Tool-steel Wire Dravving at Elevated Temperatures
Vlečenje žice iz orodnih jekel pri povišanih temperaturah
B. Arzenšek, B. Šuštaršič, G. Velikajne, Inštitut za kovinske materiale in tehnologije, Ljubljana I. Kos, K. Zalesnik, F. Marolt, Železarna Ravne, Ravne na Koroškem
Tool steels have in cold state very low workability, therefore they must be frequently recrystallization annealed during the cold drawing proeess, but some types of steels cannot be cold dravvn at aH. Their working properties are highly improved at elevated temperatures. This paper analyzes the drawability of wire, made of BRM2 (VV.NR-1.3343) steel, and it deseribes the technology of wire dravving at temperatures up to 700' C.
Orodna jekla imajo v hladnem stanju zelo slabe preoblikovalne sposobnosti, zato jih moramo med hladnim vlečenjem velikokrat rekristalizacijsko zariti, nekatera jekla pa hladnega vlečenja sploh ne prenesejo. Njihove preoblikovalne sposobnosti se precej izboljšajo pri povišanih temperaturah. V delu smo ugotavljali vlečne sposobnosti žice iz jekla BRM2 (VV.NR-1.3343) in opisali tehnologijo vlečenja žice pri temperaturah do 70(f C.
1	Introduction
High-speed tool steel of BRM2 type is mainly used for tools and spiral drills for machining of steels and alloys, and for manufacturing of high-quality wear-resistant tools. Wear resistance of steel is achieved by a great number of line secondary carbides vvhich are in the ferritic matrix of steel. Carbides do not give only a high vvear resistance of steel but they also contribute to an intensive hardening of steel in cold workability. Thus the vvire made of BRM2 steel can sustain in cold dravving almost 20% partial and approximately 45% total reduetion degree, therefore it must be more than six times intermediately recrystallization annealed in dravving from 8 to 2 mm diameter. Such a manufacturing of fine vvire is rather expensive and long lasting.
Drawability of steel is rather improved at elevated temperatures whcn vvire can be dravvn to fine dimensions vvith-out intermediate recrystallization annealing. The first dravving tests at elevated temperatures (vvorm dravving) vvere made vvith shorter lengths of vvires, and it vvas found that vvire can be dravvn at 700°C even to the diameter of 3 mm. Based on results of dravving tests to thin dimension vvires, vvhich vvill be deseribed in details, the technology for vvorm dravving of vvires in coils vvas prepared.
2	Drawability of the steel at elevated temperatures
Dravving tests of thin vvires shovved that drawability of BRM2 steel is optimal at 700°C and 15% reduetion degree. Drawability of the steel vvas determined from num-bers of dravvs vvhich vvire sustained at various temperatures and from yield stresses calculated by follovving mathematical expression:
- JL
q,n ~ A Ai'
vvhere is:
qm	yield stress in N/mm2,
F,	dravving force in N/mni2 and
AAi eross-seetion reduetion in single dravv in mm".
Yield stresses and number of dravvs, vvhich vvire of BRM2 steel sustained at various dravving temperatures, are presented in Fig. 1. The plot shovvs that the vvire sustains
2600
2400
% 2000 E z
~ 1600 cr
S 1200
in
V 800
400 0
(
Log deformation ip----—
i-1_i_l_i_l_l_l
8,28 7,58 6.43 5.46 4.63 3.93 3.35 2,80
Diameter of vvire (mm)-»-
Figure 1. Yield stresses at vvire dravving of BRM2 steel at elevated temperatures.
Slika 1. Preoblikovalne napetosti pri vlečenju žice iz jekla BRM2 pri različnih temperaturah.
the cold-dravving reduetion vvithout recrystallization annealing from 8 to 5.5 mm. It can sustain some more dravvs at temperatures up to 550°C, vvhile at 700°C it can be reduced even to 3 mm. Good workability of the steel can be judged
o 700°C « K X A-Last possible draw □ 600°C of wire					
a 500 C ■ 300°C a 20°C					
	/	/			
/					
		/ \ / _			
■ tt V					
-i Draw of 1			£ = 15 7.		
	wire : 3	5 7	9	11	
0.4 0,8 1,			2 1,	6 2	0 2.2
also by yield stresses which values at 700°C are half of the values like in cold dravving. In hot drawing of the wire the recrystallization annealing is not needed since recovery is achieved by heating wire to the drawing temperature and during the dravving process, and at higher temperatures also recrystallization probably occurs. The described tests were made with 2 to 3 m long vvire pieees on drawing bench with a dravving velocity of 0.25 m/s. Drawing conditions on a dravving bench are less demanding than in coil dravving. Therefore vvire in coil dravving sustains less dravvs.
3 Technologv of vvire dravving at elevated temperatures
The aim in preparing the technology of vvire dravving at elevated temperatures was to apply the existent equipment for cold dravving to the maximal extent. This on one side reduces the developing costs of the technology, and on the other side it enables cheaper and simpler transmission of the technology into industrial practice. The scheme of thus designed and also tested line is given in Fig. 2, while Fig. 3 shovvs the picture of it.
Heot nq	StraigM en ng	; t ■ . i ■■g
Figure 2. Scheme of vvire-dravving line at elevated temperatures. Slika 2. Shema linije za vlečenje žice pri povišanih temperaturah.
Figure 3. Picture of vvire-dravving line at elevated temperatures, built
at the Institute of Metals and Technologies in Ljubljana. Slika 3. Izgled linije za vlečenje žice pri povišanih temperaturah, ki je postavljena na Inštitutu za kovinske materiale in tehnologije v Ljubljani.
The presented line consists of three basic units:
•	equipment for conductive heating of vvire,
•	equipment for temperature measurements on vvire during the dravving and
•	dravving machine.
In front of the heating equipment also frame for uncoil-ing and straightening rollers were mounted In the vvhole installation only the conductive heating equipment for vvire was new. Wire is heated by three pairs of contact rolls. The basic characteristics of the three mentioned units are:
3.1	Heating Ec/uipment
This equipment vvas purchased at Montanstahl, Svvitzerland. Its power is 90 kVA, and this vvas chosen according to the greatest desired diameter 8 mm of heated vvire, the high-est vvire temperature 800° C, and the dravving velocity 0.28 m/s. Heating of vvire is a two-stage process. In the interval betvveen the first and second pair of rolls the vvire can be heated at most up to 500°C, and betvveen the second and the third pair the temperature is raised further to 800° C. Heating povver is adjusted manually vvhile the equipment contains also drivver vvhich prevent overheating of vvire in the čase of stoppages during the dravving process.
3.2	Equipment for Measuring Wire Temperature
Wire temperature is measured with optical pyrometer. Measuring system is not connected vvith the heating system, therefore the heating povver is regulated manually according to the registrated temperature.
3.3	Draw'ing machine
Dravving machine is designed for cold dravving of vvires thinner than 8 mm. Therefore the dravving drum is con-structed in such a way that dravvn vvire in cold dravving slips uniformly along the drum. Dravving conditions are during the cold dravving rather unchanged. In vvorm dravving the conditions are changing according to the temperature of vvorm dravving, vvire diameter, and lubrication. In the first vvorm-dravving tests vvith the described equipment, slippage of vvire on the dravving drum in the upvvard direction oc-cured, vvhile overlapping of vvire vvindings vvas experienced vvith thinner vvire. Both phenomena made dravving impossi-ble. The described troubles vvere solved by mounting spe-cial guides for dravvn vvire vvhich vvere fixed around the dravving drum. The mentioned guides can be dismounted from the drum after vvorm dravving so that the same dravving machine can be used also for cold dravving of vvire.
Efficiency of vvorm dravving of vvire does not depend only on the sliding of vvire along the dravving drum but also on the preparation of vvire surface before dravving, on the quality of lubrication, and thus also on the dravving temperature and the vvear resistance of dies.
3.4	Preparation ofWire Surface
Efficiency of conductive or contact heating depends a great deal on the quality of contacts betvveen the pair of heating rolls and the heating vvire. In bad contacts sparking, incipient melting and quenching of heated vvire on contact area can occur. Such an area does not sustain deformation in dravving, and vvire breaks. Sparking can occur on badly descaled or rusted areas of vvire, therefore the vvire surface must be vvell prepared before dravving. According to the experiences obtained so far, the sandblasted vvire surface is the most suitable one especially if it is also copperized. Sandblasted surface is rougher than a pickled one, and thus adhesion of lubricant on the vvire surface before dravving is better. Copperizing on the other hand prevents rusting of vvire after the dravving.
A oter
cooling
Tempe ra t jre measuremen t
3.5	Lubrication
In worm drawing lubrication is less effective than in cold drawing, therefore the wire cannot stand higher partial de-fomiations degree than 15%. Graphite was used as lubricant since it was in the regard to persistance and priče the only acceptable lubricant which could stand drawing also at temperatures above 500° C. Graphite has good lubrication properties but its adherence to the wire surface is unfortunately weak, therefore it was applied in form of oil paste. Paste has good lubrication power up to 650°C, at higher temperatures its lubrication power is rather reduced due to reduced adherence to the wire surface. Ground and flaky graphite was tested too. A little better lubrication was achieved with flaky graphite but it is rather more expensive. In hot dravv-ing the greatest troubles are caused in lubrication. This problem is not suitably solved, therefore some manufactur-erers of fine sections of special steel have substituted hot drawing with rather more expensive microrolling process, vvhere ali the problems with lubrication are avoided.
3.6	Wear Resistance of Dies
In dravving hard-metal dies are used so far, but they have good wear resistance only up to 300 or 400° C. At higher temperatures their wear resistance is rather reduced, therefore also ceramic dies, made of silicon carbide, were tested. The mentioned ceramic material has a very good wear resistance even up to 1000°C, but its disadvantages is a low resistance to temperature shocks and its brittleness. In cold and hot drawing tests it was found that ceramic materials stood the dravving process therefore the tests of dravving through ceramic dies vvill be continued.
In developing the technology of hot dravving of vvire a great attention was given also to the yield of dravvn vvire vvhich is lovver than in cold dravving. In hot dravving the beginning and the end of vvire coil must be dravvn cold. Since BRM2 vvire sustains only tvvo coil dravvs the cold dravvn ends should be cut off. Thus the yield of dravvn vvire is reduced. A trial vvas made to avoid cutting-off the vvire ends by vvelding another material vvith good workability on the vvire end. The mentioned solution proved as unsuitable due to too long needed times of soft annealing the vveld. Therefore the problem vvas solved by annealing the cold
dravvn vvire ends. For this purpose a special conductive equipntent for vvire-end annealing vvas built vvhich enabled annealing of 5 m lengths and thus the yield of BRM2 vvire in hot dravving vvas increased to 100% nearly.
4	Conclusion
Dravving of vvire at elevated temperatures rather differs from the cold dravving, therefore many problems appeared in de-velopment the dravving technology at elevated temperatures, vvhich vvere more or less successfully solved to such an ex-tent that already industrial dravving line for vvire coils exists. So far over 1500 kg vvire of BRM2 steel vvas dravvn from 8 mm to various finer dimensions. The finest diameter of dravvn vvire vvas 3,2 mm. The line is improved to such an extent that a large-scale production is negotiated vvith the Ravne Iron and Steelvvorks as the tool-steel producer. Dou-ble applicability of dravving equipment, i.e. for cold and hot dravving, the developed technology is much cheaper than the competitive microrolling, and it is suitable mainly for smaller manufacturers of various tool steels.
5	References
J B. Arzenšek, B. Šuštaršič, I. Kos, K. Zalesnik, F. Marolt, G. Velikajne: Tehnologija vlečenja jekla BRM2 pri povišanih temperaturah, Poročila inštituta za kovinske materiale in tehnologije v Ljubljani, Nal. št. 91-035, Ljubljana, 1992
2	K.D. Maraite: Ein Beitrag zur Optimierung des Halbvvar-mziehens, Stahl und Eisen, Umformtechnische Schriften -Band 13, 1988, Verlag Stahleisen, Dusseldorf
3	H. Tzscheutschler: Untersuchungen der Einsatzmoglich-keiten des Halbvvarmziehens, Doktor Dissertation, Aachen, 1982
4	B. Arzenšek, I. Kos, A. Godec: Vlečenje žice iz orodnega jekla Č.7680 - II. del. Poročila Metalurškega inštituta v Ljubljani, nal. št. 85-063, Ljubljana, 1985
5	B. Arzenšek. I. Kos, A. Godec: Vlečenje jekla Č.7680 prt povišanih temperaturah, Poročila Metalurškega inštituta v Ljubljani, Nal. št. 86-037, Ljubljana. 1986
b I. Kos, B. Šuštaršič, B. Arzenšek, V. Leskovšek: Razvoj tehnologije vlečenja pri povišanih temperaturah - II. del, Poročila Metalurškega inštituta v Ljubljani, nal. št. 80-018, Ljubljana. 1990