The use of creep strength-enhanced ferritic alloys like Grade 91 has become popular in fossil power plants for applications at temperatures above 566°C (1050°F). Compared to Grades 11 and 22, Grade 91 offers higher stress allowables, better ramp rate tolerance, weight reduction, and lower thermal expansion coefficients at operating temperatures. However, Grade 91's superior elevated temperature strength requires specific microstructure and metallurgical considerations. This paper highlights concerns that warrant further investigation. Initial operating stresses in Grade 91 piping systems may exceed 262 MPa (38 ksi), and lack of creep relaxation below 593°C (1050°F) could lead to weldment failures within years, especially above 159 MPa (23 ksi) after one year. While cold spring can reduce initial stresses for systems below 593°C (1050°F), creep relaxation rates up to 206 MPa (30 ksi) need study. Above 593°C (1050°F) and below 103 MPa (15 ksi), weldments may fail prematurely by Type IV creep mechanism. Long-term creep rupture studies on cross-weld and multiaxially loaded thick-walled specimens should evaluate deteriorated weldment properties, particularly below 103 MPa (15 ksi).

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