WO2001042528A1

WO2001042528A1 - Method for nitration-hardening or nitrocarburizing work pieces from steel alloys

Info

Publication number: WO2001042528A1
Application number: PCT/AT2000/000329
Authority: WO
Inventors: Walter Beck; Rudolf Mandorfer
Original assignee: Steyr Daimler Puch Fahrzeugtechnik Ag & Co Kg
Priority date: 1999-12-07
Filing date: 2000-12-05
Publication date: 2001-06-14
Also published as: DE20023102U1; AT3588U1

Abstract

The invention relates to a method for nitration-hardening work pieces from steel alloys. The aim of the invention is to achieve the highest possible long-term creep and stress rupture resistance of such steel alloys in a minimum of time and with high reliability. To this end, the inventive method comprises the following steps: a) subjecting the work piece to a nitrocarburization method in a salt bath with a cyanate content between 30 % and 80 % until the white layer reaches a thickness of 0.5 to 3 νm; b) subjecting the work piece to a gas nitriding method at a temperature of between 500 and 600° C, with a low nitride potential being adjusted so that first the white layer disintegrates while enlarging the geometrical surface. In between these two steps the work piece, for example the injection nozzle of a diesel engine, can be treated with an organic acid.

Description

METHOD FOR NITRATING or NITROCARBURING WORKPIECES FROM ALLOY STEELS

The invention relates to a method for nitriding workpieces made of alloyed steels. In particular, but not exclusively, we are thinking of finished machined workpieces of complex shape with hard-to-reach places that are exposed to very high alternating loads at high temperatures. Such workpieces are, for example, parts of injection nozzles for internal combustion engines. In the following, we will only speak of nitriding, although nitrocarburizing should also be read, unless expressly excluded.

Two types of nitriding processes are known: those in a gas atmosphere and those in a salt bath. In gas nitriding, the workpiece is exposed to an atmosphere of an ammonia-containing gas mixture at temperatures between about 500 and 600 ° C for a longer time (up to 100 hours); in salt bath nitriding a liquid bath consisting of various salts at similar temperatures. These processes are described, for example, in the brochure "Gas nitriding and gas nitrocarburizing" from AGA Gas GmbH. Above all and in general, gas nitriding has the disadvantage of being long. The lower the temperature, the longer it is - stay in the α zone of the LEHRER diagram (see below), in which the material remains tough, long-term and heat-resistant even with great hardness of the workpiece, which is stronger, the higher the steel is alloyed, especially with chrome, passivation is the increased resistance that the surface offers against the penetration of nitrogen and / or carbon atoms into the workpiece.

The disadvantage of the salt bath process is that a relatively thick and very brittle connection layer can form on the surface of the workpiece. It is described as the γ 'zone of the TEACHER diagram. Although this is particularly hard and abrasion-resistant, it is disadvantageous in the case of workpieces which are exposed to less mechanical wear, but which are supposed to be tough and heat-resistant with high strength.

Since the aforementioned passivation can be particularly intensive locally, harmful impairments of the nitrogen transfer, so-called nitration stops, occur there. Nitriding stops are surface zones of reduced nitriding and thus lower strength, which impair the process reliability of the hardening.

In the case of workpieces of complex shape, such as the nozzle body of an injection nozzle with its fine spray bores and a blind hole at the lower end, there is also the problem that nitrogen absorption depends on the local nitriding potential and thus on the local concentration of the nitriding gas. Since this is basically depleted in the vicinity of the surface of the workpiece, the stray gas exchange must be supported by additional circulation. Otherwise, insufficient hardening occurs in dead water areas, which also affects process reliability.

It is therefore the aim of the invention to remedy the disadvantages of both methods and to propose a method according to which the highest heat and fatigue strength is achieved in the shortest possible time and with high process reliability.

The method consists in that, after suitable pre-cleaning, the workpiece is first subjected to a nitrocarburizing process in a salt bath with a cyanate content between 30% and 40% until the connecting layer that forms reaches a thickness in the range from 0.5 to 3 μm; and the workpiece is then subjected to a gas nitriding process at a temperature between 520 and 580 ° Celsius, the composition of the nitriding gas being set to a low nitriding potential.

With nitrocarburizing, the very brittle compound layer that you don't really want to form initially begins, below which, as a by-product, it begins to form the diffusion layer. The latter finally has the advantageous properties desired here. However, the connection layer should remain as thin as possible. That is why this treatment is only given briefly. The accompanying diffusion layer is inevitably generally too thin. The nitriding is therefore completed in the next step. When gas nitriding namely, this connection layer is re-formed by the deep nitriding potential, releasing nitrogen atoms to the nitriding gas. What remains, however, is an enlarged geometric surface with optimally shaped crystal surfaces, which means that the passivity of the surface is overcome, the entire surface is safely activated throughout, nitriding stops can nowhere occur.

Above all, the surface can now easily and quickly absorb nitrogen atoms to form the diffusion layer. This means that the nitriding time is considerably shortened and that with a lower nitriding potential, sufficiency is found which excludes the formation of nitrides (connecting layer, nitride lines) and thus the diffusion layer remains in the α zone throughout, i.e. tough and warm. and becomes permanent. But these are exactly the properties required, and they are achieved in a much shorter time.

The low nitriding potential in the second step has several effects:

The connection layer disintegrates again, leaving behind a porous surface which accelerates the transfer of nitrogen into the workpiece,

• Local fluctuations in the concentration of the nitrate medium (reduced gas supply) have less effect, so that hardening is good even in inaccessible places on the workpiece,

• The diffusion layer remains safely in the α zone. This corresponds to a very tough structure, • The risk of accumulation of nitrogen atoms at the grain boundaries (nitride rows), which would also lead to embrittlement, is lower.

The three main goals have been achieved: maximum heat resistance, high process reliability and short nitriding time. It is surprising that, thanks to the invention, a detour leads to the destination better and faster.

Gas nitriding preferably takes place with a nitriding potential in the range between 0.08 and 0.5. As a result, you remain safely in the α zone at a nitriding temperature between 500 and 600 ° Celsius.

In a particularly advantageous procedure, the workpiece is treated with an organic acid, in particular with citric acid, after salt bath nitriding and before gas nitriding. Organic acids form metal salts, which decompose particularly easily in the subsequent gas nitriding and leave a particularly porous and additionally particularly reactive surface, which helps to further shorten the nitriding time.

The invention is described and explained below with the aid of illustrations and exemplary embodiments. They represent:

1: the TEACHER diagram, FIG. 2: an example of a workpiece to be treated by the method according to the invention,

3: The measured hardness curve for the two exemplary embodiments of the method according to the invention. 1 shows the LEHRER diagram that the various state phases of the iron-nitrogen system are a function of the nitriding temperature (ordinate) and the nitriding potential (abscissa). The nitriding potential, also called nitrogen activity in the atmosphere, is proportional to the ammonia concentration and inversely proportional to an expression of the hydrogen concentration.

The α-zone essentially corresponds to that of the iron-carbon diagram, the nitrogen atoms are deposited in the α-lattice and increase its strength and toughness without embrittlement, as long as no nitride lines form at the grain boundaries. The zone labeled γ 'corresponds to

brittle connection layer during nitriding and die zone during carbonitriding and nitrocarburizing. A trapezoid A is drawn in the diagram, which indicates the temperature range and the range of the nitriding potential for the second step of the method according to the invention. The nitriding temperature is between 500 and 600 ° Celsius, the nitriding potential between 0.04 and 0.4.

2 shows hatched the body 1 of an injection nozzle with the displaceable nozzle needle 2. The nozzle body 1 ends at the bottom in a nozzle tip 3, which has a conical needle seat surface 4 on the inside, a central blind hole 5 and spray bores 6 distributed over the circumference. The nozzle needle 2 is guided in guides 7, 8 with the highest precision. Between the two guides 7, 8 and between the guide 7 and the nozzle tip 3 there is in each case an annular space 9, 10, the upper one of which, through a channel 11, has fuel at a pressure of 2,000 bar and above, from an unillustrated set injection pump is supplied. The nozzle needle 2 is acted upon by a very strong spring (not shown) so that it rests on the needle seat 4.

The stresses to which this nozzle body is subject are extremely high and varied. The entire nozzle body is radially stretched by the internal pressure acting in the spaces 9, 10 and is thus pulsed under tension. The guides 7, 8 must be extremely precise and hard to guide the nozzle needle 2 clean. In addition to the internal pressure, the needle seat surface 4 must withstand the impacts of the flowing valve needle 2. All surfaces on which the flow is redirected or a change in cross-section takes place are also at risk of cavitation, in particular the surroundings of the spray bores 6 and the blind hole 5. It can be seen that even in the event of a gas flow with the nozzle needle removed, corresponding to the gas nitriding inside the nozzle tip 3 and especially in the spray holes 6 only a very slow movement of the nitriding gas is possible. There, above all, there is a risk of insufficient nitriding.

This workpiece was hardened in two different variants of the method according to the invention:

Example I:

2 was first thoroughly cleaned in the usual manner and then nitrocarburized in a salt bath at a temperature of 580 ° C. for 20 minutes. A suitable salt bath contains 32 to 38% of a potassium and / or sodium cyanate (CNO ^" ion). In the present case, the TENIFER" process was followed. the composition of the salt bath was also promising. TENIFER ^® is a process and trademark protected by DEGUSSA. As soon as the thickness of the connection layer had reached a μ-meter, the workpiece was removed from the salt bath, cooled in salted cold water and thoroughly cleaned in the usual way.

Next, the workpiece was gas nitrided, the composition of the ammonia-containing nitriding gas being compiled in accordance with a nitriding potential of 0.1. This nitriding potential corresponds to a residual ammonia content of the nitriding gas of 8%. In this atmosphere, the workpiece was left in the nitriding furnace at a temperature of 550 ° C. for 82 hours, then taken out, slowly cooled and finally subjected to a test which gave the value shown in FIG. 3 (curve I).

In Figure 3, the hardness curve for the needle seat on the ordinate is given as Vickers hardness (HV 0.5). The abscissa shows the distance from the edge of the needle seat in millimeters.

Example II:

The procedure was the same as in Example I, but the workpiece was treated in eight percent citric acid (citric acid dihydrate) between the first and second step. Formic acid, acetic acid or oxalic acid would also be suitable. This operation only lasted a few minutes and was carried out with moderate warming and good flow. Thanks to this bath, the time spent in the nitriding furnace for gas nitriding could be reduced to 41 hours, i.e. to about half. The finished workpiece was checked again. The measured hardness Vickers are again shown in Fig. 3, the curve is designated II. It can be seen that, despite half the time, even higher values were achieved in some cases.

Furthermore, the workpiece was also checked on all outer and inner surfaces (bores, spray holes 6, blind hole 5 in FIG. 2). All surface layers showed well matching nitriding depths and hardness values.

Claims

1. Process for nitriding workpieces made of alloyed steels, consisting of the following steps:

a) The workpiece is subjected to a nitrocarburizing process in a salt bath with a cyanate content between 30% and 80% until the connecting layer that forms reaches a thickness in the range from 0.5 to 3 μm;

b) The workpiece is then subjected to a gas nitriding process at a temperature between 500 and 600 ° Celsius, the composition of the nitriding gas being set to a low nitriding potential, so that the connecting layer disintegrates while increasing the geometric surface area.

2. The method according to claim 1, characterized in that the nitriding potential in the gas nitriding process corresponding to step b) is in the range between 0.04 and 0.4.

3. The method according to claim 1, characterized in that between the steps a) and b) the workpiece is treated with an organic acid.

4. The method according to claim 3, characterized in that the organic acid is citric acid.

5. workpiece which is nitrided by the method according to any one of claims 1 to 3.

6. Workpiece according to claim 5, which is the body of an injection nozzle.