FREE-MACHINING POWDER METALLURGY

LEAD-FREE STEEL ARTICLES AND METHOD OF MAKING SAME

FIELD OF THE INVENTION

This invention relates generally to lead-free, low sulfur free-machining articles made from powder metallurgy, having MnS micro-inclusion sizes geometrically compatible with the ISO Tolerance grade IT, and the surface condition to satisfy, and in particular to small diameter wires and bars, and to a process for making same.

DESCRIPTION OF RELATED STATE OF THE ART

High precision parts, such as for watches, instruments, or components for the automotive industry, are produced by machining small diameter, < 7.5 mm (<0.3 in), wire coils and straightened bars that are made from larger diameter, cold drawn round wire coils. They are formed from cast-and-wrought steel alloy. To attain good machinability, the coils and barstock are produced from an alloy that contains one or more free-machining additives such as lead (Pb) and/or sulfur (S). The S and/or Pb additions result in the formation of manganese sulfide (MnS) and/or Pb inclusions respectively, during the solidification of the cast alloy. But, the presence of these inclusions decreases the hot and cold workability of the products and also promotes surface defects on the machined parts.

Historically, lead has been an element reported beneficial to good machinability in the steels used to make said precision parts. However, there are many disadvantages associated with the production and use of heavily resulfurized steels, above 0.09%S, or resulfurized Pb-alloyed steels. In both cases, it has proven difficult to control the: a. fineness and even distribution (dispersion) of the MnS, and the associated Pb inclusions in the alloy matrix; b. segregation and coalescence of the MnS and lead inclusions during solidification; c. reproducibility of the process from ingot to ingot, heat to heat, and along the length of billet-strands, produced by continuous casting.

In alloys that contain both Pb and S additions, the Pb and MnS do not interact chemically with each other, although Pb often becomes physically attached to the MnS inclusions, partially coating them. Because Pb does not dissolve into or bind with iron, or other microstructural components, it is present as essentially pure standalone inclusions only. The use of lead also presents significant health and safety risks that must be addressed during mill processing operations as well as during dry

machining operations. The high vapor pressure of Pb (i.e., up to about 700 mbar at the temperature which might be reached during dry machining) causes Pb to become a major environmental and health problem in dry machining of alloy that contain that element. Furthermore, the Pb inclusions have a strong tendency to contaminate and smear all mechanically machined, polished and/or burnished part surfaces. In steels containing sulfur (S) additions, S is substantially present in the form of manganese sulfide (MnS) inclusions. Since MnS melts below the solidification temperature range of these steels, it is present as intergranular (interdendritic) inclusions only. Their sizes and distributions depend exclusively on the solidification rate of the cast ingots or strands. Consequently, it is not possible to avoid the segregation and coalescence of MnS in the as-cast structure and the formation of a wide size spectrum of MnS inclusions, ranging from fine sized inclusions to very large ones (e.g., up to about 125 μm (0.005 in) or more in length), after hot deformation of the alloy. This inclusion morphology leads to the presence of inclusions-stringers in the free-machining cast-and-wrought steels, and/or of centerline segregations in resulfurized continuously cast steels. The tolerances and surface roughness required for high precision machining coils and bars, are very tight, <IT 4-5 (i.e., <1-2 μm), and Ra <0.10 μm, where IT is the ISO Tolerance grade according to the DIN ISO 286 Standard, and Ra the surface roughness average. Further, stand-alone MnS inclusions and the associated (MnS+Pb) inclusions, form surface discontinuities at their sites of emergence on the surface of these free-machining (free-cutting) steels, thereby resulting in surface defects that are undesirable.

The morphologies of the MnS and Pb inclusions of these said steels, also limit the economically achievable precision of the cold drawn wires and straightened bars to a finished ISO Tolerance grade IT ≥6 and a surface roughness Ra ≥0.40. Further, the precision, including ovality or out-of-roundness, of the cold drawn wires and cold drawn straightened bars, relates directly to the dynamic stability and stiffness or rigidity of the machining process. Which controls the achievable precision and surface quality of the machined parts.

Lead is a very soft metal melting at 327°C (622°F). Therefore, the Pb inclusions emerging on the surface can easily smear all mechanically worked surfaces (machined, shaped, polished, and burnished). High C-steels are always heat treated (quenched and tempered) before finishing, e.g., polishing, burnishing and coating. Therefore Pb must be eliminated prior to any heat treatments. But Pb proves difficult to remove satisfactorily, even by repeated intensive ultrasonic cavitation cleaning followed by selective chemical etching. In any cases, chemical etching is always

systematically and preventively done prior to any physical, chemical, and/or galvanic coatings. As a consequence, Pb causes expensive additional works, reworks and delays. It is an important economic factor of the production of high precision parts.

SUMMARY OF THE INVENTION

In accordance with the operations of the line process, the various basic aspects embodied in this invention, are providing: I. precision machining wire coils and bars made from a steel alloy that is substantially free of Pb;

II. an alloy made from pre-alloyed metal powder that contains only S as a free- machining additive, forming a volume fraction of MnS inclusions of max 0.65% _VO ι, and carbon (C) forming a volume fraction of Fe ₃ C hypereutectoid cementite carbides at the A _C i eutectoid temperature of max 3.5% _VO ι;

III. a gas atomization powder production process producing round shaped fine powder particles containing fine evenly dispersed MnS micro-inclusions that are not greater than about 2 microns in major dimension after hot deformation, and geometrically fully compatible with the ISO Tolerance grade IT 4-5, and surface roughness Ra <0.10, required for the finished parts;

IV. a screening process of the atomized powder, to sieve the powder particles to sizes finer 100 mesh;

V. a blending process mixing the powders of several heats having essentially the same composition to produced a blended metal powder;

Vl. a HIP (Hot lsostatic Pressure) consolidation process of the powders filled in a steel canister;

VII. a hot rolling process reducing the section of the filled canister by a reduction rate of approximately 1 : > 850 to obtain rod ≥5.5 mm diameter;

VIII. a anneal process of the hot rolled rods, at an intermediate temperature between A _C i and A _CM to form a microstructure composed of spheroϊdal cementite carbides that are not greater than about 6 microns in major dimension, and evenly distributed in the ferrite matrix;

IX. a shaving process of the annealed rod wires to remove the remnants of the canister and eliminate any hot rolling surface defects, or decarburization zones;

X. a cold drawing process of the annealed rods into machining wire coils and straightened bars, which may comprise one or more cycles of cold drawing and intermediate annealing;

Xl. a wire cold drawing process capable of producing machining coils and straightened bars having a dimensional ISO Tolerance grade of IT <5-6, and a surface roughness Ra <0.10;

XII. cold drawn wires and straightened bars having a substantially uniform, fine grain size, preferably an ASTM E112 grain size of about 9 or finer;

XIII. cold drawn wires and bars having an equivalent or better machinability than that of leaded steel, but without the health, safety, and surface quality problems associated with the use of Pb;

XIV. machined parts which can be easily and safely heat treated (hardened) by quenching and tempering to the desired hardness or strength levels;

XV. machined parts which can be efficiently polished, burnished and coated without all inconveniences and additional cost associated with the presence of Pb.

XVI. machined parts which can be efficiently polished, burnished and coated without all inconveniences and additional cost associated with the presence of large MnS inclusions as found in heavily resulfurized steels.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the process according to the invention, includes the various steps listed thereafter.

I. Designing an alloy composition of the Type 1.1298 (about AISI 1095) carbon steel, that : a. is substantially free of Pb; b. has a C content forming max 3.5% _VO ι FesC hypereutectoid cementite carbides (primary carbides) at the A _C i eutectoid temperature of the steel; c. has a C content forming Fe3C carbides dissolving totally into solid solution at an A _CM temperature of ≥825°C (≥1517°F); d. has a C content forming Fe3C carbides that are not greater than about 6 microns in their major dimension compatible with the ISO Tolerance grade and surface roughness number to satisfy; e. has S as only free-machining additive, forming MnS micro-inclusions of a total volume fraction of max 0.65% _VO ι; f. has a S content forming evenly distributed MnS micro-inclusions not greater than about 2 microns in their major dimension compatible with the ISO Tolerance grade and surface roughness number to satisfy; g. has a S content forming evenly distributed fine MnS micro-inclusions geometrically fully compatible with the ISO Tolerance grade IT 4-5, and a surface roughness Ra <0.10 to satisfy.

A preferred composition of the product made by the process of the invention is, in weight (mass) percent as follows:

C 0.88-0.98

Mn 0.40-0.55

Si 0.12-0.22

P 0.030 max

S 0.010-0.090

N 0.10 max

Cr 0.25 max

Ni 0.25 max

Mo 0.25 max

Cu 0.25 max

Fe balance and usual impurities.

It is contemplated that the process described herein can be used to produce very small diameter cold drawn wires and straightened bars for machining precision parts made from other alloys. More specifically, the process can be used with martensitic stainless steels such as Alloys 1 to 4 below, each of which consists essentially of, in weight percent, about :

Alloy 1 Alloy 2 Alloy 3 Alloy 4 Type 1.4057 Type 1.4197 Type 1.4021 Type 1.4035

C 0.12-0.22 0.20-0.25 0.16-0.21 0.40-0.48

Mn 0.30-0.80 1.50 max 1.50 max 2.00 max

Si 0.20-0.60 0.60 max 1.00 max 1.00 max

P 0.030 max 0.030 max 0.030 max 0.030 max

S 0.010-0.030 0.010-0.090 0.010-0.015 0.010-0.090

Ni 1.50-2.50 0.75-1.25

Cr 15.50-17.00 12.50-14.00 12.00-14.00 12.00-14.00

Mo 0.05 max 1.00-1.20

Cu 0.25 max 0.25 max

N 0.12 max 0.12 max

Fe balance balance balance balance and usual impurities.

//. Selecting the vacuum induction melting (VIM) process to melt the steel, and then gas atomize it with nitrogen gas to produce round shaped fine powder particles. In gas atomization each metal particle (droplet) has exactly the same composition as the heat of molten steel.

III. Screening the atomized powder to preferably mesh No >100, and blending it with one or more other heats having essentially the same alloy composition to produce a blended powder.

IV. The powder blend is vibration filled into stainless steel or low carbon steel canister. The powder-filled canister is then vacuum hot outgased and sealed. The sealed canister is hot isostatically pressed (HIP'd) preferably at about 1120 ⁰ C (2048 ⁰ F) and about 100 MPa for a time sufficient to fully densify the metal powder. Argon gas is preferred as the pressurizing fluid.

V. Selecting a 3 steps hot deformation sequence comprising:

First step: hot rolling of the canister preferably at 1 150°C (2102 ⁰ F), into a billet that includes the consolidated metal powder and its canister cladding; Second step: process anneal below the A _C i temperature; annealing the hot rolled rods, at an intermediate temperature between A _C i and A _CM , forming a microstructure composed of spheroidal cementite carbides that are not greater than about 6 microns in major dimension, and evenly distributed in the ferrite matrix. The dense population of MnS micro-inclusions is believed to, first, hinder the grain growth of the ferrite matrix, second, delay the transformation of the lamellar pearlite into its spheroϊdal form, and third, impede the growth of the hypereutectoid cementite carbides, until a threshold annealing temperature situated between A _C i and A _CM is reached. Albeit, the mechanisms of this triple pinning role of the MnS inclusions are not yet fully understood, they do not appear to be primarily related to their sizes, but either to their populations (numbers) and distribution spectrum; and Third step, hot rolling the billet into the desired round rod sizes, generally <8.5 mm (<0.33 in) diameter. The total hot deformation ratio being >850.

VI. Shaving the annealed coils of rod to remove the carbon steel or stainless steel remnants of the canister cladding and eliminate any hot rolling surface defects, or decarburization zones.

VII. Cold drawing the annealed rod material into wires to produce machining coils or straightened bars. This process may comprise one or more cycles of cold drawing and intermediate annealing.

Applying either an intermediate annealing to coils of cold drawn wires in a pit or bell furnace under a protective atmosphere of argon, of 1 hr at a

temperature at least 150 ⁰ C (302 ⁰ F) below the A _C i temperature of the steel, followed by slow cooling, <25°C/hr (77°F/hr), to about 400 ⁰ C (752°C); or, a wire strand annealing at a temperature at least 70 ⁰ C (158°F) below the A _C i temperature of the steel, with an exposure to the annealing temperature not exceeding 6 min.

The time at the temperature being in both cases long enough to ensure a full softening of the ferrite matrix and short enough to avoid a growth or morphological modification of the spheroϊdal carbides and of the grain size of the ferrite.

VIII. Selecting the cold drawing process to produce machining coils of wires or straightened bars satisfying the ISO Tolerance grade IT <5-6, and a surface roughness of Ra <0.10.

IX. The cold drawn wires and straightened bars having a substantially uniform, fine grain size, preferably an ASTM E1 12 grain size number of about 9 or higher.

X. The tolerances and surface finish of the machining wires and straightened bars, having also the capability to satisfy the most severe precision requirements of ISO IT 3-4 and Ra <0.10. Requirements which cannot be met with similar Pb bearing steels, limited at their best to ISO IT 5-6 and Ra >0.20-0.40.

XI. Producing wires and bars having an equivalent or better machinability than leaded steel, but without the health, safety, and surface quality problems associated with the use of Pb.

XII. Selecting the amount of cold work hardening (cold reduction) after the last annealing to obtain machining coils and straightened bars within a specific strength range. Usually, between 750 and 900 MPa (about 107-128 kpsi) for wire diameters ≤4 mm used on rotating tools Swiss automatic lathes, and up to 1 '250 MPa (178 kpsi), for 0.50 mm (0.004 in) to 7.5 mm (0.3 in) bar diameters machined on rotating bar Swiss automatic lathes. For hard dry machining purposes the wire coils and bars may also be heat treated in the 1 '100-1 '450 MPa (156-206 kpsi) range.

XIII. Obtaining wire coils and bars having at least a similar or better machinability than comparative leaded steels. Each MnS micro-inclusion is the potential site of formation of a void developing into a microcrack in the metal cutting shear plane, thus promoting the chip formation of the metal cutting process. The dense population of MnS micro-inclusions of the steel of this invention

ensures that the MnS inter-particle distance is kept very small, typically <12 μm, thus enhancing strongly the machining efficiency.

XIV. The preferred alloy steel of this invention exhibits in the Vi to 3/4 hard (UTS <900 MPa) cold drawn condition a much better cold formability than comparative Pb-bearing carbon steels. That property makes the alloy highly amenable for cold rolling threads, stamping, e.g., sockets, cold heading, closed die forging, bending, and further cold shaping techniques.

XV. Designing a final heat treatment process (quenching and tempering) better adapted to the production of small high precision parts than the Pb-bearing steels. Compared to leaded steels, the volume fraction of retained austenite can be reduced up to 1/3 or about <10% _VO |.

WORKING EXAMPLES.

I. The composition and production processes of the products (wire coils and straightened bars) according to the present invention, are designed to provide a microstructure incorporating the following features: a. A uniform distribution of small spheroϊdal carbides, substantially all of which are not more than about 6 microns in major dimension compatible with the ISO Tolerance grade and surface roughness number to satisfy; b. A uniform distribution of fine micro-inclusions of the MnS type, substantially all of which are not more than about 2 microns in major dimension compatible with the ISO Tolerance grade and surface roughness number to satisfy; c. A uniform distribution of fine grains of the ferrite matrix of typically ASTM E 1 12 grain size No 9, preferably, ASTM No.10 or finer; and, d. A machinability at least equal or similar to leaded cast-and-wrought steel.I. Several trials were performed to determine an annealing cycle that produces the desired microstructure. Standard known annealing processes of similar grade steel, e.g., at 760 ⁰ C, have proven not adapted to anneal successfully the powder steel of this invention. They produced repeatedly undesirable microstructure consisting of a mixture of coarse and fine carbides, colonies of undesirable lamellar pearlite, which resulted in inconsistent drawability and machinability.I. Consequently, a lower carbon content has been selected to reduce the amount of cementite (carbide phase). This lower C content was determined to: a. reduce the amount of hypereutectoid carbide (primary carbides) by roughly at the A _c1 eutectoid temperature;

b. still warrant a full hardening (quenching and tempering) of the machined parts, similar to 1 % C-steels leaded steels ; and c. reduce the amount of retained austenite after hardening (quenching). Simultaneously, a lower S content has also been selected to reduce the size of the MnS micro-inclusions. It has been defined to obtain a volume fraction of the MnS micro-inclusions of <0.45% _VO |.

Because atomized small metal droplets crystallize much faster than large ones, their microstructural features, e.g., metallic dendrites and MnS micro-inclusions, are significantly finer than in coarser powder particles. To obtain the same number of MnS inclusions per unit volume, larger powder particles of mesh No. <100, requires higher S content than finer ones. No known coalescence or cluster formation of MnS particles is known to take place during either hot or cold working. Further, the relative lack of consistent drawability of the powder alloy observed with the first batch, was identified to be strongly dependent on the presence of mostly subcutaneous colonies of lamellar pearlite in the microstructure. As a consequence, it was discovered that heir dissolution requires an annealing temperature exceeding the Ad eutectic temperature to transform them into globularized cementite carbides evenly distributed in the ferrite matrix. But, if the amount of heating above the A _C i temperature is too high, the carbides become too coarse; if too low, an undesirable amount of lamellar pearlite is retained in the microstructure. Trials with two additional steel batches have shown that an annealing at 738-740 ⁰ C, during 10 hr, followed by a slow furnace cooling to 540 ⁰ C (1004°F) is adequate. In this respect, the annealing behavior of the powder metallurgy products according to this invention differs once more from the known behaviors and recommended teachings of the cast-and- wrought steels of similar C content and composition. We believe that this fundamental difference is due to the initial rapidly solidified microstructure of the alloy powder in combination with the fine dispersion of micro-inclusions of MnS, and carbides, oxides, and nitrides in the material related to the powder particles.

The machinability of the preferred steel of this invention is at least equivalent or superior to this of Pb-bearing steels. The teachings of the machining state-of-the-art of : d. cast-and-wrought steels, says that very fine sulfides are not at all expected, to provide a similar or better free-machinability than coarser sulfide particles; e. sintered powder metallurgy (not wrought) products, says that only very coarse MnS sulfides of the macro-inclusions type are expected to provide a good free- cutting behavior.

The fact that the very fine sulfides of the preferred wrought powder steel of this invention, in combination with the fine spheroϊdized carbides, delivers the desired machinability, contradicts and proves these state-of-the-art teachings not applicable to wrought powder products made according to this invention. We believe that the fine sulfide particles, or MnS micro-inclusions ≤2 μm, of the preferred steel of this invention, enhance the machinability, because they are more easily thermally activated at all the cutting speeds encountered in praxis, than the much larger MnS+Pb macro-inclusions of 10-125 μm, of the cast-and-wrought leaded-bearing steels. This thermal activation favors the chip formation process by softening the MnS micro-inclusions at all cutting speeds, from the slowest rotating speed of about 1 O00-2O00 rpm, up to the fastest ones of 25'00O rpm or more (equivalent to cutting speeds of 125 m/min (410 ft/min) or more) routinely achievable today.

Further, the size of the MnS micro-inclusions as well as of the spheroϊdal carbides of the preferred steel of this invention are both geometrically fully compatible with the dimensional tolerances, typically ISO Tolerance grade IT <4-6, and surface roughness requirements, typically Ra <0.10, routinely required in the machining of high precision parts.

Finally, it is believed that the processing according to the present invention can be applied to small diameter, machinable wire and barstock used to produce precision high parts made from other alloys including other carbon and alloyed steels as well as martensitic stainless steel, typically such as Type 1.4057 (AISI 431 ). Among possible further candidate alloys are martensitic stainless steels such as Type 1.4197, Type 1.4021 (AISI =420), Type 1.4035 (AISI =420F). Similarly, machinable austenitic stainless steels, such as Type 1.4435 and 1.4441 (AISI 316L) and other austenitic alloys of the Type AISI 300 series, could also benefit from the method of this invention.

The process according to the present invention permits to produce advantageously small diameter products for Swiss-type automatic lathes equipped with plain guide bushings (cemented carbides and ceramic) of very close tolerances, typically ± <1 μm (± <40 μin). Because, cold drawn and straightened bars of cast-and-wrought free- machining steels exhibit significantly larger scatter in diameter and tolerances, typically ISO IT ≥6, and rougher surface, Ra >0.40, than the IT <5-6 and Ra <0.10, routinely achievable with the alloy of this invention. The continuous measurement of the dimension of several batches of up to 150 kg (330 lbs) each, of machining wires

and straightened bars of the preferred alloy of this invention, did not exceed 1 μm (40 μin), corresponding to a ISO Tolerance grades of IT <3-4 and Ra <0.05. This high dimensional accuracy and reproducibility of the wires and bars products is technically and economically highly significant, because: f. It translates into a higher productivity of all machining and finishing operations, e.g., polishing, burnishing and coating; g. The idle times necessary for set-ups and adjustment are massively reduced; h. The machining tools must not be reset in process or from batch to batch of material; i. The plain guide bushings do not have to be run-up to adapt them to new mean diameter from production run to production run; j. The very close fit between the guide bushing and the gliding bar largely reduces or eliminates the dynamic micro-chatter of the couple guide bushing, resulting in a significantly better surface finish of the machined part, typically Ra <0.05-0.10; k. This very close fit also eliminates effectively the risk of damaging the bars by scratching; I. The combined very close fit and very low surface roughness of the bars and wires reduce massively the wear of the guide bushings; m. The much higher dynamic stability of the bar material ensured by the products of the invention, permits machining at significantly higher cutting speeds and feed rates, regardless of whether the operation is turning, drilling, or milling.

The quenching and tempering behaviors of small precision parts made of the preferred powder metallurgy steel of this invention differs from the behaviors and recommended teachings of the cast-and-wrought steels of similar composition. To avoid overheating, and/or coarsening of the microstructure, the quenching is preferably done at a temperature not exceeding 820 ⁰ C (1490 ⁰ F), the holding time being at the temperature being limited to 6-8 minutes. To suppress potential quenching distortions, the quenching should preferably be done in a heated oil bath at 50-90°C (122-194°F). Thence, the volume fraction of retained austenite of the preferred powder metallurgy steel of this invention is limited to typically about <10% _VO ι, compared to approximately 15% _VO ι with similar cast and-wrought Pb-bearing steels. In any cases, the higher the quenching oil temperature the smaller the risk of distortion of the quenched parts. But, correspondingly, the higher the oil temperature, the higher the volume fraction of retained austenite will be.

The peculiarities of the powder metallurgy of the preferred steel of this invention, require a holding time of preferably 45-60 minutes at the tempering temperature, or

even a double tempering of 45 minutes each, done at a slightly lower tempering temperature (e.g., -10-20 ⁰ C (-18 ⁰ F)) than a single tempering. The tempering being is done according to the desired hardness, or strength, level to achieve. The selection of the appropriate hardness levels of the heat-treated parts is usually made on the base of their function(s), and the individual appraisal of the efficiencies and yields of their finishing operations, polishing and/or burnishing (such as for axle pins). Accordingly, the final hardness number may vary between about ©500 and 800 Hv.