KR20190092493A - Stainless Steel Powder for Manufacturing Duplex Sintered Stainless Steel - Google Patents

Abstract

Embodiments of the present invention may provide new stainless steel powders suitable for the production of duplex sintered stainless steels. Embodiments of the present invention may also relate to methods for producing stainless steel powder, duplex sintered stainless steel and methods for producing duplex sintered stainless steel.

Description

Stainless Steel Powder for Manufacturing Duplex Sintered Stainless Steel

Embodiments of the present invention may provide new stainless steel powders suitable for the production of duplex sintered stainless steels. Embodiments of the present invention also relate to methods for producing stainless steel powder, methods for producing duplex sintered stainless steel as well as duplex sintered stainless steel.

Duplex stainless steels have been known in the industry for over 60 years. These are widely used in heat treated, wrought and gas sprayed powder forms in many applications requiring a combination of high strength and high corrosion resistance. However, these are not currently available in the form of water sprayed powders used in press and sintering applications.

Typical uses for duplex stainless steels include chemical process plants pipelines, petrochemical industry, power plants and automobiles. They are also used in the food processing industry, pharmaceutical process components, paper and pulp industry, desalination plants and mining. Duplex stainless steels are well known for their high resistance to inter granular corrosion (IGC) and stress corrosion cracking (SCC) in chloride media. Chloride is a serious challenge that leads to a fast corrosion medium for iron based alloys.

It is believed that high strength and high corrosion resistance properties in duplex stainless steel are obtained due to the presence of the same amounts of ferrite and austenite phases. This structure is generally austenite stabilizers such as, for example, nickel (Ni), manganese (Mn), carbon (C), nitrogen (N), copper (Cu) and cobalt (Co) and, for example, chromium ( This is achieved by using a balance of ferrite stabilizers such as Cr), silicon (Si), molybdenum (Mo), tungsten (W), titanium (Ti) and niobium (Nb).

As previously described, the high strength and high corrosion resistance of duplex stainless steel is believed to come from the balance of ferrite and austenite in the microstructure. The microstructure depends not only on the chemical properties but also on the heat treatment carried out on the material. Since N is a strong austenite stabilizer, all duplex steel compositions use N in chemistry today. N, when present in the alloy with Cr, causes the problem of forming nitrides that are detrimental to properties such as strength and corrosion resistance. In addition, during welding of duplex stainless steels, because of the slow cooling rates, an intermetallic phase, known as “Sigma”, forms in the heat affected zone (HAZ). This sigma phase is a hard supersaturated intermetallic phase containing Cr and Mo. Regions near the sigma phase are depleted of Cr and Mo, weakened and less resistant to corrosion. Often duplex stainless steels require annealing and quenching processes to reduce or eliminate this sigma phase.

In annealed or cast duplex stainless steels, the steel solidifies as ferritic steel and the austenite phase is precipitated from the ferrite during cooling of the alloy. Since the cooling rate determines the ratio of austenite and any intermetallic phases deposited in the structure, the cooling rate is important after casting or in any heat treatment.

Tempered duplex stainless steels, especially 'hot rolled' duplex stainless steels, have been used industrially since the 1930s, but they are rarely used in the powder metallurgy (PM) industry. There are also several applications where gas sprayed duplex stainless steel powders are used in hot isostatic pressed (HIP) conditions. The powders produced by the gas spray method have a spherical form. Such powders are less suitable for conventional press and sintering applications. Due to the spherical shape, they do not have enough green strength to process the green press and sintered parts. Irregularly shaped powders, such as those produced by the water spray method, have a much higher green strength because the irregular shape of the powders tends to bond the powder particles together. There is currently no water sprayed stainless steel powder available for making sintered duplex stainless steel components. Current chemical compositions used in gas sprayed powders and also annealed steels use N as the main alloying element to achieve austenite-ferrite balance and achieve the required mechanical strength. By including N in the powder, the hardness of the powder is increased, which reduces the compressibility in conventional press and sintering applications. This can reduce the green density and subsequently reduce the sintered density.

Several attempts have been made to develop sintered duplex stainless steels made from water sprayed powders. Lawley et al. ¹ attempted to develop an equivalent grade of AISI 329 and AISI 2205 with a maximum tensile strength of 578 MPa. Dobrzanski et al. ² mixed ferrite and austenite powders to produce a duplex structure with a tensile strength of 650 MPa. The same group also studied the corrosion properties of duplex stainless steels by electrochemical methods, and concluded that duplex stainless steels exhibited better corrosion resistance than their austenitic counterpart ³ . Due to their high alloy content, these steels are sensitive to composition and also to processing parameters. These alloys form intermetallic phases known as sigma, chi and gamma primes that are rich in Mo, W, N, Ni and Cr and reduce both mechanical and corrosion properties. The sigma phase is formed in the temperature range of 700 ° C to 1000 ° C, while the chi phase is formed in the range of 300 ° C to 450 ° C. The gamma (austenite) phase may begin to form at about 600 ° C.

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^{1 a} . A. Lawley, Lee. W. Wagner, C.T. CT Schade, Advances in Powdered Metallurgy and Particle Materials 2005 Part 7 Pages 78-89

² L.A. LA Dobrzanski, Jet. Z. Brytan, M. M. Actis Grande, M. M. Rosso, Material Science and Engineering Resources, Volume 28 Iss 4, April 2007 Pages 217-223

³ L.A. LA Dobrzanski, Jet. Z. Brytan, M. M. Actis Grande, M. M. Rosso, Journal of Achievements in Materials and Manufacturing Engineering, Vol. 17 Iss 1-2 Pages 317-320

A typical composition of annealed duplex stainless steel is Fe, with 21 to 23 weight% Cr, 4.5 to 6.5 weight% Ni, 2.5 to 3.5 weight% Mo, and 0.08 to 0.2 weight% N, for example for SAF 2205. Many patents exist for duplex stainless steel compositions close to this composition. Nearly all duplex stainless steels have increased corrosion resistance and strength with N content. To date, commercial uses of sintered powder metallurgy (PM) duplex stainless steels are mainly limited to the use of gas sprayed fine powders that can be used in HIP processes. The main obstacle in using low cost water sprayed powders for conventional PM use is the possibility of intermetallic and carbide precipitation due to increased N and cooling rate during sintering. Conventional sintering also requires some wetting agents or cold melting components to increase the free energy and accelerate the kinetics of austenite phase precipitation in the ferrite matrix.

Several patent documents exist that disclose sintered duplex stainless steel structures.

SE 538577 C2 (Erasteel) is made of gas sprayed powder and has a maximum of 0.030 weight% C, 4.5 to 6.5 weight% Ni, 0.21 to 0.29 weight% N, 3.0 to 3.5 weight% Mo, 21 to 24 weight % Cr, and optionally 0-1.0 wt% Cu, 0-1.0 wt% W, 0-2.0 wt% Mn, 0-1.0 wt% Si having a chemical composition of at least one wherein Si is at least 0.01 * wt% Cr And the remaining elements are Fe and unavoidable impurities—sintered duplex stainless steel.

EP 0167822 A1 (Sumitomo) discloses sintered stainless steel comprising a matrix phase and a dispersed phase and a process for the production. The dispersed phase is an austenitic metal structure and is dispersed throughout the matrix phase made of austenitic metal structure or ferrite-austenite duplex stainless steel having a different steel composition from the dispersed phase.

JP 5263199 A (Sumitomo) discloses the production of sintered stainless steel comprising a matrix phase and a dispersed phase. This method consists of austenitic stainless steel powder, austenitic-ferritic duplex stainless steel powder, austenitic-martensite duplex stainless steel powder and austenitic-ferritic-martensitic stainless steel triple phase stainless steel powder and ferrite stainless steel powder Mixing step. The powder mixture is compacted and sintered.

EP 0534864 B1 (Sumitomo) discloses sintered stainless steel made of gas sprayed steel powder having an N content of 0.10 to 0.35% by weight and having the same chemical composition as the sintered stainless steel.

Almost all duplex grades available have an N content of 0.18 to 0.40 wt% to balance the austenite-ferrite and increase the strength in the structure. Although the N content aids the above properties, it can cause obstacles by forming chromium nitride, which limits the use of duplex stainless steels in many applications in post-treatment processes such as heat treatment and welding operations. In powder form, N increases powder hardness, making it less suitable for press and sintering applications.

Embodiments of the invention avoid the use of N in chemistry, for example, less than 0.10 weight% N or less than 0.07 weight% N, or less than 0.06 weight% N, or less than 0.05 weight% N, or less than 0.04 weight% N Or, with less than 0.03% by weight N, and by achieving phase balance and strength with alternative elements, overcomes the problems with nitrides. Embodiments of the present invention may enable the preparation of water sprayed powders with suitable compressibility for use in conventional press and sintering applications. Embodiments of such compositions can also reduce precipitation of harmful 'sigma' phases; This is mainly due to the low Mo content, regardless of the cooling rate during sintering or annealing. Therefore, post-sintering heat treatments necessary to remove the "sigma" phase are minimized, and sigma phase precipitation is minimized during welding.

Embodiments of such compositions can provide similar advantages when formed by gas spraying.

Unlike conventional PM, embodiments of such compositions exhibit similar properties when processed by casting, direct metal deposition and additive manufacturing techniques.

One object of certain embodiments of the present invention is to provide an alloy powder for conventional PM that will produce a duplex structure during the sintering cycle.

Another object of certain embodiments of the present invention is to provide duplex sintered stainless steel.

Another object of certain embodiments of the present invention is to obtain a tensile strength of at least 35% higher than ferritic steels such as 430L, and to achieve twice the corrosion resistance as compared to austenitic steels such as 316L.

Another object of certain embodiments of the present invention is to provide a method for producing duplex sintered stainless steel without the need for post sintering heat treatment.

The above objects can be achieved by the following aspects and embodiments.

In a first aspect of the present invention, as a stainless steel powder, in weight%,

Up to 0.1% C,

0.5 to 3% of Si,

Up to 0.5% Mn,

20 to 27% Cr,

3 to 8% Ni

1 to 6% Mo,

Up to 3% W,

Up to 0.1% N,

Up to 4% Cu,

Up to 0.04% P,

Up to 0.04% S,

Up to 0.8% inevitable impurities,

Optionally at least one of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and

A stainless steel powder is provided which comprises or consists of the remaining Fe.

Unavoidable impurities do not include the listed elements of C, Si, Mn, Cr, Ni, Mo, W, N, Cu, P, S, B, Nb, Hf, Ti or Co. Unavoidable impurities may include impurities that are uncontrollable or difficult to control during the manufacture of the steels. They can be derived from the raw materials used and also from the process. These include Al, O, Mg, Ca, Ta, V, Te or Sn. Unavoidable impurities can be up to 0.8%, up to 0.6%, up to 0.3%. The unavoidable impurity may be O. O can be present up to 0.6%, up to 0.4% or up to 0.3%. Other inevitable impurities may be Sn, which may be present up to 0.2%, and a content of Sn greater than 0.2% is not considered inevitable in this context and therefore will be considered intentionally added.

In a preferred embodiment of the first aspect, the stainless steel powder, in weight percent,

Up to 0.06% C,

1 to 3% of Si,

Up to 0.3% Mn,

23-27% Cr,

4-7% Ni,

1-3% Mo,

0.8 to 1.5% of W,

Up to 0.07% N,

1-3% Cu,

Up to 0.04% P,

Up to 0.03% S,

Up to 0.8% inevitable impurities,

Optionally at least one of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and

A stainless steel powder consisting of the remaining Fe is provided.

In another preferred embodiment of the first aspect, the stainless steel powder, in weight percent,

Up to 0.03% C,

1.5 to 2.5% of Si,

Up to 0.3% Mn,

24 to 26% Cr,

5-7% Ni,

1 to 1.5% Mo,

1 to 1.5% of W,

Up to 0.06% N,

1-3% Cu,

Up to 0.02% P,

Up to 0.015% S,

Up to 0.8% inevitable impurities,

Optionally at least one of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and

A stainless steel powder is provided comprising the remaining Fe.

In embodiments of the first aspect, the powder is ferrite. For example, 99.5% ferrite. Small amounts of austenite, for example up to 0.5%, can be tolerated.

In embodiments according to the first aspect, the powder is produced by a water spray method.

In embodiments of the first aspect, the powder is produced by a gas spray method.

In embodiments of the first aspect, the particle size of the powder is from 53 microns to 18 microns such that at least 80 weight percent of the particles are less than 53 microns and 20 weight percent or less of the particles are less than 18 microns.

In embodiments of the first aspect, the particle size of the powder is 26 microns to 5 microns such that at least 80% by weight of the particles are less than 26 microns and 20% or less by weight of the particles are less than 5 microns.

In embodiments of the first aspect, the particle size of the powder is from 150 microns to 26 microns such that at least 80 weight percent of the particles are less than 150 microns and less than 20 weight percent of the particles are less than 26 microns.

A second aspect of the invention is a method for producing a stainless steel powder according to the first aspect,

Providing a molten metal having a chemical composition corresponding to the chemical composition of the stainless steel powder according to the first aspect;

Performing a water spray method on a stream of molten metal; And

Recovering the obtained stainless steel powder; a method for producing stainless steel powder is provided.

In a third aspect of the invention, a sintered duplex stainless steel having a chemical composition according to the first aspect and embodiments thereof are provided.

In embodiments of the third aspect, the Ni equivalent (Ni _eq ) is 5 <Ni _eq <11 and the Cr equivalent (Cr _eq ) is 27 <Cr _eq <38.

In embodiments of the third aspect, the pitting resistance equivalent number (PREN) is 28 <PREN <33.

In embodiments of the third aspect, the microstructure of the sintered duplex stainless steel is characterized by an austenite phase precipitated in the ferrite phase.

In embodiments of the third aspect, the microstructure of the sintered duplex stainless steel contains 30 to 70% austenite and 30 to 70% ferrite. In embodiments of the third aspect, the microstructure of the sintered duplex stainless steel contains at least 99.5% austenite and ferrite, for example at least 99.8% austenite and ferrite. The percentage of austenite and ferrite can be determined by ASTM E 562-11 and ASTM E 1245-03.

In embodiments of the third aspect, the microstructure of the sintered duplex stainless steel is characterized by having no sigma phases and nitrides, for example less than 1% sigma phases and nitrides.

In a fourth aspect of the present invention, there is provided a method for producing a sintered stainless steel,

Providing a stainless steel powder according to the first aspect,

Optionally mixing the stainless steel powder with a lubricant and optionally other additives,

Performing a consolidation process on the stainless steel powder or mixture to form a green component,

Performing a sintering step on a compressed green component in an inert or reducing atmosphere or vacuum for a period of from 5 minutes to 120 minutes at a temperature of 1150 ° C to 1450 ° C, preferably of 1275 ° C to 1400 ° C,

A method for producing sintered stainless steel is provided, comprising performing a cooling step to ambient temperature on the sintered component.

Examples of inert atmospheres include vacuum with nitrogen, argon, and argon backfill.

Examples of a reducing atmosphere are a hydrogen atmosphere, an atmosphere of a mixture of hydrogen and nitrogen, or an atmosphere of dissociated ammonia. In limited examples, a carbon dioxide or carbon monoxide atmosphere may be used.

In embodiments of the fourth aspect, the freezing method,

Uniaxial compression at a compression pressure of up to 900 MPa on the die to form a green component,

Ejecting the obtained compressed green component from the die.

In embodiments of the fourth aspect, the freezing method is:

One of additive manufacturing techniques such as metal injection molding (MIM), hot isostatic pressing (HIP) or binder jetting.

Methods according to the fourth aspect are one of laser powder bed fusion (L-PBF), direct metal laser sintering (DMLS) or direct metal deposition (DMD). It may include.

In embodiments of the fourth aspect, forced cooling or quenching is excluded from the cooling step.

Effect of Alloying Elements

The influence of common alloying elements on stainless steels is well known. Cr is a major element in stainless steels that forms a Cr ₂ O ₃ layer on the surface and prevents further oxygen passing through that layer, thus providing increased corrosion resistance. Ni is another major element that affects the properties of stainless steels. Ni increases the strength and toughness of the steel and also improves the corrosion resistance when present with Cr. Both Mo and W impart strength and toughness when present with Ni. Mo together with Cr and Ni also improves corrosion resistance. Si acts as an oxygen scavenger which prevents O bonding in the steel upon melting, and Si is also a strong ferrite former. Cu is an austenite stabilizer. Cu also increases the corrosion resistance of stainless steels. In particular, in conventional PM, Cu assists sintering by promoting liquid phase sintering.

Embodiments of the present invention provide not only sintered stainless steel but also powders suitable for producing sintered duplex stainless steel. Powdered and sintered stainless steels have a low or negligible N content. This eliminates the problem of the formation of harmful nitrides during the production of sintered stainless steel. Sintered stainless steel is preferably produced from compacted and sintered water sprayed powders, since a low N content can produce water sprayed powders with reasonable compressibility.

Mo is generally present in stainless steels because it strongly promotes resistance to uniform and local corrosion. Mo strongly stabilizes the ferrite microstructure. At the same time, Mo tends to precipitate Mo-rich "sigma" and "chi" phases at the ferrite-austenite grain boundaries. These are harmful phases and adversely affect strength and corrosion resistance. However, due to the lower Mo content in embodiments of the powder of the present invention, the likelihood of forming a sigma phase at any cooling rate is reduced, thereby eliminating or reducing the need for post-treatment heat treatment of annealing. This also means that no sigma phase will be formed during the welding operation, which is a common manufacturing process for duplex stainless steels.

Cr gives stainless steels their basic corrosion resistance and increases their resistance to high temperature corrosion.

Ni promotes austenite microstructures and generally increases ductility and toughness. Ni also has a positive effect as it reduces the corrosion rate of stainless steels.

Cu promotes austenite microstructures. The presence of Cu in the powder of the present invention facilitates the sintering process by enabling liquid phase sintering.

W is expected to improve resistance to pitting corrosion.

Si increases strength and promotes ferrite microstructures. It also increases oxidation resistance in strong oxidizing solutions at high temperatures and at low temperatures.

When present in powders according to certain embodiments of the present invention, B, Nb, Hf, Ti, Co may improve properties. Small additions of B can aid in liquid phase sintering. However, if excess B is present, it can form borides that are detrimental to both mechanical and corrosive properties. If Nb and Hf are present, the microstructure can be stabilized by preferential bonding with carbon to form microcarbides that remove Cr for corrosion resistance. Ti in stainless steels can increase tensile strength and toughness. Co increases the high temperature mechanical properties.

Elements such as C, Mn, S and P should be kept as low as possible in the powders of the embodiments of the invention, which vary with the compressibility of the powder and / or the mechanical and corrosion resistant properties of the sintered component. This can be negatively affected.

Other elements which are designated herein as unavoidable impurities may be allowed up to a content of 0.8% by weight of the powder according to the invention.

The composition of the powder according to embodiments of the invention is designed such that the powder produced has a ferrite structure completely (eg, at least 99.5%) in powder form and the austenite phase precipitates during the sintering cycle. This will make it possible to control the ratio of ferrite and austenite by adjusting the sintering parameters.

Ni and Cr equivalents are calculated based on the following empirical formula:

Cr _eq = Cr + 2 Si + 1.5 Mo + 0.75 W

Ni _eq = Ni + 0.5Mn + 0.3Cu + 25N + 30C

Here, Cr, Ni, etc. represent the level of each element in the alloy in weight%.

In addition, the Pitting Resistance Equivalent Number is calculated as follows:

PREN = Cr + 3.3Mo + 16N

Where Cr, Mo and N represent the levels of each element of the alloy in weight percent.

The composition is targeted such that 5 <Ni _eq <11 and 27 <Cr _eq <38. This places the alloy at the boundary of the ferrite-duplex region on the Schaeffler Diagram. At this point, the alloy is almost entirely ferrite (eg, at least 99.5%). Elements such as Mo, W and Si are supersaturated in the ferrite matrix.

Powders of embodiments of the present invention can be prepared by conventional powder preparation processes. These processes may include melting the raw materials and subsequently performing water or gas spraying, forming a so-called prealloyed powder, where all elements are homogeneously distributed in the iron matrix. In contrast to the premixed powder, the main advantage of the prealloyed powder is that two or more powders are mixed together to prevent segregation. Such segregation can cause deviations in mechanical properties, corrosion resistance and the like.

When used in the manufacture of sintered components, the powders of the embodiments of the present invention may be compressed in conventional uniaxial compression equipment at compression pressures up to 900 MPa.

Suitable particle size distributions of stainless steel powders used in conventional uniaxial compaction are such that the particle size of the powder is from 53 microns to 18 microns such that at least 80 weight percent of the particles are less than 53 microns and at most 20 weight percent of the particles are less than 18 microns. . Prior to compacting, the powders of embodiments of the present invention may be used in conventional lubricants such as, but not limited to, Acrawax, Lithium Stearate, Intralube, in amounts up to 1% by weight. It can be mixed with these. Other additives mixed up to 0.5% by weight may be machinability enhancing agents such as CaF ₂ , dolomite, bentonite or MnS.

Extrusion or additive manufacturing techniques such as metal injection molding (MIM), hot hydrostatic sintering (HIP), binder spraying, laser powder bed fusion (L-PBF), direct metal laser sintering (DMLS) or direct metal deposition (DMD) Other methods of the same solidification techniques may be used.

In the MIM process, a suitable particle size distribution of the stainless steel powder used is such that the particle size of the powder is from 26 microns to 5 microns such that at least 80% by weight of the particles are less than 26 microns and at most 20% by weight of the particles are less than 5 microns.

In a HIP or extrusion process, the appropriate particle size distribution of the stainless steel powder used is such that the particle size of the powder is from 150 to 26 microns such that at least 80% by weight of the particles are less than 150 microns and at most 20% by weight of the particles are less than 26 microns. .

The particle size distribution can be measured by conventional sieving operations according to ISO 4497: 1983 or by laser diffraction (Sympatec) according to ISO 13320: 1999.

After compaction or solidification, the compacted or solidified body is subjected to a sintering process at a sufficiently high temperature in the range from 1150 ° C. to 1450 ° C., preferably at a sufficiently high temperature in the range from 1275 ° C. to 1400 ° C. for a period of from 5 minutes to 120 minutes. Rough Depending on the shape and size of the parts to be sintered, other sintering times may be applied, such as 10 minutes to 90 minutes or 15 minutes to 60 minutes. The sintering atmosphere may be a vacuum, inert, or hydrogen atmosphere, an atmosphere of a mixture of hydrogen and nitrogen, or a reducing atmosphere such as dissociated ammonia. During the sintering process, supersaturated elements in the ferrite matrix are deposited onto the austenite phase. Austenitic will begin to precipitate at grain boundaries, will grow by further sintering, and will precipitate within the grain itself.

In contrast to other known duplex stainless steel materials, the compositions of embodiments of the invention, regardless of the rate of cooling, during cooling from elevated temperatures, sigma phases or other hard and harmful phases such as chi phase and Do not form nitrides. For example, the amount of sigma phase or other hard and harmful phases is less than 0.5%. Thus forced cooling or quenching need not be applied. In this regard, forced cooling means that the sintered parts are exposed to the cooling gas at a pressure above atmospheric pressure. Quenching means that the sintered portions are submerged into the liquid cooling medium.

Microstructures as shown in FIG. 1 will generally be formed including ferrite and austenite. The presence of both phases results in elevated mechanical and corrosion properties. Hazardous phases, such as sigma and chi, which are common for duplex stainless steels that are now known, are not formed during cooling or in fairly limited amounts are formed. As another result, this property will reduce or eliminate the formation of these phases during welding where the heat affected zone (HAZ) experiences varying cooling rates. As another result, such compositions will limit the precipitation of these phases during processes such as casting, extrusion, MIM, HIP and additive manufacturing.

Embodiments of the alloy of the present invention exhibited mechanical and corrosion properties that can be compared to or exceed the annealing and PM products made from known duplex stainless steel alloys.

In summary, certain advantages of embodiments of the present invention may include little tendency to precipitate harmful sigma and chi phases that affect mechanical and corrosion properties. This is especially important for welding. Most of the duplex stainless steel components are welded after they are formed. Welding imparts different cooling rates at different parts of the HAZ. These cooling rates tend to precipitate sigma and chi phases together with the nitrides due to nitrogen present in currently known alloys. Without these phases, post heat treatments may be removed, which typically includes annealing at temperatures above 1200 ° C. and subsequently rapid cooling. This will be difficult in most cases when the parts are welded to a larger structure and the use of duplex stainless steel is limited.

Figure 1 shows that the microstructures of the sintered stainless steel, austenite and ferrite phases of the present invention are present in equal proportions in the sintered state and the black spots are porous.
Figure 2 discloses a comparison of the final tensile strength (UTS) and corrosion properties of the sintered stainless steel of the present invention for 300 and 400 alloys (SAE grades).
3 shows a comparison of the mechanical properties of the sintered stainless steel of the present invention in different sintered states.

Example

Example 1

Stainless steel powder having a particle size of less than 325 mesh, ie 95% by weight of the particles passed through a 45 μm sieve, was mixed with 0.75% by weight of Acrawax as lubricant. The chemical analysis of the stainless steel powder is 0.01% by weight of C, 1.52% by weight of Si, 0.2% by weight of Mn, 0.01% by weight of P, 0.008% by weight of S, 24.9% by weight of Cr, 2.0% by weight of Cu, 1.3% by weight of Mo and 1.0% by weight of %, N 0.05% by weight, remaining Fe.

The powder mixture obtained was pressed in a single screw press and pressed into transverse rapture strength bars according to ASTM B528-16 at a compression pressure of 750 MPa. The pressed TRS bars were then sintered for 45 minutes at 1343 ° C. in a 100% hydrogen atmosphere at a ramp rate of 7 ° C./min. This was followed by furnace cooling at a rate of 5 ° C./min. The samples were then mounted and polished for microstructure inspection. The polished samples were then electroetched for 15 seconds with 33% NaOH at 3V. Electro etching with NaOH shows the ferrite phase to tan, the austenite to white (no effect) and the sigma phases to dark orange at grain boundaries in the ferrite matrix. The observed microstructure is as shown in FIG. 1. The microstructure shows a mixture of about 50/50 of ferrite (amber) and austenite (white). There is no trace of any sigma phase (dark orange) in the microstructure. Black spots are porous in the sample.

Example 2

Various stainless steel powders according to embodiments of the invention, and comparative samples, were prepared by water atomization. The chemical composition of the stainless steel powders is shown in Table 1. Stainless steel melts of various chemical compositions were melted in an induction furnace and a stream of water was applied to the molten metal to obtain steel powder. Subsequently, the powders obtained were dried and screened to -325 mesh. The screened powder was -45 microns, ie 95 weight percent of the powder particles were less than 45 microns. The powders were then mixed with 0.75% by weight of the lubricant Accrawax.

To test the mechanical properties, ie maximum tensile strength (UTS), yield strength (YS) and elongation, TS samples (dog bone) according to ASTM B925-15 were pressed at a compression pressure of 750 MPa. The bars were then sintered as mentioned in Example 1. Sintered bars were then tested for mechanical properties according to ASTM E8 / E8M-16a. Metallographic examination was also performed to establish the ratio between austenite and ferrite in the sintered samples. Test results are shown in Table 2 compared to known data from samples of known duplex stainless steels in tempered, (DSS 329 Wrought), and gas atomized and hip states (DSS 329 PM GA). .

Table 2 shows that the stainless steel powders according to the invention can be used to produce sintered duplex stainless steel with the desired mechanical properties.

Table 1 shows the chemical compositions of various stainless steel powders, where the type of manufacturing method and process for producing the sintered samples.

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⁴ premix of 316L, 434L and elemental powders of Si, W and Cu.

Table 2 shows the mechanical properties and metallographic structure for the sintered samples made of stainless steel powders according to Table 1.

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⁵ 316L, 434L premix and elemental powders of Si, W and Cu.

An embodiment of the powder of the present invention having the same composition as in Example 1 was also sintered at the various temperatures and atmospheres below to show the effect on the mechanical properties. This data is shown in FIG. 3.

A. 2500 ° F for 45 minutes in hydrogen gas

B. 2450 ° F. for 45 minutes in hydrogen gas

C. 2450 ° F. for 60 minutes in hydrogen gas

D. 2300 ° F. for 60 minutes in hydrogen gas

E. 2250 ° F. for 60 minutes in hydrogen gas

F. 2250 ° F. for 60 minutes in dissociated ammonia

Example 3

To perform the corrosion test, TRS bars were prepared with bars for 316 L and 434 L as representatives from austenite and ferrite grades as in Example 1. The samples were then tested for corrosion with 5% NaCl solution at room temperature according to ASTM B895-16. Corrosion was compared to the time taken for the onset of corrosion in the samples. Comparative data is shown in FIG. 2 with UTS and YS for these samples. The diameter of the bubbles in FIG. 3 indicates the number of times it took for corrosion to begin in the samples. The corrosion test on the powder of the present invention was stopped after 3700 hours because there was no sign of corrosion, which was already over three times that of 316 L samples.

Claims (18)

Up to 0.1% C,
0.5 to 3% of Si,
Up to 0.5% Mn,
20 to 27% Cr,
3 to 8% Ni
1 to 6% Mo,
Up to 3% W,
Up to 0.1% N,
Up to 4% Cu,
Up to 0.04% P,
Up to 0.04% S,
Up to 0.8% inevitable impurities,
Optionally at least one of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and
Containing the rest Fe,
Stainless steel powder. According to claim 1,
Up to 0.06% C,
1 to 3% of Si,
Up to 0.3% Mn,
23-27% Cr,
4-7% Ni,
1-3% Mo,
0.8 to 1.5% of W,
Up to 0.07% N,
1-3% Cu,
Up to 0.03% P,
Up to 0.03% S,
Up to 0.8% inevitable impurities,
Optionally at least one of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and
Containing the rest Fe,
Stainless steel powder. According to claim 1,
Up to 0.03% C,
1.5 to 2.5% of Si,
Up to 0.3% Mn,
24 to 26% Cr,
5-7% Ni,
1 to 1.5% Mo,
1 to 1.5% of W,
Up to 0.06% N,
1-3% Cu,
Up to 0.02% P,
Up to 0.015% S,
Up to 0.8% inevitable impurities,
Optionally at least one of up to 0.004% B, up to 1% Nb, up to 0.5% Hf, up to 1% Ti, up to 1% Co, and
Containing the rest Fe,
Stainless steel powder. The method according to any one of claims 1 to 3,
The stainless steel powder is ferrite,
Stainless steel powder. The method according to any one of claims 1 to 4,
The stainless steel powder is produced by water atomization,
Stainless steel powder. The method according to any one of claims 1 to 4,
The stainless steel powder is produced by the gas spray method,
Stainless steel powder. The method according to any one of claims 1 to 4,
Wherein the particle size of the powder is from 53 microns to 18 microns so that at least 80% of the particles of the powder are less than 53 microns and less than 20 microns of the particles of the powder are less than 18 microns,
Stainless steel powder. The method according to any one of claims 1 to 4,
Wherein the particle size of the powder is from 26 microns to 5 microns such that at least 80% of the particles of the powder are less than 26 microns and less than 20 microns of the particles of the powder are less than 5 microns,
Stainless steel powder. The method according to any one of claims 1 to 4,
Wherein the particle size of the powder is from 150 microns to 26 microns such that at least 80% of the particles of the powder are less than 150 microns and less than 20 microns of the particles of the powder are less than 26 microns,
Stainless steel powder. The method according to any one of claims 1 to 4,
The powder is a prealloyed powder,
Stainless steel powder. As a method for producing stainless steel powder by water spraying method,
Providing a molten metal having a chemical composition corresponding to the chemical composition of the stainless steel powder according to claim 1,
-Water spraying the stream of molten metal, and
Recovering the obtained stainless steel powder;
Method for producing stainless steel powder by water spray method. Sintered duplex stainless steel having a chemical composition according to any of claims 1 to 3, wherein
The microstructure of the sintered duplex stainless steel is characterized by an austenite phase deposited on a ferrite,
Sintered duplex stainless steel. The method of claim 12,
Ni equivalent (Ni _eq ) is 5 <Ni _eq <11, Cr equivalent (Cr _eq ) is 27 <Cr _eq <38, Cr _eq and Ni _eq is represented by the following formula:
Cr _eq = Cr + 2 Si + 1.5 Mo + 0.75 W
Ni _eq = Ni + 0.5Mn + 0.3Cu + 25N + 30C
Is calculated according to
Here, Cr, Ni, etc. are the weight% of the level of each element in an alloy,
Sintered duplex stainless steel. The method according to claim 12 or 13,
The pitting resistance equivalent number (PREN) is 28 <PREN <33, and PREN is represented by the following formula:
PREN = Cr + 3.3Mo + 16N
Is calculated according to
Where Cr, Mo and N are the weight percent of the levels of each element in the alloy,
Sintered duplex stainless steel. The method of claim 12,
Wherein the microstructure of the sintered duplex stainless steel contains 30 to 70% austenite,
Sintered duplex stainless steel. The method according to any one of claims 12 to 15,
The microstructure is free of sigma phases and nitrides,
Sintered duplex stainless steel. A method for producing duplex sintered stainless steel,
Providing a stainless steel powder according to any one of claims 1 to 10,
Optionally mixing the stainless steel powder with a lubricant and optionally other additives,
Performing a consolidation process on the stainless steel powder or the mixture to form a green component,
Performing a sintering step on the compressed green component in an inert or reducing atmosphere or vacuum for a period of 5 minutes to 120 minutes at a temperature of 1150 ° C to 1450 ° C, preferably 1275 ° C to 1400 ° C, and
-Performing a cooling step to ambient temperature for said sintered component,
Method for producing duplex sintered stainless steel. The method of claim 17,
The above freezing method,
Uniaxially compressing at a compression pressure of up to 900 MPa in the die to form a green component, and
Ejecting the obtained compressed green component from the die;
Method for producing duplex sintered stainless steel.

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