JPH04268063A - Method for nitrifying steel work under pressure - Google Patents

Abstract

PURPOSE: To provide a method for improving a work made of a steel at a temp. higher than 425°C under a pressure higher than 0.2MPa in a gas atmosphere contg. atomic nitrogen.

CONSTITUTION: This method is the one in which the gas atmosphere is composed of 5 to 90 vol.% ammonia and 95 to 5 vol.% molecular nitrogen, and nitriding is executed under a certain pressure. A compound layer with ≤50 μm thickness contg. no pores can be produced on a non-alloyed or alloyed steel in a short time without executing special pretreatment.

COPYRIGHT: (C)1992,JPO

Description

[Detailed description of the invention]

0001

FIELD OF INDUSTRIAL APPLICATION The present invention relates to a gas atmosphere containing atomic nitrogen at a temperature higher than 425°C and a pressure of 0.2 MPa.
Concerning a method for nitriding steel workpieces at higher pressures.

0002

BACKGROUND OF THE INVENTION According to the state of the art, nitrided layers are produced on steel alloy workpieces in salt baths by gas nitriding or plasma nitriding. The layer improves the corrosion resistance, wear resistance and vibration resistance of the steel. This layer generally consists of a so-called compound layer, several microns thick, which is produced by the method described above in a processing time of the order of 100 hours, on top of the nitrogen diffusion layer.

[0003] In recent years, gas nitriding in particular has made great progress.
There, the steel molding is heated in an atmosphere of nitrogen-emitting gas, preferably in an ammonia atmosphere.

From US Pat. No. 2,779,697,
Methods for nitriding steel under pressure in gaseous ammonia are known.

[0005] In this method, a pressure-resistant container is filled with a certain amount of ammonia and heated to 425°C to 640°C (800 to 1200F).
), whereby an ammonia pressure of several bars is created in the pressure vessel. Depending on the amount of ammonia, pressure and temperature, a nitride layer of 20 to 40 μm is obtained within about 15 hours. The method could of course not be realized in practice.

[0006]

OBJECTS OF THE INVENTION It is therefore an object of the present invention to produce a pore-free compound layer with a thickness of less than 50 μm on unalloyed or alloyed steel in a short time without any special pretreatment. The objective was to develop a method for nitriding steel workpieces at temperatures higher than 425° C. and pressures higher than 0.2 MPa in a gas atmosphere containing atomic nitrogen, which can be produced.

[0007]

[Means for Solving the Problems] According to the present invention, the above problems are solved by the gas atmosphere consisting of 5 to 95% by volume of ammonia and 95 to 5% by volume of molecular nitrogen, and the nitriding being carried out under constant pressure. Ru.

Ammonia, which dissociates into hydrogen and nitrogen at high temperatures, is used as the gas for releasing atomic nitrogen. Using pure ammonia without incorporating molecular nitrogen results in the formation of a clearly worse compound layer. In this method, a nitrided layer of 50 μm or less can be obtained in 4 to 5 hours.

[0009] The compound layer is almost pore-free. The ratio of γ' phase to ε phase can be adjusted by processing parameters, and in the most preferred case the γ' layer is 100%. Austenitic and high chromium-containing steels can each be nitrided without chemical pretreatment. The method is suitable for processing workpieces in any number and without geometrical restrictions. The number depends solely on the size of the furnace available. A typical method for a chamber furnace is 0.2
It can be heated to temperatures below 1200° C. at an absolute internal pressure of ~10 MPa. The process parameters, namely temperature, time, absolute pressure and partial pressure of the nitrogen release gas, can be adjusted to provide optimal process conditions for each workpiece. Values for the nitriding temperature of 500 DEG to 900 DEG C. have proven advantageous. Furthermore, it is important that the nitriding be carried out under constant pressure over the entire nitriding time. Pressure fluctuations deteriorate the good properties and reproducibility of the compound layer. The processing time depends on the type of steel and the desired layer thickness.

0010

EXAMPLES Next, the present invention will be explained in detail with reference to the following examples.

1. Steel with composition C45 was heated in a pressure chamber furnace at 70°C.
Nitriding was carried out at 0° C. with a gas mixture of 30% by volume of ammonia and 70% by volume of nitrogen at an overpressure of 2 MPa. After 1 hour, thickness 40
A nearly pore-free compound layer of .mu.m was formed.

2. In high speed steel, ammonia 80
580℃, 1M in a gas atmosphere of vol.% and nitrogen 20 vol.%
Pa, a diffusion zone with a thickness of 100 μm was obtained in 4 hours.

3. 550 in a gas atmosphere of 70 vol% ammonia and 30 vol% nitrogen on steel with composition 16MnCr5.
C. and under a pressure of 8 MPa for 2 hours, a 50 .mu.m thick compound layer almost free of pores was obtained.

Claims (1)

[Claims] [Claim 1] In a gas atmosphere containing atomic nitrogen,
A method for nitriding steel workpieces at temperatures higher than 425° C. and pressures higher than 0.2 MPa, in which the gas atmosphere consists of 5-95% by volume of ammonia and 95-5% by volume of molecular nitrogen, and the nitriding is carried out at a constant pressure. characterized by doing,
Method of nitriding steel workpieces under pressure.