innovative induction heating process line for hardening and - ATE ...

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
M. Froehlke (1) - S. Lupi (2) - S. Rettig (3) - M. Schiavon (4)
(1) SWIC, Steel Wire Industry Consulting,
Nordfeldstr. 10, D-66564 Ottweiler – Germany
(2) University of Padua - Dept. of Electrical Engineering
Via Gradenigo 6/a - 35131 - Padova (Italy)
(3) Bühler & Co. GmbH, Walzwerke und Prozesstechnologie
Hanauer Str. 1-5, D-75102 Pforzheim – Germany
(4) ATE, Applicazioni Termo Elettroniche s.r.l.
Via Soastene 4 - 36040 Brendola (VI) – Italy
Abstract. For the cold manufacturing of coil springs with high mechanical
performance it is necessary to utilise steel wire hardened and tempered with high
values of elastic limit load and tensile strength and absolutely repeatable
characteristics in the cross section as well as along the complete length of the wire.
In the last years in-line induction heat-treatment have gradually replaced conventional
furnaces in the process of hardening and tempering steel wire but, generally, with
induction furnaces the heat treatment is not executed during the starting and stopping
phases for the change of coil and wire size. In this way it is necessary to locate and get
rid of the coil ends not treated, giving rise to wire losses and waste of useful
production time.
ATE and Bühler have developed and installed an innovative continuous high capacity
heat-treating line for small steel wires (from 2 to 6 mm), which eliminates the need of
the wire crop end during the acceleration and deceleration speed ramps for the change
of coil and wire size.
1. Introduction
The main advantages of induction heating, in comparison with other systems, are the
high thermal efficiency, the fast heating process and the easy control of the operating
parameters which enables to obtain constant metallurgical characteristics.
All the above factors are the basis for the success of the induction furnaces in the inline
heat treatment of wires for spring manufacturing.
As well-known, the hardening and tempering process of the steel consists of the steps
of austenitizing, quenching and tempering: the first two are characterised by the
formation of austenite and its full transformation to hard martensite, while the third is
performed to restore toughness.
The basis for the application of the induction heating to the hardening and tempering
process lies in the fact that in these heat treatments similar properties can be obtained
using a large variety of time-temperature cycles. This means that conventional longtime
furnace heat treatment cycles can be adequately substituted by short-time,
higher-temperature ones, using induction heating [2,3].
This paper deals with the design, working conditions and metallurgical results of an
innovative installation for the completely continuous process of induction hardening
and tempering spring steel wires with small diameters ranging from 2mm to 6mm.
The main considerations arising in the treatment of such wires of small diameter are
illustrated with reference to the choice of the heating and cooling cycles and the
installation design.
7

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
Installation characteristics
The installation, corresponding to the lay-out and the component description given in
figure 1, has been designed for the continuous in-line hardening and tempering of
steel wires used in the production of springs - mainly for automotive industry - having
the following characteristics:
7
Figure 1 – Installation lay-out and description of components
2
2 / 8

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
Material: high carbon steel, preferred grades: 50 Cr V4, 55Cr Si 7, 60 Si 2 Mn, 65
Mn and similar
Dimensions: cylindrical wires, diameters from 2 to 6mm
Production rate: wire diameter 2÷3.5 mm: max 850 kg/h - wire diameter 4÷6 mm:
max 1400 kg/h with speed line up to 180m/min.
Heat treatment temperatures: hardening 920÷960°C – tempering 600°C max.
The installation is fully automated and all checking, diagnostic, recording and
working functions are controlled by two industrial PC, one for the control of the
movement of wire and the second for the control of the heat treatment process. The
two PC, with their Profibus network, have been interfaced through an insulated
DP/DP Profibus coupler allowing to a single operator to control the installation only
from one supervisor desk. For each wire size a special recipe is memorised in the
supervisor database. During production, the same or different types of wire may be
queued without stopping the line; at the proper time the control system automatically
send specific data to the different units in order to obtain, during the continuous
running, the appropriate wire heat treatment without material scrap. In fact when a
working coil is approaching its end, the “coil change procedure” is automatically
selected, and by means of the “moving accumulator” makes possible to carry out “inline”
welding of the coil ends at reduced speed.
In addition to the “coil change” it is possible also to carry out automatically, at a
reduced speed, the “wire-size change”, in the range of wire diameters allowed by the
post-Curie heating inductors. A laser device detects the new wire diameter, this value
is sent to the control system of the heat treatment section that executes the “recipe
change” at the right time, without stopping the line and without the intervention of the
operator.
Features of process
The main aim of this hardening and tempering process is to achieve an homogeneous
austenite and a well tempered martensite structure in wires of small diameters at high
speed.
Considering the range of wire diameters and the production rate, the pre-Curie and
post-Curie hardening multistage inductors are supplied by a medium frequency
converter rated 250 kW at 10kHz and a high frequency converter rated 300kW at
200kHz respectively.
In the tempering section, in order to reach rapidly a fast and uniform wire heating and
obtain a subsequently isothermal holding, a medium frequency converter rated 250kW
at 10kHz is installed to supply the multistage inductors and a resistance heated
channel furnace, 18m length rated power 45kW, is positioned along the line.
Because of the small wire diameters the isothermal holding at austenite temperature is
not required; only tempering is performed in the two steps, i.e the continuous heating
and isothermal holding.
The complete heat treatment is therefore carried out as follows:
‣ Rapid continuous inductive heating to obtain the transformation of the ferritic –
perlitic - microstructure to stable austenite
‣ Quenching to martensite
‣ Rapid continuous inductive heating to the required tempering temperature
‣ Isothermal holding by passing the wire through a resistance channel furnace for
obtaining a very homogeneous tempered martensite structure
‣ Final cooling
7
3
3 / 8

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
Figure 2 shows the temperature-time cycles for 55 SiCr spring steel wire, Ø 2.23 mm
treated at 120 m/min =2 m/s (solid line) and Ø 3.54 mm at 180 m/min = 3 m/s (dashed
line).
Figure 2 - Temperature – time diagram of the inductive hardening and tempering process of
55SiCr spring steel wire, Ø 2.23 and 3.54 mm
As shown in the diagram for the wire 2.23 mm diameter, the heating cycle is
constituted by the following phases:
1 = Inductive Pre-Curie - heating: (room temperature - 680°C, Ferrite-Perlite
structure)
2 = Inductive Post-Curie - heating: (680 – 950°C, stable Austenite structure)
3 = Polymer quenching: (at 75°C, Martensite structure)
4 = Inductive heating: (480-515°C, tempering temperature)
5 = Isothermal holding: (~ 10 s)
6 = Water cooling: (room temperature)
7
The quenching occurs immediately after the austenitizing heating. The wire moves
through the quenching system, which comprises a series of cylindrical sections that
allow the adjustment of the flow rate and a convenient distribution of the cooling
medium on the wire surface.
The wires of small diameter (∅ < 4.0 mm) had to be quenched in water containing
about 12 % Houghton AQUAQUENCH 641 polymer in order to avoid the brittle
fracture of the wire when passing the quenching system.
In figure 3 the temperature – cooling rate diagram of this quenching agent is given for
a 55 SiCr steel wire ∅ 12.5 mm, in accordance with the standard quenching test
carried out by the polymer manufacturer.
4
4 / 8

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
The maximum quenching speed is approximately 178 [K/s] at 636°C. Since below
500 °C the speed of this quenching agent is reduced, the probability of hardening
cracks is very low.
Temp. [°C]
1000
800
600
400
200
Houghton AQUAQUENCH 641 12 %
55 SiCr
d = 12.5 mm
178 K/s
at 635°C
0
0 50 100 150 200
Cooling rate [K/s]
Temperature [°C]
1000
800
600
400
200
Houghton AQUAQUENCH 641 12 %
55 SiCr
d=12.5 mm
0
0 15 30 45 60
~
Quenching time [s]
Figure 3 - Temperature - cooling rate of water +
12 % Aquaquench 641
Figure 4 - Temperature – quenching time of
water + 12 % Aquaquench 641
In figure 4 the decrease of temperature as a function of the cooling time is shown for
the same kind and concentration of the polymer. Using this concentration, the cooling
parameter λ [seconds * 10 -2 ] i.e. the cooling time from 800 to 500 °C is 0.04.
Wires of 4÷6 mm diameter were quenched in water. In figure 5 the entry of a wire ∅
4.0 mm into the first quenching unit is shown.
7
Figure 5 - Water quenching of ∅ 4.0 mm 55 SiCr wire, hardening temperature 950 °C
5
5 / 8

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
Tensile strength with the line working at rated speed.
In table 1 the tensile strength values of wires tempered inductively at nominal speed
of 120m/min for the 2.23mm diameter and 180m/min for the diameter 3.54 mm are
compared with the limits of the standards and the target of the customer. The results
refer respectively to 15 and 17 samples taken from 2 t coils for each wire diameter.
Table 1 - Values of measured tensile strength Rm, customers target and Standard limits
Tensile strength Rm [MPa]
∅ [mm] Measured Target Standard
N° JIS G 3561 (6)
2.23
1,934 – 1,958 1,960 – 2,010 1,910 ~ 2,060
∆ = 24 ∆ = 50 ∆ = 150
3.54
1,881 – 1,909 1,865 – 1,960 1,860 ~ 2,010
∆ = 28 ∆ = 95 ∆ = 150
Tensile strength during speed ramps
Figure 6 shows the measured tensile strength Rm of a 55SiCr steel wire ∅ 2.23 mm.
Samples have been taken during a test run at intervals of 43 m, with speed ramps
from 40 up to 120 m/min and to 40 m/min again.
[Rm]
2.100
2.050
2.000
1.950
1.900
1.850
1.800
0
max limit
speed
target
min limit
43,35
86,70
130,05
173,40
216,75
260,10
303,45
346,80
390,15
433,50
476,85
520,20
563,55
140
120
100
80
60
40
20
0
[mt/1’]
Rm ( Mpa)
min limit
max limit
target
speed
Figure 6 – Measured tensile strength of 55SiCr steel wire ∅ 2.23 mm during speed ramps
Rm
[mt]
7
6
6 / 8

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
Figure 7 shows the measured tensile strength Rm for the 55SiCr steel wire ∅ 3.54
mm. Samples have been taken during a test run at 20m intervals. The speed ramp
decrease from 180 to 35 m/min.
[Rm]
2.050
2.000
1.950
1.900
1.850
1.800
1.750
0
30
60
90
max limit
AVG
speed
120
150
180
210
240
270
300
330
360
410
200
180
160
140
120
100
80
60
40
20
0
[mt]
[mt/1’]
Rm ( Mpa)
AVG
min limit
max limit
speed
Figure 7 - Tensile strength of 55SiCr steel wire ∅ 3.54 mm during deceleration speed ramp
To obtain that the Rm values lies in the target range both at constant speed and during
the transient phases (acceleration and deceleration) particular care has been given to
the power control in the tempering section, introducing a temperature feedback by an
optical pyrometer and a suitable algorithm which allows for the variation of the
tempering temperature versus the wire speed.
Rm
min limit
Metallurgical results
After an accurate setting of the heat treatment parameters for the whole diameters
range and the different steel grades, a very homogeneous martensite structure, as
shown in figure 8, was obtained.
Figure 8 - Wire ∅ 3.5 mm, 55SiCr, tempered martensite,
no decarburation
7
Although the condition of the drawn wire provided by the customer was not ideal,
practically no defects of the micro-structure, as decarburation or external or internal
cracks caused by the process have been experienced; in particular, for wires of smaller
diameter as 2 or 2,5 mm, it is important to check the superficial characteristics before
the treatment to guarantee metallurgical results completely free of defects.
7
7 / 8

INNOVATIVE INDUCTION HEATING PROCESS LINE
FOR HARDENING AND TEMPERING SPRING STEEL WIRE
Conclusions
The heat-treatment line is installed in Baosteel Ergang - Shanghai and is operating
since the beginning of 2003. The positive results obtained, both regarding the
equipment reliability and the metallurgical characteristics of the finished product, are
the proof of the validity of the innovative solutions adopted.
Especially for small wires the high speed induction hardening and tempering process
needs a very accurate setting of the heat treatment parameters, in particular for the
tempering section, to achieve the specified micro-structure and mechanical properties.
Figure 9 - View of multi-stage inductors of the pre-Curie and post-Curie heating stations.
ATE, Bühler and the authors thank the contract partner Ergang Ltd., the wire manufacturing
division of the Baosteel Group – Shanghai, for the support given during the line start-up.
REFERENCES
7
1. J. REBOUX, B. LAPOSTOLLE, J.C. BRUNE: « Contribution du chauffage par induction
à la modernisation des lignes de traitement thermique dans le tréfileries des fils d’acier »,
Journées d’Etude, Versailles, 5-6 Avril 1978.
2. S.L. SEMIATIN, D.E. STUTZ : « Induction Heat Treatment of Steel », American Society
for Metals, Metals Park, Ohio (USA), 1986
3. G. CREPAZ, F. DUGHIERO, S. LUPI, E. RAMOUS: “Modern installations for the
continuous process induction hardening and tempering of steel bars”, XII UIE Congress,
Montreal (Canada), June 14-18 1992, 130-139.
4. M. FROEHLKE: “Thermo-mechanical continuous heat treatment”, Wireworld (1994),
n.3/94, 18-21.
5. I. ARTUSO, F. DUGHIERO, S. LUPI, S. PARTISANI, P. FACCHINELLI: “Installations
for the continuous induction heat-treatment of wires”, XIII UIE Congress, Birmingham
(UK), 16-20 June 1996, vol.1, n.MII, 35-43.
6. STANDARD N° JIS G 3561 OIL TEMPERED CARBON STEEL VALVE SPRING
QUALITY WIRE (1994)
8
8 / 8