WO1992007100A1

WO1992007100A1 - Process and device for affecting measurement and shape changes in the hardening of workpieces

Info

Publication number: WO1992007100A1
Application number: PCT/EP1991/001941
Authority: WO
Inventors: Ulrich Gussmann; Jakob Kiefer; Helmut Moll; Rudolf Schmid; Josef Wesener
Original assignee: Zahnradfabrik Friedrichshafen Ag
Priority date: 1990-10-15
Filing date: 1991-10-12
Publication date: 1992-04-30
Also published as: DE59102384D1; EP0553154A1; DE4133799A1; EP0553154B1

Abstract

The dimensional effect on quenching with pin or swage hardening tools is favoured and automatable both towards a closer tolerance on the final dimensions and towards a more flexible adaptability to larger variations in the raw dimensions in that there are at least two successive hardening stages (T1, T2) in shrinkage, the first of which takes place over a larger diametric range (6) and causes purely plastic deformation, while the second (T2) has to assume only partial plastic deformation. The tools involved are both fixed pins (5) with two diameter ranges (6, 7) and auxiliary force operable spreading pins (16) or corresponding matrices adjustable to smaller cylindrical dimensions during hardening. The risk of tool damage or structural breaks resulting from excessive shrinkage drops and the proportion of rejects within narrow final dimensional tolerances is reduced with a shorter and more economical plant running-in time with new workpieces.

Description

Method and device for influencing dimensional and shape changes when hardening workpieces

The invention relates to methods for

Influencing changes in size and shape when hardening rotationally symmetrical workpieces. For hardening, these workpieces are placed on mechanical aids with a predetermined diameter and cooled with a quenching medium. In the course of this cooling, the workpieces shrink to the specified diameter with plastic and elastic deformation. The invention further relates to devices for performing the proposed methods.

It is known that the formation of martensite causes a considerable increase in the hardness of steel. Hardening is a heat treatment in which the workpiece is cooled in a quenching medium, starting from the hardening temperature with an austenitic structure, in such a way that martensite is formed. When components are hardened, warpage occurs, which can be divided into an unavoidable portion resulting from structural changes and an avoidable portion that depends on the process-specific circumstances.

By using hardening devices it is possible to reduce the distortion of workpieces. With these hardening tools, a distinction is made between tied-up devices, so-called mandrel fixtures, and force-bound devices, so-called expanding tools. With the mandrel fixtures, the workpieces shrink to a predetermined diameter, so that a shape is impressed on the component. With spreading tools, this is done with diameters that are variable over time and with simultaneous application of force (details on this see essay H. Bomas and others: Influencing the warpage of case-hardened components in hardening technology reports 45 (1990/3, pages 188 to 195).

When using mandrel fixings, the diameter to which the workpieces shrink must be determined empirically. This empirically determined diameter largely determines the deviations of the workpiece from the nominal size as soon as the workpiece is detached from the fixture. Likewise, a component that is as accurate and dimensionally accurate as possible is required in the soft machined state in order to keep the deviations within tolerable limits.

The appropriate diameter must be determined empirically because the deformation of the workpiece when shrinking onto the mandrel fixture consists of a plastic and an elastic component. To make matters worse, a diameter once determined to be suitable is caused by fluctuations in the manufacturing tolerance and by other process-specific ones

Parameters (e.g. material, pre-processing, process flow) can become unsuitable. In order to keep waiting times during the production process as short as possible, it is common to have several groups of hardening tools available, each of which has slightly different diameters.

Force-bound devices are more flexible in determining the suitable diameter because variable diameters can be specified by changing the application of force. But even with these force-loaded devices, an empirical procedure is unavoidable, since the effective diameter must be maintained with pressing forces that are opposed to the shrinkage forces effective as a result of the contraction of the workpiece. The present invention has for its object to improve the known methods for influencing dimensional and shape changes during hardening so that the hardening of workpieces can be carried out more efficiently and with better results and to a greater extent reproducibly in ongoing production. The method according to the invention is intended to allow greater tolerances in the soft dimensions of a component, to avoid strong shrinkage stresses on the hardening tool and to save time and money in ongoing production.

To solve the problem, a first method according to the invention is proposed, in which the workpiece is quenched in the quenching medium in at least two time periods. It is essential that the workpiece shrinks to a first diameter at a temperature at which the workpiece is predominantly plastically deformed. The plastic deformation in this first period of time accounts for the major part of the total deformation of the component during hardening. It is important that a possible amount of elastic deformation compared to the amount of plastic deformation in this process section is negligible, as far as the target dimension of the workpiece to be observed after hardening is concerned.

The first time period is followed by a further time period in which the temperature of the workpiece is still above the martensite point and the workpiece shrinks to a second - smaller - diameter. In this section of the process, plastic deformation occurs again, measured in terms of the proportion of plastic

Deformation in the first process step, however, is significantly less. With the gradual plastic Deformation provides valuable advantages: excessive shrinkage stresses are avoided. The tolerances that must be observed when soft machining the component can be coarser. In addition, it is important to note that the proportion of elastic deformation in the overall deformation of the component also decreases, so that the set dimensions after hardening can be adhered to more precisely and reproducibly.

10 In the first stage of the process, a first

Specified diameter, the deviation from the nominal size of the workpiece is significantly larger than is the case with comparable diameters, as used in the one-step hardening process according to the prior art

15 arrive. The application of much larger ones

Hardness mandrel dimensions are only possible in connection with the variably controllable duration of the first period. The effectiveness of the first diameter is precisely limited in time and to the shrinkage profile of the one to be hardened

20 workpiece matched.

It is particularly advantageous that by changing the dwell time of the workpiece on the first diameter, deviations in the pre-dimension and in the hardness result 25 can be corrected very precisely with practically negligible effort. Such corrections are easily possible in the course of ongoing production.

The further method 30 proposed according to the invention is used when the components to be hardened can harden freely in the second method stage with a satisfactory hardening result.

The methods proposed according to the invention allow both the use of mandrel fixings and spreading tools. According to a further essential process feature, the first time period begins with the workpiece resting on the first diameter of the mechanical aid (mandrel fixture or spreading tool). After a predetermined dwell time of the workpiece on the first diameter, the workpiece is transferred to the second diameter or removed from the mechanical aid. The first period ends at this point.

A device for performing the method according to the invention is characterized in an advantageous manner in that the mechanical auxiliary means a hardening tool, for. B. a mandrel fixture or a spreading tool, the largest effective diameter of which deviates from the nominal size of the workpiece so that the workpiece is predominantly plastically deformed in a first time period, with a measuring probe being integrated in the hardening tool, which provides an initial signal for the first time period, as soon as the workpiece with the largest effective diameter comes into contact during shrinking.

It is advantageous if the hardening tool has at least two effective diameters. A particularly simple hardening tool in this case is a mandrel fixture with two diameters.

Experiments have shown that the greatest deformation in the first time period can amount to approximately 0.2% to approximately 0.5% of the soft dimension of the workpiece. Within this area of an increase in diameter, it is ensured that in the first process stage, only a plastic deformation of the workpiece takes place.

When using a mandrel fixture or a spreading tool, it is advantageous to choose the effective deformation in the second time period from about 0.05% to about 0.1% of the soft dimension of the workpiece. The effective diameters can be selected automatically in such a way that the transporter is activated and / or - in the case of a spreading tool - a dimensional change is triggered on the tool after the first time period has elapsed.

To automate the process sequence with adaptation to process-specific parameters, it is proposed according to a further advantageous feature of the invention that an adjustable probe of the

Hardening tool cooperates with a sensor for the temperature of the workpiece and an adjustable timer.

The effectiveness of the process sequence can also be increased in that the timer interacts with a process controller. This process controller automatically calculates an optimal dwell time of the workpiece on the first diameter from process-specific parameters.

According to a further advantageous feature of the invention, a device which has a particularly simple structure and with which the process sequence can be automated has a fixed mandrel or a fixed die with at least two superimposed dies as a hardening tool

Diameters. Here, both the workpiece and the hardening tool are arranged to be relatively displaceable. A conveyor that interacts with gripper pawls ensures that the workpieces are placed and removed.

The invention is not limited to the combination of features of the claims. For the specialist, there are further useful combinations of claims and individual claim characteristics from the task. The schematic drawings which illustrate the invention are explained in more detail below on the basis of two exemplary embodiments:

Fig. 1 shows the course of shrinkage in a

Quenching process on a mandrel hardening tool with two diameter stages according to FIGS. 3, 4, 5, 6;

Fig. 2 shows the workpiece or pressed part strokes relative to the different dwell times on the two diameter stages;

Fig. 3, 4 show the mandrel hardening tool with two different diameter levels for two equally large, ring-shaped workpieces, which are held in a starting position and during the first quenching phase with a press pressure piece with ejector arms guided on arms;

5, 6 show the same hardening tool in the second quenching phase and after stripping the workpieces;

FIG. 7 shows the course of shrinkage corresponding to FIG. 1, but for a spreading tool with diameters that can be adjusted during quenching;

Fig. 8 shows their respective

Changes in mandrel diameter; 9, 10 show the servo-actuated spreading tool for the assembly with five ring-shaped workpieces in each case simultaneously in the first and in the last quenching phase;

11 shows a cross section through the expanding mandrel with six outer segments that can be spread apart or contracted when the inner cone is axially displaced.

In FIG. 1, the respective shrinkage eg FW or SW of the workpieces 1 is plotted on the abscissa and the residence times T are plotted on the ordinate also valid for FIG. 2. 2 the abscissa shows the relative strokes FH or SH of the workpiece grippers 3 or mandrels 5 in relation to the different mandrel diameters 6, 7 and residence times T1, T2 etc.

At FH1, the workpiece 1 removed from the furnace is, for example, 850 ° C hot and has z. B. 0.75% oversize. With FH2 the workpiece 1 is still gripped by the gripper pawls 2 (FIG. 3) and is first lowered onto the mandrel 5 while centering it relative to its upper cone area with only a minimal temperature reduction. At FH3 the immersion in oil begins and the workpiece 1 is lowered under accelerated shrinkage at FH4 to the lower, larger mandrel diameter 6 (FIG. 4). In this position, the movement of the gripper pawls 2 is interrupted and a probe 8, which detects the workpiece 1, is activated, which, when a predetermined workpiece dimension is reached at FH5, has a preprogrammable timer (not shown separately) or a process computer for a first cooling time T1 to the lower, larger diameter range 6 turns on, which ends at FH7 after the workpiece 1 during the first holding time Tl has undergone sufficient plastic deformation in order to be better in the target dimension range after the shrinkage has ended. The plastic deformation can be seen in the curve according to FIG. 1 as a short horizontal straight line between point FH6 and FH7. At point FH7 the

Holding time Tl ended at a temperature which is still suitable for plastic deformation only. The press pressure piece lowers (according to FIG. 5) towards the mandrel 5 and begins to partially push it out of the workpiece 1 seated on a collar 9 of a mandrel guide housing 10 and thereby transfer it to the immediately following, narrower second mandrel diameter step 7. Until then, the workpiece 1 has not yet jammed more firmly on the mandrel 5 and can now shrink further within the required range on a narrower diameter 7. This second cooling time T2 begins at point FW8 and can optionally be preselected via a further time switch. In special cases, further diameter levels and cooling intervals can be used. In the last cooling phase, the austenite / martensite transformation point at FH9 is exceeded, and so the resulting lower expansion can be used to reduce the elastic residual stresses on the second mandrel diameter range 7, so that after sufficient residual cooling, the workpiece 1 at point FH10 only a small amount of force and thus can be released from its clamping by means of the press pressure piece 4 without any risk of deformation or damage while the dome 5 is completely pushed out. After that it finds out

Workpiece 1, according to point FH11, has a last small contraction due to the low pre-tension still present and is then lifted out of the hardening device to point FH12 by means of the gripping pawl 2 or transported further. From the straight course of the curve at FH6 to FH7 it can be seen that only plastic deformation takes place there. The distance between the straight lines FH8 ... FH9 from the shrink curve for unclamped material that extends below and is shown in dash-dot lines shows that here only a small part of the deformation takes place elastically.

3 to 6, which represent a preferred embodiment of a hardening tool built as a fixed mandrel 5, allow the

Workpiece handling despite the two-stage

Mandrel design 6, 7 in a particularly advantageous manner, in that the workpiece 1 is placed by a gripper arm 3 with pawls 2 on the vertically displaceable fixed mandrel 5, which is opposed by the press pressure piece 4 in the opposite direction

Mandrel guide housing 10 stored return spring 11 can be pressed down, so that finally the gripper pawls 2 rest on the collar 9 and the workpiece 1 is stripped from the mandrel 5.

In Fig. 3, the press pressure piece 4 is in the uppermost position and the gripper pawls 2 with the workpiece 1 articulated thereon are still above the fixed mandrel 5 provided with a centering cone the press pressure piece 4 longitudinally displaceable within stops. The fixed mandrel 5 is raised up to the uppermost position by the return spring 11 inserted into the mandrel guide housing 10, so that both diameter regions 6 and 7 protrude from the mandrel guide housing 10 above.

In Fig. 4, the gripper arms 3 with the workpieces 1 up to the stop of the pawls 2 on the collar 9 of the

Mandrel guide housing 10 shut down. Sit there the pawls 2 deeply into corresponding recesses 12 of the collar 9 of approximately the same height so that the workpiece 1 lies against the first, larger diameter area and has become free of the gripper pawls 2. Here it is detected by a probe 8 built directly into the fixed mandrel 5, which interacts with a time control, not shown, which limits the first, precisely controlled, time and temperature stay time T1 on this diameter range 6.

In Fig. 5, the press plunger 4 has pressed the fixed mandrel 5 against the pressure of the return spring 11 by approximately the height of the workpieces 1 into the mandrel guide housing 10 and thereby moved the workpiece 1 from the first to the narrower second mandrel diameter range 7. It no longer needs to be monitored for dimensions and temperature on this and can in itself remain here until the final cooling, if an automatic of the associated handling devices is not to be set up by appropriate timers.

In Fig. 6, the press plunger 4 and also the fixed mandrel 5 reach their lowest position, so that the fixed mandrel 5 with its second diameter region 7 is pushed into the mandrel guide housing and this

Workpiece 1 has stripped. Thereafter, the same can be picked up by the pawls 2 of the gripper arm 3 without special stripping force and transported further.

A procedure analogous to the same method principle, but also other device features, is possible with an expanding mandrel 16, which under exact time and temperature control very quickly to variable mandrel diameters during quenching, for. B. is adjustable by means of hydraulic assistants so that the Most of the dimensional adjustment is achieved while setting a first larger mandrel diameter at such a high temperature that only plastic deformation takes place. The automatable adjustability of the expanding mandrel 16 offers the possibility, by means of many small control impulses on the mandrel adjustment rod 17 from workpiece 1 to workpiece 1, to select larger or smaller adaptation stages VT as required by correspondingly shorter or longer dwell times T at the respective diameters, and so on

The degree of deformation of the workpieces can be optimally adapted to their respective raw and target dimensional deviations.

FIGS. 7 and 8 show the shrinkage path curve SW and the expansion stroke SH for such an expansion mandrel hardening over time T.

At point SH1, the workpiece coming out of the furnace is, for example, 850 ° C hot and has an excess of about 0.75%. With SH2 the placing of the workpiece 1 on the expanding mandrel 16 is completed with its largest setting and with SH3 the expanding mandrel 16 is brought to the centering dimension. With SH4, the shrinkage of workpiece 1 to a first hard mandrel dimension started before (at SH5) the workpiece 1 was flooded in oil and thereby shrinked further until a sudden reduction in spreading occurred at 'SH6, which up to a state (with SH7 ) is performed, on which only slight plastic deformation is possible. Thereafter, when the expanding mandrel 16 is stable, the residual cooling commences, which is achieved with SH8 only a slight one

Preload is ended so that an easy removal from the expanding mandrel 16, as can be seen in SH9, is possible. The finished dimension achieved after the spread in SH10 has been completely eliminated is closer to tolerance than with a mandrel set for a single diameter only. Between At points SH8 and SH9, the shrink curve was drawn in dash-dot lines, which would result without a mandrel. With this mandrel, a slight plastic and a slight elastic deformation is achieved in this temperature range with further dimensional adaptation to the target dimension.

A particularly preferred device design for such very adaptable mandrel hardening is shown schematically as an example with the aid of an expanding mandrel 16 and FIGS. 9, 10 and 11 for five identical annular workpieces 1 each. In this case, a cylindrical mandrel guide housing 18 open at the top has an inner collar 19 against which somewhat larger outer collars 20 of preferably six mandrel segments 21 arranged in a circle lie from below on the inside. The mandrel segments 21 have a greater length than the height of the workpieces 1 enclosing them during hardening and together form an outer cylinder in a circular shape, the diameter of which can be expanded to a limited extent. Towards the inside, the mandrel segments 21 in their circular arrangement form one towards the top

End converging hexagonal inner cone 23 which encloses an outer cone 25 formed by a concentric inner body 24. The mandrel segments 21 are held together by a bell cover 26 which radially extends over their upper end against spreading forces.

The latter arise by the one-piece inner body 24 fitted into the inner cone 23 of the mandrel segments 21 being displaced in the longitudinal direction against the outer cone 25. The inner body 24 can be raised or lowered vertically to a limited extent by means of an adjusting rod entering the mandrel guide housing 18 from below and exiting through the upper bell hood 26; by means of a plate 28 arranged above the bottom of the mandrel guide housing 18, both vertically supporting the bell hood 26 of the outer cone 29 and a plate 28 guiding the lower ends of the mandrel segments 21 with a vertical collar. Both the collar the plate 28 and the bell hood 26 are centered on a cone collar 29 provided with the same angle as the mandrel cones 23, 25 to the mandrel segments 21. Therefore, when the plate 28 is axially displaced, the mandrel segments 21 are pressed apart or together to the same extent at the top and bottom.

9, the plate 28 is in its highest position. In Fig. 10 in a very low position, i.e. H. set with a smaller mandrel diameter. In Fig. 11 it can be seen that the cone surfaces 23, 25 are preferably designed as surfaces perpendicular to the center bisector of the segment arches in order to avoid unintentional clamping and can be provided with radial longitudinal guide sliding blocks 30 between the cones 23 and 25.

Analogously, the same process and device features can be realized for the person skilled in the art instead of using a mandrel or a die or a die according to the same criteria, if required.

reference numeral

FW shrink path at fixed mandrel

FH stroke of press plunger or mandrel

SW shrink path with expanding mandrel

SH stroke with expanding mandrel

Tl preselectable holding time on the first diameter range 6

T2 preselectable holding time on the second diameter range 7

VT variable holding times for the expanding mandrel

Claims

Expectations

1. Method for influencing dimensions and

5 changes in shape during the hardening of rotationally symmetrical workpieces (1) which shrink in a quenching medium onto mechanical aids (5, 16) with a predetermined diameter with plastic and elastic deformation, characterized in that ° - that the quenching of the workpiece (1) in the quenching medium in At least two time periods (T1, T2) take place, specifically in a first time period (T1), the duration of which can be variably controlled, at a temperature of the workpiece (1) and a first diameter (6) of the mechanical aid, which are dimensioned in this way that the workpiece (1) shrinks under predominantly plastic deformation, in at least a subsequent subsequent 0 time period (T2) at a temperature of the workpiece still above the martensite point and a small second diameter (7), compared to the first diameter (6 ), so that the shrinking of the workpiece (1) at a time compared to the first 5 nitt (Tl) comparatively small proportionate 'plastic deformation takes place.

2. Method for influencing dimensional and shape changes during the hardening of rotationally symmetrical workpieces (1) which shrink on mechanical aids (5, 16) with a predetermined diameter in a quenching medium with plastic and elastic deformation, characterized in that the quenching of the workpiece (1) in the quenching medium takes place in at least two time periods (Tl, T2), namely 5 - in a first time period (Tl), the duration of which can be variably controlled, at a temperature of the workpiece (1) and a first diameter (6) of the mechanical aid, which are dimensioned such that the workpiece shrinks under predominantly plastic deformation ^* 0, in at least one subsequent subsequent time period (T2) in which the workpiece (1) is still above the initial temperature Martensite point at a comparatively small proportionate plastic deformation compared to the first time period (Tl) 5 cures freely.

3. The method according to claim 1 or 2, characterized in that the duration of the first 0 period (Tl) is variably controllable.

4. The method according to claims 1 and 3 or 2 and 3, characterized in that the first period (Tl) with the abutment of the workpiece (1) on the first 5 diameter (6) of the mechanical means (5, 16) and after expiry of a predetermined residence time of the workpiece on the first diameter (6) ends by transferring the workpiece (1) to the second diameter (7) or by removing the workpiece (1) from the first diameter 0.

5. Device for influencing dimensional and shape changes during the hardening of rotationally symmetrical workpieces (1) which in a quenching medium to 5 mechanical aids (5, 16) with a predetermined Shrinking the diameter, characterized in that the mechanical aid is a hardening tool (5 or 16), the largest effective diameter (6) of which deviates from the nominal size of the workpiece (1) in such a way that the workpiece predominantly plastically deforms in a first time period (Tl) and that a probe (8) is integrated in the hardening tool (5 or 16), which provides an initial signal for the first time period (Tl) when the workpiece comes into contact with the largest effective diameter (6).

6. The device according to claim 5, characterized in that the hardening tool (5 or 16) has at least two effective diameters (6, 7).

7. The device according to claim 6, characterized in that the greatest effective deformation in the first period (Tl) can be about 0.2% to about 0.5% of the soft dimension of the workpiece (1).

8. The device according to claim 5, characterized in that the effective deformation in the second section (T2) can be approximately 0.05% to approximately 0.1% of the soft dimension of the workpiece (1).

9. The device according to claim 5, with a transporter (4) for placing and stripping the workpiece (1), thereby g e k e n n z e i c h n e t that after the end of the first period (Tl) of

Carrier (4) activated and / or a change in dimension on the tool (5 or 16) is triggered in the same quenching medium.

10. The device according to claim 5, characterized g e k e n n z e i c h n e t that the in the wall of the hardening tool (5 or 16) adjustable probe (8) cooperates with a thermal sensor for the actual temperature of the workpiece (1) and a preset timer.

11. The device according to claim 5, characterized in that - the next hardness level (T2) on an immediately following second smaller diameter range (7) in the same oil bath, provided that the switching point of the probe (8) before reaching a desired dimension in the cooling state corresponding shrinkage is exceeded and that the beginning of the second hardness stage (T2) by the transporter (4) begins at a temperature that still allows plastic deformation and ends when a temperature that no longer allows plastic deformation is ended.

12. The device according to claims 5 and 10, characterized in that the timer cooperates with a process controller, the actual length of stay for actual dimensions or actual voltages or actual temperatures of the workpiece (1) in the sense of a minimum deformation in the direction of the target size the first hardening phase (Tl) is calculated and set.

13. Device according to claims 5 and 9, characterized in that the hardening tool is a fixed mandrel (5) or a fixed die with at least two directly overlying diameter regions (6, 7) and that both the workpiece (1) on the hardening tool (5 or 16) and this itself is relatively displaceable relative to a tool guide housing (10) and - a cooperating with gripper pawls (2)

Transporter (4) (e.g. a press pressure piece) is relatively displaceable both with respect to the workpiece (1) and with respect to the tool (5).

14. The apparatus of claim 13, characterized g e k e n n z e i c h n e t that several between the workpiece (1) and tool (5) embedded in the wall and interacting with the process controller probe (8) are present in the region of the first diameter range (6).

15. The device according to claims 5 and 6, characterized in that the first diameter area (6) on an expanded, power-operated expanding tool (16) and further diameter areas (7) by a step by step

Process controller controlled contraction of the expanding tool (16) can be set up.

16. The apparatus according to claim 15, characterized in that the expanding tool (16) can accommodate several identical workpieces (1) one above the other on a cylindrical outer contour, which are formed by axially displaceable identical mandrel segments (21) with a conical contour on an inner cone (23) , of which at least one mandrel segment (21) has an integrated measuring probe (8).