Method of predicting die lives
Disclosed is a method of predicting lives of dies for plastic processing of metals, typically, forging dies, to enable improved die design by predicting an important factor, low cycle fatigue life FL (shot number possible until the die lives end). The method is characterized in that the low cycle damage value “Dc” defined by the formula: Dc=σeq/(YS×softening rate), wherein, σeq is Von Misese's equivalent stress, YS is yield stress (including both of those at tension and compression), and that the following formula is introduced: FL=C1×exp(C2×DcC3), wherein, FL is shot number until the die fracture, and C1, C2 and C3 are constants depending on the material used, so as to presume the possible shot number of the die.
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1. Technical Field
The present invention concerns a method of predicting die lives. More specifically, the invention concerns predicting lives of dies for plastic processing of metals, typically, forging dies, by presuming low cycle fatigue lives, and utilizing the results for die design including choice of materials, hardness thereof and determining the die configuration so as to establish countermeasures for prolongation of the die lives.
2. Prior Art
In regard to manufacture and application of a forging die various methods of predicting damages in the die have been developed and utilized for enabling manufacture of dies having longer lives. As the method of prediction it is generally employed to calculate temperature and stress distribution in a die by finite element analysis and then substitute the calculated values for constitutive equations to presume low cycle fatigue lives and wearing. For example, Japanese Patent Disclosure No. 2002321032 discloses technique of predicting die lives on the basis of die abrasion according to an abrasion model adopting conditions inherent in forging dies.
One of the main factors causing damage and shortening of life of a forging die during using is low cycle fatigue fracture life (hereinafter referred to as “low cycle life”). The low cycle fatigue has been described to date by the formula below on the basis of the relation between the stress posed on and the frequency thereof:
LIFE=f(ε,RA)
wherein, ???is strain, and RA, reduction of area at tensile test.
More specifically, amplitude of repeated plastic deformation and number of repetition until fracture were formularized using the relation known as the “MansonCoffin's formula”. However, methods of predicting lives of dies proposed so far are not of so high accuracy.
The inventors intended to expedite the matter and noted the fact that the cause of the low cycle fatigue fracture is accumulation of strain. They succeeded in establishing a method of predicting die lives by presuming accumulated strain with a yield condition formula, in which direction of the stress posed on the die is considered, and by working out a regression formula from a low cycle fatigue curve.
SUMMARY OF THE INVENTIONThe object of the present invention is to provide a method of predicting die lives enabling design of improved dies by predicting low cycle fatigue life of dies, which give important influence to die lives.
The method according to the invention achieving the abovementioned object is a method of presuming the low cycle fatigue life properties influencing the lives of dies for plastic processing of metals to contribute to die design including choice of materials, hardness and configuration of the die.
The method of predicting die lives according to the invention is characterized in that, basically, the low cycle damage value “Dc” defined by the formula below is calculated:
Dc=σ_{eq}/(YS×softening rate)
wherein, σ_{eq }is Von Misese's equivalent stress, YS is yield stress (including both of those at tension and compression), and that the following formula expressing the low cycle fatigue life “FL” is introduced therefrom,
FL=C_{1}×exp(C_{2}×Dc^{C3})
wherein, FL is shot number until the die fracture, and C_{1}, C_{2 }and C_{3 }are constants depending on the material used, so as to presume the possible shot number of the die.
More specifically, the method according to the invention is the method of predicting lives of dies for plastic processing of metals so as to contribute to the diedesign including choice of material, hardness thereof and determining configuration of the die, and is characterized in that low cycle life tests under “tensiontension” and “tensioncompression” are carried out at respective die materials so as to comprehend the relation between the cycle and the stress amplitude, and using the results, the low cycle damage value “Dc” defined by the formula:
Dc_{fatigue}={maximum tensile stress(σ_{damage})+??×maximum compressive stressσ_{damage}}/(YS×softening rate)
wherein, “σ_{damage}” is damage stress defined as below, “?” is a constant depending on the material, and “YS” is as mentioned above:

 σ_{damage}=σ_{eq }(in case where σ_{1max}−σ_{1min}≧0)
 σ_{damage}=−σ_{eg }(in case where σ_{1max}−σ_{1min}<0)
wherein, σ_{eq }is the abovementioned Von Mises' equivalent stress, σ_{1max }is maximum main stress, and σ_{1min }is minimum main stress;
and that, on this basis, the following formula expressing the low cycle fatigue life “FL” is introduced:
FL=C_{1}×exp(C_{2}×Dc_{fatigue}C^{3})
wherein, FL is the shot number until the fracture, and C_{1}, C_{2 }and C_{3 }are constants depending on the material used, so as to presume the possible shot number of a die.
The present invention took note on the relation
Dc=σ_{eq}/(YS×softening rate)
instead of the strain, which was considered important in the known MansonCoffin's formula. In our formula, σ_{eq }is the abovementioned Von Misese's equivalent stress, YS is yield stress (including both tensile and compressive), and Dc is a criteria (critical value) of plastic flow. When the value of Dc goes up to 1.0, the plastic flow of the die will begin.
The above idea of “σ_{eq}/YS” could be extended to σ_{eq}/TS, (σ_{1max}−σ_{1max})/TS, (σ_{1max}−σ_{1max})/YS, and so on, and the results will be of no great difference. This can be expressed in the functional form as follows:
FL=f(σ_{eq}/TS), FL=f(σ_{1max}−σ_{1max})/TS), and
FL=f(σ_{1max}−σ_{1max})/YS).
wherein, TS is tensile strength.
The abovementioned relation between the life cycle FL and Dc value in the low cycle fatigue life test, Dc=σ_{eq}/(YS×softening rate), can be expressed with a regression formula by using a suitable function.
A typical example is the formula shown above in regard to the basic embodiment:
FL=C_{1}×exp(C_{2}×Dc^{C3}),
wherein, C_{1}, C_{2 }and C_{3 }are constants,
and the abovementioned formula is a materialization of this formula. Thus, by separate consideration of the Dc value into the tensile stress and the compressive stress, results fitted to the practical damage of dies can be obtained. The low cycle fatigue test is carried out with altered modes of the stress amplitude, “tensiletensile” and “tensilecompressive”, and the regression formula is thus computed. For computation of the FL it is necessary to consider stress components at various parts of the die practically used for forging. This is because both the tensile and the compressive stresses are posed during forging.
The abovementioned damage stress, σ_{damage}, is the idea introduced on the basis of the understanding that the strain occurs not only at the moment of tensile stress but also at the moment of compressive stress in view of the low cycle fatigue test and the results of stress component calculation at one shot, in other words, the above idea of damage stress resulted from the discussion in which the stress posed on the die during forging is analyzed to the tensile stress and the compressive stress. On this basis the above formula of Dc_{fatigue }was introduced. As the conclusion it can be generalized that the regression formula including the FL and the Dc_{fatigue }is the following relation:
FL=C_{4}×f(Dc_{fatigue})
The above formula of FL is of this kind. The final goal is to predict damage of dies by computer simulation using indices such as the FL value and the Dc value and to find the optimum values concerning shapes and using conditions (such as forging temperature and the extent of cooling.)
Typical steelmarks suitable for die material are the following two. It is recommended to use them with heat treatment to the hardness shown in the parentheses. Formulae of the low cycle fatigue life FL at the suitable hardness are as follows. Possible shot numbers of the die manufactured with these steels may be predicted by the formulae: “MH85” (standard set by Daido Steel Co., Ltd., HRC 61)
FL=4.0679×10^{9 }exp(−16.135×Dc_{fatigue})
“SKD61” (one of the JIS Steels, HRC 48)
FL=3.1305×10^{11 }exp(−17.239×Dc_{fatigue})
Presumption of the low cycle fatigue life of the die according to the invention enables predicting die lives with accuracy much higher than those given by the conventional damage predicting methods. Those skilled in the art may construct databases on any steel with reference to the working examples of the invention described below, calculate the low cycle fatigue life and carry out the optimum die design.
If the die enjoys a longer life, the contribution will be not only to decrease in diemanufacturing costs but also to decrease manufacturing costs of processed parts such as forged parts through reduction in time and labor for exchanging the dies
EXAMPLES The abovenoted matrix high speed tool steel, MH85, was used as the die material and the hardness was adjusted to be HRC 58.7. The material was subjected to measurement of compressive yield strength, YS, in the temperature range from room temperature to 800° C. or 700° C. to obtain the date shown in
YS_{init}=−5×10^{−6}T^{3}+0.0047T^{2}−1.5574T+2510.7 (T≦600° C.)
YS_{init}=9411202×exp(−0.0150T) (T≧600° C.)
YS_{low}=−0.0006T^{2}+0.0542T+1049.2
The above MH85 steel (HRC58.7) was subjected to also low cycle fatigue life test to observe the relation between the cycle number and the stress amplitude. The relation is shown in the graph of
The relations between the low cycle life FL and the criteria values of plastic flow Dc are illustrated in the graph of
FL=4.0679×10^{9 }exp(−16.135×Dc_{fatigue})
was introduced.
Two kinds of forging punches of the shape as shown in
Material of the Punch: MH85
Material of the Work: S53C
Temperature of the Work: 820° C.
Heatcontacting Conductance: 120 kW/m^{2}K
Forging Speed: 85 spm
Share Sliding Coefficient: 0.4
The punch as shown in
Then, the punch as shown in
Forging at 720° C.—oil cooling (
Forging at 820° C.—oil cooling (
Forging at 820° C.—water cooling (
Forging at 720° C.—oil cooling (
Forging at 820° C.—water cooling (
Forging at 820° C.—water cooling (
The results of analysis indicates that, even if the forging temperature is the same, it is preferable to enhance cooling (oil cooling→water cooling) for the die lives, and that, even if the forging temperature is high, the die lives may be prolonged by enhancing the cooling. The results of computer simulation according to the invention and the results of observation of the used punches are in good concordance, and thus, it is concluded that the present invention provides a method of prediction with high liability.
Claims
1. A method of predicting die lives enabling design of improved dies by predicting low cycle fatigue life of dies, which give important influence to die lives, characterized in that the low cycle damage value “Dc” defined by the formula below is calculated: Dc=σeq/(YS×softening rate) wherein, σeq is Von Misese's equivalent stress, YS is yield stress (including both of those at tension and compression), and that the following formula expressing the low cycle fatigue life “FL” is introduced: FL=C1×exp(C2×DcC3) wherein, FL is shot number until the die fracture, and C1, C2 and C3 are constants depending on the material used, so as to presume the possible shot number of the die.
2. A method of predicting die lives enabling design of improved dies by predicting low cycle fatigue life of dies, which give important influence to die lives, characterized in that low cycle life tests under “tensiontension” and “tensioncompression” are carried out at respective die materials so as to comprehend the relation between the cycle and the stress amplitude, and using the results, the low cycle damage value “Dc” defined by the formula: Dc fatigue = { maximum tensile stress ( σ damage ) + ?? × maximum compressive stress σ damage } / ( YS × softening rate )
 wherein, “σdamage” is damage stress defined as below, “?” is a constant depending on the material, and “YS” is as mentioned above:
 σdamage=σeq(σ1max−σ1min≧0) σdamage=−σeq(σ1max−σ1min−0)
 wherein, σeq is the abovementioned Von Misese's equivalent stress, σ1max is maximum main stress, and σ1min is minimum main stress;
 and that, on this basis, the following formula expressing the low cycle fatigue life “FL” is introduced:
 FL=C1×exp(C2×DcC3)
 wherein, FL is the shot number until the fracture, and C1, C2 and C3 are constants depending on the material used, so as to presume the possible shot number of the die.
Type: Application
Filed: May 23, 2006
Publication Date: Nov 23, 2006
Applicant: DAIDO STEEL CO., LTD. (Nagoua)
Inventors: Hiroaki Yoshida (Nagoya), Shigekazu Itoh (Nagoya), Takuma Okajima (Nagoya)
Application Number: 11/438,257
International Classification: C21D 8/00 (20060101);