Changes in Microstructure and Mechanical Properties of Cr-Mo Reactor Vessel Steels During Long-Term Service

Abstract

Changes in microstructure and mechanical properties of 1Cr-0.5Mo and 2.25Cr-1Mo steels during long-term service have been investigated. The study includes inspection of a 1Cr-0.5Mo steel reactor vessel which operated for 20 yr and tests on specimens that had been exposed to service environments. The reactor vessel, exposed at 490 to 530°C for 170,000 hr (about 20yr), showed appreciable decrease in the room temperature yield strength, impact toughness and creep rupture strength compared with the original properties. The service exposure caused changes in carbide morphology and species, forming M2C and M7C3 carbides and transferring significant amounts of Cr and Mo from the matrix to carbides; only 32 percent of the total Mo and 72 percent of the total Cr remained in the matrix. The microstructure of 2.25Cr-1Mo steel showed higher stability than that of 1Cr-0.5Mo steel, although a similar transfer of Mo and Cr from the matrix to carbides took place. The Mo and Cr contents remaining in the 2.25Cr-1Mo steel matrix after a 2-yr exposure were only 25 and 74 percent, respectively, of the total quantities in the steel. The partitioning of Mo and Cr to carbides could increase the stability of carbides and consequently reduce the carbon content in the matrix. The mechanical properties are influenced by both the change in the composition of the matrix and the change in carbide morphology. The metallographic examination provides useful information, although qualitative at this stage, on the degree of deterioration of materials.

Effect of long-term service exposure at elevated temperature on microstructural changes of 5Cr-0.5Mo steels

“…Although data relating to long-term service exposure of 1Cr-0.5Mo, [10][11][12][13][14] 2.25Cr-1Mo, [18,19,20] 0.5Cr-0.5Mo-0.25V, [13,[15][16][17] and 1Cr-1Mo-0.25V [21][22][23][24] steels are available in the literature, microstructural changes due to long-term service exposure of 5Cr-0.5Mo steels are not available in the literature.…”

THE MECHANISMS AND DETECTION OF EMBRITTLEMENT IN Cr‐Mo PRESSURE VESSEL STEELS

“…An attempt [l5] to relate hardness, grain size and impurity concentration to FATT was unsuccessful because of the low resistance of the material to transgranular cleavage and the role of elements such as Mo and Mn in altering the embrittling [17] and Shaw [2] carried out extensive testing and found Q + T material to be more susceptible than annealed material. Despite some measure of confusion in the literature, the accumulated evidence, particularly when data from different types of low alloy steels are included, indicates that those steels which have large amounts of equiaxed, as opposed to acicular, ferrite and small amounts of bainite or pearlite, are considerably less sensitive to temper embrittlement [ 18,191. Part of the reason for this apparently lower susceptibility to embrittlement may be related to the fact that these materials are not as tough in the as heat treated condition as Q + T 2 %CrlMo steels, which are susceptible to temper embrittlement.…”

Differences in service-exposed and artificially aged microstructure and tensile properties of ASTM A204 Grade C steel

“…These materials experience cyclic mechanical and thermal stresses in their service regime [8,9]. Although heat-resistant steels are expected to be resistant to any substantial microstructural changes during their service lifetime, there are examples of early degradation and failure of components due to changes in the microstructure with longer time exposure [10][11][12]. Investigation and characterisation of such microstructural degradation becomes critical for safety assessment.…”