Maraging steel article and method of manufacture

Abstract

A fully dense, powder-metallurgy produced maraging steel alloy article of prealloyed powder for use as a tool for high temperature applications. The article in the as-produced condition having a hardness less than 40 HRC to provide machinability and thereafter the article upon maraging heat treatment having a hardness greater than 45 HRC. A method for producing this article comprises compacting prealloyed powder to produce a fully dense article having a hardness less than 40 HRC and thereafter maraging heat treating to a hardness greater than 45 HRC.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The invention relates to the manufacture of a maraging steel article with a specific composition using a powder metallurgy processing method. The steel as produced by practicing this invention, either in the AS-HIP condition or HIPed and hot worked condition, is appropriate for applications involving high temperatures or cyclic heating and cooling. The steel article of the invention has a hardness of less than 40 HRC after manufacturing and after solution heat treating, allowing the article to be machined. However, after the manufacture of the article and the subsequent maraging treatment, its hardness is greater than 45 HRC. [0003] The applications for the steel article of the invention include processing of plastics or of liquid or hot solid metals, which include but are not limited to mold dies for the casting of liquid metals, mold dies for plastics, dies for forging other metals and dies for extruding. The cycl...

AI Technical Summary

Benefits of technology

[0022] Molybdenum is a key element in the strengthening of this maraging steel, as the precipitate responsible for hardening the alloy is Fe2Mo. It is also a key element in increasing the temper resistance of the alloy. Excessive quantities of molybdenum can allow the formation of detrimental delta ferrite.

[0028] Manganese is not critical for the properties of this alloy. It can be used to form manganese sulfide and therefore the content should be increased with increasing quantities of sulfur for enhanced machinability.

Problems solved by technology

The useful life of all these tool materials is typically limited by thermal fatigue cracking.

That is, as the process proceeds, more thermal fatigue cracks initiate on the surface of the tool, and existing thermal fatigue cracks grow.

The die is replaced when the extent of thermal fatigue cracking renders the produced part as being of unacceptable quality.

Fatigue cracking is related to the strength of the material.

This softening of the material will also decrease the strength of the material, making it more susceptible to thermal fatigue cracking.

Many of the tools used in the above operations are taken out of service due to the presence of thermal fatigue cracks.

However, in the case of thermal fatigue cracking, the stresses are introduced in the tool by cyclic heating and cooling.

The thermal expansion of the tool during the heating and cooling cycle introduces stresses into the tool.

1000° F. which can limit the usefulness of the material when exposed to higher temperatures.

Some conventional maraging steels have been developed in the past with good thermal fatigue resistance and strength, but when produced by conventional methods have exhibited poor hot workability during processing from ingot stage to finished form.

This poor hot workability results in either a defective final product or an insufficient yield (less than 50%) from ingot stage to finished stage to render the product commercially viable.

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