EP0552460B1 - Process for hardening of work pieces unter the action of plasma-pulses - Google Patents

Abstract

Workpieces made of steel are hardened by carburising the surface and subsequent quenching. Carburisation is carried out by means of a plasma discharge in vacuo in the presence of gaseous hydrocarbons at voltages between 200 and 2000 volts, preferably between 300 and 1000 volts, and preferably with the admixture of argon. The plasma is generated by electrodes operated in vacuo, of which the cathode serves as the workpiece holder and is operated in a pulsed manner. So as to achieve, in the process, a uniform hardness distribution even for irregularly shaped workpieces and to reduce the carbon mass flow, a) carburisation is carried out at an overall pressure between 14 and 30 mbar (1400 to 3000 Pa), b) a pulse duration of between 110 and 10,000 mu s (microseconds) is chosen, c) an internal duration of between 30 and 10,000 mu s is chosen and d) the mean power supplied to the plasma discharge is decreased, after completion of the start-up phase, by reducing the pulse duration and/or extending the interval duration, either continuously or in a stepwise manner, in such a way that the carbon content at said surface, without interruption of the pulsed operation, does not exceed at any time the saturation limit of the material for carbon in the austenitic domain. o

Description

The invention relates to a method for hardening workpieces made of steel, in particular with at least one alloy element from the group Cr, Ni, Mn, Si and Mo, by carburizing the surface and subsequent quenching, the carburizing using a plasma discharge in a vacuum in the presence of gaseous gases Hydrocarbons at voltages between 200 and 2000 volts, preferably between 300 and 1000 volts, carried out and the plasma is generated by means of electrodes operated in a vacuum, of which the cathode serves as a workpiece holder and is operated in pulse mode.

From EP-A2-0 288 680 it is known that when the delivery rate (mass flow m _c ) of the carbon is too high, carbon over-saturation occurs on the workpiece surface, which results in carbide formation. This drastically reduces the high hardness achieved during carburization. Ideally, the curves of the C content and hardness should be shown in a diagram as approximately S-shaped curves.

A reduction of the supersaturation by diffusion of the carbon into the depth of the workpiece would only be possible via extremely long-term diffusion processes. In the literature, proposed to switch several times cyclically between the carburizing phase and the diffusion phase during the process in order to give the carbon the opportunity to diffuse into the required depth of the workpiece. The cycle times are long and the timing of the switchover is difficult to perform and therefore inaccurate.

US Pat. No. 4,900,371 discloses a plasma-pulse method of the type described in the introduction, in which the repetition time is 10 ms and the pulse and pause times are each 5 ms. The specified parameters are intended to homogenize the gas and plasma distribution over the workpiece surface, but with the usual cathode voltages of 500 to 1000 V result in mass flows of carbon which, without the activation of carbon-free diffusion phases, also after a few minutes to supersaturate the surface area with carbon and would lead to undesirable carbide formation. A mass flow for carbon results from the data given $${\text{m}}_{\text{c}}{\text{= approx. 80 gm}}^{\text{-2}}{\text{.H}}^{\text{-1}}\text{.}$$

Another problem is that plasma carburizing works in the area of the so-called anomalous glow discharge, in which the current density increases disproportionately when the voltage is increased from about 200 to over 1000 volts until the anomalous glow discharge after exceeding a limit value of the voltage changed suddenly into an arc discharge (see: (1) Bell / Loh / Staines "Thermodynamic Treatment in Plasma", NEUE HÜTTE, 28th Volume, Issue 10, October 1983, pages 373 to 379; (2) Booth / Farrell / Johnson " The Theory and Practice of Plasma Carburising "HEAT TREATMENT OF METALS, 1983, pp. 45 to 52).

This process should be avoided under all circumstances, as this would damage the workpiece. Under the conditions US-A-4 900 371, a change in the abnormal glow discharge into an arc discharge cannot be ruled out with sufficient certainty.

Furthermore, a vacuum-assisted plasma carburizing process is known from US Pat. No. 4,853,046, in which the plasma discharge is operated as a pulsed glow discharge. A single pulse cycle consists of a pulse group of four or five pulses (on-pulses), followed by a pulse group consisting of two or three pulses (off-pulses). The signal curve of the individual pulses follows the rectified sine half-waves of an AC pulse. In this process, either pure methane or pure propane is introduced into the process chamber as the gas as the carbon donor medium. The electrical power supplied to the plasma during the carburizing process is effected exclusively by regulating the plasma flow. In order to prevent over-carbonization of the workpieces and the associated carbide and soot formation in the furnace, this process provides for a diffusion step.

From US-A-4 490 190 and EP-B1-0 062 550 it is known in a method of the type described in the introduction to choose a pulse duration that is much smaller than the period duration in a method operated over the duration of the method with constant power. to make two treatment parameters independent of each other, namely the plasma on the one hand and the treatment temperature on the other. However, this problem only arises with nitriding and nitro carburizing, since the treatment temperatures must be well below 600 ° C. Under the specified conditions, carburizing is not possible within economically justifiable treatment times, since this process only takes place at a useful speed at temperatures above about 800 ° C.

The invention is therefore based on the object of improving a method of the type mentioned at the outset such that with reproducible and simple process monitoring and control, even with irregularly shaped workpieces, a uniform hardness distribution is achieved and carbide formation on the surface is avoided without the interposition of a pronounced diffusion phase, that the carbon content on the surface of the workpiece can be set to any value between the carbon content of the core in the workpiece and the saturation limit of the material and that the glow discharge is reliably prevented from turning into an arc discharge.

• a) das Aufkohlen bei einem Gesamtdruck zwischen 14 und 30 mbar (1400 bis 3000 Pa) durchgeführt wird,
• b) die Impulsdauer zwischen 110 und 10 000 µs (Mikrosekunden) und die Pausendauer zwischen 30 und 10 000 µs gewählt wird, wobei das Verhälnis von Impulsdauer zu Pausendauer zwischen 0,3 und 0,02 gewählt wird und daß
• c) die der Plasmaentladung zugeführte mittlere Leistung nach Beendigung der Anfahrphase durch Verringerung der Impulsdauer und/oder durch Verlängerung der Pausendauer zurückgenommen wird, derart, daß der Kohlenstoffgehalt an der besagten Oberfläche ohne Unterbrechung des Impulsbetriebes zu keinem Zeitpunkt die Sättigungsgrenze des Werkstoffs für Kohlenstoff im Austenitgebiet überschreitet, wobei das zugeführte Prozeßgas aus einer Mischung aus 2 bis 50%, vorzugsweise 10 bis 30% Argon, 3 bis 50%, vorzugsweise 10 bis 30% Kohlenwasserstoffgas und einem Rest aus Wasserstoff gebildet wird (jeweils in Volumenprozenten).

According to the invention, the object is achieved in the method specified at the outset in that

• a) carburizing is carried out at a total pressure between 14 and 30 mbar (1400 to 3000 Pa),
• b) the pulse duration between 110 and 10,000 microseconds (microseconds) and the pause duration between 30 and 10,000 microseconds is chosen, the ratio of pulse duration to pause duration between 0.3 and 0.02 being chosen and that
• c) the mean power supplied to the plasma discharge is reduced after the start-up phase by reducing the pulse duration and / or by lengthening the pause duration, in such a way that the carbon content on said surface does not at any time exceed the saturation limit of the material for carbon in the austenite area without interrupting the pulse operation exceeds, wherein the supplied process gas is formed from a mixture of 2 to 50%, preferably 10 to 30% argon, 3 to 50%, preferably 10 to 30% hydrocarbon gas and a remainder of hydrogen (each in volume percent).

Through the measures according to the invention, the object is achieved to the full extent, that is to say the method specified at the outset is improved in such a way that, with simpler process monitoring and control, carbide formation on the surface is avoided without the interposition of a pronounced diffusion phase that the carbon content on the surface of the Workpiece can be reproducibly set to any value between the carbon content of the core in the workpiece and its saturation limit and that an envelope of Glow discharge in an arc discharge is prevented.

In particular, the mass flow m _{c of} the carbon is reduced so that its solubility in austenite is not exceeded and no carbides can be formed. The method according to the invention can be operated quasi-stationary with continuously pulsed plasma.

The oxidation-free carburizing of the surface by the plasma increases the fatigue strength, the warpage of the workpiece is reduced, and there are lower costs for the reworking of the workpieces.

Wichtig ist dabei die Einhaltung des Merkmals d). Bei einer Absenkung des besagten Verhältnisses auf 0,025 ergab sich ein Massenstrom von noch $${\text{m}}_{\text{c}}{\text{= 5 g.m}}^{\text{-2}}{\text{.h}}^{\text{-1}}\text{.}$$

The ratios of pulse duration to pause duration are between 0.3 and 0.02. At a ratio of 0.2, the mass flow of carbon was $${\text{m}}_{\text{c}}{\text{= 30 gm}}^{\text{-2}}{\text{.H}}^{\text{-1}}\text{.}$$

It is important to comply with feature d). If the ratio was reduced to 0.025, the mass flow was still $${\text{m}}_{\text{c}}{\text{= 5 gm}}^{\text{-2}}{\text{.H}}^{\text{-1}}\text{.}$$

It is particularly advantageous if the mean power after a start-up phase with the carbon content on the surface rising as quickly as possible before the said saturation limit is reached is reduced to a value at which the pulse operation continues to spread the carbon content below the saturation limit into the depth of the workpiece is continued.

The mass flow is then just as large as the migration in the workpiece through diffusion. This can speed up the process, i.e. the carburization rate can be selected very high at the beginning, but is then adapted to the diffusion rate.

• Die Zurücknahme der mittleren Leistung erfolgt kontinuierlich, oder in einer oder mehreren Stufen.
• In einem weiteren Schritt kann der Massenstrom m_c soweit reduziert oder auf 0 eingestellt werden, daß durch weitere Eindiffusion von der Oberfläche in das Werkstück innere der Rand-C-Gehalt auf den gewünschten Wert abgesenkt und (in der Kurvendarstellung des Härteverlaufs) im Randbereich ein waagerechter Verlauf eingestellt wird.
• Mit zunehmendem Druck im Prozeßraum , d.h. am oberen Ende des Druckbereichs, erfolgt noch eine bessere Anschmiegung des Plasmas an eine profilierte, strukturierte oder gar hinterschnitttene Werkstückoberfläche, wie dies beispielsweise bei Zahnrädern, Lagerkäfigen oder dergleichen der Fall ist.

The following configurations of the method according to the invention lead to further advantages:

• The average power is withdrawn continuously or in one or more stages.
• In a further step, the mass flow m _{c can} be reduced or set to 0 so that the edge C content is reduced to the desired value by further diffusion from the surface into the inside of the workpiece and (in the curve representation of the hardness curve) in the edge area horizontal course is set.
• With increasing pressure in the process space, ie at the upper end of the pressure range, there is an even better fitting of the plasma to a profiled, structured or even undercut workpiece surface, as is the case, for example, with gear wheels, bearing cages or the like.

By adding argon, part of the energy introduced is used for the ionization of argon, which makes the process advantageous.

The following gases are suitable as gaseous hydrocarbon compounds: methane, ethane, propane, ethylene and propene.

The invention is explained in more detail below with reference to FIGS. 1 to 6.

Figur 1

einen Vertikalschnitt durch eine Vorrichtung zur Durchführung des erfindungsgemäßen Verfahrens,

Figur 2

eine Parameterdarstellung der Abhängigkeit der Kohlenstoffkonzentration in verschiedenen Tiefen des Werkstücks nach unterschiedlicher Verfahrensdauer,

Figur 3

die zu Figur 2 gehörenden Härtewerte,

Figur 4

ein Diagramm über die Abhängigkeit der Kohlenstoffkonzentration ausgehend von der Oberfläche in die Tiefe des Werkstücks beim Ausführungsbeispiel,

Figur 5

ein Diagramm mit einer vergleichsweisen Darstellung des Härteverlaufs an Flanke und Fuß eines Zahns eines Zahnrades nach Anwendung eines Prozeßdrucks von 2500 Pa, und

Figur 6

ein Diagramm analog Figur 5, jedoch nach Anwendung eines Prozeßdrucks von nur 600 Pa.

Show it:

Figure 1

a vertical section through a device for performing the method according to the invention,

Figure 2

a parameter representation of the dependence of the carbon concentration in different depths of the workpiece after different process duration,

Figure 3

the hardness values belonging to FIG. 2,

Figure 4

1 shows a diagram of the dependence of the carbon concentration on the surface into the depth of the workpiece in the exemplary embodiment,

Figure 5

a diagram with a comparative representation of the hardness curve on the flank and foot of a tooth of a gear after application of a process pressure of 2500 Pa, and

Figure 6

a diagram analogous to Figure 5, but after application of a process pressure of only 600 Pa.

In Figure 1, a vacuum furnace 1 is shown with a furnace chamber 2, which is lined with a thermal insulation device 3. In front of the side walls 3a of the thermal insulation device 3 there is an electrode which is grounded and which serves as an anode 4 of the circuit. A vertical support rod 6 is passed through the furnace ceiling 2a by means of an insulating bushing 5 and carries at its lower end a plate-shaped horizontal workpiece holder which also has an electrode function, i.e. serves as cathode 7. Only one of the workpieces 8 arranged on this workpiece holder is shown.

Between the cathode 7 and the anode 4 there is a power supply 9 for the generation of the voltage pulses for the formation of the plasma. The power supply 9 is on Assigned control device 10, with which the electrical process parameters for influencing the plasma can be set.

Cathode 7 and workpieces 8 are concentrically surrounded by a resistance heating element 11, which is connected to a controllable current source 12. The energy balance of the furnace and thus the workpiece temperature is determined on the one hand by the losses and on the other by the sum of the energy contributions from the plasma and the radiation from the resistance heating element.

A supply line 13, which comes from a controllable gas source 14 and through which the desired process gases or gas mixtures are supplied, opens into the furnace chamber 2. The gas balance is determined by the gas supply, the consumption by the workpieces and possibly loss sinks, but not least by the influence of the vacuum pump 15, which is connected to the furnace chamber 2 via a suction line 16 and can also be designed as a pump set.

In the bottom 2b of the furnace chamber 2 there is an opening 17 which can be closed by a gate valve 18 and under which - connected in a vacuum-tight manner - there is a heatable oil container 19 with a quenching oil. Above the opening 17 there is an opening 20 in the cathode 7 through which the workpieces 8 can be lowered into the quenching oil by means of a manipulator (not shown). The mode of operation of this device results from the general description and from the exemplary embodiments.

FIG. 2 shows a parameter representation of the dependence of the carbon concentration at different depths of the workpiece after a different process duration in the event that one of the processes according to the prior art (Gas carburization) without interrupting the carburization by a diffusion break. The depth values in millimeters are plotted on the abscissa, starting from the workpiece surface, and the ordinate is the carbon concentration in percent by weight. The individual curves apply (from bottom to top) for the process times of 0.5, 1, 2 and 4 hours plotted under the abscissa. It can be seen that the carbon concentration on the surface has already exceeded the saturation value after a process duration of 2 h, which is reflected in a decrease in hardness according to FIG. 3.

Figure 3 shows the hardness values belonging to Figure 2. The abscissa bears the same scale, and the associated hardness values in HV are plotted on the ordinate. It can be seen that the surface hardness reaches a peak value of 800 HV after 2 h with a steep drop to the depth, but already begins to decrease again after a process duration of 3 h due to carbide formation and drops to around 700 HV after 4 h. As the experiments continue, the situation deteriorates further, which is also generally known from the literature (for example EP-A2-0 288 680).

Example 1:

In a device according to FIG. 1, cylinder pins made of the steel alloy 16MnCr5 with a diameter of 20 mm were carburized in batches as substrates. First, the device for removing the residual gases was evacuated to a pressure of 10 -3 mbar, whereupon a mixture of 15% argon, the rest of hydrogen was admitted to a pressure of 15 mbar. By simultaneously operating the resistance heater and applying a negative voltage of 600 V to the substrates, these were cleaned by sputtering and heated to 900 ° C. The pretreatment lasted 60 minutes. The gas atmosphere was then replaced by that of 5% methane, 80% hydrogen and 15% argon until a pressure of 15 mbar was reached. The actual carburizing was then carried out by means of a pulse operation in which the pulse voltage was set to 600 V, the ratio of pulse duration to pause duration was 0.07 at the power source. The first phase of the treatment period was 240 min, the substrate temperature being kept at a constant 900 ° C. by adjusting the power of the resistance heater. Then the said ratio was reduced to 0.023 with otherwise the same parameters and the carburization was continued at 900 ° C. with a corresponding adjustment of the power of the resistance heating for a period of 90 min. Arc discharges never occurred during the entire process. The cylinder pins were then inserted into the oil bath, which was kept at a temperature of 60 ° C., using a manipulator with the cylinder axis in a vertical position.

Measurements of the course of the C content led to the diagram in FIG. 4. On the abscissa, analogously to FIG. 2, the depth in mm, starting from the surface, and the ordinate, the C content in percent by weight is plotted. The curve shows the desired S-shaped course.

The hardness curve also measured, starting from the surface, was 800 HVl down to a depth of 0.4 mm in the entire area. The hardening depth at 0.9 mm was 550 HVl. This fully met the demands made.

Example 2:

The experiment according to Example 1 was repeated, but with the following modifications:

Gears with a ratio of tooth height h _Z to tooth gap width l _o of 1.5 made of the steel alloy 16CrMo4 were used as substrates. The treatment temperature was 925 ° C at a total pressure of 2500 Pa. In the first carburizing phase of 195 minutes, the ratio of pulse duration to pause duration = 0.07, in a second carburizing phase of 70 minutes = 0.04.

The results of this experiment are shown in FIG. 5: The tooth profile (hatched) and the so-called plasma border (thick black line) are shown at the top right of the window, as are the measurement locations M1 and M2. The plasma conforms extremely well to the tooth profile. The measuring location M1 is in the tooth flank, the measuring location M2 on the tooth base. The diagram shows the hardness "HVl" over the depth "t". Curve K1 shows the hardness curve at measuring point M1, curve K2 shows the hardness curve at measuring point M2. It can be seen that the measured values agree very well and that in particular the penetration depth "t" is essentially the same for M1 and M2, which is due to the good conformity of the plasma to the tooth profile.

Example 3 (comparative example):

The experiment according to Example 2 was repeated, with the only difference that the total pressure in the process chamber was reduced to 600 Pa.

The results of this experiment are shown in FIG. 6: The (identical) tooth profile (hatched) and the so-called plasma seam (thick black line) are shown here at the top right of the window, as are the measurement locations M3 and M4. The plasma only fits snugly on the tooth head and is at a significantly greater distance from the tooth profile at the tooth base. The measuring location M3 is on the tooth flank, the measuring location M4 on the tooth base. The diagram shows the hardness "HVl" over the depth "t". Curve K3 shows the hardness curve at measuring point M3, curve K4 shows the hardness curve at measuring point M4. It can be seen that the measured values deviate greatly from one another and that in particular the penetration depth "t" is significantly less with M4 than with M3, which is due to the lower conformity of the plasma in the foot region of the tooth profile.

This comparison test clearly shows that the total pressure in the process space is of considerable importance.

Claims (6)

1. Process for hardening workpieces made of steel, in particular with an alloy element pertaining to the group comprising Cr, Ni, Mn, Si and Mo, by carburising the surface and subsequent quenching, the carburising being carried out by means of a plasma discharge in a vacuum in the presence of gaseous hydrocarbons at pressures between 14 and 30 mbar (1400 to 3000 Pa) and at voltages between 200 and 2000 volts, preferably between 300 and 1000 volts, and the plasma being generated by means of electrodes operated in a vacuum, the cathode of said electrodes serving as workpiece-holder and being operated in pulsed manner, characterised in that

   a) the pulse duration is chosen between 110 and 10,000 µs (microseconds) and the pause duration is chosen between 30 and 10,000 µs, the ratio of pulse duration to pause duration being chosen between 0.3 and 0.02,

   b) the mean power supplied to the plasma discharge after conclusion of the start-up phase is lowered by reduction of the pulse duration and/or extension of the pause duration in such a way that without interruption of the pulse operation the carbon content on said surface at no time exceeds the saturation limit of the material in respect of carbon in the austenite range, the supplied process gas being constituted by a mixture consisting of 2 to 50%, preferably 10 to 30%, argon, 3 to 50%, preferably 10 to 30%, hydrocarbon gas and a remainder consisting of hydrogen (in each case in percentage by volume).
2. Process according to Claim 1, characterised in that after a start-up phase with a rate of growth of the carbon content on the surface that is as rapid as possible the mean power is lowered before said saturation limit is reached to a value at which the pulse operation is continued subject to continuous diffusion of the carbon content below the saturation limit down into the workpiece.
3. Process according to Claim 2, characterised in that in a final step the mass flow of carbon is reduced to an extent or is adjusted to 0 such that as a result of further diffusion from the surface into the interior of the workpiece the C content in the periphery is lowered to the desired value and (in the curve representation) a horizontal path is adjusted in the peripheral region.
4. Process according to Claim 1, characterised in that the mean power is lowered continuously.
5. Process according to Claim 1, characterised in that the pulse voltage supplied to the cathode is chosen between 200 and 900 V, preferably between 500 and 700 V.
6. Process according to Claim 1, characterised in that a shortfall in the power supplied to the workpieces by the plasma is supplied by a source of heat which is independent of the plasma.

Images