Development of a Modified A286 Alloy With Improved Strength at Elevated Temperature and an Evaluation of Its Hydrogen Embrittlement Susceptibility

A modified A286 alloy was developed with the aim of improving elevated temperature strength. The alloy was evaluated using a tensile test and a creep rupture test. Microstructure observations of the modified A286 alloy revealed that W and Ti carbides were precipitated at the grain boundaries and that no η phases were observed. The modified alloy showed higher tensile strength in a test at 625°C and a longer rupture time in a creep rupture test at 650°C relative to the conventional A286 alloy. Hydrogen embrittlement susceptibility was evaluated using a low-strain-rate tensile test with hydrogen-charged specimens. From a comparison of the reduction of area, it was determined that the hydrogen embrittlement resistance of the modified A286 was equal to or greater than that of conventional A286.

Failure Analysis of T91 Finishing Superheater Tube Bursting for Improper Initial Microstructure and their Life Evaluation

The failure analysis was performed for T91 finishing superheater bursting tube. It is concluded that improper initial microstructure resulted from non-standard heat treatment contributes to poor creep properties at high temperature and subsequent tube bursting. In addition, life evaluation was carried out on the basis of creep-rupture test result of experimental steel with similar improper microstructure. It is in accordance with the practical condition and provides significant guidance for safety and operation supervision.

Structural Changes in a 9%Cr Creep Resistant Steel during Creep Test

Structural changes in a 9%Cr martensitic steel after 1%, 4% creep and creep rupture test at 650°C and stress of 118 MPa were examined. Heat treatment provided the formation of tempered martensite lath structure (TMLS) in the steel. The precipitations of second phase particles along block and lath boundaries provide effective stabilization of the TMSL under annealing/aging condition. This structure hardly changed under creep conditions in grip portion of crept sample. Significant coarsening of both the second phase particles and the martensite laths takes place in neck portion. In addition, the latter ones lose their original morphology and are replaced by large strain-induced subgrains. It should be noted that the increase of subgrain size is in almost direct proportion to the particle growth during the creep to 4% strain. The rapid growth of martesite laths followed by their evolution to deformation subgrains takes place within the tertiary creep regime.

MECHANICAL PROPERTIES OF SQUEEZE INFILTRATED AS52 MAGNESIUM MATRIX COMPOSITES

Aluminum borate whisker (9( Al 2 O 3)·( B 2 O 3)) reinforced AS52(with and without strontium modification) metal matrix composites (MMC) were fabricated by the squeeze infiltration method. Creep rupture test was carried out at 150°C and 100 MPa condition. Results showed that Alborex reinforcement and modified polygonal shape Mg 2 Si phase contributed to the enhancement of mechanical properties and creep resistance. Creep rupture time was increased 11% and minimum creep rate was decreased 17% in the composite. At the Alborex/matrix interface, uniform thin layer of MgO was formed. The initiation of micro-voids at the reinforcement/matrix or Mg 2 Si /matrix interface was followed by their growth and coalescence to macro-cracks.