ONSOEIEN MORTEN I (NO)
AKSELSEN ODD MAGNE (NO)
LOU DECHUN (NO)
ONSOEIEN MORTEN I (NO)
AKSELSEN ODD MAGNE (NO)
| CN1827835A | 2006-09-06 |
115917/CH
CLAIMS
1. Boron containing paste with B 4 C, SiC and NaF for coating of metal components to achieve protective layer(s) on its surface through heating, c h a r a c t e r i s e d b y a powder mixture comprising of the following (wt%)
3-90% B 4 C,
0-85% SiC,
5-20% NaF, 2-10% (NH 2 ) 2 CO,
2-5% active carbon
0-5 % RECI, where RE is cerium (Ce) or lanthanum (La), mixed with a binder solution which provides the paste consistence.
2. Boron containing paste according to claim 1 , wherein the binder solution is 1-20% shellac in 80-99% ethanol, this binder solution is added in amounts of 20- 40% to the powder mixture.
3. Boron containing paste according to claim 1 , where the powder mixture comprises
5-10% B 4 C, 68-83% SiC, 5-10% NaF, 3-6% (NH 2 ) 2 CO, 2-5% active carbon and 1-2% RECI.
4. Boron containing paste according to claims 1 , where the powder mixture comprises 20-40% B 4 C, 30-60% SiC, 10-20% NaF, 5-10% (NHs) 2 CO, 2-5% active carbon and 2-5% RECI.
5. Boron containing paste according to claims 1-2, where the powder mixture comprises
70-82% B 4 C, 10-20% NaF, 3-7% (NHs) 2 CO and 2-5% active carbon.
6. Application of boron containing paste according to claim 3, for boronizing of steel and cast iron components heat treated at a temperature from 850 to 1100 0 C for a period up to 10 hours.
7. Application of boron containing paste according to claims 4 or 5, for boronizing of steel and cast iron components heat treated at a temperature from 650 to 850 0 C for a period up to 40 hours.
8. Application of boron containing paste according to claim 5, for boronizing of a metal surface made in Ni and Ni alloys, or Co and Co alloys heated to a temperature from 700 to 1100 0 C for up to 10 hours.
9. Application of boron containing paste to provide protective layer(s) on a metal surface characterised by • the metal component is supplied with paste on the surfaces to be protected according to claims 1 -5,
• the metal component with paste is dried at a temperature up to 100 0 C,
• the metal component with paste is heated to a temperature in the range from 600 to 1100 0 C for up to 40 hours, and excess paste is removed. |
Self-protective boron containing paste and application of this on metal components
The present invention relates to a paste for boronizing of metal components to provide diffusion coatings that enhance the resistance against wear and corrosion in certain environments.
It is well known to boronize metals to achieve better surface protection of the metal. The boron sources are various chemical compounds such as amorphous or crystalline boron, ferroboron, boron carbide or borates (e.g., borax). The reactive compounds are often chlorine- or fluorine-containing species. Examples are alkaline compounds with Cl and F. The compound most frequently used is potassium tetrafluoroborate, KBF 4 . The inert species have often been SiC, SiO 2 or AI 2 O 3 . Such mixtures can be in the form of powders, pellets (see DE 21 27 096, 1971) or paste (e.g., DE 26 33 137). The pellets and the paste also comprise of binder and water. The paste has the advantage that it easily adheres to the surface of the metal substrate to be coated.
US patent 2002/0036030 A1 describes boronizing agent in paste form to provide boronized layer on metal surfaces. The paste contains species that release boron atoms and activating agents, and the rest is inert refractory fillers together with water and eventually other agents to form the paste. It also contains additives such as CaCO 3 and/or LiCO 3 , at least one compound from the Group I (alkaline) or Group Il (earth alkaline) metal nitrites, and one compound from the group of water soluble alkaline- and earth alkaline metal borates.
The disadvantage with these pastes is that they must all be used with protective (inert gas) or reducing atmosphere during the heat treatment boronizing process. The most usual boronizing process is the pack boronizing.
The present invention relates to a paste that is much more easy to use. The paste is self-protective, which means that it can be applied without the need for protective or reducing atmosphere.
Another advantage is that corrosion attack of the metal substrate is avoided prior to drying compared with normal water containing paste.
In the article "Surface modification of steel and cast iron to improve corrosion resistance in molten aluminium" by D. C. Lou, O. M. Akselsen, M. I.
Onsøien, J. K. Solberg, J. Berget, Surfaces & Coatings Technology 200 (2006) p. 5282-5288, it is shown experimental results that demonstrates that the boronizing reduces the corrosion rate substantially when the paste contains boron releasing species (B 4 C), activating species (KF 4 B, (NH 2 ) 2 CS and NaF), filling compound (AI 2 O 3 , graphite and SiC) and a sticky solution of cellulose.
The present invention brings forward a boron containing paste with addition of B 4 C, SiC and NaF, for coating the surface of a metal substrate by heat treatment to achieve a protective layer on the metal surface, where the paste composition comprises of (wt%):
3-90% B 4 C,
0-85% SiC,
5-20% NaF,
2-10% (NH 2 ) 2 CO, 2-5% active carbon,
0-5 % RECI, where RE (rare earth metal) is cerium (Ce) or lanthanum (La), together with a binder that provides the paste consistence.
LIST OF FIGURES
Fig. 1. Principles of boronizing using self-protective paste coating (schematic): (a) metal substrate with deposited 3-5 mm thick paste, (b) drying at low temperature (<100 0 C), (c) formation of protective glassy layer at the paste surface during heating and diffusion of boron into the surface of the metal, (d) diffusion layer is formed.
Fig. 2. Boronized layer on carbon steel: (a) AIS1 1040, (b) AISI 1060. Fig. 3. Boronized layer on carbon steel: (a) 0.2% C, (b) 0.4% C.
Fig. 4. Boronized layer on tool steel (QRO90).
Fig. 5. Boronized layer on ductile iron. Boronized at (a) 850 0 C, (b) 950 0 C. Fig. 6. Boronized layer on ductile iron at 650 0 C for 24 hours. Fig. 7. Boronized layer on tool steel: (a) QRO90, (b) H13. Fig. 8. Boronized layer on tool steel (QRO90). Treated at 650 0 C for 20 hours.
Fig. 9. Boronized layer on duplex stainless steel (22Cr-5Ni). Treated at 950°C for 4 hours.
Fig. 10. Boronized layer on nickel. Treated at 950 0 C for 4 hours.
Fig. 11. Boronized layer on cobalt. Treated at 950 0 C for 4 hours.
Fig. 12. Boronized layer on nickel alloys: (a) Nimonic 90, (b) lnconel 625.
Detailed description of the invention
Depending on the material to be boronized, different paste compositions can be used (Table 1 ). For boronizing of low alloy steel and cast iron at temperatures above 850 0 C, which is the most conventional application of the paste, it is not necessary to use more than 5-10 wt% B 4 C, preferably 5-7 wt%. The amount of SiC should be high, i.e., 68-83 wt%, preferably 76-82 wt%. The compounds SiC, B 4 C and NaF contribute to the formation of a glassy layer which protects the surface of the metal substrate from reaction with the air in the furnace. In many cases it will be rather beneficial to carry out the boronizing heat treatment at lower temperature to avoid internal stresses and deformation of the metal substrate. By the use of high content of B 4 C, i.e., 70-82 wt%, preferably 75-80 wt%, it is possible to perform boronizing in the temperature range of 600-850 0 C within a time period of up to 40 hours. This is regarded to be an important embodiment of the present invention. If deformation is not critical high boronizing temperature will be selected to shorten the duration to less than 10 hours.
Paste with intermediate content of boron containing species, e.g., with 20- 40 wt% B 4 C, can also be used in the temperature range of 600-850 0 C. However, with the high costs of B 4 C in mind it may be desirable to use a lower amount, depending on the metal substrate type and heat treatment temperature.
Table 1. Preferred chemical composition (in wt%) of the paste and corresponding temperature range for boronizing of different substrates.
The NaF content in the paste is 5-10 wt%. NaF is a reactive component that is preferred in stead of KBF 4 , since NaF will give lower exhaust of fluorine due to its higher melting point.
Urea, (NH 2 ) 2 CO, is added in amounts of 2-10 wt%. Urea is preferred rather than thiourea, (NH 2 ) 2 CS, which is very harmful to health.
The RECI is added as diffusion aid in boronizing of steel and cast iron. It does not give similar effect for Ni- or Co-alloys.
Active carbon may also contribute to the diffusion process, and is recommended used in the range of 2-5 wt%.
5 As binder, 1-20 wt% shellac is dissolved in 80-99% ethanol. This solution is added in amounts of 20-40 wt% to the powder mixture. This is very beneficial when compared with organic binder that needs to be dissolved in water, which during use may cause corrosion attacks on the metal surface. Other binders may also be used, provided that they can be dissolved in ethanol. An example of PFo resin has been tested and the thickness of the diffusion layer and phase types were the same as using shellac as the binder. This is because the binders do not take chemical reaction in the paste, and only provides the paste consistence.
The diffusion coatings that are formed may consist of one or more layers of metal borides. They have high hardness, and they are relatively homogeneouss with low fraction of porosity. The actual type of boride that forms, and thus the resulting hardness, is strongly dependent on the type of material being boronized and the associated heat treatment procedure. The thickness of the diffusion coating varies from a few μm to 300 μm.
Another advantage is that NaF can not be dissolved in ethanol, whicho means that hydrogen fluoride (HF), which can be formed when using water in the paste, can be avoided, and thus, reduce exhaust of environmentally harmful waste.
Another aspect of the invention is the technique of supplying the metal substrate with the protective layers, which comprises of the following main stages:5 • supplying paste according to any of the claims 1 -5, on the metal surfaces to be coated,
• drying the metal substrate with paste coating at a temperature within the range from room temperature to 100 0 C,
• heating the metal substrate covered with paste to temperature within theo range from 600 to 1100 0 C for up to 40 hours, removing excess paste.
After boronizing the excess paste can be removed easily with a brush or in rinsing water.
Boronizing according to the present invention can be applied to objects in steel and cast iron, Ni and Ni alloys and Co and Co alloys. It is a huge advantage that the paste can be applied at lower temperatures
(600-850 0 C) if the metal substrates need to satisfy tolerance requirements, and the deformation of the component is critical.
Extensive cleaning of the sample to be boronized, as necessary in normal boronizing, is not required in the present case due to the addition of ethanol and urea. In addition, rust removal is not necessary due to the oxide removal action by the paste. The surfaces which are to be boronized are covered by a 3-5 mm thick layer of paste as illustrated in Fig. 1. Then the paste is dried at room temperature or in an incubator up to 100 °C.
The principles involved in boronizing with self-protective paste are schematically shown in Fig. 1. In Fig. 1a the specimen top surface is coated with
3-5 mm thick paste layer, then (Fig. 1 b) the paste is dried at low temperature
(<100 0 C). In Fig. 1c a glassy layer is formed on the paste surface to protect the metal sample from reacting with the furnace atmosphere, and boron atoms diffuse into the metal surface. In Fig. 1d the completion of the diffusion layer is shown. The self-protection is achieved over the entire temperature range as briefly described below.
Low temperature (< 600 0 C):
Within the temperature range between 150 and 570 0 C, urea will decompose. Several reactions are possible:
(NH 2 ) 2 CO (liq) → CO (g) + [N] + 2H 2 (g) (1 )
2CO (g) → [C] + CO 2 (g) (2)
The gases H 2 and CO provides reducing atmosphere, and may remove the oxide layer on the metal surface. Nitrogen [N] and carbon [C] may diffuse into the
surface of the substrate and increase the solubility of boron, and hence, increase the thickness of the diffusion layer.
In the ternary system Fe-H 2 -H 2 O and Fe-CO-CO 2 , the following reactions may take place:
4H 2 ( g) + Fe 3 O 4 (S) → 3Fe (s) + 4H 2 O (g) (3)
H 2 (g) +FeO (s) → Fe (8) + H 2 O (g) (4)
CO ( g) + Fe 3 O 4 (β) → 3FeO (8) + CO 2 (g) (5)
CO (g ) + FeO (S) → Fe (s) + CO 2 (g) (6)
The gases which form in these reactions may also contribute to reduce the oxide on the substrate surface, and protect it against re-oxidation.
High temperature (> 600 0 C):
When the temperature exceeds 600 0 C, the B 4 C and SiC compounds may oxidise due to reaction with the oxygen in the furnace atmosphere. The reaction product may, together with the NaF compound, form a glassy reaction product on the surface of the paste, while the paste between the glassy layer phase and the substrate will still be in powder shape.
After the heat treatment the remaining paste will be removed from the metal surface by light scratching with a brush or rinsing in hot water when the substrate has been cooled down to room temperature.
Applications
Some possible applications of self-protective paste are: • Coating of parts subjected to wear, erosion and corrosion in the paper and pulp industry
• Coating of tools for metal cutting and forming
• Coating of components used to handle in aluminium melt
• Coating of wear parts (bearings, drill bits, knives, cutters, tools for thermal spraying and components in welding equipment, etc.)
• Chemical and petrochemical industry: ball valve, tubing, pipe parts, bend, fittings, dies/jets, etc. • Transport industry: coating of engine parts (piston ring, cylinder lining, cam shaft, connection rods, valve seat, etc.)
• Hydro power: parts of turbine valve
Examples (all amounts in wt%) Example 1
Substrate of conventional carbon steels (AISI 1040 and AIS1 1060) was boronized at 850 0 C for 4 hours in muffle furnace in air atmosphere with a paste. The powder mixture consists of 6% B 4 C, 5% NaF, 4% (NH 2 ) 2 CO, 3% active carbon, 2% RECI, 80% SiC (all in wt%). Finally, an amount of 20-30% ethanol with 1% shellac was added into the powder mixture to make the paste. After boronizing the excess paste is easily removed by brush or in rinsing water. No corrosion attack on the substrate was found. The boronized layer consisted of Fe 2 B with thickness of 50- 60 μm, cf. Fig. 2, which shows an optical micrograph of boronized (a) AIS1 1040 and (b) AIS1 1060 steel grades.
Example 2
Substrate of conventional low carbon (0.2 %C) and medium carbon steel (0.4 %C) was boronized at 750 °C for 6 hours in muffle furnace in air atmosphere with a paste. The powder mixture consists of 80% B 4 C, 10%NaF, 5%(NH 2 ) 2 CO, 5% active carbon. Finally, an amount of 20-30% ethanol with 1 % shellac was added into the powder mixture to make the paste. After boronizing the excess paste is easily removed by brush or in rinsing water. No corrosion attack on the substrate was found. The boronized layer consisted of Fe 2 B with thickness of 20-40 μm, cf. Fig. 3, which shows optical micrograph of boronized (a) low carbon steel (0.2 %C) and (b) medium carbon steel (0.4 %C).
Example 3
Substrate of tool steel (QRO90) was boronized at 700 °C for 20 hours in muffle furnace in air atmosphere with a paste. The powder mixture consists of 35% B 4 C, 15% NaF, 10% (NHa) 2 CO, 3% active carbon, 2% RECI, 35% SiC. Finally, an 5 amount of 20-30% ethanol with 1 % shellac was added into the powder mixture to make the paste. The boronized layer consisted of Fe 2 B with thickness of 10-15 μm, Fig. 4, which shows optical micrograph of QRO90 tool steel boronized at 700 0 C. o Example 4
Substrate of ductile iron was borided at 850 and 950 °C for 4 hours in muffle furnace in air atmosphere with a paste. The powder mixture consists of 7% B 4 C, 5% NaF, 4% (NH 2 ) 2 CO, 3% active carbon, 2% RECI, 79% SiC. Finally, an amount of 20-30% ethanol with 10% shellac was added into the powder mixture to makes the paste. After boronizing, excess paste was removed with brush and no corrosion attack can be seen. The borided layer is Fe 2 B with thickness of 60-80 μm and 250-300 μm for 850 and 950 °C, respectively. This is shown by the micrographs contained in Fig. 5 for the ductile iron borided at (a) 850 °C and (b) 950 0 C.o
Example 5
Substrate of ductile iron was borided at 650 °C for 24 hours in muffle furnace with a paste. The powder mixture consists of 75% B 4 C, 10%NaF, 5% ReCI, 5% (NH 2 ) 2 CO, 5% active carbon. Finally, an amount of 20-30% ethanol with 1%5 shellac was added to make the paste. After boronizing the excess paste was removed in hot water, and no corrosion attack on the substrate was found. The borided layer was Fe 2 B with thickness of 20-30 μm, as shown in Fig. 6.
Example 6o Substrate of tool steel (QRO 90 and H13) was borided at 1020 °C for 1 hour in muffle furnace in air atmosphere with a paste. The powder mixture consists of 5% B 4 C, 5% NaF, 4% (NH 2 ) 2 CO, 3% active carbon, 1% RECI, 82% SiC. Finally, an amount of 20-30% ethanol with 1% shellac was added into the powder mixture to
make the paste. After boronizing excess paste was removed with brush, and no corrosion attack was seen. The borided layer was Fe 2 B for the QRO90 steel and FeB + Fe 2 B for the H13 steel, both coatings with thickness of 50-70 μm. This is shown in Fig. 7 for the two tool steels: (a) QRO 90 and (b) H13 substrates.
Example 7
Substrate of tool steel (QRO90) was borided at 600-650 0 C for 20-40 hours in muffle furnace in air atmosphere with a paste. The powder consists of 80% B 4 C, 10%NaF, 5%(NH 2 ) 2 CO, 5% active carbon. Finally, an amount of 20-30% ethanol with 1% shellac was added into the powder mixture to make the paste. After boronizing the excess paste was removed in hot water. No visual corrosion attacks were found. The boronized layer was Fe 2 B with thickness of 4-7 μm. This is shown in Fig. 8.
Example 8
Substrate of duplex stainless steel (22Cr-5Ni) was borided at 950 °C for 4 hours in muffle furnace in air atmosphere with a paste. The powder mixture consists of 5% B 4 C, 5% NaF, 5% (NH 2 ) 2 CO, 5% active carbon, 2% RECI, 78% SiC. Finally, an amount of 20-30% ethanol with 1 % shellac was added into the powder mixture to make the paste. After boronizing the excess paste was removed with steel brush. No visual corrosion attacks were found. The borided layer was (Fe 1 Cr)B + (Fe 1 Cr) 2 B, and had thickness of 50-60 μm. This is illustrated by the micrograph in Fig. 9.
Example 9
Substrate of nickel was borided at 950 0 C for 4 hours in muffle furnace in air atmosphere with a paste. The powder mixture consisting of 80% B 4 C, 10%NaF, 5%(NH 2 ) 2 CO, 5% active carbon. Finally, an amount of 20-30% ethanol with 1% shellac was added to make the paste. After boronizing the excess paste was removed with hot water. There was no corrosion attack visible on the substrate. The layer consisted of Ni 2 B with thickness of 100-130 μm, as shown in Fig. 10.
Example 10
Substrate of cobalt was bonded at 950 °C for 4 hours in muffle furnace in air atmosphere with a paste. The powder mixture consists of 80% B 4 C, 10%NaF, 5%(NH 2 ) 2 CO, 5% active carbon. Finally, an amount of 20-30% ethanol with 1% shellac was added into the powder mixture to make the paste. After boronizing the excess paste was removed in hot water. No corrosion attacks were found on the substrate. The borided layer was CoB with thickness of 100-200 μm, as indicated in the optical micrograph in Fig. 11.
Example 11
Substrate of superalloys (Nimonic 90 and lnconel 625) was borided at 950 °C for 3 hours in muffle furnace in air atmosphere with a paste. The powder mixture with the following chemical composition: 80% B 4 C, 10%NaF, 5% (NH 2 ) 2 CO, and 5% active carbon. Finally, an amount of 20-30% ethanol with 1% shellac was added into the powder mixture to make the paste. After boronizing the excess paste was removed in hot water. Corrosion attack on the substrate was not found. The borided layer was CrB and Ni 2 B in both alloys with thickness of 50-60 μm. This is shown in the optical micrograph in Fig. 12 for Nimonic 90 (Fig. 12a) and lnconel 625 (Fig. 12b).