1. Field of the Invention
The present invention relates to methods for making radiant tubes in general and, more particularly, to a method for making a nickel-base radiant tube comprising straight and elbow tube sections having different thicknesses.
2. Description of the Related Art
Radiant tubes used to heat industrial furnaces are well known. Usually, in a radiant tube furnace, products of combustion from a burner are confined within a radiant tube with the material to be heated neighboring or surrounding the radiant tube. The heat from the burner is indirectly transferred to the material via the radiant tube, primarily by radiation and secondarily by convection. In a radiant tube furnace, the material being heated is isolated from the products of combustion of the burner, allowing a protective atmosphere to be maintained surrounding the material.
Gas fired radiant tubes have been in use in various heating systems. These tubes vary in shape depending on type of application, burners and fuel being fired. Typically, U-shaped, P-shaped or W-shaped radiant tubes are used, each including two (in case of the U-shaped radiant tubes) or four (in case of the W-shaped radiant tubes) straight tube sections (or legs) and semi-toroidal elbow tube sections connected by a weld joint. Moreover, one of the straight tube sections of the typical radiant tube is a firing leg equipped with a burner, while the other is an exhaust leg or alternative firing being used to uniformly heat the tube. The firing leg of the radiant tube is the hotter leg since the majority of the combustion occurs therein.
The efficiency, fabrication cost and service life of these tubes are very important in decreasing fuel cost, emissions and repair frequency. The burner and operating temperature dictate the alloy design of the system. The burner mixes fuel gas with air/oxygen to generate heat in the firing leg and hot gases traverse through the remaining part of the radiant tube to give a desirable uniform temperature along the whole length of the radiant tube. In reality, achieving uniform temperature throughout the length of the radiant tube is not practical. There are regions with high temperature called hot spots, and at these hot spots the designed temperature may exceed and lead to a premature failure. To avoid such failures in conventional regenerative burners, firing is carried out at both ends of the radiant tube at optimum firing intervals. The change in flow directions is expected to reduce hot spots. Upgrading old burners with a regenerative mechanism is an expensive task; therefore highly alloyed tubes are used as a counter measure.
Nickel-base radiant tubes, including W-shaped radiant tubes, have been in use at a continuous annealing processing line (CAPL) and other furnaces for heat treatment of sheet steels. The CAPL production lines are being used to heat treat sheet steels and to transform microstructure of cold rolled sheet steel to produce DP, TRIP and other advanced high strength steel (AHSS) grades.
In this heating device, or furnace, the mixing of fuel gas and air (20% oxygen) and firing takes place at an entry tube section of the radiant tube, i.e., in the firing leg. As a result the entry tube section (firing leg) and the adjacent elbow tube section attain the highest temperatures during operation. Conventionally, the tube sections of the radiant tube consist of composite materials: a highly alloyed firing leg and a lean alloyed exhaust leg. For example, nickel base super alloy radiant tubes have been used in day to day operation of the CAPL for annealing of sheet steels. These tubes have to withstand high operating temperatures ˜1,100° C. and associated thermal stresses during service with the radiant tube. The fabrication of these tubes involves welding of centrifugally cast straight tube sections (legs) with static cast elbow tube sections. The radiant tubes have to withstand service temperatures for a minimum period of four to six years. However, there are occasions of tube failures associated with poor welds. Some of the radiant tubes were found to fail prematurely within one year after installation. At the same time, on average, each W-shaped radiant tube costs $5,000 to $14,000 and companies often spend $750,000 per year to replace failed radiant tubes.
As shown in FIG. 1, a conventional W-shaped radiant tube 2, mounted to a refractory wall 3 of an industrial furnace, includes straight tube sections (or legs) 4a-4d fabricated by centrifugal casting and elbow tube sections 6a-6c fabricated by static casting. The straight tube sections 4a-4d are joined to the elbow tube sections 6a-6c by weld joints 5. It will be appreciated that the straight tube section 4a is an entry tube section of the radiant tube, or a firing leg, while the straight tube section 4d is an exit tube section, or an exhaust leg. As further illustrated in FIGS. 1 and 2, the straight tube sections 4a-4d have the same outer and inner diameters and a thickness t (typically, about 9 mm). Similarly, the elbow tube sections 6a-6c have the same outer and inner diameters and a thickness T in cross-section (typically, about 15 mm). However, there is a variation in thickness between the straight tube sections 4a-4d and the elbow tube sections 6a-6c. More specifically, the elbow tube sections 6a-6c are thicker than the straight tube sections 4a-4d. In other words, the thickness T of the elbow tube sections 6a-6c (typically, T is about 15 mm) is substantially bigger than the thickness t the straight tube sections 4a-4d (typically, t is about 9 mm). Alternatively, the elbow tube sections 6a-6c may be thinner than the straight tube sections 4a-4d. 
Typically, the straight tube sections 4a-4d of the radiant tube 2 are made from special nickel-base alloy, such as Ni—Cr-14W, Ni—Cr-1.5W or HK40, while the elbow tube sections 6a-6c thereof are made from similar or other nickel-base alloy. Specifically, in the exemplary embodiment of FIG. 1, the hottest straight tube section (the firing leg) 4a of the radiant tube 2 is made from a special alloy of high nickel content, such as Ni—Cr-14W, the straight tube section 4b subsequent to the firing leg 4a is made from special alloy, such as Ni—Cr-1.5W, while the straight tube sections 4c and 4d subsequent to the straight tube section 4b are both made from an austenitic Fe—Cr—Ni alloy, such as HK40 alloy that has been a standard heat resistant material for over four decades. The special alloy of high nickel content, as understood and used herein, refers to any special alloy having nickel content of more than 35 weight percent nickel (>35% Ni).
Similarly, the hottest elbow tube section 6a of the radiant tube 2 adjacent to the firing leg 14a is made from a special alloy of high nickel (>35% Ni) content, while the subsequent elbow tube sections 6b and 6c of the radiant tube 2 adjacent to the straight tube sections 4b-4d are both made from the HK40 alloy.
Conventionally, the straight tube sections 4 and the elbow tube sections 6 are weld joined using the design shown in FIG. 2. The adjacent ends of the straight tube sections 4 and the elbow tube sections 6 are machined to ‘V’ groove 7 and welded using Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW) or Shielded Metal Arc Welding (SMAW) (also known as Manual Metal Arc Welding (MMAW)) process with nickel filler wire/rod. Due to mismatch between the straight tube sections 4 and the elbow tube sections 6, as well as, in a few cases, insufficient root penetration and thermal fatigue loading, the straight tube sections 4 fail prematurely at weld joints 5. Moreover, when the high nickel (>35% Ni) content alloy is used in fabricating the radiant tube 2, the high nickel content and shape of radiant tubes make welding difficult. The radiant tubes 2 in the CAPL furnace were found to fail prematurely at weld joints 5. In some cases the radiant tubes 2 were failing within two years of installation, while the average design life of the radiant tube 2 is 4 to 6 years.
Thus, typical weld joints of radiant tubes are susceptible to improvements that may enhance their strength and decreases cost of production with increased life of the radiant tubes.