UDC 621.783.28:621.78.061

USE OF VACUUM FURNACES IN HEAT TREATMENT

J. Oleinik1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 36 – 40, December, 2004.

Vacuum furnaces with a cylindrical chamber, with a rectangular-cross-section chamber, and special-purpose

furnaces produced by SECO�WARWICK are considered. Examples of the use of vacuum furnaces for harden-

ing, tempering, and carburizing are given. Prospects for improvement of the furnaces are considered.

INTRODUCTION

Furnaces of various designs are used in many industries

for heat treatment of parts. The SECO�WARWICK Com-

pany produces vacuum furnaces (horizontal and vertical) for

tool, aircraft, power, electronic, and other industries and for

centers engaged in heat treatment. These vacuum furnaces

are highly universal because they make it possible to perform

hardening of high-, medium-, and low-alloy steels on one

hand and ensure uniform and rapid heating and cooling on

the other hand. They are also characterized by optimum out-

put and efficiency.

Standard products of SECO�WARWICK are furnaces

with cylindrical heating chamber, but the Company also has

experience in manufacturing furnaces with chambers of rect-

angular cross section. The aim of the present paper consists

in acquainting the readers with the design of furnaces pro-

duced by the Company for heat treatment of tools and for

hardening parts from steels of the high-strength low-alloy

(HSLA) group.

FURNACES WITH HEATING CHAMBERS

OF RECTANGULAR CROSS SECTION

Figure 1 presents a furnace with the chamber in the form

of a rectangular parallelepiped. The cooling gas is fed to the

parts at a pressure of 1 MPa (10 bar) through a hatch above

the charge or below it. Every 20 – 30 sec the direction of the

gas feed is changed. This pattern of cooling is known as a re-

versible one.

The gas is fed through the entire cross section of the

hatch, which is comparable with the surface of the horizontal

section of the charge (plane A in Fig. 2a ).

It is impossible in principle to ensure a uniform linear

velocity of gas circulation through the cross section of the

charge. In order to improve circulation in vacuum furnaces,

blinds or other structural elements for breaking the gas flow

are built into the entrance and exit hatches. This solution in

combination with reversible feed of gas to the charge (first

from top and then from bottom) improves to a certain degree

the uniformity of the gas feed and of the cooling of the

charge, though the principle of reversal (for example, in an

interval of 20 sec) causes some nonuniformity itself. When

the gas is fed at a low speed, a considerable part of it passes

through the space between the charge and the walls of the

heating chamber (bypassing the charge), which prevents cre-

ation of a significant rate of gas circulation in the entire vo-

lume of the charge. Dense placing of the charge promotes

uniformity of circulation of the gas mass through the charge.

If the charge is not dense, for example, consists of individual

parts, the conditions for uniform circulation are worsened.

Some producers try to improve the linear velocity by

equipping their furnaces with gas circulators with high vo-

Metal Science and Heat Treatment Vol. 46, Nos. 11 – 12, 2004

5540026-0673/04/1112-0554 © 2004 Springer Science+Business Media, Inc.

1 SECO�WARWICK Ltd., Poland. Fig. 1. Vacuum furnace with chamber of rectangular cross section.

lume efficiency in the critical period of the cooling cycle.

However, this solution is not simple, because it requires a

powerful engine for the circulator, which gives rise to prob-

lems with power supply and increases the cost of the system.

Why is the linear velocity of circulation of the cooling

gas so important for the possibilities of a vacuum furnace? It

is known that the heat transfer coefficient � [V�(m2 � K)] re-

sponsible for the cooling rate of the charge depends on the

linear velocity of the gas w [m�sec] and on the pressure of

the cooling gas p [bar] according to the equation

� = C w 0.7p 0.7� – 0.39c p0 31. �0.69,

where � is the thermal conductivity, cp is the specific heat ca-

pacity, and � is the density. These parameters characterize

the kind of cooling gas.

Increase in the linear velocity, for example, from 6 to

10 m�sec causes the same effect within the boundaries of the

charge as growth in the pressure of the cooling gas from 6

to 10 bar.

Furnaces with rectangular heating chambers meet the re-

quirements on heat treatment of medium- and low-alloy tool

steels.

It is assumed that the production of furnaces with rectan-

gular chambers is less expensive and takes less time than that

of furnaces with cylindrical heating chambers. In this con-

nection it is expected that furnaces with rectangular cham-

bers should cost less than furnaces with cylindrical chambers.

VACUUM FURNACES WITH CYLINDRICAL

HEATING CHAMBERS

A system of convection heating developed by

SECO�WARWICK has been used in such furnaces for

8 years. Vacuum furnaces with cylindrical chambers (see

Fig. 3) and a nozzle system for feeding the cooling gas to the

region of the charge are characterized by uniform feed of the

mass of the cooling gas to the surface of the parts. The linear

velocity of the gas leaving the nozzles is reduced in the

working space of the charge, and then the gas passes through

the charge over its axis, i.e., through section B (see Fig. 2b ).

This section is much smaller than the circulation plane A

(Fig. 2a ) in a rectangular heating chamber. This trick ensures

higher linear velocities in the region of the charge, and the

structure makes it possible to develop very high cooling

rates. The gas is fed through a system of nozzles arranged

around the charge and in the front wall of the heating cham-

ber. Hot gas arrives through the rear wall. Our many-years

experience shows that the uniformity of cooling also affects

the parameters of the zone of exit of hot gases from the heat-

ing chamber.

A simple engineering solution in furnaces with a rectan-

gular cross section of the chamber consists in arranging the

zone of exit of hot gases over the perimeter of the surface of

contact of the heating chamber and the furnace wall

(Fig. 4a ). In this case the gas flow circulates over the axis of

the charge, not reaching the end of its length. This improves

the parameters of gas circulation through the charge but cre-

ates a certain nonuniformity of the circulation, which is more

considerable than in the best furnaces with rectangular heat-

ing chamber.

In furnaces with cylindrical chambers the zone of suck-

ing of the gas is positioned in the center of the axis of the

charge, which makes it possible to keep the circulation of the

gas flow in the region of the charge over its entire length.

Use of Vacuum Furnaces in Heat Treatment 555

À

Bà b

Fig. 2. Direction of circulation (flow) of cooling gas: a) furnace

with working chamber of rectangular cross section; b ) furnace with

cylindrical chamber.

à

b

Fig. 3. Vacuum furnace with cylindrical chamber: a) appearance;

b ) chamber.

This approach has resulted in the creation of a cylindrical

furnace with maximum cooling rate and maximum unifor-

mity. Figure 5 presents the results of tests on cooling a

charge consisting of steel bars in nitrogen at a pressure of

10 bar. The test was performed in a chamber 600 � 600 �

900 mm in size. The charge consisted of 220 bars

� 25 � 300 mm with a net mass of 340 kg. For such a charge

the coefficient � turned out to be 0.6!

It is natural that cooling is always connected with a cer-

tain nonuniformity. For the test in question this is presented

in Fig. 6. For comparison, we can state that in the best design

with reversible type of cooling � = 0.8 (see Fig. 5). In this

case the charge also consists of parts with a diameter of 20,

50, 75, and 100 mm, for which we give the values of �

(in the middle of a part) in Fig. 5.

The furnaces of SECO�WARWICK possess the highest

cooling rate possible at the maximum uniformity of cooling

in the entire volume of densely and loosely packed charges.

A wide range of standard sizes makes it possible to choose

the furnace with requisite output for specific heat treatment

modes. It is sometimes necessary to harden a charge consist-

ing of vertically mounted plates. In this case it is expedient to

direct the flow of the heating gas in parallel to the principal

plane of the plates. This can be ensured by closing the noz-

zles built into the side walls of the heating chamber.

EXAMPLES OF APPLICATION OF VACUUM

FURNACES WITH CYLINDRICAL HEATING

CHAMBERS

1. Heat treatment of molds and dies from steels serving

at high temperatures. Vacuum furnaces with cylindrical

heating chambers are especially advantageous for hardening

molds and dies. Dies from steel H13 406 � 406 � 406 mm

(� 16� ) in size and about 530 kg in mass are heat treated in a

furnace with a working chamber 600 � 600 � 900 mm in size

at a pressure of 9 bar of the cooling gas and ensure a mean

cooling rate of 80 K�min; the cooling rate in a furnace with a

working chamber 900 � 800 � 1200 mm in size is 50 K�min.

Heat treatment of tools always requires isothermal hard-

ening in an automated mode. This is ensured by rapid cool-

ing of the charge to a temperature inconsiderably exceeding

that of martensitic transformation, and then slow cooling of

the core with a hold at constant surface temperature until the

moment of leveling of the temperatures in the core and on

the surface. Then the charge is subjected to interim cooling

with passage through the martensitic transformation (or to a

programmed hold at the temperature of the bainitic transfor-

mation with further cooling). Such a process yields an opti-

mum structure in the tools at minimum residual strain. The

process of isothermal hardening in the furnace can be con-

trolled automatically.

2. Heat treatment of tools from tool steels. At the pre-

sent time heat treatment of tools is performed in vacuum fur-

naces that ensure gas cooling at a pressure of 10 bar. Until re-

cently the process was performed in furnaces developing a

gas pressure of 6 bar (still earlier of 2 bar).

Furnaces with a cooling gas pressure of 2 bar are still

used in the aircraft, power, and other industries. Furnaces

with a gas pressure of 10 bar meet the requirements of the

tool industry and their price is comparable with that of fur-

naces developing a pressure of 6 bar.

3. Heat treatment of parts from steels of type HSLA. In

the heat treatment of tools from tool steels the cooling rate

556 J. Oleinik

à

b

Exit for hot gases

Gas exitCooling zone

Fig. 4. Diagram of gas flows in the chamber of a conventional fur-

nace (a) and in a chamber designed by SECO�WARWICK (b ).

1

2

3 4

� 0.01, sec�

d, mm

d = 25 mm

Air

Oil

0.2 0.3 0.4 0.5 0.7 1 2 3 4

300

200

10080

60

50

40

30

20

Fig. 5. Cooling conditions of a charge (steel bars with diameter d )

in oil, in air, and in nitrogen under a pressure of 10 bar in vacuum

furnaces 600 � 600 � 900 mm in size with cylindrical chamber (1, 3 )

and with rectangular-cross-section chamber (2, 4 ): 1, 2 ) � for cool-

ing from 800 to 500°C; 3, 4 ) � for cooling from 1000 to 200°C.

obtained at a cooling gas pressure of 10 (12) bar is very im-

portant only in the upper temperature range. For steels of

type HSLA (see Table 1) this rate has to be preserved until

200°C (Fig. 5). This is ensured in hardening of steels 300M

and 4340M (the Russian counterpart is steel 40Kh2N2MA)

in the cooling gas at a pressure of 6 bar and higher.

For example, the bodies of hydraulic distributors 220 �

140 � 65 mm in size from steel 4340 with a mass of one part

of about 8 kg are heat treated in nitrogen at pressure of 9 bar.

The net weigh of the charge is about 400 kg. After hardening

the surface hardness is 52 – 54 HRC and after tempering at

450°C it is 42 – 44 HRC.

SPECIAL FURNACES

These are elevator vacuum furnaces of SECO�WARWICK

with heating chamber � 1500 � 1500 mm and maximum

mass of the charge of 2000 kg. They serve for heat treatment

of circular dies from steel DIN 1.4034 (the Russian counter-

part is steel 45Kh13), from steel 36NCD16 of the AFNOR

standard (0.35% C, 1.8% Cr, 4% Ni), or steel DIN 1.4021

(the Russian counterpart is steel 20Kh13). The dies have a

diameter of up to 1500 mm, a height of up to 300 mm, and a

cross section of up to 125 mm and are used in presses for

granulating fodder, processed artificial materials, and other

materials. Such furnaces are characterized by very uniform

heating due to the use of wide flat heating elements arranged

at 360° around the charge and on the hearth and in the ceiling

of the heating chamber. Convection heating is used at a low

temperature. In order to ensure uniform heating in the iso-

thermal cycle, the design involves rotation of the hearth with

the charge during cooling. Heat treatment in such furnaces

ensures minimum deformations and final hardness at a level

no lower than 54 – 56 HRC for steel 1.4034, 50 HRC for

steel AFNOR 36NCD16, and 48 HRC for steel 1.4021. In re-

cent years six furnaces of this type were shipped to industrial

enterprises of several countries.

FURNACES WITH CONVECTION HEATING

It is important to use convection heating for charges

heated in a low temperature range. SECO�WARWICK pos-

sesses a developed system for convection heating for fur-

naces with Kon Flap® cylindrical heating chamber. The

combination of a system of convection heating with flat heat-

ing elements with large surface and low specific surface load

arranged around the charge in one furnace is a knowhow of

SECO�WARWICK, which ensures rapid and uniform heat-

ing of the charge. Figure 7 presents comparative heating

curves for a charge in vacuum and under conditions of con-

vection heating.

Use of Vacuum Furnaces in Heat Treatment 557

t, °C

1

2

�, min

150

100

50

0 1 2 3 4 5 6

Fig. 6. Uniformity of temperature distribution in the process of

cooling of a charge 340 kg in mass consisting of bars � 25 � 300 mm

(the working space of the chamber is 600 � 600 � 900 mm in size):

1, 2 ) chambers with rectangular and cylindrical cross section, re-

spectively.

88 min

59 min

600

400

200

600

400

200

0

Time

Time

Saved time 33%

t, °C

t, °C

à

b

Fig. 7. Heating curves for a charge in vacuum (a) and under condi-

tions of convection heating (b ) plotted from readings of eight

thermocouples mounted in different places of the charge.

TABLE 1. Hardness of Steels after Nitriding and

Hardening

Grade of steel

(in AISI)Russian counterpart HRC hardness

4130 30KhM 26 – 28

4135 35KhM�35KhML 37 – 40

4140 40KhM 46 – 48

4340M 40Kh2N2MA 55 – 57

300M – 59 – 61

Note. The results are given after measuring the hard-

ness of a part 40 mm in diameter after nitriding at a

pressure of 5 atm.

A vacuum furnace equipped with a system for convec-

tion heating has enhanced output due to accelerated heating

of charges (especially densely packed ones) in the range of

750 – 800°C and combines hardening and repeated temper-

ing of the tools in one process at the design ( 5°C) and even

higher accuracy of temperature distribution.

The mean workload of a vacuum furnace permits tough-

ening in the same furnace. In a production with high work-

load of the vacuum furnace (high-temperature treatment pro-

cesses) tempering should be performed in a less expensive

special furnace. For these purposes SECO�WARWICK pro-

duces horizontal retort furnaces with vacuum “rinsing” at a

maximum temperature of 700°C. The furnaces operate at

from 150 to 700°C with a guaranteed temperature distribu-

tion of no less than 5°C.

Figure 8 presents two furnaces with retorts of type VTR

(600 � 900 � 600 mm) and maximum mass of a charge of

600 kg.

PROSPECTS

We have already mentioned that the cooling rate of the

charge is and will be the most important for development of

heat treatment in vacuum furnaces. An innovation is the use

of engines of water-cooled blowers with overload (operation

at maximum speed) for a short period (5 – 10 min). Supply

of the blower engine through an inverter also opens new pos-

sibilities, which allows a furnace with 10-bar pressure to ope-

rate on helium instead of nitrogen. This gas requires much

less power, and the use of the inverter makes it possible to

start the engine at a helium feed rate of, say, 4500 liters�min,

which doubles the volume of the circulating gas. In combina-

tion with the positive thermal properties of helium (or a he-

lium-nitrogen mixture) this may give a replacement coeffi-

cient � 0.3. This simple solution makes it possible to work

with nitrogen (� = 0.6) and helium (� = 0.3) and create sys-

tems for vacuum carburizing in single-chamber vacuum fur-

naces. Such installations will permit simultaneous conduc-

tion of vacuum carburizing and hardening for 17CrNiMo6

(DIN 1.6587), 20NiCrMo5 (DIN 1.6557), 27MnCr5, and

other alloys.

558 J. Oleinik

Fig. 8. Furnaces with VTR-type retort (600 � 900 � 600 mm) for

heat treatment of charges up to 600 kg in mass.