Mechanical properties and microstructural evolution of modified 9Cr-1Mo steel after long-term aging for 50,000 h

Abstract

The mechanical properties and microstructural evolution of modified 9Cr-1Mo steel have been studied to investigate steel property changes after long-term isothermal aging at 600 o C for 50,000 h. The microhardness and strength were maintained constantly after aging but the impact energy was dramatically reduced by 62 % during the aging period. From the viewpoint of microstructural evolution after the aging process, Cr-enrichment and Fe-depletion took place within the M23C6-type precipitates in the as-aged steel and V-depletion also happened within the VX-type precipitates after aging. In addition, the precipitates of the M2Mo-type Laves phase and the segregation of the impurity atoms would be formed during the long-term aging period. It was considered that the sharp reduction of the impact energy could be related to the formation of the Laves phases and the impurity segregation after aging at 600 o C. The phase stability was also verified by the specific heat results up to 950 o C from a DSC test. It was concluded from this study that the modified 9Cr-1Mo steel would keep its microstructural stability at 600 o C during the long-term aging period of 50,000 h, which was equivalent to the in-service life of the SFR fuel cladding.

Assessment of microstructure stability and mechanical properties of X10CrWMoVNb9-2 (P92) steel after long-term thermal ageing for high-temperature applications

“…• C. Similar results of microstructural degradation in 9%Cr steel were also observed in [14,15]. Moreover, the analysis of the microstructure of P92 steel after thermal ageing revealed the existence of near-boundary precipitate-depleted zones (Fig.…”

“…13), which may result from coarsening of M 23 C 6 carbides and Laves phase, but also from the probable presence of Z-phase precipitates in the microstructure, which results in a reduction in the effect of precipitation and solution strengthening. The similar impact of long-term thermal ageing on changes in mechanical properties in 9%Cr steel was also observed in [8,[12][13][14][15]24]. The effect of the time of ageing at 600 and 650…”

“…The main reason for the decrease in impact energy of the examined steel should be sought in precipitation and growth of the particles of Laves phase precipitated at the grain/lath boundaries, which is also implied by other researchers [12,15,24]. The decrease in ductility of the examined steel was also affected by the increase in an amount of M 23 C 6 carbides precipitated at the grain boundaries, which form the socalled "network of precipitates" in places, but also the degradation of tempered martensite lath microstructure (Figs.…”

Effect of Long-Term Service on Microstructure and Mechanical Properties of Martensitic 9% Cr Steel

“…T/P91 steel was introduced into the power industry in the 1990s, mostly as material for seamless hot-rolled pipes (Ref 1, 2). Many longterm studies were performed on this steel, such as but not limited to, the assessment of the stability of the microstructure and mechanical properties (Ref [3][4][5], low-cycle fatigue resistance (Ref [6][7][8], and weldability (Ref 9,10). The result of these studies was that the steel was put into service in power units without any major problems.…”

Laves Phase Formation and Its Effect on Mechanical Properties in P91 Steel

“…Laves phase precipitates in Cr-depleted regions adjacent to M 23 C 6 carbides or on the martensite packet and the grain boundaries without the presence of M 23 C 6 carbide in the vicinity, because the high W and Mo contents in such regions are beneficial to the formation of Laves phase. The formation of Laves phase will deteriorate the creep properties [14] and impact toughness [15] in P91 steel. However, the others report that Laves phase has not been found in P91 steel during creep test at 873 K [16].…”