EP0721995A2 - Use of an iron based alloy for plastic molds - Google Patents

Abstract

Iron-based alloy contains (in wt.%): 0.25-1, pref. 0.4-0.8, C; max. 1 Si; max. 1.6, pref. 0.3-0.8, Mn; 0.1-0.35, pref. 0.12-0.29, N; max. 1, pref. 0.002-0.8, Al; max. 2.8 Co, 14-25, pref. 16-19, Cr; 0.5-3, pref. 0.8-1.5, Mo; max. 3.9, pref. max 1.5 Ni; 0.04-0.4, pref. 0.05-0.2, V; max. 3 W; max. 0.18 Nb; max. 0.2 Ti; and balance Fe plus melting impurities. The sum of C+N is 0.5-1.2, pref. 0.61-0.95. The alloy may contain 0.02-0.45, pref. 0.2-0.3 S. The alloy may be provided with a surface coating of carbide and/or nitride and/or oxide of Ti and/or V and/or Al.

Description

The invention relates to the use of a chromium-containing, martensitic iron-based alloy for plastic molds.

Iron-based alloys with a chromium content of more than 12% are mainly used for the production of corrosion-resistant plastic molds for processing chemically attacking molding compounds. Depending on the required or desired material hardness, temperable Cr steels with approx. 13.0% Cr and approx. 0.2 or approx. 0.4% by weight C are used, for example according to DIN material numbers 1.2082 and 1.2083. These iron-base alloys, which essentially contain carbon and chromium, can be used economically for less stressed molds, but have the disadvantage that insufficient tool life is achieved for highly corrosive molding compounds and plastics with wearing additives.

By increasing the chromium content to approx. 14.5% by weight and increasing the carbon content to approx. 0.48% by weight as well as adding approx. 0.25% by weight of molybdenum in accordance with DIN material number 1.2314 better corrosion-resistant iron-based alloys can be obtained for plastics processing. Such materials are usually sufficiently resistant to chemical attack in practical use, but, particularly in the case of molding compositions containing mineral fibers, do not have sufficient resistance to wear.

Improved usage properties of plastic molds with regard to oxidation / corrosion and wear can be achieved through comparatively high chrome contents, high carbon contents as well as molybdenum and vanadium contents of the steel used. A typical iron-based alloy for highly stressed plastic tools is material no. 1.2361 according to DIN. However, when tools or molds are made from this alloy, a material distortion or an uneven dimensional change can occur, which often involves expensive reworking or the removal of the require processed part. Such a non-uniform dimensional change, as is known to the person skilled in the art, is essentially brought about by a deformation texture or a line arrangement of the carbides. If, as has been proposed, the carbon content and thus the carbide content in the matrix is reduced, the wear resistance of the material also decreases, which increases the removal of the mold under high friction stress and reduces the service life. Another disadvantage of a high carbon content is the low elongation and the toughness of the steel.

The object of the invention was to avoid the above disadvantages and to propose a chromium-containing, martensitic iron-based alloy for thermally tempered plastic molds with high corrosion resistance, which molds can be produced economically with little dimensional change and have improved usage properties.

To achieve this object, the use of an iron-based alloy with the composition according to claim 1 for the production of thermally tempered plastic molds with a hardness of at least 45 HRC, preferably from 50 to 55 HRC, and with high corrosion resistance is proposed.

The advantages achieved by the invention are essentially to be seen in the fact that the molded part or the workpiece shows largely isometric dimensional changes during a heat treatment. Furthermore, the corrosion resistance of the material is improved and its matrix is more homogeneous. Both the mechanical properties and, surprisingly, the wear resistance of the plastic molds made from the alloy used according to the invention are significantly increased. The reason for this improvement in the properties of the mold material is seen in the fact that the iron-based alloy contains nitrogen, which element is on the one hand a strong austenite former and on the other hand causes nitride-forming elements to produce intermetallic hard phases. The concentrations of all essential alloy elements are synergistic with one another, taking into account the effect of nitrogen the solidification, on the precipitations, on the conversion kinetics in a heat treatment and on the corrosion and cracking behavior of the iron-based alloy, so that when the material is used according to the invention for the production of thermally tempered plastic molds, these have significantly improved performance properties. This applies in particular to the high-gloss polishability of the plastic mold, which is often necessary, inter alia when the mold is used in the electronics industry. All the reasons for this have not yet been fully clarified scientifically, but the following relationships have been found: during solidification and deformation and a conventional heat treatment, the concentration differences of chromium in the matrix of the molding material used according to the invention are small and the carbide content is also low compared to nitrogen-free martensitic chromium steels low, which results in a high corrosion resistance and obviously a particularly good high-gloss polishability. However, Cr contents lower than 14% by weight lead to a sudden increase in chemical attack, in particular due to organic acids. At chromium contents above 25% by weight, embrittlement phenomena of the material were observed when used for plastic molds, the best long-term results being found at Cr concentrations of 16.0 to 18.0% by weight.

A minimum content of 0.5% by weight of molybdenum is important to support the corrosion resistance or the stabilization of the surface passive layer, but contents higher than 3.0% by weight can have a ferrite-stabilizing effect, making it difficult to harden the alloy . Particularly good results, also with regard to the effect of molybdenum nitride (Mo ₂ N) on the mechanical material properties, but in particular on the wear resistance, were found at contents in the range from 0.3 to 1.5% by weight of Mo.

Vanadium has a very high affinity for both carbon and nitrogen. The fine dispersed monocarbides (VC) or the mononitrides (VN) and the mixed carbides are advantageously effective in the range from 0.04 to 0.4% by weight of vanadium with regard to the material properties of the material in the tempered state, whereby particularly good hardness values and high tempering resistance with good dimensional stability of the shape were achieved in the range between 0.05 and 0.2% by weight of V, which is presumably due to the germinating effect of the small, homogeneously distributed vanadium compounds.

The total effect of carbon and nitrogen in the iron-based alloy is essential in the selected concentration ranges of the alloy metals. At minimum concentrations of either carbon and / or nitrogen of 0.25 or 0.1% by weight, the sum of the contents must be at least 0.5% by weight in order to bring about an advantageous interaction of the alloying elements, as previously mentioned . With a total content in the range from 0.5 to 1.2% by weight of C + N, it has surprisingly been found that the fatigue strength in particular in the case of alternating stresses such as occurs in the case of plastic forms due to the filling cycles is significantly increased. This is probably due to the stabilization of the passive layer in the atomic or micro range caused by nitrogen and thus a avoidance of crack initiation by local material attack. Nitrogen atoms, which will be examined in more detail, could have a beneficial effect on the material's alternating corrosion stress, as was found. Furthermore, with the above minimum total content, the cubic body-centered lattice obviously begins to be destabilized, so that there are no remaining areas with alpha and delta structure in the coating, which eliminates the tendency of the material to crack corrosion. With the same hardness and wear resistance, alloying the chromium-containing martensitic steel with carbon and nitrogen results in a lower carbide content, the matrix having increased strength, which significantly improves the performance properties of a highly stressed plastic mold. Total values of carbon and nitrogen higher than 1.2% by weight bring about an extraordinarily great hardness in the case of complex tempering and deep-freezing treatments of the mold, but they also suddenly increase the risk of breakage.
In a range from 0.61 to 0.95% by weight of the total content of carbon and nitrogen of the iron-based alloy, the longest service lives were achieved with thermally tempered plastic molds with a material hardness of 50 to 55 HRC, especially when processing strongly chemically aggressive ones Pressing compounds and plastics with wearing additives determined. It was surprising that the adhesion of the plastic product or compact in the mold, in particular in the case of high production numbers, was significantly lower than at low nitrogen concentrations in the alloy, which made it easier to eject the compact. The cause of a reduction in sliding friction on the mold wall has not yet been fully clarified.

Tungsten contents of up to 3.0% by weight improve the hardness and wear resistance, but higher values have a disadvantageous effect on the machinability and the annealing behavior of the material due to the high carbon affinity of the tungsten.

Niobium and / or titanium are monocarbide and mononitride formers in higher proportions; up to a concentration of 0.18% by weight or 0.2% by weight, however, these elements are mainly stored in mixed carbide, improve the mechanical properties of the steel and significantly reduce the risk of overheating. Higher levels can increase the brittleness of the molds, particularly when the carbon content exceeds 0.7% by weight.

Cobalt and nickel improve the material toughness in low contents of up to 2.8% by weight and 3.9% by weight, respectively, with nickel, an austenite-forming element, preferably not exceeding a concentration value of 1.5% by weight because of the hardenability should.

As is known per se, an improvement in the machinability of the material can be achieved by alloying with sulfur, the most favorable values being found in a concentration range according to claim 2.

Extensive work has shown that, for further hardening or increasing the wear resistance of the surface of the plastic molds made from an iron-based alloy used according to the invention, it is advantageous if, in particular, a hard material layer, preferably produced by a CVD or PVD process, is formed on the work surface.

For the purpose of further clarification, the invention is illustrated by means of examples which are described in are summarized in a table, described below. Eight iron-based alloys were used for plastic molds of the same design, which are particularly high, but of the same chemical and wear, whereby the result values of the mold from the DIN material no To be able to clearly display shapes made of different materials. The respective values are rounded total values. The corrosion behavior, the mechanical properties, the fatigue strength, the hard material coating and the number of wear and tear are better with higher result values, less dimensional stability and better polishability of the material are indicated by lower identification numbers.

Claims (3)

Use of an iron-based alloy consisting of in% by weight C 0.25 to 1.0, preferably 0.4 to 0.8 Si to 1.0 Mn to 1.6, preferably 0.3 to 0.8 N 0.10 to 0.35, preferably 0.12 to 0.29 Al to 1.0, preferably 0.002 to 0.8 Co to 2.8 Cr 14.0 to 25.0, preferably 16.0 to 19.0 Mo 0.5 to 3.0, preferably 0.8 to 1.5 Ni to 3.9, preferably to 1.5 V 0.04 to 0.4, preferably 0.05 to 0.2 W to 3.0 Nb to 0.18 Ti to 0.20 with the proviso that the sum of the concentration of carbon and nitrogen has a value A of in% by weight
gives at least 0.5 and at most 1.2, preferably at least 0.61 and at most 0.95, the rest iron and melting-related impurities for the production of thermally tempered plastic molds with a hardness of at least 45 HRC, preferably from 50 to 55 HRC and with high Corrosion resistance and / or high gloss polishability. Use of an iron-based alloy according to claim 1, which contains in% by weight 0.02 to 0.45, preferably 0.20 to 0.30 sulfur, according to the purpose according to claim 1. Use of an iron-based alloy according to one of claims 1 or 2 with a surface, in particular a work surface, on which at least partially a hard material layer, preferably made of carbide and / or nitride and / or oxide in individual or mixed forms, in particular the elements titanium and / or vanadium and / or aluminum.

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