DK157691B - PROCEDURE FOR IONIC NITRIDATION OF A STEEL SUBSTANCE THAT BEFORE DEFORMED PLASTIC - Google Patents

Abstract

1. A method for the ionic nitriding of pieces of steel of any type, which comprises the following four stages in succession : a) a prior cold-working operation on the piece, the final degree of cold-working of which is between 10% and 40% ; b) a first ionic nitriding sequence with a duration of between one hour and 10 hours, carried out at a temperature t1 of between 450 degrees and 520 degrees, in a gaseous mixture consisting of nitrogen and hydrogen, which is such that the nitrogen partial pressure p1 is between 10 and 35 Pascal and such that the total gas pressure is between 200 and 650 Pascal ; c) a second ionic nitriding sequence the duration of which is between 40 and 70 hours, carried out at a temperature t2 of between 500 degrees and 580 degrees C and at least 10 degrees C and at most 50 degrees C above t1 in a gaseous mixture consisting of nitrogen and hydrogen, which is such that the nitrogen partial pressure p2 is between 60 and 80 Pascal, and that the total gas pressure remains between 200 and 650 Pascal ; and d) a final cooling in vacuo.

Description

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The present invention relates to ionic nitriding of steel.

It is known that ionic nitriding of steel is a process different from gas nitriding.

The latter, which is of a thermochemical nature, is classically carried out between 500 and 550 ° C in an atmosphere of ammonia, optionally diluted with nitrogen.

This results in a surface hardening of the steel, which is significant due to the presence of two successive nitrided layers: (a) In the surface a combination layer formed under saturation with nitrogen and consisting of iron nitrides of two different types 10 (Fe2 3N and Fe ^ N ), the first being closest to the surface and the second more in depth.

(b) Just below the combination layer, a nitrogen diffusion layer which is much thicker than the previous one and strongly marked by precipitation limits in the system of carbon nitrides of iron and other metals derived from the combined action of the nitrogen introduced and of the carbon in the steel.

These two layers are sensitive to crack initiation and propagation: the combination layer due to the occurrence of cracks at the boundary between the two nitride categories and rapid peeling of the constituents near the surface; the diffusion layer due to the precipitation of carbon nitrides which are not resistant to the mechanical and thermal stresses to which the two nitrided layers are subjected.

The nature and extent of the diffusion zone have an effect on the more or less extensive increase in the overall resistance properties of the workpiece, in particular on its dynamic resistance to alternating stresses and to a lesser extent on its resistance to wear and on its properties.

Finally, under the diffusion zone there is the base material which is not affected by the nitrogen diffusion and which should consequently remain unchanged during the nitriding treatment. To this end, the base material itself should have the greatest possible resistance without the risk of being overridden under the thermal conditions during nitriding. Hereby, this base material is then capable of providing sufficient mechanical support for the entirety of the two relatively thin nitriding layers so that the total resistance of the workpiece becomes sufficiently large.

The just mentioned cracking and peeling problems caused by gas nitriding can be largely avoided by ionic nitriding, which is of an electrothermal nature.

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The ionic nitriding of steel workpieces uses an electric charge induced by a potential difference of between 400 and 700 volts between the workpiece to be treated and the cathode, and the reactor wall which constitutes the anode in a room placed under reduced pressure , generally between 5,100 and 1,000 Pascals, of a gas consisting of a mixture of nitrogen and hydrogen which may, as the case may be, contain between 1 and 35% by volume of nitrogen, optionally with a little hydrocarbon gas, at temperatures between 430 and 600 ° C.

Examples of treatment cycles are previously described in French Patent Specification No. 2,446,326.

The process for ionic nitriding of elongated hollow steel bodies of all types according to the patent is characterized by the following seven successive steps: (a) placing the container containing the hollow bodies and their anode under vacuum, (b) a purification under much reduced pressure less than 0.8 millibars accompanied by a slight rise in temperature below 100 ° C using a gas mixture of nitrogen and hydrogen containing between 1 and 4% by volume of nitrogen, 20 (c) increasing the temperature to 430-540 ° C accompanied by a (c) a holding period at a constant temperature between 430-540 ° C for a duration between 1-6 hours and preferably in the vicinity of 4 hours for the entire time with the gas mixture described above (b) under a pressure between 2.5-10 millibars, (e) a new temperature rise from 430-540 ° C to 460-570 ° C with a gas mixture of nitrogen and h. (f) a holding period at a constant maximum temperature of 460-570 ° C for a duration of 25-70 hours and under the treatment pressure of 2.5-10 millibars below the above (e) gas mixture described, and (g) final cooling under vacuum.

The items so treated by ionic nitriding are characterized in that they exhibit a pore-free and single-phase composition outer layer, a maximum of 10 microns in thickness, and a diffusion zone largely free of nitride networks and boundaries.

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Thus, they are much less sensitive to cracking and peeling than subjects treated by gas nitriding.

One of the major advantages of ionic nitriding is that the dependence between the treatment temperature, which is a function of the 5 parameters of the electric discharge (voltage, intensity, gas pressure) and the activity of the reactive environment which depends on the gas mixture used , is realized.

This is known to those skilled in the art.

On the other hand, it is also known that in the case of thermochemical treatments, a prior plastic deformation of the steel has a very large influence on the diffusion process of the interstitial elements such as the carbon and nitrogen. Thus, in the case of nitriding, cold deformation of steel or deformation at moderate temperatures, not exceeding 600 ° C, results in a noticeable increase in the thickness of the nitrided layers for a given temperature and holding time.

However, this favorable effect of a prior plastic deformation of the steel on the kinetics of the growth of the nitrided layers is accompanied by a significant disadvantage with respect to the usual nitriding conditions. Thus, the precipitates of cementite or of carbon compounds present in the cold-deformed steel are partially dissolved while maintaining the nitriding treatment temperature (430-600 ° C) and the carbon present in the solid solution. facilitates the growth of the carbon nitrides both in the combination layer and in the nitrogen diffusion layer. Hereby penetrations of the combination layer are obtained in the diffusion layer, partly along the grain boundaries and partly along the cold deformation lines. These compositions, which are naturally brittle, degrade the toughness and fatigue resistance of the nitrided layers, both mechanically and thermally.

The object of the present invention is to take advantage of the beneficial effect of a prior cold deformation on the kinetics of the growth of the nitrided layers, avoiding any risk of excessive brittleness but without performing any special treatment to relieve the stresses.

Thus, the present invention relates to a process for the ionic nitriding of steel parts of all types, characterized by the following four successive steps: (a) a prior cold deformation of the workpiece to a final deformation degree of between 10 and 40% .

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4 (b) a first ionic nitriding sequence of a duration between 6 and 10 hours carried out at a temperature of between 500 and 520 ° C in a gas mixture of nitrogen and hydrogen, where the nitrogen partial pressure pi is between 10 and 35 Pascal, and the total gas pressure is between 200 and 650 Pascal, (c) another ionic nitriding sequence of a duration between 40 and 70 hours performed at a temperature t 2 between 500 and 580 ° C and at least 10 ° C and not more than 50 ° C higher than ten, which second sequence is carried out in a a gas mixture of nitrogen and hydrogen, wherein the nitrogen portion of the pressure p 2 is between 60 and 80 Pascal, and the total gas pressure is between 200 and 650 Pascal, and (d) a final cooling under vacuum.

According to a particular feature of the present invention, the longer the proportion of alloying elements in the steel to be treated, the longer the ionic nitriding sequence and the duration of the second sequence within the previously stated durations. This is because the restoration of the structure after the cold deformation occurs more slowly in steel with a larger proportion of alloying elements, such as stainless steel or heat-resistant steel.

According to an embodiment of the invention, which may be particularly applicable to alloy steel, such as stainless steel or heat-resistant steel, the article is subjected to the same cold deformation as before a first nitriding sequence for a period of between 6 and 10 hours at a time. temperature between 500 and 520 ° C in a gas mixture of nitrogen and hydrogen, where the nitrogen partial pressure is between 30 and 35 Pascal and the total gas pressure is between 200 and 650 Pascal. Next, the blank is subjected to a second ionic nitriding sequence for a period of between 40 and 70 hours at a temperature between 500 and 580 ° C in a mixture of nitrogen and hydrogen, with the nitrogen partial pressure being between 60 and 80 Pascal, and preferably about 70 Pascal, and the total gas pressure is between 200 and 650 Pascal.

It should be noted that if, in all cases of using the process of the invention, the gas mixture always consists of nitrogen and hydrogen, the relative amounts of these two gases should be controlled at least in the first ionic nitriding sequence as a function of the composition of the steel, and especially of its content of alloying elements which attract nitrogen such as chromium, vanadium, titanium and aluminum. The higher the content of these elements, the greater the nitrogen partial pressure should be for one and the same total working pressure.

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The duration of the first sequence is of great importance. The superficial decarbonization induced by this treatment step, if continued for too long, may cause modifications to the mechanical properties of the nitrided layers. In particular, the penetration of nitrogen into steel 5 is facilitated by the reduction in the amount of carbon compounds, and this can result in a noticeable reduction in surface hardness with a consequent deterioration of the following properties: resistance to impression, wear and mechanical or thermal fatigue.

To avoid this risk, it is not necessary to continue the first 10 ionic nitriding sequence for too long, which is why the period of maintaining the temperature during this sequence should not exceed 10 hours.

This first sequence is followed by a second sequence during which the activity of the reactive environment is modified as a function of the thickness of the combination layer which one wishes to obtain in the surface. During this sequence, the temperature and residence times should be adjusted, taking into account the beneficial effect of the prior cold deformation on the kinetics of the growth of the nitrided layers. This phenomenon, controlled by the nitrogen diffusion in the treated subject, can be illustrated by a graphical representation of the course of the ratio _e (where e is the thickness of the surface layers in micrometers and t is the residence time in hours) as a function of the reciprocal value of the absolute temperature. This imaging allows the residence time required during the second treatment sequence to be determined to obtain a nitrided layer of given thickness at a given temperature as characterized by the growth kinetics.

As the first sequence of the ionic nitriding treatment took place, the recovery of the cold deformation structure was greatly delayed by the introduction of nitrogen into the system, as the dislocations serving as favorable positions for the nitrogen penetration were inhibited in their displacement of the formed precipitates of nitrides.

However, the second sequence, when set well as just mentioned, completes in a reduced time the restoration of the structure deformed by cold deformation.

35 It is furthermore extremely interesting to judge the interest in the method according to the invention to make the following comparison of two identical steel items, both of which have undergone the same prior plastic deformation by cold hammering to a final deformation degree of 30%: 6

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first, a first blank subjected to a relaxation treatment at 500 ° C for 2 to 4 hours in a traditional controlled atmosphere furnace for partial recovery of the cold-deformed structure, followed by a conventional type ionic nitriding at temperatures between 500 and 500 ° C. 560 ° C in a gas mixture of nitrogen and hydrogen, where the nitrogen partial pressure is equal to 60 Pascal, - secondly, subject to the two successive sequences of ionic nitriding according to the invention, the first sequence in the form of an ionic nitriding for 6 to 10 hours at approx. 500eC in a gas mixture of 10 nitrogen and hydrogen, where the nitrogen partial pressure is between 10 and 15 Pascal, and the second sequence performed at 500 to 560eC in a gas mixture of nitrogen and hydrogen under a nitrogen partial pressure of 60 Pascal.

In such a comparison, it is observed that for the same thickness of the nitrided layer, the gain in the total processing time of the second blank (according to the invention) relative to the first is remarkable, depending on the ratio between 10 and 50%.

To illustrate this comparison, reference is made to two steel tubes "35 CD 12" (0.35% C, 3% Cr, 0.80% Mo), the design of which is made by cold forging to a degree of strain deformation of 30%.

The voltages measured by radiocrystallography on the surface of each of these two tubes in such a state are as follows (the longitudinal voltage, i.e. along the axis of the workpiece, being denoted by σι, and the peripheral stress, i.e. the voltage perpendicular to the workpiece axis with σθ): σ L = - 740 MPa 25 σ β = - 400 MPa

The first tube then undergoes a relaxation at 500 ° C for three hours in a conventional controlled atmosphere oven. The stresses then appear to have fallen to the following values: σ L = - 210 MPa 30 σ Θ = - 170 MPa

There is thus a good partial restoration of the cold-deformed structure.

The first tube finally undergoes a classical ionic nitriding under the conditions previously stated.

The kinetics of the growth of the nitrided layers by this classical ionic nitriding, as previously defined, are represented by curve 1 in FIG. 1 (see later).

The second tube undergoes the ionic nitriding in two successive 7

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sequences according to the invention under the previously stated conditions.

The kinetics of the growth of the layers by this ionic nitriding according to the invention are represented by curve 2 in FIG. First

In FIG. 1, the reciprocal value of the absolute treatment temperature (the temperature of the second sequence in the example of the invention) as the abscissa is plotted, and as the ordinate the ratio of the nitrided thickness to the square root of the time of the ionic nitriding.

As can be clearly seen, curve 2 is for a given temperature above 490 ° C, constant above curve 1. For a certain nitriding time, 10 is seen e.g. for a treatment at 520 ° C, the nitrided thickness is approx. 1.25 times greater according to curve 2 than according to curve 1, and that this coefficient increases with the treatment temperature. Thus, the kinetics of the growth of the nitrided layers is clearly improved by the method of the invention.

15 Another way of illustrating the foregoing comparison is shown in FIG. 2, in which, as a linear scale, the temperatures of the ionic nitriding (for the second sequence of the example according to the invention) and as ordinate in logarithmic scale the nitriding times in hours, showing two obtained lines of nitriding depths, respectively, for the first tube (solid lines) and the second tube, according to the invention (dashed lines) for values of 200, 300, 400, 500 and 600 micrometers, respectively.

From this FIG. 2, for example, to obtain a thickness of the nitrided layer of about 500 micrometers with a nitriding treatment of about 540 ° C is required - 60 hours maintaining this temperature for the classical treatment after relieving the stresses and - only 42 hours stay at this temperature for the treatment according to the invention.

The ionic nitriding process of the invention thus represents a very remarkable advance.

This progress is equally significant if the reactive atmosphere is regulated in such a way that only a nitrogen diffusion layer without a combination layer is obtained in the surface of the nitrided substance. This result 35 can be obtained by controlling the two sequences of the ionic nitriding as follows: - A first sequence of a duration between 6 and 10 hours, depending on the nature of the steel to be treated, carried out at a temperature ten between

Claims (3)

A method of ionic nitriding of steel parts of all types characterized by the following four successive steps: (a) a prior cold deformation of the workpiece to a final deformation degree of between 10 and 40%, (b) a first ionic nitriding sequence of a duration between 6 15 and 10 hours carried out at a temperature ten between 500 and 520 ° C in a gas mixture of nitrogen and hydrogen, where the nitrogen partial pressure pi is between 10 and 35 Pascal and the total gas pressure is between 200 and 650 Pascal, (c) another ionic nitriding sequence of a duration between 40 and 70 hours carried out at a temperature t 2 between 500 and 580 ° C and at least 10 ° C and not more than 50 ° C higher than ten, which second sequence is carried out in a gas mixture consisting of nitrogen and hydrogen, where the nitrogen partial pressure p 2 is between 60 and 80 Pascal, and the total gas pressure is between 200 and 650 Pascal, and 25 (d) a final cooling under vacuum. The method of ionic nitriding according to claim 1, characterized in that the final degree of prior cold deformation of the article to be treated is of the order of 30%. Method for ionic nitriding according to any one of claims 1 and 2, characterized in that the longer the first sequence of ionic nitriding and the duration of the second sequence, the greater the proportion of alloying elements in the steel to be. processed, is. 35