Geometrical and mechanical properties of warm-extruded components

Journal of Mechanical Working Technology, 2 (1978) 205--215

205

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

GEOMETRICAL AND MECHANICAL PROPERTIES OF WARM-EXTRUDED COMPONENTS*

U. DIETHER

Institute for Metalforming, University of Stuttgart (W. Germany) (Received March 16, 1978)

Industrial Summary Warm extrusion of steel is a relatively new technique which in recent years has gained increasing importance. This technique aims at combining the advantages of cold extrusion (high accuracy and surface quality) with those of hot extrusion (low load requirements), while possibly avoiding the disadvantages of both techniques. When employing warm extrusion, a good knowledge of the expected properties of the extruded components is of importance. An investigation was undertaken therefore, to establish the effect of working temperature and reduction in area on the geometrical and mechanical properties of warm-extruded components. Results of this investigation, relating to the forward extrusion of rod in the temperature range of from 773 K (500°C) to 1073 K (800°C), are presented for various case-hardening and heat-treatable steels. It has been found that in particular the ductility, the impact value and also the strength can be improved remarkably with warm extrusion. Under certain working conditions the properties of warm-extruded components are such that heat treatment after extrusion can be avoided.

1. Introduction Cold extrusion of steel is limited by the admissible stress of the tools and by the formability of the material. It is well k n o w n that, apart from the range of blue brittleness and transformation temperature, the flow stress of the material decreases with increasing temperature whereas the formability increases. Steels t h a t cannot be formed by cold extrusion are mainly formed, therefore, by hot forging/hot extrusion. Compared with cold extrusion, hot extrusion has the disadvantage of lower accuracy and lower surface quality, due to scale, shrinkage, and distortion. Procedures are explored, therefore, t h a t permit forming in the range between room temperature and hot-forging temperature, thus making use of the advantages of b o t h cold and h o t extrusion, while possibly avoiding the disadvantages of both techniques. In the warm extrusion of steel -- which is a relatively new procedure -- so *Paper presented at the International Conference on W a r m Working, Sunderland, U.K., September 11--12, 1978.

206

far only a few practical experiences have been gathered. Apart from presenting a survey of today's knowledge and techniques of warm working, Ref. 1 also includes some mechanical (hardness), and geometrical properties of warmextruded components. Data concerning the influence of extrusion temperature on accuracy and surface quality were published in Refs. 2 and 3. The effects of temperature and reduction in area on the mechanical properties and on the microstructure of components produced by warm extrusion are described in Refs. 4--6. It can be said that at the present time only a little detailed knowledge is available, and that this rather limited information does not permit a prediction of the properties of warm-extruded components. An investigation was undertaken, therefore, to establish the properties of warm-extruded components; some of the results from this investigation are presented hereafter. 2. Test conditions

The properties of components that had been produced by forward extrusion of rod were determined. The reduction in area e A = (Ao - A , )]Ao w a s in the zone 0.6 ~< e A ~ 0.85. All the components produced had a shank diameter of 15 mm, and thus specimens taken from the shank to establish the mechanical properties were of uniform dimensions. The die cone angle was 2a = 90 °. Tests were carried out on a mechanical crank press, where at the beginning of the process the ram speed was approximately 250--300 mm/s. Lubrication was effected by coating with colloidal graphite dispersed in water. Temperatures reported are always those of the slugs in the furnace. 3. Properties o f the extruded workpieces 3.1 M a c r o g e o m e t r i c a l p r o p e r t i e s

As a rule, the diameter of the tool and the diameter of the workpiece do not correspond. This results mainly from two effects: Apart from elastic deformation -- which also occurs in cold extrusion -- in warm extrusion the contraction of the c o m p o n e n t during cooling becomes effective. (i) Due to the decreasing radial stress the expansion of the tool decreases with rising temperature. (ii) Furthermore, in the forward extrusion of rod, the shrinkage of the components -- which increases when cooling from a higher extrusion temperature -- results in a reduction of the shank diameter. Thus with rising temperature the two influences reinforce each other in their effects on the shank diameter of warm-extruded components. Figure 1 demonstrates the dependence of the shank diameter on the slug temperature and on the reduction in area, for material 20 MnCr 5. The absolute values o f the shank diameter d are not plotted, but instead the value related to the diameter of the die dwz , i.e. (d - d w z ) / d w z .

207

I\__!

Z

•, . \

£A =0.7

,0:,

0

diometer of the die: dwz =14.93

-4

dwz = 14.92

dwz =14.92

__~r

I/ I J4 ,~-"--

-6 ~ " " ~ ~

, .

o773Kiso~l_

/r--

~ 873 K (50O~C)

~ '.~--.J.,.._.,,~/ _, ~

•

wa~ITso%) lo73~i,oo'cl-

-lO moteriob ZOMn Er 5

L

-lz 0

ZO

40 mm

60

ZO

z,D mm 60

0

ZO

40 mm 60

oxioI direction z

Fig. 1. Dimensional accuracy after forward extrusion of rod. Material: 20 MnCr 5.

The values of the shank diameter, taken after extrusion at 293 K (i.e. after cold extrusion) were above those of the tool diameter, over the entire shank length. In warm-extruded components, however, the contraction of the c o m p o n e n t due to cooling dominates, i.e. the shank diameter is smaller than the tool diameter. The shank diameter d does not remain constant along the length of the component, but the qualitative variation of the diameter with the shank length z is identical in cold- and warm-extruded components. The largest diameter occurs at the free end of the shank -- which is extruded at the beginning of the process -- and can be explained by the m a x i m u m extrusion load occurring at the beginning of the process. The minimum values appear as a rule in the central zone of the shank, where the highest increase of temperature can be observed. The diameters established at the rear end of the shank are slightly greater, which results from the increased heat transfer to the relatively cooler undeformed part of the component, and also to the tool, due to the longer-duration contact at b o t t o m dead centre. At a slug temperature of 1023 K (750°C) the tendency to a decreasing shank diameter with increasing temperature is interrupted. When exceeding this temperature the transformation of the body-centred cubic a-iron into the facecentered cubic 7-form will take place, which transformation is linked with a change of volume (reduction of volume) due to the closer package of 7-iron.

208

3.2 Mechanical properties 3.2.1 Distribution of hardness The material investigated was 20 MnCr 5, which can be extruded at room temperature, and the initial hardness of which in the spheroidized condition can be obtained from Table 1. Employing longitudinally-bisected, forwardextruded components, the hardness was determined in the axial and the lateral directions (Fig. 2). The following distribution of hardness along the axis of the c o m p o n e n t was established: in the undeformed part before the shoulder there is already a considerable increase of hardness. The hardness at the shoulder end of the shank is about equal to the values recorded over most of the length of the shank. Worth noting is the relatively unimportant difference of hardness between the cold-extruded c o m p o n e n t and the component extruded at a slug temperature of 773 K (500°C). The greatest increase in hardness appears in the surface layers of the shank (right-hand side of Fig. 2). This pattern applies at all temperatures and can be observed in the entire shank area, and also in the shoulder and head-piece.

TABLE 1 Heat t r e a t m e n t , c o m p o s i t i o n a n d m e c h a n i c a l p r o p e r t i e s in t h e initial state, o f t h e materials tested Material

20 MnCr 5 15 CrNi 6

Cf 53

42 CrMo 4

Heat t r e a t m e n t : spheroidising 9 7 3 K / 1 5 h / c o o l i n g in f u r n a c e t o 8 7 3 K t h e n air cooling Material characteristics in t h e initial s t a t e a f t e r spheroidising I m p a c t value a K ( J / c m 2) H a r d n e s s H V 10 Tensile s t r e n g t h R m ( N / m m 2) P r o o f stress Rp0.: ( N / m m :) U n i f o r m e l o n g a t i o n b e f o r e n e c k i n g Ag (%) E l o n g a t i o n at f r a c t u r e As (%) R e d u c t i o n o f area at f r a c t u r e Z (%)

165 149 516 300 24 41 65

185 145 576 291 22 38 65

43 178 631 320 22 33 46

68 173 645 354 22 38 57

0.20 0.31 1.10 0.21 0.33 1.0

0.14 0.24 0.48 0.11 0.23 1.46 1.49

0.54 0.27 0.81 0.028 0.043

0.43 0.28 0.75 0.30 0.31 1.1

C o m p o s i t i o n (%) C Si Mn P S Cr Ni Mo A1

0.23 0.36

209

Z80

Z6O

_.--.,//

240

/

2Z0

measurement at z: 25 m m 200

180 Ar slug temperature: o 293K ( RT )

160

140

A 773 K o 873 K * 973 K '~I073 K

•

...%.

o?

A0

moterial:2OMnCrS

(500'C) (600°C). {70O°C) (800"C)

initial hardness : HV10 =149

d~ =15mm

--I -10

O

10

ZO

30

axial direction z

40

SO mm 60

5

EA: A°.--A.-~-LA=0 6

I 4

~ 2

A0 I 0

I Z

4

6

r odius r

Fig. 2. D i s t r i b u t i o n o f h a r d n e s s a f t e r f o r w a r d e x t r u s i o n o f rod. Material: 20 M n C r 5.

After cold extrusion, the hardness of the c o m p o n e n t is approximately 1.75 times its initial value, whilst after extrusion at a slug temperature of 773 K (500°C) the factor is 1.6, and after extrusion at a slug temperature of 1073 K (800°C) the factor is still approximately 1.2.

3.2.2 Material characteristics obtained in tensile tests The test pieces for the tensile tests, of 6 mm diameter, were taken from the shank of the warm-extruded components. The following characteristics were determined in the tensile tests: tensile strength Rm, p r o o f stress Rp0.2, uniform elongation before necking Ag, elongation at fracture As, and reduction of area at fracture Z. The characteristics of the material in the initial condition are included in Table 1. Figure 3 demonstrates the change of these characteristics due to warm extrusion, for material 42 CrMo 4. The tensile strength R m decreases with increasing slug temperature, reaches a minimum at 1023 K (750°C), and above this temperature continues to increase. The p r o o f stress Rp0.2 and the tensile strength Rm are very close for a slug temperature of 773 K (500°C), b u t with increasing slug temperature, however, these values b e c o m e distinctly different. As a result, with increasing temperature the ratio of p r o o f stress to tensile strength Rpo.2/Rm decreases, and above 1023 K (750°C) drops

210

1200 N

EA =0.6

1000

materic

I 42 Cr Mo/.

EA= 0.7

./

Rm

Rm

RPO'z°'~

~

000

\

600

's ~ o

f::~:l' _ _ I l ~ :!~-~ tensile test

I

'%.

specimen B6 ~30

gA = Ao-A1 AO dl =lSmm

0 u',

60 c

..= ~o

J

40

.'-2_

',o

o

0.26 c~

0.10 ac

0.10

A5 o"' A g , n o.~

0~

700

0

(a)

E

800 900 slug lemperalure l R

J 1000 K 1100 0

700

000 900 slug temperature T

'-

EA =0.85

EA =0.8

N

-=m-~m c~

0 ~o

1000 K 1100

c

Rm

%.

materio

4Z Cr Mo 4 I

N.

£x:

R,~ o.. Rp0,z o,,.,

A~

v~

gprobe B6.30 ¢o

d1 : 15 mm I

3:

60----Z

1"-4 =/I

==

~#40 0.26

~

z0

O.lO A5

0

(b)

A

Ag ' n°"" 700 800 900 slug temperature

AO, n°~ I 1000 K 1100

Te

700

0.~0 .~

800 900 1000 K 1100 s[ug temperature Te

Fig. 3. M e c h a n i c a l p r o p e r t i e s o f w a r m - e x t r u d e d c o m p o n e n t s . M a t e r i a l : 4 2 C r M o 4. (a) F o r s t r a i n s o f e A = 0 . 6 a n d 0 . 7 , ( b ) e A = 0 . 8 a n d 0 . 8 5 .

211

1.0 0.8 0.0

moterinl: 15 CrNi 6

___

moteriQl: 42 CrMa 4

0.4 - - -

? 700 o

800 900 1000 K 1100 slug temperature TR

1.0

%

O2

tensile test specimen B 6 x3O

÷-

0.6 m a t e r i a l : 20

MnCr5 o EA=0.6 ~' CA : 0.7 • EA :0.8 • EA = 0,85

0/,

0

EA= Ao-AI Ao di:15mm

700 BOO go0 1000 K 1100 slug temperature TR

Fig. 4. R a t i o o f p r o o f stress t o tensile s t r e n g t h f o r w a r m - e x t r u d e d c o m p o n e n t s . Materials: 15 CrNi 6, 20 M n C r 5 a n d 42 CrMo 4.

rapidly. Figure 4 demonstrates t h a t materials 20 MnCr 5, and 15 CrNi 6 show a similar behaviour. The quantities characterising ductility -- uniform elongation before necking Ag, elongation at fracture As, and reduction of area at fracture Z - also increase with rising temperature. (Bottom part of Figs. 3 (a} and (b).) Here too there is an exception in the range of slug temperature above 1023 K (750°C). In this range the extrusion temperature reaches the transformation temperature and there is a distinct decrease in the values of the quantities characterising ductility. The effect of the reduction in area e A on the values from the tensile test is unimportant. (The region investigated is 0.6 ~< eA ~ 0.85). Figures 5(a}, (b) and (c) show t h a t the same can be said for the casehardening steels 20 MnCr 5 and 15 CrNi 6, and also for the unalloyed heattreatable steel Cf 53. 3.2.3 I m p a c t value a K

The impact value a K was determined from DVM-samples, taken from the shank of the extruded component. Prior to extrusion the materials were annealed. The initial impact values are contained within Table 1. Figures 6(a) and (b) show the alterations of the impact value resulting from warm extrusion, for materials 20 MnCr 5, 15 CrNi 6, Cf 53, and 42 CrMo 4. The m a x i m u m of the impact value figures for material 20 MnCr 5 is always found in the components t h a t were extruded at a slug temperature of 1073 K (800°C). For the other three materials 15 CrNi 6, Cf 53, and 42 CrMo 4, there

212

1000

o

I

£A=O.O

N

~

BOO

~

600

I OMn Cr5

moter~

refit2

E:A=0.8

Rm

Rm

Rp0.z o.

~

R~o.~X,~ " ~ - ~

~'~e~.._..._~

ea

EA = 400

~

I ~

Ao

d I :15mm I

!':

al

I

%

60

.o"" ,.-o""-

o - 7

Z

.p,

Y

"<~° 40

i::-

~o

A5

0 _A

(a)

I

~ E mm B00

•"~

As

700

AQ,n

._A ......

'

BOO g00 1000 K slug temperature l R

1100 0

700

o-,.--

026 o

o--- ~

0.18 ,~ 0.10

, I

i 800 gO0 1000 K slug iemperature lR

0 c 1100 ~

EA= 0.8

EA=O,6

N

=

_c,"

.o,.,,.-n

o..-

1000

N

1o---o

.o~.- .o

material: 15 Cr Ni 6 Rm

Rm

Rpo.z ~,:

600

EA Ao_A'~"o,, d I = 15 mm

400

~ .

i

e,nsi!e test specimen 00x30 i

_&

g0 % ~0

Z

Z

o---

or

.~ 40 g~

A, ~o.~ - ' ~

o',-20

OLA (b)

I

Ag,n 700

800

900

slug temperature T R

1000 K llO0

0

•

_.

"X

io.~0

ALo~--'1~~lo.to

700

800

900

slug temperature lR

1000 K

.E

~ 1oi

1100

213 1200 N

-:

I000

materia C153

EA =0,6

£A =0.8

Rm o,,,

.~_ 8O0

Rp 0.2 cx,,.

~

;+

Rm o.,. '°--~o...o/o

600 - - - -

+"01i,e ,esl

"~ 4oo

I ; A = ~ d 1 = 15 mm

]

6O

/o

c

/

z

0.26 io..-.-..o

As

o-- ~

-n"'~°" -°'-'-°

~

-o-----o

ko, n o-. ~ . . - o - -

0~ (c)

Ag, n ° " 700

~ r

800 900 lOgO K slug temper0ture TR

0.I0

._k 1100 0

700

000

0.18 ,~'

900

1000

0'~ K 1100

slug temperature T e

Fig. 5. M e c h a n i c a l p r o p e r t i e s o f w a r m - e x t r u d e d c o m p o n e n t s . Material: (a) 2 0 M n C r 5, ( b ) 15 CrNi 6, (c) Cf 53.

occurs, for a slug temperature exceeding 1023 K (750°C), a material
Some geometrical and mechanical properties of components after forward extrusion of rod in the temperature range of from 773 K (500°C) to 1073 K (800°C) were determined for various case-hardening and heat-treatable steels. Apart from the elastic expansion of the c o m p o n e n t under load, the contraction of the c o m p o n e n t during cooling particularly influences the macrogeometrical properties of extruded components. The values characteristic of ductility recorded in tensile tests were found to increase with rising temperature, whereas the tensile strength decreases.

214

3O0

r I °o,.r,o,:,oMocr5

]

T

[

F-q---I j__j

250

o >

"~ 150 o

100

5O 0

(a)

slug temperature T R

ZSO

I

slug lemperoture 1 R

I

I

material: Cf 53 ZOO

AtOll ~

150

dI :15 mm OVM-somple I

CA : 0.85

&8 I00 CA= 0.85

&g

50 3.0 ~ 0

(b)

L 0

700

800

I

material: 42 Cr Mo 4

gO0

u~

1000 K 1100 0

slug temperature T R

•- j / ~ ~J 7"J 1

I

700

800

gO0

1000 K 1100

slug lemperature l R

Fig. 6. Impact values for warm-extruded components. Materials: (a) 20 MnCr 5 and 15 CrNi 6, (b) Cf 53 and 42 CrMo 4.

This tendency is interrupted at a slug temperature of 1023 K (750 ° C) and, dependent upon the material, can be quite distinct. The dependence of the deformation values and the values characteristic of strength upon the reduction in area is unimportant. The increase of the impact value with rising temperature is interrupted at a slug temperature of 1023 K (750°C), except in the case of material 20 MnCr 5. There exists some influence of the reduction of area on the range of the impact value. In many cases the properties after warm extrusion are such that heat treat-

215

ment after extrusion can be avoided; for practical application of warm extrusion the following can then be recommended: (i) If good impact values, and high values characteristic of ductility -- such as elongation at fracture and reduction of area at fracture -- are of importance, then the most favourable range of slug temperature is b e t w e e n 973 K (700°C) and 1023 K (750°C). (ii) If higher values characteristic of strength (such as higher tensile strength) are required, extrusion should take place at a lower slug temperature, in the range b e t w e e n 773 K (500°C) and 873 K (600°C). (iii) In all events, a slug temperature of 1023 K (750°C) should not be exceeded, otherwise deterioration of the values characteristic of ductility, and particularly, of the impact value, is to be expected. Furthermore the required extrusion load -- and with this the stress of the tools -- increases above 1023 K (750°C).

References 1 R. Geiger, E. Dannenrnann and J. Stefanakis, Untersuchungen zurn Halbwarrnfliesspressen yon Stahl, Berichte aus dern Institut fiirUmformtechnik, Universit~itStuttgart, No. 41, Girardet, Essen, 1976. 2 H. Lindner, Massivumforrnen yon Stahl zwischen 600 und 900°C "Halbwarrnschrnieden". Fortschr.-Ber. VDI-Z., Vol. 2, No. 7, VDI-Verlag, D~sseldorf, 1966. 3 H. Grotz, Warrnfliesspressen yon Stahl, Dr.-Ing. Dissertation,Technische Hochschule, Hannover, 1966. 4 E. Schlowag and W. P6hlrnann, Untersuchungen zurn Halbwarrnriickw~/rtsfliesspressen von Stahl, Der Maschinenbau, 18 (1969) 385. 5 K. Yuasa and Y. Murata, Mechanical properties and microstructure of warm-extruded carbon steels,Metall. Met. Form., 41 (1974) 290. 6 V.I. Dorosko, V.M. Lescinskij and A.A. Andrjuscuk, Untersuchungen der rnechanischen Eigenschaften yon unlegierten und niedriglegiertenSt~/hlen nach dern Halbwarrnfliesspressen, Metalloved. Techn. Obrab. Met., Moskva, (2) (1976) 57 (in Russian).