EP2881492B1 - Method for carburising metal deep drawn article or a bent pressed article made of austenitic stainless steel - Google Patents

Description

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

For deep-drawing articles and stamped-bent articles in the context of the invention, customary 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 a variety of fields of technology such. used 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 those parts having a high aspect ratio (aspect ratio) and / or containing nitrogen, has been poor both in mechanical resistance, weldability and corrosion resistance. Provide remedy here carbon-based process. However, these are only partially suitable for scooping deep-drawn or stamped-bent articles. It is caused by the surface hardening with carbon contaminants that can no longer be economically removed according to the current industrial standard in scooping parts. If one uses known and established methods for surface hardening on articles having a very thin wall thickness and a high aspect ratio, it is not possible to produce edge-layers that are industrially reproducible and meet the quality requirements.

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

So shows, for example, the US 2012/111454 a high-temperature process for carburizing stainless steel ingots. Carburization temperatures of 760 ° C - 1200 ° C are used in this process. Processes with such high temperatures are not applicable to the surface hardening of thin-walled deep-drawing and stamped-bent articles, since they cause a thermal deformation of the structures, some of which are very filigree, and thus make them useless.

The US 6,461,448 shows a method for carburizing a steel article, wherein said steel article is treated in a Schmelzalkalibad. 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 liquid treatments in filigree articles due to incomplete surface wetting lead to unsatisfactory results.

The EP 0 678 589 B1 discloses in this context a method of carburizing 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 the resulting removal of surface is even desirable for articles with high wall thickness due to a corresponding material wealth, it can not be compensated for thin-walled thermoforming and stamping bending articles and leads to the irreversible destruction of the article. In addition, the gases used there due to their high toxicity, their high corrosiveness and their highly environmentally hazardous properties enormous demands on the reactor to be used, storage and work safety.

The EP 1 553 204 A1 discloses a method of edge hardening an austenitic steel part. The method is characterized in that the steel part is heated in a first process step to a temperature between 200 ° C and 500 ° C for a time between 10 minutes and 3 hours. The heating takes place in an atmosphere consisting of between 0.5 to 20% by volume of a halogen gas or a halide gas and the balance of nitrogen, hydrogen or an inert gas to activate the steel surface. After activation of the steel surface, the surface of the steel part is nitrocarburized in a second process step. This is done within a temperature range of 430 ° C to 600 ° C for 20 minutes or more within a mixed atmosphere comprising ammonia for nitrification and carbon monoxide and / or methane for carburizing.

D3 discloses a method for carburizing a steel part. For the initial activation of the steel surface, the steel part is heated in a first process step to a temperature between 500 ° F and 600 ° F in a halogenated hydrogen atmosphere, preferably HCL or HF. Instead of said gas treatment, the activation with strong bases, the coating with iron or a treatment in the cyanide melt bath can be used for the activation. For carburization, the thus activated steel part is then initially heated to a temperature of 1125 ° C. Said initial carburizing temperature is cooled to 925 ° F in the course of the carburizing process. In particular, hydrocarbon gases such as methane, ethane and propane, oxygen-containing gases such as carbon monoxide and carbon dioxide or mixtures of these gases such as synthesis gases can be used as the carburizing gas.

The EP 1 193 413 A1 discloses a method for carburizing an austenitic steel roller bearing. In a first method step, said roller bearing is activated at a temperature between 200 ° C and 400 ° C a fluorine-containing gas, preferably NF3 in the nitrogen. In a second process step, the activated rolling bearing is treated at a temperature between 460 ° C and 520 ° C with a carburizing gas. Said carburizing gas may consist of either RX gas or a gas containing an unsaturated hydrocarbon, preferably acetylene or ethylene.

The EP 0 497 409 A2 discloses a method of edge hardening an austenitic steel part. In a first process step, the steel part is either at a temperature between 250 ° C and 400 ° C in a fluoride-containing gas atmosphere or at a temperature between 100 ° C. and heated at 250 ° C in a fluorine-containing atmosphere. Further, the steel part activated in the first step would be heated in a second process step to a temperature of 530 ° C and treated in an atmosphere consisting of nitrogen and hydrogen over a period of 30 minutes. In a third process step, the activated steel part is then nitrocarborated in an ammonia-containing gas mixture at a temperature of 480 ° C to 700 ° C over a period of three to five hours. The ammonia-containing atmosphere may be formed in particular of ammonia and RX gas or ammonia, carbon dioxide and RX gas.

All of the above-mentioned methods have in common that they use for depassivation or activation of the surface layer of the steel part to aggressive process steps containing halogen-containing, in particular fluorine-containing gases. As already stated, however, these process conditions are not suitable for deep-drawn parts with small wall thicknesses below 2000 μm.

It is therefore the object of the invention to provide an effective method for carburizing 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. Preference is given here to a temperature of 110 ° 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 potentially existing hazard potential, resulting from the contact of the oxygen-containing normal atmosphere with the first gas mixture is 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.

During carburization, it is of utmost importance to remove or depassivate this passivating chromium oxide layer in order to allow homogeneous diffusion of the carbon into the edge area 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 lead to an undesirable, reduced 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 a further feature of the invention, the first gas mixture consists of H2 and N2. 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 affecting the morphology of the surface of the filigree 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 furnace in the case of an identity between the 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 article is heated to the target temperature, the second temperature, for the carburization. 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. According to the invention, the second temperature is 450 ° C to 550 ° C and 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 takes place during the heating phase to the second temperature by a gas mixture consisting of H ₂ , N ₂ and a carbon-containing gas from an unsaturated hydrocarbon. In particular, in connection with the low heating rate may preferably be a particularly slow and therefore mild and easy to control 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 before the article is placed in the oven or during the second process step. For this purpose, solids and / or liquids are used which form acid reaction products in conjunction with the reaction gases, which would give a pH <7 when introduced into water, as the application of the substances has proven particularly advantageous directly on or in the article surface , As a result, local depassivation processes, which initiate and promote uniform depassivation earlier, already occur at low temperatures.

As the carbon-containing component of the second gas mixture according to the invention, an unsaturated hydrocarbon, in particular ethyne used.

According to the invention, elemental nitrogen is used in the second gas mixture.

In addition, it has been found that the use of elemental hydrogen as a constituent of the second gas mixture, in particular in conjunction with the depassivation additives, leads to the formation of particularly homogeneous boundary 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 deep-drawn part is kept constant at the second temperature. The third step in this context serves to carburize the thin-walled deep-drawn part. It has been found that the second temperature advantageously allows a careful construction of the surface layer to be hardened. The diffusion of the carbon into the edge region of the deep-drawn part takes place slowly at these temperatures, is therefore easy to control and causes the formation of a homogeneous carbon-rich surface layer. Too high a temperature is to avoid in any case, as it comes to the formation of irregular layers and the formation of carbide 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 replaced by a third gas mixture, which is particularly suitable for gentle carburizing under mild conditions. Particularly advantageous in this context, the use of a gas mixture has proven, which is composed of a hydrogen-containing gas, a nitrogen-containing gas and a carbon-containing gas. It is further provided according to the invention to add a further carbon-containing component to this gas mixture, whereby the formation of a homogeneous carbon-rich surface layer is promoted by the two different carbon components in a synergetic manner. As the carbon-containing component of the second gas mixture according to the invention an unsaturated hydrocarbon, in particular ethyne, is used , According to the invention, the second gas mixture contains elemental nitrogen.

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 here 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 10 hours is required to generate a 10-40 microns thick edge layer.

According to a preferred feature of the invention, the control unit, which has a corresponding device for time recording, releases the furnace for the fourth method step after the expiration of a freely selectable carburizing time. 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 third gas mixture by a fourth gas mixture. In particular, the choice of a slightly reducing gas mixture is considered advantageous. According to the invention, the fourth gas mixture consists of a hydrogen-containing gas and a nitrogen-containing gas. It is envisaged here that the fourth gas mixture is formed from H2 and N2. To ensure a weak reduction potential, the composition of the fourth gas mixture advantageously from 5% to 25% H2 and 75% to 95% N2, more preferably 5% to 10% H2 and 90% to 95% N2 and more preferably 5% H2 and 95% N2. It has been found that the cooling according to the invention of the thin-walled deep-drawn part effectively prevents the escape of the carbon from the hardened edge layer.

Furthermore, the invention relates to an edge-hardened thermoforming article with very small wall thicknesses.

By means of the method according to the invention, it is now possible for the first time to harden industrially reproducible thin-walled stainless steel articles, in particular deep-drawn articles, with a high length-diameter ratio with a thin wall thickness and to cure them in excellent quality.

The deep-drawing article produced by the method according to the invention has a soft, elastic core with a hardness of 350 to 400 HV1 and a hard carbon-rich surface layer.

According to one feature, 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 produced by the method according to the invention has mechanical properties of unprecedented quality.

Thus, the thermoforming article produced by means of the method according to the invention has in its edge region a carbon-rich layer with a hardness of 700 to 1000 HV0.01 and with a layer thickness of 10 to 40 μm.

According to another feature, the corrosion and abrasion resistance of the thermoforming article is better than that of the starting material. The former, in particular, is surprising in that carburizing generally worsens the corrosion properties of a steel product.

Claims (5)

1. A thin wall carburizing process of a deep-drawn article or a bent pressed article made of austenitic rustproof stainless steel,

   in which the article which comprises a wall thickness of less than 2000 µm at least in some portions, is introduced into an oven 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 oven will be replaced by a first gas mixture, wherein the first gas mixture consists of N₂ and H₂,

   and in which the article will be 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, N₂ and a carbonic gas from an unsaturated hydrocarbon,

   and in which the article will be held on the second temperature in a third process step,

   wherein the second gas mixture will be replaced by a third gas mixture, wherein the third gas mixture consists of a hydrogen-bearing gas, nitrogen-bearing gas, a first carbonic gas and a second carbonic gas, wherein the second carbonic gas is a gas different from the first carbonic gas,

   and in which the article will be cooled down to a third temperature comprised between 50°C and 80°C in a fourth process step,

   wherein the third gas mixture will be replaced by a fourth gas mixture, wherein the fourth gas mixture consists of N₂ and H₂.
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 that if a freely selectable residual oxygen value is achieved, the second process step will be initiated.
3. A method according to one of the preceding claims 1 or 2, characterized in that if the second temperature is achieved, 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 the fourth process step will be initiated after a freely selectable duration of treatment has come to an end.