The present invention relates to a method for applying a resistant hard coating including at least

18% Cr, at least 20% Co, and up to 45% Fe on a sur- face of base material including Fe, and a resistant hard coating.

Resistant hard coatings are required in many- industrial applications, e.g. where surfaces of machine parts are sliding against each other and/or the environment is corrosive. Methods for coating such machine parts are e.g. known from EP 0 456 847 Al and US 6,723,674 B2.

A situation were surfaces of machine parts are sliding against each other can be found within the food production industry in e.g. scraped surface heat exchangers used for the production of e.g. margarine, dairy spread, mayonnaise and similar products. Naturally, the machine parts, usually manufactured from carbon steel alloys or other steel alloys with anti- corrosive properties, require a hard coating to resist the wear between sliding parts and provide sufficient durability to obtain a profitable production. However, for machine parts for use within the food production industry special care has to be taken in order to ensure production in a hygienic manner without undesired contamination. Thus, the machine parts have hitherto been coated by use of a complicated and rather expensive process.

An object of the present invention is to pro- vide a simple and cost effective method for coating machine parts made from base materials including Fe.

Consequently, the invention provides a method for applying a resistant hard coating including at least 18% Cr, at least 20% Co, and up to 45% Fe on a

surface of base material including Fe, which method comprises the steps of: a) cleaning the surface of the carbon steel alloy; and b) plasma spraying a powder comprising at least 30% Cr onto the surface to form a first coating; and c) fusing said first coating by subjecting the first coating to a laser beam that ap- plies a substantially uniform amount of energy in a limited area on the coating surface in a substantially linear movement to fuse adjacent tracks on the first coating in order to form a second coating; wherein the said adjacent tracks are fused with an overlap, said overlap being in the range of 0.1-1.5 mm, thereby forming the second coating as a single substantially uniform layer on the surface of the base material. The base material is a metallic mate- rial, preferably a carbon steel or other steel alloys based on iron. For some embodiments it is preferred to use steel alloys with anti-corrosive properties.

Thus, the method according to the invention makes it possible to coat a surface with a single layer comprising Cr and Co in a relatively simple operation in such a way that dilution of material from the carbon steel alloy into the coating material is low. This effect has surprisingly been achieved by in one step plasma spray coating material onto the sur- face of the carbon steel alloy and in a subsequent step fusing the plasma sprayed coating by means of a laser.

In order to obtain good adherence of the sur-

face to be coated it is initially cleaned, e.g. by- sand-blasting and degreasing with a suitable degreas- ing agent .

The plasma sprayed coating can be applied with any suitable commercially available plasma spray gun, e.g. a single wire arc plasma gun (SWAP gun) or a vacuum plasma gun (VP gun) .

The plasma sprayed coating is relatively porous and the subsequent fusing by laser melts the coating and causes densification so that a porosity of less than 1.6% can be obtained. A coating with low porosity is desirable for applications within the food production industry as a low porosity in material in contact with food products contributes to ensuring good hygienic conditions.

Moreover, the fusing of adjacent tracks with an overlap within a range of 0.1-1.5 mm serves to ensure that the entire coating layer is fused to minimize porosity. The treatment also contributes to elirninat- ing cracks and binding errors in the coating.

The coating applied according to the invention has excellent properties in respect of hardness, durability and corrosion. The properties are at least partly due to the high content of chromium and cobalt and the fact that the dilution is surprisingly low so that the content of iron in the coating (which at least partly originates from the base material) is no higher than 45%. Dilution is caused by melted material from the base material including Fe that mi- grates into the coating material during the laser fusing.

A suitable powder to be used in the plasma spraying process may have the following composition

10-16% W, 25-45% Cr, 0.1-1% Mn, 12-16% Fe, 32-40% Co, and 8-13% Ni. The composition may further comprise minor amounts of Si and Mo. The relatively high amounts of Cr and Co give rise to the good corrosive and wear resistant properties present in the final coating.

Moreover, the overlap of the adjacent tracks results in a uniform coating substantially free of cracks, and it is preferred that the overlap is in the range of 0.25-1.25 mm.

In a preferred embodiment of the method according to the invention the width of the tracks is in the range of 4 to 21 mm. Keeping the width of the tracks within these ranges ensures a good quality of the coating and that a satisfactory time for production is obtained.

Although the limited area may be of any desired shape e.g. circular, square, etc. it is preferred that the limited area in which the laser beam applies a substantially uniform amount of energy is a rectangular area which has a width in the range of 0.5 to 12 mm and a length in the range of 4 to 21 mm. Consequently, the length of the rectangular area corresponds to the width of the tracks, fused by the Ia- ser. Moreover, it is also preferred that the width of the rectangle is substantially parallel to the direction of the substantially linear movement of the limited area within which the laser energy forms the fused tracks in the coating material . Preferably the rectangular area has a width in the range of 6.0 to

10 mm and a length in the range of 7 to 18 mm.

In order to obtain a reasonable production time balanced against the obtained quality of the final

coating it is preferred that the velocity of the substantially linear movement is in the range of 200 to 3000 mm/min. , more preferred in the range of 400 to 2000 mm/min. Although any suitable laser can be utilised in the method it is preferred that the laser has an effect in the range of 1 to 7 kW, more preferably in the range of 2 to 4 kW. The effect of the laser is a question of balance between proper fusing of the coating layer and avoiding excessive melting of the base material in order to minimize the effect of dilution. The laser can for instance be a CO ₂ laser, a YAG laser, or a diode laser.

In a preferred embodiment of the method accord- ing to the invention the adjacent tracks are located on the inner surface of a tubular member and form a helical pattern on the inner surface. In this fashion the substantially linear movement is in fact slightly curved in order to follow the curved surface of the tubular member. However, when viewed on a short section the movement is substantially linear. In such an arrangement the plasma sprayer and the laser can be mounted on rods which are led into the tubular member. It has been found that the best results are ob- tained by keeping the plasma sprayer and the laser in a static position and rotating and sliding the tubular member in a suitable manner to meet with the above stated requirements for width of tracks and overlaps etc. Of course, it is also possible to ro- tate and move the plasma sprayer and/or the laser and keep the tubular member in a static position.

In order to obtain a surface suitable for use within e.g. the food production industry it is pre-

ferred that the coated surface is subjected to a further treatment by honing, polishing, turning and/or grinding. In this manner a smooth surface can be obtained which is easy to clean and maintain, and fur- thermore has low friction when sliding against other parts .

If a thickness of the coating in excess of approximately 0.8 mm is desired, steps b and c of the method are preferably repeated one ore more times in order to obtain the desired thickness of the coating. In this manner it is possible to obtain a coating according to the invention with a thickness of several mm. The coating, thus, ^' comprises two or more layers and the preferred thickness of each individually layer is in the range of 0.1 to 0.8 mm, preferably in the range of 0.2 to 0.6 mm.

The arrangement of plasma spraying equipment and laser equipment applied in the method according to the invention may naturally comprise further equipment like e.g. cooling devices, inert gas supplies to obtain non-oxidation zones and similar equipment normally used in plasma spraying and laser fusing processes.

In one aspect, the invention thus relates to use of the method for coating the inner surface of a metallic tube, preferably a carbon steel alloy tube, a stainless steel tube or a duplex metal tube. The metallic tube or carbon steel alloy tube may be a chilling or heating tube for scraped surface heat ex- changers and may e.g. have an inner diameter in the range of 50 to 400 mm, preferably in the range of 125 to 350 mm.

In a further aspect, the invention relates to a

resistant hard coating including at least 18% Cr, at least 20% Co, and up to 45% Fe on a surface of base material including Fe, wherein the coating constitutes at least one layer on the surface of the base material and has a porosity of less than 1.6% and a hardness of at least 200 HV (HV Vickers hardness according to EN 1043) . Preferably, the coating is obtained by the above-described method.

The coating is particularly suitable for use in devices to be used within the food production industry, in particular devices having parts sliding against each other e.g. scraped surface heat exchangers. The coating according to the invention is particularly advantageous as it can be applied as a sin- gle layer using the method discloses above. Moreover, the coating according to the invention is a hard, resistant coating that can be applied to a surface of base material by use of a cost-effective method. One preferred base material including Fe is carbon steel alloy. Carbon steel alloys as such are cheap and strong materials with good heat transfer properties. However, carbon steel alloys are susceptible to wear and corrosion. Therefore, it is desirable to coat exposed surface parts of the carbon steel alloys with a hard resistant coating according to the invention.

The coating according to the invention preferably has a porosity of less than 1.0%, preferably less than 0.8%, more preferably less than 0.5%. A low porosity is desirable for coatings to be used in con- nection with food production as low porosity of the coating makes the coated surface easier to clean and minimizes the risk of uncontrolled bacterial growth.

For the purpose of making the coating suffi-

cient wear resistant it is preferred that the hardness of the coating is at least 300 HV, more preferably at least 450 HV, even more preferably at least 500 HV. Moreover, in order to ensure that the coating has a sufficient long life it is preferred that the coating has a thickness in the range of 0.10 to 0.80 mm, more preferably in the range of 0.15 to 0.75 mm, most preferably in the range of 0.25 to 0.65 mm. A suitable thickness of the coating also contributes to minimizing the risk of cracks and other errors in the coating, which will diminish the value of the coating. Accordingly, it is preferred that the coating is substantially free of cracks. Although the coating according to the invention may comprises a wide range of components as long as the content of Cr is at least 18%, the content of Co is at least 20%, and the content of Fe is up to 45%, it is, however preferred that the coating comprises 1-2% Si, 3.5-5.5% W, 20.0-23.5% Cr, 0.5-2.0 Mn, 28.0- 45.0% Fe, 22.0-29.5% Co, and 4.8-7.4% Ni. Coatings having such compositions appear to have excellent properties in respect of hardness and resistance to wear and corrosion. For some embodiments it may be desirable to apply a coating with a thickness in excess of 0.8 mm. For such embodiments it is preferred that the coating comprises two or more layers, where each layer has a porosity of less than 1.6% and a hardness of at least 200 HV. Consequently, when a thick coating is required, it is advantageous to have the coating applied as two or more layers where each layer has a maximum thickness of 0.8 mm. In this manner it is

possible to obtain a thick low-porous, hard and substantially error-free coating. The coating may have a thickness of several mm, e.g. 1.5 mm. 2.0 mm or 3.0 mm. Although the base material including Fe may be any iron based alloy, e.g. carbon steel, stainless steel or duplex steel, having any desired shape it is, however, in one specific embodiment preferred that the coating according to the invention is placed on the inner surface of a metallic tube, preferably a carbon steel alloy tube. The carbon steel alloy tube may e.g. be a chilling tube for use in a scraped surface heat exchanger. Consequently, the invention also relates to use of a coating according to the inven- tion on the inner surface of a chilling tube or scraped surface heat exchanger. The coating has proven to have excellent properties for use on the inner surface of a chilling tube and a scraped surface heat exchanger due to the coating's outstanding properties in respect of resisting wear and corrosion during use of the chilling tube and scraped surface heat exchanger .

In a preferred embodiment the base material comprises different alloys joined together e.g. by welding. In such an embodiment, a centre piece may be constituted by carbon steel and one end piece may be constituted by stainless steel and the other end piece may be constituted by duplex steel .

In case of a tube, the major part of the tube may be carbon steel provided with end pieces of stainless steel and/or duplex steel . In a preferred embodiment of a tube for a scrapped heat exchanger, most of the tube is made from carbon steel, one end

is made from stainless steel and the other end is made from duplex steel . The two end pieces of stainless steel and duplex steel are welded to the carbon steel. The interior part of the tube, includ- ing the end parts of stainless steel and duplex steel, is coated with a coating according to the invention. The advantage of the embodiment is that chroming of the end pieces can be avoided and due to lesser shrinkage of the stainless steel and/or duplex steel it is easier to obtain the desired dimensions for connection.

Furthermore, the invention also relates to use of a coating according to the invention in apparatus for food production. Apparatus for food production may be subjected to rather harsh treatment, in particular when they are cleaned, as they have to meet with the requirements for hygiene in the food production industry, therefore, resistant and durable coatings are desirable for such apparatus. Accordingly, such apparatus may be for the production of food for animals, e.g. pet food or food for farm animals. Such apparatus may also be for the production of food for humans. The food products may be within a large variety of products, e.g. margarine, mayonnaise, ketchup, dairy spread, cheese spread, minced meat, dough, ice cream etc.

Although the invention has been described in particular in relation to carbon steel alloys, it is never the less clear that the coating can, in princi- pie, be applied to any metal alloy surface suitable to receive a hard resistant coating according to the invention.

The invention will now be described in further

details with reference to an example and drawings in which:

Figure 1 schematically depicts an arrangement for plasma spraying on a surface on the inside of a tubular member.

Figure 2 schematically depicts an arrangement for laser fusing a surface on the inside of a tubular member. Figure 3 shows a SEM picture of a cut through a plasma sprayed coating.

Figure 4 shows a SEM picture of the coating after laser fusing.

Figure 5 shows a preferred embodiment according to the invention

Plasma spraying is a technique where a powder is sprayed onto a surface through a heating zone constituted by plasma. The powder melts at least partly and adheres to the surface. The temperature in the heating zone is typically in the range of 1000 to 1300 ⁰ C.

Laser fusing is a technique where a material is fused by a laser beam. The laser beam is typically directed towards the material to be fused by means of mirrors and optics.

Plasma spraying and laser fusing are well-known techniques .

In the context of this invention it is desir- able to perform plasma spraying and laser fusing as separate steps as plasma spraying is a "dirty" process that might cause impurities that could spoil the function of the laser equipment if the processes were

performed in a single step.

The final coating is the coating ready for use in honed and optionally polished conditions. The final coating may be constituted by a single layer or comprise two or more layers.

All percentages are weight percentages unless for porosity where the percentages are volume percentages .

Figure 1 depicts an arrangement for plasma spraying powder onto a surface on the inside of a tube. Figure IA depicts a side view of the arrangement and figure IB depicts an end view of the arrangement .

A flexible arm 1 is located on a control sta- tion 2 at one end. At the other end the flexible arm

I is equipped with a plasma spray device 3. The contour of a tubular member 4 is indicated by dotted lines. The tubular member 4 is placed in rollers 5 on a bench 6 which can move in the direction indicated by arrow 7.

Figure 2 depicts a similar arrangement for laser fusing the plasma sprayed surface on the inside of the tube. Figure 2A depicts a side view of the arrangement and figure 2B depicts an end view of the arrangement.

A flexible arm 11 is located on a control station 12 at one end. At the other end the flexible arm

II is equipped with a laser device 13 and a supply for inert gas 14. The contour of a tubular member 15 is indicated by dotted lines. The tubular member 15 is placed in rollers 16 on a bench 17 which can move in a direction indicated by arrow 18.

As mentioned, the drawings are only schemati-

cal, and means for cooling, controlling etc. have been omitted for reasons of simplicity.

By adjusting the rotation speed of the rollers and the speed of movement of the bench in figure 2, it is possible to obtain adjacent laser fused tracks on the inside surface of the tube in a helical pattern.

Example 1

A layer according to the invention was produced using the method according to the invention. The equipment used was arranged as outlined in figure 1 and 2. The plasma spray equipment was a plasma spray gun (Sulzer Metco F4 with powder supply)

The laser equipment was a high power CO ₂ laser (Rofin Sinar SR170) with line focus mirrors designed to treat a rectangular area with dimensions 7.0x1.0 mm.

A tube with a length of 1800 mm and an inner diameter of 150 mm manufactured from carbon steel alloy (carbon steel 52-4, DIN1630) had the inner surface sandblasted and cleaned with a degreasing agent (TRI) before it was placed in the equipment for plasma spraying.

A standard powder composition (Stellite no. 6) with the following composition:

W 13.21% Mo 0.87%

Cr 26.50% Mn 0.17%

Fe 13.94%

Co 35.44%

Ni 9.87% was mixed with further 20% Cr and 1.2% Si (as fusing aid) in powder form and plasma sprayed onto the inner surface of the tube at a temperature of approximately 1150 ⁰ C, forming a coating with a porosity of about 21% (see figure 3) . The thickness of the layer was approximately 0.4 mm.

The tube was subsequently placed in the equip- ment for laser fusing. The rectangular focus area

(7.0-1.0 mm) applied an energy of 4kW to the treated area in tracks with a width of 7 mm and overlaps of

0.5 mm. The treatment speed was 300 mm/min.

The resulting coating had the following compo- sition:

Si 1.68%

W 3.94%

Mo 0.07%

Cr 21.58% Mn 1.28%

Fe 39.85% Co 25.70% Ni 5.89%

The coating had a porosity of about 1% (see figure 4) and hardness of about 510 HV. The thickness of the layer was about 0.35 mm.

The coating appeared as a helical pattern and after honing, where about 0.05 mm of the material was removed, the coating had a smooth mirror-like finish. Eddy current tests and penetration tests showed that the coating was free of cracks.

Example 2

A tube as depicted in figure 5 was subjected to a treatment corresponding to the treatment in example 1. The tube is for use in a scrapped surface heat exchanger. The tube 100 also had a length of 1800 mm and an inner diameter of 150 mm. The centre part of the tube 101 is carbon steel alloy (carbon steel 52- 4, DIN1630) . The first end 102 is made from stainless steel (stainless steel 1.4571 DIN EN 10028-7) . The first end 102 is the entrance end shaped as a flange 103 (for connecting the tube with a feeding pump - not shown) The second end 104 is made from duplex steel (duplex steel 1.4462 DIN EN 10028-7). The first end 102 and the second end 104 are attached to the centre part 101 by welded portions 105. The whole interior surface 106 of the tube 100, including the interior surface of the first end 102 and the interior surface of the second end 103, was coated with a coating 107 as described above.

During manufacture, the first end 101 and the second end 102 had less tendency to shrink than the centre part 103, thereby maintaining the tube 100 at dimension suitable for connection with other parts with standard dimensions.