MANGANESE CONTAINING MATERIALS HAVING HIGH TENSILE STRENGTH

Abstract: The invention concerns an iron-based powder for power-metallurgically producing components by powder compacting and sintering. The powder contains 0.25-2.0 % by weight of Mo, 1.2-3.5 % by weight of Mn, 0.5-1.75 % by weight of Si and 0.2-1.0 % by weight of C and not more than 2 % by weight of impurities including less than 0.25 % by weight of Cu. The invention also includes a method for preparing sintered components from this iron powder, as well as the sintered products.

Description: MANGANESE CONTAINING MATERIALS HAVING HIGH TENSILE STRENGTH The present invention relates to an iron-based pow¬ der for producing components by compacting and sinter¬ ing. Specifically the invention concerns powder composi¬ tions which are essentially free from nickel and which, when sintered, give components having valuable proper¬ ties, such as high tensile strength. The components can be used in e.g. the car industry. The invention also concerns a powder-metallurgically produced component of this powder as well as a method of powder-metallurgi- cally producing such a component. Nickel is a relatively common alloying element in iron-based powder compositions in the field of powder metallurgy, and it is generally known that nickel im¬ proves the tensile strength of the sintered components which have been made by iron powders containing up to 8 % of nickel. Additionally, nickel promotes sintering, increases the hardenability and has a positive influence on the elongation at the same time. A currently marketed powder, the use of which re- suits in products having properties similar to those ob¬ tained with the product according to the present inven¬ tion, is Distaloy®AE, which contains 4% per weight nickel. There is however an increasing demand for powders which do not contain nickel as, for instance, nickel is expensive, creates dusting problems during the process¬ ing of the powder, causes allergic reactions in minor amounts. From an environmental point of view the use of nickel should thus be avoided. An object of the present invention is thus to pro¬ vide a nickel-free powder composition having, at least in some respects, essentially the same properties as compositions containing nickel. A second object is to provide a low-cost, environ¬ mentally acceptable material. A third object is to provide sintered products which after both low and high temperature sintering have tensile strength values superior to those obtained with Distaloy®AE. According to the present invention, metal powders which, in addition to iron, contain 0.25 - 2.0 % by weight of Mo, 1.2 - 3.5 % by weight of Mn and 0.5 - 1.75 % by weight of Si, 0.2 - 1.0 % by weight of C and 2 % by weight of impurities exhibit very interesting proper¬ ties. Thus, tensile strengths up to 1200 MPa can be ob¬ tained, when the metal powders according to the inven¬ tion are compacted and then sintered at high tempera- tures. A preferred iron-based powder composition according to the invention contains 0.5-2 % by weight of Mo, 1.2-3 % by weight of Mn, 0.5-1.5 % by weight of Si, 0.3-0.9 % by weight of C, and less than 2 % by weight of impuri- ties including less than 0.25 % by weight of Cu. In ad¬ dition to Cu, the impurities can consist of Cr, Ni, Al, P, S, 0, N, Be, B etc. in amounts less than 0.5 % by weight, respectively. Mo might be used as metal powder, partially pre-al- loyed with Fe or prealloyed with Fe. When Mo is added to the iron powder, the hardenability of the compressed ma¬ terial increases and it is recommended that the amount of Mo should be at least 0.25 % by weight. As, however, increasing amounts of Mo result in decreased compressi- bility and, accordingly, decreased density, the amount of Mo should preferably be less than about 2.0 % by weight. Furthermore, too high amounts of Mo, especially in combination with high amounts of C, make the sintered material hard and brittle and the strength of the mate- rial will decrease. Mo is preferably added in the form of a prealloyed base powder, which makes it possible to obtain a more homogenous microstructure consisting of bainite and martensite in the sintered material. According to an especially preferred embodiment of the invention, Mo is added in the form of Astaloy Mo or Astaloy 85 Mo (available from Hoganas AB, Sweden) which contain 1.5 and 0.85 % Mo, respectively. Mn and Si improve the hardenability. Suitably these elements are added in amounts above 1.2 and 0.5 % by weight, respectively. However, too high amounts of Mn and Si, such as above 3.5 and 1.75 % by weight, respec¬ tively, result in decreased compressibility and can cause oxidation problems. High amounts of Mn and Si in a prealloyed base powder have a strong solution-hardening effect whereas these elements added in elementary form have a high affinity to oxygen. If, however, these elements are added in the form of a master, their affinity to oxygen is reduced and they become less sensitive to oxidation. Thus, according to a preferred embodiment of the invention, Mn and Si are added in the form of an Fe-Mn-Si-master consisting of 10-30% by weight of Si, 20-70% by weight of Mn, the balance being Fe and having a weight ratio Mn/Si between 1 and 3. Such a master may mainly consist of, for example, (Fe,Mn)3Si and (Fe,Mn)5Si3 and is disclosed in EP 97 737. The master alloy also gives an improved com¬ pressibility and the microstructure of the sintered ma¬ terial becomes more homogenous, due to the fact that, during sintering, the Fe-Mn-Si-master forms a transient liquid phase which accelerates sintering, facilitates diffusion, increases the amount of martensite and makes the pores rounder. With the master alloy it is possible to avoid the large shrinkage normally caused by silicon and get a dimensional change close to zero. Alterna- tively, Mn and Si can be added in the form of ferro-man- ganese and ferrosilicon. If the amount of C, which is normally added as a graphite powder, is less than 0.2%, the tensile strength will be too low, and U the amount of C is above 1.0%, the sintered component will be too brittle. Components prepared from compositions according to the present in¬ vention wherein the C content is relatively low exhibit good ductility and acceptable tensile strength, whereas products prepared from compositions having higher amounts of C have lower ductility and increased tensile strength. The graphite addition has to be made with re¬ spect to the sintering atmosphere. The more hydrogen in the atmosphere the more graphite has to be added due to greater decarburization. As some carbon normally disap¬ pears during sintering, the carbon content of the sin¬ tered product will be somewhat less than the carbon con¬ tent of the iron-based powder. Thus, the carbon content of the sintered products normally varies between 0.15 and 0.70 % by weight. As possible impurities Ni, Cu and Cr may be men¬ tioned. These elements can be present in amounts less than 0.25 % by weight, respectively, but should prefer¬ ably be present only as traces, i.e. up to 0,1 % by weight of the composition. Other possible impurities are Al, P, S, 0, N, Be, B in amounts as indicated in the claims. The total amount of impurities should be less than 2 % by weight but is preferably less than 1% by weight. The influence of the addition of different amounts of Mo, Mn/Si and C is disclosed in Figs 1, 2 and 3, re¬ spectively. In addition to the iron-based powders, the present invention also concerns methods of producing components by using these new powders as well as the components produced. The powder-metallurgical method is carried out in a conventional way known to the man skilled in the art and includes the steps of compacting, sintering and optionally recompacting and sintering and/or quenching and tempering of the powder. The compacting step could be carried out both as a cold and warm compacting step and the sintering step could be carried out as low-tem¬ perature sintering as well as high-temperature sinte¬ ring. The sintering atmosphere as well as the sintering times have an impact on the pro-perties of final product as is well known in the art. In this context it can be mentioned that WO 80/01083 discloses alloy steel articles having a compo¬ sition similar to the composition of the present pro¬ ducts. These known products are, however, conventional, wrought, pore free products prepared by casting. A special subsequent heat treatment, auste pering is made in order to obtain products having a substantially comp¬ lete bainite structure. In addition to the ranges of the alloying elements, These known products differ from the product prepared according to the present invention in several respects, such as the type of starting mate¬ rials, the process routes and the microstructure. It has quite unexpectedly been found that materials having tensile strengths up to about 1200 MPa can be ob- tained by using the new iron-based compositions. These remarkably high values can be obtained, for instance, by high temperature sintering between about 1200°C and 1280°C for periods of about an hour in hydrogen atmos¬ phere. Noteworthy is also the fact that the compressed bodies made of the iron-based powder according to the present invention, when subjected to low-temperature sintering, i.e. sintering between 1110°C and 1150°C, are also distinguished by very high tensile strengths up to 1000 MPa. It has also been observed that unexpextedly high tensile strengths are obtained at relatively mode¬ rate densities, such as 6.8 - 7.0 g/cm 3 . Additionally, it has been found that the new compositions exhibit good stability in dimensional change at different densities. In brief, the high tensile strength of the sintered products according to the invention in combination with the low cost of the powder and modest influence on the environment makes the present invention especially in¬ teresting. The invention is described more in detail in the following example. EXAMPLE Different alloying compositions with respect to Mo, Mn, Si and C have been tested. A master with a composi¬ tion of 45% Mn, 21% Si and Fe balance has been used in the following trials. The master alloy, graphite and in some trials also Mo powder were admixed to ASC100.29, Astaloy 85 Mo or Astaloy Mo. Tensile testbars were com¬ pacted at 600 MPa followed by sintering at 1250°C for 30-60 minutes in a mixed hydrogen nitrogen atmosphere. Optimal strength properties were achieved at molybdenum contents of 0.25-2.0%, Fig. 1. The hardenability is too low at small additions whereas the density becomes too low at higher molybdenum additions. The Mo content is preferably between 0.5 and 2%. In addition to iron and different amounts of Mo, the tested powder contained 2.8% Mn, 1.2% Si and 0.7% graphite. Smaller master additions result in a low harden¬ ability of the material and thereby a low strength. High alloying additions lead to a large volume of master which decreases the compressibility and will also result in increased swelling of the material. The strength will thereby decrease due to the lower density. The manganese and silicon additions are optimal between 1 and 3.5% Mn and between 0.5 and 1.75% Si, respectively, Fig 2. In addition to iron and varying amounts of Mn, Si, the tested powder included 0.85% Mo and 0.7% graphite. The analysed carbon content depends on the amount of graphite added and also on which sintering atmosphere that has been used. The higher hydrogen content used the larger decarburisation. The carbon content of the sin- tered product is optimal between 0.15 and 0.7%, Fig. 4. In these trials this corresponds to 0.3-0.9% graphite in the powder composition. Fig. 3. The tested iron-based powder contained 0.85% Mo, 1.8% Mn, 0.8% Si and varying amounts of graphite. The strength of the material is increased by in¬ creasing sintering temperature and time. This is mainly due to a better diffusion of the admixed alloying ele¬ ments, which improves the hardenability and thereby the strength of the material. This effect can be seen in Fig. 5 for a powder consisting of iron, 0.85% Mo, 1.8% Mn, 0.8% Si and 0.5-0.7% graphite. The dimensional change at different densities is stable for the newly developed material. This is a great benefit when producing components having a great inter- nal density variation. It becomes easier to keep narrow tolerances by using a dimensionally stable material. Fig. 6 discloses the variation of the dimensional change for Fe-0.85Mo-l.8Mn-0.8Si- (0.6-0.7C) compacted at 400, 600 and 800MPa. Sintering was performed at 1120°C and 1250°C. The variation in dimensional change is 0.03% and 0.12%, respectively, in the density range 6.6-7.1 g/cm 3 .

Claims: CLAIMS 1. An iron-based powder for producing porous com¬ ponents by powder compacting and sintering, comprising in addition to iron 0.25 - 2.0 % by weight of Mo 1.2 - 3.5 % by weight of Mn 0.5 - 1.75 % by weight of Si 0.2 - 1.0 % by weight of C and not more than 2, preferably not more than 1 % by weight of impurities including less than 0.25 % by weight of Cu 2. A powder according to claim 1, c h a r a c ¬ t e r i s e d in that the amount of Mo is 0.5 - 2.0 % by weight. 3. A powder according to claim 1 or 2, c h a - r a c t e r i s e in that the amount of Mn is 1.2 - 3.0 % by weight. 4. A powder according to anyone of claims 1-3, c h a r a c t e r i s e d in that the amount of Si is 0.5 - 1.50 % by weight. 5. A powder according to anyone of claims 1-4, c h a r a c t e r i s e d in that the amount of C is 0.3 - 0.9 % by weight. 6. A powder according to anyone of claims 1-5, c h a r a c t e r i s e d in that Mn and Si are added in the form of ferromanganese, ferrosilicon or a silicon- manganese-iron master. 7. A powder according to claim 6, c h a r a c ¬ t e r i s e d in that the weight ratio manganese/silicon of the silicon-manganese-iron master varies between 1 and 3. 8. A powder according to anyone of the preceding claims, c h a r a c e r t e r i s e d in that Mo is ad¬ ded in the form of a prealloy of Fe and Mo. 9. A powder according to anyone of the preceding claims, c h a r a c t e r i s e d in that it includes inevitable impurities in the following ranges Cr < 0.25 Cu < 0.25 Ni < 0.25 Al < 0.20 P < 0.05 S < 0.05 0 < 0.03 N < 0.02 Be < 0.01 B < 0.02 Others < 0.5 10. A powder-metallurgically produced porous compo¬ nent, which in addition to iron comprises 0.25 - 2.0 % by weight of Mo 1.2 - 3.5 % by weight of Mn 0.5 - 1.75 % by weight of Si 0.15 - 0.70 % by weight of C and not more than 2 by weight of impurities including less than 0.25 % by weight of Cu. 11. A method of powder-metallurgically producing sintered porous components, c h a r a c t e r i s e d by using an iron-based powder comprising 0.25 - 2.0 % by weight of Mo 1.2 - 3.5 % by weight of Mn 0.5 - 1.75 % by weight of Si 0.2 - 1.0 % by weight of C and not more than 2 by weight of impurities including less than 0.25 % by weight of Cu; compacting the powder into the desired shape and sintering the compact at a temperature above 1120°C.

Inventors: LINDBERG CAROLINE, LINDBERG, CAROLINE
Applicants: HOEGANAES AB, LINDBERG CAROLINE, LINDBERG, CAROLINE
Application year: 1995
Main classification: B22F1
IPC codes: B22F1, C22C33, C22C38