Spheroidization is a change in the microstructure of steels after exposure in the 850°F to 1400°F (440C to 760°C) range. In carbon steels, the carbide phases are unstable and may agglomerate from their normal plate-like form to a spheroidal form.
Softening (Spheroidization) | Materials And Corrosion Control
Damage Mechanism |
Spheroidization (Softening) |
Damage Description |
Spheroidization is a change in the microstructure of steels after exposure in the 850°F to 1400°F (440C to 760°C) range. In carbon steels, the carbide phases are unstable and may agglomerate from their normal plate-like form to a spheroidal form. In low alloy steels like 1Cr-0.5Mo the small, finely dispersed carbides convert to large agglomerated carbides. Spheroidization may cause a loss in strength and/or creep resistance.
· Metal chemistry, microstructure, exposure time, and temperature are critical factors. · The rate of spheroidization depends on the temperature and initial microstructure. Spheroidization can occur in a few hours at 1300°F (552°C), but may take several years at 850°F (454°C). · Annealed steels are more resistant to spheroidization than normalized steels. Coarse-grained steels are more resistant than fine-grained. Fine grained silicon-killed steels are more resistant than aluminum-killed. · Spheroidization and graphitization are competing mechanisms which occur at overlapping temperature ranges. At temperatures above about 1025°F (552°C), graphitization may occur after spheroidization. Below 1025°F (552°C), graphitization occurs before the steel is fully spheroidized. |
Materials & Equipment |
Materials
All commonly used grades of carbon steel and low alloy steels including C-0.5Mo, 1Cr-0.5Mo, 1.25Cr-0.5Mo, 2.25Cr-1Mo, Equipment · Spheroidization can occur in piping and equipment after exposure to temperatures above 850°F (454°C). · The loss in strength may be as high as about 30% but failure is not likely to occur except under very high applied stresses, in areas of stress concentration, or in combination with other damage mechanisms. · The loss in strength is usually accompanied by an increase in ductility which allows for deformation at stress concentrations. · Spheroidization affects hot wall piping and equipment in the FCC, catalytic reforming and coker units. Fired heater tubes in boilers or process units may be affected by a loss in creep strength, but equipment, in general, is seldom renewed or repaired because of spheroidization. |
Control Methodology |
· Spheroidization is difficult to prevent except by minimizing long-term exposure to elevated temperatures.
· Spheroidization is not visible or readily apparent and can only be observed through metallography. The pearlitic phase undergoes a time dependent transformation from partial to complete spheroidization · In the case of the 5% to 9% CrMo alloys, spheroidization is the process of transforming the carbides from their original finely dispersed morphology to large agglomerated carbides. |
Monitoring Techniques |
· Spheroidization can only be found through field metallography or removal of samples for metallographic observation. A reduction in tensile strength and/or hardness may indicate a spheroidized microstructure. |
Inspection Frequency |
· Set based on the metal chemistry, microstructure, exposure time, and temperature |
KPIs |
None |
Reference |
· API RP-571 (2003) |
Carbonate Stress Corrosion Cracking | Materials And Corrosion Control