AT527435A2 - Process for the production of alloyed sintered steel - Google Patents

Abstract

Method for producing a group 6 element and/or group 5 element alloyed sintered steel, comprising the steps: • Providing a powder mixture comprising o 1 to 6 wt. % of a powdered master alloy, wherein the master alloy has a melting point of max. 1300 °C, o 0.1 to 2 wt. % of powdered group 6 and/or group 5 element as elemental powder or as ferroalloy powder or as carbide powder, o 0.1 to 1.0 wt. % carbon, o up to 1.0 wt. % of a pressing aid, o remainder: powdered iron or powdered iron alloy • Producing a green compact from this powder mixture and • Sintering the green compact at a maximum temperature of 1100 to 1250 °C.

Description

PROCESS FOR THE PRODUCTION OF ALLOYED SINTERED STEEL

The present invention relates to a method for producing a group 5 or group 6 element alloyed sintered steel, in particular a molybdenum alloyed sintered steel. In addition, the invention relates to the use of a master alloy for alloying sintered steel. Furthermore, the invention relates to a master alloy for alloying

of sintered steel.

Background of the invention

To harden steel and prevent temper embrittlement, refractory metals such as molybdenum or vanadium are increasingly being added to steel as alloying elements. Due to the very high melting temperature of molybdenum in particular compared to iron, master alloy processes have now also been described for the production of alloyed sintered steels. In these processes, a master alloy (a "master alloy") with a significant content of the metal to be alloyed (e.g. molybdenum or vanadium) is produced, whereby the master alloy is then mixed in powder form with metallic iron powder, pressed and sintered (see e.g. Zapf, G., Dalal, K.: Modern developments in powder metallurgy, 1977, p. 129; Schlieper, G., Thümmler, F.: Powder Metallurgy International, vol. 11, 1979, p. 172; Banerjee, S., et al.: Progress in Powder Metallurgy, vol. 13, 1980, p. 143). A uniform distribution of the molybdenum- or vanadium-containing master alloy in the powder mixture ensures a good distribution of the molybdenum and vanadium in the sintered steel. However, since in this process the molybdenum and even more the vanadium in the master alloy is present as a thermodynamically stable carbide, high

sintering temperatures required.

Brief description of the invention

Molybdenum-alloyed sintered steels are known from the state of the art, but the production of molybdenum (Mo)-alloyed sintered steel, in which the Mo is distributed homogeneously in the finished sintered steel, is technically and energetically complex due to the high melting point of Mo (2623 °C). The sintering of powder mixtures with elemental Mo or ferromolybdenum powder requires very high sintering temperatures and thus complex furnace units in order to achieve the uniform distribution of the alloying element in the steel matrix required for the required mechanical properties. As mentioned, the previously described master alloy process is also energetically complex with high required sintering temperatures. In this case, high sintering temperatures also lead to

that the sintered steel swells noticeably or tends to warp, which in terms of dimensional stability

alloyed sintered steels.

The object of the present invention is therefore to provide a method for producing a low refractory metal alloyed sintered steel, in which the required sintering temperature is lower than in the above-mentioned methods and the dimensional stability is improved.

This object is achieved by a method for producing a group 6 element or group 5 element alloyed sintered steel, comprising the steps: e providing a powder mixture comprising o 1 to 6 wt.% of a powdered master alloy, wherein the master alloy has a melting point of max. 1300 °C, o 0.1 to 2 wt.% of powdered group 6 and/or group 5 element as elemental powder or as ferroalloy powder or as carbide powder, o 0.1 to 1.0 wt.% carbon, o up to 1.0 wt.% of a pressing aid, Oo remainder: powdered iron or powdered iron alloy e producing a green compact from this powder mixture and

e Sintering of the green body at a maximum temperature of 1100 to 1250 °C.

The naturally occurring elements of group 6 and group 5 are understood as group 6 and group 5 elements. The group 6 elements in the sense of the invention are therefore the metals chromium, molybdenum and tungsten, the group 5 elements are vanadium, niobium and tantalum. The group 6 element molybdenum is preferably provided within the scope of the invention. Therefore, in addition to the term group 6 element alloyed sintered steel, molybdenum alloyed sintered steel is described below in particular;

The same applies to vanadium among group 5 elements.

Low refractory metal alloy or low molybdenum alloy in the sense of the invention means that the finished sintered steel does not contain more than 2 wt.% of refractory metal or Mo. Powdered iron has a purity of > 99 wt.%. An iron alloy in the context of the invention contains at least 90 wt.% iron.

For the purposes of the invention, steel is defined with reference to EN 10020:2000—-07 as

Material whose mass fraction of iron is greater than that of any other element,

whose carbon content is a maximum of 2 wt. % and which may contain other elements.

Molybdenum carbide powder or vanadium carbide powder can be added.

According to the inventors' studies, the distribution of group 6 element or group 5 element occurs in such a way that the master alloy melts first and is distributed in the powder mixture during melting. The powdered group 6 element or group 5 element dissolves in the melting phase of the master alloy and is distributed together with the master alloy in the green body. In this way, the sintering temperature can be kept low because only the melting temperature of the master alloy has to be reached. In addition, the dimensions of the body hardly change during sintering, so that the dimensional stability is higher compared to other state-of-the-art processes. Similar results are possible when alloying iron with chromium or tungsten or niobium or tantalum, whereby the master alloy is then chromium-free or tungsten-free, niobium-free or tantalum-free and has a melting point of max. 1300 °C and if the chromium or tungsten, niobium and/or tantalum as

elemental powder or as corresponding ferroalloy powder.

In contrast to known master alloy approaches (as described e.g. in Raquel de Oro Calderon et. al; Powder Metallurgy Progress, Vol.18 (2018), No.2, p.121-127 http://dx.doi.org/10.1515/pmp2018-0014), the element to be alloyed is not introduced into the master alloy itself, but is introduced as a powder either directly (preferred) or as

Ferroalloy powder or added as carbide.

With this process it is now possible to easily alloy elements with high melting points, but also those that form high melting carbides and are difficult to alloy due to the melting or decomposition temperatures. Specific alloying methods for individual group 6 and group 5 elements are given below.

Melting points and decomposition points are listed:

Cr: congruent melting at 1857°C Cr23C6: peritectic decomposition at 1576°C Cr7C3: congruent melting at 1766°C

e NbC, TaC: W.G.Moffatt: “The Handbook of Binary Phase Diagrams”. Genium Publishing Corp., Schenectady NY (1984)

The process according to the invention now also allows these metals or metal carbides

easy to alloy.

The Masteralloy master alloy is intended to promote the distribution of the group 6 or group 5 element in the iron phase. Alloys in which the distribution of the Masteralloy melt in the green body is ideal have proven to be ideal. Masteralloy master alloys that have proven to be suitable are those that consist of

e 10 to 60 wt.% iron,

e 0 to 20 wt.% silicon,

e 0O0to 6.7 wt. % carbon,

e 10 to 60 wt. % manganese,

ee max 30 wt. %, preferably maximum 20 wt. % other elements

consist.

Other elements include Cu, Ni, Co, Sn and Al in small amounts.

Preferably, the content of the group 6 or group 5 element to be alloyed in the master alloy is < 0.1 wt. %, particularly preferably < 0.01 wt. %.

The distribution of the powdered group 6 or group 5 element is ideal when it

a particle diameter dso of < 10 µm.

In an advantageous embodiment, the powder mixture is prepared in several steps: In a first step, the powdered Masteralloy master alloy and the powdered iron or the powdered iron alloy are mixed for preferably 10 to 30 minutes; then the powdered Group 6 or Group 5 element is added and mixed again for the same time. Carbon is then added and the mixture thus obtained is mixed again — preferably for 10 to 30 minutes. The pressing aid is then added and the mixture thus obtained is mixed again — preferably for 10 to 30 minutes. This procedure

ensures the optimal distribution of all components in the finished sintered steel.

The green part is typically produced by pressing the powder mixture into the desired shape by axial die pressing. This can be done in molds by

Apply a pressure of preferably 500-800 MPa.

Preferably, the sintering process is started at an initial temperature of 600 °C in order to

Press aids must be completely removed.

In addition, it has proven advantageous if the sintering temperature is increased from the initial temperature to the maximum temperature at a rate of 5 to 15 K/min, preferably

about 10 K/min.

If a pressing aid is used, the pressing aid should be removed from the green body before sintering by heating it - preferably to a setting temperature of 600 °C as mentioned above. This step is preferably carried out under a protective gas atmosphere. For example, heating to remove the pressing aid can be carried out under

nitrogen atmosphere.

The sintering process is preferably carried out under an atmosphere consisting of nitrogen and

Hydrogen, with a dew point below -20°C.

e 10 to 60 wt.% iron,

e 0 to 20 wt.% silicon,

e 0O0to 6.7 wt. % carbon,

e 10 to 60 wt. % manganese,

ee max 20 wt. % other elements

for alloying sintered steel.

Detailed description of the invention

The invention is illustrated by examples, figures and the following

Figure description explained in more detail.

Fig. 1 shows a classic sintering process for refractory elements. Fig. 2 shows a known sintering process for refractory elements with Masteralloy.

Fig. 3 shows a sintering process according to the invention.

Fig. 1 shows a classic sintering process in which powdered iron or a powdered iron alloy (base powder) is sintered with a group 6 element or group 5 element (e.g. Mo, Cr, V or W) and carbon (C). The base powders investigated were ASC 100.29 (Fe), Astaloy 0.85 Mo (Fe_0.85Mo), Astaloy CrA (Fe_1.8Cr) and Astaloy CrM (Fe 3Cr_0.5Mo). Mo, Cr, V and W were investigated as group 6 element or group 5 element.

The base powder is mixed with carbon and elemental, powdered group 6 element or group 5 element. In the heating phase, the mixture is heated from about 20 °C to 1250 to 1300 °C, which leads to the formation of carbide. If a eutectic reaction takes place with the base powder, an intermediate melt formation with secondary pores occurs; after sintering, the group 6 element or group 5 element is distributed largely homogeneously in the iron matrix. However, the eutectic reaction requires temperatures of 1250 to 1370 °C, depending on the alloying element. If the sintering process takes place below this temperature threshold, with pure solid phase diffusion, the result is a sintered and alloyed powder steel. In this case, however, the group 6 element or group 5 element is only distributed locally, not over the entire body, and the desired effect of this

Elements on the properties of the sintered steel is not achieved.

The base powder is mixed with carbon and powdered MA. In the heating phase, the mixture is heated from about 20 °C to 1120 to 1180 °C. The master alloy melts and spreads over the open pores. After the sintering process with solid phase diffusion at 1120 to 1180 °C, a sintered and alloyed powder steel is obtained. The elements Mn and Si are distributed over the entire body, but not the group 6 element or group 5 element, which is still present at least partially as undissolved carbide. For a homogeneous distribution of these latter elements, sintering temperatures of at least 1250 °C are also required here. Only when the master alloy powder MnMA is used are sintering temperatures of 1120-1180 °C sufficient to bring about homogenization of the alloy elements; however, in this case, the beneficial effect

of Group 5 or Group 6 elements.

Fig. 3 shows a master alloy sintering process according to the invention, in which powdered iron or a powdered iron alloy (base powder) is sintered with a master alloy powder and a group 6 element or group 5 element in powder form. The master alloy powder is free of the group 6 element or group 5 element to be sintered (e.g. Mo, Cr, V or W). Carbon (C) is also present. The base powders mentioned in Fig. 1 were also examined as base powders. MnMA (Fe-Si-C-Mn) was used as the master alloy. Mo, Cr, V and W were used as the group 6 element or group 5 element.

examined.

The base powder is mixed with carbon, powdered MA and powdered group 6 element or group 5 element. In the heating phase, the mixture is heated from about 20 °C to 1120 to 1180 °C. The master alloy melts and dissolves the elemental group 6 element or group 5 element. This solution is distributed over the open pores. After the sintering process with solid phase diffusion at 1120 to 1180 °C, a sintered and alloyed powder steel is present. The group 6 element or group 5 element is distributed over the entire body. In contrast to the example in Fig. 2, a complex master alloy, in which an alloy with the group 6 element or group 5 element is used, can be dispensed with.

element must be presented.

“solubilizer” and transport medium.

The production of the sintered steel bodies follows the well-known production route for powder metallurgical molded parts, i.e. mixing the starting powder with the addition of a suitable pressing aid, pressing by uniaxial die pressing in pressing tools with the appropriate geometry, thermal removal of the pressing aid in a furnace unit under a suitable atmosphere and sintering under inert or reducing protective gas. If the sintering furnace is suitably equipped, the sintered material can be quenched and thus hardened by blowing cold inert gas immediately after leaving the high-temperature zone of the furnace; this is followed by tempering in an appropriate

tempered outlet zone of the furnace or in a separate tempering furnace.

In the specifically described method according to the invention, a master alloy powder of the described composition is mixed with a pure iron powder or a low-alloy steel powder as a base powder and graphite powder as a carbon carrier; furthermore, a small amount of the powder of a group 6 and/or group 5 element is added, as well as the pressing aid. The mixing process is advantageously carried out in stages by first mixing the base powder with the master alloy powder, then the group 6/group 5S element powder, after a further mixing process the graphite powder and after

After further mixing, the lubricant is added and mixed until complete.

Pressing takes place in a press tool shaped according to the geometry of the component to be manufactured at a pressure of 500 to 800 MPa, depending on the relative density to be achieved. The green body thus obtained, which typically contains 8-12% porosity, is first thermally dewaxed and then sintered in the furnace unit. During the heating process to the sintering temperature, which is preferably between 1120 and 1180°C, the added master alloy powder melts and forms an intermediate liquid phase that is able to dissolve the added group 6 or group 5 element. Under the effect of capillary forces, the liquid phase with the group 6 or group 5 element contained in it is distributed in the pore network of the body; this means that the alloying elements are distributed macroscopically homogeneously in the body. The elements then diffuse into the

particles of the base powder, causing the liquid phase to disappear.

properties are unfavorable.

The method according to the invention thus makes it possible to alloy high-melting group 6 or group 5 elements with the iron-based matrix metal even at moderate sintering temperatures and thus to increase the positive effect of the group 6 or group 5 elements

to fully exploit the properties of sintered steel.

Examples of implementation:

For this investigation, material compositions and manufacturing parameters were used which showed that the systems already produced usable mechanical properties without the addition of Mo or V. This was specifically investigated to see whether and to what extent

the extent to which the addition of these elements brings about further improvement.

Example 1:

The following powders are mixed:

- 4 wt. % Masteralloy powder Fe-40%Mn-9%Si-1.5%C, high-pressure water atomized - 2 wt. % Masteralloy powder Fe-33%Mn-7.5%Si-3.44%C, high-pressure water atomized

- 0.5 wt. % Mo powder <32 μm, elemental - 0.471 wt. % graphite powder

- Rest: pure iron powder water atomized

The mixture is mixed with 0.6% ethylene bisstearamide as a lubricant and pressed in a press tool with a floating die at 600 MPa pressure to form cuboid-shaped test specimens according to ISO 5754. As a reference, similar test specimens are produced, but without Mo addition.

The pellets of both compositions produced in this way are dewaxed in a furnace with a tubular, gas-tight retort made of heat-resistant steel for 30 minutes at 600°C under pure nitrogen and then sintered in another furnace with a tubular, gas-tight retort made of a superalloy for 60 minutes at a temperature of 1180°C and in the

water-cooled outlet zone of the furnace at approx. 0.4 K/s to room temperature.

For each composition, a portion of the test specimens is characterized in the sintered state, another portion is heated again to 1100°C in the latter furnace, held for 30 minutes and then cooled in a quenching device with pure nitrogen so that a cooling rate of 3 K/s linearized between 900° and 100°C is obtained. Immediately afterwards, the

Material is annealed in air at 180°C for 60 minutes. These samples are also characterized.

The following properties are measured:

AS (as sintered): sintered state; GQ (gas quenched): gas quenched and tempered Sintered density | Hardness AS | Hardness GQ | Impact strength- Impact strength-

g/cm” HV30 HV30 AS strength GQ J/cm-2 J.cm-2 With Mo addition 7.01+0.01 | 339+ 14 | 379+11 7.1+0.6 11.7+ 0.6 Without Mo 7.00+ 0.01 | 264+4 338 +4 8.0+ 0.8 74+1.2

Example 2:

The following powders are mixed:

4 wt. % Masteralloy powder Fe-40%Mn-9%Si-1.5%C, high-pressure water atomized

2 wt. % Masteralloy powder Fe-33%Mn-7.5%Si-3.44%C, high-pressure water atomized 0.5 wt. % V powder <90 um, elemental

0.471 wt. % graphite powder

Rest: pure iron powder, water atomized

11718

The powder mixture is mixed with lubricant as described in Example 1, pressed,

dewaxed and sintered or, in some cases, subsequently gas quenched and tempered.

The following properties are measured:

AS (as sintered): sintered state; GQ (gas quenched): gas quenched and tempered

Sintered density | Hardness Hardness Impact strength Impact strength

g/cm” AS GQ AS GQ HV30 HV30 J/cm-2 J.cm-2 With V-added | 7.01+0.01 | 296+22 | 376 + 15 7.7+03 6.2 + 0.4 Without V 7.00+£0.01 | 264+4 | 3384 8.0+ 0.8 74+1.2

Claims (16)

1. A method for producing a group 6 element and/or group 5 element alloyed sintered steel, comprising the steps: e providing a powder mixture comprising o 1 to 6 wt.% of a powdered master alloy, wherein the master alloy has a melting point of max. 1300 °C, o 0.1 to 2 wt.% of powdered group 6 and/or group 5 element as elemental powder or as ferroalloy powder or as carbide powder, o 0.1 to 1.0 wt.% carbon, o up to 1.0 wt.% of a pressing aid, Oo remainder: powdered iron or powdered iron alloy e producing a green compact from this powder mixture and e Sintering of the green body at a maximum temperature of 1100 to 1250 °C. 2. Process according to claim 1, characterized in that the master alloy is an alloy consisting of e 10 to 60 wt.% iron, e 0 to 20 wt.% silicon, e 0 to 6.7 wt. % carbon, e 10 to 60 wt. % manganese, ® max. 30 wt. %, preferably maximum 20 wt. % other elements. 3. A method according to claim 1 or claim 2, characterized in that the group 6 element is selected from the group consisting of molybdenum, chromium and tungsten, preferably the group 6 element is molybdenum and the group 5 element is selected from the group consisting of vanadium, niobium and tantalum, preferably the Group 5 element vanadium. 4. Method according to one of claims 1 to 3, characterized in that the Masteralloy powder has an average diameter of d50 < 45 µm. 5. Process according to one of claims 1 to 4, characterized in that the powdered group 6 or group 5 element has an average diameter of dso < 45 µm, preferably dso < 10 µm. 6. Method according to one of claims 1 to 5, characterized in that the Preparation of the powder mixture comprises several steps, whereby in a first step the powdered master alloy and powdered iron are mixed for preferably 10 to 30 minutes, after which the powdered group 6 or group 5 element is added and the resulting mixture is mixed again - preferably for 10 to 30 minutes -, after which carbon is added and the resulting mixture is mixed again - preferably for 10 to 30 minutes -, after which the pressing aid is added and the resulting mixture is mixed again - preferably for 10 to 30 minutes — is mixed. 7. Method according to one of claims 1 to 6, characterized in that the Green compact is produced by pressing the powder mixture into the desired shape. 8. Method according to one of claims 1 to 7, characterized in that the sintering is started at an initial temperature of 600 °C. 9. Method according to one of claims 1 to 8, characterized in that the maximum sintering temperature is between 1100 and 1200°C. 10. A method according to claim 8 or claim 9, characterized in that the sintering temperature is increased from the initial temperature to the maximum temperature at a rate of 5 to 15 K/min, preferably about 10 K/min. 11. Method according to one of claims 1 to 10, characterized in that a pressing aid is present, wherein the pressing aid is heated prior to sintering preferably at a maximum of 600 °C from the green body. 12. Method according to claim 11, characterized in that the heating for Removal of the pressing aid takes place under a nitrogen atmosphere. 13. Method according to one of claims 1 to 12, characterized in that the sintering under an atmosphere consisting of nitrogen and hydrogen. 14. Method according to one of claims 1 to 13, characterized in that the Masteralloy master alloy contains less than 0.01 wt.% of Mo, Wo, V, Nb and Ta. 15. Use of a Masteralloy master alloy consisting of e 10 to 60 wt.% iron, e 0 to 20 wt.% silicon, e 0O0 to 6.7 wt.% carbon, e 10 to 60 wt. % manganese, ee max. 30 wt. %, preferably max. 20 wt. % other elements for alloying sintered steel with a group 6 and/or group 5 element. 16. Use according to claim 15, characterized in that the group 6 and/or Group 5 element is selected from the group consisting of Mo, W, V, Nb and Ta, preferably from the group consisting of Mo, W and V.