Free-machining powder metallurgy steel articles and method of making same
A method of making a small diameter elongated steel article such as wire or strip is disclosed. The method includes the step of melting a steel alloy having the following weight percent composition C 0.88-1.00 Mn 0.20-0.80 Si 0.50 max. P 0.050 max. S 0.010-0.100 Cr 0.15-0.90 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. and the balance is iron and usual impurities. The method includes melting the alloy, atomizing the molten alloy to make a pre-alloyed metal powder, consolidating the metal powder to substantially full density, and then hot working the consolidated metal powder to form an intermediate elongated article. The method further includes a multi-step heat treating process. A small diameter, elongated steel article having enhanced machinability is also disclosed.
1. 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 A cm 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 A 1 temperature to the A 1 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 A 1 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 A 1 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.
2. The method as claimed in claim 1 comprising the step of cold drawing the elongated intermediate article after said heat treating step to reduce the cross-sectional area of said elongated intermediate article to provide an elongated article having a small cross section for precision machining of parts.
3. The method as claimed in claim 1 wherein the hot working step comprises hot rolling the intermediate article to reduce the cross-sectional area of the intermediate article before said heat treating step.
4. The method as claimed in claim 1 wherein in the step of atomizing the steel alloy comprises the step of atomizing the alloy with nitrogen gas.
5. The method as claimed in claim 1 wherein the step of consolidating the steel powder comprises hot isostatic pressing of the steel powder.
6. The method as claimed in claim 1 wherein the steel alloy has the following weight percent composition:
C
0.92-0.98
Mn
0.20-0.80
Si
0.12-0.22
P
0.030
max.
S
0.010-0.090
Cr
0.30-0.60
Ni
0.10-0.25
Mo
0.25
max.
Cu
0.10-0.23
V
0.035-0.060
N
0.060
max.
O
0.040
max.
and the balance is iron and usual impurities.
7. A small diameter elongated steel article, consisting essentially of fully consolidated, prealloyed metal powder formed from 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;
wherein the consolidated metal powder 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.
8. The steel article as claimed in claim 7 wherein the article comprises wire having a diameter of up to 15 mm.
9. The steel article as claimed in claim 8 wherein the article comprises wire having a diameter of up to 6.5 mm.
10. The steel article as claimed in claim 7 wherein the steel alloy has the following weight percent composition:
C
0.92-0.98
Mn
0.20-0.80
Si
0.12-0.22
P
0.030
max.
S
0.010-0.090
Cr
0.30-0.60
Ni
0.10-0.25
Mo
0.25
max.
Cu
0.10-0.23
V
0.035-0.060
N
0.060
max.
O
0.040
max.
and the balance is iron and usual impurities.
11. The steel article as claimed in claim 7 wherein vanadium is about 0.025% to about 0.060%.