WO2000063454A1

WO2000063454A1 - Corrosion-free iron-nickel alloy for residual-current circuit-breakers and clockworks

Info

Publication number: WO2000063454A1
Application number: PCT/EP2000/003349
Authority: WO
Inventors: Johannes Tenbrink
Original assignee: Vacuumschmelze Gmbh
Priority date: 1999-04-15
Filing date: 2000-04-13
Publication date: 2000-10-26
Also published as: EP1169487A1

Abstract

An alloy on the basis of 40-55 wt. % nickel and iron becomes corrosion-proof by alloying it with 0.5-6 wt. % chromium. The surfaces of the components produced by said alloy are hardened by nitrating. The alloy is particularly useful for the use in relays and clockworks. The concentration of Ca, Mg and S together amounts to less than 0.009 wt. %.

Description

description

Corrosion-free iron-nickel alloy for residual current circuit breakers and watch drives

The invention relates to an alloy based on Nikkei and iron with a nickel content of 40 to 55 wt .-% and a chromium content between 0.5 and 6 wt .-%.

Such an alloy is known from US-A-5, 135, 588. The known alloy based on iron and nickel has a Ni content of 40-52% by weight, a Cr content of 0.5-5% by weight and concentrations of sulfur, oxygen and boron below 0.005% by weight .-% on. Due to its good magnetic properties, the alloy is suitable as a material for magnetic cores.

A disadvantage of the known alloy is that individual corrosion centers and blooming can occur on the surface of the body made from this alloy, which are particularly troublesome when smooth surfaces are important. This is particularly the case with precision parts such as relays for residual current circuit breakers or stepping motors for watch drives. There, small geometric deviations on the surface of a component made from the known alloy can result in the inoperability of the component in question.

Starting from this prior art, the object of the invention is to create an alloy with improved corrosion resistance that is suitable for precision mechanical applications.

This object is achieved in accordance with the invention in that the concentration of Ca, Mg and S together is less than 0.009% by weight. -%. The low content of Ca, Mg and S significantly improves the corrosion resistance of the alloy. The sharp deoxidizing agents calcium and magnesium generally form oxides in the melt, which have a corrosive effect on the surface. Sulfur also forms sulfides, which also form corrosion centers on the surface of a component made from this alloy. The low concentration of calcium, magnesium and sulfur therefore significantly improves the corrosion resistance of the iron-nickel alloy, so that it can also be used for precision mechanical components.

Further advantageous compositions and uses of the alloy according to the invention are the subject of the dependent claims.

The invention is explained in more detail below with reference to the accompanying drawings. Show it:

Figure 1 is a schematic representation of a relay for a residual current circuit breaker;

FIG. 2 shows a divided stator of a stepping motor for a clockwork;

Figure 3 is a graph showing the dependence of corrosion resistance on the Cr content; and

Figure 4 is a diagram showing the dependence of the saturation induction on the Cr content.

Figure 1 is a representation of a relay 1 for a residual current circuit breaker, which has an armature 2 and a yoke 3. Both the armature 2 and the yoke 3 are made of an alloy based on iron and nickel. A permanent magnet 4 is connected to the yoke 3 and generates a magnetic flux which holds the armature 2 on the yoke 3. A magnetic coil 5 is generated by a trip coil 5, which is dependent on the fault current and counteracts the flow of the permanent magnet 4. Infoigen that a return spring 6 pulls the armature 2 from the yoke 3 and interrupts the circuit to be monitored.

Functional surfaces 7 of the yoke 3 and functional surfaces 8 of the armature 2 are usually ground during the manufacture of the relay 1 in order to ensure a precise function of the relay 1 with the lowest possible output power. It is also essential for a precise function of the relay 1 that the functional surfaces 7 and 8 are and remain free of impurities. If, however, due to the corrosion

Form functional surfaces 7 and 8 impurities, due to the unavoidable relative movement between the armature 2 and the yoke 3, local material is raised between the armature 2 and the yoke 3. As a result, the air gap between the armature 2 and the yoke 3 can change, so that the release power of relay 1 is changed.

The situation is similar with watch drives. FIG. 2 shows a stepping motor 9 for a clock run, which has a divided stator 10. The stator 10 comprises a U-shaped coil core 11, which is closed by a return part 12. A circular rotor opening 13 is formed in one of the legs of the coil core 11. The cross section of the coil core 11 is further reduced by notches 14 in the region of the rotor opening 13. Typically, the cross section of the coil core 11 at this point is only about 0.5 x 0.5 mm ² . As a result, the stator is magnetically divided, so to speak. The stator can also be divided mechanically by interruptions or gaps. A rotor, not shown in FIG. 2, made of a permanent magnet is located in the rotor opening 13. An excitation coil 15 is also applied to the coil core 11, by means of which a magnetic flux is generated in the coil core 11 in the stator 10. In the area of the notches 14, the stator 10 quickly saturates, so that the magnetic flux enters the rotor opening 13 in the area of the notches. This sets the rotor in rotation.

If corrosion-related contaminants form in the area of the notches 14, the magnetic flux entering the rotor opening 13 changes, which impairs the functionality of the watch mechanism 9. The same applies if impurities form in bulges 16 of the rotor opening 13 or occur in the area of the rotor opening at all.

As can be seen from FIG. 3, the corrosion resistance of an alloy based on iron and Nikkei improves with increasing chromium content.

The results of experiments are shown in FIG. 0.7 mm thick strips of NiFeCr alloy were subjected to various tests.

On the one hand, this is a climate test of the total duration of 48 hours with alternating three hours at 55 ° C with 95% relative humidity and three hours at 20 ° C with 45% relative humidity.

The salt spray test is a test in which the strips are stored in a desiccator over a previously boiled-up 20% NaCl salt solution for 24 hours.

FIG. 3 shows the percentage of parts obtained with rust points in the respective test as a function of the Cr content. There is a clear dependence of the corrosion resistance on the Cr content of the alloy, with a Cr content of about 1.5% corrosion resistance in these tests.

However, the saturation production is reduced by the increasing Cr content.

FIG. 4 shows the course of the induction Bio and B ₂₀ at a magnetic field strength of 10 A / cm and 20 A / cm. Up to a Cr content of <6% by weight, the saturation reduction can be kept above one Tesla. Below a Cr content of 3%, the saturation modulation is above 1.35 Tesla. This is sufficient for a functional watch drive.

In order to prevent the above-mentioned impurities and blooming, it is also necessary to limit the concentration of calcium, magnesium and sulfur in the alloy. The results of various tests are summarized in Table 1. The alloy AI is a first comparative example, which once again clarifies that the chromium content must not fall below a minimum limit for high corrosion resistance. The alloys B1 to B5 represent preferred exemplary embodiments with an optimal nickel content. Alloys Cl to C4 are further comparative examples in which the concentrations of Ca, Mg and S have been increased beyond the permissible upper limit. Alloys Dl to D4 are further exemplary embodiments of the invention, with nickel concentrations deviating from the optimum nickel content. Compared to the alloys B1 to B5, this sometimes results in poorer magnetic properties. In the case of a high nickel content, as in the exemplary embodiment D1, this is based on the fact that the induction which can be achieved decreases at a higher nickel content beyond the optimum nickel content. If the nickel content is below the optimum nickel content, on the other hand, it is necessary to choose high concentration ar Cr in order to obtain the required corrosion resistance: a. However, a high chromium content affects the Saturation induction disadvantageously. At nickel contents above 55% by weight, the saturation induction also deteriorates due to the high nickel content to values that are inadequate for many applications. This is particularly clear from the exemplary embodiments D3 and D4. While in embodiment D3 the salt spray test was negative due to the Cr content of 2.97% by weight, the alloy according to embodiment D4 proved to be both in the climatic test and in the due to the high Cr content of 5.9% by weight Salt spray test as corrosion-resistant.

In addition to the concentrations in% by weight given in Table 1, the alloys each contain 0.4-0.6% by weight of manganese and 0.1-0.3% by weight of silicon, the rest being Fe. The magnetic properties were determined on punched rings with an outer diameter of 28.5 mm and an inner diameter of 20 mm and a thickness of 0.7 mm after final annealing at 1150 ° C. for five hours under hydrogen. The punched rings produced in this way were also used to assess the corrosion resistance. The purity index was determined on sections of finally annealed alloy samples, the total characteristic value K _{0 being determined in} accordance with DIN 50602.

DIN 50602 also describes the testing of non-metallic inclusions in the form of oxides and sulfides. For this purpose, a longitudinal metallographic section of the alloy to be examined must be made. An area of at least 100 sqmm is used for evaluation. In the process K, the inclusions are determined from a defined inclusion size and the degree of purity of the alloy is given by a summarizing characteristic value K, which characterizes the area of the inclusions and which corresponds to a weighted sum of the total area of the individual types of inclusions. The test according to K0 is the most sensitive test. In order to achieve reliable corrosion resistance and high permeability, in particular a high maximum permeability μ _ma above 80,000, the concentrations of Ca, Mg and S must be limited. The following upper limits result:

For calcium 0.0025% by weight, preferably 0.0015% by weight; for magnesium 0.0025% by weight, preferably 0.0015% by weight; for sulfur 0.004% by weight, preferably 0.002% by weight.

Alloys that comply with these limit values show reliable corrosion resistance and a high maximum permeability. By limiting the concentration of the sharp deoxidizing agents Ca and Mg, the formation of hygroscopic oxides is reduced, so that there is no formation of corrosion centers on the surface of workpieces made with these alloys. Due to the low content of S, only little corrosion-triggering sulfides are formed. In addition, the low concentration of oxides and sulfides makes it easier to move the Bloch walls, which results in high permeability.

Freedom from coarse contaminants such as non-metallic inclusions and slag residues is also essential. With relay 1 for residual current circuit breakers, the functional surfaces of yoke 3 and armature 2 are ground. If such impurities are ground in the process, this generally leads to a reduced service life of the relay 1. Because of the unavoidable relative movement between armature 2 and yoke 3 when opening and closing the relay 1, local contamination of material occurs between the armature 2 when such impurities are present and the yoke 3. As a result, the air gap between the armature 2 and the yoke 3 is changed, and with it the release power of the relay 1. The desired number of operating cycles of, for example, 4000 switching games with a change in the output power of less than ± 20% can then be achieved can no longer be achieved. In order to test the suitability of the alloys listed in Table 1, alloy B3 was used with 47.5% by weight: Ni, 1.77% by weight Cr, 0.48% by weight Mn, 0.20 % By weight Si, 0.002% by weight S and <0.001% by weight Mg, <0.001% by weight Ca,

<0.001% by weight of Al, remainder Fe and inevitable impurities 20 anchors 2 and 3 yokes each. In a measurement with a DC bias of 0.45 Tesla, a voltage drop across the 20 pairs of armature 2 and yoke 3 was found at a test current of I _eff = 10 mA

Winding achieved on average 17 ± 1 mV. For comparison, the same measurement was carried out with a standard alloy with the composition 47.5% by weight of Ni, 0.5% by weight of Mn, 0.2% by weight of Si, balance Fe and manufacturing-related impurities. The voltage drop was 17.5 ± 1.5 mV. This shows that the alloy B3 can be used to manufacture parts for sensitive residual current circuit breakers which, in terms of their magnetic properties, are in no way inferior to residual current circuit breakers with standard alloys.

The alloys B1 to B5 listed in Table 1 are also suitable as a material for the stator 10 in the stepping motors 9 for watch drives if special corrosion resistance is required.

It is in particular possible to do without the coating of the stator 10 which has been customary to prevent corrosion.

XI (0H In connection with relay 1 for residual current circuit breakers, the service life of relay 1 can be significantly improved by forming a hard surface layer through nitriding.

After the final glow at temperatures above 1000 ° C necessary for the adjustment of the soft magnetic properties of the armature 2 and the yoke 3, the material is very soft. The Vickers-Harte HV is usually around 100. As already mentioned, the functional surfaces 8 of the armature 2 and the yoke 3 are then ground in order to ensure a precise function of the relay 1. However, since the functional surfaces 8 are worn by the ultimately unavoidable relative movement between the armature 2 and the yoke 3, the air gap between the functional surfaces 8 is changed. The resulting change in the shear of the hysteresis loop changes the effective permeability of the magnetic circuit and thus the sensitivity of relay 1. The tripping power therefore changes with an increasing number of switching cycles. If a change of +/- 20% of the originally set trigger power is exceeded, relay 1 is no longer usable. The term service life is therefore understood to mean the number of switching cycles, after which the draw power deviates by a maximum of 20% from the initially set draw power.

The use of a chromium-containing nickel-iron alloy, such as an alloy with 47.5% by weight Ni, 1.9% by weight Cr, 0.45% by weight Mn, 0.15% by weight Si and a limited content of Ca, Mg and S initially has the advantage of improved corrosion resistance. Alloying with Cr also opens up the possibility of targeted surface hardening by nitriding. This means that Oberflacnenharten can be set with a Vickers-Harte HV above 20C. The connection layer formed by nitriding must not become too thick, since it would act as an air gap in the magnetic circuit of relay 1. In addition, sιc ° increases the coercive field strength H _c by nitriding. However, if the connecting layer is thinner than 25 μm, preferably 10 μm, the increase in the coercive force H _c is within reasonable limits.

Table 2:

Table 2 shows examples of parts made of an alloy based on nickel, iron and chromium, the surface of which has been treated by nitriding.

It should be noted that nitriding in a temperature range of 400 to 800 ° C, preferably between 480 and

600 ° C, layer thicknesses below 10 μm can be achieved.

It should also be noted that in addition to nitriding with the aid of NH3, an additional C or O activator in the process gas, that is to say nitrocarbons or oxynitriding, is also possible. Surface treatment with nitriding increases the service life of relay 1 considerably. Around 5000 switching cycles can be carried out with less than +/- 20% change in the output power.

Claims

claims

1. Alloy based on nickel and iron with a nickel content of 40 b s 55 wt .-% and a chromium content between 0.5 and 6 wt .-%, characterized in that the concentration of Ca, Mg and S together less than

0.009% by weight.

2. Alloy according to claim 1, characterized by a content of

Ca <0.0025% by weight,

Mg <0.0025% by weight, S <0.004% by weight.

3. Alloy according to claim 2, characterized by a content of Ca <0.0015% by weight,

Mg <0.0015% by weight, S <0.002% by weight.

4. Alloy according to one of claims 1 to 3, characterized by a content of O <0.01% by weight, N <0.005% by weight, C <0.02% by weight.

Alloy according to one of Claims 1 to 4, characterized by a content of Mn <1% by weight and a content of Si <1% by weight.

6. Alloy according to one of claims 1 to 5, characterized by a content of Cr> 1.5 wt .-%.

7. Alloy according to one of claims 1 to 6, characterized by a content of Cr <3 wt .-%.

8. Alloy according to one of claims 1 to 7, characterized by a content of Ni> 45 wt .-%.

9. Alloy according to claim 8, characterized by a Ni content between 46.5 wt .-% and 48.5 wt .-%.

10. Part made of an alloy according to one of claims 1 to 9, characterized in that a surface of the part has a nitride-containing connecting layer with a thickness co 25 microns.

11. Part according to claim 10, characterized in that the thickness of the surface layer is ω 10 microns.

12. Relay for a residual current circuit breaker with a yoke (3) and a yoke (3) closing armature (2), characterized in that the armature (2) and / or the yoke (3) made of an alloy according to one of claims 1 to 9 are made.

13. stepper motor for watch drives with a stator (10), characterized in that the stator (10) is made at least in sections from an alloy according to one of claims 1 to 9.

14. A method for treating the surfaces of a part made of an alloy according to one of claims 1 to 9, characterized in that a connecting layer with a layer thickness below 25 microns is formed at least partially on the surface of the part by nitriding in an atmosphere containing NH3.

15. The method according to claim 14, characterized in that a connecting layer is formed with a layer thickness below 10 microns.

16. The method according to claim 14 or 15, characterized in that the increase in the coercive force H _c is limited by the formation of the connection layer to 300 mA / cra.

17. The method according to claim 16, characterized in that the increase in Koezitivfeidstarke is limited to less than 100 mA / cm.

18. The method according to any one of claims 14 to 17, characterized in that the surface treatment is carried out in the temperature range from 400 to 800 ° C.

19. The method according to claim 18, characterized in that the surface treatment is carried out in the temperature range from 480 to 600 ° C.

20. The method according to any one of claims 14 to 19, characterized in that the surface is nitrocarburized.

21. The method according to any one of claims 14 to 19, characterized in that the surface is oxynitrided.