Coarsening Of M23C6 Research Articles

Hardness Variation in P92 Heat‐Resistant Steel based on Microstructural Evolution during Creep

AbstractIn order to optimize the strength of P92 heat‐resistant steel, the variation in hardness and microstructural evolution during creep were investigated. The results show that before crept for 1429 h at 873 K, the coarsening of M23C6 is the main factor to decrease the hardness. When the creeping time prolongs from 1429 to 6063 h, the increase of hardness is mostly attributed to the precipitation of a large amount of Laves phase. Thereafter, the coarsening of Laves phase leads to the decline in hardness. The precipitation hardening resulting from MX drops distinctly in spite of the slight growth during creep. It is important to control the growth of MX, decrease the coarsening velocity of M23C6 and keep Laves phase to be fine.

Prediction of creep life of X10CrMoVNbN-91 (P-91) steel through short term stress relaxation test methodology

The methodology proposed here adopts short-term stress relaxation testing (SRT) to predict creep strength/life of X10CrMoVNbN 91 (P-91) steel. SRT has been performed at three temperatures (853, 873, 898 K), and at each temperature two prestrains (0.7 and 1.2%) were applied with a hold time of 7 h. The initial data, stress versus time, were converted to stress versus inelastic strain rate by computation and compared with conventional steady state creep rate. Creep life was predicted from the SRT results using the Monkman-Grant relationship, and compared with creep data. The comparison showed the predicted life to agree extremely well with the creep-rupture life (≈30 000 h). The calculated activation energy for SRT (719 kJ mol-1) was found to be higher than the creep (415 kJ mol-1). Detailed SEM and TEM studies revealed that the degradation of microstructure in terms of coarsening of M23 C6, and martensitic lath grains, had taken place and was commensurable to long term creep exposure.

Effect of ferrite content on the creep strength of 12Cr-2Mo-0.08C boiler tubing steels

The effect of ferrite content on the creep of a 12Cr-2Mo-0.08C, ferritemartensite, heatresisting steel was investigated. Creep tests were performed at 550, 600, and 650 °C for up to 10,000 hours on four steels containing, respectively, 51, 38, 23, and 12 pct ferrite. The variation in ferrite content was obtained through nickel additions of 0.6, 1.3, and 2.0 pct. Rupture time, minimum creep rate, elongation, and reduction in area are reported. The effect of ferrite content was found to be small but significant. At short rupture times, increasing ferrite content reduces creep strength. At long exposure times, however, increasing ferrite content increases creep strength. This effect is explained by the precipitation of Laves phase, Fe2Mo, and possibly Cr2N, in the ferrite during testing. This precipitation progressively strengthens the ferrite relative to martensite. In addition, martensite is continually weakening because of the coarsening of M23C6, the only precipitate found in the martensite. Nickel at the 2.0 pct level radically decreased creep resistance in these steels by accelerating aging of the precipitate structure in martensite.