Electric heating – Metal heating – Nonatmospheric environment at hot spot
Reexamination Certificate
1999-03-23
2001-12-18
Ryan, Patrick (Department: 1725)
Electric heating
Metal heating
Nonatmospheric environment at hot spot
C219S073200, C219S073000, C219S072000, C219S076100, C219S136000, C219S137200
Reexamination Certificate
active
06331688
ABSTRACT:
The present invention concerns a metal powder for submerged arc welding. Specifically the invention concerns the use of high-alloy metal powders for surface coating of a metal substrate by the submerged arc welding method.
The submerged arc welding (SAW) method is a well known method, the principle of which is described in eg Chapter 28 of Welding Handbook No 2, 4th Edition, (1958) Section 2, Welding Processes, which is hereby incorporated by reference.
At present the SAW method is considered to be a very rational method for welding and surface coating. Even though this method is highly rational there is an interest to increase its productivity. One way of increasing the productivity is to use multi-wire systems in combination with high amperage. Another way is to add carefully measured amounts of metal powders to the weld joint during the whole welding process. The metal powder can be added either by “forward feed” which means that the powder is fed in front of the flux feed and the electrode wire, or by “direct feed” which means that the powder is magnetically attached to the electrode wire(s). The metal powders currently used are essentially unalloyed metal powders, whereas the elements of the desired for the final joint and the compostion thereof is decided by the electrode wire(s) or electrode cord(s), which contain high amounts of alloying elements.
According to the present invention it has now been found that the SAW method can be improved by using a prealloyed, atomised metal powder containg high amounts of alloying elements in combination with at least one melting electrode wire or cord consisting of unalloyed or low-alloy metal. It is essential that the powder is prealloyed as otherwise it will not be possible to obtain a uniform coating. Another important feature is that the powder is provided on the outside of or separate from the electrode wire used in the SAW method.
Especially important advantages are obtained when the method according to the present invention is applied for surface coating of metal substrates such as low alloy steel substates which have to be coated in order to increase the hardness, the strength, the oxidation and the wear resistance. The problems with currently used coating methods concern low productivity and comparatively high dilution of the metal substrate surface. A consequence of the high dilution is that several layers and large amounts of the alloying material are required in order to obtain a surface coating having the desired composition and properties. In brief this means that the coated substrates obtained by the currently used methods are expensive and the productivity is low.
U.S. Pat. No. 2,810,063 discloses a method, wherein a metal powder is used for submerged arc welding. The powders specifically used in this patent include ferrochromium, ferromanganese and ferromolybdenum in combination with a soft iron powder and a sodium silicate. A distinguishing feature according to this patent is the presence of the iron powder which is essential in order to make the alloying elements magnetically associated with the electrode rod or wire.
This disadvantage, which in practice makes the method according to the U.S. patent unacceptable for industrial applications, where a high degree of uniformity is required, can be avoided by using a prealloyed metal powder according to the present invention.
A method similar to that according to the U.S. Pat. No. 2,810,063 is disclosed in the U.S. Pat. No. 2,191,471.
By using a prealloyed powder according to the present invention, is has been found that not only a very uniform structure can be obtained but also that the microstructure will be finer. The risk of obtaining microcracks in the coating is thus reduced and the strength is higher. By supplying the prealloyed powder separately, i e by forward feeding the powder, or on the outside of the electrode surface i.e. by feeding the powder magnetically attached to the outside of electrode(s), the melted substrate surface is efficiently cooled and the melting bath can be reduced. The amounts of expensive alloying elements can be decreased due to the fact the alloying elements are not diluted to the same extent as when the major part of the alloying elements are included in the melting electrode wire or cord. An additional advantage is that the surfacing or coating rate can be increased. It has thus been found the for a given area to be coated the productivity is three times as high as when the open arc method is used. In comparison with coating according to the conventional SAW method, i.e. where the material of the electrode is comparatively high alloyed and the metal powder essetially unalloyed, the present invention offers an improvment of about 50 percent.
The type and the amount of metal powder are selected in such a way that the powder in combination with the electrode wire gives the desired coating composition of the substrate. The powder, which can be magnetic or non-magnetic powder, can be selected from a wide variety of available powders. If the powder is magnetic it can be fed or transported to the substrate together with and fixed on a (magnetic) electrode wire. Magnetic powders are especially preferred for coating rolls. If non-magnetic, the metal powder is fed directly to or in front of the melting bath on the substrate.
An essential feature is that the powder is prealloyed and contains high amounts of alloying elements, which in this context means that amount of alloying elements of the powder is higher than that of the wire. The amount of the alloying elements should be least 2% by weight of the powder in order to be effective. The amount of alloying elements of the metal powder as well as the feed of the metal powder and the electrode wire are selected in view of the desired composition of the final coating of the substrate. The powder feeding is usually between 30% and 80% of the total filler metal, i.e. the powder metal and the wire metal. By using gas or water atomised prealloyed powders, a high degree of accuracy of the coating composition can be obtained.
Examples of suitable alloying elements in iron-based powders are Cr, Ni, Mo, Mn, V, Nb, Si, N, C, Co, Ti and W.
An example of a high-alloy iron-based powder which is well suited for surface coating according to the invention is a powder essentially consisting of iron and 0-55% by weight of Cr, 0-50 by weight of Ni, 0-35 by weight of Mo, 0-15 by weight of Mn, 0-17 by weight of V, 0-15 by weight of Nb, 0-5 by weight of Si, 0-0.8 by weight of N, 0-8 by weight of C, 0-55 by weight of Co, Ti 0-15 and 0-65 by weight of W.
Preferred amounts of the alloying elements are 12-35% by weight of Cr, less than 25% by weight of Ni, less that 15% by weight of Mo, 0.05-15% by weight of Mn, 0-8% by weight of V, 0-8% by weight of Nb, 0-10.0% by weight of Si, less than 0.5% by weight of N and 0.005-5% by weight of C.
More specifically this powder could include 20-34% by weight of Cr, 0-18% by weight of Ni, 0,5-8% by weight of Mo, 0.1-5% by weight of Mn, 0.1-2.0% by weight of V, 0.1-2.0% by weight of Nb, 0.05-3.0% by weight of Si, 0.005-0.4% by weight of N and 0.005-0.8% by weight of C.
Another interesting powder composition which is useful according to the invention is an iron-based powder including 0-40% by weight of Co, 7-17% by weight of W, 5-15% by weight of Mo, 0.05-2.5% by weight of Mn, 1-6% by weight of V, 0.05-2.5% by weight of Si, less that 0.40% by weight of N and 1-4.0% by weight of C.
According to the invention, the main purpose of the melting electrode wire is to provide sufficient heat for melting the metal powder and the substrate surface. A special advantage is that, if combined with different types of metal powders having different alloying elements, the same electrode wire can be used for different types of coatings.
When iron-based, the electrode wire can by unalloyed or low-alloy electrode wire, such as S
1
or S
2
according to DIN 8557. All unalloyed or low-alloy electrode wires including metal cord and flux cord electrodes, i e hollow electrode, can, however, be used according
Hallén Hans
Johansson Karl-Erik
Burns Doane Swecker & Mathis L.L.P.
Elve M. Alexandra
Hoganas AB
Ryan Patrick
LandOfFree
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