Effect of interpass temperature on the structure and properties of multipass weldments in high performance nickel alloys

Abstract

Nickel alloys comprise an important group of engineering materials which are used primarily for their exceptional resistance to corrosion and their ability to maintain good mechanical strength over a wide temperature range, (both low and high) in demanding industrial applications. Welding is a primary fabrication process for these alloys. It has been a generally accepted practice to maintain a maximum interpass temperature of 200°F or lower when multipass welding many nickel alloys to prevent defects such as cracking or loss of corrosion resistance. This practice has been based on recommendations by many of the nickel alloy producers. A low maximum interpass temperature can increase the welding time which increases fabrication costs. According to the author's industry contacts and based upon the author's industrial experience as well as the author's examination of the literature, there has been little or no systematic research on the effect of interpass temperature for multipass welding of nickel alloys. In fact, the same is true for the establishment of the basic robotic welding parameters using the new generation of digital power supplies for these alloys. This dissertation presents research on the effect of interpass temperature on two nickel alloys; HASTELLOY® C-2000® and HASTELLOY® B-3®. Welding parameters were also developed for these alloys and also for HAYNES® 230® alloy using Gas Metal Arc Welding, GMAW, as a single process for both the root and fill weld passes. Weldments were made at 5 different interpass temperatures, 100°F - 500°F, in 100°F increments, for these alloys in thicknesses of 0.25 inch and 0.5 inch. Transverse weld specimens were then tested according to AWS B4.0:2007 using tensile, bend, and hardness tests. Transverse weld specimens were corrosion tested according to ASTM G28A for the HASTELLOY C-2000 alloy and the HASTELLOY B-3 alloy was subjected to 20% HCl at 149°C for 96 hours in an autoclave. The specimens were also examined using optical light microscopy for intergranular corrosion attack, weld fusion, cracking, and heat affected zone (HAZ) microstructure effects. (HASTELLOY, HAYNES, C-2000, B-3, and 230 are registered trademarks of Haynes International, Inc.) No significant loss of tensile strength was found at any of the higher interpass temperatures. All ultimate tensile strengths for both alloys were above the ASME Boiler and Pressure Vessel Code Section IX minimum. All samples passed 2T transverse face bend tests. Some lack of fusion was observed at the root of some samples at random interpass temperatures. No noticeable change in the HAZ microstructure or cracking was observed at the highest interpass temperature for both the HASTELLOY C-2000 and the HASTELLOY B-3 alloys. No significant corrosion attack was found along the weld, face or root sides, for both alloys at the higher interpass temperature of 500°F. It was concluded that a higher interpass temperature could be specified for these alloys without any appreciable loss of strength, weld soundness, loss of corrosion resistance, or detrimental effect to microstructure. It was also shown that the GMAW process could be used as a sole welding process but more development is needed to decrease process variability in the root pass and to develop a complete welding procedure specification.