On some aspects of dynamic recrystallization in several manufacturing processes

On some aspects of dynamic recrystallization in several

manufacturing processes

Citation for published version (APA):

Dautzenberg, J. H., Dijck, van, J. A. B., & van der Wolf, A. C. H. (1979). On some aspects of dynamic

recrystallization in several manufacturing processes. (TH Eindhoven. Afd. Werktuigbouwkunde, Laboratorium voor mechanische technologie en werkplaatstechniek : WT rapporten; Vol. WT0459). Technische Hogeschool Eindhoven.

Document status and date: Published: 01/01/1979

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by J.H. Dautzenberg and J.A.B. van Dijckt

submitted by A.C.H. van der Wolf

"Eindhoven University Pressl l _{PT-report nr. PTO-459}

-1-ON SOME ASPECTS OF DYNAMIC RECRYSTALLIZATI-1-ON IN SEVERAL MANUFACTURING PROCESSES

by J.H. Dautzenberg and J.A.B. van Dijck, submitted by A.C.H. van der Wolf,

UNIVERSITY OF TECHNOLOGY, EINDHOVEN, THE NETHERLANDS.

In some processes like dry sl iding wear, grinding and cutting, thespecific

energy (=E ) is relatively high in comparison with for example deep

sp

drawing (table 1).

Assuming that all the energy is necessary for plastic deformation it holds

....

JadE

=

E _sp

with: a

=

effective stress

d£

=

increment of effective strain

Transforming equation (1) with the Nadai relation:

-n

a

=

c£

with: c

=

material constant

n

=

strain hardening exponent

results in:

n+1)

£

=

Table 1 shows the calculated values of

E

for the various processes.

(1)

(2)

For measuring these deformations the usual method of gratings is impossible. So they use the grain thickness before and after the process to determine

the deformation [1]. That is only permitted if the material does not

re-crystallize during the process. It is known that in case of materials with low stocking fault energy [2] (for instance copper, nickel, steel at high temperatures) it is possible that the material during the deformation at

high homologous temperature (= a fraction of the absolute temperature of

This process is known as dynamic recrystallization.

This type of process has in contrary with the weI 1 known static recrystal,J ization f1 ,41 the following characteristic qual ities:

, It has no incubation time

. It starts exclusive during the deformation process at a definite value

of the effective stress

0

dependant of strain rate and temperature

• It can give much smaller grains than in the static case.

In constant rate tensile tests, this dynamical recrystallization brings about

a stress-strain curve with a characteristic shape (figure 1). The process

starts after some critical

O.

In the case of high strain rates it gives a

strain hardening to a peak stress followed by work softening to a steady state level. The work softening is caused by the beginning of dynamic

re-crystallization and the stable flow from the continuous dynamic recrystallizatior At low strain rates the flow curve is periodic due to recurrent cycles of

re-crystallization. For a range of testing conditions the critical 0 is uniquely

defined by the Zener-Hollomon parameter Z [4]:

.

Z ==

E

exp (Q/RT)

Q

=

activation energy for dynamic recrystall ization

£

=

strain rate

R

=

8,2 J °K- 1

T

=

OK

In a double logarithmic plot the critical

0

increases I inearly with Z at

low and maximum

Z

values. At high

Z

values a deviation towards a weaker stress

dependence is normally observed [4],

The size of dynamically recrystal I ized grains (= d) increases monotoniously

with decreasing critical

0

and can be described by the phenomenological

relationship:

(j :: a + Ad-m

o (5)

where a • A and m are empirical constants with m having values in the range

o

of 0.5-0.8.

The physical processes underlying dynamic recrystallization are rather poorly

understood [5],

In the processes like grinding dry sliding wear and cutting (in the second shear zone) one would expect very long shaped grains (pure shear). For that

-3-d » -3-d o

with d _o

=

initial grain thickness

d

=

grain thickness after the deformation test.

(6)

In al I these processes one finds very fine equiaxial crystals, that means recrystall ized crystals with very different orientations [6].

Figure 2 shows an electron micrograph of a thin foil of copper material which has undergone sliding friction with the tool material during cutting. This figure shows cl~arly elongated grains with locally dynamic recrystallized grains.

Figure 3 is the electron diffraction pattern of another part of the same foil representing fully dynamic recrystallized material. It confirms that the crystals are very small.

Figure 4 shows a photo micrograph of the undeformed copper material (attention please for the different magnifications in figure 2 and 4).

The very small grains are the cause of the high hardness and the too low value of the effective strain by grain thickness measurements of materials undergoing processes I ike grinding. dry sl iding wear and cutting (second shear zone). The technical appl ications of dynamic recrystallization have to be found in particular in grain refinement.

N

.

e

e

z Ib 9 £ ' 0.40 sec •

--t

"0.06'5 sec I (: . 0.0011 ,~c ' - TRUE STRAIN t

Figure 1. The influence of dynamic recrystall ization on the stress-strain curves by different strain rates of a plain 0.25% C steel in the austenitic state at 1100

°c

[3].

Figure 2. Electron micrograph of a thin foil of copper deformed by sliding across the tool material by cutting (cutting velocity 0.05 m/s;

cut width 4 mm; cut depth 0.1 mm; cool ing: cutting oil; magnification 32.000 x).

-5-Figure 3. Selected area electron diffraction pattern of another part of

2

the same foil as figure 2 (area approximately 3 ~m ). Acceleration voltage 100 kV.

Figure 4. Photo micrograph of the structure of undeformed copper. Magnification 230 x.

Table 1. Different methods of machining with their different characteristic va 1 ues

[71.

Process Specific energy Involved volumen EffeCtive strain

i

J

fmm3

j

t

mm3

j

-

£

Cutting" 0.5-3 1-2.104 'V 1

Grinding 40-60 0.1-100 'V 20

Dry s I j ding wear > 200 0.1-100 'V 400

Forming 0.2-1.5 1-10

9

'V 0.3

Electro discharge

machining 200-1000 0.01-10

-*

The raported effective strain applies for the primary shear zone. In the second shear zone the strain is much higher.

-7-Li terature

1. J.H. Dautzenberg and J.H. Zaat, Wear 23,9. (1973).

2. S. F 1 ugge ,

Handbuch der Physik Bd VII/2 Springer (1958).

3.

H.J. McQueen and J.J. Jonas,

Plastic deformation of materials (edited by R.J. Arsenault) Academic Press New York (1975).

4. W. Roberts and B. Ahlblom, Metal1urgica 26, 801, (1978).

5.

G. Gottstein, D. Zabardjadi and H. Mecking, Metal Science 223 (1979).

6. J.H. Dautzenberg, Wear, to be published.

7. J.A.W. Hijink,

Lecture script Ve 20