Current trends in the design of large pressure vessels require massive sections that operate under high stresses and at high temperatures. To attain good through-section fracture toughness values at the higher yield strength required for such components, steels with higher hardenability, and thus greater heat treatment potential, are used. However, during the fabrication process or later during the vessel’s actual service, certain of these steels exhibit microcracking and/or severe embrittlement.Research efforts with the steels have resulted in the identification and characterization of some of these problems.In particular, temper embrittlement, creep embrittlement, and stress-relief cracking have been shown to be causes of cracking and low toughness.