Abstract:
A method for the carburization of a deep-drawn article or a stamped-bent article made of austenitic rustproof stainless steel includes inserting the article into an oven in a first process step and heating the article to a first temperature, wherein an oxygen containing standard atmosphere that is present in the oven is replaced by a first gas mixture, and in which the article is heated up to a second temperature in a second process step, wherein the first gas mixture is replaced by a second gas mixture, and in which the article is maintained on the second temperature in a third process step, wherein the second gas mixture is replaced by a third gas mixture, and in which the article is cooled down to a third temperature in a fourth process step, wherein the third gas mixture is replaced by a fourth gas mixture.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit and priority of European patent application No. EP13196076.7, filed Dec. 6, 2013. The entire disclosure of the above application is incorporated herein by reference. 
       FIELD 
       [0002]    The invention relates to a method for the carburization of a deep-drawn part or a stamped-bent part made of austenitic rustproof stainless steel and having a small wall thickness, which is usual for such parts, in at least some areas. 
       BACKGROUND 
       [0003]    Usual small wall thicknesses of deep-drawn parts and stamped-bent parts in the sense of the invention are beneath 2000 μm. Such stainless steel parts are manufactured from very thin sheet metals by means of tensile compression reshaping or stamping-bending and sometimes take very filigree structures. Depending on the used method, parts having a varying or constant wall thickness can be produced, whereby these ones then entirely comprise a wall thickness of less than 2000 μm or they have such a wall thickness in at least some areas. 
         [0004]    These filigree items are used in the most different fields of technique, such as for example as bearing covers in gearboxes, valve seats in ABS systems or as sample carriers for hazardous substances in high-precision measurements and are subject to extreme mechanical, thermal and chemical stress. The demand for corrosion resistant materials comprising a high hardness is therefore accordingly high. 
         [0005]    The quality of such hardened items, in particular of such parts which have a high length diameter ratio (aspect ratio) and/or which contain nitrogen, has however been poor so far with respect to the mechanical resistance, the suitability for welding as well as the corrosion resistance. Methods based on carbon provide remedy. But they are only suitable to a limited extend for scooping deep-drawn or stamped-bent parts. Soilings are caused by the surface hardening by means of carbon, which soilings can no more economically be removed from scooping parts according to the up-to-date industrial standard. If established methods for surface hardening known from the state of the art are used for items having a very thin wall thickness and a high aspect ratio, no industrially reproducible surface layers which meet the quality requirements can be produced. 
         [0006]    The reason for these results has over all to be seen in the partially extreme treatment conditions of the established methods. 
         [0007]    US 2012/111454 thus shows for example a high temperature method for the carburization of rustproof steel ingots. In this method, carburization temperatures comprised between 760° C. and 1200° C. are used. Methods using such high temperatures cannot be used for the surface hardening of thin-walled deep-drawn and stamped-bent items, since they cause a thermal deformation of the partly very filigree structures and thus make them unusable. 
         [0008]    U.S. Pat. No. 6,461,448 shows a method for the carburization of a steel article, in which the said steel article is treated in a molten alkaline bath. Such an aggressive kind of treatment causes filigree items in the sense of the invention to get partially considerable corrosion damages because of the thin wall thickness, leading to a highly inhomogeneous surface layer. Furthermore it has been proved that liquid treatments of filigree items lead to unsatisfying results because of an incomplete surface wetting. 
         [0009]    In this connection EP 0 678 589 B1 discloses a method for the carburization of an austenitic metal. Herein, a fluorine-based gas is applied to the metal. Fluorine-based gases are highly corrosive due to their reactivity and as a result they act aggressively on the surface of the metal. While the thus caused surface removal is even desired for articles having a high wall thickness and a corresponding high amount of material, this surface removal cannot be compensated in thin-walled deep-drawn and stamped-bent items and leads to the irreversible destruction of the article. Furthermore, the gases used there are highly toxic, highly corrosive and comprise properties that are highly endangering the environment and thus they place enormous requirements on the reactor to be used, the storage and the operational safety. 
       SUMMARY 
       [0010]    It is therefore the object of the invention to provide an effective method for the carburization of thin-walled deep-drawn or stamped-bent stainless steel articles. 
         [0011]    For achieving this aim, an invention comprising the features according to the present disclosure is proposed. Further advantages and features will become apparent from the dependent claims. 
         [0012]    The invention advantageously proposes a method comprising mild conditions which are adapted to the particularities of thin-walled deep-drawn and stamped-bent articles. 
     
    
     DETAILED DESCRIPTION 
       [0013]    According to the invention, the article is inserted into an oven for carrying out the method. It has been found that oxygen and water residues especially interfere with the surface hardening. For excluding these disruptive factors, the article is heated up to a temperature which is above the boiling temperature of water. Herein, a temperature comprised between 110° C. and 140° C. is preferred and 120° C. is most preferred. 
         [0014]    Furthermore, according to the invention, the oxygen containing atmosphere in the oven will be replaced by a first gas mixture. Therefore, the oven advantageously comprises gas inlets and gas outlets. 
         [0015]    According to a preferred manner of carrying out the process, it can be provided to flood the oven with an inert gas before introducing the first gas mixture. Herein, the oxygen displacement will be advantageously accelerated and a possible hazard potential resulting from the contact of the standard atmosphere containing oxygen with the first gas mixture will be reduced. Known chemically unreactive gases such as in particular nitrogen or argon will be preferably used as inert gas. 
         [0016]    Non-rusting stainless steels inter alia comprise chrome as an alloying element. Due to the contact with atmospheric oxygen, a passivating and corrosion resistant chromium (III) oxide layer is formed on the material surface. 
         [0017]    During the carburization it is of enormous importance to remove or depassivate this passivating chromium oxide layer, in order to enable a homogenous diffusion of the carbon into the surface zone of the stainless steel. If this is not assured due to lacking depassivation, the diffusion will be impeded in the area having an intact chromium oxide layer and the consequence will be an inhomogeneous hardness distribution in the resulting surface layer. Furthermore, a lacking depassivation in the areas having an intact chromium oxide layer leads to the formation of defect sites in the surface area. These defect sites ultimately lead to an undesired reduced corrosion resistance of the steel. 
         [0018]    According to a preferred feature of the invention, the first gas mixture therefore has reducing characteristics, in order to avoid a further oxidation of the chrome. Furthermore, this gas mixture already initiates the depassivation of the surface. According to another preferred feature of the invention, the first gas mixture is at least composed of a hydrogen containing gas and a nitrogen containing gas and especially preferred are H 2  and N 2 . It has been found that this gas mixture, in particular in connection with the mild temperature of the first process step, has an especially mild and advantageous effect on the chromium oxide layer without having a detrimental effect on the morphology of the surface of the filigree articles. 
         [0019]    According to a preferred feature of the invention, the oxygen concentration will be measured continuously or at intervals by means of a sensor. Herein, a control unit connected to the sensor compares the actual value continuously or at intervals to a freely selectable set point and in case of an identity between the actual value and the set point, the control unit enables the oven to carry out the second process step. The method according to the invention is advantageously highly simplified hereby and minimizes possible sources of error for the user in this manner. 
         [0020]    According to the invention, a second process step is provided, in which the article is heated up to the target temperature, the second temperature, for the carburization. The second temperature is preferably selected such that this one is clearly beneath the recrystallization temperature of highly cold formed iron alloys (680° C.). Herein, a possible modification of the morphology of the surface is effectively prevented, whereby the formation of a homogenous surface layer is promoted. The second temperature is preferably comprised between 450° C. and 550° C. and is most preferably 500° C. The heating up phase especially serves to the gentle and complete depassivation of the chromium oxide layer. 
         [0021]    It is advantageous to select the heat-up rate, at least in certain temperature ranges, as low as possible, in order to assure a uniform depassivation. In this connection, the applicant has discovered that the quality of the resulting surface layer of thin-walled deep-drawn parts significantly suffers from a high heat-up rate. In a certain temperature range, the heat-up rate is preferably comprised between 0.5 and 1° C./min, more preferably between 0.5 and 0.7° C./min and most preferably 0.5° C./min. The temperature range, in which this low heat-up rate is selected, is preferably comprised between 420° C. and 550° C., more preferably between 450° C. and 500° C. and most preferably between 480° C. and 500° C. 
         [0022]    According to a feature of the invention, the first gas mixture will be replaced by a second gas mixture in the second process step. Herein, it has been found that a mild depassivation of the thin-walled deep-drawn parts during the heat-up phase to the second temperature will be preferably realized by a gas mixture that is at least composed of a hydrogen containing gas, a nitrogen containing gas as well as a carbon containing gas. In particular in connection with the low heat-up rate, an especially slow and thus mild and well controllable depassivation of the chromium oxide layer can be preferably achieved. 
         [0023]    According to an advantageous feature of the invention, the article will be treated with additives which selectively or entirely dissolve the passive layer. These additives especially refer to salt compounds and/or organic substances and acidifiers which are applied to the good or in the oven in solid or liquid form. Herein, the application takes preferably place before the article is inserted into the oven or during the second process step. For this purpose, solids and/or liquids are used which form acid reaction products in connection with the reaction gases, which reaction products would result in a pH value of &lt;7 if they were introduced into water. Herein, the application of the substances directly onto or into the article surface has proved to be especially advantageous. Hereby, local depassivation processes which early initiate and promote a uniform depassivation will be initiated already at low temperatures. 
         [0024]    As carbon containing component, preferably carbon oxides, saturated, unsaturated, aliphatic, cyclic, heterocyclic and/or aromatic hydrocarbons can be added to the second gas mixture. Herein, the use of unsaturated hydrocarbons, such as especially ethyne, is highly preferred. 
         [0025]    As nitrogen containing component, preferably elementary nitrogen, ammonia, amines, amides, imides, nitriles and/or nitrogen oxides can be added to the second gas mixture. 
         [0026]    Herein, it has been found that the use of elementary hydrogen as a constituent of the second gas mixture, in particular in connection with the depassivation additives, leads to the formation of especially homogenous surface layers. 
         [0027]    According to a preferred feature of the invention, the temperature will be measured continuously or at intervals by means of a sensor. Herein, the control unit connected to the sensor compares the actual value continuously or at intervals to a freely selectable set point for the second temperature and in case of an identity between the actual value and the set point, the control unit enables the oven to carry out the third process step. The method according to the invention is advantageously highly simplified hereby and minimizes possible sources of error for the user in this manner. 
         [0028]    According to the invention, a third process step is provided, in which the deep-drawn part is constantly kept on the second temperature. In this connection, the third process step serves to the carburization of the thin-walled deep-drawn part. It has been found that the second temperature advantageously enables a gentle formation of the surface layer to be hardened. The diffusion of the carbon into the surface area of the deep-drawn part takes place slowly at these temperatures, can thus be easily controlled and causes a homogenous surface layer that is rich in carbon to form. A too high temperature has to be avoided in any case, since due to the high diffusion speed and the high kinetic energy of the involved molecules, uneven layers and carbide particles will be formed. 
         [0029]    According to the invention, the second gas mixture will be replaced by a third gas mixture which is especially suitable for a gentle carburization under mild conditions. In this connection, the use of a gas mixture which is at least composed of a hydrogen containing gas, a nitrogen containing gas as well as a carbon containing gas has proved to be advantageous. It can be preferably provided to add another carbon containing component to this gas mixture, whereby the formation of a homogenous surface layer which is rich in carbon will be promoted in a synergetic manner by the two different carbon components. 
         [0030]    As a first carbon containing component, preferably carbon oxides, saturated, unsaturated, aliphatic, cyclic, heterocyclic and/or aromatic hydrocarbons can be added to the third gas mixture. Herein, the use of unsaturated hydrocarbons, such as especially ethyne, is highly preferred. 
         [0031]    As a second carbon containing component, preferably carbon oxides, saturated, unsaturated, aliphatic, cyclic, heterocyclic and/or aromatic hydrocarbons can be added to the third gas mixture. Herein, the use of carbon oxides, such as especially carbon monoxide, is most preferred. 
         [0032]    As nitrogen containing component, preferably elementary nitrogen, ammonia, amines, amides, imides, nitriles and/or nitrogen oxides can be added to the third gas mixture. 
         [0033]    According to a preferred feature of the invention, the individual concentrations of the gas components will be measured continuously or at intervals by means of respective sensors. Herein, the control unit connected to the sensors compares the respective actual values continuously or at intervals to freely selectable set points for the respective concentration of the gas component and compensates deviations within a fault tolerance continuously or at intervals. The process control is advantageously simplified hereby and allows providing constant process conditions, which is of decisive importance for the formation of a homogenous surface layer rich in carbon. 
         [0034]    Herein, the layer thickness of the surface layer rich in carbon can be set by means of the duration of gassing. Advantageously, a period of time comprised between 2 and 10 hours is required for generating a surface layer having a thickness of 10-40 μm. 
         [0035]    According to a preferred feature of the invention, the control unit, which comprises a corresponding device for measuring the time, will enable the oven to carry out the fourth process step after a freely selectable carburization time has elapsed. The method according to the invention is advantageously highly simplified hereby and minimizes possible sources of error for the user in this manner. 
         [0036]    According to the invention, a fourth process step is provided, in which the deep-drawn part is cooled down to a third temperature. Herein, it is preferably provided to cool down the deep-drawn part to a temperature comprised between 50° C. and 80° C. and most preferably to 60° C. 
         [0037]    Herein it has been found that the selection of the atmosphere in which the cooling down process takes place is of decisive importance for the formation of a homogenous surface layer. It is therefore provided according to the invention to replace the third gas mixture by a fourth gas mixture. The selection of a slightly reducing gas mixture is especially considered to be advantageous. According to a preferred embodiment of the invention, the fourth gas mixture is composed of at least a hydrogen containing gas and a nitrogen containing gas. Herein, it is especially preferred that the fourth gas mixture is composed of H 2  and N 2 . In order to assure a weak reduction potential, the composition of the fourth gas mixture advantageously contains 5% to 25% H 2  and 75% to 95% N 2 , more preferred 5% to 10% H 2  and 90% to 95% N 2  and especially preferred 5% H 2  and 95% N 2 . It has been shown that the cooling down according to the invention of the deep drawn part effectively prevents an escape of the carbon from the hardened surface layer. 
         [0038]    The invention furthermore relates to a surface hardened deep-drawn article having very small wall thicknesses. 
         [0039]    For the first time it has become possible by means of the method according to the invention to harden thin-walled stainless steel articles, especially deep-drawn articles having a high length diameter ratio and a small wall thickness in an industrially reproducible manner and with excellent quality. 
         [0040]    The deep-drawn article according to the invention comprises a soft elastic core having a hardness comprised between 350 and 400 HV1 and a hard surface layer rich in carbon. 
         [0041]    According to a feature essential for the invention, the surface layer is free of defect sites and/or particles, completely closed over the circumference and comprises an essentially flat surface. 
         [0042]    As a result, the thin-walled deep-drawn article according to the invention comprises mechanical properties of a hitherto unattained quality. 
         [0043]    Thus, the deep-drawn article according to the invention comprises a surface area comprising a layer rich in carbon and having a hardness of 700 to 1000 HV0.01 and a layer thickness comprised between 10 and 40 μm. 
         [0044]    According to another feature essential for the invention, the corrosion and the abrasion resistance of the deep-drawn article are better than the ones of the starting product. In particular the first aspect is surprising in so far as a carburization usually deteriorates the corrosion properties of a steel product.