Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C12/00—Solid state diffusion of at least one non-metal element other than silicon and at least one metal element or silicon into metallic material surfaces

Definitions

• the technical sector of the present invention is that of chromium-based coatings for steel with high resistance to friction wear and of processes using chromium-plating in the vapor phase.
• French patents 2,018,609 and 2,450,286 describe boriding processes according to which a metal part is subjected to a borating activator at a temperature between 850 ° C. and 1100 ° C. for a sufficient time. An iron boride layer is thus obtained, which improves the wear resistance of the treated steel.
• chromium to the surface of the material to be treated is done by means of halides which are the only chromium compounds found in the vapor state at diffusion temperatures.
• the layer formed by chromization consists of carbides of chrome. These carbides are, in the part the most superficial, the carbide M 23 C 6 and, in the underlying layers, the carbide M 7 C 3 M here designates a metal which, apart from chromium (Cr), can be iron (Fe), molybdenum (Mo), vanadium (V) etc ... We know that the respective hardnesses of M 23 C 6 and M 7 C 3 are of the order of 1200 HV 0.01 and 2 100 HV 0.01 .
• French patent 2,439,824 proposes a process according to which an ionic nitriding of a steel containing at least 0.2% carbon is first carried out, then a conventional chromization by gas route at a temperature between 850 ° C. and 1100 ° C.
• Nitriding requires ion bombardment of the sample at a temperature between 450 and 570 ° C at a pressure of 2.5 to 8 millibar. It is specified that such a treatment makes it possible to almost completely eliminate the carbide M 7 C 3 at the origin of the formation of cracks or flaking of the steel. This would form a single surface layer of carbide M 23 C 6 greater than 30 microns.
• the said layers may also contain 7 Cr 2 N nitrides when the halogen introduced into the cement is ammonium chloride NH 4 Cl. These coatings have resistance to l 'quite significant wear but the morphology and internal stresses of M 7 C 3 carbide limit their capacity.
• the nitriding mode ionic or gaseous
• carbonitrides had no major influence on the nature of the constituent phases of the layers obtained.
• the predominant criterion turns out to be in fact the initial thickness of the nitride layer.
• the coatings obtained are divided into two main sub-layers. One, on the surface, consists of the majority phase Cr 2 (C, N) and the carbide M 23 C 6 .
• the other, underlying, consists, for approximately half of the thickness of the coating, of carbide M 7 C 3 . But this carbide again retains a columnar structure harmful to good wear resistance.
• the layers obtained by chromizing or nitriding treatment followed by chromizing comprise a sublayer of chromium carbide M 7 C 3 . In its crystallized state, this carbide reduces the wear resistance of the chromized layers.
• the object of the present invention is to provide a person skilled in the art with a new coating and a process for obtaining it in which the formation of the carbide M 7 C 3 with its basaltic crystallization is avoided.
• the subject of the invention is therefore a coating for steel with high resistance to friction wear, characterized in that it comprises at least two surface layers, the first of which on the surface consists essentially of the phase (Cr, Fe) 2 B and the other internal mainly constituted by the phase (Fe, Cr) 2 B, Cr, Fe, B, denoting respectively chromium, iron, boron.
• the (Cr, Fe) 2 B phase may contain another phase of the M 23 C 6 type in which M mainly represents the chromium optionally substituted by iron, vanadium, molybdenum, nickel, manganese.
• the thickness of the first layer can be at least 12 ⁇ m and the thickness of the second at least 18 ⁇ m.
• This coating is preferably applied to a steel whose carbon content is at least 0.15%, by mass.
• the invention also relates to a process for obtaining a coating on a steel, characterized in that in a first stage, a boronization of the steel in the austenitic state is carried out at a temperature lower than or substantially equal to 950 ° C. for a time greater than or substantially equal to 4 hours and in that in a second step, chromization is carried out at a temperature less than or substantially equal to 980 ° C. for a time greater than or substantially equal to 10 hours, steps possibly followed by a heat treatment of quenching and tempering.
• Boronization can be carried out in the presence of a borating agent of the B 4 C + Na 2 B 4 07 type in a vinyl binder in order to produce a layer of iron borides FeB and Fe 2 B at least 30 ⁇ m thick.
• the . layer of iron borides has a thickness close to 40 ⁇ m.
• the chromization can be carried out in the vapor phase in the presence of a cement comprising an iron-chromium powder 60-40, an anti-sintering agent (Al 2 O 3 ) a transporter (NH 4 Cl) and a hydrogen flow rate of approximately 300 liters per hour.
• a cement comprising an iron-chromium powder 60-40, an anti-sintering agent (Al 2 O 3 ) a transporter (NH 4 Cl) and a hydrogen flow rate of approximately 300 liters per hour.
• the boriding is carried out at 950 ° C for 4 hours and the chromization at 950 ° C for 15 hours.
• the method according to the invention is applied to a low alloy steel of the 35 C D4 type.
• the main advantage of the invention is to avoid for the first time the formation of the carbide M 7 C 3 both in the surface layer and in the deeper layers.
• Another advantage lies in the fact that the invention makes it possible to provide coatings containing on the surface a phase notably containing chromium boride, which has never been obtained before.
• those skilled in the art have for the first time a coating containing chromium boride, the tribological properties of which are also known, without significantly modifying the chromization treatment.
• the method according to the invention can be applied to all types of steel regardless of their carbon content.
• a boriding treatment is first carried out. This treatment, regardless of the technique used (powder carburizing, salt baths, EKABOR, slury coating, ionic, etc.), must result in a compact layer of iron boride of at least 15 ⁇ m.
• the barrier layer which, during chromization, will make it possible to build up sublayers based on chromium boride Cr 2 B and iron boride enriched in chromium (Fc, Cr) 2 B. its thickness, the initial iron boride layer more or less limits the carbon flux which can, after having migrated through it, combine with the chromium deposited during chromization. Together with the boride Cr 2 B, only the carbide M 23 C 6 can then be formed; but this provided that the initial layer of iron boride has a sufficient thickness.
• thermochemical treatments heat treatments necessary for acquiring the internal characteristics of the substrate can be carried out.
• heat treatments necessary for acquiring the internal characteristics of the substrate can be carried out.
• the treatment process is carried out conventionally, but under the aforementioned conditions, by "slury-coating" in the presence of B 4 C + Ta 2 B 4 O 7 in a vinyl binder.
• the three samples include a compact layer of FeB and Fe 2 B iron borides with respective thicknesses of 4, 15 and 40 ⁇ m.
• cements is not the subject of the invention since The nature of the cements is not the subject of the invention since these cements come under known techniques. But the order of the treatments and the temperatures reached condition the results.
• the steel substrates are austenitized at 850 ° C, quenched in oil and returned to 250 ° C for 2 h.
• the layer of iron boride was consumed and gave rise to the formation of a chromium carbide coating M 23 C 6 and M 7 C 3 without particularly appreciable improvement in the state of crystallization of the latter carbide.
• the carbide layer M 7 C 3 has almost disappeared. Its thickness is less than 1 ⁇ m.
• the M 23 C 6 carbide in the form of an independent sublayer remains. But it appears here the chromium boride Cr 2 B with enrichment in chromium of the iron boride Fe 2 B.
• the coating system is completely different.
• the initial thickness of iron boride no longer lets the carbon reach the surface with a sufficient content compared to the transport kinetics of chromium. And because of this, the formation of the carbide layer M 7 C 3 is retracted, the diffusion path in the system Fe, Cr, .B, C at the isotherm T of chromization being then modified.
• the layer over a thickness of approximately 16 ⁇ m, consists of the Cr 2 B and Cr 23 C 6 phases in which iron is substituted (Fe ⁇ 18%). Underlying this layer is the iron boride enriched in chromium by diffusion (Fe, Cr) 2 B.
• the layer configuration for which the Cr 2 B phase was formed (sample 3) gives particularly advantageous wear resistance results compared to those obtained after direct chromization.
• test results presented here are penalizing, because in fact the wear in this case was limited to the layer M 7 C 3 , the layer Cr 2 B could not be reached.
• the coating according to the invention can therefore be applied to any metal part whose wear must be negligible compared to another part whose wear is significant. This is for example the case of a barrel tube and the belt of a shell.
• the morphology of the layers is typically that of the M 23 C 6 phase.
• crystals of chromium carbonitride Cr 2 (C, N) are very rare in the case of borochromization.
• the coating of sample 1 consists of two sub-layers M 23 C 6 and M 7 C 3 with the same appearance as the chromized layers.
• the coating of sample 2 consists of three sublayers M 23 C 6 , M 7 C 3 and (Cr, Fe) 2 B before. find the general morphology of the layer of iron boride which was not completely consumed during the exchanges.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Other Surface Treatments For Metallic Materials (AREA)
• Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)

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• 1981
  • 1981-10-06 FR FR8118779A patent/FR2514032A1/fr active Granted
• 1982
  • 1982-09-30 EP EP82401775A patent/EP0077703B1/de not_active Expired
  • 1982-09-30 US US06/428,662 patent/US4469532A/en not_active Expired - Fee Related
  • 1982-09-30 DE DE8282401775T patent/DE3268575D1/de not_active Expired

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