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

A method of making a small diameter elongated steel article comprising the steps of

• melting a steel alloy having the following weight percent composition in a melting furnace:
• C
• 0.88-1.00
• Mn
• 0.20-0.80
• Si
• 0.12-0.22
• P
• 0.050
• max.
• S
• 0.010-0.100
• Cr
• 0.30-0.90
• Ni
• 0.10-0.50
• Mo
• 0.25
• max.
• Cu
• 0.08-0.23
• V
• 0.025-0.15
• N
• 0.060
• max.
• O
• 0.040
• max.
• the balance being iron and usual impurities;
• atomizing the steel alloy with an inert gas to form a prealloyed steel powder;
• consolidating the steel powder to substantially full density to form a powder compact;
• hot working the powder compact to form an elongated intermediate article;
• heat treating the intermediate article by performing the following steps:
• a) heating the intermediate article at a first temperature in the range from about 40° C. below to about 25° C. above the alloy's Acm temperature for about 45-90 minutes per inch of thickness of the intermediate article;
• b) cooling the intermediate article from the first temperature at a rate sufficient to transform the alloy to one or more of martensite, upper bainite, lower bainite, and combinations thereof in said intermediate article;
• then
• c) heating the intermediate article at a second temperature in the range from about 150° C. below the alloy's A1 temperature to the A1 temperature for a time sufficient to precipitate a plurality of fine carbides in the matrix material of the alloy;
• d) cooling the reheated intermediate article from the second temperature;
• then
• e) heating the intermediate article at a third temperature of about 10-50° C. above the alloy's A1 temperature for about 1.5-6 hours per inch of thickness;
• f) cooling the intermediate article from the third temperature at a rate of about 5-80° C./hour to an intermediate temperature of about 100-400° C. below the A1 temperature;
• and then
• g) air cooling the intermediate article from the intermediate temperature to room temperature;
• whereby the elongated steel article has a microstructure consisting essentially of:
• a) a ferritic matrix having a substantially uniform distribution of fine grains characterized by a grain size number of at least about 8 as determined in accordance with ASTM Standard Specification E 112;
• b) a plurality of fine carbides uniformly distributed throughout the ferritic matrix, said carbides being substantially spheroidal in shape and not greater than about 4 μm in major dimension;
• c) a plurality of sulfides uniformly distributed throughout the ferritic matrix, said sulfides being not greater than about 2 μm in major dimension;
• and
• d) the microstructure is substantially free of lamellar carbides and carbide networks.