JP3030419B2 - Non-destructive estimation method for toughness - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-destructive inspection method for materials such as molds and shafts, and more particularly to a method for determining the toughness of such materials by quenching cooling rate (CR), hardness (H) and quenching. The present invention relates to a non-destructive evaluation method based on temperature (T).

[0002]

2. Description of the Related Art Toughness is one of the characteristics to guarantee the quality of products such as molds and shafts after heat treatment. Toughness refers to the strength of a material to brittle fracture. Notch embrittlement, temper embrittlement, low-temperature embrittlement, and the like are known as brittle fractures. Therefore, in general, it is necessary to evaluate the toughness of a material in order to guarantee the safety against notch embrittlement, temper embrittlement, and low-temperature brittle fracture. Here, as a parameter for evaluating toughness, a Charpy impact value (kg
M / cm ² ), Izod impact value (ft · lb), absorbed energy, shelf energy ((upper) s
self-energy, brittle transition temperature (energy transition temperature, fracture surface transition temperature, 15 ft-lb transition temperature, etc.), fracture toughness (plane strain fracture toughness, energy release rate, elasto-plastic fracture toughness). For example, in the case of so-called hot dies used for hot working of metal materials and the like, their shapes have recently become larger and their use environment has become increasingly severe. Therefore, the importance of quality assurance (hardness and toughness) of the mold after heat treatment has been reconsidered. That is, if the hardness for improving abrasion resistance and the toughness for preventing cracking can be accurately non-destructively evaluated, it is possible to secure the opposing characteristics in an optimal combination according to the mold use conditions. Quality assurance and long life of the mold can be advanced. However, although hardness is currently measured, there is no method for non-destructive evaluation of toughness, and therefore, it is not possible to accurately grasp insufficient toughness caused by a decrease in quenching cooling rate due to enlargement of molds. In some cases, a large crack is caused in the early stage of use, which is a problem.

Therefore, in order to solve such a problem and to assure the quality of the mold after tempering, it is necessary to establish a method capable of non-destructively evaluating the toughness of the mold after heat treatment.
According to conventional studies on toughness and heat treatment, toughness (C
It is qualitatively known that h) is affected by tempering hardness (H), quenching cooling rate (CR) and quenching temperature (T). Therefore, the relationship between these various quantities (Ch, H, T, CR) is determined in advance, and the hardness, the quenching temperature and the quenching cooling rate are measured. It can be estimated non-destructively. The inventors of the present invention use a quenched and tempered material of a 0.4C-5Cr-Mo-V hot die steel from a standard quenching temperature (1020 ° C) to obtain hardness (H) and toughness (Charpy impact strength). Value (C
h)) and quenching temperature (CR)
A method of non-destructively estimating the toughness (Ch) from the relational expression [(Ch = f (Ht, H)] by examining the relationship with the cooling time (half-cooling time (Ht)) up to 00 ° C. (“Iron and Steel”, 1975 (1989) No. 5, p.
833-p. 840). According to this report, Ch is represented in the form as in equation (1).

Ch = δ ₁ + γ ₁ · H (kg · m / cm ² ) (1) δ ₁ = 17.3−2.56 · log (Ht) γ ₁ = −0.266 + 0. 0249 · log (Ht) where HRC43 ≦ H ≦ HRC51, T = 1020 ° C. Therefore, Ht is determined and H is determined by a hardness test to obtain the Ch of the material quenched and tempered from a quenching temperature of 1020 ° C. Can be non-destructively evaluated.

[0005]

The above-mentioned report by the present inventors is the result when the quenching temperature (T) is set to the standard quenching temperature of the used material of 1020 ° C. However, the actual quenching temperature of the mold is usually 1020 when toughness is prioritized.
Lower than ℃ (up to about 1015 ℃), if higher strength is higher than 1020 ℃ (up to about 1035 ℃)
Is set to Tempering hardness (H) at different quenching temperatures
, But the microstructures are different. On the other hand, the quenching cooling rate (CR) has a large effect on the microstructure, and the toughness (C
h) is sensitive to microstructure. Therefore, when the quenching temperature differs, CR and Ch are estimated to be different. Therefore,
Using the Ch-Ht-H relational expression for T = 1020 ° C. obtained in the report of the present inventors, the quenching temperature was 102
It seems that Ch at a temperature different from 0 ° C. cannot be accurately estimated. Accordingly, an object of the present invention is to provide a nondestructive evaluation method capable of accurately estimating the toughness of a material quenched from various temperatures within a practical quenching temperature range of a mold or the like.

[0006]

Means for Solving the Problems In order to achieve the above objects, the present inventor first sets the quenching temperature within the practical range.
In order to clarify the overall relationship of CR-HT, the relationship between these various quantities in a range wider than the practical quenching temperature was examined.
Next, in this range, a method for non-destructively evaluating Ch by expressing Ch quantitatively and comprehensively in the form of a function of CR, H, T (Ch = f (CR, H, T)) was studied. . As a result, in the practical quenching temperature range, it was found that Ch can be estimated non-destructively comprehensively by measuring the quenching temperature, obtaining the quenching cooling rate, and measuring the hardness by a hardness test. I came to

That is, according to the present invention, the quenching cooling rate (CR), the tempering hardness (H) and the quenching temperature (T) are used as parameters, and a function of these quantities [Ch = f (CR,
H, T)], a non-destructive method for estimating toughness (Ch) of a material is provided.

The present invention mainly evaluates the toughness of a material after heat treatment against notch toughness fracture. However, by applying the present invention, low-temperature brittle fracture of a material at a low temperature lower than room temperature, and impurities (D, S, As, Sb
Etc.) may be considered to evaluate the toughness against temper brittle fracture caused by the above.

Here, the materials to which the present invention is applied include SKD5, SKD6, SKD61, SKD62, and SKD5.
There are KD7, SKD8, etc., which are materials whose heat treatment structure is affected by the quenching cooling rate. Further, the quenching temperature (T) in the case where the present invention is applied is not particularly limited. For the purpose of evaluating the austenitized state of the material, for example, measuring the atmosphere temperature in a furnace with a thermocouple, or the like. Measured by means. But,
In order to more accurately grasp the austenitized state, it is preferable to use quenching parameters including not only the quenching temperature but also the holding time.

[0010] In addition, the method of measuring the hardness in applying the present invention is not particularly limited, and includes "Brinell hardness", "Vickers hardness", "Knoop hardness", "Rockell hardness", Various hardness parameters such as "Shore hardness" and "Echo tip hardness" can be used.
The hardness of the target material is not particularly limited. For example, in the case of a hot mold, the Rockwell hardness is about HRC43 to HRC51, but in the case of a cold mold or HSS, it is about HRC70. In some cases, the rotor material or the like may be lower than the HRC 43, and in any case, the present invention can be applied to all cases.

The overall relational expression Ch = f (CR, H, T) of various quantities used in the present invention is determined for each material to which the present invention is applied and whose material properties are evaluated. Here, the method of specifying the quenching cooling rate (CR) used in the present invention is not particularly limited.
A cooling time to ℃, ie a half-cooling time (Ht) can be used. In addition, 300 ° C from the quenching temperature
For example, a cooling time up to the above can be used.

Further, the method of measuring the cooling rate is not particularly limited. For example, the cooling rate may be obtained by actual measurement using a thermocouple, or may be obtained by numerical analysis using a finite element method or the like. Further, the cooling rate may be estimated using Barkhausen noise (hereinafter abbreviated as BHN) parameter. As such a Barkhausen noise parameter, a parameter based on the total output voltage of BHN generated in the magnetization process (sum of squares of all BHN output voltages (V
p) or the effective value (R) which is the average output voltage of the BHN output voltage.
MS)), parameters based on the instantaneous output voltage of BHN (maximum instantaneous output voltage (Vh), etc.) and BHN
A parameter (spectrum) or the like obtained by frequency analysis of N can be used.

[0013]

The chemical composition of 0.4C-5Cr-Mo shown in Table 1
-T = 990,1050 ° C ± 5.0 ° C for -V steel, 3
After holding for 0 min, quenching was carried out with a half-cooling time of 3, 15, 45 and 100 min.
1 and 2 show the results of the relationship between Ch and H of the material tempered to C52 determined by the Charpy test. Note that C
h is the average value of the impact values of the five Charpy test pieces, and the variation is indicated by an error bar in the figure.

[0014]

[Table 1]

As shown in FIGS. 1 and 2, Ch is T = 9.
In both cases of 90 and 1050 ° C., it depends on Ht and H, and in any H, Ch decreases with an increase in Ht, and in any Ht, C decreases with an increase in H.
h is decreasing. Such a qualitative relationship of Ch-H-Ht has already been reported by the present inventors ("Iron and Steel", 75th (1989) No. 5, p. 833-p. 84).
0) The same applies to the relationship between these quantities when T = 1020 ° C. However, as shown in FIG. 3, Ch also depends on T, and the quantitative relationship of Ch-HT differs when T differs. Therefore, in order to quantitatively determine Ht in the actual quenching temperature range other than T = 1020 ° C., Ch-H containing T must be used.
It is necessary to find a comprehensive and quantitative relationship of -Ht-T.

Therefore, in the present invention, various test pieces having different quenching temperatures, quenching cooling rates and tempering hardness are prepared using the materials shown in Table 1, and Ch is measured by a Charpy test to determine Ht and T. Ch-H relationship when constant (FIGS. 1 and 2), Ch-T relationship when Ht and H are constant (FIG. 3), and Ch-Ht relationship when H and T are constant. Focusing on (FIG. 4), the comprehensive and quantitative relationship of Ch-Ht-HT over a wider range than the actual quenching temperature range was examined. As a result, according to the present invention, Ch-Ht-H
−T is defined as the function of these quantities (Ch = f (Ht,
H, T)) can be expressed comprehensively and quantitatively. Therefore, the toughness can be estimated by obtaining the quenching temperature (T), the tempering hardness (H), and the quenching cooling rate (CR), and substituting these variables into this function.
Thus, according to the present invention, the toughness (Ch) is changed to T, H, H
By expressing in the form of a function of t, the toughness (Ch) of the material can be quantitatively and comprehensively determined without individually determining the Ch-Ht-H relational expression at various quenching temperatures in the actual quenching temperature range. Can be estimated non-destructively.

[0017]

Next, an embodiment of the present invention will be described. 1 Test material The test material was a 0.4C-5Cr-Mo-V steel having a chemical composition shown in Table 1. 0.4 of this chemical component
C-5Cr-Mo-V steel was melted in an electric arc furnace, hot-formed to a forging ratio of 6 or more, and then annealed at 860 ° C. This test piece was sampled in the forging direction from a position intermediate between the center and the corner, and was used in the practice of the present invention. 2 Shape of test piece and heat-treated Charpy test piece were prepared as follows. Dimension is 10m
The notch is a U notch (R1 mm) having a size of mx 10 mm x 55 mm and a depth of 2 mm.

As the heat treatment of the test piece, T = 960, 990, 1
After performing the austenitizing treatment while maintaining the temperature at 020, 1050 ° C., and 1080 ° C. for 30 minutes, Ht was changed to Ht =
It was selected from four stages of 3, 15, 45, and 110, cooled at a constant speed by program control, and then tempered twice.

[0019] 3. Estimation of toughness (Ch: Charpy impact value)
Constant then performed Charpy test to determine the relationship between the Ch-Ht-H-T. As a result, the relationship of Ch-Ht-H in the special case of T = 1020 ° C., which has been clarified so far, is expressed by a function including T (Ch = f
(Ht-H, T)).

In obtaining this relational expression, first, C
Focusing on the fact that the relationship between h and H is a linear expression as shown in FIGS. 1 and 2, the relationship between the two is represented by a linear expression shown in Expression (5), and the coefficients (δ ₂ , γ ₂ ) are investigated. In examining such coefficients (δ ₂ , γ ₂ ), Ch and log (H
Paying attention to the fact that the relationship of t) is a linear relationship of FIG.
The respective coefficients (C ₀ , D ₀ ) and (C ₁ , D ₁ ) were examined. In calculating the coefficients of (C ₀ , D ₀ ) and (C ₁ , D ₁ ), the relationship between Ch and T is substantially the same as ΔT shown in FIG. 3 (the actual quenching temperature (1020 ° C.)). ) And the difference between the standard quenching temperature) and the quadratic equation.

Ch = δ₂+ Γ₂・ H (kg ・ m / cm²) ... (5) δ₂= C₀+ D₀・ Log (Ht) C₀= 16.6-0.377 · (T-1020)-5.21 · 10^-5・ (T-1020)²  D₀= −2.14-0.0157 · (T−102) −1.29 · 10^-5・ (T-1020)²  γ₂= C₁+ D₁・ Log (Ht) C₁= −0.252 + 4.67 · 10^-4・ (T-1020) D₁= 0.0171 + 4.20 · 10^-4・ (T-1020) However, HRC43 ≦ H ≦ HRC51,990 ° C ≦ T ≦ 1050 ° C

Materials subjected to various heat treatments are prepared, hardness (H) is determined by a hardness test, quenching temperature (T) is determined from furnace atmosphere temperature, and quenching cooling rate (Ht) is determined by a thermocouple. The non-destructive estimation value of Ch (Ct) obtained by substituting these quantities (Ht, H, T) into equation (5)
h) and the actual measured value of Ch (C
h) is shown in FIG. As shown in FIG. 5, the calculated values and the experimental values are almost the same. Therefore, the hardness (H) and the quenching temperature (T) are measured, the quenching cooling rate (Ht) is determined, and these variables are substituted into the equation (5) to obtain the normal quenching temperature of the mold. In the range (1015 to 1035 ° C), Ch was able to be comprehensively and quantitatively subjected to nondestructive evaluation. In addition, this example is a result of the Charpy impact value (Ch) at room temperature.
The relational expression of HT is established in a form different from that of this embodiment. Even in such a case, the toughness can be evaluated nondestructively by the present invention.

[0023]

As described above, according to the non-destructive estimation method of toughness of the present invention, the relationship among toughness (Ch), quenching temperature (T), quenching cooling rate (CR) and hardness (H) is expressed as Ch = f (CR,
H, T) in a comprehensive and quantitative manner, and by substituting these various quantities (CR, H, T) into this relational expression, non-destructive evaluation of toughness was made. There is no need to individually examine the relationship between CR, H and toughness (Ch) at temperature, and an excellent effect that industrial application can be easily achieved is achieved. In particular, according to the present invention, there is an advantage that the toughness can be accurately non-destructively evaluated in the range of the quenching temperature of the mold actually performed irrespective of the quenching temperature.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a Charpy impact value (Ch) and a hardness (H) at a quenching temperature T = 990 ° C.

FIG. 2 is a diagram showing a relationship between a Charpy impact value (Ch) and a hardness (H) at a quenching temperature T = 1050 ° C.

FIG. 3 Charpy impact value (Ch) and quenching temperature (T)
FIG.

FIG. 4 Charpy impact value (Ch) and half-cooling time (H
It is a figure which shows the relationship with t).

FIG. 5 Non-destructively calculates the non-destructive quenching rate (Ht), quenching temperature (T) and hardness (H) by substituting the quenching cooling rate (Ht), the quenching temperature (T) and the hardness (H) into the toughness non-destructive estimation formula obtained by implementing the present invention. It is a figure which shows the result of having compared the estimated value (Ch) of the Charpy impact value calculated | required, and the measured value (Ch) calculated | required by the destructive test.

Claims (1)

(57) [Claims] 1. A quenching cooling rate (CR), a tempering hardness (H), and a quenching temperature (T) are used as parameters, and a material [Ch = f (CR, H, T)] is used as a function of these quantities. Non-destructive estimation method for toughness, characterized by estimating toughness (Ch) of steel.