Contribution of Microstructural Factors to Hardness Change during Creep Exposure in Mod.9Cr-1Mo Steel

Abstract

The effect of microstructural factors on hardness was investigated in normalized, tempered, aged and crept materials for Mod.9Cr-1Mo steel, using nanoindentation and microhardness tests. Nanohardness and microhardness decreased during tempering, aging and creep exposure. Dislocation spacing, lath width, high angle boundary (block and packet boundary) spacing and inter-particle spacing increased during tempering, aging and creep exposure. A converted Vickers hardness was introduced to compare directly nanohardness and microhardness to Vickers hardness. The converted Vickers hardness increased with indent size in all the materials tested. Hardness at an indent size less than 1 mm mainly consists of dislocations inside lath grains. Hardness at an indent size larger than 1 mm originates from not only dislocation but precipitates and high angle boundaries such as block and packet boundaries. Comparing the converted Vickers hardness with lath width and high angle boundary spacing in normalized material with no precipitates, it was found that the lath boundary does not contribute to hardness. The difference in converted Vickers hardness between tempered and aged material was obviously large at the indent size, greater than inter-particle spacing. The decrease in hardness during aging is caused by increase in inter-particle spacing due to coarsening and coalescence of precipitates. On the other hand, not only changes in precipitates but also increase in high angle boundary spacing and dislocation spacing contribute to decrease in hardness during creep exposure.

On the effect of long-term creep on the microstructure of a 12% chromium tempered martensite ferritic steel

“…11a) is mainly associated with the decrease in the density of free dislocations (at constant subgrain size), and this is fully in line with our observations [16] and with previous work on the evolution of dislocation density during short-term creep [5]. Our results on the evolution of subgrain sizes agree well with results reported in the literature [22,23].…”

“…Carbides become mechanically important during creep [7], where they stabilize subgrain boundaries. Our results are also in line with a recent very careful assessment of microstructure of a 9% Cr tempered martensite ferritic steel (OIM and TEM) in combination with nano-, micro-and macrohardness measurements [22], where the strong decrease in room temperature hardness during long-term creep is mainly associated with subgrain coarsening. It was suggested [22] that the small decrease in hardness in the thread of the creep specimen (which we also observe in Fig.…”

“…Our results are also in line with a recent very careful assessment of microstructure of a 9% Cr tempered martensite ferritic steel (OIM and TEM) in combination with nano-, micro-and macrohardness measurements [22], where the strong decrease in room temperature hardness during long-term creep is mainly associated with subgrain coarsening. It was suggested [22] that the small decrease in hardness in the thread of the creep specimen (which we also observe in Fig. 11a) is mainly associated with the decrease in the density of free dislocations (at constant subgrain size), and this is fully in line with our observations [16] and with previous work on the evolution of dislocation density during short-term creep [5].…”

“…populations form during their thermomechanical treatments and change during long-term exposure and creep (e.g. [9,[20][21][22][24][25][26]). The systems attempt to reach a minimum of their overall Gibbs free energies [38,39] and in many cases it is difficult to predict the nature of this equilibrium.…”

Changes in contributions of matrix and block boundary strengths to macroscopic hardness of high Cr ferritic steel during creep

“…They were the decrease in dislocation density and coarsening of laths and/or subgrains rather than the decrease in solidsolution and precipitation strengthening, considering the relatively short creep times. However, Sawada et al [6] has revealed that lath boundaries have almost no influence on the strength mechanism in Mod.9Cr-1Mo steel. Therefore, the decrease in dislocation density seems to be the most significant factor of the decrease in Hm at the present test condition.…”