U.S. patent application number 11/438257 was filed with the patent office on 2006-11-23 for method of predicting die lives.
This patent application is currently assigned to DAIDO STEEL CO., LTD.. Invention is credited to Shigekazu Itoh, Takuma Okajima, Hiroaki Yoshida.
Application Number | 20060260722 11/438257 |
Document ID | / |
Family ID | 36688519 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060260722 |
Kind Code |
A1 |
Yoshida; Hiroaki ; et
al. |
November 23, 2006 |
Method of predicting die lives
Abstract
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=.sigma..sub.eq/(YS.times.softening rate), wherein,
.sigma..sub.eq 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=C.sub.1.times.exp(C.sub.2.times.Dc.sup.C3), wherein, FL is shot
number until the die fracture, and C.sub.1, C.sub.2 and C.sub.3 are
constants depending on the material used, so as to presume the
possible shot number of the die.
Inventors: |
Yoshida; Hiroaki; (Nagoya,
JP) ; Itoh; Shigekazu; (Nagoya, JP) ; Okajima;
Takuma; (Nagoya, JP) |
Correspondence
Address: |
ARMSTRONG, KRATZ, QUINTOS, HANSON & BROOKS, LLP
1725 K STREET, NW
SUITE 1000
WASHINGTON
DC
20006
US
|
Assignee: |
DAIDO STEEL CO., LTD.
Nagoua
JP
|
Family ID: |
36688519 |
Appl. No.: |
11/438257 |
Filed: |
May 23, 2006 |
Current U.S.
Class: |
148/500 |
Current CPC
Class: |
B21J 13/02 20130101;
B21J 5/00 20130101 |
Class at
Publication: |
148/500 |
International
Class: |
C21D 8/00 20060101
C21D008/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 23, 2005 |
JP |
2005-150266 |
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=.sigma..sub.eq/(YS.times.softening rate) wherein, .sigma..sub.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:
FL=C.sub.1.times.exp(C.sub.2.times.Dc.sup.C3) wherein, FL is shot
number until the die fracture, and C.sub.1, C.sub.2 and C.sub.3 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 "tension-tension" and "tension-compression" 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 .times. .times. tensile .times. .times. stress
.times. .times. ( .sigma. damage ) + ?? .times. .times. .times.
maximum .times. .times. compressive .times. .times. stress .times.
.times. .sigma. damage } / ( YS .times. softening .times. .times.
rate ) ##EQU1## wherein, ".sigma..sub.damage" is damage stress
defined as below, "?" is a constant depending on the material, and
"YS" is as mentioned above:
.sigma..sub.damage=.sigma..sub.eq(.sigma..sub.1max-.sigma..sub.1min.gtore-
q.0)
.sigma..sub.damage=-.sigma..sub.eq(.sigma..sub.1max-.sigma..sub.1min-
-0) wherein, .sigma..sub.eq is the above-mentioned Von Misese's
equivalent stress, .sigma..sub.1max is maximum main stress, and
.sigma..sub.1min is minimum main stress; and that, on this basis,
the following formula expressing the low cycle fatigue life "FL" is
introduced: FL=C.sub.1.times.exp(C.sub.2.times.Dc.sup.C3) wherein,
FL is the shot number until the fracture, and C.sub.1, C.sub.2 and
C.sub.3 are constants depending on the material used, so as to
presume the possible shot number of the die.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] 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.
[0003] 2. Prior Art
[0004] 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. 2002-321032
discloses technique of predicting die lives on the basis of die
abrasion according to an abrasion model adopting conditions
inherent in forging dies.
[0005] 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(.epsilon.,RA) wherein, ???is strain, and RA,
reduction of area at tensile test.
[0006] More specifically, amplitude of repeated plastic deformation
and number of repetition until fracture were formularized using the
relation known as the "Manson-Coffin's formula". However, methods
of predicting lives of dies proposed so far are not of so high
accuracy.
[0007] 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 INVENTION
[0008] The 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.
[0009] The method according to the invention achieving the
above-mentioned 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.
[0010] 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=.sigma..sub.eq/(YS.times.softening rate) wherein, .sigma..sub.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.sub.1.times.exp(C.sub.2.times.Dc.sup.C3) wherein,
FL is shot number until the die fracture, and C.sub.1, C.sub.2 and
C.sub.3 are constants depending on the material used, so as to
presume the possible shot number of the die.
[0011] 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 die-design including choice of
material, hardness thereof and determining configuration of the
die, and is characterized in that low cycle life tests under
"tension-tension" and "tension-compression" 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.sub.fatigue={maximum tensile
stress(.sigma..sub.damage)+??.times.maximum compressive
stress|.sigma..sub.damage|}/(YS.times.softening rate) wherein,
".sigma..sub.damage" is damage stress defined as below, "?" is a
constant depending on the material, and "YS" is as mentioned above:
[0012] .sigma..sub.damage=.sigma..sub.eq (in case where
.sigma..sub.1max-.sigma..sub.1min.gtoreq.0) [0013]
.sigma..sub.damage=-.sigma..sub.eg (in case where
.sigma..sub.1max-.sigma..sub.1min<0) wherein, .sigma..sub.eq is
the above-mentioned Von Mises' equivalent stress, .sigma..sub.1max
is maximum main stress, and .sigma..sub.1min is minimum main
stress; and that, on this basis, the following formula expressing
the low cycle fatigue life "FL" is introduced:
FL=C.sub.1.times.exp(C.sub.2.times.Dc.sub.fatigueC.sup.3) wherein,
FL is the shot number until the fracture, and C.sub.1, C.sub.2 and
C.sub.3 are constants depending on the material used, so as to
presume the possible shot number of a die.
BRIEF EXPLANATIO OF THE DRAWINGS
[0014] FIG. 1 is a graph illustrating dynamic compressive yield
strength of heat-treated state (HRC 60) and softened state (HRC
29.6) of MH85 steel, which is a matrix type high speed steel
provided by Daido Steel Co., Ltd., depending on the
temperature;
[0015] FIG. 2 is a graph prepared by plotting the relation between
the cycle number and the stress amplitude at low cycle fatigue life
test of the MH85 steel (HRC 58.7);
[0016] FIG. 3 is a figure showing the concept of "tension-tension"
and "tension-compression" of the tests for preparing the graph of
FIG. 2;
[0017] FIG. 4 is a graph prepared by plotting the relation between
the plastic flow criteria value "D.sub.c" and the low cycle fatigue
life FL of the MH85 steel (HRC 58.7);
[0018] FIG. 5 is a graph obtained by plotting the relation between
the Dc value (Dc.sub.fatigue) and the low cycle fatigue life FL of
the die material of FIG. 4;
[0019] FIG. 6 is a computer graphics (hereinafter referred to as
"CG") showing the relation between the low cycle fatigue life FL
and the intensity of cooling (A: mild cooling, B: strong cooling)
of a forging punch obtained from the data of a working examples of
the invention;
[0020] FIG. 7 is a CG like FIG. 6 showing the relation between the
low cycle fatigue life FL and the manner of cooling the forging
punches (A: forging at 820.degree. C.--oil quenching, B: forging at
820.degree. C.--water quenching, and C: forging at 920.degree.
C.--water quenching) also obtained from the data of a working
examples of the invention;
[0021] FIG. 8 is a CG like FIG. 6 showing the relation between the
low cycle fatigue life FL and the forging temperature of the
forging punches (A: forging at 820.degree. C.--oil quenching, B:
forging at 820.degree. C.--water quenching, and C: forging at
920.degree. C.--water quenching) also obtained from the data of a
working examples of the invention.
DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS
[0022] The present invention took note on the relation
Dc=.sigma..sub.eq/(YS.times.softening rate) instead of the strain,
which was considered important in the known Manson-Coffin's
formula. In our formula, .sigma..sub.eq is the above-mentioned 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.
[0023] The above idea of ".sigma..sub.eq/YS" could be extended to
.sigma..sub.eq/TS, (.sigma..sub.1max-.sigma..sub.1max)/TS,
(.sigma..sub.1max-.sigma..sub.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(.sigma..sub.eq/TS),
FL=f(.sigma..sub.1max-.sigma..sub.1max)/TS), and
FL=f(.sigma..sub.1max-.sigma..sub.1max)/YS). wherein, TS is tensile
strength.
[0024] The above-mentioned relation between the life cycle FL and
Dc value in the low cycle fatigue life test,
Dc=.sigma..sub.eq/(YS.times.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.sub.1.times.exp(C.sub.2.times.Dc.sup.C3), wherein,
C.sub.1, C.sub.2 and C.sub.3 are constants, and the above-mentioned
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, "tensile-tensile" and
"tensile-compressive", 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.
[0025] The above-mentioned damage stress, .sigma..sub.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.sub.fatigue was introduced. As the
conclusion it can be generalized that the regression formula
including the FL and the Dc.sub.fatigue is the following relation:
FL=C.sub.4.times.f(Dc.sub.fatigue)
[0026] 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.)
[0027] Typical steel-marks 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.times.10.sup.9
exp(-16.135.times.Dc.sub.fatigue) "SKD61" (one of the JIS Steels,
HRC 48) FL=3.1305.times.10.sup.11
exp(-17.239.times.Dc.sub.fatigue)
[0028] 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.
[0029] If the die enjoys a longer life, the contribution will be
not only to decrease in die-manufacturing 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
[0030] The above-noted 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.degree. C. or 700.degree. C. to obtain the date shown in FIG.
1. The relations between the compressive yield strength and
temperature T were as follows:
YS.sub.init=-5.times.10.sup.-6T.sup.3+0.0047T.sup.2-1.5574T+2510.7
(T.ltoreq.600.degree. C.) YS.sub.init=9411202.times.exp(-0.0150T)
(T.gtoreq.600.degree. C.)
YS.sub.low=-0.0006T.sup.2+0.0542T+1049.2
[0031] 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 FIG. 2. "Tensile-tensile" and "tensile-compressive" of the
stress amplitude in this graph mean the manner of posing stress as
shown in upper and lower parts of FIG. 3.
[0032] The relations between the low cycle life FL and the criteria
values of plastic flow Dc are illustrated in the graph of FIG. 4 in
both the cases of "tensile-tensile" and "tensile-compressive". The
stress posed on the die varies continuously, depending on the
location in the die, among the typical cases shown in FIG. 3, and
unified treatment of the typical cases gave the "improved Dc value"
in FIG. 5, which is the graph showing the relation between the
Dc.sub.fatigue and the low cycle life FL. From regression analysis
of this graph the above noted formula: FL=4.0679.times.10.sup.9
exp(-16.135.times.Dc.sub.fatigue) was introduced.
[0033] Two kinds of forging punches of the shape as shown in FIG. 6
and FIG. 7 were manufactured with MH85 steel, and the punches were
subjected to wear tests on a horizontal type parts former. The
forging consists of two steps, the first for upsetting and the
second for backward extrusion. The type and the extent of the
damage of the die can be learned by observing the state of damaging
after the second step. Stress-thermorelated elastoplasticity
analysis (MSC/Super Form 2004) was carried out under the following
conditions:
Material of the Punch: MH85
Material of the Work: S53C
Temperature of the Work: 820.degree. C.
Heat-contacting Conductance: 120 kW/m.sup.2K
Forging Speed: 85 spm
Share Sliding Coefficient: 0.4
[0034] The punch as shown in FIG. 6 (in this case, without
consideration of stress direction, tensile only) was used and the
cooling conditions were controlled to strong and weak by adjusting
flow rates of the forging oil. The results of simulation are as
shown in FIGS. 6A (weak cooling) and 6B (strong cooling), from
which it is concluded that cooling should be strong. After the
testing the punch was cut along the axis thereof to observe the
texture. In case of weak cooling there was observed plastic flow at
the part "R" of the tip of the punch. (This was so judged from the
fact that the stripes made by corrosion curved at the surface.) On
the other hand, no sign of the plastic flow was observed in case of
strong cooling. Comparison of the Dc values in the cases of weak
and strong cooling, the Dc value of the weak cooling was higher at
the tip of the punch.
[0035] Then, the punch as shown in FIG. 7 was used to carry out
forging of backward extrusion. (In this case the direction of the
stress is considered.) Conditions for forging and cooling were
chosen as follows, and the possibility of occurring damage in the
punch due to the plastic flow was simulated by a computer. The
results are shown in FIGS. 7A-C and FIGS. 8A-C.
Forging at 720.degree. C.--oil cooling (FIG. 7A)
Forging at 820.degree. C.--oil cooling (FIG. 7B)
Forging at 820.degree. C.--water cooling (FIG. 7C)
Forging at 720.degree. C.--oil cooling (FIG. 8A)
Forging at 820.degree. C.--water cooling (FIG. 8B)
Forging at 820.degree. C.--water cooling (FIG. 8C)
[0036] The results of analysis indicates that, even if the forging
temperature is the same, it is preferable to enhance cooling (oil
cooling.fwdarw.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.
* * * * *