WO1990006198A1

WO1990006198A1 - Iron-based powder for producing sintered components

Info

Publication number: WO1990006198A1
Application number: PCT/SE1989/000712
Authority: WO
Inventors: Björn Lindqvist; Patrick Mcgeehan
Original assignee: Höganäs Ab
Priority date: 1988-12-06
Filing date: 1989-12-06
Publication date: 1990-06-14
Also published as: CA2004625A1

Abstract

An iron-based powder prealloyed during atomisation contains dissolved Mo for producing, under normal compacting and sintering conditions in powder metallurgical production, precision-moulded components with good dimensional accuracy, hardenability and strength, as well as high density.

Description

IRON-BASED POWDER FOR PRODUCING SINTERED

COMPONENTS

The present invention relates to an iron-based powder prealloyed during atomisation and intended for use in the powder metallurgical production of precision-moulded com¬ ponents with good dimensional accuracy, hardenability and strength, as well as high density.

Above all in the car industry, there is a demand for sintered components of reduced weight but with intact or improved strength. To meet this demand, new powder metal¬ lurgical alloys are being developed which often have a higher density and better homogeneity.

About 90% of all damage to sintered goods is caused by fatigue fractures. These are best avoided by surface hardening, since most fractures are initiated in the sur¬ face. The hardening induces compressive strain in the sur¬ face layer to check crack growth. The core, which is more ductile, is subjected to relatively low tensile strain which, however, is of practically no consequence.

The alloying components used today for surface har¬ dening of powder metallurgical materials mainly comprise Ni, Cu, Mo, C and, to some extent, Cr, Mn and N. The ele¬ ments C and/or N are usually added during a heat-treatment operation after sintering.

There are two common types of alloying powder, namely powder mixtures and so-called atomised powders.

The powder mixtures are prepared in that a powder of the alloying substance, either in the form of a metal, or a compound degradable during the sintering process, is ad¬ mixed to the iron powder. The so-called atomised steel powders are produced in that a melt of steel containing the desired alloying components is decomposed into powder. One disadvantage of powder mixtures is, however, the risk of segregation which is due to the fact that powders ^• with different characteristics, e.g. particle size, are intermixed without being mechanically linked to one an- other. This segregation results in a varying composition of the compacts produced from the powder mixture, and therefore also in varying dimensional changes when these compacts are being sintered. Another disadvantage of pow- der mixtures is their tendency to give off dust, especial¬ ly when the alloying substance has a very fine particle size. Naturally, this may cause serious environmental pro¬ blems.

There is, on the other hand, no risk of segregation for the atomised powders, since every particle thereof has the desired alloy composition, and furthermore the risk of dusting is not very great since there is no alloying sub¬ stance of small particle size. The prealloyed, atomised powders suffer, however, from another major disadvantage, namely their low compressibility which is due to the solu¬ tion hardening effect of the alloying substances on every powder particle.

A high compressibility is essential when one wishes to obtain a component of high density; a prerequisite of high strength. The compressibility of a powder mixture, on the other hand, is practically identical with that of the constituent iron powder.

The addition of Ni and Cu to a powder mixture in¬ creases the hardenability of the sintered material, but the slow diffusion of, above all, Ni gives the end product a very heterogeneous Ni distribution with areas of soft, residual austenite, because Ni stabilises the austenitic phase. Cu diffuses slightly better than Ni in austenite, and it furthermore melts, which means that a more homoge- neous distribution of Cu than of Ni is obtained in the sintered material. An addition of Cu causes a swelling, and consequently a lower density, of the resulting sin¬ tered compact.

When Mo, which substantially increases the hardenabi- lity, is added, the sintering is more effective than when Cu or Ni is added, because Mo stabilises ferrite, and in the Mo/Fe boundary layer, the sintering is more rapid. However, to obtain a homogeneous Mo distribution in the material, a high-temperature sintering during a very long time is required, which is very expensive. It takes even longer to level out alloy variations of Ni and Cu than of MO.

When Mn and Cr are added, a very careful process con¬ trol and very pure process gases are required in high-tem¬ perature sintering, so as to avoid an oxidation of Cr or Mn and, consequently, a loss of the hardenability-increas- ing effect of the dissolved Mn and the nitride effect of Cr.

Hence, to obtain a homogeneously alloyed material, an atomised powder is needed in which the above-mentioned, heavier alloying components are present in solid solution, i.e. prealloyed in the molten phase.

The object of the present invention is to provide an iron-based powder for producing sintered components with good dimensional accuracy, hardenability and strength, as well as high density, said powder having none of the above-mentioned disadvantages of segregation, dusting and low compressibility, due to the solution hardening effect. The present invention has now shown that if the mate¬ rial is prealloyed with Mo only, the compressibility is merely slightly affected, compared to when pure Fe powder is used, despite the fact that the alloyed (substitution- dissolved) Mo has a much larger atomic size than Ni, and therefore should produce a higher increase in hardness (Example 1). A minimum amount of 0.5% by weight Mo is re¬ quired to obtain a surface hardness sufficient for prac- tical use. 2.5% by weight molybdenum constitutes the upper limit for the amount of Mo which can be prealloyed, if the density requirements concerning the finished component are to be met in pressing with compacting pressures common within powder metallurgical production. Furthermore, a content higher than 2.5% by weight entails a more substan¬ tial shrinkage during sintering, and therefore also a poorer dimensional accuracy of the finished components. The upper limit is 2.5% by weight Mo for reasons of com¬ pressibility, dimensions, stability and costs. According to the invention, the preferred amount of Mo is 0.75-2% by weight. At these values, excellent compressibility, sur- face hardness and hardenability are obtained. An important factor in this context is that Mo is the only alloying substance present in the prealloyed powder, since the pow¬ der otherwise would obtain a lower compressibility. There¬ fore, the amount of inevitable impurities must be kept at a low level. The sum of Mn, Cr, Si, Cu, Ni and Al must not exceed 0.4% by weight, the amount of Mn being 0.25% by weight at the most. Furthermore, the amount of C must not exceed 0.02% by weight. The powder is produced by atomisa¬ tion of a pure Fe material with 0.5-2.5% by weight Mo into a powder having a maximum particle size of 212 μm, mainly below 150 μm. Thereafter, the powder is annealed at a tem¬ perature of 700-1200°C in a reducing atmosphere. Example 1

Four steel powders A-D having the same particle size were produced by atomisation of a melt containing the me¬ tallic alloying components. Then, a lubricant was admixed to the powders, whereupon these were compacted. The densi¬ ty of the resulting compacts was determined as a function of the compacting pressure.

Powder composition:

As is apparent from Fig. 1, powder B, which has been prealloyed with molybdenum, only has a marginal effect on the compressibility compared to a pure iron powder, in this Example powder A. Powder C, which has been prealloyed with Ni, has a much more negative effect on the compressibility, despite the fact that literature as well as known relation¬ ships of ultimate strength, hardness and compressibility indicate that the use of nickel should be more favourable if one wishes to avoid a lowering of the compressibility (see Fig. 2). Powder D, which is a powder available on the market and intended for the production of hardened compo¬ nents, has a considerably poorer compressibility than pow¬ der B. Example 2

Three powders E-G were compacted and hot-forged to full density, the purpose being to examine the hardenabi¬ lity of the materials at one and the same density level. The samples produced were cylinders having a diameter of 25 mm and a height of 25 mm. The forged samples were car- burised for 30 min. and at 890°C in endogas with a carbon potential corresponding to a carbon content of 0.8% by weight, whereupon the samples were quenched in oil of 60°C. Powder composition:

(admixed as graphite)

(admixed as graphite)

As is apparent from Fig. 3, powder G has a very high surface hardness compared to powders E and F, a character¬ istic of a good material for surface hardening operations.

Fig. 4 shows the hardening profile of the same mate¬ rials as above. As can be seen, powder G not only has a high surface hardness, but also a much greater hardening depth than the powders E and F. A great hardening depth is a property characterising a material suited for sur¬ face hardening operations.. Example 3

Fig. 5 illustrates the compressibility in the form of density obtained at a compacting pressure of 410 MPa for an atomised steel powder alloyed with 1.5% by weight molybdenum as a function of the carbon content and the manganese content.

As is apparent from this Example, the amount of dis¬ solved carbon greatly affects the compressibility and should therefore be as low as possible. Furthermore, the manganese content has a negative effect on the compress¬ ibility, in particular when exceeding 0.20-0.25% by weight.

Claims

1. An atomised, prealloyed and iron-based powder for powder metallurgical production of precision-moulded com¬ ponents with high hardenability, high density, good dimen¬ sional accuracy and high strength, c h a r a c t e r ¬ i s e d in that it contains, as alloying component, dis¬ solved molybdenum to an amount of 0.2-2.5% by weight.

2. Atomised powder as claimed in claim 1, c h a ¬ r a c t e r i s e d in that the dissolved molybdenum amounts to 0.75-2% by weight.

3. Atomised powder as claimed in claim 1 or 2, c h a r a c t e r i s e d in that the amount of carbon is less than 0.02% by weight.

4. Atomised powder as claimed in claims 1-3, c h a ¬ r a c t e r i s e d in that the maximum amount of the im¬ purities manganese, chromium, silicon, copper, nickel and aluminium is 0.4% by weight.

5. Atomised powder as claimed in claim 4, c h a ¬ r a c t e r i s e d in that the maximum amount of manga¬ nese is 0.25% by weight.

6. Atomised powder as claimed in any one of the pre¬ ceding claims, c h a r a c t e r i s e d in that the particle size is less than 212 μm, preferably 150 μm.