EP1424402B1 - Process for avoiding the adhesion of metal parts during annealing - Google Patents

Abstract

Annealing metal parts, especially steel parts containing chromium and manganese, comprises acquiring the dew point of the hydrogen-steam atmosphere during heat-maintaining and cooling, and automatically regulating by adjusting with empirically determined theoretical values.

Description

The invention relates to a method for adhesive-free annealing of metal parts, in particular of chromium and manganese-containing steel parts, according to the features of the preamble of claim 1.

During annealing of chromium or manganese-containing steel parts in a defined hydrogen-steam atmosphere in a treatment furnace, such as in a bell-type furnace, a thin chromium oxide layer (Cr 2 O 3) and / or spinel layer (Fe Cr 2 O 4) is formed on the surface of the annealed material during the heating phase ,

This resulting from oxidation cover prevents during the held in the holding phase recrystallization of a, from local diffusion bonds, so-called adhesives, resulting gluing 'of Glühgutteile at their contact surfaces and thus the quality of the finished product diminishing surface defects. A corresponding procedure is off DE 195 31 447 A1 known. By selective oxidation of chromium in a chromium-rich steel according to the equation:

2Cr + 3 H2O = Cr2O3 + 3H2

Oxide layer formed on the surface of the annealing depending on the temperature, time and gas and Glühteil composition can be decomposed during annealing by a too low water content of the steam-hydrogen protective gas.

The pure hydrogen used in the annealing of alloy steel usually as a protective gas with a dew point less than 70 degrees Celsius, contains only up to 2 vpm (0.002 vol .-%) of water in the form of water vapor.

The minimum water content in such a hydrogen protective gas can lead to the fact that during the holding or cooling phase of the annealing process, the oxidic cover layer of the glow part according to the equation:

Cr2O3 +3 H2 = 2Cr + 3H2O

as far as or completely decomposed, whereby their desired blocking and isolation function is eliminated.

In order to obtain a clean, metallically bright steel surface, the dew point of the water vapor-hydrogen mixture (PH 2 O / PH 2 ratio), which is dependent on the annealing temperature and the composition (alloy constituent (s) of the annealed material, must not be exceeded.

For this reason, until now, during the annealing, the hydrogen-hydrogen content of the protective gas atmosphere has been determined by means of oxygen or lambda probes or with continuously measuring dew-point measuring devices.

From the US 5,772,428 discloses a method of reducing the carbon content by annealing using a closed loop to control the hydrogen / water content. To determine this content, an oxygen sensor is preferred over a dew point meter.

The optimum for the respective annealing phase water vapor content in the furnace atmosphere can by addition of oxygen-containing media, such as water, nitrous oxide, air, oxygen or carbon dioxide so by additives that do not cause undesirable reactions, such. an embroidering and carburizing effect in the annealing, be adjusted.

In the known method for the adhesive-free annealing of metal parts, in particular of steel parts containing chromium and manganese, it would be desirable if the composition of the hydrogen-steam protective gas could be controlled during the holding and cooling phase so that the formed by oxidation during the heating phase , a sticking of the

Annealing annealing preventing and no undesirable surface discoloration causing oxide layer is retained.

Furthermore, it would be advantageous if the successful in a hydrogen-steam furnace atmosphere annealing of steel, the previously necessary, costly post-purification of the furnace gas could be omitted.

The invention is therefore based on the object to provide a method for adhesive-free annealing of metal parts, in particular of chromium and manganese-containing steel parts, under a protective gas atmosphere containing hydrogen, which is reproducible and leads to the further processing of the annealing not impairing oxide layers.

The object of the invention is achieved by a method having the features specified in the claim 1.

Advantageous developments of the invention are specified in the subclaims.

According to the invention, the dew point of the introduced in the treatment chamber of the furnace hydrogen protective gas is detected during the holding and cooling phase of the annealing process by measuring devices and generates an actual signal from the detected dew point of the hydrogen-water vapor mixture.

The actual signal is fed to a control unit and compared in this with a setpoint signal formed in the control unit from the stored nominal values.

In the case of deviations between the actual and desired signal, the water vapor content in the hydrogen atmosphere is changed by the addition of oxygen-containing media until the actual signal corresponds to the desired signal.

As oxygen-containing media can include water, carbon dioxide, nitrous oxide, air or pure oxygen - ie media that does not cause undesirable reactions, such as For example, an embroidering or carburizing in annealing, cause - be introduced into the treatment room of the furnace.

For measuring the water content in the furnace atmosphere continuously measuring dew point measuring devices are used.

The target signal is based on empirically determined process parameters, such as the process temperature and the annealed material composition, which are determined empirically as the desired value and which optimally correspond to the respective process conditions.

By the annealing of metal parts according to the invention, in particular of steels containing chromium and manganese, a quality product which has clean, scale, annealing and decarburization-free annealed surfaces can be provided.

Fig. 1

eine Diagramm-Darstellung der Stabilität von Chromoxid in Abhängigkeit vom Sauerstoffpartialdruck der Wasserstoffatmosphäre und der Temperatur;

Fig. 2

eine Diagramm-Darstellung der Stabilität von Chromoxid in Abhängigkeit vom Sauerstoffpartialdruck der Wasserstoffatmosphäre und der Temperatur;

Fig. 3

eine Diagramm-Darstellung der Kinetik der Chromoxidbildung einer Fe-20%Cr-Legierung im Wasserstoff/Wasser-Gemisch in Abhängigkeit von der Temperatur und Zeit;

Fig. 4

eine Diagramm-Darstellung der Kinetik der Chromoxidschichtbildung einer Fe-20%Cr-Legierung im Wasserstoff/Wassergemisch in Abhängigkeit von der Temperatur und Zeit.

The invention will be explained in more detail with reference to an embodiment shown in the drawing.
Show it:

Fig. 1

a diagram of the stability of chromium oxide as a function of the oxygen partial pressure of the hydrogen atmosphere and the temperature;

Fig. 2

a diagram of the stability of chromium oxide as a function of the oxygen partial pressure of the hydrogen atmosphere and the temperature;

Fig. 3

a diagram of the kinetics of chromium oxide formation of an Fe-20% Cr alloy in the hydrogen / water mixture as a function of temperature and time;

Fig. 4

a diagram of the kinetics of chromium oxide layer formation of a Fe-20% Cr alloy in hydrogen / water mixture as a function of temperature and time.

FIG. 1 shows the stability of chromium oxide as a function of the oxygen partial pressure in the temperature interval from 600 ° C. to 800 ° C.

2Cr + 1.5 O2 = Cr2O3

The chemical activity of chromium depends on the concentration in the alloy as well as on other factors. There is a connection that the less chromium is contained in the alloy, the lower the chromium activity. Consequently, the required oxygen partial pressure, which must prevail in the gas atmosphere, is greater, in order to just form chromium oxide: $${{\mathrm{P}}_{\mathrm{O}2}}^{*}={\left({\mathrm{K}}_{\mathrm{P}1}*{{\mathrm{a}}_{\mathrm{cr}}}^2/{\mathrm{a}}_{\mathrm{cr}2\mathrm{O}3}\right)}^{1.5}$$

There are three alloys with chromium concentrations of 1, 10 and 20 percent by weight of chromium, it being assumed that the percentage by weight of chromium also corresponds to the activities (a _cr = 0.01, 0.10 and 0.20). The chromium oxide activity has always been 1, that is, it forms pure oxide, supposedly.

A constant hydrogen / water gas mixture has a specific oxygen partial pressure at a particular temperature:

2H2O = 2H2 + O2

It was based on 4 realistic mixtures that can be used in heat treatment practice (dew point corresponds to: -30 ° C, -40 ° C, -50 ° C, -60 ° C). The dew point is known to be a practical measure of the water concentration and can therefore be used as a concentration.

1. 1.) log P_O2 ^* = -30 atm und 660°C bei Taupunkt -60°C
2. 2.) log P_O2 ^* = -26 atm und 765°C bei Taupunkt -50°C
   An den beiden Schnittpunkten sieht man, dass je höher die Behandlungstemperatur ist, desto höher kann auch der Taupunkt und somit die Wasserkonzentration sein, um eine Chromoxidation zu behindern.
   Eine weitere Kurve ist eingetragen, hierbei handelt es sich um eine Wasserstoffatmosphäre, die 1 Vol.-% Kohlenmonoxid enthält und eine Kohlenstoffaktivität von 1 besitzt:
3. 3.) log P_O2* = -24,3 atm und 787°C bei 1 Vol.-%CO
   Der Schnittpunkt der beiden Aktivitätskurven Gas/Legierung liegt bei der mit 1 Gew.- % Cr Legierung, die beiden anderen Chromlegierungen werden nur oberhalb dieser Temperatur von 787°C reduziert.
   In Fig. 2 ist die Stabilität von Chromoxid in Abhängigkeit vom Sauerstoffpartialdruck im Temperaturintervall von 750°C bis 950°C dargestellt:
   
           2Cr + 1,5 O2 = Cr2O3
   
   
   Folgende Schnittpunkte ergeben sich in diesem Temperaturbereich, bei denen die Chromoxidschichten auf den Legierungen reduziert werden:
4. 4.) log P_O2* = -21,4 atm und 874°C bei Taupunkt -30°C mit 1 Gew.-% Cr
5. 5.) log P_O2* = -22,5 atm und 883°C bei Taupunkt -40°C mit 10 Gew.-% Cr
6. 6.) log P_O2* = -21,7 atm und 923°C bei Taupunkt -40°C mit 20 Gew.-% Cr
7. 7.) log P_O2* = -23,7 atm und 835°C bei 1 Vol.-%CO mit 10 Gew.-% Cr
8. 8.) log P_O2* = -23,6 atm und 850°C bei 1 Vol.-%CO mit 20 Gew.-% Cr

If the oxygen partial pressure of a hydrogen / water mixture is above the chromium oxide stability line of a chromium alloy, chromium oxide forms on the alloy surface. However, if this activity line is undershot, the chromium oxide is reduced. Thus, for each prevailing system and for each alloy / gas atmosphere, there is an intersection point characterized by a temperature and an oxygen partial pressure. An alloy with a chromium activity of 0.10 has z. B. the following intersections:

1. 1.) log P _O2 ^* = -30 atm and 660 ° C at dew point -60 ° C
2. 2.) log P _O2 ^* = -26 atm and 765 ° C at dew point -50 ° C
   At the two intersections, it can be seen that the higher the treatment temperature, the higher the dew point, and thus the water concentration, can be to hinder chromium oxidation.
   Another curve is plotted, this being a hydrogen atmosphere containing 1% by volume of carbon monoxide and having a carbon activity of 1:
3. 3.) log P _O2 * = -24.3 atm and 787 ° C at 1 vol.% CO
   The point of intersection of the two activity curves gas / alloy is 1 wt% Cr alloy, the other two chromium alloys are only reduced above this temperature of 787 ° C.
   FIG. 2 shows the stability of chromium oxide as a function of the oxygen partial pressure in the temperature interval from 750 ° C. to 950 ° C.
   
   2Cr + 1.5 O2 = Cr2O3
   
   
   The following intersections result in this temperature range, where the chromium oxide layers on the alloys are reduced:
4. 4.) log P _O2 * = -21.4 atm and 874 ° C at dew point -30 ° C with 1 wt% Cr
5. 5.) log P _O2 * = -22.5 atm and 883 ° C at dew point -40 ° C with 10 wt% Cr
6. 6.) log P _O2 * = -21.7 atm and 923 ° C at dew point -40 ° C with 20 wt% Cr
7. 7.) log P _O2 * = -23.7 atm and 835 ° C at 1 vol.% CO with 10 wt.% Cr
8. 8.) log P _{O2 *} = -23.6 atm and 850 ° C at 1 vol.% CO with 20 wt.% Cr

On the basis of the listed intersection points, it can be clearly seen that with increasing operating temperatures the moisture content of the hydrogen atmospheres can increase and the chromium oxide layer is nevertheless reduced. Above 1000 ° C, all 3 alloys are bright annealed with a hydrogen atmosphere whose dew point is -30 ° C. The recrystallization temperature of high-alloy chromium steels is generally about 1050 ° C., so that the metal parts are bright annealed. A dew point of minus 30 ° C or higher is easily achievable in industrial furnaces.

FIGS. 3 and 4 show the growth rates of chromium oxide formation as a function of the oxidation time in the temperature interval from 500 ° C. to 1070 ° C.

2Cr + 1.5 O2 = Cr2O3

The temperatures and holding times adapted to the respective heat treatment necessitate the formation of different chromium oxide layer thicknesses on the metal parts. The transparency of the surface of a metal part is ensured at a chromium oxide layer thickness of up to about 0.030 microns. At a chromium oxide layer thickness of greater than about 0.030 micrometers, the metal part surface turns yellowish and with further increasing chromium oxide layer thickness turns blue-violet green and loses its transparency.

For chromium-containing steels annealed at a temperature of approximately 700 ° C., the entire holding phase is approximately 10 hours during which a 0.030 micron thick oxide layer can form at a temperature of 660 ° C. The forming chromium layer thickness is - with low holding times - within the transparent glow part surface ensuring layer thickness. Such heat-treated steels thus have a metallically bright surface.

FIG. 4 shows the growth rate of chromium oxide formation as a function of the oxidation time in the temperature interval from 750 ° C. to 1070 ° C.

2Cr + 1.5 O2 = Cr2O3

With a total holding time of 0.5 hours and a temperature of less than 960 ° C, the alloy surfaces remain bare, but are still passivated, which prevents diffusion bonding. If the stability limit of the chromium oxide is exceeded, then the oxide layer formed during the heating phase is reduced again. Since the formation and decomposition kinetics are almost equal in this layer thickness range of chromium oxide, the reduction time is significantly smaller due to the higher temperatures, so that the reduction is completed after a short time and there is enough time for the formation of welds.

In continuously operated continuous furnaces very low dew points of -50 ° C often occur, so that the formation of chromium oxide is already completed at about 800 ° C and the reduction of the oxide is begun. Has by this time a chromium oxide layer of, for example, 0.10 microns formed, the chromium oxide layer can be reduced again before reaching the holding temperature of 1050 ° C.

Claims (5)

1. Method for the sticker-free annealing of metal parts, in particular of steels containing chromium and manganese, in a hydrogen/water vapour atmosphere, comprising the phases of heating, holding and cooling, wherein the oxide layer, which is formed on the surface of the product being annealed during the heating phase and reduces the extent to which the annealed parts stick together, is retained during the cooling phase, characterized in that the dew point of the hydrogen/water vapour atmosphere is recorded during the holding and cooling phases and is automatically controlled to match empirically determined setpoint values, wherein the dew point is determined using a unit which measures the dew point continuously.
2. Process according to Claim 1, characterized in that the process temperature is recorded as a setpoint value.
3. Process according to Claim 1, characterized in that the composition of the product being annealed is recorded as a setpoint value.
4. Process according to Claim 1, characterized in that the quantity of hydrogen/water vapour furnace gas introduced is recorded as a setpoint value.
5. Process according to Claim 1, characterized in that the dosing time of the hydrogen/water vapour furnace gas is recorded as a setpoint value.

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