EP1424402A1 - 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 glue-free annealing of metal parts, especially of steel parts containing chrome and manganese, according to Features of the preamble of claim 1.

When taking place in a defined hydrogen-water vapor atmosphere Annealing steel parts containing chrome or manganese in one Treatment furnace, such as in a hood furnace, is during the Heating phase on the surface of the annealing material a thin layer of chrome oxide (Cr2O3) and / or spinel layer (FeCr2O4).

This top layer created by oxidation prevents during the Hold phase performed recrystallization annealing from local Diffusion welding, so-called glue, resulting gluing of the annealing parts on their contact surfaces and thus the quality of the Finished product reducing surface defects.

By selective oxidation of chromium in a chromium-rich steel according to the equation: 2Cr + 3 H2O = Cr2O3 + 3H2 The oxide layer formed on the surface of the annealing material as a function of the temperature, time and gas and glow element composition can be decomposed during the annealing due to an insufficient water content of the steam-hydrogen protective gas.

The pure one usually used as a protective gas when annealing alloy steel Hydrogen with a dew point below minus 70 degrees Celsius only contains up to to 2 vpm (0.002 vol%) water in the form of water vapor.

The minimum water content in such a hydrogen shielding gas can lead to the fact that during the holding or cooling phase of the annealing process the oxide cover layer of the annealing part corresponds to the equation: Cr2O3 + 3 H2 = 2Cr + 3H2O as far as or can be completely decomposed, which eliminates their desired locking and isolation function.

In order to obtain a clean, metallic bright steel surface, that of the Annealing temperature and the composition (alloy component / s) of the annealing material dependent dew point of the water vapor-hydrogen mixture used as protective gas (PH2O / PH2 ratio) must not be exceeded.

For this reason, the water vapor-hydrogen content is so far during the glow the protective gas atmosphere using oxygen or lambda sensors or with continuously measuring dew point measuring devices.

The optimal water vapor content for the respective glow phase Furnace atmosphere can be increased by adding oxygen-containing media, such as for example water, nitrous oxide, air, oxygen or carbon dioxide Additives that do not cause undesired reactions, e.g. an embroidery and Carburize in the annealing material can be adjusted.

In the known method for glue-free annealing of metal parts, it would be especially steel parts containing chrome and manganese desirable if the composition of the hydrogen-water vapor protective gas during the holding and cooling phase could be regulated so that the formed by oxidation during the heating phase, a sticking of the Annealing parts preventing and no undesirable surface discoloration causing oxide layer is retained.

It would also be advantageous if in a hydrogen-steam oven atmosphere annealing of steel the previously necessary costly post-cleaning of the furnace gas could be omitted.

The invention is therefore based on the object of a method for adhesive-free Annealing metal parts, especially those containing chromium and manganese Steel parts, under a protective gas atmosphere containing hydrogen To provide that is reproducible and for the further processing of the Annealing leads not impairing oxide layers.

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

Advantageous developments of the invention are specified in the subclaims.

According to the dew point of the in the treatment room of the furnace introduced hydrogen protective gas during the holding and cooling phase of the Annealing process recorded using measuring devices and from the recorded dew point of the Hydrogen-water vapor mixture generates an actual signal.

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

If there are deviations between the actual and target signals, the Water vapor content in the hydrogen atmosphere by adding changed media containing oxygen until the actual signal corresponds to the target signal.

As oxygen-containing media, i.a. Water, carbon dioxide, nitrous oxide, air or pure oxygen - i.e. Media that do not like undesirable reactions for example embroidering or carburizing in the annealing material, - in the Treatment room of the furnace can be introduced.

To measure the water content in the furnace atmosphere find measuring devices such as for example, commercially available oxygen or lambda sensors or continuously measuring dew point meters use.

The target signal is empirically determined as the target value, the respective one Process conditions optimally corresponding process parameters, for example the Process temperature and the composition of the annealing material.

By annealing metal parts, in particular chromium and Steels containing manganese can be a clean, scale, temper and Quality products with decarburization-free annealing surface provided become.

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 is explained in more detail below with reference to an embodiment shown in the drawing.
Show it:

Fig. 1

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

Fig. 2

a diagram showing 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 an Fe-20% Cr alloy in the hydrogen / water mixture as a function of temperature and time.

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 and other influencing factors. There is a connection that the less chromium the alloy contains, the lower the chromium activity. As a result, the required partial pressure of oxygen, which must prevail in the gas atmosphere, is also greater in order to just form chromium oxide: P _O2 ^* = (K _P1 * a _cr ² / a _Cr2O3 ) ^1.5

Three alloys with chromium concentrations of 1, 10 and 20 weight percent chromium are shown, it being assumed that the mass fraction of chromium also corresponds to the activities (a _cr = 0.01; 0.10 and 0.20). The chromium oxide activity was always assumed to be 1, ie pure oxide is formed.

A constant hydrogen / water gas mixture has a certain oxygen partial pressure at a particular temperature: 2H2O = 2H2 + O2

4 realistic mixtures were used, which are in the Heat treatment practice can occur (dew point corresponds to: -30 ° C, -40 ° C, - 50 ° C, -60 ° C). As is well known, the dew point is a practical measure of water concentration and can therefore be used as a concentration.

1.) log P_O2 ^* = -30 atm und 660°C bei Taupunkt -60°C

2.) log P_O2 ^* = -26 atm und 765°C bei Taupunkt -50°C

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 surface of the alloy. However, if the activity falls below this level, the chromium oxide is reduced. Thus there is an intersection for each prevailing system and for each alloy / gas atmosphere, which is characterized by a temperature and an oxygen partial pressure. An alloy with a chromium activity of 0.10 has e.g. B. the following intersections:

1.) log P _O2 ^* = -30 atm and 660 ° C at dew point -60 ° C

2.) log P _O2 ^* = -26 atm and 765 ° C at dew point -50 ° C

At the two intersections you can see that the higher the treatment temperature the higher the dew point and thus the water concentration can be, to hinder chromium oxidation.

3.) log P_O2 ^* = -24,3 atm und 787°C bei 1 Vol.-%CO

Another curve is entered, this is a hydrogen atmosphere that contains 1 vol .-% carbon monoxide and has a carbon activity of 1:

3.) log P _O2 ^* = -24.3 atm and 787 ° C with 1 vol.% CO

The intersection of the two activity curves gas / alloy lies at 1% by weight Cr alloy, the other two chrome alloys are only above this Temperature reduced from 787 ° C.

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

4.) log P_O2 ^* = -21,4 atm und 874°C bei Taupunkt -30°C mit 1 Gew.-% Cr

5.) log P_O2 ^* = -22,5 atm und 883°C bei Taupunkt -40°C mit 10 Gew.-% Cr

6.) log P_O2 ^* = -21,7 atm und 923°C bei Taupunkt -40°C mit 20 Gew.-% Cr

7.) log P_O2 ^* = -23,7 atm und 835°C bei 1 Vol.-%CO mit 10 Gew.-% Cr

8.) log P_O2 ^* = -23,6 atm und 850°C bei 1 Vol.-%CO mit 20 Gew.-% Cr

The following intersections result in this temperature range at which the chromium oxide layers on the alloys are reduced:

4.) log P _O2 ^* = -21.4 atm and 874 ° C at dew point -30 ° C with 1 wt .-% Cr

5.) log P _O2 ^* = -22.5 atm and 883 ° C at dew point -40 ° C with 10 wt .-% Cr

6.) log P _O2 ^* = -21.7 atm and 923 ° C at dew point -40 ° C with 20 wt .-% Cr

7.) log P _O2 ^* = -23.7 atm and 835 ° C at 1 vol.% CO with 10 wt.% Cr

8.) log P _O2 ^* = -23.6 atm and 850 ° C at 1 vol .-% CO with 20 wt .-% Cr

From the intersections listed, you can clearly see that with increasing Operating temperatures increase the moisture content of the hydrogen atmosphere can be reduced and the chrome oxide layer can still be reduced. Above of All 3 alloys with a hydrogen atmosphere, whose Dew point is -30 ° C, bright annealed. The recrystallization temperature of high-alloy chrome steels is usually about 1050 ° C, so the metal parts be bright annealed. A dew point of minus 30 ° C or higher is in technical Easy to achieve furnace systems.

3 and 4 show the growth rates of the 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 cause the formation of different chromium oxide layer thicknesses on the Metal parts. The transparency of the surface of a metal part is with a chromium oxide layer thickness guaranteed up to about 0.030 microns. With a chromium oxide layer thickness The surface of the metal part changes color if it is larger than approx. 0.030 micrometer yellowish and with increasing chrome oxide layer thickness blue-violet-green and loses its transparency.

For chromium-containing steels annealed at a temperature of approx. 700 ° C this is entire holding phase about 10 hours, in which at a temperature of 660 ° C can form a 0.030 micron thick oxide layer. The forming Chrome layer thickness - with short holding times - lies within that of a transparent one Glow part surface ensuring layer thickness. Such heat-treated steels therefore have a shiny metallic surface.

4 shows the growth rate of the 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 bright, but are still slightly passivated, thereby preventing diffusion welding. If the stability limit of the Below chromium oxide, then the formed during the heating phase Oxide layer reduced again. Because the kinetics of education and decomposition in this Layer thickness range of chromium oxide are almost the same size, is the reduction time noticeably smaller due to the higher temperatures, so the reduction after completed in a short time and enough time for the formation of Welding is given.

In continuously operated continuous furnaces, very low levels are often found Dew points of -50 ° C, so that the chromium oxide formation already at approx. 800 ° C is completed and the reduction of the oxide is started. Has up to at this point in time a chromium oxide layer of, for example, 0.10 micrometers formed, the chromium oxide layer before reaching the holding temperature of 1050 ° C can be reduced again.

Claims (6)

Process for the glue-free annealing of metal parts, in particular steels containing chromium and manganese, in a hydrogen-water vapor atmosphere with the phases of heating, holding and cooling, in which the oxide layer formed on the surface of the annealing material during the heating phase prevents the annealing parts from sticking together is retained during the cooling phase, characterized in that the dew point of the hydrogen-water vapor atmosphere is recorded during the holding and cooling phases and is automatically regulated in accordance with empirically determined target values. A method according to claim 1, characterized in that
the process temperature is recorded as the setpoint. A method according to claim 1, characterized in that
the composition of the annealing material is recorded as the setpoint. A method according to claim 1, characterized in that the entered amount of hydrogen-water vapor furnace gas is recorded as the target value. A method according to claim 1, characterized in that
the dosing time of the hydrogen-steam oven gas is recorded as the setpoint. A method according to claim 1, characterized in that the respective oxygen partial pressure of the hydrogen-water vapor atmosphere is recorded as a measured variable.

Images