Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
1998-04-08
2002-04-23
Sheehan, John (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
Reexamination Certificate
active
06375762
ABSTRACT:
The invention pertains to a base material for the production of blank blades, particularly for circular saws, cutoff wheels, gang saws, as well as for cutting and scraping devices, consisting of a base steel enriched with carbon starting from its surface consisting of two broad surfaces, two end face surfaces and two long edge surfaces, wherein the base steel has a basic carbon content of less than 0.3 wt % carbon.
It is conventional to use tool steels with a carbon content between 0.5-1.0 wt % or low-alloyed structural steel (as steel for tempering) in order to produce a base material for the production of blank blades, particularly for circular saws, cutoff wheels, gang saws, as well as for cutting and scraping devices. The heat treatment of these materials is then done with the objective of obtaining a homogeneous texture and a uniformly high hardness over the entire thickness range. The necessary toughness of the base materials is achieved by a controlled tempering, the latter, however, necessarily being connected to losses of hardness. Depending on the purpose of use and the specific load on the base material, for saws, for instance, hardness values between-roughly 37-50 HRC are produced.
Particularly in the hot-rolling process of a typically used tool or tempering steel and in the austenitization treatment of it for hardening, the carbon diffuses out of the boundary layer of the material. A decarbonization of the surface results, so that the decarbonized boundary layer with low hardness has to be ground away after heat treatment.
In order to improve service life, a large number of saws are hard-chrome-plated, tipped with hard metal or diamonds or stellitized. The tipping is done by soldering or sintering. These measures lead to clear improvements of service life without, however, influencing the inherent strength of the blank blades. The manufacturing costs of these saws are markedly increased by the measures for increasing service life. This necessarily leads to a reduction of the teeth or number of segments, which worsens the cutting quality and increases the noise emission.
In the corporate publication “Sie+Wir” of the Stahlwerke Südwestfalen, No. 14/1975, manufacturing processes for various types of saws are described, reference being made to the fact that there is always a demand for a sheet which is as free of strains as possible with low decarbonization values and homogeneous texture formation. The steels used must have a very fine-grained texture with good tenacity after hardening and tempering, so that the very high centrifugal and shearing forces that appear can be securely absorbed.
The typification of the saws in the aforementioned corporate publication relies on a customary division into three groups, corresponding to the material to be cut. According to the material group, different requirements are placed on the properties of the saws. These groups are:
1. saws for wood and plastic (circular wood saws, hard metal tipped circular saws, forestry and gang saws;
2. saws for metal (segmented circular saws, cutoff saws, circular hot sawing machines);
3. saws for stone (diamond-tipped circular saws, diamond-tipped slab saws).
One of the requirements of saw blades is the presence of a high bending stiffness or shape stability. To stabilize slab, band, circular, and quick-cutting saw blades as well as diamond discs, in particular to compensate for strains produced by nonuniform heating in the tool body, a known procedure consists in producing internal strains in certain zones deliberately by tensioning the blade (“Comparative studies on the tensioning of circular saw blades with machines and flattening hammers,” in the special issue of Holz als Roh- und Werkstoff, Vol. 21 (1963), pp. 135-144). Such a generation of internal strains can be accomplished in hardened steel disks or bands by cold hammering with a hammer or mechanically by rolling or pressing, but in any case, it represents an elaborate processing step in manufacturing.
The thermochemical enrichment of iron and steel materials with carbon has been known for some time, and is referred to as case-hardening; If nitrogen is introduced into the material at the same time, one speaks of carbonitriding. An overview of caburizing, with special emphasis in regard to a mathematical modeling of it, is provided, for instance, by the article “The carburizing process” in Härterei Technische Mitteilungen, Vol. 50 (1995) No. 2, pp. 86-92. The carburizing process can take place in a gaseous medium, in a salt bath or in powder and is generally performed at temperatures between 900-1000° C. As carbon donors, agents are employed here whose carbon activity must be higher than that of the iron material. The carbon emitted from the carburizing agent diffuses into the boundary layer of the workpiece to be carburized. A characteristic carbon concentration profile results, according to the selected process parameters, such as temperature and treatment time, as well as the carbon activity of the carburizing agent and the composition of the iron material. The carbon concentration declines continuously with increasing distance from the boundary, until it reaches the initial level of the material in the inside of the material. The carburizing depth A
t
is to be considered a characteristic parameter of significance for practice in this regard. The carburizing depth A
t
is defined as the vertical distance from the surface up to a boundary characterizing the thickness of the layer enriched with carbon. The carbon content at which this boundary is assumed to exist is subject to standardization (cf. DIN EN 10 052) and is generally agreed to be 0.35 wt % carbon. The carburizing depth A
t
of a material increases with increasing duration of carburizing of a workpiece, the geometry of the latter also playing a role. For convex-curved workpiece surfaces, at edges or points, therefore, a greater carburizing depth A
t
occurs, since a comparatively smaller volume-is available to the carbon diffusing in from all sides. Thereby an excess carbonization can occur, which is characterized by the separation of carbides or by an undesired residual austenite content after hardening.
A method of this class for producing highly alloyed strip steel which is used for quick-cutting and tool steel as used for, among other things, the purpose of manufacturing blades and cutters found in razor blades or metal saw blades, has become known from DE-OS 2,431,797. The high content of alloy elements and the type of alloy elements, e.g. 12-13 wt % chromium, whereby a high hot hardness can be achieved, corresponds to this purpose of the strip steel for metal saws or razor blades, classified in the second group-according to the division above. Highly alloyed steels with additional high carbon content are difficult to process using hot and cold rolling in the manufacturing process, i.e., they are at risk for cracking and fracturing. Therefore a strip material with low carbon content is first either sintered or cold-rolled and subsequently enriched with carbon, either over its entire surface or partially, in the edge area. The carbon enrichment is done over the entire cross section or thickness of the strip material. Thus a carbon concentration corresponding in its level to the carbon concentration of tool steels results with almost a constant profile over the entire thickness of the strip material, slight corresponding to the foreseen usage of the material.
From AT-PS 372,709, a cutting tool, specifically a saw, made of alloyed steel is known, which is enriched in the area of its working surfaces or teeth with 1.8-2.2 wt % carbon to a depth of 0.02-0.10 mm, the carbon content at a depth of 0.15-0.25 mm reaching the carbon content. The steel alloy consists of iron with the unavoidable impurities and contains 0.1-0.3 wt % carbon, 0.2-2.0 wt % silicon, 0.5-1.5 wt % manganese, 5.0-7.0 wt % chromium, 1.0-2.0 wt % tungsten, 1.0-2.0 wt % molybdenum, 0-2.0 wt % vanadium, 0-0.5 wt % titanium, and 0-0.5 wt % niobium. To produce the cutting tool, the workpiece blank, specific
Carl Aug. Picard GmbH & Co. KG
Kilpatrick Stockton LL
Sheehan John
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