Boronizing agent in paste form

Details Boronizing agent in paste form Boronizing agent in paste form

2001-09-24

2003-01-07

Sheehan, John (Department: 1742)

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

C148S280000, C148S217000, C148S283000, C252S385000

Reexamination Certificate

active

06503344

ABSTRACT:

INTRODUCTION AND BACKGROUND
The present invention relates to a boronizing agent in the form of a paste for the production of boride layers on metallic work pieces and materials. The purpose of this paste is, in particular, to produce single-phase, hard and tenacious boride layers on ferrous materials in order to increase wear resistance and in order to improve the corrosion resistance of such workpieces.
Boronizing has long been known as a process for protecting iron, steel and refractory metals from wear. Dense, uniform layers of particular borides, for example the borides FeB, Fe
2
B on iron, are produced by elemental boron diffusing into the surface of the workpiece being treated and reacting with the base material. In comparison with the pure metals, the borides have considerably modified properties, in particular most borides are very hard, corrosion-resistant and thus extremely wear-resistant. Since they are produced by diffusion and a solid-state reaction, the boride layers are solidly bonded to the base material. With regard to wear resistance, some boronized steels are, for example, superior to steels treated by nitriding or carburizing.
Numerous means and industrial processing variants have accordingly been developed in the past by means of which boride layers may be produced, in particular on steel.
In practice, boronizing is predominantly performed in solid Boronizing agents. In this case, the parts to be treated are packed in iron boxes in powder mixtures which substantially consist of boron-releasing substances, activating substances with the remainder being inert, refractory extenders. The sealed boxes are heat treated for a certain period, wherein the desired boride layers are formed on the parts in a direct solid-state reaction or by transport of the boron in the gas phase. This is well known in the art.
Boronizing is conventionally performed at temperatures of between 800 and 1100° C. and in particular between 850 and 950° C. Achievable layer thicknesses of the boride layers are normally in the range between 30 and 300 &mgr;m.
Boron-releasing substances which may be considered for use as boronizing agents are amorphous and crystalline boron, ferroboron, boron carbide and borates such as borax. Suitable activating substances are chloride or fluoridereleasing compounds such as alkali metal and alkaline earth metal chlorides or fluorides. Fluoroborates, such as in particular potassium tetrafluoroborate, are particularly widely used as activators. Typical extenders are aluminum oxide, silicon dioxide and silicon carbide. Boronizing agents of this type are described, for example, in German Patent 17 96 212. A typical composition which has hitherto proved successful as a boronizing agent contains approx. 5 wt. % of boron carbide, 5 wt. % of potassium tetrafluoroborate and 90 wt. % of silicon carbide. Boronizing agents of the stated type are normally used as powder mixtures. They may, however, also be formulated as pellets (for example DE-OS 21 27 096) or as pastes (for example DE-OS 26 33 137). In the case of pellets and pastes, the compositions also contain subordinate quantities of binders and water respectively.
Processes have furthermore been developed which operate with gaseous boronizing agents such as diborane, boron halides or, alternatively, in molten salt baths with boron carbide and borax as the boron-releasing substances. These latter-stated processes have not become well established due to the toxicity of the compounds and due to processing disadvantages, such as the elaborate control measures required to ensure a uniform boronizing action. Recent attempts to produce boride layers using plasma processes are not suitable for all applications due to the influences of charging and complex geometric shapes. Plant and equipment costs are moreover very high. Solid boronizing agents, some of which are also used in paste form, thus still retain their dominant position for surface boronizing since they have the advantages of being simple to use and providing good boride layers.
The most common boronizing processes using known solid boronizing agents do, however, have the disadvantage that they demand highly elaborate processing technology in order to produce single-phase iron boride layers in particular on ferrous materials (c.f. for example EP 0 387 536 B1).
Since the two borides Fe
2
B and FeB have differing properties and multi-phase layers usually have poorer properties than single-phase layers, efforts are made to produce single-phase layers when boronizing.
Thus, in particular, the FeB phase, which has a higher boron content, is substantially more brittle than the Fe
2
B phase, which has a negative influence on the wear resistance of the boronized components. In boride layers thicker than 50 &mgr;m, an FeB case is also readily formed, which should, if at all possible, be avoided for the stated reason.
Using hitherto known boronizing pastes, it has previously been possible under conventional processing conditions to obtain single-phase layers only of a thickness of less than 50 &mgr;m. If thicker boride layers are to be obtained, it is necessary to perform post-diffusion by a complex heat treatment operation under a vacuum or in a salt bath or special boronizing agents are required (for example according to German patent application 198 30 654.7). Moreover, fluoride emissions are found in the exhaust gases from conventional boronizing pastes. Both post-diffusion and fluoride emissions result in layer porosity, which has a negative impact on layer properties.
With many materials, known boronizing pastes result in corrosive attack on the coated workpiece during the drying phase. As a result, paste residues adhere so strongly to the surface of the workpiece after treatment that cleaning the components with water is not sufficient and an additional jet cleaning operation is required, wherein there is also a risk that the boride layer which has been produced will also be affected. Such corrosive attack may be so severe that it has not previously been possible to use paste boronizing on certain grades of steel as it results in corrosive loss of material.
Known boronizing pastes are not stable in storage, in particular at elevated temperatures, due to dissociation of the activator KBF
4
accompanied by a reduction in pH.
An object of the invention is accordingly to provide a boronizing agent in the form of paste with which, in particular on ferrous materials, produces a virtually exclusively single-phase boride layer containing Fe
2
B.
Moreover, another object is also to reduce the content of water-soluble fluorides in these boronizing agents in paste form and that correct use should be accompanied by reduced fluoride emissions.
A further object is also to reduce the porosity of the boride layer formed.
Still further, it is an object to prevent corrosive attack and thus also facilitate cleaning of the components as well as to improve the storage stability of the boronizing paste.
SUMMARY OF THE INVENTION
It has surprisingly now been found that, in boronizing agents in paste form which include boron-releasing substances, activating substances and the remainder of inert, refractory extenders together with water and optionally auxiliaries required for paste formulation, these disadvantages may be overcome by the addition of small quantities of certain additives.
It has firstly been discovered that the porosity of the boride layer may be distinctly reduced by the addition of calcium carbonate and/or lithium carbonate, for example calcium carbonate. This brings about extended component service life. Hydrogen fluoride emissions are additionally reduced by fluorides, for example HF, being bound as CaF
2
. The optionally produced CaF
2
moreover brings about the positive effects described in German patent application 198 30 654.7.
It has moreover been discovered that corrosive attack by the boronizing paste on all investigated grades of steel may be completely suppressed by the addition of alkali metal or alkaline earth metal nitrites, for example sodium nitrite. As a r

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