Warm compaction of steel powders

Details Warm compaction of steel powders Warm compaction of steel powders

2002-07-30

2004-03-30

Mai, Ngoclan (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Loose particulate mixture containing metal particles

Reexamination Certificate

active

06712873

ABSTRACT:

FIELD OF INVENTION
The present invention concerns steel powder compositions as well as the compacted and sintered bodies obtained thereof. Specifically the invention concerns stainless steel powder compositions for warm compaction.
BACKGROUND ART
Since the start of the industrial use of powder metallurgical processes i.e. the pressing and sintering of metal powders, great efforts have been made in order to enhance the mechanical properties of P/M-components and to improve the tolerances of the finished parts in order to expand the market and achieve the lowest total cost.
Recently much attention has been paid to warm compaction as a promising way of improving the properties of P/M components. The warm compaction process gives the opportunity to increase the density level, i.e. decrease the porosity level in finished parts. The warm compaction process is applicable to most powder/material systems. Normally the warm compaction process leads to higher strength and better dimensional tolerances. A possibility of green machining, i.e. machining in the “as-pressed” state, is also obtained by this process.
Warm compaction is considered to be defined as compaction of a particulate material mostly consisting of metal powder above approximately 100° C. up to approximately 150° C. according to the currently available powder technologies such as Densmix™, Ancorbond™ or Flow-Met™.
A detailed description of the warm compaction process is described in e.g. a paper presented at PM TEC 96 World Congress, Washington, June 1996, which is hereby incorporated by reference. Specific types of lubricants used for warm compaction of iron powders are disclosed in e.g. the U.S. Pat. Nos. 5,154,881 (Rutz) and 5,744,433 (Storström).
Until recently it has been observed that the general advantages with warm compaction have been insignificant as only minor differences in e.g. density and green strength have been demonstrated in the case of stainless steel powders. Major problems encountered when warm compacting stainless steel powders are the high ejection forces and the high internal friction during compaction.
However, as disclosed in the U.S. Pat. No. 6,365,095 (Bergkvist), it was recently found that stainless steel powders may be subjected to warm compaction with good results provided that the stainless steel powder is distinguished by very low oxygen, carbon and silicon levels. The widely used standard qualities having higher levels of these elements could however not be successfully warm compacted i.e. the properties of the warm compacts were not significantly better than the green density of a corresponding body compacted at ambient temperature.
It has now unexpectedly been found that also standard stainless steel powders can be compacted at elevated temperatures with good results. In comparison with the stainless steel powders disclosed in the above US patent the standard stainless powders are generally characterised in a higher amount of oxygen, carbon and silicon. These powders are also easier to produce and accordingly cheaper. According to the present invention it has thus, contrary to the teaching in the SE publication, been found that these standard powders can be compacted to high green densities without the use of excessively high compaction pressures. The high green density is valuable when the product is subsequently sintered as it is not necessary to use high sintering temperatures and accompanying high energy consumption in order to get a high sintered density which is normally necessary in order to get good mechanical properties. Additionally high sintering temperatures induce strains in the material which in turn gives poor dimensional stability.
SUMMARY OF THE INVENTION
In brief the process of preparing high density, warm compacted bodies of a water atomised standard stainless steel powder according to the present invention is based on the discovery that specific amounts of lubricants have to be used in the stainless steel powder composition which is subjected to the compaction at elevated temperature. Minor amounts of selected additives included in the composition contribute to the unexpected finding that standard stainless steels can be successfully compacted.
DETAILED DESCRIPTION OF THE INVENTION Type of powder
Preferably the powders subjected to warm compaction are pre-alloyed, water atomised powders which include, by percent of weight, 10-30% of chromium. The stainless steel powder may also include other elements such as, molybdenum, nickel, manganese, niobium, titanium, vanadium. The amounts of these elements may be 0-5% of molybdenum, 0-22% of nickel, 0-1.5% of manganese, 0-2% of niobium, 0-2% of titanium, 0-2% of vanadium, and at most 1% of inevitable impurities and most preferably 10-20% of chromium, 0-3% of molybdenum, 0.1-0.4% of manganese, 0-0.5% of niobium, 0-0.5% of titanium, 0-0.5% of vanadium and essentially no nickel or alternatively 5-15% of nickel, the balance being iron and unavoidable impurities (normally less than 1% by weight). Examples of stainless steel powders which are suitably used according to the present invention are 316 LHC, 316 LHD, 409 Nb, 410 LHC, 434 LHC. The standard steel powders used according to the present invention generally include more than 0.5% by weight of Si and normally the Si content is 0.7-1.0% by weight. This feature distinguishes standard stainless powders from the stainless powders used for the warm compaction according to the U.S. Pat. No. 6,365,095 (Bergkvist) mentioned above.
AMOUNT OF LUBRICANT
The amount of lubricant in the composition to be compacted is an important factor for the possibility to get a satisfactory result. It has thus been found that the total amount of lubricant should be above 0.8% by weight, preferably at least 1.0% by weight and most preferably at least 1.2% by weight of the total powder composition. As increasing amounts of lubricant decrease the final green density due to the fact that the lubricants normally have much lower density than the metal powder, lubricant amounts above 2.0% by weight are less important. In practice it is believed that the upper limit should be less than 1.8% by weight. A minor amount, such as at least 0.05 and at most 0.4% by weight of the lubricant should preferably be a compound having high oxygen affinity.
TYPE OF LUBRICANT
The lubricant may be of any type as long as it is compatible with the warm compaction process. Examples of such lubricants are disclosed in e.g. the U.S. Pat Nos. 5,154,881 (Rutz) and 5,744,433 (Storström), which are referred to above and which are hereby incorporated by reference. Preliminary results have also shown that lubricants conventionally used for cold compaction, such as EBS, may be used for warm compaction of the standard steel powders according to the present invention although the flow properties of such powder compositions are inferior.
So far however the most promising results have been obtained by using a type of lubricants disclosed in the copending patent application SE02/00762 PCT. These type of lubricants include an amide component which can be represented by the following formula
D—C
ma
—B—A—B—C
mb
—D
wherein
D is —H, COR, CNHR, wherein R is a straight or branched aliphatic or aromatic group including 2-21 C atoms
C is the group —NH (CH)
n
CO—
B is amino or carbonyl
A is alkylen having 4-16 C atoms optionally including up to 4 O atoms
ma and mb which may be the same of different is an integer 1-10
n is an integer 5-11.
Examples of preferred such amides are:
CH
3
(CH
2
)
16
CO—[HN(CH
2
)
11
CO]
2
—HN(CH
2
)
12
NH—[OC(CH
2
)
11
NH]
2
—OC(CH
2
)
16
CH
3
CH
3
(CH
2
)
16
CO—[HN(CH
2
)
11
CO]
2
—HN(CH
2
)
12
NH—[OC(CH
2
)
11
NH]
3
—OC(CH
2
)
16
CH
3
CH
3
(CH
2
)
16
CO—[HN(CH
2
)
11
CO]
3
—HN(CH
2
)
12
NH—[OC(CH
2
)
11
NH]
3
—OCCH
2
)
16
CH
3
CH
3
(CH
2
)
16
CO—[HN(CH
2
)
11
CO]
3
—HN(CH
2
)
12
NH—[OC(CH
2
)
11
NH]
4
—OC(CH
2
)
16
CH
3
CH
3
(CH
2
)
16
CO—[HN(CH
2
)
11
CO]
4
—HN(CH
2
)
12
NH—[OC(CH
2
)
11
NH]
4
—OC(CH
2
)
16
CH
3
CH
3
(CH
2

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