Nickel-base radiant tube and method for making the same

A nickel-base radiant tube comprising:

• a straight tube section and a curved elbow tube section, at least one of said straight tube section and said curved elbow tube section of said radiant tube made of high nickel content alloy;
• said straight tube section having a first center line, an inner radius and an outer radius both substantially constant along an entire length of said straight tube section and a mating end portion having an end face;
• said curved elbow tube section having a second center line, an inner radius substantially constant along an entire length of said curved elbow tube section and an outer radius varying along a portion of the length of said curved elbow tube section;
• said inner radius of said curved elbow tube section being substantially equal to said inner radius of said straight tube section;
• said curved elbow tube section including a mating end portion having an end face, an outer radius substantially equal to said outer radius of said straight tube section, a main portion having an outer radius substantially different than said outer radius of said straight tube section, and a sloped transitional portion disposed between said mating end portion and said main portion of said curved elbow tube section;
• said outer radius of said mating end portion of said curved elbow tube section being substantially constant along an entire length of said mating end portion;
• said mating end portion of said straight tube section being adjacent to and axially aligned with said mating end portion of said curved elbow tube section so that said end faces of said straight tube section and said curved elbow tube section facing each other to form a circumferentially extending weld groove;
• and
• a circumferentially extending butt welding joint occupying said weld groove for weldingly connecting said mating end portions of said straight tube section and said curved elbow tube section together at said weld groove.