FIELD OF THE INVENTION

The present invention refers to an iron-based stainless overlay weld having a wear and corrosion resistant top coating. The invention also refers to a powder, or a powder mixture, suitable for producing the wear and corrosion resistant top coating by e.g. laser cladding as well as a method for producing the iron-based overlay weld.

BACKGROUND

In overlay welding, also known as cladding, weld cladding, weld overlay or hardfacing, various metal powders may be used to provide the material covering the surface, thus creating the functional layer on the object to be protected. The powder used can be a so called pre-alloyed powder wherein the alloying elements are homogeneously distributed within each powder particle or a powder mixture wherein powder constituents with different compositions are mixed with each other.

Stainless steel is soft and has good corrosion resistance and good weldability but poor wear resistance. The wear resistance can be improved by adding carbides such as Cr3C2, WC or TiC. A problem however is that when adding sufficient amounts of either of the carbides, the possibility to clad the materials decreases as the material becomes very prone to crack with increased amounts of carbides. The reason for this is twofold. Since some of the carbides are dissolving, they release carbon into the matrix, resulting in increased hardness and brittleness as well as loss in corrosion resistance as carbon consumes Cr from the stainless matrix. But also, the mere presence of large amounts of solid-state carbides decreases the flexibility and toughness of the material and introduces cracks when the molten matrix material solidifies.

When adding the material to cover a surface in form of powders at overlay welding or cladding, most of the powders are molten when fed into the welding melt pool.

When the melt pool solidifies the structure of the coating is formed. A prerequisite to maintain the corrosion resistance for a coating, made from an iron-based chromium containing powder, is that the matrix contains enough chromium to be able to re-pacify the thin chromium oxide layer if this is damaged. Typically, this means the matrix must contain above 11 % by weight of chromium dissolved in the matrix. To increase hardness in iron-based alloys carbon is usually added as it has a large influence on the hardenability. Excessive carbon content in a stainless steel will damage the ability of the material to re-pacify the surface, because carbon will react with chromium present in the matrix forming chromium carbides, and there will not be enough chromium to form a passive layer of chromium oxide. This will destroy the corrosion resistance of the material. Carbon does not have to consume chromium throughout the material to destroy the corrosion resistance, since local depletion of chromium may cause localized corrosion. Such depletion of chromium may occur at the grain boundaries in the material, where the diffusion rates of the atoms are higher, thus facilitating the formation of chromium carbides. This phenomenon is called grain boundary corrosion. Localized depletion of chromium will also happen if carbon is replaced with another interstitial alloying element able to react with chromium, such as boron.

Cracks through the coating is catastrophic for the corrosion resistance if the cracks connect the surface with the substrate. Corrosive media will in such case reach the substrate and corrode the bonding zone, causing rust to penetrate to the surface, and in worst case cause the coating to spall of. This will visually and functionally destroy the coating.

To increase wear resistance in stainless coatings without depleting the matrix from chromium, carbides with high melting points may be added. This allows the carbides to be dispersed in the molten metal without being substantially decomposed, and at the subsequent solidification, a structure of embedded carbides in the matrix material is formed. However, there will always be some decomposition or dissolution of the carbides, depending e.g. of type of carbides used and welding parameters applied. When the material solidifies, secondary carbides may be formed from carbon, originating from carbides that has been dissolved during the welding process. These secondary carbides distributed in the matrix are much finer, compared to the added carbides in the powder mixture, thus increasing the hardness of the matrix. However, the presence of fine secondary carbides may also increase the risk that cracks are formed during the subsequent cooling of the welded material. There is also a risk that the carbides reformed are chromium carbides or chromium containing carbides, thus depleting the matrix next to the carbide interface of chromium. Thus, there is a need for improvement related to overlay welding to form a corrosion and wear resistant iron- based top coating, based on stainless powder containing carbides. OBJECTS OF THE INVENTION

The aim of the present invention is to solve the above described problems and obtain the below described objects.

It is an object of the present invention to provide a powder mixture suitable to be used in an overlay welding process, especially suitable in a laser overlay welding process.

It is a further object of the invention to provide an overlay welding process for coating a metal surface such as cast iron.

It is yet another object of the present invention to provide an overlay weld having improved corrosion and wear resistance. The overlay weld being substantially free of cracks.

SUMMARY OF THE INVENTION

The invention provides a mixture of a stainless-steel powder, at least one chromium carbide and at least one other carbide having a melting point higher than the at least one chromium carbide and the stainless-steel powder. Said mixture being suitable to be used in an overlay weld process to form a wear and corrosion resistant top coating on a metal substrate. The coating having surprisingly high matrix hardness and surprisingly high carbide content while being substantially free from cracks.

The top coating per se is also provided as well as a method for producing the overlay weld with the top coating.

The present invention includes the following aspects. Further aspects and features of the invention will become more apparent in view of the detailed description.

It is disclosed herein in one of the aspects a powder mixture suitable for a wear and corrosion resistant top coating of an overlay weld containing stainless steel powder and at least one of a chromium carbide and at least one other carbide from the group of WC, W2C, TiC, NbC, Mo2C or VC.

It is further disclosed herein in another aspect, a powder mixture according to wherein the total content of carbides is at least 20% by weight and up to 60% by weight and wherein the weight ratio between chromium carbide and said other carbide is from 3:2 and up to 1 :5. It is further disclosed herein a powder mixture according to any of the previous aspects wherein the d50 value of the particle size distribution is between 20 pm and 75 pm and the d90 value of the particle size distribution is between 30 pm 120 pm measured according to SS-ISO 13320-1.

It is further disclosed herein a powder mixture according to any of the previous aspects wherein the chromium carbide is chosen from the group of Cr3C2 and Cr7C3.

It is further disclosed herein a powder mixture according to any of the previous aspects wherein the stainless-steel powder is austenitic.

It is further disclosed herein a powder mixture according to the previous aspect wherein the stainless-steel powder has a composition of 304 or 316 according to AISI/SAE stainless steel designation.

It is further disclosed herein a powder mixture according to any of the preceding aspects wherein the stainless-steel powder is ferritic or martensitic or ferritic/martensitic.

It is further disclosed herein a powder mixture according to the previous aspect wherein the stainless-steel powder has a composition of 400 series such as 420 according to AISI/SAE stainless steel designation.

It is further disclosed herein a powder mixture according to the previous aspect wherein the stainless-steel powder has a composition of 400 series such as 430 according to AISI/SAE stainless steel designation.

It is further disclosed herein a method for producing an overlay weld comprising the steps of:

- obtaining a substrate or a substrate coated with a buffer layer and a powder mixture according to any of the previous aspects;

- feeding the powder mixture into a weld pool containing a melted substrate or a melted buffer layer;

- cooling the melt pool thereby obtaining an overlay weld with a wear and corrosion resistant top coating. It is further disclosed herein the method according to the previous aspect wherein energy to form the melt pool is supplied in form of a laser beam.

It is further disclosed herein a wear and corrosion resistant iron-based top coating containing at least 30% by volume of at least two dispersed carbides and having a matrix hardness of at least 430, preferably at least 500 FIV measured according to SS-EN ISO 6507-1 :2006 HV0,1 and being substantially free from cracks.

It is further disclosed herein a wear and corrosion resistant iron-based top coating according to the previous aspect wherein at least one of the dispersed carbide is chosen from the group of Cr3C2 or Cr7C3 and at least one of the dispersed carbide is from the group of WC, W2C, TiC, NbC, Mo2C or VC.

It is further disclosed herein a wear and corrosion resistant iron-based top coating according to any of the previous aspects of the wear and corrosion resistant iron- based top coating having a corrosion resistance of at least 24 hours in salt spray chamber measured according to ASTM B 117-16.

It is further disclosed herein a Use of a powder mixture according to any of aspects of the disclosed powder mixture in a laser cladding process.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows a schematic picture of the overlay weld and the sample for determine number of cracks.

Figure 2 presents the total carbide content, matrix hardness and presence of cracks in top coatings, according to Example 1.

Figure 3 shows the overlay weld having a top coating based on mixture 2, subjected to corrosion test for 24 hours, according to Example 2.

Figure 4 shows the overlay weld having a top coating based on mixture 12, subjected to corrosion test for 24 hours, according to Example 2.

Figure 5 shows a cross section of the overlay weld having a buffer layer made from 316L, according to Example 3. DETAILED DESCRIPTION

The inventors of the present invention have found that by carefully combine high melting point carbides, defined as carbides having a melting point above 2 000°C such as WC, W2C, TiC, NbC, Mo2C or VC with chromium carbides with lower melting point, a corrosion and wear resistant substantially crack free top coating can be obtained.

In the overlay weld process according to the present invention, almost all carbides that are dissolved will be chromium carbides, the released amount of carbon is matched with a corresponding amount of chromium, thus maintaining the corrosion resistance. At the same time, the released carbon or small secondary carbides in the matrix increases the hardness of the same.

This is achieved by providing a powder mixture containing or consisting of the specified carbides in specified proportions and a stainless-steel powder. The mixture to be used in an overlay welding process, preferably a laser cladding process.

Powder mixture

The powder m ixture contains at least 10% by weight at least one of chrom ium carbides, preferably at least one of Cr3C2 or Cr7C4. A least one of a high melting point carbide is contained in the powder mixture. Examples of suitable high melting point carbides are WC, TiC, NbC or VC, preferred high melting point carbides are TiC or WC.

The carbides are thoroughly mixed with suitable stainless-steel powder. Examples of such stainless-steel powder are an austenitic stainless-steel powder such as grade 304 or grade 316 defined in AISI/SAE stainless steel designation, or a ferritic stainless steel powder such as grade 430 defined in AISI/SAE, or a martensitic stainless steel powder such as grade 420 defined in AISI/SAE or a ferritic/martensitic stainless steel powder of 400 series defined in AISI/SAE, or mixtures thereof. Stainless steel powder is here defined as a powder having a content of at least 50% by weight of iron and at least 11 % by weight of chromium.

The content of chromium carbide(s) in the powder mixture is between 10-30% by weight, the content of stainless-steel powder is 40-80% by weight, and the content of high melting point carbide(s) is between 10-30 % by weight. The weight ratio between chromium carbide(s) and high melting point carbide(s)s is between 0.2 and 1.5, preferably between 0.2 and 1 .3 The powder m ixture contains at least 95% by weight, preferably at least 97% by weight, most preferably at least 99% by weight of the totality of the carbide powders and the stainless-steel powder.

The particle size distribution of the powder mixture having d50 value of between 20 pm and 75 pm and having d90 value of between 30 pm 120 pm, measured according to SS-ISO 13320-1.

Process

Laser Cladding or Laser Metal Deposition is a process where material is added onto a material surface for repair and/or to improve surface properties as wear and/or corrosion resistance. The added material can be deposited as powder or wire into a molten pool created by the high intensity laser beam. The added material solidifies onto the surface of the substrate, creating a coating which is metallurgically bonded and less than 10% diluted with the substrate material. In order to cover a specific surface, strings of coating are deposited adjacent to each other with an overlap normally between 10%-95%. The physical properties of the coating are controlled by a combination of process parameters such as laser power, laser focal spot size, translation speed, powder feed rate, overlap and shielding gas flow.

In the overlay welding process according to the present invention, the powder mixture is supplied to the weld pool. The powder mixture can be applied on the substrate, optionally previously coated with a buffer layer, and energy is supplied to form the melt pool. Alternatively, the powder mixture is directly fed into the melt pool. Energy to form the melt pool is supplied by an electrical arc or a laser beam. In some cases, a buffer layer between the substrate and the corrosion and wear resistant top coating is needed. This is especially needed in the case when the substrate is a cast iron material, such as in brake discs for cars, otherwise there will be a risk that carbon from the cast iron would diffuse into the stainless-steel overlay weld and impair the corrosion resistance. The buffer layer also provides an additional corrosion resistant effect, as it blocks the possibility for the corrosion media to get in contact with the cast iron if there are cracks or other defects in the top coating. Corrosion media in contact with the cast iron substrate will lead to corrosion and possibly spalling of the protective layer. If the wear and corrosion resistant top coating is to be applied directly on a stainless steel or a low carbon steel substrate, a buffer layer may not be necessary. A laser cladding process is especially suitable since such weld overlay process offers small heat affecter zones, HAZ, low dilution of the top coating from the substrate, or from the buffer layer, and low thermal deformation of the substrate.

Wear and corrosion resistant top coating.

The top coating is characterized by having high content of carbides, at least 30% by volume. Furthermore, the matrix hardness is at least 430 HV, preferably at least 500 HV, measured according to SS-EN ISO 6507-1 :2006 HV0,1 , and being substantially free from cracks. In this context substantially free from cracks means that no more than 4 cracks/cm in the top coating is tolerated, determined according to the following test procedure.

A buffer layer is cladded on a substrate and a subsequent top coating is cladded on the buffer layer. The width of the overlay weld shall be at least 20 mm. A cross section of the overlay weld, perpendicular to the clad direction, is prepared. The number of cracks having an extension in the clad direction and penetrating throughout the top coating is determined by optical microscopy at a magnification of 30 times. The minimum width of the cross section examined is 20 mm. Figure 1 shows a schematic picture of the overlay weld and the sample for determine number of cracks.

For the top coating to obtain corrosion resistance, it must be substantially free from cracks. Many defects in the coating make it possible for the corrosive media to reach the substrate. The top coating shall withstand at least 24 hours at corrosion test in salt spray chamber according to ASTM B 117-16.

Suitable applications are for various components in hydraulic systems such as pistons or cylinders, pumps, tools such as drills, components in braking systems for automobiles where the combination of corrosion resistance and wear resistance is crucial for the function.

EXAMPLES

The following non limiting examples aim to illustrate the invention but in no means restrict the scope of the claimed invention.

The tests were made on cast iron substrate. Two different buffer layer materials were used. Buffer layer a) Stainless steel Powder from Hoganas AB, 430L 6-02, which is a gas atomized 430 L (Fe Balance+ Cr 17 % + Si 1 %) powder with a particle size 20-53 pm. b) Stainless Steel Powder from Hoganas AB, 316 L 20-53 pm, which is a gas atomized powder 316 L (Fe Balance + Cr 17 % + Ni 12 % + Mo 2,5 % + Mn 1 ,5 % + Si 0,8 %) powder with a particle size 20 - 53 pm.

Powder mixture for top coating

Stainless steel powder from Hoganas AB, 430L 6-02, which is gas atomized 430 L (Fe Balance+ Cr 17 % + Si 1 %) powder with a particle size 20-53 pm.

Chromium Carbide powder from Hoganas AB, Amperit 580.002 (Cr3C2) with a particle size of 45-90 pm.

High melting point carbide powder, Titanium Carbide powder from Hoganas AB, Amperweld TiC 10-45 pm.

The following powder mixtures were prepared:

1 ) Balance 430 L + 15 % Cr3C2 + 10 % TiC

2) Balance 430 L + 10 % Cr3C2 + 20 % TiC

3) Balance 430 L + 15 % Cr3C2 + 15 % TiC

4) Balance 430 L + 20 % Cr3C2 + 10 % TiC

5) Balance 430 L + 25 % Cr3C2 + 10 % TiC

6) Balance 430 L + 25 % Cr3C2

7) Balance 430 L + 10 % TiC

8) Balance 430 L + 15 % TiC

9) Balance 430 L + 30 % Cr3C2

10) Balance 430 L + 30 % Cr3C2

11 )Balance 430 L + 25 % TiC

12) Balance 430L + 35 % Cr3C2 Example 1

On buffer layers formed from of 430 L 6-02 powder from Hoganas AB on substrates of cast iron, top coatings were cladded with the aid of a diode laser, Laserline LDF 7000- 40, with a laser focal spot size of 1 mm in diameter. The powders or powder mixtures, forming the buffer layers and top coatings, were supplied from a powder feeder via hoses to a continuous coaxial nozzle which distributed the powders or powder mixtures onto the laser cladding process area.

The different powder mixtures required individual adjustments to find the optimal process window, thus the range of applied process parameters were: line energy of 1-2 J/mm, laser power density of 2000 - 3500 W/mm ² , powder feed rate of 0.4 - 0.7 kg/kWh.

The different top coatings were investigated metallographically and the total carbide content per volume, matrix hardness and cracks in the coatings were determined. The total carbide content and matrix hardness are a very good indication of wear resistance. It is desired to have as high amount of carbides as possible and as high matrix hardness as possible, while the coating still being substantially free from cracks. The number of cracks in the coating was used as an indication of the weldability. Cracks above 4/cm indicates that coating is not weldable, and that the corrosion resistance is destroyed by cracks through the coating. The results are presented in the following table 1 .

Figure 2 shows matrix hardness and total carbide volumes for the top coatings and indicates if the top coatings are substantially free from cracks or not. The Example discloses that the powder mixture according to the present invention provides a top coating having a remarkable high content of carbides, high hardness and still being substantially free from cracks.

Table 1.

Example 2 Buffer layers

Stainless-steel Powder from Hoganas AB, 430L 6-02.

Powder mixtures for top coatings

Powder mixture no. 2: Balance 430L + 10 % Cr3C2 + 20 % TiC Powder mixture no. 12: Balance 430L + 35 % Cr3C2

Overlay welds were prepared according to Example 1.

The overlay welds were subjected to corrosion resistance tests in salt spray chamber according to ASTM B 117-16.

Figure 3 shows the overlay weld having a top coating based on powder mixture no. 2 subjected to corrosion test for 24 hours. No visible corrosion can be detected.

Figure 4 shows the overlay weld having a top coating based on powder mixture no. 12 subjected to corrosion test for 24 hours. Several corrosion spots are visible where the material is cracked down to the substrate. Example 3

Buffer layers

Stainless-steel Powder from Hoganas AB, 316L 20-53 pm.

Powder mixtures for wear coatings

Powder mixture no. 10, Balance 430L + 30 % Cr3C2

Powder mixture no. 6, Balance 430L + 25 % Cr3C2

Overlay welds were prepared according to Example 1 .

The metallurgical bonds between the top coatings and the austenitic stainless steel buffer layers were visually examined. It was found that there were no significant differences in the metallurgical bonds regardless the buffer layers were made of 316L powder or 430L powder, as in Example 1 . Also, hardness values and number of cracks were of similar magnitude for the overlay welds having 316L buffer layers compared to those having 430L buffer layers. Thus, Example 3 shows that the results obtained in Example 1 and Example 2 are consistent and not dependent on the type of buffer layer. Figure 5 shows a cross section of the overlay weld having a buffer layer made from 316L powder and a top coating made from powder mixture No. 10. It is evident from figure 5 that the top coating is firmly bonded to the buffer layer.