Berg, Sigurd (Skanörgatan 7, Höganäs, S-263 57, SE)
Engström, Ulf (307 State Street, Johnstown, PA, 15905, US)
Berg, Sigurd (Skanörgatan 7, Höganäs, S-263 57, SE)
|1.||A wateratomised, prealloyed steel powder comprising, in addition to iron and inevitable impurities, by wt %, 1.31. 7 % by weight of Cr 0.150. 3 % by weight of Mo 0.090. 3% by weight of Mn not more than 0.01 by weight of C not more than 0.25% of O.|
|2.||A alloy steel powder according to claim 1 comprising 1.351. 65 % by weight of Cr 0.170. 27 % by weight of Mo 0.090. 25% by weight of Mn not larger than 0.006 by weight of C 3.|
|3.||A alloy steel powder according to claim 1 or 2 further comprising one or more of the elements Ti, B, V and Nb as prealloyed elements.|
|4.||A alloy steel powder according to claim 3 including, in % by weight, 0.010. 04 of Ti, 0. 010. 04 of B, 0.050. 3 of V and not more than 0.1 of Nb.|
|5.||A alloy steel powder according to any one of the claims 14 admixed with at least one element selected from the group consisting of Cu and Ni.|
|6.||A alloy steel powder according to claim 5, wherein particles of at least one element selected from the group consisting of Cu and Ni are bonded to the alloy steel powder.|
|7.||A alloy steel powder according to claim 5 or 6, wherein the amount of Cu is 0.54. 0 % by weight.|
|8.||A alloy steel powder according to claim 5 or 6, wherein the amount of Ni is 0.58. 0 % by weight.|
|9.||Alloy steel powder according to any one of the preceding claims admixed with graphite.|
|10.||A alloy steel powder according to any one of the preceding claims admixed with a lubricant.|
|11.||A alloy steel powder according to claim 10 characterized in that the lubricant is a cold compaction lubricant selected from the group consisting of metal soaps and waxes.|
|12.||A alloy steel powder according to claim 11 wherein the lubricant is selected from the group consisting of waxes, such as ethylenebisstearamide, metal stearates, such as zinc stearate, fatty acid derivates such as oleic amide and glyceryl stearate.|
|13.||Method for producing sintered components including a microstructure of low temperature bainite comprising the steps of providing a prealloyed, wateratomised powder comprising, in addition to iron and inevitable impurities, by wt %, 1.31. 7 % by weight of Cr 0.150. 3 % by weight of Mo 0.090. 3% by weight of Mn not more than 0. 01 by weight of C not more than 0.25% of O ; mixing the powder with graphite and optionally with one or more additives selected from the group consisting of alloying elements including, lubricant, binder, hard phase material, machinablity improving agent and flow enhancing agent compacting the obtained mixture; and sintering the obtained green body.|
|14.||Method according to claim 13 wherein the compation is performed without internal lubrication.|
|15.||Method according to claim 13 wherein the powder is admixed with a lubricant.|
|16.||Method according to claim 13 wherein the compation is performed with a combination of internal and external lurication.|
|17.||Method according to any one of the claims 1215 wherein the compation is performed at ambient temperature.|
|18.||Method according to any of the claims 1317, wherein the sintering is performed in a reducing atmosphere at temperatures above about 1050°C.|
|19.||A sintered component having a microstructure comprising at least 50, preferably at least 70 % and most preferably at least 90 % of lower bainite.|
Field of invention The present invention concerns a pre-alloyed iron based powder. Particularly the in- vention concerns a pre-alloyed iron based powder including small amounts of alloy- ing elements which permits a cost effectively manufacture of sintered parts for an increasing P/M market.
Background of the invention In industry the use of metal products manufactured by compacting and sintering metal-powder compositions is becoming increasingly widespread. A number of dif- ferent products of varying shapes and thickness are being produced and the quality requirements are continuously raised at the same time as it is desired to reduce the costs. This is particularly true for P/M parts for the automotive market, which is an important market for the P/M industry and for which cost is a major driving force.
Another factor of importance is the possibility of recycling scrap from the automotive industry and to consider the effect on the environment. Known alloying systems which have gained wide acceptance within this field have frequently included alloy- ing elements such as Ni and Cu. However, nickel is a strong allergen and is also con- sidered to have other detrimental medical effects. A problem with copper is that, during recycling of scrap used for steel manufacture, copper is accumulated. In many steel qualities copper is however not suitable and scrap without copper or with a minimum of copper would be required. Iron-based powders having low amounts of alloying elements without nickel and copper are previously known from e. g. the US patents 4 266 974,5605559, 5666634 and 6 348 080.
The purpose of the invention according to the U. S. patent 4 266 974 is to provide a powder satisfying the demand of high compressibility and to provide a sintered body having good hardenability and good heat-treatment properties, such as carburising.
The most important step in the production of the steel alloy powder produced according to this patent is the reduction annealing step (col. 5 line 15).
The US patents 5605559 and 5666634 both concern steel powders including Cr, Mo and Mn. The alloy steel powder according to the US patent 5605559 comprises, by wt %, about 0.5-2% of Cr, not greater than about 0. 08% of Mn, about 0.1-0. 6% of Mo, about 0.05-0. 5% of V, not greater than about 0. 015 of S, not greater than about 0.2% of O, and the balance being Fe and incidental impurities. The US patent 5 666 634 discloses that the effective amounts should be between 0.5 and 3% by weight of chromium, 0.1 and 2% by weight of molybdenum and at most 0.08% by weight of manganese.
A serious drawback when using the inventions disclosed in these US patents 5605559 and 5666634 is that cheap scrap cannot be used as this scrap normally includes more than 0.08% by weight of manganese. In this context the patent 5 605 559 teaches that "when Mn content exceeds about 0.08 wt% oxide is produced on the surface of alloy steel powders such that compressibility is lowered and hardenability increased be- yond the required level..... Mn content is preferably not greater than about 0.06 % wt. " (col 3 47-53). This teaching is repeated in the US patent 5 666 634 disclosing that"a specific treatment is used in order to reduce the Mn content to a level not lar- ger than 0. 08% by weight during the course of the steel making" (col. 3 line 40-44).
Another problem is that nothing is taught about the reduction annealing and the pos- sibility to obtain the low oxygen and carbon content in water-atomised iron powders including elements sensitive to oxidation, such as chromium, manganese. The only information given in this respect seems to be in example 1, which discloses that a final reduction has to be performed. Furthermore the US patent 5 666 634 refers to a Japanese Patent Laid-open No. 4-165002 which concerns an alloy steel powder in- cluding in addition to Cr also Mn, Nb and V. This alloy powder may also include Mo in amounts above 0. 5 % by weight. According to the investigations referred to in the US patent 5 666634, it was found that this Cr-based alloy steel powder is disadvanta- geous due to the existence of the carbides and nitrides which act as sites of fracture in the sintered body.
The possibility of using powders from scrap is disclosed in the US patent 6 348 080 which discloses a water-atomised, annealed iron-based powder compris- ing, by weight %, Cr 2.5-3. 5, Mo 0.3-0. 7, Mn 0.09-0. 3, O <0. 2, C<0. 01 the balance being iron and, an amount of not more than 1%, inevitable impurities. This patent also discloses a method of preparing such a powder. Additionally the US patent 6261514 discloses the possibility of obtaining sintered products having high tensile strength and high impact strength if powders having this composition is warm com- pacted and sintered at a temperature > 1220° C.
The present inventors have now unexpectedly found that lower ranges of alloying elements, especially chromium, give unexpected improvements as regards the possibilities of annealing and sintering. For example a higher partial pressure of oxygen during sintering can be allowed. The maximum partial pressure of oxygen that can be allowed when sintering components produced of powder according to the present invention is at most 3xi 0-17 atm, whereas during sintering of components produced of powder according to US patent 6348080 (Arvidsson) the maximum allowable partial pressure is as low as 5x10-18 atm.
Additionally, when comparing green bodies prepared from these known powders with green bodies prepared from the new powders according to the present invention, it was found that compacted bodies prepared from the new powders are distinguished by an unexpectedly high green strength. This is particularly true when die wall lubrication is used. Green strength is one of the most important physical properties of green parts. The importance of this property increases as P/M parts increase in size and geometry becomes more complex. Green strength increases with increasing compact density and is influenced by type and amount of lubricant admixed to the powder. The green strength is also influenced by the type of powder used. A high green strength is required in order to prevent compacts from cracking during the ejection from the compacting tool and prevent them from getting damaged during the handling and the transport between the press and the sintering furnace. Presently used compacts having a relatively high green strength are advantageously prepared from sponge iron powders whereas difficulties have been met as regards the preparation of
compacts of atomised powders in spite of the fact that an atomised powder is more compressible and hence gives a higher green density.
Objects of the invention A first object is to provide a new pre-alloyed powder including low amounts of alloying elements.
A second object is to provide a pre-alloyed powder which can be compacted at ambient temperature to high green strength at moderate compaction pressures.
A third object is to provide a new pre-alloyed powder which can be cost effectively compacted and sintered in industrial scale.
A fourth object is to provide a pre-alloyed powder which can be produced from cheap scrap.
A fifth object is to provide a new pre-alloyed powder suitable for producing sintered components with a microstructure essentially consisting of low-temperature bainite.
A sixth object is to provide a new pre-alloyed powder including low amounts of alloying elemements, with good compressibility, good hardenability and oxygen content below 0.25 %.
Summary of the invention According to the present invention these objects are achieved by using a pre-alloyed, water-atomised steel powder comprising, 1.3-1. 7 % by weight of Cr, 0.15-0. 3 % by weight of Mo, 0.09-0. 3% by weight of Mn, not larger than 0.01 by weight of C not larger than 0.25% of O the balance being Fe and inevitable impurities.
According to a more preferred embodiment of the invention the powder has the com- position 1. 35-1. 65 % by weight of Cr 0. 1 S-0. 25 % by weight of Mo, 0.09-0. 25% by weight of Mn not larger than 0.006 by weight of C.
The invention also concerns compacted and sintered products prepared from this powder optionally admixed with other alloying elements and lubricants, binders, hard phase materials, flow enhancing agents, machinability improving agents.
Detailed description of the invention Preparation of the new powder.
The alloy steel powder of the invention can be readily produced by subjecting ingot steel prepared to have the above-defined composition of alloying elements to any known water-atomising method. It is preferred that the water-atomised powder is pre- pared in such a way that, before annealing, the water-atomised powder has a weight ratio O : C between 1 and 4, preferably between 1.5 and 3.5 and most, preferably be- tween 2 and 3, and a carbon content between 0.1 and 0.9% by weight. For the further processing according to the present invention this water-atomised powder could be annealed according to methods described in PCT/SE97/01292 (which is hereby in- corporated by reference) Another process which can be used for the preparation of low oxygen, low carbon iron-based powders including low amounts of easily oxidised alloying elements is disclosed in the co-pending Swedish application 9800153-0.
A distinguishing feature which has been observed concerning the appearance of the annealed powder particles is that the particle shape is slightly more irregular com- pared with the particle shape of water atomised plain iron powder.
Amount of Cr The component Cr is a suitable alloying element in steel powders, since it provides sintered products having an improved hardenability but not significantly increased ferrite hardness. To obtain a sufficient strength after sintering and still maintain a good compressibility a Cr range of 1.3 to 1.7 is suitable. A higher chromium content decreases the compressibility and also increases the risk of forming unwanted car- bides. A lower content decreases the hardenability.
Amount of Mn The component Mn improves the strength of steel by improving hardenability and through solution hardening. However, if the amount of Mn exceeds 0.3%, the ferrite hardness will increase through solid solution hardening. If the amount of Mn is less than 0. 08 it is not possible to use cheap scrap that normally has an Mn content above 0.08%, unless a specific treatment for the reduction of Mn during the course of the steel manufacturing is carried out. Thus, the preferred amount of Mn according to the present invention is 0.09-0. 3%. In combination with C contents below 0. 01% this Mn interval gives the most interesting results.
Amount of Mo The component Mo serves to improve the strength of steel through the improvement of hardenability and also through solution and precipitation hardening. To the given chemical composition the Mo addition in the range of 0.15 to 0.3 is sufficient to move the perlite noose in the CCT-diagram to the right making it possible to form a bainitic structure at normally used cooling rates.
C amount The reason why C in the alloy steel powder is not larger than 0. 01% is that C is an element, which serves to harden the ferrite matrix through interstitial solid solution hardening. If the C content exceeds 0.01% by weight, the powder is hardened considerably, which results in a too poor compressibility for a powder intended for commercial use.
O amount The amount of O should not exceed 0.25 % by weight. O content is preferably limited to less than about 0.2 wt % and more preferably to less than about 0. 15 wt %.
Other elements Other elements which may be included in the pre-alloyed powder are Ti, B, V and Nb. Ti, V and Nb can form carbides which will give precipitation hardening effects.
B has the same effect as carbon, a solution hardening effect, and can form borides
with Ti, Nb and V giving a precipitation hardening effect. The amounts of these elements are preferably, in % by weight, 0.01-0. 04 of Ti, 0.01-0. 04 of B, 0.05-0. 3 of V and not more than 0.1 of Nb.
Ni and/or Cu may be admixed with the new powder. Alternatively particles of Cu and/or Ni may be adhered to the particles of the new powder by using a bonding agent. Ni and/or Cu may also be diffusion bonded to the particles of the new powder.
The addition of Ni and/or Cu improves the hardenability. Additive amounts of these alloys are limited to about 0.5-8 wt % of Ni and about 0.5-4 wt % of Cu.
Figure 1 shows a CCT diagram and figure 2 shows the phase amounts at different cooling rates for a material prepared from the new powder with 0.5 % of carbon with 2 % Cu addition. The good hardenability is demonstrated in these figures.
Furhermore elements such as P, B, Si, Mo and Mn may also be admixed with the new powder.
Graphite Graphite is normally added to powder metallurgical mixes in order to improve the mechanical properties. Graphite also acts as a reducing agent decreasing the amount of oxides in the sintered body further increasing the mechanical properties. The amount of C in the sintered product is determined by the amount of graphite powder mixed with the alloy steel powder of the invention. Typically graphite is added in the amounts up to 1 % by weight.
Lubricant A lubricant may also be admixed with the powder composition to be compacted. The presently most interesting application of the new powder seems to be for the production of sintered parts compacted at ambient temperature (= cold compaction), but also warm compaction is possible.
Representative examples of lubricants used at ambient temperature (low temperatur lubricants) are ; KenolubeTM, ethylene-bis-stearamide (EBS) and metal stearates, such
as zinc stearate, fatty acid derivates such as oleic amide and glyceryl stearate, and polyethylene waxes.
Representative examples of lubricants used at elevated temperatures (high temperature lubricants) are, polyamides, amide oligomers, polyesters or lithium stearate. The amount of lubricants added is normally up to 1 % by weight.
Other additives Other additives which optionally may be admixed with the powder according to the invention include hard phase material, machinablity improving agent and flow enhancing agents.
Compaction and sintering Compaction may be performed in a uniaxially pressing operation at ambient or elevated temperature at pressures up to 2000 MPa although normally the pressure varies between 400 and 800 MPa.
After compaction, sintering of the obtained component is performed at a temperature of about 1000° C to about 1400° C. Sintering in the temperature range of 1050° to 1200° C leads to a cost effective manufacture of high performance components. Further increase of the sintering temperature, above 1200° C, high temperature sintering, leads to further improvement of the mechanical properties.
The sintering times may be comparatively short, i. e. below 1 h, such as 45 minutes.
Usually the sintering time is about 30 minutes.
Depending on i. e. the composition of the iron-based powder and the amount of graphite added, the density and cooling rates typical for sintering furnaces, i. e. 0.5-2 C°/s lead to fully bainitic structures.
By decreasing the cooling rate below 0. 5°C/s and/or decreasing the amount of admixed graphite a microstructure consisting of a mixture of ferrite, perlite and bainite in different amounts can be obtained. By increasing the cooling rate above
2°C/s hardening, i. e. a microstructure consisting of more than 50 % martensite is obtained.
Best combination of strength and toughness is achieved when the microstructure of the sintered component mainly consists of lower bainite. This structure is achieved when the sintering is performed at above 1050°C with a carbon content of 0.55-1. 0 %.
Bainite consists of non-lamellar aggregates of ferrite and carbides. The principle variants of bainite in steels are called upper and lower bainite. The distinction between upper and lower bainite is based on whether the carbides are distributed between the individual ferrite regions (upper bainite) or within them (lower bainite). The diffusion rate of carbon during formation of lower bainite is so slow that the carbon atoms cannot move fast enough to avoid getting trapped inside the fast growing ferrite platelets. For a plain iron-carbon system the formation of upper bainite occurs above 350 °C. Below this temperature lower bainite is obtained. By addition of the alloying elements this temperature changes. The new powder makes it possibel to obtain sintered products including at least 50, preferably at least 70 % and most preferably at least 90 % of lower bainite in a simple and cost effective way.
Tables 6-8 show that when increasing the carbon content of the sintered product from about 0.2 %, tensile and yield strength increase and elongation and impact strength show a minimum at about 0.6 % of carbon.
Combined increase in tensile and impact strength is achieved above about 0.55 % of carbon when cooling at a cooling rate of 0. 8°C/s. This combined increase of strength and toughness in sintered products having a carbon content of about 0. 55%-1 % is unique for the material as it can be achieved with sintering in industrial scale in conventional sintering furnaces, such as mesh belt furnaces with or without rapid cooling units, pusher furnaces, roller furnaces or walking beam furnaces.
Sinterhardening Sinter hardening is a process which might be a powerful tool in reducing the costs.
New types of sintering furnaces allow low alloy steel parts to be sintered with neutral carbon potential (without decarburization or carburization) and then to be hardened in
a rapid cooling zone. The heat treatment is achieved by high speed circulation of a water cooled protective gas in the rapid cooling zone of the furnace with cooling rates of up to 7°C/sec achievable between 900°C and 400°C. This results in at least 50 % martensitic structure in the PM steels. In order to benefit by the advantages of the sinter hardening the selection of alloying system is of outmost importance.
The invention is further illustrated by the following non-limiting examples.
Example 1 This example illustrates the improvement in green density and green strength of compacted components when using the new powder compared with components compacted with a known powder according to US patent 6348080. Samples for determining green strength and green density were moulded at three different compacting pressure with the aid of external lubrication (die wall lubrication) and internal lubrication (zinc stearate and Advawax) according to table 1-4.
Table 1 Known powder Compaction Die wall Internal lubrication Pressure Lubrication (g/cm3) (MPa) Green Density Green Density (g/cm3) 0. 8 % Zn stearate 0. 6 AdvawaxTM 400 6. 43 6. 52 6. 65 600 6. 93 6. 96 7. 07 800 7. 25 7. 17 7. 24 From the following table 2 the corresponding results obtained with a powder accord- ing to the present invention. The powder consisted of 1.5 % by weight of Cr, 0.2 % by weight of Mo and 0.11 % by weight of Mn.
Table 2 New powder Compaction Die wall Internal lubrication Pressure Lubrication Green Density (MPa) Green Density (g/cm3) (g/cm3) 0. 8 % Zn stearate 0. 6 % AdvawaxTM 400 6. 55 6. 61 6. 75 600 7. 04 7. 02 7. 17 800 7. 32 7. 21 7. 38
From a comparison of the results listed in the tables 1 and 2 it can be seen that higher green densities are obtained with the new powder.
The following tables 3 and 4 disclose the corresponding green strengths for the known and new powders, respectively. The green strength which is obtained espe- cially when the new powder is compacted in a lubricated die is remarkably higher than when the previously known powder was used.
Table 3 Known powder Compaction Die wall Internal lubrication Pressure Lubrication Green Strength (MPa) (MPa) Green Strength (MPa) 0. 8 % Zn stearate 0. 6 % Advawax 400 11. 08 8. 76 20. 32 600 19. 92 13. 46 28. 98 800 27. 40 15. 25 27. 64 Table 4 New powder Compaction Die wall Internal lubrication Pressure Lubrication Green Strength (MPa) MPa Green Strength (MPa) 0.8 % Zn stearate 0. 6 % AdvawaxTM 400 21. 5 11. 3 19. 3 600 38. 2 17. 3 29. 5 800 53. 9 18. 8 32. 2
Example 2 This example discloses mechanical properties of samples produced from the new powder with addition of 1 % by weight of Cu. The powders which included 0.6% graphite were compacted at 600 MPa. The sintered density for the obtained material was about 6.95 g/cm3.
The samples were sinterhardened at a cooling rate of 2. 5° C/s. Tensile strength, yield strength, hardness and elongation were measured. Table 5 shows that the mechanical properties of samples prepared from the new powder including only 1 % Cu by weight are as good as the standard material FL 4608 according to USMPIF standard including 2 % of Cu.
Table 5 Added Cu Added TS YS HRC A (%) (%) graphite (%) (MPa) (MPa) 1 0. 6 923 784 30 0. 50 2 0. 6 863 682 33 0. 21 FL 4608 0. 6 900 787 27 0. 27
Example 3 The example illustrates the mechanical properties of samples prepared from the powder according to the invention with added graphite at an amount of 0.2 %. 0.4 %. 0.6 %. 0.8 % and 0.85 %, respectively.
Samples were compacted at 400 MPa, 600 MPa and 800 MPa respectively. 0.8 %, by weight, of ethylene-bis-stearamide was used as lubricant. The compaction was performed by a uniaxially compaction operation at ambient temperature. The samples were sintered at 1120° C for 30 minutes in an atmosphere of 90 % nitrogen, 10 % of hydrogen. Cooling was performed at a cooling rate of about 0. 5-1° C/s.
Sintered density (SD), tensile strength (TS), yield strength (YS), elongation (A), impact energy (IE), carbon content (C) and oxygen content (O) of the sintered samples were measured according to table 6-8.
Table 6-Compacting pressure 400 MPa Graphite SD TS YS A IE C microstucture % g/em Pa MPa % % 0. 2 6. 56 258 187 2. 94 10. 6 0. 21 Ferriteaperiite 0. 4 6. 55 50 366 1. 29 8. 3 0. 39 Upper bainite+perlite 0.6 6.55 574 474 0.82 8.6 0.58 Upper bainte+lower bainite 0.8 6.55 598 478 1.12 9.8 0.75 Lower bainite 0.85 6.5 599 481 1.01 10.3 0.80 Lower bainite Table 7-Compacting pressure 600 MPa Graphite SD TS YS A% IE C microstucture % g/cm3 MPa MPa % J % 0.2 6.97 331 225 4.89 22.5 0.18 Ferrite+perlite 0.4 6.94 561 443 1.90 14.7 0.37 Upper bainite+perlite 0.6 6.93 723 584 1.13 14.1 0.56 Upper bainite+lower bainite 0.8 6.91 758 603 1.47 16.6 0.75 Lower bainite 0. 85 6.90 737 567 1.50 16.1 0.77 Lower bainite Table 8-Compacting pressure 800 MPa Graphite % SD TS YS Il C microstucture % g/cm3 MPa MPa % J % 0.2 7.17 374 244 6.35 32.2 0.18 Ferrite+perlite 0.4 7.13 611 476 2.32 19.5 0.37 Uppe rbainite+perlite 0.6 7.10 779 627 1.13 16.0 0.56 Upper bainite+lower bainite 0.8 7.07 816 631 1.84 19.7 0.73 Lower bainite . 85 7. 06 813 621 1. 66 19. 2 0. 78 Lower bainite
Table 6-8 show that tensile strength, impact strength and elongation increase for samples with a carbon content of above about 0.5 %. This phenomena is due to the formation of low temperature bainite. The fact that low temperature bainite can be formed in this way makes the new powder unique.