A method for preventing the emergence of crack formation and a device for practising the method

FIELD OF THE INVENTION AND PRIOR ART

The present invention relates to a method for preventing the emergence of crack formation in the cutting surfaces of a rod- shaped object of metallic material which under the action of an impact member is subjected to high-velocity cutting. The invention also relates to a device for practising such a method.

It is previously known, for instance from WO 97/00751 A1 , to provide a striking machine utilising hydraulics in order to achieve a rapid impact effect on an object, such as a rod or wire of metallic material, for cross-cutting thereof. The impact or the thrust applied against the object in that connection has such an impulse and brings about such a transfer of energy to the object that the cutting tool used for performing said cross-cutting under the action of the impact member of the striking machine does not have to pierce the object in order to achieve the cross-cutting. The hydraulically actuated striking machine is a further development of the previously used pneumatically actuated or spring-actuated striking machines. By utilising hydraulics for controlling the striking machine, it has been possible to substantially increase the striking velocity and thereby the cutting capacity.

Striking machines of the above-mentioned type are with advan- tage used for cutting rod-shaped objects of metallic material into several shorter units. In this connection, the rod-shaped object

is fed forward a given distance between each actuation of the impact member of the striking machine. In connection with high- velocity cutting of rod-shaped object of hard metallic materials, such as for instance iron-based materials, there is a tendency to emerge crack formation in the cutting surfaces when the thickness of the rod-shaped object is relatively large, approximately in the order of 60 mm or more. Up to now, no solution to this problem of crack formation in connection with high-velocity cutting has appeared, and it has consequently been difficult or even impossible to cut thick rod-shaped objects of hard metallic materials with satisfying cutting quality by means of high-velocity cutting.

OBJECT OF THE INVENTION

The object of the present invention is to achieve a solution to the above-mentioned problem with crack formation in the cutting surfaces in connection with high-velocity cutting of thick rod- shaped objects of hard metallic materials.

SUMMARY OF THE INVENTION

According to the invention, said object is achieved by means of a method having the features defined in claim 1 and a device having the features defined in claim 5.

It has surprisingly been established that the emergence of crack formation in the cutting surfaces in connection with high-velocity cutting of a thick rod-shaped object of a hard metallic material with bcc or hep structure can be eliminated by subjecting the rod-shaped object to a relatively low temperature increase in the cutting zones. Closer studies and analysis of this surprising phenomenon have shown that the emergence of crack formation is eliminated when the rod-shaped object is preheated to such an extent that an intended cutting zone of the object, through which cutting zone a cut is intended to be executed by means of

a cutting tool under the action of an impact member, obtains a temperature which at the moment of cutting exceeds a transition temperature T ₀ defined as the temperature at which, under the prevailing striking conditions, the curve for the cleavage fracture stress σ _f of the metallic material of the rod-shaped object intersects the curve for the yield stress σ _s of this metallic material. For the majority of hard metallic materials, this transition temperature T ₀ is located between 50-500 ⁰ C at the striking conditions normally occurring in connection with high-velocity cutting.

In this description and the subsequent claims, the expression bcc structure refers to body centre cubic crystal structure (bcc = body centre cubic). In this description and the subsequent claims, the expression hep structure refers to hexagonal close- packed crystal structure (hep = hexagonal close-packed).

In this description and the subsequent claims, the expression high-velocity cutting refers to a cutting method where an object is cut under the action of an impact member, which with very high velocity transfers a powerful impact impulse to the object via a cutting tool. In connection with high-velocity cutting, the cutting tool is at the moment of cutting affected with a quantity of energy of a certain preset magnitude in order to achieve the desired cutting effect, instead of being affected with an applied force as in connection with conventional cutting. The velocity of the cutting tool at the moment of cutting is in connection with high-velocity cutting 10-200 times higher than in connection with conventional cutting. In connection with high-velocity cutting, which cutting method also is denominated HVCU (High Velocity Cutting), adiabatic softening is used. A concentrated quantity of mechanical energy during very short periods of time is supplied to the metallic material to be cut. This will cause locally high temperatures. Since the process takes place very rapidly, the thermal dissipation from the material subjected to the cutting to the surrounding elements is almost none-existent (that is the reason for the expression "adiabatic softening"). The adiabatic

softening makes it possible to obtain very even and good cutting surfaces in the object subjected to cutting. In connection with high-velocity cutting, the cutting conditions are so chosen that adiabatic softening occurs in the object in the zones where cuts are executed. The impact member hitting the cutting tool should have a velocity of between 3 and 10 m/sec at the moment of impact.

It is previously known to preheat metallic material in connection with conventional mechanical cutting in order to reduce the force required for the cross-cutting of the material and the wear on the cutting tool. However, in this case a preheating to very high temperatures in the order of 1000-2000 ⁰ C is involved in order to achieve softening of the material. In connection with high-veloc- ity cutting, it is not desirable with such a softening of the material or any other temperature-related structural change in the material, and such preheating has therefor not been utilized in connection with high-velocity cutting. The present invention is based on the surprising realization that a preheating to a tem- perature level that is essentially lower than the one applied in connection with preheating in connection with other cutting methods will have a most favourable effect on the quality of the cuts in connection with high-velocity cutting and makes possible an elimination of the problem with crack formation in connection with high-velocity cutting of thick rod-shaped objects of hard metallic materials.

Preferred embodiments of the inventive method will appear from the independent claims and the subsequent description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be more closely described by means of embodiment examples, with reference to the appended drawings. It is shown in:

Fig 1 a schematic perspective view of a device according to the present invention.

Fig 2 a schematic longitudinal cut through the device of fig 1 , and

Fig 3 a schematic diagram of how the yield stress and the cleavage fracture stress of a metallic material with bcc or hep structure vary with the temperature at a certain deformation velocity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig 1 schematically illustrates a device 1 according to the pre- sent invention for high-velocity cutting of rod-shaped objects. This device 1 comprises a cutting appliance 2, which has a housing 3 in which a cutting tool 4 is arranged. The cutting tool 4, which is indicated by broken lines in fig 1 , comprises two tool parts 4a, 4b abutting against each other, and a hole 5, which extends through the two tool parts and in which a rod-shaped object to be cut is intended to be received. Said hole 5 consequently comprises a first hole section extending through a first one 4a of said tool parts and a second hole section extending through a second one 4b of said tool parts. The housing 3 has an infeed opening 6, which via a cavity 7 is connected to the cutting tool 4. Via said infeed opening 6 and cavity 7, a rod- shaped object 8 is intended to be fed to the cutting tool 4 in the direction of the arrow P1 by means of a not shown feeding appliance.

The cutting appliance 2 further comprises an impact head 10 which is displaceably arranged in relation to the housing 3, the impact head 10 being displaceable in directions indicated by the arrow P2, i.e. upwards and downwards in figs 1 and 2. The im- pact head 10 is intended to be acted upon by an impact member 1 1 (see fig 2) of an striking appliance known per se, schemati-

cally indicated at 12 in figs 1 and 2, which preferably utilizes hydraulics in order to accelerate the impact member. Consequently, the impact member 1 1 is preferably a hydraulically actuated impact ram. The striking appliance may with advantage be of the type shown in WO 97/00751 A1. The impact member 1 1 is arranged to hit against the impact head 10 with a very high velocity. The impact force from the impact member 1 1 is transferred to the first tool part 4a via the impact head 10. This first tool part 4a is arranged to be displaceable under the action of the impact head 10 and impact member 1 1 so as to be displaced in relation to the second tool part 4b, under the impact action from the impact member 1 1 , in a direction crosswise to the centre axis of the hole 5 and thereby achieve a cross-cutting of a rod-shaped object 8 received in the hole 5. The first tool part 4a is consequently arranged in the housing 3 and displaceable in the striking direction P2 of the impact member.

The impact head 10 and the tool part 4a are with advantage two separate parts, as illustrated in figs 1 and 2. However, the im- pact head 10 and the tool part 4a could be integrated with each other if so considered suitable.

In connection with the execution of a cutting operation, the impact head 10 and the first tool part 4a are displaced in relation to the second tool part 4b under the action of the impact member 1 1 , as mentioned above. In the example shown in figs 1 and 2, this displacement takes place downwards. After a performed cutting, the impact head 10 and the first tool part 4a are to return to the starting position shown in fig 2 in order to make pos- sible a new feeding of the rod-shaped object and cutting-off of the new section thereof. For this purpose, the cutting appliance 2 comprises a returning member, a part of which being indicated at 13 in fig 2. The returning member suitably comprises a hydraulically actuated piston 14, or alternatively a spring device, which after a performed cross-cutting returns the impact head 10 and the first tool part 4a to the starting position shown in fig

2. The returning member 13 suitably also comprises a damper, not shown, for clamping the motion of the impact head 10 and the first tool part 4a when the first tool part 4a has been displaced a sufficient distance for achievement of a cross-cutting of the rod-shaped object 8. The damping is suitably achieved by means of hydraulics.

In metallic material with bcc or hep structure, the yield strength σ _s of the material depends on the deformation velocity, i.e. the elongation rate and the temperature in the material. In metal materials with bcc or hep structure, there is also a cleavage fracture stress σ _f which depends on the deformation velocity and the temperature in the material. Fig 3 illustrates in a schematic diagram how the yield strength σ _s (curve B) and the cleavage fracture stress σ _f (curve A) of a metallic material with bcc or hep structure vary with the temperature at a certain deformation velocity. In connection with high-velocity cutting, the deformation velocity of a material depends on the mass of the impact member of the striking appliance and the striking velocity of the im- pact member against the impact head. The cleavage fracture stress σ _f of the material increases slowly with increasing material temperature T, as illustrated by the curve A, whereas the yield stress σ _s of the material at the beginning goes down slowly with increasing material temperature T so as to then flatten out, as illustrated by the curve B. The curve A for the cleavage fracture stress σ _f of the material intersects the curve B for the yield stress σ _s of a material at a certain temperature T ₀ , here denominated transition temperature. At the transition temperature T ₀ , a fracture executed in the material with the deformation velocity in question changes from ductile to brittle fracture.

It has been established that there is a risk that crack formation will occur in the cutting surfaces in connection with high-velocity cutting of a rod-shaped object of metallic material with bcc or hep structure if the temperature in the cutting zones of the object, i.e. the zones through which cuts are intended to be exe-

cuted under the action of the impact member of the striking appliance, at the respective moment of cutting is lower than the transition temperature T ₀ at the prevailing striking conditions and that such crack formation is avoided if the temperature in the cutting zones of the object at the respective moment of cutting is higher than the transition temperature T ₀ in question.

In this description and the subsequent claims, the expression cutting zone refers to a zone of a rod-shaped object 8 through which a cut is intended to be executed by means of the cutting tool 4 under the action of the impact member 1 1 of the striking appliance.

In this description and the subsequent claims, the expression moment of cutting refers to the moment when the cutting tool under the action of the impact member 11 of the striking appliance hits against a rod-shaped object 8 in order to cut-off a section therefrom.

In this description and the subsequent claims, the expression striking conditions refers to the mass of the impact member and the velocity with which the impact member 1 1 hits against the impact head 10 in order to cut-off a section from a rod-shaped object.

According to the invention, the rod-shaped object 8 is preheated by means of a suitable heating appliance 20 so that an intended cutting zone of the object obtains a temperature T _κ which at the moment of cutting exceeds the transition temperature T ₀ , i.e. ex- ceeds the temperature at which, under the prevailing striking conditions, the curve A for the cleavage fracture stress σ _f of the metallic material of the rod-shaped object intersects the curve B for the yield stress σ _s of this metallic material. It is realized that no preheating is necessary in those cases when the transition temperature T ₀ is lower than the prevailing ambient temperature T _R in the room where the cutting is performed.

A certain safety margin δT is with advantage applied. In this case, the rod-shaped object is preheated so that an intended cutting zone obtains a temperature T _K which at the moment of cutting exceeds the transition temperature T ₀ with the predetermined safety margin δT, i.e. T _κ = T ₀ + δT, as illustrated in fig 3.

For the majority of hard metallic materials with bcc or hep structure, the transition temperature T ₀ is located between 50- 500 ⁰ C at the striking conditions normally occurring in connection with high-velocity cutting. The rod-shaped object is therefore suitably preheated to such an extent that the intended cutting zones will obtain a temperature T _κ which at the respective moment of cutting amounts to a value between 50-500 ⁰ C depend- ing on the material type of the rod-shaped object and the prevailing striking conditions. For iron and iron-based materials, the transition temperature T ₀ is located between 100-200 ⁰ C at the striking conditions normally occurring in connection with high- velocity cutting. In connection with high-velocity cutting of rod- shaped objects of iron or iron-based materials, the object is therefore suitably preheated to such an extent that the intended cutting zones will obtain a temperature T _κ which at the respective moment of cutting amounts to a value between 100-200 ⁰ C.

The device 1 according to the present invention comprises a heating appliance 20, by means of which an intended cutting zone of a rod-shaped object 8 can be preheated to a desired temperature before the feeding thereof to the cutting tool 4. The heating appliance 20 should be located just before the cutting tool 4, as seen in the feeding direction P1 of the object. Several different types of heating appliances are conceivable for this purpose. The heating appliance 20 is with advantage an induction heating appliance arranged to preheat a rod-shaped object 8 by means of induction heat, as illustrated in figs 1 and 2. In this case, the heating appliance 20 comprises one or several inductance coils, through which the rod-shaped object 8 that is to

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be preheated prior to the high-velocity cutting in the cutting appliance 2 is conveyed during its course up to the cutting tool.

The invention is of course not in any way restricted to the preferred embodiments described above. On the contrary, many possibilities to modifications thereof should be apparent to a person skilled in the art without thereby deviating from the basic idea of the invention as defined in the appended claims.