EP0652300B1 - Case nitriding for producing a high-strength austenitic skin in stainless steels - Google Patents

Case nitriding for producing a high-strength austenitic skin in stainless steels Download PDF

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EP0652300B1
EP0652300B1 EP94114659A EP94114659A EP0652300B1 EP 0652300 B1 EP0652300 B1 EP 0652300B1 EP 94114659 A EP94114659 A EP 94114659A EP 94114659 A EP94114659 A EP 94114659A EP 0652300 B1 EP0652300 B1 EP 0652300B1
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nitrogen
nitriding
skin
austenitic
case
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EP0652300A1 (en
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Hans Prof. Dr.-Ing. Berns
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces

Definitions

  • the invention relates to a heat treatment process for forming an austenitic surface layer with ⁇ 0.30 wt.% Of dissolved nitrogen in near net shape parts of stainless steel by nitriding at a temperature between 1000 ° C and 1200 ° C in a nitrogen-containing gas atmosphere.
  • a heat treatment process of the aforementioned type is known from DE-A-25 18 452 known. So revealed the DE-A-25 18 452 a process for austenitizing an iron alloy containing about 21 to 45% manganese and about 10 to 30% chromium, wherein the surface of the solid state alloy is at a temperature of at least 927 ° C above a nitrogen content at least in the surface of about 0.85 to 3% sufficient period of exposure to nitrogen exposure.
  • the manganese content in the with the help of from the DE-A-25 18 452 known alloys heat treatment process is 21 to 45%. Since manganese is an austenite and increases the solubility of nitrogen in the steel, amounts greater than 21% are required.
  • a heat treatment process for forming an austenitic surface layer with ⁇ 0.30 wt.% of dissolved nitrogen in near-net shape parts of X5 CrNiMo 17 12 2 or X2 CrNiMoN 22 5 3 is proposed as stainless steel, by nitriding at a Temperature between 1000 ° C and 1200 ° C in a nitrogen-containing gas atmosphere and subsequent cooling at a rate such that nitride precipitation is avoided.
  • the present invention dispenses with a consistently high nitrogen content in the steel. Instead, only the edge zone of stainless steel parts close to the final shape is enriched with dissolved nitrogen by a heat treatment so that a high-strength but tough austenitic surface layer forms over a core structure of ferrite, austenite, martensite or a mixture of two or three of these structural components.
  • the heat treatment according to the invention consists of a nitridation in a nitrogen-emitting gas atmosphere at a temperature between 1000 and 1200 ° C.
  • Temperature, pressure and duration of the treatment are chosen so as to form a surface layer of certain thickness, the nitrogen content of which in the surface is between a lower limit of 0.3% by weight and an upper limit given by incipient nitride precipitation during nitriding.
  • the subsequent cooling takes place so quickly that even in this period no nitride precipitation occurs.
  • the invention is a heat treatment process for forming an austenitic surface layer with ⁇ 0.30 wt
  • Nitrogen in near-net shape parts of X5 CrNiMo 17 12 2 or X2 CrNiMoN 22 5 3 by embossing at a temperature of between 1,000 ° C and 1,200 ° C in a nitrogen-containing gas atmosphere and subsequent cooling at such a rate as to avoid nitride precipitation is proposed. It is stabilized by the inward diffusion of nitrogen de austenitic phase in the boundary layer, so that martensitic or ferritic microstructures in the edge zone to austenite.
  • the strength of the surface layer is increased by the solid solution hardening of the austenite with nitrogen without embrittlement occurring. Due to the achieved combination of strength and toughness, the austenitic surface layer according to the invention is suitable for increasing the wear resistance, in particular in the case of stress due to impact wear. Cavitation and drop impact, as occurs, for example, in turbomachines.
  • Ferritic-austenitic stainless duplex steels are often used for high-speed pump wheels in aggressive media. Their biphasic structure has the required high insertion limit. A common type of failure is cavitation wear. As can be seen from Fig. 2, a content of 1.4 wt.% Nitrogen in the peripheral zone of this material is achieved by embroidering in nitrogen gas at 1150 ° C and a pressure of 1 bar. After cooling, a fully austenitic edge layer can be seen over a ferrite-austenitic core structure in FIG. 3. This edge layer was subjected to a Kavitationsverschl employ phenomenon compared to the non-embroidered core material.
  • a bubble field is generated by an ultrasound oscillator at 20 kHz and an amplitude of 40 ⁇ m in distilled water, which leads to implosions on the sample surface.
  • the amount of wear is shown as weight loss over the load duration in FIG.
  • a wear rate of 0.0358 (mg / 10 3 s) results for the non-embroidered steel it is 1.53 (mg / 10 3 s). Due to the edge attachment, a decrease of the wear rate by a factor of 43 is achieved. From the example of a current density potential curve, it can be seen from FIG. 5 that the resistance to wet corrosion in artificial seawater is slightly improved by the edge impacting. At approximately the same passive current density results for the embroidered sample an increase in the breakdown potential compared to the non-embroidered sample.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
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  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
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Description

Die Erfindung betrifft ein Wärmebehandlungsverfahren zur Bildung einer austenitischen Randschicht mit ≥ 0,30 Gew.% an gelöstem Stickstoff in endformnahen Teilen aus nicht rostendem Stahl durch Aufstickung bei einer Temperatur zwischen 1.000°C und 1.200°C in einer stickstoffhaltigen Gasatmosphäre.The invention relates to a heat treatment process for forming an austenitic surface layer with ≥0.30 wt.% Of dissolved nitrogen in near net shape parts of stainless steel by nitriding at a temperature between 1000 ° C and 1200 ° C in a nitrogen-containing gas atmosphere.

Ein Wärmebehandlungsverfahren der vorgenannten Art ist aus der DE-A-25 18 452 bekannt. So offenbart die DE-A-25 18 452 ein Verfahren zur Austenitisierung einer etwa 21 bis 45 % Mangan und etwa 10 bis 30 % Chrom enthaltenden Eisenlegierung, wobei die Oberfläche der im festen Zustand vorliegenden Legierung bei einer Temperatur von wenigstens 927°C über einen zur Erhöhung des Stickstoffgehaltes wenigstens in der Oberfläche aus etwa 0,85 bis 3 % ausreichenden Zeitraum der Einwirkung von Stickstoff ausgesetzt wird.A heat treatment process of the aforementioned type is known from DE-A-25 18 452 known. So revealed the DE-A-25 18 452 a process for austenitizing an iron alloy containing about 21 to 45% manganese and about 10 to 30% chromium, wherein the surface of the solid state alloy is at a temperature of at least 927 ° C above a nitrogen content at least in the surface of about 0.85 to 3% sufficient period of exposure to nitrogen exposure.

Der Mangangehalt in den mit Hilfe des aus der DE-A-25 18 452 bekannten Wärmebehandlungsverfahrens zu behandelnden Legierungen beträgt 21 bis 45 %. Da Mangan ein Austenitbilder ist und die Löslichkeit des Stickstoffes im Stahl erhöht, sind Mengen von mehr als 21 % erforderlich.The manganese content in the with the help of from the DE-A-25 18 452 known alloys heat treatment process is 21 to 45%. Since manganese is an austenite and increases the solubility of nitrogen in the steel, amounts greater than 21% are required.

Im Unterschied zur DE-A-25 18 452 wird nun mit der Erfindung ein Wärmebehandlungsverfahren zur Bildung einer austenitischen Randschicht mit ≥ 0,30 Gew.% an gelöstem Stickstoff in endformnahen Teilen aus X5 CrNiMo 17 12 2 oder X2 CrNiMoN 22 5 3 als nicht rostendem Stahl vorgeschlagen, und zwar durch Aufstickung bei einer Temperatur zwischen 1.000°C und 1.200°C in einer stickstoffhaltigen Gasatmosphäre und nachfolgende Abkühlung mit einer solchen Geschwindigkeit, daß eine Nitridausscheidung vermieden wird.In contrast to DE-A-25 18 452 Now, with the invention, a heat treatment process for forming an austenitic surface layer with ≥ 0.30 wt.% of dissolved nitrogen in near-net shape parts of X5 CrNiMo 17 12 2 or X2 CrNiMoN 22 5 3 is proposed as stainless steel, by nitriding at a Temperature between 1000 ° C and 1200 ° C in a nitrogen-containing gas atmosphere and subsequent cooling at a rate such that nitride precipitation is avoided.

Gelöster Kohlenstoff und Stickstoff steigern in nichtrostenden Stählen die Härte des Martensits, die Streckgrenze des Austenits und bewirken eine Stabilisierung der austenitischen Phase. Während die Zugabe von Kohlenstoff den Widerstand nichtrostender Stähle gegen Naßkorrosion verschlechtert, bewirkt Stickstoff eine Verbesserung dieser Eigenschaft. Der Nutzung dieser günstigen Wirkung des Stickstoffs steht seine gegenüber Kohlenstoff wesentlich geringere Löslichkeit in der Stahlschmelze unter Normaldruck entgegen. Daher werden heute druck- oder pulvermetallurgische Verfahren angewendet, um nichtrostende Stähle mit einem Stickstoffgehalt zwischen 03, und 3 Gew. % herzustellen. Diese Verfahren sind jedoch gegenüber einer offenen Stahlerschmelzung mit erheblich höheren Kosten verbunden.Dissolved carbon and nitrogen in stainless steels increase the hardness of martensite, the yield strength of austenite and stabilize the austenitic phase. While the addition of carbon degrades the resistance of stainless steels to wet corrosion, nitrogen causes an improvement in this property. The use of this beneficial effect of nitrogen is counteracted by its significantly lower solubility in the molten steel compared to carbon under atmospheric pressure. Therefore, today, pressure or powder metallurgy processes are used to produce stainless steels having a nitrogen content between 03 and 3 wt%. However, these processes are associated with significantly higher costs compared to open steel smelting.

Die vorliegende Erfindung verzichtet auf einen durchgehend hohen Stickstoffgehalt im Stahl. Statt dessen wird nur die Randzone endformnaher Teile aus nichtrostendem Stahl durch eine Wärmebehandlung so weit mit gelöstern Stickstoff angereichert, daß sich eine hochfeste, aber zähe austenitische Randschicht über einem Kerngefüge aus Ferrit, Austenit, Martensit oder einem Gemisch aus zwei oder drei dieser Gefügebestandteile bildet. Die erfindungsgemäße Wärmebehandlung besteht aus einer Aufstickung in einer stickstoffabgebenden Gasatmosphäre bei einer Temperatur zwischen 1000 und 1200°C. Temperatur, Druck und Dauer der Behandlung werden so gewählt, daß sich eine Randschicht bestimmter Dicke bildet, deren Stickstoffgehalt in der Oberfläche zwischen einer Untergrenze von 0,3 Gew.% und einer Obergrenze liegt, die durch die beginnende Nitridausscheidung während der Aufstickung gegeben ist. Die nachfolgende Abkühlung erfolgt so rasch, daß auch in diesem Zeitraum keine Nitridausscheidung auftritt. Durch eine anschließende Auslagerung bei einer Temperatur ≤ 650°C ist eine Aushärtung der Randschicht möglich.The present invention dispenses with a consistently high nitrogen content in the steel. Instead, only the edge zone of stainless steel parts close to the final shape is enriched with dissolved nitrogen by a heat treatment so that a high-strength but tough austenitic surface layer forms over a core structure of ferrite, austenite, martensite or a mixture of two or three of these structural components. The heat treatment according to the invention consists of a nitridation in a nitrogen-emitting gas atmosphere at a temperature between 1000 and 1200 ° C. Temperature, pressure and duration of the treatment are chosen so as to form a surface layer of certain thickness, the nitrogen content of which in the surface is between a lower limit of 0.3% by weight and an upper limit given by incipient nitride precipitation during nitriding. The subsequent cooling takes place so quickly that even in this period no nitride precipitation occurs. By subsequent removal at a temperature ≤ 650 ° C curing of the surface layer is possible.

Im Patent DE 40 33 706 ist das Einsatzhärten mit Stickstoff beschrieben, bei dem nach Aufstikken eines martensitischen, nichtrostende Stahles durch Härten eine harte martensitische Randschicht Ober einem duktilen Kern erzeugt wird. Dieses Verfahren wird zur Behandlung von nichtrostenden Wälzlagern, Getriebeteilen und Werkzeugen verwendet sowie für nichtrostende Pumpenteile und Ventile in partikelbeladenen Fluiden. In all diesen Fällen kommt es auf höchste Druckfestigkeit und Härte der Randschicht an, die aber mit einer erheblichen Versprödung einhergeht. Das Ziel der vorliegenden Erfindung ist dagegen die Schaffung einer möglichst hochfesten, aber zähen austenitischen Randschicht Ober einem duktilen oder harten Kern (Fig. 1), wobei mit der Erfindung ein Wärmebehandlungsverfahren zur Bildung einer austenitischen Randschicht mit ≥ 0,30 Gew.% an gelöstem Stickstoff in endformnahen Teilen aus X5 CrNiMo 17 12 2 oder X2 CrNiMoN 22 5 3 als nicht rostendem Stahl durch Aufsticken bei einer Temperatur zwischen 1.000°C und 1.200°C in einer stickstoffhaltigen Gasatmosphäre und nachfolgender Abkühlung mit einer solchen Geschwindigkeit, daß eine Nitridausscheidung vermieden wird, vorgeschlagen wird. Dabei wird durch die Eindiffusion von Stickstoff de austenitische Phase in der Randschicht stabilisiert, so daß martensitische oder ferritische Gefügeanteile in der Randzone zu Austenit umwandeln. Gleichzeitig wird durch die Mischkristallhärtung des Austenits mit Stickstoff die Festigkeit der Randschicht erhöht, ohne daß eine Versprödung auftritt. Aufgrund der erreichten Kombination von Festigkeit und Zähigkeit eignet sich die erfindungsgemäße austenitische Randschicht zur Erhöhung des Verschleißwiderstandes insbesondere bei einer Beanspruchung durch Prallverschleiß. Kavitation und Tropfenschlag, wie sie z.B. in Strömungsmaschinen auftritt.In the patent DE 40 33 706 Case hardening with nitrogen is described, in which after hardening a martensitic stainless steel, a hard martensitic surface layer is produced above a ductile core. This process is used to treat stainless steel rolling bearings, gear parts and tools, as well as stainless pump parts and valves in particle-laden fluids. In all these cases, it depends on the highest compressive strength and hardness of the surface layer, which is accompanied by a significant embrittlement. The aim of the present invention, however, is to provide a high-strength, but tough austenitic surface layer above a ductile or hard core (FIG. 1), wherein the invention is a heat treatment process for forming an austenitic surface layer with ≥ 0.30 wt Nitrogen in near-net shape parts of X5 CrNiMo 17 12 2 or X2 CrNiMoN 22 5 3 by embossing at a temperature of between 1,000 ° C and 1,200 ° C in a nitrogen-containing gas atmosphere and subsequent cooling at such a rate as to avoid nitride precipitation , is proposed. It is stabilized by the inward diffusion of nitrogen de austenitic phase in the boundary layer, so that martensitic or ferritic microstructures in the edge zone to austenite. At the same time, the strength of the surface layer is increased by the solid solution hardening of the austenite with nitrogen without embrittlement occurring. Due to the achieved combination of strength and toughness, the austenitic surface layer according to the invention is suitable for increasing the wear resistance, in particular in the case of stress due to impact wear. Cavitation and drop impact, as occurs, for example, in turbomachines.

Im folgenden wird die Erfindung anhand eines Ausführungsbeispieles beschrieben. Für schnellaufende Pumpenräder in aggressiven Medien werden vielfach ferritisch-austenitische nichtrostende Duplexstähle verwendet, deren zweiphasiges Gefüge die erforderliche hohe Steckgrenze mit sich bringt. Eine häufige Versagensart ist der Verschleiß durch Kavitation. Wie aus Fig. 2 ersichtlich, wird durch Aufsticken in Stickstoffgas bei 1150°C und einem Druck von 1 bar ein Gehalt von 1,4 Gew.% Stickstoff in der Randzone dieses Werkstoffes gelöst. Nach dem Abkühlen ist eine vollaustenitische Randschicht über einem ferritisch-austenitischem Kerngefüge in Fig. 3 zu erkennen. Diese Randschicht wurde im Vergleich zu dem nicht aufgestickten Kernwerkstoff einer Kavitationsverschleißprüfung unterzogen. Dabei wird durch einen Ultraschsllschwinger bei 20 kHz und einer Amplitude von 40µm in destilliertem Wasser ein Blasenfeld erzeugt, das zu Implosionen an der Probenoberfläche führt. Der Verschleißbetrag ist als Gewichtsverlust über der Belastungsdauer in Fig. 4 wiedergegeben. Für die erfindungsgemäß aufgestickte Randschicht ergibt sich eine Verschleißrate von 0,0358 (mg/103s) für den nicht aufgestickten Stahl beträgt sie 1,53 (mg/103s). Durch die Randaufstikkung wird damit eine Abnahme der Verschleißrate um den Faktor 43 erreicht. Am Beispiel einer Stromdichtepotentialkurve ist aus Fig. 5 zu entnehmen, daß der Widerstand gegen Naßkorrosion in künstlichem Meerwasser durch die Randaufstikkung leicht verbessert wird. Bei ungefähr gleicher Passivstromdichte ergibt sich für die aufgestickte Probe eine Erhöhung des Durchbruchpotentials gegenüber der nicht aufgestickten Probe.In the following the invention will be described with reference to an embodiment. Ferritic-austenitic stainless duplex steels are often used for high-speed pump wheels in aggressive media. Their biphasic structure has the required high insertion limit. A common type of failure is cavitation wear. As can be seen from Fig. 2, a content of 1.4 wt.% Nitrogen in the peripheral zone of this material is achieved by embroidering in nitrogen gas at 1150 ° C and a pressure of 1 bar. After cooling, a fully austenitic edge layer can be seen over a ferrite-austenitic core structure in FIG. 3. This edge layer was subjected to a Kavitationsverschleißprüfung compared to the non-embroidered core material. In this case, a bubble field is generated by an ultrasound oscillator at 20 kHz and an amplitude of 40 μm in distilled water, which leads to implosions on the sample surface. The amount of wear is shown as weight loss over the load duration in FIG. For the surface layer embroidered on according to the invention, a wear rate of 0.0358 (mg / 10 3 s) results for the non-embroidered steel it is 1.53 (mg / 10 3 s). Due to the edge attachment, a decrease of the wear rate by a factor of 43 is achieved. From the example of a current density potential curve, it can be seen from FIG. 5 that the resistance to wet corrosion in artificial seawater is slightly improved by the edge impacting. At approximately the same passive current density results for the embroidered sample an increase in the breakdown potential compared to the non-embroidered sample.

Übertragen auf ein Pumpenrad bedeuten diese Prüfergebnisse, daß die hohe Streckgrenze des ferritisch-austenitischen Duplexgefüges im Kern erhalten bleibt und damit die Tragfähigkeit bei hoher Umdrehungsgeschwindigkeit. Gleichzeitig wird die Kavitationsverschleißrate durch die aufgestickte austenitische Randschicht solange erheblich gesenkt, bis diese aufgezehrt ist. Was die Kosten betrifft, so entfällt die bei Duplexstählen übliche Wärmebehandlung bestehend aus Lösungsglühen bei 1020 bis 1100°C und Abschrecken. An ihreWhen transferred to a pump wheel, these test results mean that the high yield strength of the ferritic-austenitic duplex structure in the core is maintained, and thus the load capacity at high rotational speed. At the same time, the cavitation wear rate is considerably reduced by the embroidered austenitic surface layer until it is consumed. As far as the costs are concerned, the heat treatment customary in duplex steels, consisting of solution annealing at 1020 to 1100 ° C. and quenching, is eliminated. At yours

Stelle tritt das Aufsticken und Abkühlen, so daß nur der Mehraufwand für eine längere Behandlungsdauer und die Gasatmosphäre anfällt.Spot occurs the Aufsticken and cooling, so that only the extra effort for a longer treatment time and the gas atmosphere is obtained.

Claims (3)

  1. A method of thermal treatment for forming an austenitic skin with ≥ 0.30 % by wt. of dissolved nitrogen in near to final shape components of X5 CrNiMo 17 12 2 or X2 CrNiMoN 22 5 3 in the form of stainless steel by nitriding at a temperature between 1000 and 1200°C in a nitrogen-containing gas atmosphere and subsequent cooling at such a speed that nitride deposition is prevented.
  2. A method of thermal treatment as claimed in Claim 1, characterised in that the pressure in the gas atmosphere during the nitriding process differs from normal pressure.
  3. A method of thermal treatment as claimed in Claim 1 or 2, characterised in that the skin is hardened by subsequent re-heating to a temperatures 650°C.
EP94114659A 1993-10-05 1994-09-17 Case nitriding for producing a high-strength austenitic skin in stainless steels Expired - Lifetime EP0652300B1 (en)

Applications Claiming Priority (2)

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DE4333917 1993-10-05
DE4333917A DE4333917C2 (en) 1993-10-05 1993-10-05 Edge embroidery to create a high-strength austenitic surface layer in stainless steels

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EP0652300A1 EP0652300A1 (en) 1995-05-10
EP0652300B1 true EP0652300B1 (en) 2007-11-28

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EP (1) EP0652300B1 (en)
JP (1) JPH07188733A (en)
CN (1) CN1058758C (en)
CZ (1) CZ240094A3 (en)
DE (1) DE4333917C2 (en)
ES (1) ES2296286T3 (en)
PL (1) PL178509B1 (en)
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