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.
Each of these forms of embrittlement or cracking results in a distinct behavior of the steel: steels susceptible to temper embrittlement when operated for long times in the temperature range of 650-1050°F may severely embrittle, causing the temperature at which the material undergoes the ductile to brittle transition to increase several hundred degrees. Steels susceptible to creep embrittlement, when operated under load for long times in the temperature range of 800-1100°F, may exhibit a severe reduction in the stress-rupture ductility of the material. Steels susceptible to stress-relief cracking may exhibit cracks which appear to start at the outside surface of a weld at the fusion boundary. These cracks occur with heating in the temperature range of 200-1800°F, depending on the alloy considered.
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| Number of Pages : | 31 |
| Published : | 1973 |
