EP0175157B1 - Process for boriding metal and metal alloys by means of solid boriding agents - Google Patents

Abstract

A process is provided for boriding metals and metal alloys in a fluidized bed at a temperature of from about 580 DEG to about 1300 DEG C. As a boriding agent, there was used a granular material comprising essentially spherical particles having a particle size of from about 0.025 to about 5.00 mm, which granular material was manufactured by spray drying a preferably aqueous suspension or dispersion based on materials that yield boron, and which can contain fillers, extenders and binders.

Description

It is known to produce very hard surfaces from borides on metals or metal alloys by reaction with boron-containing materials. This surface hardening can be achieved by means of gaseous substances such as diborane or boron halides, liquid media such as borax melts or solid borating agents on metal surfaces. For toxicological, economic and technological reasons, only solid borating agents could prevail in practice, i. H. Powders and pastes.

Methods for boronizing metals and metal alloys using powders or pastes are described in DE-PS 17 96 215 (H. Kunst, Elektroschmelzwerk Kempten GmbH; issued July 26, 1973), DE-PS 21 46 472 (W. Fichtl et al. , Elektroschmelzwerk Kempten GmbH, issued on September 7, 1978), DE-PS 22 08 734 (G. Wiebke et al. Elektroschmelzwerk Kempten GmbH, issued on July 31, 1980) and DE-AS 23 61017 (E. Preuschen, Vac- Hyd Processing GmbH, announced on August 30, 1979) described in detail.

In powder boriding, the parts to be borated are arranged in containers and tightly surrounded with boron-releasing powder. The containers are then placed in a preheated oven and kept at temperatures around 800 ° C to 1100 ° C, then cooled and then emptied.

In paste boriding (DE-AS 23 61 017), a layer of the borating agent that is as uniformly thick as possible is applied to the workpiece, dried and treated for several hours at temperatures of approximately 800 ° C. to 1100 ° C.

The borating agent usually contains crystalline or amorphous boron, boron carbide, ferroboron, borax or mixtures of at least two of these components as a boron-releasing substance, for example carbon black as fillers. Silicon carbide, silica, aluminum or magnesium oxide and, in particular, complex fluorides such as potassium tetrafluoroborate as activators. The temperature treatment is carried out in chamber, pot, belt conveyor, chain conveyor or vacuum ovens.

In the case of powder borating processes, the parts to be treated have to be packed and unpacked, which is associated with dust pollution. The warm-up and cool-down periods are relatively long due to the poor heat transfer through the boron powder. An excess of the relatively expensive boron medium is usually used. Paste borating requires a very evenly thick application of the paste. Drying the paste is also time-consuming.

Because of the known good heat exchange, the use of fluidized bed technology would therefore be advantageous. The implementation of coating processes in a fluidized bed is also already known. For example, US Pat. No. 3,405,000 describes a method for coating non-ferrous metal objects using a fluidized bed. The metal objects in the fluidized bed, the particles of which consist of metals, among which boron is also mentioned, are heated to 750 ° C. to 1,250 ° C. The fluidized bed is generated by an inert gas stream to which up to 50 vol.% Iodine vapor is added as a reactant. In addition, vacuum must be used in the reactor so that a uniform, smooth coating can be produced on the metal objects to be coated. In carrying out this process are called "iodine Transp _o rt _r eaktio _nen place .the is, some of the particles combine with the iodine to form volatile iodides, which are then decomposed into contact with the to be coated metal Ioberflächen and on this create a metal coating. In addition to the coating particles made of metals or metal compounds, inert particles based on oxide can also be present.

This patent therefore teaches the teaching of a metal coating process in a fluidized bed, in which solid and gaseous reactants react to form volatile compounds, which then decompose in the specified temperature range when they come into contact with the metal surfaces to be coated, forming a metal coating thereon, this thermal decomposition reaction being caused by Application of negative pressure is favored. Suggestions for a boronization process in a fluidized bed, in which a boride diffusion layer is to be generated by exclusively solid reaction participants in a thermochemical reaction on the metal surfaces to be borated from iron and non-ferrous metals, could not be derived from this prior art. The activators required to trigger this thermochemical reaction in the temperature range customary for boronization processes cannot be introduced into the reaction space as gaseous reactants, but must be available as solids in close contact with the boron-releasing solids.

The production of boride diffusion layers from solid boron dispensers and solid activators is known per se from the powder packing methods mentioned above, as is the fact that inert fillers are necessary for the control or control of the boriding action, which in particular in the case of ferrous metals form the formation of single-phase Fe ₂ B - Allow diffusion layers.

However, the use of solid boron dispensers, activators and fillers in the form of a simple powder mixture, as in the powder packing methods, is impossible under the known erosive conditions of a fluidized bed.

The object of the invention is therefore to provide a method for producing a solid borating agent To develop a granular form which enables a metal to be easily and quickly borated in a fluidized bed with fluidizing gas which is not involved in the chemical reaction. The shape and size of the particles alone are not decisive, but rather the combination of the components required for boronizing (boron donors, activators, fillers) in one and the same particle. This is to ensure that the essential components hit the surface to be borated at the same time, but also that premature disintegration of the granules under the hard fluidized bed conditions is avoided.

This object is achieved according to the invention by a process for the production of boron granules, which consists of almost spherical particles with a particle size of 0.025 mm to 5.0 mm based on boron-releasing substances, activators, fillers and binders, the boron granules by spray drying an aqueous suspension from the constituents mentioned using emulsifiers, auxiliaries and binders, selected from mono-, di- and / or polysaccharides, at temperatures between 120 ° C to 750 ° C. The boron granules produced by this process are preferably used for boronizing metals and metal alloys in a fluidized bed at a temperature of 580 ° C. to 1300 ° C., the fluidized bed being generated with fluidizing gas which is not involved in the chemical reaction.

A borating agent in granular form is known from DE-A-2 127 096 (H. Krzyminski, Deutsche Gold- und Silber-Scheideanstalt, published December 14, 1972). However, due to its cylindrical grain, it is unsuitable for a fluidized bed process. The known boron powders cannot be used in this process because of their grain size and grain distribution. In principle, all solid formulations of borating agents produced according to the invention can be used for boronizing in the fluidized bed, the almost spherical grain of which can be kept in a fluidized state at the reaction temperature in the flowing gaseous medium. Almost spherical particles with a grain size of 0.05 to 2.0 mm are preferred.

The boron granules produced according to the invention can be formulated, for example, from all powders which have hitherto been successfully used for boronizing metals. As boron-releasing substances, they can contain amorphous or crystalline boron, boron carbide, borax or metal borides, or mixtures of at least two of these substances. Boron carbide is particularly preferred. Soot, silicon carbide, aluminum, magnesium and silicon oxides, silicates, non-boronable metals, their mixtures or similar substances can serve as fillers which are also extenders. As activators, the borating agents can contain all substances, individually or in a mixture, which were previously used as activators in the boriding of metals and their alloys. Complex fluorides, in particular potassium tetrafluoroborate, are preferred.

According to the invention, a method that is not typical for this purpose is used to granulate the borating agent: spray drying. This process is generally used to make highly disperse and redispersible particles, i.e. H. Particles of low mechanical stability are used. The spray drying of the boron mixture, however, forms particles which are mechanically stable and, owing to their almost spherical geometry, their particle size, their narrow particle size distribution and their dimensional stability, are particularly suitable for use in a fluidized bed process under reaction conditions. Before the spray drying, binders, a dispersing agent which is inert towards the powder constituents and emulsifiers, are added to the powder to be granulated. Saccharides, disaccharides, polysaccharides and mixtures of at least two of these substances are preferred as binders. Water is preferred as the dispersing agent which is inert towards the powder components for environmental and cost reasons. Based on the weight of the boriding agent and stabilizer to be granulated, 10 to 100 percent by weight, preferably 20 to 70 percent by weight, of dispersing agent are added. It is possible to use more dispersants, but it requires higher energy consumption or lower throughput when spray drying. Emulsifiers can be added to the mixture to be granulated. Although not absolutely necessary, auxiliary substances such as protective colloids, anti-foaming agents and spraying aids can be added. Binder is preferably used in amounts of 2 to 30 percent by weight, based on the sum of the weight of dry granules, i.e. H. boron-releasing substance, fillers and activators, emulsifiers, auxiliaries and binders used; Amounts between 5 and 20 percent by weight are particularly preferred. The amounts of boron-imparting substance can be between 2 and 90 percent by weight, based on the dry granulate, depending on the affinities of the surfaces to be borated. The activator is used in amounts of 1 to 15, preferably 3 to 8 percent by weight. Larger amounts of activator have no advantages.

In the boriding process in the fluidized bed, the boriding granulate can be used as the only bulk material, but it can also be used in a mixture with a granulate which is inert to the boron-releasing substance. Such inert granules can consist, for example, of the fillers mentioned above.

The fluidized bed boriding process is carried out in a retort made of a gas-tight material which is stable at the reaction temperature, preferably in ceramic or ceramic-coated retorts.

Inert gases and gas mixtures or reducing gases are preferably used as fluidizing gases and gas mixtures used. Examples of inert gases or gas mixtures are nitrogen, argon and their mixtures. Examples of reducing gases or gas mixtures are hydrogen, ammonia cracking gas, forming gas (5-30% hydrogen, 70-95% nitrogen), hydrocarbons, mixtures of at least two of these reducing gases and mixtures of at least one reducing gas with at least one inert gas.

The boriding process is carried out at temperatures from 580 ° C. to 1300 ° C., preferably at 580 ° C. to 1100 ° C., in particular at 800 ° C. to 1100 ° C. The fluidized bed boring process allows a continuous or semi-continuous procedure for boring individual and serial parts, also in connection with subsequent treatments. In general, it is advisable to preheat the workpieces to be borated before the actual boriding step. Borier granules that have largely been used up can be removed from the fluidized bed during the process, for example by suction or pneumatic conveying; Unused borating agent can be added to the reactor at any time. Fully continuous operation can be achieved, for example, by guiding the boric agent stream in the moving bed. The boriding process can be followed by other process steps that have proven themselves in metal treatment. Boronizing steels can be followed, for example, by diffusion annealing, austenitizing, quenching and / or tempering.

In comparison with the powder boriding process, in which mostly a large excess of borating agent is used, the fluidized bed process allows a more economical use of the relatively expensive boriding medium. The fluidized bed boron creates a closed boride layer of uniform thickness. The fluidized bed process can borate all metals and metal alloys that could also be borated in the previously known processes. Examples of these metals or metal alloys are iron, cobalt, nickel, titanium, steels, hard metal and alloys which contain iron, cobalt, nickel and / or titanium. A single-phase iron boride layer is achieved on the surface of iron-containing alloys or iron, ie the iron boride formed consists essentially of Fe ₂ B. Most other processes produce two-phase layers, one phase of which contains Fe ₂ B and the other of which FeB. Tensions can occur in such two-phase layers containing iron boride, which ultimately lead to cracks.

Examples A. Preparation of the Borier Granules

• 20 950 g Siliziumcarbid
• 810 g Borcarbid (unter der geschützten Bezeichnung « Tetrabor der Firma Elektroschmelzwerk Kempten GmbH, München, BRD, handelsüblich)
• 1 160 g Kaliumtetrafluoroborat
• 2 000 g einer 50 Gew. %-igen wäßrigen Saccharose-Lösung
• 13 000 g Wasser und
• 0,2 g Emulgator (unter der geschützten Bezeichnung « Targo 1128 X der Firma Benckiser und Knapsack, Ladenburg, BRD, handelsüblich) wird bei 30 °C aufgerührt und dem Sprühturm, der auf ca. 350 °C vorgeheizt ist, von oben langsam zugeführt. Ein trockenes Granulat fällt mit ca. 60 °C an. Es besteht aus nahezu kugelförmigen Partikeln einer Korngröße zwischen 0,080 mm und 0,220 mm.

A suspension of

• 20 950 g silicon carbide
• 810 g boron carbide (commercially available under the protected name “Tetrabor of Elektroschmelzwerk Kempten GmbH, Munich, FRG)
• 1 160 g of potassium tetrafluoroborate
• 2,000 g of a 50% by weight aqueous sucrose solution
• 13,000 g of water and
• 0.2 g of emulsifier (under the protected name “Targo 1128 X from Benckiser and Knapsack, Ladenburg, Germany, commercially available) is stirred at 30 ° C. and slowly added to the spray tower, which is preheated to approx. 350 ° C., from above . A dry granulate is obtained at approx. 60 ° C. It consists of almost spherical particles with a grain size between 0.080 mm and 0.220 mm.

B. fluidized bed boriding

• 1 den Absaugstutzen,
• 2 ein feines Maschensieb,
• 3 ein Thermoelement,
• 4 und 5 Tragstangen bzw. Aufhängung für das Werkstück,
• 6 die Ausmauerung,
• 7 Heizelemente,
• 8 die Behälterwand,
• 9 das Wirbelbett,
• 10 das zu borierende Werkstück,
• 11 ein Thermoelement,
• 12 grobkörniges SiC/A1₂0₃ (zur besseren Verteilung des Wirbelgases)
• 13 eine Lochplatte,
• 14 die Gas-Ausgleichs- und Mischkammer und
• 15 die Gaszuführung.

The workpieces were heated to the desired reaction temperature for boronizing. The reaction is carried out in an externally heated fluidized bed, the inner wall of which is made of ceramic. A preferred embodiment of a usable apparatus is shown in Fig. 1. Mean here

• 1 the suction nozzle,
• 2 a fine mesh screen,
• 3 a thermocouple,
• 4 and 5 support rods or suspension for the workpiece,
• 6 the lining,
• 7 heating elements,
• 8 the container wall,
• 9 the fluidized bed,
• 10 the workpiece to be borated,
• 11 a thermocouple,
• 12 coarse-grained SiC / A1 ₂ 0 ₃ (for better distribution of the fluidizing gas)
• 13 a perforated plate,
• 14 the gas compensation and mixing chamber and
• 15 the gas supply.

Example 1 :

A plate made of Ck 45 steel was suspended in a boriding agent prepared according to A in a fluidized bed at 920 ° C. and kept at this temperature for 2 hours. After this time . cooled the sample in the raised shaft of the fluidized bed furnace in the gas atmosphere. Forming gas (95% nitrogen, 5% hydrogen) was used as the fluidizing gas. The surface of the sample was free of borating agent. Under these boriding conditions, a single-phase boride layer with a thickness of approx. 100 µm was created.

Example 2:

Pawls and switch cams made of St 37 K steel were borated according to Example 1, but for 3 hours, at 920 ° C. in a fluidized bed. Forming gas (90% nitrogen, 10% hydrogen) was used as the fluidizing gas.

The cut examination showed a single-phase boride layer thickness of approx. 140 _IL m.

Example 3:

42 CrMo 4 steel gears were 45 min. long at 860 ° C with borating agent prepared according to A using forming gas (90% nitrogen, 10% hydrogen) as fluidizing gas.

Then the gears were removed from the fluidized bed and then quenched in an oil bath. The gears had a single-phase boride layer 30 wm thick. The duration of the treatment from preparation to the end of curing was approximately 2 hours. According to the previously known methods, a treatment cycle of at least two days was necessary to achieve an equivalent result.

Claims (3)

1. Process for preparing borating granules which are composed of virtually spherical particles having a particle size of 0.025 mm to 5.0 mm based on boronreleasing substances, activators, fillers and binders, by spray drying an aqueous suspension of the constituents mentioned using emulsifiers, auxiliary substances and binders selected from mono-, di- and/or polysaccharides, at temperatures between 120°C and 750 °C.

2. Process according to claim 1, in which borating granules are prepared which are composed of virtually spherical particles having a particle size of 0.05 mm to 2.0 mm.

3. Use of borating granules prepared according to claim 1 or 2 for borating metals and metal alloys in a fluidized bed at a temperature of 580 °C to 1 300 °C, the fluidized bed being produced with fluidizing gas which does not participate in the chemical reaction.

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