SINTERED METAL OBJECT COMPRISING METAL FIBERS

Abstract

A sintered metal object comprises metal fibers in a nonwoven web arrangement. The metal fibers comprise stainless steel fibers having a duplex microstructure. The duplex microstructure is a mixed microstructure of austenite and ferrite. The stainless steel fibers are bonded at at least part of their contacting points by means of sinter bonds.

Claims

1. Sintered metal object, comprising metal fibers in a nonwoven web arrangement, wherein said metal fibers comprise stainless steel fibers having a duplex microstructure, wherein the duplex microstructure is a mixed microstructure of austenite and ferrite, wherein said stainless steel fibers are bonded at at least part of their contacting points by means of sinter bonds.

2. Sintered metal object as in claim 1, wherein no sigma phase is present in said stainless steel fibers.

3. Sintered metal object as in claim 1, wherein said stainless steel fibers having a duplex microstructure, have an austenite percentage between 30 and 70 percent by volume.

4. Sintered metal object as in claim 1, wherein said stainless steel fibers having a duplex microstructure are super duplex stainless steel fibers.

5. Sintered metal object as in claim 1, wherein in the duplex microstructure of said stainless steel fibers, the average grain size of the austenite grains is smaller than half of the equivalent fiber diameter of said stainless steel fibers.

6. Sintered metal object as in claim 1, wherein said stainless steel fibers have over their volume a substantially uniform weight percentage of chromium and molybdenum.

7. Sintered metal object as in claim 1, wherein said stainless steel fibers that have a duplex microstructure are substantially free of nitrides.

8. Sintered metal object as in claim 1, wherein said stainless steel fibers having a duplex microstructure have an equivalent diameter between 1.5 and 100 μm.

9. Sintered metal object as in claim 1, comprising more than one layer of metal fibers in a nonwoven web arrangement, wherein at least two layers of said more than one layer of metal fibers comprise stainless steel fibers having a duplex microstructure; wherein the duplex microstructure is a mixed microstructure of austenite and ferrite; wherein said stainless steel fibers are bonded at at least part of their contacting points by means of sinter bonds wherein the average equivalent diameter of the stainless steel fibers having a duplex microstructure of a first of said at least two layers, differs from the average equivalent diameter of the stainless steel fibers having a duplex microstructure of a second of said at least two layers.

10. Sintered metal object as in claim 1, wherein said metal object comprises one or more metal meshes sintered into the sintered metal object.

11. Method to manufacture a sintered metal object as in claim 1, comprising the steps of making an unsintered metal object comprising a nonwoven web comprising duplex stainless steel fibers; sintering the metal object in a sinter oven at a sintering temperature, wherein the sintering temperature is between 1000 and 1300° C., in a sintering atmosphere comprising nitrogen, wherein the nitrogen partial pressure of the sintering atmosphere is between 10 to 100 mbar; cooling down the sintered metal object in the sinter oven after sintering, wherein the cooling down in the temperature range between 950° C. and 650° C. is accomplished within less than 1.5 hours.

12. Method as in claim 11, wherein cooling from the sintering temperature down to 650° C. is performed in a time period less than 1.5 hours.

13. Method as in claim 11, wherein cooling from the sintering temperature down to 650° C. is performed in an atmosphere comprising nitrogen gas.

14. Method as in claim 11, wherein at least part of the cooling down of the sintered metal object from the sintering temperature is realised at least in part by the introduction of nitrogen gas in the sinter oven.

15. Filtration membrane comprising a sintered metal object as in claim 1.

Description

MODE(S) FOR CARRYING OUT THE INVENTION

[0047] As an example, metal fiber webs have been made out of super duplex stainless steel 1.4410 (according to EN10088:2005). Two different nonwoven webs have been made. A first web out of super duplex 1.4410 fibers of equivalent diameter 12 μm; and a second web out of super duplex 1.4410 fibers of equivalent diameter 22 μm. The super duplex fibers have been made by means of machining from an ingot, as is described in WO2014/048738A1. With each of the fiber types, a nonwoven web of weight 300 g/m.sup.2 was made using wet webbing technology.

[0048] The first nonwoven web (fibers of equivalent diameter 12 μm) and the second nonwoven web (fibers of equivalent diameter 22 μm) have each been heated in a sintering oven in vacuum up to 1150° C. During the increase of temperature, stops have been integrated for rinsing with an inert gas, e.g. nitrogen (N.sub.2), although also argon can be used. The webs have been sintered at nitrogen (N.sub.2) partial pressure 30 mbar (which was also the total pressure) at 1150° C. during 75 minutes. Cooling from the sintering temperature 1150° C. down to 650° C. was performed in 45 minutes. This cooling was performed under nitrogen partial pressure of 30 mbar. The webs have been further cooled down to room temperature.

[0049] Other exemplary samples have been made using bundle drawn fibers out of duplex stainless steel alloy 1.4462 (according to EN10088:2005), which is equivalent to grade 2205. The web out of these fibers was sintered at 1100° C. in otherwise similar process parameters as for the first and the second web examples.

[0050] All samples made, showed that the mixed microstructure of austenite and ferrite was present in the fibers of the sintered web. Both phases (austenite and ferrite) were present in about equal percentages by volume. Nor via XRD (X-ray diffraction), nor via metallographic analysis, a sigma phase could be detected in the stainless steel fibers of the sintered web. The stainless steel fibers in the sintered web were substantially free of nitrides. The average grain size of the austenite grains in the fibers in the web showed to be smaller than half of the equivalent fiber diameter of the fibers. The sintered webs showed to have excellent corrosion resistance when tested in an environment containing chloride ions.

[0051] It is also possible to sinter the 300 g/m.sup.2 nonwoven web out of 12 μm equivalent diameter fibers out of super duplex 1.4410 and two 300 g/m.sup.2 web out of 22 μm fibers out super duplex 1.4410 stacked together, thereby producing a layered sintered metal fiber web. It is e.g. also possible to put a stainless steel wire mesh out of super duplex 1.4410 between the nonwoven webs of different average equivalent diameter; and sinter so that the nonwovens webs and the mesh are sintered into one sintered metal object comprising 12 μm equivalent diameter and 22 μm equivalent diameter stainless steel fibers of duplex microstructure. Such products can be used as filtration membrane in a filter.