EP0132602B1 - Salt bath for the currentless production of wear-resistant boride layers - Google Patents

Abstract

There is described a salt bath based on an alkali and/or alkaline earth metal halide with which there can be produced without the use of current adherent and wear resistant boride coatings on metallic workpieces. This bath contains gaseous boron monofluoride or a compound from which there is formed intermediately boron monofluoride. Advantageous there is used a salt bath containing 30-60% BaCl2, 10-25% NaCl, 1-20% boron oxide or borate, 10-30% NaF, and 1-15% B4C.

Description

The invention relates to a salt bath based on alkali and / or alkaline earth halides for the currentless production of wear-resistant boride layers on metallic materials at 650 to 1100 ° C. It is used in particular to produce single-phase, hard and adhesive boride layers on steels to increase wear resistance and improve corrosion resistance.

Boronizing for: Wear protection of steel and refractory metals has been a well-known process. Diffusion of the element boron into the surface of the treated workpiece and reaction with the base material creates dense, uniform layers of the respective boride, on iron, for example, the borides FeB and Fe ₂ B. The borides have significantly changed properties compared to the pure metals, in particular they are Most borides are very hard, corrosion-resistant and therefore extremely wear-resistant. The boride layers are firmly connected to the base material by diffusion. With regard to their wear resistance, borated steels, for example, are partly superior to steels treated by nitriding or carburizing.

A large number of technical process variants have therefore been developed in the past according to which boride layers, in particular on steel, can be produced.

In practice, boriding in solid boriding agents is used almost exclusively. The parts to be treated are packed in iron boxes in a boron-releasing powder, usually mixtures of boron carbide, aluminum oxide, silicon oxide and the like, with activating additives such as ammonium fluoride or potassium borofluoride (e.g. DE-C-1,796,216). The boxes are tightly sealed and annealed for a while, the desired boride layers being formed in direct solid-to-solid reactions or by transporting the boron over the gas phase.

These powder processes have a number of disadvantages.

All parts must be carefully inserted into the powder by hand. Furthermore, the powders sinter together strongly during annealing, so that the borated parts are very difficult to remove and must additionally be cleaned. At the same time, large amounts of boron powder are required, which makes the process extremely expensive. After all, you have to count on uneven layers when boring in powders. A quality control is not possible by examining a single part, since this is not representative of the batch, because the quality of the parts essentially depends on the care taken when inserting them into the boron powder. Small parts, parts with thin holes, undercuts etc. cannot be borated in the powder at all or only with extreme effort.

There has been no shortage of attempts to compensate for these disadvantages by other methods. Attempts were made to apply the boron powder to the parts in the form of a slurry or paste, to evaporate the solvent and to anneal the parts in the resulting crust from boroning residues (eg H. Kunst, O. Schaaber, Härtereitechn. Mitt. 22 [1967], 275-284).

However, these methods, known as paste processes, are only modifications of powder boriding and have the additional disadvantage that large amounts of stubborn residues have to be removed from the parts after treatment and that even application of the paste is extremely difficult, especially in the case of parts of complex shape.

It is equally difficult to avoid blistering when applying paste or crumbling off when annealing.

An attempt was therefore also made to borate in gaseous media, for example using boron halide / hydrogen mixtures (EP-A-76488). Although boride layers are obtained in this way, they are technically unusable or can only be produced in a very complex manner. When boronizing with boron halides, there is always uncontrollable corrosion of the base material, since it reacts with the boron halide to form metal halide and boride. This creates perforated, undernourished boride layers. Boronizing with diborane is technically almost impossible due to the extreme explosiveness and high toxicity of this gas. In addition, boronizing with the gaseous media mentioned is also uneconomical because of the high prices of the boron compounds. For these reasons, attempts have been made to avoid the disadvantages mentioned by boriding in liquid media, especially in molten salts. For example, melts based on alkali and alkaline earth chlorides with B ₂ 0 ₃ , borax or KBF _{4 have been} described. In such melts, however, a material can only be borated if electrolysis is carried out at the same time. The workpieces to be borated are connected cathodically, the crucible or a graphite rod serves as an anode. These methods have the disadvantage that different current densities at com _- plicated produce uneven layer thickness parts. In addition, oxygen, chlorine or fluorine is generated on the anode, which causes severe corrosion. Furthermore, charging is difficult because electrical contacting of the individual parts is required. For these reasons, electrolytic boriding processes in molten salts have not been able to be introduced in industry.

On the other hand, very little is known about boriding in molten salts without electrolysis. In the hardening techn. Mitt. 17 (1962) 131-140 describes a melt of 80% NaCl, 15% NaBF ₄ and 5% B ₄ C, but at the same time it is pointed out that the NaBF ₄ dissolved in the melt very quickly becomes NaF and BF ₃ disintegrates, which escapes. Due to this instability of the melt, no boroning effect can be obtained that is constant over time; the melt becomes inactive very quickly. DE-OS 3118585 specifies a process for boronization in molten salts without electrolysis, in which the boron required for the boronization is released by reacting borax with silicon carbide. Because of the oxidation of SiC to Si0 ₂ by atmospheric oxygen or by digestion of SiC with borate, it forms in such melts, however, an impenetrable silicate blanket will soon appear on the surface of the bath.

A salt bath is known from SU-A-953002, which consists of sodium halide, sodium borate and boron carbide. Because of the high borate content, these melts are highly viscous and poorly water-soluble. In addition, two-phase FeB / Fe ₂ B layers are obtained, which are undesirable.

Electroless boron salt baths are also known which contain boric acid and fluoroborate in addition to boron carbide (GB-B-959 533) or an alkali or alkaline earth metal halide and fluoroborate (US-A-3634145). However, these salt baths have also not been able to establish themselves in practice.

It was therefore an object of the present invention to develop a salt bath based on alkali and / or alkaline earth halides for the electroless generation of wear-resistant boride layers on metallic materials at temperatures of 650 to 1100 ° C., which is simple and inexpensive to operate, without crusts forms the bath surface and provides adherent boride layers that consist of single-phase Fe ₂ B layers, particularly in the case of steels.

This object is achieved according to the invention in that the salt bath contains boron monofluoride or compounds from which boron monofluoride is formed as an intermediate under bath conditions.

The boron monofluoride which acts as a borating agent can be added to the melt from the outside or can advantageously be generated in the melt itself. In the former case, the gaseous boron monofluoride produced in a known manner by heating boron trifluoride with finely divided boron is introduced into the salt melt during the boronization process.

Electroless boronation baths that are particularly easy to operate are obtained when the boron monofluoride is generated in the molten salt itself. Surprisingly, it was found that one can borate in an inert, readily water-soluble and low-viscosity melt from alkali and alkaline earth chlorides if boroning agent, such as boron carbide powder, which is suspended therein is activated by trifluoroboroxal and caused to release boron monofluoride, which in turn is caused on the component surface disintegrates and in this way transfers the boron from the boron carbide to the workpiece.

The required trifluoroboroxol (BOF) ₃ is also generated in the melt itself. This is based on the knowledge that (BOF) ₃ can be produced very well in an inert melt from alkali / alkaline earth chlorides by reacting boron oxide or borates with alkali / alkaline earth fluorides, the presence of barium ions in particular exerting a positive influence. The trifluoroboroxol that is produced in this way in a very slow reaction and in a concentration that is hardly measurable converts with the boron carbide suspended in the melt to form the boronizing agent, the boron monofluoride BF.

It is therefore preferable to use salt melts which consist of 30 to 60% by weight of barium chloride, 10 to 25% by weight of sodium chloride, 1 to 20% by weight of boron oxide and / or alkali borates and / or alkaline earth borates, 10 to 30% by weight Sodium fluoride and 1 to 15 wt .-% boron carbide.

The trifluoroboroxol formed by reacting boron-oxygen compounds with fluorides causes a slow, controlled digestion of the boron carbide, whereby boron-active boron monofluoride is released, which boron can release on the workpiece surface through decay. Instead of boron carbide, other known borating agents, such as amorphous boron or calcium boride, can also be used.

The boriding effect of the melts can be influenced above all by variations in the concentration of boron oxide or borate and in alkali metal / alkaline earth metal fluoride, as well as by changing the temperature and - to a small extent - by changing the concentration of the boron carbide. It has thus been shown that the salt melts according to the invention make it possible to produce layers of Fe ₂ B on steel without the undesirable boron-rich phase FeB occurring.

Salt melts which are composed of 40-55% by weight BaC1 ₂ , 5-15% by weight B ₂ 0 ₃ , alkali and / or alkaline earth borate, 18-25% by weight NaF, 15-20% are preferably used. -% NaCI and 4-10 wt .-% B ₄ C exist.

The molten salts according to the invention enable extremely simple work in practice. The salt mixture is melted in a melting crucible made of heat-resistant steel and the B ₄ C is kept in suspension by introducing an inert gas stream, for example nitrogen. The workpieces to be borated are attached to a charging frame, preheated to 350 ° C with hot air, for example, and then hung in the melt. Steels produce uniform, very wear-resistant, single-phase layers of Fe ₂ B, whereby the layer thickness can be varied depending on the base material and the duration of the treatment. The parts are removed from the melt and quenched, for example in a quenching bath made of sodium and potassium nitrate, which is customary in hardening technology, and then rinsed with water. In this way, no fluoride gets into the waste water.

The salt bath according to the invention can thus be easily integrated into the existing infrastructure of a salt bath hardening shop, without significant investments or additional wastewater treatment being required. The method of operation largely corresponds to that of salt bath coal or salt bath nitriding. The melts are composed of relatively cheap components. A boriding process is thus available which can compete with the known industrial processes of salt bath nitriding and salt bath coaling in terms of operation and costs.

The following examples show salt bath compositions for carrying out boronations.

example 1

100 kg of a salt mixture of 50 kg BaC1 ₂ , 15 kg NaF, 20 kg NaCl, 5 kg B ₂ 0 ₃ and 10 kg B ₄ C powder are melted in a crucible furnace of size 30/80 and the boron carbide is suspended by introducing an inert gas stream . With a treatment at a treatment time of 2 hours, an FeB-free boride layer of Fe ₂ B with a thickness of 60 μm is obtained on CK 15 steel.

Example 2

100 kg of a salt mixture of 50 kg BaC1 ₂ , 25 kg KF, 15 kg NaCl, 5 kg B ₄ C powder are melted in a crucible furnace of size 30/80 and the boron carbide is kept in suspension by introducing an inert gas stream, for example nitrogen. At a treatment temperature of 850 ° C. and a boriding time of 2 hours, an FeB-free Fe ₂ B boride layer of 30 μm thick is obtained on CK-15 steel.

Example 3

Particularly good boride layers provide molten salts with the following composition: 50 kg BaC1 ₂ , 16 kg NaCl, 10 kg B ₂ 0 ₃ , 18 kg NaF and 6 kg B ₄ C.

Claims (3)

1. A salt bath based on alkali and/or alkaline earth halides for the currentless production of wear- resistant boride coatings on metallic materials at temperatures of from 650 to 1,100°C, characterised in that it contains boron monofluoride or compounds from which boronmonofluoride can be obtained indirectly under bath conditions.

2. A salt bath according to Claim 1, characterised in that it is composed of from 30 to 60% by weight of barium chloride, from 10 to 25% by weight of sodium chloride, from 1 to 20% by weight of boron oxide and/or alkali borates and/or alkaline earth borates, from 10 to 30% by weight of sodium fluoride and from 1 to 15% by weight of boron carbide.

3. A salt bath according to Claims 1 and 2, characterised in that it is composed of from 40 to 55% by weight of barium chloride, from 15to 20% by weight of sodium chloride, from 5 to 15% by weight of boron oxide and/or alkali borates and/or alkaline earth borates, from 18 to 25% by weight of sodium fluoride and from 4 to 10% by weight of boron carbide.