EP2881493B1 - Process for the nitrocarburization of a deep drawn article or a bent pressed article made of austenitic stainless steel - Google Patents

Description

The invention relates to a process for nitrocarburizing a thermoformed article or a stamped article made of austenitic stainless steel with a wall thickness which is at least partially suitable for such articles.

Nitrocarburizing is a process for surface hardening of metals, in which the metal to be hardened is enriched in its edge region with carbon and nitrogen. It has long been established in the art, which is why it does not require a separate documentary evidence at this point.

In the case of thermoformed articles and stamped and bent parts in the sense of the invention, conventional low wall thicknesses are below 2000 μm. Such stainless steel articles are made of very thin sheets by tensile or compression bending and sometimes take on very filigree structures. Depending on the method used, articles with varying or constant wall thickness can be produced, whereby they then have a wall thickness of less than 2000 μm, at least in regions or as a whole.

These filigree articles are used in the most diverse areas of technology, such as bearing covers in gearboxes, valve seats in ABS systems or as sample carriers for hazardous substances in high-precision measurements where they are exposed to extreme mechanical, thermal and chemical stresses. The demand for high hardness corrosion resistant materials is therefore correspondingly high.

However, the quality of such cured articles, especially parts having a high length to diameter ratio (aspect ratio), leaves much to be desired in terms of both mechanical resistance and corrosion resistance. If one applies known and established methods for surface hardening on articles with very thin wall thickness from the prior art, then no industrially reproducible and quality claims sufficient edge layers can be produced.

Electron-microscopic and metallographic investigations of cross-sections of nitrocarburized thermoforming articles have shown that edge layers produced in this way do not form a peripherally closed structure. Rather, these edge layers are highly rugged and have a plurality of edge layer penetrating channels in which there is direct contact between the environment and the uncured core portion of the thermoformed article. Likewise, the edge region produced in this way has numerous defects and coarse-grained particles. Due to this heterogeneous design, both the hardness of the surface layer and its corrosion properties are subject to such high fluctuations that an industrial application is possible only to a very limited extent.

The reason for these results is to be found above all in the sometimes extreme treatment conditions of the established methods.

So shows the DE 44 42 382 C1 a method of nitrocarburizing a stainless steel article, wherein said steel article is treated in a melt alkali bath. Such an aggressive type of treatment leads to filigree articles in the context of the invention due to the small wall thickness sometimes severe corrosion phenomena, which has a highly inhomogeneous edge layer result. In addition, it has been found that fluid treatments in filigree thermoformed articles result in unsatisfactory results due to incomplete surface wetting.

The EP 0 588 458 discloses in this context a method for nitriding an austenitic metal. Here, the metal is exposed to a fluorine-containing gas. Fluorine-containing gases are highly corrosive due to their reactivity and, as a result, aggressively affect the surface of the metal. While with it caused surface removal is even desirable for articles with high wall thickness due to a corresponding material wealth, it can not be compensated for thin-walled thermoforming articles or stamped and bent articles and leads in part to the irreversible destruction of the surface of the article. In addition, because of their high toxicity, their high corrosiveness and their highly environmentally hazardous properties, the gases used there place enormous demands on the reactor to be used, storage and occupational safety.

It is therefore the object of the invention to provide an effective method for nitrocarburizing thin-walled deep-drawn or stamped bent stainless steel articles.

To solve this problem, an invention with the features of claim 1 is proposed. Further advantages and features emerge from the subclaims.

With the invention, a method with mild conditions is proposed in an advantageous manner, which are tailored to the specifics of thin-walled thermoforming and stamping bending articles.

In accordance with the invention, the article is introduced into an oven for carrying out the method. It has been shown that in particular oxygen and water residues interfere with the surface hardening. To exclude these disturbing factors, the article is heated to a temperature which is above the boiling point of water. Essential here is a temperature of 100 ° C to 140 ° C, more preferably 120 ° C.

In addition, the oxygen-containing atmosphere in the furnace is replaced in accordance with the invention by a first gas mixture. The furnace therefore advantageously has gas inlets and gas outlets.

According to a preferred process control, it may be provided to flood the furnace with an inert gas before the introduction of the first gas mixture. In this case, the oxygen displacement is accelerated in an advantageous manner and a possibly existing hazard potential, resulting from the contact of the oxygen-containing normal atmosphere with the first gas mixture, lowered. As the inert gas, it is preferable to use known chemically unreactive gases such as, in particular, nitrogen or argon.

Stainless steels include, among others, chromium as an alloying constituent. Upon contact with atmospheric oxygen, a passivating and corrosion-resistant chromium (III) oxide layer is formed on the surface of the material.

In the case of surface hardening processes, in particular nitrocarburization, it is of enormous importance to remove or depassivate this passivating chromium oxide layer in order to allow homogeneous diffusion of the carbon and nitrogen into the edge region of the stainless steel. If this is not ensured by a lack of depassivation, the diffusion into regions with an intact chromium oxide layer is inhibited and, as a consequence, an inhomogeneous hardness distribution of the resulting surface layer occurs. In addition, a lack of depassivation in areas with intact chromium oxide layer promotes the formation of defects in the edge area. As a consequence, these defects result in undesirable, reduced wear resistance and impair the corrosion resistance of the steel.

Thus, according to one feature of the invention, the first gas mixture has reducing properties in order to avoid further oxidation of the chromium. In addition, this gas mixture already initiated the depassivation of the surface. According to one feature of the invention, the first gas mixture is H ₂ and N ₂ . It has been found that this gas mixture, in particular in conjunction with the mild temperature of the first process step, exerts a particularly mild and advantageous effect on the chromium oxide layer without adversely altering the morphology of the surface of the filigree steel articles.

According to a preferred feature of the invention, the oxygen concentration is measured continuously or at intervals by means of a sensor. A control unit connected to the sensor checks the actual value here continuously or at intervals with a freely selectable desired value and releases the oven in the case of an identity between actual and desired value for a second method step. The inventive method is thereby greatly simplified in an advantageous manner and minimizes in this way possible user-side sources of error.

According to the invention, a second method step is provided, in which the thermoforming or stamped bent article is heated to the target temperature, the second temperature, for the surface hardening. The second temperature is chosen so that it is well below the recrystallization temperature of strongly cold-worked iron alloys (680 ° C). A possible change in the morphology of the surface is thereby effectively prevented, whereby the formation of a homogeneous surface layer is promoted. The second temperature is 450 ° C to 550 ° C, and more preferably 500 ° C. The heating phase serves in particular the careful and complete depassivation of the chromium oxide layer.

It is advantageous to select the heating rate as low as possible, at least in certain temperature ranges, in order to ensure uniform depassivation. The Applicant has found in this context that the quality of the resulting edge layer of thin-walled deep-drawn parts suffers in a special way under a high heating rate. Preferably, the heating rate in a certain temperature range between 0.5 and 1 ° C / min, more preferably between 0.5 and 0.7 ° C / min and more preferably 0.5 ° C / min. The temperature range in which this low heating rate is selected is preferably 420 ° C to 550 ° C, more preferably 450 ° C to 500 ° C, and most preferably 480 ° C to 500 ° C.

According to a feature of the invention, the first gas mixture is replaced by a second gas mixture in the second method step. It has been found that mild depassivation of the thin-walled deep-drawn parts during the heating phase to the second temperature by a gas mixture consisting of a hydrogen-containing gas, a nitrogen-containing gas and a carbon-containing gas. Particularly in connection with the low heating rate, a particularly slow and therefore mild and readily controllable depassivation of the chromium oxide layer can be achieved.

According to a preferred feature of the invention, the article is treated with additives which selectively or completely dissolve the passive layer. In particular, it means salt compounds and / or organic substances and acid generators, which are applied in solid or liquid form to the product or in the oven. The application is preferably carried out either before the article is placed in the oven or during the second process step. For this purpose, solid and / or liquids are used, which in Compound with the reaction gases form acidic reaction products that would give a pH <7 when introduced into water. The application of the substances directly on or in the article surface has proved to be particularly advantageous. As a result, local depassivation processes, which initiate and promote uniform depassivation earlier, occur even at low temperatures.

As the carbonaceous component, carbon oxides, saturated and unsaturated hydrocarbons can be added to the second gas mixture. Particularly preferred here is the use of carbon oxides, in particular carbon monoxide.

As the nitrogen-containing component elementary nitrogen, ammonia and nitrogen oxides can be added to the second gas mixture. Particularly preferred here is the use of ammonia.

It has also been found that the use of elemental hydrogen as part of the second gas mixture leads to the formation of particularly homogeneous surface layers.

According to a preferred feature of the invention, the temperature is measured continuously or at intervals by means of a sensor. The control unit connected to the sensor checks the actual value here continuously or at intervals with a freely selectable setpoint value for the second temperature and releases the oven in the case of an identity between the actual and setpoint values for a third method step. The inventive method is thereby greatly simplified in an advantageous manner and minimizes in this way possible user-side sources of error.

According to the invention, a third method step is provided in which the article is kept constant at the second temperature. The third process step is in this context the nitrocarburizing of the thin-walled article and leads by carburizing and nitriding of the edge region of the steel article to form a hardened surface layer. It has been found that the second temperature advantageously allows a careful construction of the hardened surface layer. The diffusion of the carbon and nitrogen in the edge region of the deep-drawn part takes place slowly at these temperatures, is therefore well controlled and causes the Structure of a homogeneous surface layer. The method according to the invention promotes, in particular, the formation of a boundary layer which consists of a carbon-rich phase closer to the core and an outer nitrogen-rich phase. Too high a temperature is to be avoided in any case, as it comes to the formation of irregular layers and the formation of carbide and / or nitride particles due to the high diffusion rate and the high kinetic energy of the molecules involved.

In accordance with the invention, the second gas mixture is used for nitrocarburizing the thermoforming article. The second gas mixture causes, especially in conjunction with the mild temperature, a particularly gentle and controlled construction of the boundary layer according to the invention, formed from a carbon-rich and a nitrogen-rich phase.

It has been found that in particular the use of carbon monoxide in this process leads to a particularly homogeneous carbon-rich phase.

Furthermore, it has been found that the use of ammonia promotes the formation of a particularly homogeneous nitrogen-rich phase.

According to a preferred feature of the invention, the individual concentrations of the gas components are measured continuously or at intervals by means of respective sensors. The control unit connected to the sensors checks the respective actual values continuously or at intervals with freely selectable desired values for the respective concentration of the gas component and compensates for changes within a fault tolerance continuously or at intervals. In this way, the process control is advantageously simplified and allows the provision of constant process conditions, which is for the construction of a homogeneous carbon-rich surface layer of crucial importance.

The layer thickness of the carbon-rich surface layer can be adjusted over the gassing time. Advantageously, a period of 2 to 20 hours is required to generate a 5 to 35 .mu.m thick edge layer.

According to a preferred feature of the invention, the control unit which provides for Time recording on a corresponding device has, after an arbitrary Nitrocarburierungszeit a free oven for the fourth step. The inventive method is thereby greatly simplified in an advantageous manner and minimizes in this way possible user-side sources of error.

According to the invention, a fourth method step is provided in which the deep-drawn part is cooled to a third temperature. It is provided here, the deep-drawn part to a temperature of 50 ° C to 80 ° C and more preferably cool to 60 ° C.

It has been found that the choice of atmosphere in the cooled is crucial for the formation of a homogeneous surface layer. It is therefore provided according to the invention to replace the second gas mixture by a third gas mixture. In particular, the choice of a slightly reducing gas mixture is considered advantageous. According to one embodiment of the invention, the third gas mixture of H ₂ and N ₂ . To ensure a low reduction potential, the composition of the third gas mixture advantageously consists of 5% to 25% H ₂ and 75% to 95% N ₂ , more preferably 5% to 10% H ₂ and 90% to 95% N ₂ and especially preferably 5% H ₂ and 95% N ₂ . It has been found that the inventive cooling of the thin-walled deep-drawn part effectively prevents escape of both the carbon and the nitrogen from the hardened surface layer and promotes the inclusion of both components in dissolved form.

Furthermore, the invention relates to a nitrocarburized thermoforming article made of austenitic stainless steel with a standard for thermoforming articles, at least partially low wall thickness.

By means of the method according to the invention, it is now possible for the first time to harden a thermoformed article with thin wall thickness in an industrially reproducible manner and in excellent quality.

The deep-drawing article according to the invention has a soft or depending on the cold deformation cold-strengthened elastic core and a hard edge layer on.

According to a feature essential to the invention, the edge layer is free of defects and / or particles, circumferentially completely closed and has a substantially planar surface.

As a result, the thin-walled thermoforming article according to the invention has mechanical properties of unprecedented quality.

Thus, the corrosion and abrasion resistance of the thermoforming article in the inventive way better than that of the starting material. This is made possible by the structure according to the invention of the surface layer. The surface layer consists of an outer nitrogen-rich phase and a carbon-rich phase located between the core and the nitrogen-rich phase. The carbon-rich phase in this case has a hardness of 700 to 1000 HV0.025 and a layer thickness of 2.5 to 15 microns. The nitrogen-rich phase, however, has a hardness of 1000 to 1400 HV0.025 and a layer thickness of 2.5 to 20 microns.

In this way, a hardness gradient is advantageously formed from the soft core to the harder carbon-rich phase to the even harder nitrogen-rich phase. The material is thereby advantageously flexible in its core region and cured in its edge region. Characterized in that a two-phase edge layer is formed with different hardness, also the edge layer has an increased flexibility with simultaneous enormous hardness.

Claims (6)

1. A method for the thin-wall nitrocarburization of a deep-drawn article or a punched-bent article made of austenitic stainless steel,
   in which the article, which comprises a wall thickness in at least some areas of less than 2000 µm, is introduced into a furnace in a first process step and heated up to a first temperature comprised between 100°C and 140°C,
   wherein an oxygen-bearing atmosphere which is present in the furnace will be replaced by a first gas mixture, wherein the first gas mixture consists of H₂ and N₂,
   and in which the article is heated up to a second temperature comprised between 450°C and 550°C in a second process step,
   wherein the first gas mixture will be replaced by a second gas mixture, wherein the second gas mixture consists of H₂, a nitrogen-bearing gas of the group comprising elementary nitrogen, ammonia and nitrogen oxides and of a carbon-bearing gas of the group comprising carbon oxides, saturated hydrocarbons and unsaturated hydrocarbons,
   and in which the article is maintained at the second temperature in a third process step,
   wherein the article will be treated with the second gas mixture,
   and in which the article is cooled down to a third temperature comprised between 50°C and 80°C in a fourth process step,
   wherein the second gas mixture will be replaced by a third gas mixture, wherein the third gas mixture consists of H₂ and N₂.
2. A method according to claim 1, characterized in that the residual oxygen content will be measured by means of a sensor during the first process step and if a freely selectable residual oxygen value is achieved, the second process step will be automatically initiated.
3. A method according to one of the preceding claims, characterized in that if the second temperature is reached, the third process step will be automatically initiated.
4. A method according to one of the preceding claims, characterized in that the article will be treated with at least one depassivating salt compound.
5. A method according to one of the preceding claims, characterized in that after the expiry of a freely selectable treatment period, the fourth process step will be automatically initiated.
6. A nitrocarburized deep drawn article made of austenitic stainless steel and nitrocarburized by a method according to one of the claims 1 through 5,
   comprising a small wall thickness of less than 2000 µm in at least some areas, which wall thickness is usual for deep drawn articles,
   a core
   as well as an edge layer,
   wherein the edge layer is harder than the core and
   wherein the edge layer is free of defect sites and/or particles, is circumferentially closed and comprises an essentially flat surface,
   and wherein the edge layer is formed by an outer nitrogen rich phase and a carbon rich phase arranged between the core and the nitrogen rich phase,
   and wherein the abrasion resistance of the edge layer is higher than the one of the starting material,
   wherein the carbon rich phase comprises a hardness comprised between 700 and 1000 HV0,025 and a layer thickness comprised between 2.5 and 15 µm,
   and wherein the nitrogen rich phase comprises a hardness comprised between 1000 and 1400 HV0,025 and a layer thickness comprised between 2.5 and 20 µm.