PROCEDE DE NITRURATION DE PIECES D'ACIER SOUS PRESSION

PROCESS FOR NITRIDING STEEL WORKPIECES UNDER PRESSURE

Abrégé anglais

A b s t r a c t
In gas nitriding under pressure, substantially non.
porous, thick nitride layers are obtained in a short time
providing a mixture of 5 to 95% by volume ammonia and 95 to
5% by volume nitrogen is used and the nitriding process is
carried out under a constant pressure above 0.2 MPa.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for nitriding steel workpieces in a gas
atmosphere containing atomic nitrogen at temperatures above
425°C and under pressures above 0.2 MPa, characterized in
that the gas atmosphere consists of 5 to 95% by volume
ammonia and 95 to 5% by volume molecular nitrogen and in that
nitriding is carried out under constant pressure.

Description

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A process for nitriding steel workpieces under pressure
Description:
This invention relates to processes for nitriding steel
workpieces and in particular, to such processes in a gas
atmosphere containing atomic nitrogen at temperatures above
425C and under pressures above 0.2 MPa.
At present, nitride layers are formed on workpieces of
steel alloys in a salt bath, by gas nitriding or by plasma
nitriding. These layers improve the resistance to corrosion,
wear resistance and vibration resistance of the steels. They
normally consist of a so-called coupling layer a few
micrometres thick over a nitrogen diffusion layer which is
generally formed by the above-mentioned processes over
process times of the order of 100 hours.
In recent years, gas nitriding in particular has
received a considerable impetus, the steel workpieces being
heated in an atmosphere of nitrogen-releasing gases,
preferably in an atmosphere of ammonia.
US-PS 2,779,697 describes a process for nitriding steels
under pressure in gaseous ammonia.
In this process, a pressure vessel is filled with a
certain quantity of ammonia and heated to temperatures of 425
to 640C (800 to 1,200F) so that an ammonia pressure of a
few bar is built up in the pressure vessel. Nitride layers
between 20 and 40 ~m thick are formed in about 15 hours,
depending on the quantity of ammonia, the pressure and the
temperature. However, this process has not been successfully
used in practice.
Accordingly, the problem addressed by the present
invention was to provide a process for nitriding steel
workpieces in a gas atmosphere containing atomic nitrogen at
temperatures above 425C and under pressures above 0.2 MPa,
in which non-porous coupling layers up to 50 ~m in

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thickness could be formed in a short time on unalloyed and
alloyed steels without any need for speclal pretreatment.
According to the invention, the solution to this
problem is characterized in that the gas atmosphere con-
sists of 5 to 95% by volume ammonia and 95 to 5% by volume
molecular nitrogen and in that nitriding is carried out
under constant pressure.
Ammonia which dissociates into hydrogen and nitrogen
at relatively high temperatures is used as the gas releas-
ing atomic nitrogen. The use of pure ammonia without
incorporation of molecular nitrogen leads to distinctly
poorer coupling layers. In this way, nitriding layers up
to 50 ~m thick are o~tained in 4 to 5 hours.
These coupling layers are substantially non-porous.
The ratio of the 7' phase to the ~ phase can be established
through the process parameters and, in the most favorable
case, is 100% ~ phase. Austenitic and high-chrome steels
can be nitrided without any chemical pretreatment. This
process is suitable for treating workpieces in any numbers
and without any geometric restrictlon. The number is
determined solely by the available furnace size. A chamber
furnace can typically be heated to temperatures of up to
1,200C for an absolute internal pressure of 0.2 to 10 MPa.
~he treatment parameters of temperature, time, absolute
pressure and partial pressure of the nitrogen-releasing gas
-may be adJusteJ tG establish optimal treatment condl~lons
for each material. Nitriding temperatures of 500 to 900C
have proved to be favourable. In addition, it is important
that the entire nitriding process be carried out under
constant pressure. Pressure variations adversely affect
the favorable properties and reproducibility of the coup-
ling layers. The treatment time is determined by the type
of steel and the desired layer thickness.
The invention is illustrated by the following Ex-
amples.

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Example 1
A steel of the composition C 45 is nitrided with a gas
mixture of 30% by volume ammonia and 70% by volume nitrogen
in a pressure-tight chamber furnace under an excess pressure
of 2 MPa and at a temperature of 700C. After one hour, a
substantially non-porous 40 ~m thick coupling layer has
formed.
Example 2
In the case of fast steels, a 100 ~m thick diffusion
zone is obtained in 4 hours at 580C/l MPa pressure using a
gas atmosphere of 80% by volume ammonia and 20% by volume
nitrogen.
Example 3
A substantially non-porous 50 ~m thick coupling layer is
obtained on a steel of the composition 16MnCr5 after 2 hours
at 550C/8 MPa pressure in a gas atmosphere of 70% by volume
ammonia and 30% by volume nitrogen.