EP0097737A1 - Powder metallurgy process for producing parts having high strength and hardness from Si-Mn or Si-Mn-C alloyed steel - Google Patents

Abstract

The invention relates to a method for the powder-metallurgical production of moldings of high strength and hardness from silicon-manganese or silicon-manganese-carbon alloyed steels. The previously known manufacturing processes for such steels have not brought the hoped-for results. The invention is intended to provide a method for the powder-metallurgical production of molded parts of high strength and hardness as well as sufficient toughness from sintered steels, in which only easily available alloy elements, the purchase of which is ensured in the long term, are used. The shaped parts are said to be able to be manufactured to size by the invention during the sintering process, i.e. they should not suffer any shrinkage and / or no loss of strength due to anti-shrinkage measures due to the addition of alloying elements. This is achieved in that the alloy elements Si and Mn or Si, Mn and C are made of ferrosilicon, ferromanganese or a silicon-manganese-iron master alloy with silicon and manganese contents in the ranges of 10 to 30% by weight. Si, 20 to 70% by weight Mn, remainder Fe and graphite in powder form are mixed with an iron powder and the powder mixture is pressed in a manner known per se and sintered and cooled at a temperature in the range from 1150 ° C. to 1250 ° C. under an inert gas atmosphere .

Description

The invention relates to a method for the powder-metallurgical production of moldings of high strength and hardness from silicon-manganese or silicon-manganese-carbon alloyed steels.

Since the strength of unalloyed sintered iron is relatively low (even at the highest density, only around 300 MPa is achieved), alloying measures must be taken to meet higher strength requirements.

The elements Cu, Ni, Mo, P and C are mainly used as alloying elements. A whole range of alloying elements that are successfully used in melt metallurgy can only be used to a limited extent in powder metallurgy. These are the alloy elements with an affinity for oxygen, such as Cr, Mn, Si and Ti, of which Si and Mn are particularly interesting because of their low price and their long-term availability.

Si is known as a solidifying solidifying agent. Significant increases in strength can also be achieved with Si in Fe-based alloys produced by powder metallurgy [Hoffmann, G., Thümmler, F., Zapf, G .; Sintering, Homogenization and Properties of a-Phase Iron-Aluminum and Iron-Silicon Alloys ; Powder Metallurgy, 3rd European Powder. Met. Symp. 1971 Conf. Supplement, Pt 1, 335-361J. However, there are two disadvantages contrary: On the one hand, Si tends to oxide formation due to its high affinity for oxygen in technical sintering atmospheres. This can be countered by using an Fe-Si pre-alloy as the alloy carrier [process for producing sintered materials based on iron; German Auslegeschrift 1 928 9301. However, sintered steels produced by this process are not suitable for molded parts, since during their sintering a strong shrinkage occurs, which has a negative effect on the dimensional accuracy of the parts. Attempts have been made to compensate for the shrinkage by alloying further elements (Cu, Al). However, this is only partially successful and is also associated with losses in the strength properties [see Hoffmann, G., Thümmler, F., Zapf, G.: Sintering, Homogenization and Properties of α-Phase Iron-Aluminum and Iron-Silicon Alloys _; Powder Metalurgy, 3rd European Powder. Met. Symp. 1971 Conf. Supplement, Pt. 1, page 1281.

Mn has also found its way into powder metallurgy as an alloying element. Since it is also very affinity for oxygen, special protective measures are also required here. It is known, for example, to introduce Mn over carbide master alloys [process for producing homogeneous manganese alloyed sintered steels; German patent specification 24 56 7811. These master alloys are stable up to the sintering temperature, which protects the alloy elements against oxidation. Since Mn is not itself a strong carbide former, these master alloys must be used inevitably contain carbide-forming elements such as Cr, Mo or V. These elements are very expensive, which greatly increases the alloying costs. The high hardness of the carbides also causes increased tool wear.

The common use of Si and Mn is mentioned in the literature. However, the sintered steels produced thereafter are said to have "no surprising results" [etchisen, G., Hewing, J .: sintered steels containing copper and nickel with further alloy additives; Industrie-Anzeiger Vol. 92 (1970) pages 241 - 244 u. 431 - 434, here: p. 434].

The invention has for its object to provide a method for powder metallurgical production of moldings of high strength and hardness and sufficient toughness from sintered steels, in which only more readily available alloy elements, the purchase of which is ensured in the long term, are used. The shaped parts are said to be able to be manufactured to size by the invention during the sintering process, i.e. they should not suffer any shrinkage and / or no loss of strength due to anti-shrinkage measures due to additions of alloying elements.

• von 0,3 bis 3 Gew.-% Si;
• von 0,3 bis 4 Gew.-% Mn;
• von 0 bis 0,5 Gew.-% C

entsprechen. In einer besonderen Ausbildung der Erfindung wird ein Massenverhältnis Mangan zu Silizium zwischen 1,5 und 3 verwendet. Vorteilhafterweise wird ein Ferrosilizium mit 15 Gew.-% Si und/oder ein Ferromangan mit 80 bis 84 Gew.-% Mn verwendet. Produkte, welche mit die nesten mechanischen Eigenschaften zeigen, werden erhalten, wenn eine Silizium-Mangan-Eisen-Vorlegierung der Zusammensetzung 19 Gew.-_Si 41 Gew.-% Mn und 40 Gew.-% Fe verwendet wird. In einer anderen Ausführung des erfindungsgemäßen Verfahren werden die gesinterten Formteile einer zusätzlichen Wärmebehandlung, bestehend aus Härten und Anlassen, unterzogen.The object is achieved by the method according to the invention in that the alloying elements silicon and manganese or silicon, manganese and carbon are used via the alloy carriers ferrosilicon, Ferromanganese or a silicon-manganese-iron master alloy with silicon and manganese contents in the ranges 10 to 30% by weight of Si, 20 to 70% by weight of Mn, remainder Fe and graphite in powder form are mixed with an iron powder and the powder mixture pressed in a manner known per se and sintered and cooled at a temperature in the range from 1150 ° C. to 1250 ° C. under a protective gas atmosphere. The alloy carriers and the graphite are mixed in such quantities that they represent the mass fractions in the powder mixture

• from 0.3 to 3 wt% Si;
• from 0.3 to 4 wt% Mn;
• from 0 to 0.5 wt% C

correspond. In a special embodiment of the invention, a mass ratio of manganese to silicon between 1.5 and 3 is used. A ferrosilicon with 15% by weight Si and / or a ferromanganese with 80 to 84% by weight Mn is advantageously used. Products showing the nest with mechanical properties when a silicon-manganese-iron master alloy 19 weight _S i is 41 wt .-% Mn and 40 wt .-% Fe used the composition will be obtained. In another embodiment of the method according to the invention, the sintered molded parts are subjected to an additional heat treatment consisting of hardening and tempering.

At sufficiently high cooling speeds, high strength and hardness are achieved with the simple sintering technique.

• - Sinterung und Homogenisierung nicht durch Oxidation der Legierungselemente beeinträchtigt werden,
• - eine homogene Verteilung der Legierungselemente bei technisch relevanten Temperatur/Zeit-Bedingungen für die Sinterung erreicht wird,
• - die Legierungselemente wirksam zur Erhöhung der Festigkeit führen,
• - die gefertigten Teile während der Sinterung eine möglichst geringe Maßänderung erfahren.

The method according to the invention has the effect that

• Sintering and homogenization are not impaired by oxidation of the alloy elements,
• a homogeneous distribution of the alloying elements is achieved under technically relevant temperature / time conditions for the sintering,
• - the alloying elements effectively increase the strength,
• - The manufactured parts experience the smallest possible dimensional change during sintering.

• - Niedrige Legierungskosten gegenüber herkömmlich legierten Sinterstählen und hohe Zugfestigkeiten (>600MPa). Nur wenige der bekannten relativ hoch legierten Zusammensetzungen erreichen Zugfestigkeiten von 700 MPa mit Einfachsintertechnik ohne zusätzliche Wärmebehandlung (z.B. Sinterstahl mit 4,5 % Cu, 5 % Ni : R = 600 MPa).
• - Die mechanischen Eigenschaften sind in einem Temperaturbereich von 1150 - 1250 C relativ unempfindlich gegenüber der Sintertemperatur. Dies ermöglicht z.B. eine exakte Abgleichung der Maßänderung durch Wahl einer geeigneten Sintertemperatur, ohne starke Änderung des mechanischen Verhaltens.
• - Die Anwesenheit von Si und Mn bewirkt eine so starke Härtbarkeitszunahme, daß bereits bei Abkühlung aus der Sinterhitze ein Vergütungsgefüge entsteht, wodurch sich eine weitere Wärmebehandlung erübrigt. Solche Sinterstähle wurden bisher unter Verwendung der teuren Legierungselemente Ni und Mo hergestellt.

During the homogenization, a temporary liquid phase already occurs at temperatures of around 1000 ^° C., which is why oxide layers that may be present on the alloy particles cannot become a diffusion barrier and, moreover, the alloy elements are rapidly distributed. The addition of Mn and C compensates for the shrinkage previously caused by Si, so that overall there are relatively minor dimensional changes or dimensional stability. Si and Mn act as mixed crystal hardeners. Mn also strongly inhibits the tendency to change and Si influences the C diffusion, so that the combination of the alloying elements Si-Mn- (C) leads to high strength values. A tempering structure with a hardness of up to 300 HV20 can be achieved with an average cooling rate of 30 K / min from the sintering heat, i.e. without additional heat treatment, even when the furnace is cooled. If the cooling is slow enough, a hardening effect can be suppressed, so that the parts can be calibrated. The potential of the alloy elements used can then be harnessed by additional heat treatment.

• - Low alloy costs compared to conventionally alloyed sintered steels and high tensile strengths (> 600MPa). Only a few of the known relatively high-alloy compositions achieve tensile strengths of 700 MPa with simple sintering technology without additional heat treatment (for example sintered steel with 4.5% Cu, 5% Ni: R = 600 MPa).
• - The mechanical properties are relatively insensitive to the sintering temperature in a temperature range of 1150 - 1250 C. This enables, for example, an exact adjustment of the dimensional change by choosing a suitable sintering temperature without a major change in the mechanical behavior.
• - The presence of Si and Mn causes such a strong increase in hardenability that a tempering structure already arises when the sintering heat cools, making further heat treatment unnecessary. Such sintered steels have hitherto been produced using the expensive alloy elements Ni and Mo.

The method according to the invention is explained in more detail below on the basis of a few exemplary embodiments in conjunction with the following FIGS. 1 to 8.

)Example 1:

Different amounts of Si, Mn and C were made into a common iron via the alloy carriers mentioned powder added and mixed together with a conventional press-relieving additive. The powder mixture was pressed at a pressure of 600 MPa into test bars which were sintered for 60 minutes at a temperature of 1180 ⁰ C in a hydrogen atmosphere. The mechanical properties of some selected compositions are shown in Figures 1 and 2 (R _m = tensile strength in MPa, R _p0.1 = yield strength in MPa). Optimal combinations of properties are achieved with mass fractions of 1 to 2% by weight of Si, 2 to 3% by weight of Mn, 0.2 to 0.3% by weight of C. The change in length of the test rods caused by the sintering can be seen from FIG. Only when the Si / Mn> 2 ratio does the shrinkage increase sharply.

With the same Si and Mn contents, tensile strengths of 350 to 6 ₀ 0 _MP a, hardnesses of 100 to 210 HV20, and elongations at break of 11 to 2% are achieved without carbon. These alloys are also dimensionally stable in certain ranges (for example 2% Si, 2 to 4% Mn) under the specified production conditions.

Example 2:

With a composition with an optimal combination of properties, the sintering temperature was varied under the same conditions as in Example 1. Mechanical properties and dimensional changes are shown in FIGS. 4 and 5. Strength and yield strength are almost independent of the sintering temperature in the area examined, and hardness and elongation at break are also not sensitive. With increasing sintering temperature, this initially rises slight swelling recedes until even slight shrinkage occurs. This opens up the possibility of adjusting the dimensional behavior via the sintering temperature without significantly changing the mechanical properties (R = tensile strength in MPa; R _p0.1 = yield strength in MPa; A = elongation at break in%).

Example 3:

Parts that need to be calibrated require very slow cooling from the sintering heat. Strength and hardness then do not reach the values shown in Examples 1 and 2. However, the alloy elements used enable heat treatment.

The alloy used in Example 2 was sintered in hydrogen at 1200 ° C. for 60 minutes and then tempered. Tempering treatment: Austenitized at 1000 C, 60 min. In argon, quenched in oil, annealed 60 min. In argon at the specified temperatures (see Figure 6). Optimal strength and hardness values are achieved at tempering temperatures of 200 to 300 ° C, the elongation at break is still measurable (> 1%).

Example 4:

• Zunächst wurde eine Vorlegierung hergestellt mit folgender Zusammensetzung:
  

Production of alloys using a Si-Mn-Fe pre-alloy:

• First, a master alloy was made with the following composition:
  

The master alloy was melted in a vacuum furnace and, after cooling, mechanically comminuted to a particle size below 25 μm (average particle size about 10 μm). The comminution can take place in a crusher, in a vibrating disk mill or in a ball mill.

Different amounts of this master alloy produced in this way were mixed on the one hand without added carbon, on the other hand with added carbon in the form of graphite-iron powder mixtures and processed in the usual way. A pressing pressure of 600 MPa, a sintering temperature of 1180 ° C, a sintering time of one hour and a hydrogen atmosphere were used as the protective gas atmosphere.

The mechanical properties and the dimensional changes of the silicon-manganese or silicon-manganese-carbon alloyed sintered steels produced according to this are shown in FIGS. 7 and 8. Figure 7 shows the optimization of the content of the master alloy used, an optimal content of about 8 wt .-% (these are manganese and silicon contents in the sintered alloy of 3.3 wt .-% manganese and 1.5 wt .-% silicon) was determined.

FIG. 8 shows the influence of the carbon content in the optimal alloy (8% by weight pre-alloy in sintered steel).

• - Bessere Verdichtung der Pulvermischungen, die die Vorlegierung enthalten, woraus sich eine geringere Porosität der Teile ergibt,
• - sehr gleichmäßige Verteilung der Legierungselemente, da die Legierungsträger als feinste Teilchen vorliegen,
• - gleichmäßiges Sinterverhalten der Legierungsträger durch deren gleichbleibende Zusammensetzung.

As FIGS. 7 and 8 show, sintered steels with excellent combinations of properties are obtained with Si-Mn-Fe master alloys. Tensile strengths and yield strengths can be achieved which are even higher than those of the sintered steels produced using ferrosilicon and ferromanganese as alloy carriers. Further improvements in the sintered steels according to the invention may possibly be achieved if the final one. Heat treatment and the use of double sintering technology can be optimized. The better properties are due to:

• Better compaction of the powder mixtures containing the master alloy, which results in lower porosity of the parts,
• very uniform distribution of the alloy elements, since the alloy carriers are present as very fine particles,
• - Uniform sintering behavior of the alloy carriers due to their constant composition.

Dimensional stability can be achieved by suitably adjusting the composition and sintering temperature.

The Si-Mn-Fe master alloys in the specified ranges for the silicon and manganese content also lead to the formation of a liquid phase in the specified range of technical sintering temperatures during the sintering process, which also contributes to the substantial improvement of the properties of the invention sintered steels is responsible. Lower levels of manganese and silicon require too high proportions of a master alloy to achieve optimum alloy levels. This makes the powder mixtures more difficult to press, which leads to a decrease in density. Higher contents, particularly of silicon, no longer lead to the formation of a liquid phase in the range of the sintering temperatures up to approx. 1250 ° C.

• Die Vorlegierungen sind relativ einfach herzustellen, ihre Schmelztemperatur ist verhältnismäßig niedrig, die Gußblöcke sind äußerst spröde und daher sehr einfach und bis zu hoher Feinheit mechanisch zu zerkleinern. Das Sinterverhalten wird bei Anwendung von Vorlegierungen günstiger.

The advantages of the specified master alloys are as follows:

• The master alloys are relatively easy to manufacture, their melting temperature is relatively low, the casting blocks are extremely brittle and therefore very easy to mechanically comminute to high fineness. The sintering behavior becomes more favorable when using master alloys.

Legend for the figures:.

• Kurve 1: Fe + 3Mn + 0,25C
• Kurve 2: Fe + 2Mn + 0,20C

Fig. 1: tensile strengths (R) of some alloys depending on the silicon content:

• Curve 1: Fe + 3Mn + 0.25C
• Curve 2: Fe + 2Mn + 0.20C

• Kurve 3: Fe + 3Mn + 0,25C
• Kurve 4: Fe + 2Mn + 0,20C

Elongation at break (A) depending on the silicon content:

• Curve 3: Fe + 3Mn + 0.25C
• Curve 4: Fe + 2Mn + 0.20C

• Kurve 5: Fe + 3Mn + 0,25C
• Kurve 6: Fe + 2Mn + 0,20C

Fig. 2: _Strain limits (R _p0.1 ) depending on the silicon _content :

• Curve 5: Fe + 3Mn + 0.25C
• Curve 6: Fe + 2Mn + 0.20C

• Kurve 7: Fe + 3Mn + 0,25C
• Kurve 8: Fe + 2Mn + 0,20C

Hardness values (Vickers hardness HV20) depending on the silicon content:

• Curve 7: Fe + 3Mn + 0.25C
• Curve 8: Fe + 2Mn + 0.20C

• Kurve 9: Fe + 3Mn + 0,25C
• Kurve 10: Fe + 2Mn + 0,20C
• Kurve 11: Fe + 1 Mn + 0,20C

Fig. 3: Dimensional changes (Δl / l _o ) of different alloys depending on the silicon content:

• Curve 9: Fe + 3Mn + 0.25C
• Curve 10: Fe + 2Mn + 0.20C
• Curve 11: Fe + 1 Mn + 0.20C

• Dehngrenzen (Kurve 13),
• Härtewerte (Kurve 14) und

Fig. 4: tensile strengths (curve 12)

• Yield strengths (curve 13),
• Hardness values (curve 14) and

Elongation at break (curve 15) of the same alloy (Fe + 1 Si + 3Mn + 0.25 C) depending on the sintering temperature used in the production.

Fig. 5 dimensional changes (curve 16) of one and the same alloy (Fe + 1Si + 3Mn + 0.25C) depending on the sintering temperature used in the production.

Fig. 6: tensile strengths (curve 17), hardness values (curve 18) and elongations at break (curve 19) of one and the same alloy (Fe + 1Si + 3Mn + 0.25C) depending on the tempering temperature.

Fig. 7: tensile strengths (R _m ): curve 20, yield strengths (R _p0.1 ): curve 21, hardness values (HV ₂₀ ): curve 22, elongations at break (A): curve 23 and dimensional changes (Δ1 / 1 _o ): curve 24 different Si-Mn-alloyed sintered steels (without C addition) depending on the content of the master alloy (19 Si-41Mn-40Fe) in the powder mixture to be sintered.

Fig. 8: Tensile strengths: curve 25, yield strengths: curve 26, hardness values: curve 27, elongation at break: curve 28 and dimensional changes: curve 29 of sintered steels with 8% by weight of pre-alloy (19Si-41Mn-40Fe) depending on their C- Salary.

Claims (7)

1. A process for the powder-metallurgical production of moldings of high strength and hardness from silicon-manganese or silicon-manganese-carbon-alloyed steels, characterized in that the alloying elements silicon and manganese or silicon, manganese and carbon are used via the alloy carriers ferrosilicon, ferromanganese or a silicon-manganese-iron master alloy with silicon and manganese contents in the ranges 10 to 30% by weight of Si, 20 to 70% by weight of Mn, remainder, Fe and graphite in powder form, mixed with an iron powder and the powder mixture in compressed manner known per se and is sintered at a temperature in the range of 1150 ° C to 1250 ⁰ C under a protective gas atmosphere and cooled. 2. The method according to claim 1, characterized in that the admixture of the alloy carrier and the graphite is carried out in such amounts that they are the mass fractions in the powder mixture

correspond. 3. The method according to claim 1, characterized in that a mass ratio of manganese to silicon between 1.5 and 3 is used. 4. The method according to claim 1, characterized in that a ferrosilicon with 15 wt .-% Si is used. 5. The method according to claim 1, characterized in that a ferromanganese with 80 to 84 wt .-% Mn is used. 6. The method according to claim 1, characterized in that a silicon-manganese-iron pre-alloy of the composition 19 wt .-% Si 41 wt .-% Mn 40 wt .-% Fe is used. 7. The method according to claim 1, characterized in that the sintered moldings are subjected to an additional heat treatment consisting of hardening and tempering.

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