranges of the materials.

(5)
k_i ^a：各材料成分の表面硬さに対する効果
C_i：各材料成分の濃度[重量%] 5.1 Surface Hardness The surface hardness can be expressed as Equation (5) by ignoring the second and higher order terms of the material component and approximating it by a linear expression of the material component.

(Five)
k _i ^a : Effect of each material component on surface hardness
C _i : Concentration of each material component [% by weight]

The result of performing multiple regression analysis ignoring components with low significance is shown in FIG.
Cr and Mo tend to increase the surface hardness.

(6)
k_i ^b：各材料成分の内部硬さに対する効果 5.2 Internal Hardness FIG. 12 shows the result of a similar multiple regression analysis for the internal hardness.
Si and Mo tend to prevent internal hardness from softening.

(6)
k _i ^b : Effect on internal hardness of each material component

(7)
の形で表される。拡散係数Dの対数が材料成分の1次式で近似
できるとした場合、拡散係数Dは材料成分の指数関数となるので、

(8)
図１３に示すように、Vは窒化深さを増加させる傾向がある。 5.3 Nitriding depth (diffusion coefficient)
The diffusion coefficient is

(7)
In the form of Assuming that the logarithm of the diffusion coefficient D can be approximated by a linear expression of the material component, the diffusion coefficient D is an exponential function of the material component.

(8)
As shown in FIG. 13, V tends to increase the nitriding depth.

To summarize the results of the multiple regression analysis described above, in order to satisfy the conditions that the nitriding depth of the valve spring is 30 μm or more, the surface hardness is Hv700 or more, and the internal hardness is Hv550 or more, the contained components and the nitriding temperature T (absolute The relationship between the temperature K) and the time t (second) must satisfy all the following three inequalities.
(-1.5985Si + 0.202Cr + 0.5238Mo + 2.1414) T +
(1204.9Si + 2.2Cr-867.4)> 700 (9)
(-0.2275Si + 1.8458Mo-0.4153) T +
(236.4Si-1053.6Mo + 734.7)> 550 (10)
2√ (t) exp ((40.4C-2.8Mn-15.5V + 17.2Mo-7.9Ni-21.5)-
(253.5C-105.3V + 127Mo-40.7Ni-96.4) * 1000 / R / T)> 0.03 (11)
By modifying these equations, the second invention is derived.

  FIG. 14 shows a comparison between the actually measured values of the parameters (surface hardness, internal hardness, nitriding depth) and the values calculated by the prediction formula. The measured and predicted values agree very well.

6. Conclusion The hardness distribution other than the compound layer on the surface can be approximated by an error function.
・ Prediction formula of hardness distribution was made by multiple regression analysis.
Cr and Mo tend to increase the surface hardness.
Si and Mo tend to prevent softening of the internal hardness.

FIG. 3 is a schematic diagram of a nitrogen concentration distribution after a nitriding treatment. 9 is a table of diffusion coefficient parameter values of each nitride phase in the case of pure iron. 3 is a table of solid solution limit values of each nitride phase in the case of pure iron. The figure of the pattern (a) of the density | concentration two-step nitriding process, and the pattern (b) of the simple nitriding process. Table of calculated values of compound layer depth and diffusion layer depth for both patterns. 4 is a graph showing the relationship between the nitrogen concentration in steel and hardness. 5 is a graph of a measured value of a hardness distribution of a cross section of a sample surface and an approximate curve thereof. Surface cross-sectional micrograph (× 900) of a sample subjected to nitriding treatment in Pattern 1 and Pattern 2. 7 is a table of measured values of the depth of the compound layer and the depth of the diffusion layer of the sample subjected to the nitriding treatment in Pattern 1 and Pattern 2. The table which shows the range of the spring material component used by the multiple regression analysis. 4 is a graph showing the effect of Si, Cr, and Mo on surface hardness. 4 is a graph showing the effect of Si and Mo on internal hardness. 9 is a graph showing the effect of C, Mn, Mo, and V on the diffusion coefficient. 7 is a graph showing a comparison between a predicted value and an actually measured value according to the obtained regression equation.

Claims (7)

  A method for manufacturing a valve spring, comprising performing a nitriding treatment in a nitriding gas atmosphere of a first concentration range, and thereafter performing a nitriding treatment in a nitriding gas atmosphere of a second concentration range having a lower concentration.   The method according to claim 1, wherein the nitriding gas is an ammonia gas.   3. The method according to claim 1, wherein the first concentration range is 50 to 100%, and the second concentration range is 0 to 50%. 4.   The method for manufacturing a valve spring according to any one of claims 1 to 3, wherein the change of the concentration range is repeated two or more times. After quenching and tempering a coil spring made of steel containing C: 0.50 to 1.00%, Si: 1.20 to 2.50%, Mn: 1.0% or less in weight ratio, all of the following formulas (1) to (3) A nitriding treatment at a temperature T (absolute temperature K) and time t (s) that satisfies the following.
(1) When (-1.60Si + 2.14)> = 0, T> {700- (1205Si-867)} / (-1.60Si + 2.14)
If (-1.60Si + 2.14) <0, T <{700- (1205Si-867)} / (-1.60Si + 2.14)
(2) T <{550- (236Si + 735)} / (-0.23Si-0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-21.5)-(254C-96.4) * 1000 / R / T)} ²
R: gas constant (8.31 J / molK) Hardening and tempering a coil spring made of steel containing C: 0.50 to 1.00%, Si: 1.20 to 2.50%, Mn: 1.0% or less, Ni: 0.5% or less, Cr: 0.40 to 1.50% by weight Thereafter, a nitriding treatment is performed at a temperature T (absolute temperature K) and a time t (s) satisfying all of the following expressions (1) to (3).
(1) When (-1.60Si + 0.20Cr + 2.14)> = 0, T> {700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
When (-1.60Si + 0.20Cr + 2.14) <0, T <{700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
(2) T <{550- (236Si + 735)} / (-0.23Si-0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-7.9Ni-21.5)-(254C-40.7Ni-96.4) * 1000 / R / T)} ² C: 0.50 to 1.00% by weight, Si: 1.20 to 2.50%, Mn: 1.0% or less, Ni: 0.5% or less, Cr: 0.40 to 1.50%, Mo: 0.5% or less, V: 0.60% or less After quenching and tempering a coil spring made of steel to be processed, a nitriding treatment is performed at a temperature T (absolute temperature K) and a time t (s) that satisfy all of the following equations (1) to (3). Manufacturing method of a valve spring.
(1) When (-1.60Si + 0.20Cr + 2.14)> = 0, T> {700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
When (-1.60Si + 0.20Cr + 2.14) <0, T <{700- (1205Si + 2.2Cr-867)} / (-1.60Si + 0.20Cr + 2.14)
(2) When (-0.23Si + 1.85Mo-0.42)> = 0, T> {550- (236Si-1054Mo + 735)} / (-0.23Si + 1.85Mo-0.42)
When (-0.23Si + 1.85Mo-0.42) <0, T <{550- (236Si-1054Mo + 735)} / (-0.23Si + 1.85Mo-0.42)
(3) t> {0.015 / exp ((40.4C-2.8Mn-15.5V + 17.2Mo-7.9Ni-21.5)-(254C-105V + 127Mo-40.7Ni-96.4) * 1000 / R / T)} ²

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