CA2004625A1 - Iron-based powder for the manufacture of sintered components - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention describes an iron-based powder, pre-alloyed by atomization, containing dissolved Mo for production in the normal pressing and sintering processes used in the powder-metallurgical manufacture techniques of precision parts with high density, good dimensional accuracy, hardenability and strength. The amount of Mo that is pre-alloyed into the powder is about 0.5-2.5% by weight.

Description

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~ C~p~p9 BACRGROUND OF T~ IN~N~ION
The present invention pertains to an iron-based powder, alloyed by melting and subsequent atomization, for use in the manufacture by powder-m~tallurgical methods of precision parts with high density, good~dimensional accuracy, hardenability, and strength. More particularly, it relates to a powder composition of iron pre~alloyed with molybdenum as substantially the onIy alloyinq element.
Requirements that sintered metal parts have reduced weight but with preserved or e~en increased strength are primarily imposed in the automobile:industry.
To satisfy these re~uirements, ~ew powder metallurgical alloys, often with higher density and better homogeneity, h~ve been developed.
Althou~h such sintered parts ar~ genexally strong and defect-fr~e, ~ailures nonetheless have been known to occur. About 90~ of all failures in aintered materials are 20 caused by fatigue fracture. The best way to avoid~fatigue ~ ;
~ailure is sur~ace hardening; ~ince in ~ost cases the ~ :
fracture is initiated at the surfac~. As;a r-sult~of the hardening, compressive stre~ses are induced in the ~urface layer which inhibit crack growth. ~The ~o~e ductile~core;is exposed:to relatively low t~nsile ~tre~ , which~
essentially lack signi~icance.~
The all~vying ele~nt~ used t~day ~or:the surface hardening ~puw~er-~et~llurgical ma~erials are primarily :
Ni, Cu, ~o, C, and to a:~erta~n~degree, Cr,~ Mnj and~N. The :~

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~ 2 -elements c and/or N are usually added in a heat treatment operation after sintering.
There are two common types of alloying methods, i.e., powder ~ixtures and so-called atomized powdersO
Po~der ~xtur~ are prepared by mixing the base iron powder wit~ a po~dered form of the metal to be alloyed, eithar as the metal itself or as a compound which breaks down during the sint~ring process. The 60~called atomized steel powders are produced Prom a melt of iron and the desired alloying ele~ents, which melt i5 then sprayed into droplets (atomizing) that ~olidify upon cooling to form relatively homogeneous particles of the metallic components.
one of the disadvantages of powder mixtures is the risk of segregation which exists, depending on whether powders with different characteristics (e.g., different particle sizes or densities) are mixed with one another without being mechanically combined. This segregation leads to varying compositions of the pressed bodies produced from the powder mixtures, and as a result of this, leads to varying dimensional changes during their sintering. Another disadvantage of the powder mixtures is their tendency to form dust, ~specially when the alloying substance is present in a very small particle size.
Naturally, this may also cause seriou environmental problems.
on the other hand, the atomized powders are entirely free of the risk of segregation, ~ince each particle ~as the d~sired alloying composition. The risk of dust formation is ~lso not ~o gr~at, ~ince no alloying substance with a small p~rticle cize i~ included. The pre-alloyed atomized powders, however, have another disadvantage - low co~pressibility due to the solution-hardening effect that the alloying substances have on ~ach powder particle.
High c~mpressi~ility is e~ential to obtain a part with high density, which is a prerequisite for high :

strength. The compressibility of a simple mixture of elemental powders on the other hand, is almost the same as that of the iron powder included in it.
If Ni a~d Su are add~d to a powdPr ~ixture, the hardena~ility of ~h~ ~in~red ma~erial increases, but because of th~ slow dif~usion of Ni, the ~inal product has a highly heterogeneous di~tribution o~ Ni with regions of soft residual austenite due to the ~act that Ni stabilizes the austenitic phase. Cu dif~uses somewhat b~tter than Ni into austenite, and in addition, ~u ~elts, which means that one should get a more ho~ogeneous diskr~bution of Cu than of Ni in the sintered material~ ~he addi~ion of Cu causes swelling and thus lower density of the resultant sintered body.
In the case of the a~di ion of Mo, which increases hardenability, the sintering is ~ore ef~ective than with the addition o~ Cu or Ni due to the fact that Mo stabilizes ferrite and the sintering occur~ more rapidly at the Mo/Fe interface. To obtain a homogeneous distribution of Mo in the material, however, high-temperature sintering for a long period time is required,~which raises the production costs. An even longer time is required to adjust the alloying variations of Ni and Cu than for Mo.
The addition o~ Mn and Cr reguires extremely accurate process control and extremely clean process gases during high-temperature sintering to avoid oxidation o~ Cr of Mn and consequently the loss of the hardenability-increasing effect by ~he dis~olved Mn and the nitriding ~f~ect cause by Cr.
To obtain a homogeneous alloyed material, there~ore, an atomized powder i~ required in which the ;~
above-mentioned heavier alloying ele~ents exist in solid solution, i~e., pre-alloyed in ~he ~olten phase.
The obj~ctive of ~he present invention is an -~
iron-based powde~ ~or ~he production of sintered parts with high densi~y, good di~ensional accuracy, hardenability and strength, which lack the above-mentioned disadvantages with .:

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regard to segregation, dust formation and low compressibility due to the solution~hardening effect.

8~MB~Y OF ~ INV~N~IO~
According ~o the pre~en~ invention, it has been found that when the iron ~aterial i simply pre alloyed with Mo, the compressibility o~ ~he re~ulting powder is not significantly chanyed compared to that of pure Fe powder, despite the fact that the alloyed-in (dissolved) Mo h~s a significantly greater atomic size than Ni or other heretofore used alloying elements and would otherwise be expected to increase the hardness of the powder. For the surface hardness of the final ~intered product to reach a practically useful value, a ~inimum guantity of 0.5 wt.% ~o is required to be pre-alloyed or otherwise pre~ent in the powder mixture. At a content of 2.5 wt.~ of molybdenum, the practical upper limit for the quantity of Mo that should be pre-alloyed is reached with r~spect to the density requirement of the finished part. Furthermore, a higher content than 2.5 wt.% leads to greater ~hrinkage during sintering and consequently poorer dimensional accuracy of the finished part. The upper limit of about 2.5 wt.% Mo is therefore established for reasons of compressibility, dimen~ional stability and cost. The quantity of Mo preferred according to the invention is 0.75-2.0 wt.%. More prëferred is a quantity of about O.75-1.5 wt.~. A composition having about 0.8-0.9 wt.% Mo has been found to be particularly useful for the operations and purposes herein describ~d. At these values, good compressibility, sur~ace hardness, and hardenability are achieved.
What is also of importance to the invention is that the powder i5 substantially free of ~le~ents pre~
alloyed with the iron other than ~lybdenu~ ~ince ~o include others could unduly lowgx th4 compre~ibility of the powder~ Thus, it is ~vident that impurities should be kept at a low level. The total weight of impuritie such .

as Mn, Cr, Si, Cu, ~i and Al should not exceed 0.4 wt.%, while Mn itsel~ should be no more than 0.25 wt.%.
Furthermore, the C content should nQt ~xceed o.02 wt.%
The powder of this inven~ion is produced by atomizing a ~elt of subs~antially pure Fe materi~l containing 0,5-2.5 wt.~ ~o to produce a powder with a ~aximum particle size of about 212 ~m, preferably below 150 ~m. The powder is ~hen annealed at a temperature o~
700-1,200C in a reducing ~tmosphere. The powder is then capable of use in traditional powder-metallurgical methods involving compaction and sintering.

BPcIEF D$!SCRIPT~ON OF q~ DRalJI~lG~
FIG. 1: is a graph depicting density achieved as a function of compacting pressure for powders A-D o~ the examples described herein;
FIG. 2: is a graph depicting literature data on the effect of different alloy~ng substances on the fracture limit of iron;
FIG. 3: is a graph depicting the macro hardness `
of test bodies manufactured according to Example 2 described herein;
FIG. 4: is a graph depicting hardness profile o~
test bodies manufactured according to Example 2 described ~
herein; and ~:
FIG. 5: is a ~raph depict~ng the effect of manganese and carbon content on compreasibility.

D~T~ILED D~C~IP~I0~ 0~ T~ INY~ 0 Ex~mpl~
Four iron-based steel powders, identified as powders A-D, of equal gr~in ~ize are produced by atomization of a melt contai~ing the indic~ted metallic - :
alloying elements. This pouder is mixed with a lubricant :~
and compacted. The de~sity o~ ~h~ resulting powdered bodîQs was determined as a function of the cvmpacting ~:
35 pressure. ~:

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Composition o~ the Powders:
Powder A: 99.2 wt.% Fe O.8 wt.% lubricant Po~er B: 9~.7 wt.~ Fe ~.~ w~.~ ~o 0.8 wt.~ lubricant Powder C: 97.7 wt.% ~
1.5 wt.% Ni 0.8 wt.% lubricant ~owder D: 97.15 wt.% Fe 1.0 wt.% Cr 0.8 wt.% Mn 0.25 w~.% ~o O.8 wt.~ lubricant.
Figure 1 shows that the c~mpressibility (attained compacted densityl of powder B~ which i~ prP-alloy~d with molybdenum, is only marginally di~ferent ~rom that of a pure iron powder~ powder A. Powder C, which is pre-alloyed with Ni, gives a signi~icantly greater negative effect on compressibility, despite the fact that the known literature and the known relation~hips betw~en the fracture limit, hardness and compres~ibility sugg~st that nickel should be more favorable if one desires to avoid a decrease in compressibility; see Figure 2. Powder D, which is a commercially ~vailable powder intended ~or:the manufacture of hardened components, displays a significantly poorer compressibility than powder B.

~mp~ 2 Three powder~ E-G are pres~ed and hot f~rged to full density to ~nvestigate the hardenability o~ ~he material at one and the ~ame density level. The:~amp~es were cylinders ~ith dia~eter~ of 2~ ~ and heights of 25 ~;:
mm. The for~ed samples were csrburized a~ 890~C for 30 ~ 5 minutes in endogas with a carbon poten~ial corresponding to :
.. .

2~ ;~i a carbon content of 0.8 wt.%, after which the samples were quenched in oil at 60-C.

Composition of the Powder:
Powder E: 99.9 wt.% Fe 0.2 wt.~ (mixe~ in as graphite~
Powde~ F: ~7.75 wt.% F~
1.0 w~.% Cr 0.8 wt.% Mn 0.25 wt.% ~o 0.2 wt.% C (mixed in as graphite) Powder G: 98.3 wt.% Fe 1.5 wt.% Mo 0.2 wt.~ C (mixed in as graphite) Figure 3 shows that material G has a very high surface hardness, compared to materials E and F, which is a characteristic of a good material for sur~ace hardening operations.
Figure 4 shows the hardness profile in the same material as above. It is seen here that material G, besides having a high sur~ace hardness, also has a strikingly greater depth of hardness than materials E and F. A great hardness depth is also one of the characteristics of a material suitable for surface hardening operations.
. .
Ex~mple 3 Figure 5 ~hows the co~pr2ssi~ y in the form of the den~ity achieved at a compacting pressure of 410 MPa for an atomized steel powder alloyed with 1.5 wt.%
molybdenum as a function of the carbon content and manganese content.
The example shows that the ~uanti~y o~ carbon dissolved in has a great ~ffect on the compressibility and therefore should be as low a~ possi~le~ ~urthermore, the manganese con~ent influence~ the cc~pressibility negatively and espec;ally ab~ve a content of 0.20-0.25 wt.~.

6;~S

Example 4 A particularly preferred composition of the present invention is prepared from an iron-based melt having about 0.~5 wt.% ~o; no mor~ than about 0.08 wt.% Ni, 0~20 wt.~ ~n, and 0.10 wt.% ~u; no more than about 0.2 wt.%
other impurities; and the ~alance iron~ ~t compac~ing pressures of 30, 40, and 50 tons per square inch (0.5 wt.
zinc stearate added as lubricant), ~uch a composition attains green densities of about 6.75, 7.05, and 7.25 g/cc, respectively. These densities are hi~her ~han densities attained with comparable alloying mixtures containing about O.5-0.65 wt.% Mo but containing Ni levels of 0.4 wt.% or more.

Claims (8)

1. Atomized pre-alloyed Fe-based powder for powder-metallurgical processes containing dissolved molybdenum in an amount of 0.5-2.5 wt.% as an alloying element.

2. Atomized powder of Claim 1 containing 0.75-2.0 wt.% molybdenum.

3. Atomized powder of Claim 1 containing 0.8-0.9 wt.% molybdenum.

4. Atomized powder of Claim 1, 2, or 3, containing less than about 0.02 wt.% carbon.

5. Atomized powder of Claim 1, 2, or 3 wherein the total amount of any contained manganese, chromium, silicon, copper, nickel and aluminum is no greater than about 0.4 wt.%.

6. Atomized powder of Claim 4 wherein the total amount of any contained manganese, chromiumr silicon, copper, nickel and aluminum is no greater than about 0.4 wt.%.

7. Atomized powder of Claim 5 wherein the amount of any contained manganese is no greater than about 0.25 wt.%.

8. Atomized powder if claim 1, 2, 3, or 4 having a particle size less than about 212 µm.