Title of Invention

"A PROCESS FOR PREPARING AN IRON-BASED ALLOY"

Abstract

A process for preparing an iron-based alloy, preferably having at least partially a mechanically resistant coating of carbide and/or nitride and/or oxide in single or mixed form, in particular of the elements titanium and/or vanadium for producing thermally treated plastic molds wherein said process comprises mixing, in the manner as known in the art the following ingredients in weight-%: C varying in the range from 0.25 to 1.0; preferably from 0.4 to 0.8; N varying in the range from 0.10 to 0.35; preferably from 0.12 to 0.29; Cr varying the in the range from 14.0 to 25.0; preferably from 16.0 to 19.0; Mo varying in the range from 0.5 to 3.0; preferably from 0.8 to 1.5; V varying in the range from 0.04 to 0.4; preferably from 0.05 to 0.2; Si up to 1.0; Mn up to 1.6; preferably from 0.3 to 0.8; Al up to 1.0; preferably from 0.002 to 0.8; Co up to 2.8; Ni up to 3.9; preferably up to 1.5; W up to 3.0; Nb up to 0.18; and/or Ti up to 0.20; Wherein the;sum of the concentration of carbon and nitrogen varies from 0.5 to 1.2 weights%, preferably from 0.61 to 0.95, the remainder comprises iron and melt-related impurities.

Full Text

The present invention relates to a process for preparing an iron-based alloy, preferably having at least partially a mechanically resistant coating of carbide and/or nitride and/or oxide in single or mixed form, in particular of the elements titanium and/or vanadium for producing thermally treated plastic molds. This invention also related to thermally treated plastic molds produced from the alloys of the presently disclosed process. Iron-based alloys with a chromium content of more than 12% are mainly used for producing corrosion-resistant plastic molds for processing chemically-reactive molding compounds. Depending on the required or desired hardness of the material, heat-treatable Cr steel with approximately 13.0% Cr and approximately 0.2 or approximately 0.4 weight-% c, for example in accordance with DIN Material Number 1.2082 and 1*2083, are employed. These iron-based alloys essentially containing carbon and chromium are easily economically usable for less stressed molds,, but they have the disadvantage that a sufficient service life of the tool cannot be attained for highly corrosive molding compounds with wear-causing additives. Iron-based alloys for processing of plasties which are more corrosion-resistant can be obtained by increasing the chromium content to approximately 14.5 weight-% and an increase of the carbon content to approximately 0.48 veight-%, as well as the addition of approximately 0.25 weight-% of molybdenum in accordance with DIN Material Number 1.2314. In practical use, such materials are mostly sufficiently resistant to chemical reactions but have, particularly in connection with molding materials containing mineral fibers, insufficient resistance to wear. Improved use properties of plastic molds in respect to oxidation/corrosion and wear can be attained by comparably large chromium contents, large carbon contents as well as molybdenum and vanadium content of the steel used. The material NO. 1.2361 in accordance with DIN constitutes an iron-hased alloy typical for this in connection with Highly-stressed plastic tools. However, in the course of producing tools or molds froia this alloy it is possible for a material distortion or uneven dimensional changes to occur, which often requires expensive finishing work or discarding of the worked part. As one skilled in the art knows, such uneven dimensional changes are essentially caused fey a defenaation texture or a linear arrangement of the carbides, if now, as has been proposed,, the carbon content and thus the carbide portion in the matrix is reduced, the wear resistance of the materia], in particular is also reduced, because, of which the wear of the wold under large directional stress is increased and the service life reduced. A further disadvantage of a high earbon content lies in low stretching ability and a. reduced toughness of the steel. It was the object of the invention to avoid the above disadvantages and to propose a chromium-containing aartensitic iron-based alloy for thermally treated plastic molds with a high corrosion resistance, which molds can be economically produced with little dimensional changes and have improved use properties. For the attainment of this object, the use of an iron-based alloy of the composition in accordance with claim 1 is proposed by the invention for the production of thenaally treated plastic aolds of a hardnesis of at least 45 Rockwell C, preferably SO to 55 Rockwell C, and of high corrosion resistance* Accordingly, the present invention relates to a process for preparing an iron-based alloy, preferably having at least partially a mechanically resistant coating of carbide and/or nitride and/or oxide in single or mixed form, in particular of the elements titanium and/or vanatiium.for producing thermally treated plastic molds. wherein said process comprises mixing, in the manner as known in the art the following ingredients in weight-% : C varying in the range from 0.25 to 1.0; preferably from 0.4 to 0.8; N varying in the range from 0.10 to 0.35; preferably from 0.12 to 0.29; Cr varying in the range from 14.0 to 25.0; preferably from 16.0 to 19.0; Mo varying in the range from 0.5 to 3.0; preferably from 0.8 to 1.5; V varying in the range from 0.04 to 0.4; preferably from 0.05 to 0.2; Si up to 1.0; Mn up to 1.6; preferably from 0.3 to 0.8; Al up to 1.0; preferably from 0.002 to 0.8; Co up to 2.8; Ni up to 3.9; preferably up to 1.5; W up to 3.0; Nb up to 0.18; and/or Ti up to 0.20; wherein the sum of the concentration of carbon and nitrogen varies from 0.5 to 1.2 weight %, preferably from 0.61 to 0.95, the remainder comprises iron and melt-related impurities. The advantages achieved by means of the invention are essentially to be seen in that the molded part or the workpiece shows to a large extent isometric dimensional changes in the course of heat treatment. The corrosion resistance of the material is furthermore improved and its matrix has greater homogeneity. The mechanical properties and, completely surprisingly also the wear resistance of tho plastic molds made from the alloy used in accordance with tho invention, are clearly increased. The reason for this improvement in properties of the mold material is seen in that the iron-based alloy contains nitrogen, which element one the one hand is a strong austenite carrier and, on the other hand,, by means of nitride-forming elements causes the creation of inter-metallic hard phases by moanc of nitride-forming elements. In the process, the concentrations of all essential alloy elements arc synergetically matched to each other, taking into consideration the effect of nitrogen on the solidification, on the precipitation products, the conversion kinetics during heat treatment and the corrosion and cracking behavior of the iron-based alloy, so that with the use in accordance with the invention of the material for producing thermally treated plastic molds the latter have considerably improved use properties. This applies in particular to the capability to polish the plastic sold to a high gloss which is often required, among others when using the mold in the electronic industry. Not all tho reasons for this have been completely explained scientifically, however, tho following connections woro founds during solidification and deformation, as well as conventional heat treatment tho differenced in the concentration of chromium in the matrix of the mold material used in accordance with the invention are small and the carbide proportion is also low in comparison with nitrogen-free martensitic chromium steels, which causes a high corrosion resistance and obviously a particular capability for high gloss polishing. However, lower Cr contents than 14 weight-% result in a sharply increased chemical reaction, in particular by organic acids, with chromium contents above 25 weight-%, signs of embrittling of the material when used as plastic molds were observed, wherein the best long time results were noted with Cr concentrations of 16.0 to 18.0 weight-%. To aid the corrosion resistance or the; stabilization of the surface passive layer, a minimum content of 0,5 w«ight-% of molybdenum is important, but higher concentrations than 3.0 weight-% can have a ferrite-stabilizing effect, because of which heat treatment of the alloy is made more difficult. Particularly good results also in regard to the effect of molybdenum nitride (Mo2N) on the mechanical properties of the material, and in particular on the wear resistance, were found with contents in the range between 0.3 to 1.5 weight-% Mc. Vanadium has a very high affinity for carbon as well as nitrogen. The fine, dispersely distributed monocarbides (VC) or mononitrides (VN) and the mixed carbides are advantageously effective in the range between 0.04 to 0.4 weight-% of vanadium in respect to the material properties of the material in the heat-treated state, wherein particularly good hardness values and high tempering properties with good dimensional stability of the mold were achieved in the range between 0.05 to 0.2 weight-%, which is probably a result of the germ effect of the small, homogeneously distributed vanadium compounds. The summing effect of carbon and nitrogen in the iron-based alloy is of essential importance in the selected areas of concentration of the alloy metals, with a minimum concentration of either carbon and/or nitrogen from 0.25 to 0.1 weight-%, the sum of the contents must be at last 0.5 weight-% in order to cause an advantageous interaction of the alloy elements, as mentioned before, with the sum of the contents in the range between 0.5 to 1.2 weight-% c + N, it was surprisingly found that the fatigue strength in particular during changing stresses, such as occur in plastic molds because of the filling cycles, was considerably increased. Most likely this is the result of stabilising the passive layer in the atomic or microscopic range and thus the prevention of an initiation of cracks because of local material reaction. As has been found, nitrogen atoms could have an advantageous effect during changes in the corrosive stress of the material, something which will have to be investigated more closely. Furthermore, with the above sua of tho contents, a destabilisation of tho cubic body centered lattice is obviously started, so that in a simple manner under hoot treatment no areas with alpha and dolta structures remain, which prevents the tendency of tho material to corrosive stress craclting, With tha came hardness and wear resistance, a reduced carbide content is provided by alloying the chromium-containing martensitic steel with carbon and nitrogen, therein the matrix has an increased otupdinese which considerably improves the too properties of a highly stressed plastic mold. Although cum valuoc of carbon and nitrogen higher than 1.2 weight-l cauoo oxtraordinary hardness during elaborate tempering and deep chilling treatment of the mold, they suddenly increase tho danger of their breaking. Within a range between 0.61 to 0.95 weight-% of the sum of the content of carbon and nitrogen of tho iron-baaed alloy, the longest service life of heat-treated plastic molds made from this were determined at a material hardness of between 50 and 55 Rockwell C, in particular when processing strongly chemically-reactive moldling compounds with wear-causing additives. It was surprising here that the adhesion of the plastic product or the molded body to the mold was considerably less than with 10's nitrogen concentrations in the alloy, particularly with high production numbers, which made the ejection of the molded material considerably easier. The cause for the reduction of the sliding friction on the mold wall has not yet been completely clarified. Tungsten contents up to 3.0 weight-% improve hardness and wear resistance, however, higher values have a negative effect on workability and tempering behavior of the material because of the great affinity of tungsten to carbon. Niobium and/or titanium are monocarbide and mononitride formers at higher proportions, however, up to a concentration of 0.18 weight-% or 0.2 weight-% these elements arc primarily stored in mixed carbide, improve the mechanical properties of the steel and considerably reduce the danger of overheating. Higher contents can increase the brittleness of the mold, in particular with a carbon content above 0.7 weight-%. Cobalt and nickel in small amounts up 1:0 2.8 weight-* or 3.9 weight-% improve the toughness of the material, wherein nickel, an austenite-formlng elements, should not exceed a concentration value of 1.5 weight- for the sake of hardening capability. An improvement in the workability of the material can be achieved, as known per se, by adding sulfur to the alloy, wherein the most advantageous values were found in a concentration range in accordance with clain 2. As extensive work has shown, it is advantageous for further hardening or increasing the wear resistance of the surface of the plastic molds made from an iron-based alloy in accordance with the invention, if a mechanically resistant coating, preferably produced by a sans of a CVD or„ PVD process, is formed on the working surface in particular, yor further clarification, the invention will be described below by means of examples which have been compiled in a table. For this purpose eight iron-based alloys were use for plastic molds which werta designed the same way and were particularly strongly, but in the same way, stressed chemically and by wear, wherein the resulting values for the mold made from the DIN Material No. 1.2361, which is part of the prior art, were set at 100% in order to be able to show by comparison essential property values of other molds made from different materials. The respective values are rounded-off sum values, in this case the corrosion behavior, the mechanical properties, the fatigue strength, the mechanically resistant coating and the wear resistance number are better with higher resulting values, reduced dimensional stability and improved high-gloss polishing capability of the material sire indicated by reduced characteristic numbers. Chemical composition Study results (Table Removed) A Corrosion resistance B....-.Dimension changes C………….Mechanical Properties D ... Standing time" duration E Hard material inspection F Wear durability value G High gloss polishing capability (k-nuaeral) WE CLAIM: 1. A process for preparing an iron-based alloy, preferably having at least partially a mechanically resistant coating of carbide and/or nitride and/or oxide in single or mixed form, in particular of the elements titanium and/or vanadium forproducing thermally treated plastic molds. wherein said process comprises mixing, in the manner as known in the art the following ingredients in weight-% : C varying in the range from 0.25 to 1.0; preferably from 0.4 to 0.8; N varying in the range from 0.10 to 0.35; preferably from 0.12 to 0.29; Cr varying in the range from 14.0 to 25.0; preferably from 16.0 to 19.0; Mo varying in the range from 0.5 to 3.0; preferably from 0.8 to 1.5; V varying in the range from 0.04 to 0.4; preferably from 0.05 to 0.2; Si up to 1.0; Mn up to 1.6; preferably from 0.3 to 0.8; Al up to 1.0; preferably from 0.002 to 0.8; Co up to 2.8; Ni up to 3.9; preferably up to 1.5; W up to 3.0; Nbup to 0.18; and/or Ti up to 0.20; wherein the sum of the concentration of carbon and nitrogen varies from 0.5 to 1.2 weight %, preferably from 0.61 to 0.95, the remainder comprises iron and melt-related impurities. 2. A process as claimed in claim 1, wherein, in weight %, from 0.02 to 0.45 weight %, preferably 0.20 to 0.30 weight % sulfur is optionally added. 3. A process for preparing an iron-based alloy for producing thermally treated plastic molds substantially as herein described with reference to the foregoing examples.

Documents:

95-del-1996-abstract.pdf

95-del-1996-claims.pdf

95-del-1996-complete specification (granted).pdf

95-del-1996-correspondence-others.pdf

95-del-1996-correspondence-po.pdf

95-del-1996-description (complete).pdf

95-del-1996-form-1.pdf

95-del-1996-form-13.pdf

95-del-1996-form-2.pdf

95-del-1996-form-3.pdf

95-del-1996-form-4.pdf

95-del-1996-form-6.pdf

95-del-1996-pa.pdf

95-del-1996-petition-137.pdf

95-del-1996-petition-138.pdf