Method of treating the surfaces of mould parts for casting moulds consisting of a steel material

Abstract

With the method according to the invention, mould parts for casting moulds for light metal casting can be treated such that the danger of crack formation in the region of the surface sections of the mould part coming into contact with the light metal melt during casting is reduced to a minimum. This is achieved in that by means of nitriding treatment on the mould part a nitride-hardened edge layer adjoining its free surface is generated which is harder than the inner core region of the mould part and comprises a diffusion layer adjoining the core region and a compound layer located on the diffusion layer and adjoining the free surface of the mould part and in that at least one section of the surface of the mould part is mechanically processed by machine hammer peening, in the case of which a hammer tool, which, at a certain impact frequency, carries out an impact movement along a movement axis which is aligned in relation to the free surface at a certain acute angle, is guided continuously over the free surface of the mould part following a track determined in a preceding design step such that the compound layer is removed by the impacting stress in the impact region of the hammer tool.

Claims

1. A of treating the surfaces of mould parts consisting of a steel material for casting moulds which are provided for the casting-related manufacture of cast parts from a light metal melt, comprising the following work steps: a) Nitrate treating the mould part to generate a nitride-hardened edge layer adjoining the free surface of the mould part which has a greater hardness than the inner core region of the mould part and comprises a diffusion layer adjoining the core region and a compound layer located on the diffusion layer and adjoining the free surface of the mould part; b) mechanical processing of at least one section of the free surface of the mould part, wherein the mechanical processing is carried out by machine surface hammering, in the case of which a hammer tool, which performs an impact movement along a movement axis at a certain impact frequency, said movement axis being aligned in relation to the free surface at a determined acute angle, is guided continually over the free surface of the mould part following a track determined in a preceding design step such that the compound layer is removed by the impact stress in the impact region of the hammer tool.

2. The method according to claim 1, characterised in that the nitride treatment is carried out as gas nitriding, in the case of which the mould part is held under a nitrogen-containing atmosphere, as bath nitriding or as plasma nitriding.

3. The method according to claim 1, characterised in that the hammer tool has a spherically vaulted impact surface, with which the hammer tool impacts the mould part.

4. The method according to claim 3, characterised in that the hammer tool consists of a carbide material at least on its section impacting the surface to be processed.

5. The method according to claim 4, characterised in that the hammer tool has a hardness of at least 2000 HV on its section consisting of carbide material.

6. The method according to claim 1, characterised in that the movement axis of the hammer tool forms an angle of 15-50° with a surface normal established on the respectively processed section of the free surface of the mould part.

7. The method according to claim 6, characterised in that the angle formed is 20-45°.

8. The method according to claim 7, characterised in that the angle formed is 30°±7°.

9. The method according to claim 1, characterised in that the impact frequency of the hammer tool is 20-500 Hz.

10. The method according to claim 1, characterised in that the stroke of the impact movement performed by the hammer tool is up to 2 mm.

11. The method according to claim 1, characterised in that in work step b) at least 90% of the compound layer present after work step a) on the section of the free surface of the mould part processed in work step b) is removed.

12. The method according to claim 1, characterised in that the impression distance of the impressions generated by the hammer tool on the free surface is >0 to 1 mm.

13. The method according to claim 1, characterised in that the travel speed, at which the hammer tool is moved, is 400-6,000 mm/min.

14. The method according to claim 1, characterised in that the track, which the hammer tool follows in work step b), runs in the manner of a meander pattern or in the manner of a spiral pattern.

15. The method according to claim 1, characterised in that the hammer tool is guided by a numerically-controlled adjustment device with two or more movement axes along the previously specified track.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail in the following with reference to a drawing representing an exemplary embodiment. The schematic drawings show the following:

(2) FIG. 1 is a mould part, which serves as a water jacket core for the casting-related manufacture of an engine block for a combustion engine in a perspective view.

(3) FIG. 2a-f are surface sections of the mould part according to FIG. 1, which have been subjected to processing according to the invention, in each case in a plan view;

(4) FIG. 3 is the mould part in a view from above;

(5) FIG. 4a is the mould part in the case of processing of one of its surface sections represented in FIG. 2 by a hammer tool in a view from above;

(6) FIG. 4b is the mould part in the case of processing of the surface section by the hammer tool in a frontal view on the end face of the moulded part;

(7) FIG. 4c is the mould part in the case of processing of the surface section by the hammer tool in a perspective view;

(8) FIG. 5 is a hammer tool used for processing sections of the surface of the mould part according to FIG. 1 in a longitudinal section;

(9) FIG. 6 is a section of a cut of a near-surface cross-sectional region of the mould part in the nitride-hardened state;

(10) FIG. 7a,7b are an enlarged representation of surface sections after processing by means of machine hammer peening;

(11) FIG. 8a is a scanning electron microscope image of a surface cut-out of a mould part treated according to the invention;

(12) FIG. 8b is a scanning electron microscope image of a surface cut-out of a mould part not treated according to the invention;

(13) FIG. 8c is a scanning electron microscope image of a surface cut-out of a mould part not treated after gas nitriding;

(14) FIGS. 9a-9c are schematic representations of the position and alignment of the tilt angle βt, of the working angle βa and of the impact angle βi; and

(15) FIG. 10 is a schematic representation of the course of the tracks P, PV in the processing of the examples.

DESCRIPTION OF THE INVENTION

(16) The mould part W (water jacket core) provided to cast an engine block for a combustion engine in a pressure casting mould has been manufactured in a conventional manner by machining from a steel block that consisted of the steel 1.2340 (“E38K”).

(17) The mould part W provided in this manner has been held in a conventionally performed gas nitriding process under a correspondingly conventionally composed NH3 containing nitriding atmosphere.

(18) An edge layer R adjoining the surface O of the mould part W has resulted on the mould part by means of the gas nitriding. The edge layer R comprises a diffusion layer D, whose hardness is increased by nitrogen atoms diffused into this diffusion layer D compared to the hardness of the core material K of the mould part W.

(19) In addition, the edge layer R comprises a compound layer V, which is present after gas nitriding on the surface O of the mould part W and covers the diffusion layer D of the edge layer R (FIG. 6). In FIG. 8c, this compound layer V is discernible as the white coating (“white layer”) completely covering the surface section shown there.

(20) After the nitriding treatment, the mould part W was placed into a conventional pressure casting mould in a test operation. Under the standard operational conditions, engine blocks were then cast using said mould part in the pressure casting mould. In this case, the mould part W regularly underwent temperature fluctuations of 111-377° C. The cycle time was 100 s.

(21) After a usage duration of 10,000 shots, i.e. 10,000 cast parts, the free surfaces O of the mould part W were examined for crack formation on their inner and outer side. The sections 1-6 of the surfaces O have been found to be critical, with both the surface sections on the inner side “internal” and on the outer side “external” being affected in these sections (see FIGS. 1 and 3).

(22) In order to avoid such crack formations, the sections 1-6 internal and 1-6 external have been processed with a hammer tool S in the case of a further embodiment of the mould part W following an edge layer hardening also carried out in the above-described manner. A standard hammer tool S was also used, as has been used already in the case of the methods which are described in the above-mentioned articles. An example of a specific embodiment for such a tool is represented in WO 2007/016919 A1. Therefore, only the elements of this tool S required for the understanding of the invention are explained here.

(23) The hammer tool S accordingly had a plunger 10, which has, at its free end, a spherical insert 11 made of carbide material as the hammer head, which has a spherically bulged impact surface 12, with which the hammer tool S impacts on its impact surface in the surface section 1-6 processed in each case during the processing. The plunger 10 is mounted in axial slide bearings 13, 14 such that it carries out a linear movement along a movement axis X aligned coaxially thereto.

(24) For the purposes of processing taking place on the inner (“internal”) and on the outer (“external”) side of the surface sections 1-6, the hammer tool S has been aligned in relation to the respective surface section 1-6 such that the movement axis X of its impact movement formed angles (tilt angle βt, working angle βa, impact angle βi) with the surface normal FN on the respective section 1-6 “internal”/“external”, which were in the range of 25° to 45° in each case. (FIGS. 4a-4c)

(25) The parameters of the MHP processes carried out on the individual surface sections 1-6 are indicated in Table 1.

(26) The other essential operating parameters of the hammer tool S are listed in Table 2, with the time required for the processing of the respective surface section 1-6 being designated with the cycle-time. The course of the tracks P, PV are meander shaped as shown in FIG. 10.

(27) In the case of the processing, the hammer tool S was guided in a meander-shaped track P uninterrupted over the respectively processed section 1-6 of the free surface O of the mould part W.

(28) FIG. 7a shows a representative enlarged cut-out of one of the surface sections 2 internal, 3 external and 4 external, in the case of which the movement axis X of the tool S was aligned at an angle of 30° to the surface normal FN on the respective surface section. The meander sections of the track P travelled by the tool S overlapping one another over a respectively 0.1 mm-wide overlapping region are clearly discernible just like the fact that the white compound layer V in the dark looking region processed according to the invention has been largely completely i.e. up to at least 95% removed by the machine hammer peening using the hammer tool S.

(29) For comparison, the hammer tool S has been aligned perpendicular (angles βt, βa, βi=0) to the respectively processed surface section on the surface sections 2 external and 5 external of the cooling jacket W such that the movement axis X coincides with the surface normal FN. The parameters of the comparative processing are otherwise consistent with the parameters indicated in Table 2.

(30) It is discernible from FIG. 7b that in the case of the surface section processed in this manner, although the surface section which the hammer tool S has processed following a track PV is also visible by the dark colouring, the region processed by the tool S is still covered by the white compound layer V. Accordingly, unlike the surface section represented in FIG. 7a and processed according to the invention, there is still the danger that microcracks are formed in the nitride layer which form the start for cracks penetrating into the mould part 2.

(31) FIG. 8c shows a surface section of the mould part W directly after gas nitriding. The compound layer V can be discerned as a white coating there, which covers the entire surface section.

(32) FIG. 8b shows a surface section of the mould part W after an MHP treatment, in the case of which the hammer tool S has been aligned perpendicular to the surface to be processed (β=0°). The white compound layer V has been visibly removed only incompletely there such that the darker looking surface of the diffusion layer D takes in only a smaller part of the represented surface section.

(33) FIG. 8a shows a surface section of the mould part W following an MHP treatment carried out according to the invention, in the case of which the hammer tool S has been aligned at an angle of 45°. The substantially completely exposed darker surface of the diffusion layer D takes in the entire surface section here.

(34) With the method according to the invention, mould parts for casting moulds for light metal casting can therefore be treated such that the danger of crack formation in the region of the surface sections of the mould part coming into contact with the light metal melt during casting is reduced to a minimum. This is achieved according to the invention in that in a work step a) by means of nitriding treatment on the mould part a nitride-hardened edge layer adjoining its free surface is generated which is harder than the inner core region of the mould part and comprises a diffusion layer adjoining the core region and a compound layer located on the diffusion layer and adjoining the free surface of the mould part and in that in a work step b) at least one section of the surface of the mould part is mechanically processed by machine hammer peening, in the case of which a hammer tool, which, at a certain impact frequency, carries out an impact movement along a movement axis which is aligned in relation to the free surface at a certain acute angle, is guided continuously over the free surface of the mould part following a track determined in a preceding design step such that the compound layer is removed by the impacting stress in the impact region of the hammer tool.

(35) TABLE-US-00001 TABLE 1 impact cycle processed surface depth β time surface section position [mm] [°] [s] [mm.sup.2] 1 internal 0.35 25 411 804 1 external 0.35 25 438 809 2 internal 0.35 30 1158 2277 2 external 0.35 0 1104 2181 3 internal 0.35 40 356 711 3 external 0.35 30 502 547 4 internal 0.35 40 1145 2255 4 external 0.35 30 1090 2153 5 internal 0.35 40 1286 2140 5 external 0.35 0 1090 2153 6 internal 0.35 40 245 477 6 external 0.35 25 164 311

(36) TABLE-US-00002 TABLE 2 impact frequency 200 Hz stroke 0.35 mm moving mass 280 g travel speed 1,200 mm/min distance of the impressions 0.1 mm overlapping of the meander sections 0.1 mm diameter of the ball insert 11 6 mm speed at which the hammer tool is 1200 mm/min moved along the track P, PV

REFERENCE NUMERALS

(37) βa working angle β1 impact angle βt tilt angle B contact point of the hammer head 11 with the surface O, to be processed, of the mould part W D diffusion layer E_VF plane “travel direction-surface normal” E_QV normal to the surface to be processed and transverse to the plane aligned in the travel direction VR FN surface normal K core region of the mould part W O surface of the mould part W P,PV track followed by the hammer tool S during the processing R nitride-hardened edge layer S hammer tool W compound layer VR travel direction of the hammer tool S along the respective track P, PV W mould part (water jacket core) X movement axis of the hammer tool S 1-6 sections of the free surface O of the mould part W 10 plunger 11 ball-shaped insert (hammer head) 12 impact surface 13,14 slide bearing