Electric heating – Metal heating – By arc
Reexamination Certificate
2000-03-21
2001-10-23
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
C219S076120, C219S076140, C219S076150
Reexamination Certificate
active
06307178
ABSTRACT:
High-alloy heat-resistant steels are generally used for the production installations for industrial manufacture of basic chemicals. Examples of such installations are hydrogen reformers, installations for synthesis of ammonia, methanol and olefins, cracking installations and heat-treatment furnaces. In these installations the materials are used for reformer pipes, headers, lines, furnace rollers and other components. Heat-resistant cast-steel grades can be used at temperatures in excess of 600° C. and are insensitive in contact with aggressive gaseous substances. The materials generally have the following composition: 0.3 to 0.5 wt % carbon, 1.0 to 2.5 wt % silicon, <1.5 wt % manganese, 20 to 50 wt % chromium and 10 to 70 wt % nickel. The phosphorus and sulfur contents of these steels are very low. By virtue of their high carbon content and a high content of chromium and nickel alloying elements, these materials have excellent heat resistance and good mechanical characteristics even at high temperatures.
In the use of these alloys, however, the problem arises that, because of the high operating temperatures and the effect of the different gas mixtures present in the installations, they gradually become aged and embrittled after a certain operating time. As a result, intermetallic phases are precipitated and carbide precipitates are formed. The materials also become carburized or nitrided.
In turn, the mechanical characteristics of the materials are considerably altered. The tensile strength and elongation after fracture decrease greatly, and a loss of elongation is suffered.
From the prior art it is known that heat-resistant cast-steel grades should be welded without preheating if at all possible or with only slight preheating to lukewarm condition, and that postweld treatment is not required. Such information can be found, for example, in DIN 17465 of August 1993, Table 6.
The “Manual for Welding Engineering”, 5th Edition, March 1939, pages 84, 89 also states that heat-resistant steels have very low thermal conductivity and greater thermal expansion than unalloyed steels. It is therefore recommended that welding always be performed with the method that inputs the least quantity of heat to the workpiece. From page 89 it can be inferred that these materials must be welded in the cold condition.
The article of R. Malisius entitled “Shrinkages, stresses and cracks during welding” in the “Welding Engineering” textbook series, page 20, paragraph 2.52, “Heat-resisting steels”, 1957 also contains information on the welding of heat-resistant steels. It is stated that, in order to reduce the sensitivity of the heat-resistant steels to thermal stresses, it can be expedient slowly to heat the parts to be welded to about 200° C. before welding. In addition, postweld annealing with slow uniform heating and cooling is recommended in order to anneal out the very high internal stress and the hardness variations.
The article by Heribert Wirtz entitled “The behavior of steels during welding” in the “Welding Engineering” textbook series, 44, 1st Edition, March 1968, pages 143 to 147 also states that heat-resistant austenitic steels must be welded in cold condition if at all possible. It is indicated that high preheating temperatures in combination with the input welding heat cause local hot spots, which result in cracking. This article recommends, for the welding of root passes, preheating in the range of 100 to 150° C., or for thick-walled workpieces up to 200° C. This temperature, however, must not be exceeded.
To ensure trouble-free operation over the long term, individual parts of the installations must be replaced after a certain operating time. For this purpose it is frequently necessary to make welded joints between old and new parts. Because of the differences between the mechanical characteristics of different old parts and also of old parts and new parts, considerable problems can develop in joining such parts by welding, or even in weld surfacing of old shaped parts with new material. The hot-cracking tendency of the materials is worsened by the increased carbon content. The formation of a carbide network due to carburization leads to embrittlement of the material, and so internal welding stresses cannot be relieved by local yielding and therefore lead to cracks.
In joining different old parts as well as old and new parts by welding, and also in weld surfacing, therefore, cracks are frequently formed to the extent that the welded workpiece is no longer usable. This causes not only high material costs but also considerable production stoppages, since the installations must be shut down during these welding tasks. The prior art methods described in the foregoing therefore have not proved practical, since cracks have frequently been observed in the joined parts during welding thereof.
The technical object of the invention is to provide, for welding of shaped bodies of carburized heat-resistant steel, a process which permits simple joining of carburized heat-resistant steel to new parts by welding or even permits weld surfacing, without the development during welding of cracks that lead to material damage.
This technical object is achieved in that the parts to be welded are preheated to temperatures of 700 to 900° C., preferably 800 to 900° C. before welding, and are welded with current intensities of 50 to 200 A.
It was surprisingly found that welding of carburized heat-resistant steel with corresponding new parts or even other carburized heat-resistant old parts is possible in simple manner when the parts to be welded are preheated to the cited high temperatures before welding and are welded at high current intensities. By virtue of these newly discovered preheating and welding parameters, the weldability of carburized shaped parts can be restored, while at the same time the internal stresses are relieved and crack-free welds can be achieved.
The welding method according to the invention can be used both for weld surfacing and for joining by welding.
In a preferred embodiment, weld surfacing is performed by the TIG welding technique. This is an electric inert-gas-shielded welding process using nonconsumable tungsten electrodes.
Preferably chromium-containing and nickel-containing alloys or nickel-base alloys are used as filler metals for welding. Especially preferred are the following alloys:
Thermanit Nicro 82 SG-Ni CR 20 Nb (2.4648)
Thermanit Nicro 82 E-Ni Cr 19 Nb (2.4648)
Sandvik Sanicro 71 EL-Ni Cr 19 Nb
Thermanit 25/35 Nb Si (1.4853)
Thermanit 25/35 R (1.4853) 25/35 Nb
or alloys with composition corresponding to that of the base metal.
In a preferred embodiment, the heat-resistant steel is an austenitic steel. Preheating is preferably performed starting from the weld side wall toward the base metal, and the soaking depth during preheating can be 10 to 50 mm.
In a preferred embodiment, the buildup of layers during welding can take place from the outside, in which case the workpieces to be welded together are reheated after each weld pass, so that the preheating temperature of 700 to 900° C. is maintained throughout the entire welding process. The thickness build-up per pass is preferably 6 to 8 mm.
Preferably a postweld heat treatment at temperatures of 850 to 900° C. is performed after joining by welding, in order to achieve effortless removal of the welding slag. After welding, the weld is allowed to cool slowly to room temperature. Preferably the weld is thermally insulated for this purpose.
The parts to be replaced are preferably welded together without solution annealing. Solution annealing is understood as heat-treatment, at about 1200° C., of the workpieces to be welded together, for a relatively long period of about 4 hours, followed by cooling to room temperature. In the case of welding of hollow shaped bodies, build-up of the side-wall structure is preferably performed by welding at 60 to 140 A and build-up of the filler passes and top pass at 90 to 200 A. Preheating is performed with an oxy-gas flame. Tacking of the parts to be welded is performed after heatup. Furth
Dunn Tom
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pittman Zidia
Ruhr Oel GmbH
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