Effects of the Hot Isostatic Pressing Process on Crack Healing of the Laser Repair-Welded CM247LC Superalloy
This study investigated the effects of the hot isostatic pressing process on the crack healing, microstructure, and mechanical properties of the laser repair-welded CM247LC precipitation-hardened superalloy. To ensure the strength of the repair-welded area, this study used the direct re-melting approach, and simulated the repair welding with the filler addition, which has a chemical composition matching that of the base superalloy. As expected, different types of cracks, including the solidification crack in the weld fusion zone and various types of liquidation cracks in the heat-affected zone, were observed. Through a proper hot isostatic pressing healing process, all cracks in the weld fusion zone and the heat-affected zone of the repair-welded pieces were healed. At the same time, some nano- and micro-sized carbides tended to form discontinuously along the healed crack trace. A micro-hardness test was conducted in the repair-welded area, and the results were similar to that of the as-cast material.
Development of an Improved Stainless Steel to Ferritic Steel Transition Joint
The objective of this study was to develop an improved transition weld joint between stainless steel and ferritic steel piping to circumvent the failures and concerns with joints made by existing technology. The primary concern of mismatch in coefficients of thermal expansion between the ferritic and austenitic steels and the high stresses that it imposes at the interface between ferritic steel and weld metal led to an extensive elastic stress analysis involving base metal combinations, filler metals, and weld joint geometries. This analysis indicated that, for pipe welds, joint stress could be reduced considerably by using a transition material with an intermediate coefficient of thermal expansion between the 2 1/4 Cr-1 Mo ferritic steel and the austenitic stainless steel. Alloy 800H was selected as an appropriate intermediate material for this application.
Laser beam welding of 2205 duplex stainless steel with metal powder additions
Laser beam welding with the aid of metal powder additions to the weld pool was carried out to modify the ferrite/austenite (α/γ) ratio of the weld metal of 2205 duplex stainless steel (DSS). The α content in the weld metal of DSS welds could be controlled by the proper flow rate of nickel powder through a coaxial nozzle. This process had the advantage of using only a small amount of filler metal, i.e. a few grams per minute of nickel powder, in the welding process. Imapct and notched tensile tests were utilized to evaluate mechanical properties of laser welds. The notched tensile test was also carried out in hydrogen under a slow displacement rate. The susceptibility to hydrogen embrittlement (HE) was estimated from the loss in notched tensile strength and correlated with the microstructure of a given laser weld. On the whole, the susceptibility to HE decreased with increasing the γ content of DSS welds. Autogeneous laser welds containing the highest α content of all welds tested were most susceptible to HE. The base material with banded α+γ structures was susceptible to HE and exhibited severe secondary cracks mainly along α/γ phase boundaries. Although laser welds produced at a flow rate of 3 g/min nickel powder had similar α content to the base material, they were more resistant to HE owing to randomly distributed α and γ phases in the weld metal. The impact energy of laser welds at low temperatures (−75 to −100°C) along with the hardness test could also be used to check if the proper amount of nickel powder was added in laser welding of DSSs.