CS336091A3 - Process of nitriding steel workpieces under pressure - Google Patents

Description

Μ 60-fy

Method of nitriding steel workpieces ρόα rxaseju t

CT Technique

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BACKGROUND OF THE INVENTION The present invention relates to a process for nitriding steel workpieces with an atomic nitrogen containing atmosphere at temperatures above 400 ° C and pressures above 0.2 MPa.

Background Art

According to the state of the art, nitriding layers of alloyed steel workpieces are formed in a salt bath, by nitriding with gas or by plasma nitriding. These layers improve corrosion resistance, abrasion resistance and cyclic strength. They generally consist of a so-called few-µm thick layer above the nitrogen diffusion layer, which is produced in the order of hundreds of hours above. In recent years there has been a particularly large increase in gas-nitriding, in which steel parts are heated in nitrogen-splitting atmospheres, preferably in an ammonia atmosphere. U.S. Pat. No. 2,779,697 discloses a method for nitriding steel in pressurized ammonia gas.

In this case, the pressure vessel is filled with some ammonia and heated to 425 to 640 ° C, thereby increasing the ammonia pressure in the pressure vessel to several tenths of MPa. In this process, nitride layers of from 20 to 40 [mu] m are obtained over about 15 hours, depending on the amount of ammonia, pressure and temperature. However, this practice could not be enforced in practice. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a process for nitriding steel workpieces in a gaseous atmosphere comprising atomic nitrogen at temperatures above 425 ° C and pressures above 0.2 MPa at which up to 50 µm non-porous compound layers can be produced on unalloyed materials. alloyed steels without special pre-treatment-

SUMMARY OF THE INVENTION

This object is achieved according to the invention in that the gaseous atmosphere consists of 5 to 95% by volume of ammonia and 95 to 5% by volume of molecular nitrogen and that the nitriding takes place at constant pressure.

Ammonia is used as the atomic nitrogen-providing gas, which dissociates to hydrogen and nitrogen at higher temperatures. The use of pure ammonia without the addition of molecular nitrogen leads to clearly worse nitriding layers. Over 4 to 5 hours, nitride layers of up to 50 µm are obtained.

These compound layers are almost free of pores. The Y'-kε-phase ratio is adjustable by operating parameters and reaches 100% γ-phase in the most favorable case. Austen steels and steels with high chromium content can be nitrided without prior chemical treatment. This procedure is suitable for editing workpieces without their geometry limitation and in any number. The number is determined only by the furnace size available. The chamber furnace can usually be heated up to 1200 ° C at an internal absolute pressure of 0.2 to 10 MPa. The operating parameters, temperature, time, absolute pressure, and the partial pressure of the nitrogen-separating gas can be adjusted to provide optimum operating conditions for each workpiece. In addition, it is important to perform nitriding during the nitriding time at constant pressure. Pressure fluctuations deteriorate the good properties and reproducibility of the compound layers The treatment time depends on the type of steel and the desired thickness. The following examples are intended to explain the invention in more detail. EXAMPLES Example 1

The steel of composition C 45 is nitrided in a pressure chamber of 700 ° C with a gas mixture consisting of 30% by volume of ammonia and 70% by volume of nitrogen at an overpressure of 2 MPa. After one hour, a 40 µm thick pore-free compound layer was formed. EXAMPLE 2 A diffusion zone of 100 [mu] m thickness is obtained at 580 [deg.] C. in a gaseous atmosphere consisting of 80% by volume ammonia and 20% by volume nitrogen at a pressure of 1 MPa. Example 3

An almost non-porous compound layer of 50 .mu.m thick is obtained in 2 hours at a temperature of 550 DEG C. in a gaseous atmosphere of 70% by volume ammonia and 30% by volume of nitrogen at a pressure of 8 MPa. * r '

Claims (1)

PATENT REQUIREMENT 1 under (alz & vr) A method for nitriding steel workpieces in a gaseous atmosphere containing atomic nitrogen at temperatures above 425 ° C and pressures of up to 0.2 MPa, characterized in that the gaseous atmosphere consists of 5 to 95% by volume of ammonia and 95 to 5% by volume of molecular nitrogen and that the nitriding takes place at a constant pressure. • -iiga, ΑΛ3Γ907.λΖΞίν Λ ovyn ....... "Ί 16 iX '9 0 1