DE19937785A1 - Roll for coffee grinder or similar, with basic crystal structure including pearlite and cementite phases - Google Patents

Abstract

The roll is made of hard cast iron. Its basic crystal structure includes both pearlite and cementite phases. The surface layer is subjected to plasma diffusion treatment. It may be shell cast, with or without a high alloying of chromium, or may be centrifugally cast. The surface layer may be subjected to plasmanite treatment. The roll may be treated in a nitriding furnace.

Description

The invention relates to a roller in a grinder, but in particular not exclusively for chopping seeds, coffee or cocoa beans or on their organic materials. Such rollers come especially for shredders Tasks in the food, chemical or pharmaceutical industry for use.

For crushing seeds, coffee or cocoa beans or other organic Materials have been used for many years by cast rollers, which are characterized by a special Manufacturing process outside have a layer that is significantly harder than its core is. It can be rollers that are used in the die casting process against mold are poured and which have a greatly increased surface area in an outer shock zone  to have hardness. However, these can also be centrifugal casting rolls which are in a high Speed rotating mold are made, starting with the material for a cladding layer and then the material for the core and the tenons in the mold can be filled.

Depending on the desired shredding effect, the roller surface can be moved view of the respective input product can be designed very differently. In many cases, corrugated rollers are used for shredding tasks where one End product with the narrowest possible particle size distribution is required, so that a Screening and reprocessing and the associated dust waste avoided become. The cross-section of these corrugated rollers has an approximately sawtooth-shaped profile. For the fine grinding of various products, they are also smooth, i.e. non-ripe felt rollers used.

All grinders have a long service life for the rollers worth a small amount in order to be able to produce continuously for as long as possible To have reprocessing work and as long as possible a perfect function of the Grinder without influencing the grain size spectrum, the degree of heating etc. due to roller wear. The more precise the shredding task is, the more Progressive wear and tear on the result of the grinding has a more negative effect noticeable.

Proceeding from this, the object of the invention is wear resistance of rollers in grinders.

The task is carried out by a roller in a grinder with the characteristics of the An Proverb 1 solved. Advantageous embodiments of the roller are in the subclaims Chen specified.

According to the invention, the roller is in a grinder, in particular for grinding of seeds, coffee or cocoa beans or other organic materials, from egg Made of a hard cast material, which has a basic structure with the two phases pearlite and has cementite and the surface layer of which has been subjected to plasma diffusion treatment. The surface layer is preferably plasma nitrided.

The hard cast material has a basic structure in which the two phases pearlite and ze mentit side by side, which results in a relatively large area results in medium hardness. So far, this material has not given rise to Verbes efforts. However, the invention is based on the consideration that due to the quite different hardness values in the two phases preventable wear. In a shredding process, e.g. B. the Zerklei nern of coffee beans, it is to be expected that the zones of soft Ren pearlite are removed and the hard zones of cementite remain as webs ben. If this process is sufficiently advanced, eventually  break off the hard bars. This will result in a rough surface that will Wear can increase even further.

The plasma diffusion treatment of the material now results in one Surface layer in which the different hardnesses of the different phases are largely balanced. Structural studies show that in the basic structure the hard cast material of the Geflegenteil Perlit a relatively low hardness z. B. of <400 Hk0.01 (Hk ≘ hardness Knoop, see below) and the structural component cementite one relatively high hardness z. B. has about 1500 Hk 0.01. By plasma diffusion treatment, preferably plasma nitriding, but a surface is formed on the basic structure surface layer with high hardness everywhere. B. of approximately 1150 Hk 0.01. The hardness is over the entire surface layer is essentially uniform and depends very little depends on whether the structure component pearlite before nitriding on the surface or cementite was present. As a result, the roller according to the invention is too expect that the originally soft structural components after treatment show significantly less abrasive wear and the previously very hard and brittle structural components have a significantly higher ductility after treatment exhibit. Overall, the roller thus has a very uniform surface area on a high level of hardness, so that little wear and tear a long service life is to be expected.  

Plasma diffusion treatment is a method known per se, which is primarily serves to make steels more wear-resistant. For plasma diffusion treatment ge hears plasma nitriding as well as plasma carburization (carburizing plasma carburizing) or plasma working (plasma boriding). In the plasma diffusion process the chemical composition of the surface layer is determined by single or Diffusion of one or more elements changed. This thermochemical heat Treatment can be carried out easily and safely with reasonable effort the. The substance is taken up from the plasma state. For this purpose, the differ foundation elements activated in a glow discharge, so that with relative low treatment temperatures can be worked. When plasma nitriding becomes a low pressure glow discharge within a vacuum apparatus Activation of the diffusible species (here: nitrogen) ignited. This species hit the workpiece and diffuse. Depending on the process parameters they either connect to the atoms of the workpiece and thus form a ver binding layer and / or they are deposited in a diffusion layer in the atomic lattice and thus change the surface properties of the workpiece. Since this is only done in a thin layer, there are almost no dimensional changes.

The comparative effect of plasma diffusion treatment on the heterogeneous The structure of a cast material was previously unknown.  

It is also particularly advantageous that with corrugated rollers with fine surface structures hardening without dimensional change and thus impairing the grinding parameters is possible.

The invention is explained in more detail below using an exemplary embodiment:
First, a roller made of gray cast iron (GGL), bales are poured against mold, and the cones in a sand mold if necessary. The material is so coordinated in the analysis that, taking into account the wall thicknesses in the area of the mold, a depth zone (also called "working zone") defined in terms of depth is created in pure white solidification. This white outer zone has a pearlitic basic structure (in the case of alloyed qualities, intermediate stage structure) with a high cementite content without free graphite. The structure of this hard shell casting is pearlitic in the working zone, pearlitic / sorbitic with 70% cementite and graphite-free for alloyed qualities. About 10% of the diameter of the roller, which is usually in the range of 100 to 300 mm, is taken up by the working zone. The extent of the adjoining transition zone with increasingly free graphite towards the core is determined by suitable coordination of the analysis and casting data. The core structure corresponds to a gray cast iron with a base of pearlite or, in the case of alloyed qualities, from an intermediate structure with free graphite.

For optimal heat transfer, friction rollers for the food and paint industry with a very short transition zone and core structure with coarse gra phit be made.

The rollers can also be cast exclusively as a roller body without a pin the. In the - if necessary machined inside the roller body - can then continuous axles or steering knuckles made of steel of the desired quality be shrinking.

The rollers can in particular have a corrugated structure on the surface be put.

The following mechanical properties are typically achieved:

HV ≘ hardness Vickers

The following physical properties are achieved:

Finally, the roller is plasma nitrided by being at 560 ° C for 12 hours Nitriding furnace is treated. Other parameters of plasma diffusion treatment are: Gas composition: nitrogen, technical, from the bottle; Plasma discharge chip voltage: 500 V; Vacuum: 5 mbar (500 Pa).

The plasma nitrided roller has a basic structure in which the structural component is pearlite a relatively low hardness of less than 400 Hk0.01 and the structural component Ze mentit has a high hardness of approx. 1500 Hk0.01. The surface is outside layer (connection layer) with a uniform hardness of approx. 1150 Hk0.01.  

Hk denotes hardnesses that result from a hardness measurement according to Knoop ben. In contrast to the hardness measurement according to Vickers, which uses an indenter The shape of an isosceles pyramid is used in the hardness test Knoop is a diamond pyramid with a rhombic base (Longitudinal edge angle 172 ° 30 'and transverse edge angle 130 °) used. This gives get an impression with the diagonal ratio 7: 1. The hardness measurement according to Knoop is therefore particularly suitable for low-load hardness testing, where only ge ringer test forces only very small impressions arise, their evaluation in particular can cause difficulties in the Vickers hardness test. The hardness test Knoop is therefore particularly suitable for measuring hardness on thin layers. In deviation from the stipulations of Brinell and Vickers, the knee hardness Hk as the quotient of the test force F to the area of the impression at the height of the material top area calculated. Low load hardness values are also due to the dependency of the measured values from the test force specified the test forces.

The attached pictures are micrographs of a sample of the plasma nitrided Roller, each in 414x magnification.

Figure 1 shows a metallographic cut of a sample of the corrugated roller after plasma nitriding in the area of the tooth base. In the edge area you can see the connection layer made of iron nitrides. There is soft pearlite in the dark areas and hard cementite in the light areas.

Figure 2 shows a metallographic section of the same sample through a tooth flank. One can see a microstructure distribution corresponding to that of Figure 1.

Claims (10)

1. Roller in a grinder, especially for grinding seeds, coffee or cocoa beans and other organic materials made from a cast iron is made of a basic structure with the two phases perlite and Has cementite and the surface layer is plasma diffusion treated. 2. Roller according to claim 1, which is a hard cast, high chrome alloy shells is a cast iron or a centrifugal cast roller. 3. Roller according to claim 1 or 2, in which the pearlite in the basic structure has a hardness of a maximum of 500 Hk0.01, preferably a maximum of 400 Hk0.01, and the cementite has a hardness of 1400 to 1600 Hk0.01, preferably of about 1500 Hk0.01. 4. Roller according to one of claims 1 to 3, the surface layer of which is plasma nitrided is. 5. Roller according to claim 4, the 4 to 20 hours, preferably about 12 hours 450 to 650 ° C, preferably about 560 ° C, has been treated in the nitriding. 6. Roller according to one of claims 1 to 5, wherein the surface layer is a Depth of 1 to 50 microns, preferably 5 to 20 microns, especially about 10 microns.   7. Roller according to one of claims 1 to 6, wherein the surface layer is a Hardness from 1000 to 1300 Hk0.01, preferably from about 1150 Hk0.01. 8. Roller according to one of claims 1 to 7, which is a corrugated roller. 9. Roller according to one of claims 1 to 8, the egg with a further roller nem of claims 1 to 7 or another roller a pair of rollers of the meal works. 10. pair of rollers according to claim 9, with at least one further pair of rollers, in particular according to claim 9, is arranged in the grinder.