EP3538676B1 - Method for the heat treatment of a workpiece consisting of a high-alloy steel - Google Patents

Description

The present invention relates to a method for heat treatment of a workpiece made of a high-alloy steel.

State of the art

To increase the fatigue strength, the corrosion resistance and the wear resistance of metallic components, it is known to nitride them in areas close to the surface. Nitriding causes different nitrides to separate out within the metallic material in the surface area. This leads to the build-up of internal compressive stresses, which in some cases assume very high values in the edge area. Depending on the surface distance, the residual stresses decrease with increasing distance from the edge area. The presence of internal compressive stresses leads to improved fatigue strengths. Nitriding is also used for high-alloy steels, especially for components such as nozzle bodies, valve bodies or throttle plates.

Due to the high oxygen affinity of their alloying elements, high-alloy steels form a natural oxide layer of a few nanometers. This oxide layer is formed when it comes into contact with air and consists, for example, of chromium oxide, vanadium oxide, iron oxide and other oxides. Since the oxide layer is very compact and partially diffusion-tight, subsequent diffusion of nitrogen at elevated temperatures, in particular between 480 ° C. and 590 ° C., can be negatively influenced and even completely prevented. Inhomogeneous connecting layers as well as diffusion layers with different functional Properties are the result. The naturally occurring oxide layer can be removed chemically, for example by pickling with an acid, before the actual nitriding process. Furthermore, the oxide layer can also be removed mechanically by brushing and / or grinding, or else electrically by applying a corresponding voltage.

The formation of an oxide layer on the high-alloy steel that comes into contact with air has disadvantages when it comes to finishing the surface or removing the oxide layer. Acid pickling often results in local scars as a result of different oxide layer thicknesses, or during mechanical post-processing, the residues have to be removed using a complex cleaning or removal process. It has also been found that, in the case of components with complex geometries, chemical, mechanical or electrical aftertreatment often does not bring about the desired effect due to the complex real geometry. In particular, blind holes are difficult to access, and optimal removal of the oxide layer cannot be ensured here. This inevitably leads to defects after nitriding or to inhomogeneous connection and diffusion layers and could lead to premature component failure.

From the EP 1 122 331 B1 a method for heat treatment of metallic workpieces, in particular for nitriding or nitrocarburizing of alloyed iron materials, is known. First, the workpieces are heated in a nitriding furnace to a temperature between 400 ° C and 500 ° C in an ammonia-containing gas atmosphere. The workpieces are then heated to a temperature between 500 ° C. and 700 ° C. in a gas atmosphere containing ammonia and an added oxidizing agent. The workpieces are exposed to this temperature and this gas atmosphere for a period of up to 5 hours.

From the EP1612290A1 a method for gas nitriding a workpiece is also known.

From Menthe E. et al .: "Further investigation of the structure and properties of austenitic stainless steel after plasma nitriding", Surf. & Coat. Tech., ISSN: 0257-8972, NL, Vol. 116-119, (1999-09-01), pp. 199-204 Investigations of the structure and properties of austenitic stainless steel after plasma nitriding are known.

From Jordan D. et al. : "Low torr-range vacuum nitriding of 4140 steel", Heat Treat. Prog., ISSN: 1536-2558, US, Vol. 8, No. 2, (2008-02-29), pp. 33/8 a method for low-pressure gas nitriding is known.

Task and solution

It is the object of the invention to further develop a heat treatment of a workpiece made of a high-alloy steel.

According to the invention, this object is achieved by a method according to the main claim. Further advantageous refinements emerge from the subclaims that refer back to them.

Disclosure of the invention

In the context of the invention, a method for heat treatment of a high-alloy steel was developed.

The workpiece, which consists of a high-alloy steel, is heated to a first temperature in a vacuum environment, the first temperature being kept constant during a first holding phase, the workpiece then being heated to a second temperature that is higher than the first temperature, the second temperature during is kept constant in a second holding phase, and wherein the workpiece is quenched after the second holding phase, preferably in gaseous or evaporating media. A surface of the workpiece, in particular also the inner contours, are flowed around a surface of the workpiece during the first holding phase in a first treatment step with a process gas that emits hydrogen and / or a process gas mixture for cleaning and activation of the surface, the surface during the first holding phase in a second treatment step with a process gas that emits nitrogen and / or process gas mixture is flowed around to form a thin nitride-containing layer, and the nitride-containing layer is provided to optimize a downstream gas nitriding process.

The heat treatment according to the invention is subdivided into the manufacturing process of a workpiece consisting of a high-alloy steel behind the initial soft machining, in particular the production of the workpiece from a blank. Subsequent to the heat treatment according to the invention, in particular hardening, the tempering of the workpiece takes place, for example, in an evacuable, oxygen-free tempering furnace. In other words, the tempering of the workpieces is a second heat treatment. Before the final hard machining of the workpiece to the finished component and the associated final dimension setting The third heat treatment step takes place by grinding, hard turning or similar processes, in which the properties required for the workpiece, in particular the workpiece surface, are set by means of gas nitriding at preferably 480-590 ° C by diffusing nitrogen into the workpiece.

The vacuum environment is a technical vacuum with a pressure of at most 50 mbar (1 mbar = 100 Pa). The vacuum furnace is hermetically sealed and a pump connected to the interior of the vacuum furnace creates the vacuum ambient conditions in the vacuum furnace. The naturally formed oxide layer or passive layer is broken up by the hardening process according to the invention and the surface of the high-alloy steel is cleaned. Carrying out the process in a vacuum or an oxygen-free atmosphere prevents or slows down the formation of a new passive layer and / or the repassivation of the high-alloy steel. A depletion of hardness-increasing alloying elements close to the edge is thus additionally avoided.

The term "holding phase" is to be understood as the constant holding of a temperature at which the workpiece assumes the internal temperature of the vacuum furnace for carrying out the first and second treatment step. During the first holding phase, a process gas and / or process gas mixture that emits hydrogen flows around the high-alloy steel in a first treatment step. The injection of the gas preferably takes place constantly. A pulsed, variable or pressure-controlled course of the flow is also conceivable.

The flow around the workpiece in the first treatment step represents a cleaning and activation step in order to promote the diffusion of nitrogen into the surface of the steel in the second processing step due to the surface cleaned and activated as a result and the high temperature in the vacuum furnace. The first temperature for the first treatment step is between 800 and 1090 ° C., preferably 900 ° C., in order to ensure optimum interaction of the process gas and / or process gas mixture which emits hydrogen with the surface of the To ensure workpiece. In the first treatment step, the oxide layer is broken up and repassivation of the surface is prevented with the aid of the vacuum. The surface of the workpiece is therefore highly reactive towards the diffusion of nitrogen in the second treatment step.

After completion of the first treatment step, the second treatment step begins at the constant first temperature of the furnace. A nitrogen-releasing process gas and / or process gas mixture flows around the high-alloy steel to form a nitride-containing layer.

Advantageously, pure nitrogen (N2) or else ammonia (NH ₃ ) or a mixture of nitrogen / ammonia is used. Alloyed or high-alloy steels are particularly suitable for nitriding, since the alloying elements of these steels preferentially combine with the atomic nitrogen to form nitrides. On the other hand, unalloyed steels can form brittle nitriding layers that tend to flake off during nitriding. Steels with a carbon content between 0.3 and 0.6 mass% and alloying elements such as chromium or vanadium, which form surface layer nitrides at high temperatures, are particularly suitable for nitriding.

The advantage resulting from the so-called pre-nitriding in the hardening process according to the invention over conventional production methods of workpieces made of high-alloy steels is that due to the vacuum environment and the cleaning and activation by the process gas and / or process gas mixture emitting hydrogen during nitriding in the second treatment step of the hardening process, a homogeneous and forms a dense nitride layer on the surface. This nitride layer can be regarded as a seed layer or passivation layer, since the actual nitriding step only takes place after the tempering and before the hard machining of the workpiece.

Furthermore, the pre-nitriding in the hardening process also optimizes the gas nitriding in the downstream manufacturing step. Due to the homogeneous seed layer from the hardening process, a more compact connecting layer with a correspondingly lower proportion of pores is formed during gas nitriding in the chamber furnace. The nitriding effect, which is described with the help of the so-called nitriding index, is accordingly higher due to the pre-nitriding in the hardening process. The nitriding index results from the partial pressures of the nitrogen emitting Process gas and / or process gas mixture and the partial pressure of the hydrogen. The higher the nitriding index, the greater the potential for nitride formation. If the nitrogen content in the material exceeds the maximum solubility of the nitrogen in the base material, nitrides are formed. These nitride precipitates form the connecting layer directly on the surface. Starting from the surface, a decreasing nitrogen gradient forms, this area is called the diffusion layer. In this area there are small nitride precipitates as well as nitrogen dissolved in the metal lattice. In steels, iron forms iron nitrides and in high-alloy steels, for example, chromium and vanadium combine to form corresponding nitrides. Since a nitrided seed layer is present as a result of the pre-nitriding in the hardening process, a lower nitriding index is required in the nitriding process, which simplifies and simplifies the process management. The gas nitriding process can also be shortened and / or carried out at lower temperatures, which also makes the process more cost-effective.

In addition, the nitrided layer makes the tempering process less sensitive after the hardening process, as a renewed temperature increase below the transformation temperature reduces stresses, depending on the composition of the steel, further special carbides can be precipitated and a lower hardness can be set without the alloying elements on the surface of the base material interacting to risk with the furnace atmosphere.

During the second treatment step, the hardening process changes from the first holding phase to the second holding phase. In the second holding phase, the high-alloy steel is heated to the second temperature. The second temperature is also to be understood as the austenitizing temperature. At room temperature, the high-alloy steel is essentially in the form of ferrite and carbide, which converts to austenite at high temperatures and the carbides partially dissolve. The aim is to take advantage of the high solubility of carbon at high temperatures in austenite.

At the austenitizing temperature, carbon diffuses into the austenite lattice. If the high-alloy steel is then quenched, the carbon can no longer diffuse out of the grid and, due to the increase in volume, distorts it tetragonally, which essentially forms martensite. The greater the quenching speed, the higher the martensite content. To initiate the quenching process, the second treatment step ends with the second holding phase.

Furthermore, the duration of the second treatment step, the second temperature of the high-alloy steel during the second treatment step and / or the nitrogen partial pressure on the surface of the high-alloy steel during the second treatment step are selected so that the nitride-containing layer with a thickness of less than 2 μm, preferably with a thickness of 0.001 µm to 1 µm.

The nitride-containing layer preferably has sheet-like or crystalline nitrides. Chromium can form sheet-like nitrides, iron preferentially forming crystalline nitrides.

The hydrogen-releasing process gas and / or process gas mixture flows around the surface with a first treatment pressure and the nitrogen-releasing process gas and / or process gas mixture flows around the surface with a second treatment pressure, the respective treatment pressure being in a pressure range between 10 mbar and 3000 mbar. The selected pressure range is strongly dependent on the properties of the workpiece.

Furthermore, the first treatment pressure is lower than the second treatment pressure. The higher the second treatment pressure, the greater the potential for nitride formation in the area of the workpiece near the edge and the deeper the nitrogen diffuses into the workpiece.

Further measures improving the invention are shown in more detail below together with the description of a preferred exemplary embodiment of the invention on the basis of figures.

Working examples

• Figur 1 den Verlauf der Temperatur T und des Druck p über die Zeit bei einem Ausführungsbeispiel des erfindungsgemäßen Verfahrens, und
• Figuren 2 bis 5 die erfindungsgemäßen Verfahrensschritte zur Wärmebehandlung eines aus einem hochlegierten Stahl bestehenden Werkstücks.

It shows:

• Figure 1 the course of the temperature T and the pressure p over time in an exemplary embodiment of the method according to the invention, and
• Figures 2 to 5 the method steps according to the invention for the heat treatment of a workpiece consisting of a high-alloy steel.

Figure 1 shows an example of the process control for an embodiment of the method according to the invention. The left ordinate 4 describes the temperature axis, the right ordinate 5 describes the partial pressure axis and the abscissa 6 describes the time axis. The upper continuous curve denotes the course of the temperature T over time. The lower continuous curve denotes the course of the partial pressure p over time. Sections A1, H1, A2, H2, F as well as B1 and B2 are defined along the time axis, in which different activities take place.

In a first heating phase A1, the workpiece S is initially heated from room temperature to a temperature T1 of 900 ° C. The heating rate is essentially constant. The vacuum furnace in which the process is carried out is under a technical vacuum, with a negative pressure of less than 50 mbar ( Figure 2 ). Furthermore, it is also conceivable that the vacuum is only generated after a certain temperature has been reached.

In the first holding phase H1 following the first heating phase A1, the first temperature T1 is kept constant at approximately 900 ° C. During the heating phase A1, no process gas or process gas mixture G1, G2 containing hydrogen or nitrogen is fed in. During the first holding phase H1, the first treatment step B1 begins, in which a hydrogen-containing process gas or process gas mixture G1 with a first treatment pressure P1 flows around the workpiece S. The first treatment pressure corresponds to P1 the hydrogen partial pressure acting on the surface 1 of the workpiece S. The partial pressure corresponds to the pressure that the individual gas component, in this case hydrogen, would exert if it were alone in a given volume. The flow of the hydrogen-containing process gas or process gas mixture G1 is constant ( Figure 3 ). During the first treatment step, the naturally formed oxide layer 7 or passive layer of the high-alloy steel is broken up and the surface 1 of the workpiece S is cleaned and activated against the diffusion of nitrogen in the subsequent second treatment step B2.

The first treatment step B1 is followed by the second treatment step B2, in which a nitrogen-containing process gas or process gas mixture G2 with a second treatment pressure P2 flows around the workpiece S. The second treatment pressure P2 corresponds to the nitrogen partial pressure acting on the surface 1 of the workpiece S. The flow of the nitrogen-containing process gas or process gas mixture G2 is constant ( Figure 4 ). The second treatment pressure P2 is higher than the first treatment pressure P1, the respective treatment pressure P1, P2 being between 10 mbar and 3000 mbar.

During the second treatment step B2, the first holding phase H1 is followed by a second heating phase A2 with a subsequent second holding phase H2. The heating rate is constant. The workpiece S is first heated from the first temperature T1 to the second temperature T2, which is then kept constant. The second temperature T2 corresponds to the austenitizing temperature of the workpiece S. In the edge area, while maintaining the austenitizing temperature, a phase transformation to an austenitic structure takes place. In the second treatment step B2, which continues from the first holding phase H1, the nitrogen-containing process gas or process gas mixture G2 continues to flow around the workpiece S in the second holding phase H2 with a second treatment pressure P2 and constant flow. The second holding phase H2 corresponds to a nitriding phase. Due to the second temperature T2, atomic nitrogen diffuses from the nitrogen-containing process gas or process gas mixture G2 into the surface 1 of the workpiece S and combines with nitride-forming alloying elements such as for example chromium, vanadium or iron. The duration of the second treatment step B2, the second temperature T2 of the workpiece S during the second treatment step B2 and the second treatment pressure B2 on the surface 1 of the workpiece S during the second treatment step B2 influence the thickness of the nitride-containing layer 2, which is between 0.001 µm and 1 µm located ( Figure 5 ).

The second holding phase H2 and the second treatment step B2 are then followed by a quenching phase F for setting an essentially martensitic structure. The vacuum furnace 3 and the workpiece S are quenched to room temperature.

the Figures 2 to 5 describe the method steps according to the invention for heat treatment of a workpiece S made of a high-alloy steel in sectional drawing according to the in Figure 1 shown and explained process management.

Claims (3)

1. Method for the heat treatment of a workpiece (S) consisting of a high-alloy steel, wherein the workpiece (S) is heated to a first temperature (T1) in a vacuum environment, wherein the first temperature (T1) is kept constant during a first holding phase (H1), wherein the component (S) is then heated to a second temperature (T2) that is higher than the first temperature (T1), wherein the second temperature (T2) is kept constant during a second holding phase (H2), and wherein the workpiece (S) is quenched following the second holding phase (H2), wherein a surface (1) of the workpiece (S) is subjected to a circulating flow of a hydrogen-releasing process gas and/or process gas mixture (G1) for cleaning and activating the surface (1) during the first holding phase (H1) in a first treatment step (B1), wherein the surface (1) is subjected to a circulating flow of a nitrogen-releasing process gas and/or process gas mixture (G2) for forming a nitride-containing layer (2) during the first holding phase (H1) in a second treatment step (B2), and wherein the nitride-containing layer (2) is provided to optimize a downstream gas nitriding process,
   characterized in that the first holding phase (H1) is switched over to the second holding phase (H2) during the second treatment step (B2), in that the surface (1) is subjected to a circulating flow of the hydrogen-releasing process gas and/or process gas mixture (G1) at a first treatment pressure (P1) and the surface (1) is subjected to a circulating flow of the nitrogen-releasing process gas and/or process gas mixture (G2) at a second treatment pressure (P2), wherein the respective treatment pressure (P1, P2) is in a pressure range of between 10 mbar and 3000 mbar, in that the second treatment step (B2) ends with the second holding phase (H2), in that the first temperature (T1) amounts to at least 800°C to 1090°C, and preferably 900°C, during the first holding phase (H1) and in that the second temperature (T2) is selected as the austenitization temperature of the workpiece (S), wherein the first treatment pressure (P1) is the hydrogen partial pressure and the second treatment pressure (P2) is the nitrogen partial pressure, and wherein the first treatment pressure (P1) is less than the second treatment pressure (P2).
2. Method according to Claim 1, characterized in that the duration of the second treatment step (B2), the second temperature (T2) of the workpiece (S) during the second treatment step (B2) and/or the second treatment pressure (P2) at the surface (1) of the workpiece (S) during the second treatment step (B2) are selected such that the nitride-containing layer (2) is formed with a thickness of less than 2 µm, preferably with a thickness from 0.001 µm to 1 µm.
3. Method according to one of the preceding claims, characterized in that the nitride-containing layer (2) comprises sheet-like or crystalline-deposited nitrides.

Images