JP2008045975A - Carbon concentration distribution measuring method, and manufacturing method of carburized member using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a carbon concentration distribution measuring method capable of determining simply a carbon concentration distribution in a steel after carburization by nondestructive inspection, and a manufacturing method of a carburized member using it. <P>SOLUTION: In the measuring method, a regression curve showing in a dimensionless way a relation between the depth from the surface of the steel after carburization and the carbon concentration at the depth is assumed, and a carbon concentration axial direction of the regression curve is multiplied by (Q-P) and P is added thereto, and the depth axial direction of the regression curve is multiplied by (B-A)/äρ×S×(Q-P)}, to thereby determine the carbon concentration distribution in the steel after carburization. In this case, Q is a surface carbon concentration (wt.%) in the steel after carburization; P is a surface carbon concentration (wt.%) in the steel before carburization; B is a steel weight (g) after carburization; A is a steel weight (g) before carburization; ρ is a density (g/m<SP>3</SP>) of the steel; and S is a surface area (m<SP>2</SP>) of the steel. The manufacturing method has an inspection process for determining the depth from the surface at which a constant carbon concentration is acquired by using the measuring method, and performing nondestructive inspection of the carburized member. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a carbon concentration distribution measuring method and a carburized member manufacturing method using the same.

  Conventionally, carburizing treatment of steel has been widely performed in order to increase the strength of steel. In such carburized steel, it is important to manage the carbon concentration distribution in the carburizing process in order to keep the quality constant. Therefore, the carbon concentration distribution from the steel surface after carburizing in the depth direction is obtained.

  For example, Non-Patent Document 1 discloses a method of measuring the carbon concentration distribution of carburized steel by cutting out a cross section of carburized steel and measuring the carbon concentration at each depth using EPMA.

  Further, Non-Patent Document 2 discloses a method for measuring the carbon concentration distribution of carburized steel by cutting the surface of carburized steel, collecting chips each having a certain depth, and chemically analyzing the chips. Yes.

Makio Kato, Takashi Kano, “Effects of Carburized Layer Hardness and Carbide on Residual Stress Distribution after Shot Peening of Highly Concentrated Carburized Steel”, Electric Steelmaking, Electric Steel Research Group, February 2006, Vol. 77, No. 1, p. 67-77 Susumu Kanbara and 4 others, “Estimation of carbon concentration distribution in carburized steel”, Heat Treatment, Japan Heat Treatment Technology Association, December 1983, Vol. 23, No. 6, p. 337-341

  However, both of the above-described methods require a lot of time and man-hours for sample preparation, and cannot be inspected without destroying the steel material. Therefore, there is a problem that the carbon concentration distribution of the steel after carburization cannot be easily obtained. there were.

  The problem to be solved by the present invention is to provide a carbon concentration distribution measuring method capable of easily obtaining the carbon concentration distribution of steel after carburizing by nondestructive inspection and a method of manufacturing a carburized member using the carbon concentration distribution measuring method. .

In order to solve the above problems, the carbon concentration distribution measuring method according to the present invention assumes a regression curve that represents the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at the depth in a dimensionless manner. The carbon concentration axis direction of the regression curve is multiplied by (QP) and P is added, and the depth axis direction of the regression curve is multiplied by (BA) / {ρ × S × (QP)}. The gist is to determine the carbon concentration distribution of the steel after carburizing.
However,
Q: Surface carbon concentration (wt%) of steel after carburizing
P: Surface carbon concentration of steel before carburization (wt%)
B: Weight of steel after carburization (g)
A: Weight of steel before carburizing (g)
ρ: density of steel (g / m ³ )
S: Surface area of steel (m ² )

  In this case, the regression curve is preferably an approximate decreasing exponential function.

  On the other hand, the manufacturing method of the carburized member according to the present invention is characterized by having an inspection step of obtaining a depth from the surface having a constant carbon concentration by using the above measuring method and performing non-destructive inspection of the steel after carburizing. To do.

  According to the carbon concentration distribution measuring method according to the present invention, the steel before and after carburizing is assumed on the assumption of a regression curve representing the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at that depth. From the weight of the steel, the surface area of the steel, and the surface carbon concentration of the steel before and after carburizing, the carbon concentration distribution in the depth direction from the surface of the steel after carburizing is obtained. Therefore, the carbon concentration distribution can be easily obtained by nondestructive inspection.

  In this case, if the regression curve is an approximate decrease exponential function, the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at that depth is properly represented.

  On the other hand, the method for manufacturing a carburized member according to the present invention uses the measurement method described above to determine the depth from the surface at which the carbon concentration is constant, and has an inspection process for nondestructive inspection of the steel after carburization. Becomes simple and the productivity of the carburized member with stable quality is improved.

  Hereinafter, an embodiment of the present invention will be described in detail.

  The carbon concentration distribution measuring method according to the present invention (hereinafter sometimes referred to as the present measuring method) is a regression that represents the relationship between the depth from the surface of steel after carburizing and the carbon concentration at that depth in a non-dimensional manner. Assuming a curve, the carbon concentration axis direction of this regression curve is multiplied by (Q−P), P is added, and the depth axis direction is multiplied by (B−A) / {ρ × S × (Q−P)}. Thus, the carbon concentration distribution of the steel after carburizing is obtained.

  Up to now, cut the cross section of steel after carburizing and measure the carbon concentration of the cross section with EPMA, or cut the surface of the steel after carburizing little by little to collect chips of a certain depth from the surface for chemical analysis. The carbon concentration distribution in the depth direction from the surface of the steel after carburization is obtained by destructive inspection such as soldering.

  The carbon concentration distribution obtained by the above destructive inspection may be for steel after carburizing under different carburizing conditions, but these are the depth from the surface of the steel after carburizing and the carbon concentration or hardness at that depth, etc. It is known that there is a certain tendency when expressed in a graph showing the relationship between the carbon concentration and the physical properties correlated. That is, it has been known that the shape of the carbon concentration distribution curve is exponentially reduced by the carbon concentration distribution obtained by the above destructive inspection.

  Then, it was found that when a plurality of graphs showing the carbon concentration distribution obtained by the destructive inspection are expressed in a non-dimensional manner, these graphs result in graphs having almost the same shape and size.

  Therefore, this measurement method empirically assumes a regression curve that represents the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at that depth in a dimensionless manner.

  A method of making dimensionless to assume the regression curve can be freely determined. For example, it is preferable to make the carbon concentration axis dimensionless by expressing the carbon concentration as a ratio to the surface carbon concentration, and to make the depth axis dimensionless by expressing the depth as a ratio with an arbitrary depth.

  When the carbon concentration axis is made dimensionless by the ratio to the surface carbon concentration, the maximum value of the carbon concentration is 1 and the minimum value is 0. On the other hand, for the depth axis, for example, there is no depth axis so that the area surrounded by a curve obtained by making both axes (carbon concentration axis and depth axis) dimensionless and the area surrounded by both axes becomes 1. It is preferable to dimension.

  FIG. 1 shows an example of an assumed regression curve. This regression curve represents the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at that depth in a dimensionless manner. The y-axis is a carbon concentration axis, and the carbon concentration is expressed as a ratio to the surface carbon concentration. On the other hand, the x-axis is a depth axis, and the density after the ratio of the depth to an arbitrary depth is set so that the area obtained by making the both axes dimensionless and the area surrounded by both axes becomes 1. Expressed by multiplication.

  When the regression curve shown in FIG. 1 is approximated by an exponential decreasing function, the following approximate expression is obtained, and the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at the depth is appropriately represented.

(Formula 1)
y = exp (−C × x ^D )
C and D are constants

  This measurement method measures the carbon concentration distribution of steel after carburization having an unknown carbon concentration distribution based on the regression curve assumed as described above. The method is described below.

  First, the carbon concentration axis direction of the regression curve is multiplied by (QP), P is added, and the depth axis direction is multiplied by (BA) / {ρ × S × (QP)}. Q to S are shown below.

Q: Surface carbon concentration (wt%) of steel after carburizing
P: Surface carbon concentration of steel before carburization (wt%)
B: Weight of steel after carburization (g)
A: Weight of steel before carburizing (g)
ρ: density of steel (g / m ³ )
S: Surface area of steel (m ² )

  The depth axis direction is multiplied by (BA) / {ρ × S × (QP)} because the depth axis direction that is dimensionless in the regression curve is a unit of depth (length). It is to make it.

Since A and B are the weight of steel, respectively, and B-A is the weight increase of steel by carburizing, the unit of A, B, and B-A is (g). S represents the surface area of the steel, and the unit is (m ² ). Since P and Q are respectively the surface carbon concentrations of steel and QP is the increase in the surface carbon concentration of steel due to carburization, the units of P, Q and QP are (g / g). ρ represents the density of the steel, and the unit is (g / m ³ ).

Therefore, if the depth axis direction is multiplied by (BA) / {ρ × S × (QP)}, [g] / {[g / m ³ ] [m ² ] [g / g]} = [M], and the depth axis is a unit of depth (length).

  On the other hand, the reason why the carbon concentration axis direction is multiplied by (QP) is to make the carbon concentration axis direction a unit of weight concentration (wt% = g / g), and is reduced to (QP) times. The reason for adding P is to maximize the surface carbon concentration after carburizing and minimize the surface carbon concentration before carburizing.

  As described above, it is possible to obtain a carbon concentration distribution curve in which the depth axis is the unit of depth (m) and the carbon concentration axis is the weight concentration (wt% = g / g). That is, in this measurement method, if the parameters A to P are measured, the carbon concentration distribution of the steel after carburizing can be obtained.

Here, the parameters A to P are measured as follows. That is, before carburizing the steel, the weight A (g) of the steel before carburizing, the surface area S (m ² ) of the steel, and the surface carbon concentration P (wt% = g / g) of the steel before carburizing are measured. Then, after carburizing, the weight B (g) of the steel after carburizing and the surface carbon concentration Q (wt% = g / g) of the steel after carburizing are measured. Note that ρ is the density of the steel and is a constant.

  The surface carbon concentration can be measured by emission spectroscopy or fluorescent X-ray spectroscopy. In the emission spectroscopic analysis method, the surface of a steel material is excited by a discharge between the steel material and a counter electrode to emit light, and this is spectroscopically dispersed to measure the intensity of the wavelength spectrum of a target specific element (such as carbon). Thus, qualitative and quantitative analysis of each element is performed. In addition, X-ray fluorescence spectroscopy is performed by measuring the intensity of the wavelength spectrum of a target specific element (such as carbon) by spectroscopically analyzing the fluorescent X-rays generated by irradiating the steel surface with X-rays. Qualitative and quantitative analysis of elements is performed.

  Since any of the analysis methods is a method for analyzing only the surface of the steel, it does not require cutting of the cross section of the steel or surface cutting, and does not analyze the steel by breaking it. Thus, for example, it is possible to perform qualitative and quantitative analysis on a flow material flowing in a carburized steel production line. Further, since the conventional method is not performed using EPMA, the analysis cost is not so expensive.

  Next, a method for manufacturing a carburized member according to the present invention (hereinafter sometimes referred to as the present manufacturing method) will be described. This manufacturing method has the inspection process which calculates | requires the depth from the surface used as a fixed carbon concentration using this measuring method, and nondestructively inspects the steel after carburizing.

  In this manufacturing method, the carburized member is manufactured by carburizing steel. The steel used for this manufacturing method can illustrate steel types, such as SCr420, SCM420, SNCM220, SNCM420, for example.

  As the carburizing method, a commonly performed method such as gas carburizing, vacuum carburizing, or plasma carburizing can be used. In gas carburizing, hydrocarbon gas such as natural gas, propane, butane, and acetylene is modified to produce a carburizing gas mainly composed of CO, and thereby carburizing steel. Vacuum carburizing is a type of gas carburizing, and carburizing treatment is carried out by introducing hydrocarbon gas directly into the furnace without reducing the carburizing gas under reduced pressure.

  Carburization diffuses and infiltrates carbon on the surface of steel. In the above carburizing method, carburizing treatment is performed by appropriately setting the carburizing temperature, carburizing time, and diffusion time.

  The carburized member manufactured by this manufacturing method is preferably in a certain standard. In order to judge whether a certain standard is met, the surface carbon concentration and the depth from the surface at which the carbon concentration is constant are evaluated.

  The surface carbon concentration is preferably in the range of 0.6 to 0.9%. More preferably, it is 0.7 to 0.8% of range. This is because if the surface carbon concentration is less than 0.6%, the surface hardness is low, and if it exceeds 0.9%, carbides along the grain boundaries are generated and the strength is lowered.

  On the other hand, the depth from the surface at which the carbon concentration is constant is determined using this measurement method. In addition, since this measurement method is as above-mentioned, description is omitted here.

  The constant carbon concentration is not particularly limited. However, since the carburized member having a surface carbon concentration in the range of 0.6 to 0.9% is evaluated, for example, the carbon concentration is preferably 0.35%. At this time, it is preferable that the depth from the surface which becomes 0.35% carbon concentration exists in the range of 0.5-0.8 mm. This is because the carburization depth is sufficiently deep, so that the carburization depth can be applied to, for example, a gear to which high surface pressure is applied.

  According to this manufacturing method, since the carbon concentration distribution of the steel after carburizing can be easily obtained by nondestructive inspection, the quality inspection is simplified and the productivity of the carburized member with stable quality is improved.

  Hereinafter, in the Examples, it is confirmed that the carbon concentration distribution is obtained with the same accuracy as the carbon concentration distribution measuring method using the conventional EPMA by this measuring method.

However,
Q: Surface carbon concentration (wt%) of steel after carburizing
P: Surface carbon concentration of steel before carburization (wt%)
B: Weight of steel after carburization (g)
A: Weight of steel before carburizing (g)
ρ: density of steel (g / m ³ )
S: Surface area of steel (m ² )
It is.

(Example 1-9)
First, the regression curve shown in FIG. 1 was approximated as (Equation 2) below.

(Formula 2)
y = exp (−10766 × x ^2.2 )

Next, before carburizing the gears made of the steel types specified in JIS SCr420, the surface area S (m ² ), the weight A (g), and the carbon concentration P (wt%) were measured and vacuum carburized. In the vacuum carburizing process, the inside of the furnace is depressurized to 10 Pa, and after the furnace temperature becomes constant at 950 ° C., acetylene gas is supplied for 30 minutes while maintaining the furnace pressure at 1500 Pa, and then the supply of acetylene gas is stopped. For 90 minutes. After cooling to room temperature, the weight B (g) and surface carbon concentration Q (wt%) of the gear subjected to vacuum carburization were measured.

  Next, with respect to the above (Equation 2), the carbon concentration side axial direction is multiplied by (Q−P), P is added, and the depth side axial direction is set to (BA) / {ρ × S × (Q−P). )} To create a curve representing the relationship between the depth from the surface of the carburized steel and the carbon concentration at that depth. Using this curve, the surface carbon concentration at which the depth was 0 and the depth at which the carbon concentration was 0.35% were determined. The results are shown in Table 1.

(Reference Example 1-9)
Using EPMA, a cross section of the same gear as the carburized gear in the above example was measured, and the surface carbon concentration at which the depth was 0 and the depth at which the 0.35% carbon concentration was obtained were obtained. The results are shown in Table 1.

  As shown in Example 1-9 and Reference Example 1-9 in Table 1, the surface carbon concentration measured by this measurement method and the depth value at which the 0.35% carbon concentration is obtained are the surface carbon measured by EPMA. The concentration was almost the same as the depth at which the carbon concentration was 0.35%. At this time, in order to confirm the reliability of this measurement method, as shown in Example 1-7, the experiment was repeated seven times. All the results were almost the same as the results measured by EPMA. became.

  From the above, it was confirmed that the carbon concentration distribution of the carburized steel material can be easily obtained by nondestructive inspection by this measurement method, and the carbon concentration at a predetermined depth can be obtained with the same accuracy as EPMA. .

  Next, an inspection was made as to whether or not the carburizing standard was met from the surface carbon concentration and the depth at which the carbon concentration was 0.35%. The carburized standard range is such that the surface carbon concentration at the tooth surface pitch point of the gear is in the range of 0.7 to 0.8%, and the 0.35% carbon depth is in the range of 0.5 to 0.8 mm. Was passed. The results are shown in Table 1.

  From Table 1, as shown in Example 8, it was found that when the carburization amount (BA) was small, the depth of 0.35% carbon concentration was 0.30 mm, which did not satisfy the standard. Further, as shown in Example 9, it was found that when the carburization amount (BA) was large, the surface carbon concentration was 0.91%, which did not satisfy the standard.

  On the other hand, as shown in Example 1-7, it was found that when the carburization amount (BA) was set appropriately, the surface carbon concentration and the depth at which the carbon concentration was 0.35% satisfied the standard.

  As described above, it was confirmed that this measurement method can also inspect whether or not the carburization standard is met.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

It is an example of the regression curve which expressed the reduction | restoration curve showing the relationship between the carbon concentration with respect to the depth from the surface and the depth from the surface defined empirically as non-dimensional.

Claims (3)

Assuming a regression curve that represents the relationship between the depth from the surface of the steel after carburizing and the carbon concentration at that depth,
While adding P by multiplying the carbon concentration axis direction of the regression curve by (QP),
A carbon concentration distribution measuring method, wherein the depth axis direction of the regression curve is multiplied by (BA) / {ρ × S × (QP)} to obtain a carbon concentration distribution of the steel after carburizing.
However,
Q: Surface carbon concentration (wt%) of steel after carburizing
P: Surface carbon concentration of steel before carburization (wt%)
B: Weight of steel after carburization (g)
A: Weight of steel before carburizing (g)
ρ: density of steel (g / m ³ )
S: Surface area of steel (m ² )   The carbon concentration distribution measuring method according to claim 1, wherein the regression curve is an approximate exponential decrease function.   A method for producing a carburized member, comprising: an inspection step for obtaining a depth from a surface having a constant carbon concentration by using the measurement method according to claim 1 and performing a nondestructive inspection of the carburized member.

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