Meltless preparation of martensitic steel articles having thermophysically melt incompatible alloying elements

Abstract

An article of iron base metal base metal alloyed with an alloying element is prepared by mixing a chemically reducible nonmetallic base-metal precursor compound of the iron base metal and a chemically reducible nonmetallic alloying-element precursor compound of an alloying element to form a compound mixture. The alloying element is preferably thermophysically melt incompatible with the iron base metal. The method further includes chemically reducing the compound mixture to a metallic alloy, without melting the metallic alloy, and thereafter consolidating the metallic alloy to produce a martensitic-composition consolidated metallic article, without melting the metallic alloy and without melting the consolidated metallic article.

Description

[0001] This invention relates to the preparation of metallic-alloy articles, specifically martensitic iron-base alloy articles, without melting of the metallic alloy. BACKGROUND OF THE INVENTION [0002] Martensitic steels having a martensitic microstructure are one important class of the ferritic steels. The martensitic steels have wide application in the aircraft propulsion industry and are also used in other industries such as the automotive industry. Metallic articles made of martensitic steels are fabricated by any of a number of techniques, as may be appropriate for the nature of the metal and the article. In one common approach, metal-containing ores are refined to produce molten metal, which is thereafter cast. Ore refinement may take place separately for each of the major alloying elements, or in combination for more than one element. Elements and combinations of elements may take many intermediate forms before being melted to form the final alloy. The metal is refined as nec...

AI Technical Summary

Benefits of technology

[0031] Another important benefit of the present approach is improved inspectability as compared with cast-and-wrought product. The martensitic steel articles produced by the present approach are of a fine microstructural size and are free of microstructures discussed previously that inhibit inspectability. As a result, they exhibit a significantly reduced noise level during ultrasonic inspection, and permit inspection for smaller irregularities. The reduction in size of irregularities that may be detected allows larger articles to be fabricated and inspected, thus permitting more economical fabrication procedures to be adopted, and / or the detection of smaller irregularities. By reducing the noise associated with the inspection procedure, larger diameter intermediate-stage articles may be processed and inspected. Processing steps and costs are reduced, and there is greater confidence in the inspected quality of the final product. The final article that contains fewer and smaller irregularities also results in improved mechanical properties.

[0032] The present approach also offers important benefits when used to make alloys of iron-base metal with conventional alloying elements that are not thermophysically incompatible with the base metal. Conventional melting-and-casting technology of commercial-scale heats of alloys, starting from ores of the metals, inevitably results in levels of impurity elements in the alloys. In some cases, the presence of the impurity elements produces highly undesirable effects on the properties of the alloys in service. In some cases the adverse effects of minor amounts of these elements has become evident only as the applications of the alloys becomes ever-more demanding. The present approach reduces, and in some cases eliminates entirely, the presence of such minor levels of impurity elements, due to the low-impurity nature of the starting materials and the low processing temperatures that are used, which limits the migration of impurity elements into the alloy. As a result, the strength, fatigue properties, and oxidation / sulfidization / corrosion resistance of the alloys are improved, as compared with the nominally same alloys produced by conventional techniques.

[0033] The present approach thus allows the production of new alloys that cannot be made with the present melting-and-casting technology because of thermophysical incompatibility. It also allows the production of existing alloys that can be made by melting-and-casting technology, but at lower cost and better quality than possible with the existing melting-and-casting technology, and with a more-desirable phase distribution.

[0034] The present approach is advantageously applied to make martensitic steel articles. Contamination and other impurity elements that are almost unavoidable in conventional casting practice, and which may have major adverse effects on the properties of the material, may be eliminated with the present approach. The structure is more uniform and homogeneous than may be produced by conventional casting and working techniques. For the material produced by the present approach that replaces conventionally cast material, there is a reduced incidence of irregularities such as those produced by segregation and inclusions (e.g., white spots, freckles, eutectic nodules, and banding) during conventional casting operations, and those associated with remelted / recycled material. The cost is also reduced due to the elimination of processing steps associated with casting. The reduction in the cost of the final product achieved by the present approach also makes the martensitic steels more economical. Properties are also improved. Material made by the present approach that is a replacement for conventional wrought articles realizes these same benefits. Additionally, large-sized specialty articles, whose size is limited only by compaction capability, may be made while avoiding microsegregation and macrosegregation. Reduced thermomechanical work is required to produce fine microstructures, and there is reduced loading on the mechanical working equipment. More complex processing may be used, because of the initially fine microstructure.

Problems solved by technology

The melting operation, which often involves multiple melting and solidification steps, is costly and imposes some fundamental limitations on the alloy content and hence the mechanical properties of the final martensitic-composition articles.

The method circumvents problems, which cannot be avoided in melting practice or are circumvented only with great difficulty and expense.

The present approach permits a uniform alloy to be prepared without subjecting the constituents to the circumstance, which leads to the incompatibility, specifically the melting process.

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