Free-machining powder metallurgy steel articles and method of making same

摘要

A small diameter, elongated steel article, comprising fully consolidated, prealloyed metal powder is disclosed. The consolidated metal powder has a microstructure that has a substantially uniform distribution of fine grains having a grain size of not larger than about 9 when determined in accordance with ASTM Standard Specification E 112. The microstructure of the consolidated metal powder is further characterized by having a plurality of substantially spheroidal carbides uniformly distributed throughout the consolidated metal powder that are not greater than about 6 microns in major dimension and a plurality of sulfides uniformly distributed throughout the consolidated metal powder wherein the sulfides are not greater than about 2 microns in major dimension. A process for making the elongated steel article is also disclosed.

主权项

1. A method of making a small diameter elongated steel article having good machinability and dimensional stability comprising the steps of
melting a steel alloy in a melting furnace; atomizing the steel alloy with an inert gas to form a prealloyed steel powder having the following weight percent composition, aboutC0.88-0.98Mn0.40-0.55Si0.12-0.22P0.030 max. S0.010-0.075N0.060 max. Cr0.25 max.Ni0.25 max.Mo0.25 max.and the balance of the alloy is iron and usual impurities:
consolidating the steel powder to substantially full density to form a powder compact; hot working the powder compact to form an elongated intermediate article; heating the intermediate article under time and temperature conditions that provide a microstructure characterized by a substantially uniform dispersion of fine carbides and fine sulfides; cold drawing the elongated intermediate article after said heating step to reduce the cross-sectional area of said elongated intermediate article; and then straightening the reduced cross-sectional area article to provide an elongated article having a small cross section for precision machining of parts.