Crack Prevention in Cobalt Based Alloy Hardfacing for Refinery Equipment

Literature Focus and Core Result

The 2000 paper in Welding Technology reviews the application and cracking problems of cobalt based alloy hardfacing in refinery equipment. The core message is that cobalt based overlays can provide excellent high temperature hardness and wear resistance, but they are susceptible to cracking if the process is not carefully controlled. The authors emphasize strict adherence to hardfacing procedure, sequence, material cleanliness, and base metal control.

This topic is important for refinery and petrochemical equipment such as valve seats, pump shafts, agitator components, and high temperature valve trim. Cobalt based alloys, commonly related to Stellite families, are often used where galling, erosion, and hot hardness are required. However, their high alloy content, limited ductility, and high thermal stresses make them demanding to weld.

Crack Mechanism and Prevention

Cracking in cobalt based hardfacing can arise from hot cracking, solidification cracking, strain age cracking, or hydrogen assisted cracking depending on the base metal, dilution level, and cooling path. The molten pool can have low fluidity and a wide solidification range, which increases susceptibility to centerline cracking. High residual stress from dissimilar metal overlay can also open cracks if the deposit is restrained.

Crack type	Likely cause	Prevention
Hot cracking	Wide solidification range and poor fluidity	Control dilution, preheat, and interpass temperature.
Crater cracking	Arc stop in high stress zone	Use crater fillers or overlap termination.
Hydrogen cracking	Moisture or surface contamination	Dry electrodes, clean base metal, and controlled cooling.
Strain cracking	Rigid restraint and high residual stress	Reduce restraint and apply post heat when qualified.

The paper highlights the importance of controlling harmful elements in both the hardfacing alloy and the base metal. Sulfur, phosphorus, and surface contaminants can increase cracking susceptibility. In refinery equipment, oil residues, scale, and previous service deposits must be removed thoroughly before overlay. A clean joint is not a minor detail for cobalt alloys; it is often the difference between a sound deposit and a cracked repair.

Engineering Practice and Quality Control

In practice, the hardfacing sequence should be designed to minimize restraint. Multiple small passes with controlled interpass temperature are often better than large beads that trap stress. For critical valve seat overlays, a metallographic or dye penetrant examination after finishing is valuable because surface cracks may be very fine. Magnetic particle testing may not be effective if the overlay is austenitic or non magnetic, so penetrant testing is often the more reliable method.

The procedure should also control dilution. Excessive dilution from a plain carbon or low alloy steel base metal can change the cobalt alloy chemistry and create brittle transition zones. A buttering layer or intermediate pass may be required when the base metal is highly diluting or when the final service requires low iron content in the wear surface.

The study supports a prevention first philosophy. For refinery equipment, the cost of a cracked overlay is not only rework but also potential leakage, erosion, and shutdown. The engineering lesson is that cobalt based hardfacing should be managed with the same discipline as a critical repair welding procedure. Material traceability, surface preparation, heat input, sequence, and final examination must be integrated into one controlled system.