U.S. patent application number 11/406379 was filed with the patent office on 2006-10-19 for method of predicting damage of dies.
This patent application is currently assigned to DAIDO STEEL CO., LTD.. Invention is credited to Shigekazu Itoh, Hiroaki Yoshida.
Application Number | 20060230881 11/406379 |
Document ID | / |
Family ID | 36648542 |
Filed Date | 2006-10-19 |
United States Patent
Application |
20060230881 |
Kind Code |
A1 |
Yoshida; Hiroaki ; et
al. |
October 19, 2006 |
Method of predicting damage of dies
Abstract
Disclosed is a method of predicting damage of dies for plastic
processing of metallic materials, typically, forging dies, by
predicting brittle fracture ("great crack" or "initial crack")
dominating die lives contribute to die design including choice of
materials, hardness and configuration of the die. The method is
characterized in that the die design is carried out by choosing the
condition that none of the anticipated values of brittle fracture,
F.sub.c1 to F.sub.c3, calculated by the formulae 1 to 3 below
exceed the critical values depending on the material used.
F.sub.c1=(.sigma..sub.m/.sigma..sub.eq) [formula 1]
F.sub.c2=(.sigma..sub.m/.sigma..sub.1 max) [formula 2]
F.sub.c3=(.sigma..sub.1max/.sigma..sub.eq) [formula 3]
.sigma..sub.m: mean normal stress loaded to the tensile side of the
die .sigma..sub.eq: Von Misese's equivalent stress
.sigma..sub.1max: maximum principal stress
Inventors: |
Yoshida; Hiroaki; (Nagoya,
JP) ; Itoh; Shigekazu; (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.
Nagoya
JP
|
Family ID: |
36648542 |
Appl. No.: |
11/406379 |
Filed: |
April 19, 2006 |
Current U.S.
Class: |
76/107.1 |
Current CPC
Class: |
B21J 13/02 20130101 |
Class at
Publication: |
076/107.1 |
International
Class: |
B21K 5/20 20060101
B21K005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 19, 2005 |
JP |
2005-120660 |
Claims
1. A method of predicting damage of dies used for plastic
processing of metallic materials by predicting "great crack" caused
by brittle fracture dominating the die lives so as to contribute to
die design including choice of die materials, hardness of the
material and determination of die shape, which comprises carrying
out the die design by selecting the conditions that none of the
anticipated brittle fracture values F.sub.c1 to F.sub.c3 calculated
by the formulae below exceed the critical values depending on the
materials: F.sub.c1=(.sigma..sub.m/.sigma..sub.eq) [formula 1]
F.sub.c2=(.sigma..sub.m/.sigma..sub.1 max) [formula 2]
F.sub.c3=(s.sub.1max/s.sub.eq) [formula 3] where, .sigma..sub.m:
mean normal stress loaded to the tensile side of the die
.sigma..sub.eq: Von Misese's equivalent stress .sigma..sub.1max:
maximum principal stress.
2. The method of predicting damage of die according to claim 1,
wherein the material of the die is SKD61 steel and F.sub.c1=0.7,
F.sub.c2=0.5 and F.sub.c3=1.25.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention concerns a method of predicting
damages of dies. More specifically, the invention concerns
predicting damages in dies for plastic processing of metals,
typically, forging dies by predicting "great crack" damage caused
by brittle fracture which dominates die 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 die lives.
[0003] 2. Prior Art
[0004] At manufacturing and application of forging dies various
methods of predicting damages in dies have been developed and
utilized for enabling manufacture of dies of 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 predict low cycle fatigue lives and abrasion. 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] Fracture factors causing damage of forging dies during using
are four, namely, "great crack" or static brittle fracture, plastic
flow, abrasion and low cycle fatigue destruction. The brittle
fracture is a sudden phenomenon occurring at an initial stage
before substantial use of the die, and also called "initial crack",
which is a fatal damage. However, methods of predicting damage of
dies proposed so far are not effective for this kind of brittle
fracture. As to ductile fracture there has been proposed Cockroft's
formula, Oyane's formula and Ayada's formula, which have been
acknowledged. These formulae are, however, not applicable to the
brittle fracture. Thus, there has been demand for formula or
formulae which enable effective prediction of damage of dies caused
by brittle fracture.
SUMMARY OF THE INVENTION
[0006] The object of the present invention is to provide a method
of predicting damage of dies enabling design of improved dies by
predicting brittle fracture which give, among various factors
causing damage to forging die, fatal influence to die lives.
[0007] The method according to the invention achieving the
above-mentioned object is a method of predicting "great crack" by
brittle fracture which dominates 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 damages of dies according to the invention is
characterized in that the die design is carried out by choosing the
condition that none of the predicted values of brittle fracture,
F.sub.c1 to F.sub.c3, calculated by the formulae 1 to 3 below
exceed the critical values determined on the basis of the material
used. F.sub.c1=(.sigma..sub.m/.sigma..sub.eq) [formula 1]
F.sub.c2=(.sigma..sub.m/.sigma..sub.1 max) [formula 2]
F.sub.c3=(.sigma..sub.1max/.sigma..sub.eq) [formula 3] [0008]
.sigma..sub.m: mean normal stress loaded to the tensile side of the
die [0009] .sigma..sub.eq: Von Misese's equivalent stress [0010]
.sigma..sub.1max: maximum principal stress
BRIEF EXPLANATIO OF THE DRAWINGS
[0011] FIG. 1 is a plan view illustrating the shape and dimension
of a smooth test piece of the tensile test pieces prepared for
constructing database of the material properties prior to
conducting the present invention;
[0012] FIG. 2 is a plan view like FIG. 1 illustrating the shape and
dimension of the test piece having a surrounding V-notch with notch
angle of 30.degree.;
[0013] FIG. 3 is a plan view like a part of FIG. 2 illustrating the
detail of the notched part of the test piece with notch angle of
90.degree.;
[0014] FIG. 4 is a plan view like a part of FIG. 3 illustrating the
detail of the notched part of the test piece with notch angle of
120.degree.;
[0015] FIG. 5 is a graph obtained by plotting mean normal stress
(.sigma..sub.m) corresponding to equivalent stress (.sigma..sub.eq)
based on the data given by tensile tests in working example of the
present invention;
[0016] FIG. 6 is a graph obtained by plotting mean normal stress
(.sigma..sub.m) corresponding to maximum principal stress
(.sigma..sub.1max) based on the data given by tensile tests in
working example of the present invention;
[0017] FIG. 7 is a graph obtained by plotting maximum principal
stress (.sigma..sub.1max) corresponding to equivalent stress
(.sigma..sub.eq) based on the data given by tensile tests in
working example of the present invention;
[0018] FIG. 8 is a section view illustrating the shape of a
ring-die and a work used at hot forging a final gear (an automobile
part);
[0019] FIG. 8 is data of a working example and a computer graphics
(hereinafter referred to as "CG") obtained by FEM analysis showing
distribution of the critical values F.sub.c1 of brittle fracture in
a ring-die before improvement by the invention;
[0020] FIG. 10 is a CG like FIG. 9 showing distribution of the
critical values F.sub.c2 of brittle fracture in a ring-die before
improvement by the invention;
[0021] FIG. 11 is a CG like FIG. 9 showing distribution of the
critical values F.sub.c3 of brittle fracture in a ring-die before
improvement by the invention;
[0022] FIG. 12 is a CG like FIG. 9 showing distribution of the
critical values F.sub.c1 of brittle fracture in a ring-die after
improvement by the invention;
[0023] FIG. 13 is a CG like FIG. 9 showing distribution of the
critical values F.sub.c2 of brittle fracture in a ring-die before
improvement by the invention;
[0024] FIG. 14 is a CG like FIG. 9 showing distribution of the
critical values F.sub.c2 of brittle fracture in a ring-die before
improvement by the invention;
DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS
[0025] The factors dominating the brittle fracture of dies are
three, as noted above, i.e., mean normal stress (.sigma..sub.m),
equivalent stress (.sigma..sub.eq) and maximum principal stress
(.sigma..sub.1max). It can be hence said that there is generally
the following relation: Fc=f(.sigma..sub.m, .sigma..sub.eq,
.sigma..sub.1max)
[0026] All the dominating factors are considered in the above
formulae 1 to 3. In practical use of these formulae an improved
formula or formulae (such as those with adjusted coefficients) may
be found by experience. They will give the same effect as those
discussed above, and thus the invention includes the embodiments
using such formulae.
[0027] By predicting the damages of dies it will be possible to
establish effective countermeasures to the brittle fracture
(so-called "great crack" or "initial crack"), to which, though it
is an important factor, no conventional method of predicting
damages has not been confronted. Those skilled in the art will be
able to manufacture the optimum die by constructing databases in
regard to the respective steels with reference to the working
examples described below, by choosing the condition where all the
predicted brittle fracture values F.sub.c1 to F.sub.c3 do not reach
the critical limits, and by designing the dies. If the dies enjoy
prolonged lives it will contribute to decrease in processing costs
of various forged products through not only reducing the die cost
itself but also saving time and labor for exchanging the worn
dies.
[0028] The method of predicting damages of dies according to the
invention may exhibit the performance to the dies for forming. It
will be, however, applicable to other dies such as those for
die-casting, which are used under similar environment of high
temperature and high stress. Through the prediction of damages of
dies desired properties of die materials may be known as a matter
of course and the indices for developing the die materials can be
obtained. Thus, the invention may contribute to development of
alloy technologies.
EXAMPLES
Example 1
[0029] SKD61, one of the steels for hot processing tools, was used
as the die material and the hardness was adjusted to be HRC 46, 49
or 52. JIS No. 4 tensile test pieces were prepared and some of them
were subjected to machining to provide surrounding V-shaped notches
of depth 50%. The shapes and dimensions of the test pieces are
shown in FIG. 1 (smooth surface), FIG. 2 (notch angle 30.degree.),
FIG. 3 (90.degree.) and FIG. 4 (120.degree.). Curvature of the
bottom of the notches is 0.2 mm.
[0030] The test pieces were subjected to tensile tests to determine
mean normal stress (.sigma..sub.m) at which fracture occurs and at
the same time whether the fracture is ductile fracture or brittle
fracture was recorded. The equivalent stress (.sigma..sub.eq) and
the maximum principal stress (.sigma..sub.1max) were calculated. By
plotting the mean normal stress (.sigma..sub.m) in correspondence
of the equivalent stress (.sigma..sub.eq) there was obtained FIG.
5, by plotting the mean normal stress (.sigma..sub.m) in
correspondence of the maximum principal stress (.sigma..sub.1max),
FIG. 6, and plotting the maximum principal stress
(.sigma..sub.1max) in correspondence of the equivalent stress
(.sigma..sub.eq), FIG. 7, respectively.
[0031] Based on the graphs of FIGS. 5 to 7 the critical values
C.sub.1 to C.sub.3 for the brittle fracture of SKD61 steel were
determined as follows: F.sub.c1=(.sigma..sub.m/.sigma..sub.eq)=0.7
[formula 1] F.sub.c2=(.sigma..sub.m/.sigma..sub.1 max)=0.5 [formula
2] F.sub.c3=(s.sub.1max/s.sub.eq)=1.25 [formula 3]
Example 2
[0032] The prediction of die life according to the present
invention was carried out in regard to a ring-die, a die for
limiting the outer surface of the work forged by a punch and a
counter punch from top and bottom used for hot forging a final
gear, an automobile part, having the cross section shown in FIG. 8.
A used die made of SKD61 steel was inspected and it was found that
crack of the ring-die occurred from the outer surface.
[0033] The following three CG's were obtained by analyzing
distribution of the critical fracture values F.sub.c1 to F.sub.c3
on the basis of the above database by computer simulation (FEM
analysis).
FIG. 9 (former shape) F.sub.c1=(.sigma..sub.m/.sigma..sub.eq)
FIG. 10 (former shape) F.sub.c2=(.sigma..sub.m/.sigma..sub.1
max)
FIG. 11 (former shape) F.sub.c3=(.sigma..sub.1
max/.sigma..sub.eq)
[0034] All the CG's showed that in some parts of outer surface of
the ring-die F.sub.c1 to F.sub.c3 exceed the critical values. The
fact agrees with the results of the above inspection.
[0035] Then, supposing the cases where the outer diameter of the
ring-die is increased to prevent the brittle fracture, the computer
simulation was carried out again. Three CG's as mentioned below
were obtained. The CG's showed that there existed no longer the
parts where the F.sub.c1 to F.sub.c1 exceeded the critical
values.
FIG. 12 (improved shape)
F.sub.c1=(.sigma..sub.m/.sigma..sub.eq)
FIG. 13 (improved shape) F.sub.c2=(.sigma..sub.m/.sigma..sub.1
max)
FIG. 14 (improved shape) F.sub.c3=(.sigma..sub.1
max/.sigma..sub.eq)
* * * * *