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Grain boundary migration in recrystallised Armco iron during grain growth has been studied by vacuum etching method. It was found that grain growth in recrystallised α-ferrite is closely related with substructures within the ferrite grains, grains containing no substructures usually grow at the expense of those possessing substructures.Through tracing and analysing the remaining grooves of the migrating grain boundaries and together with other observations, we have confirmed the following theories suggested by previous investigators.1. The main driving force for grain boundary migration is interfacial energy of the grains.2. The manner in which grains in metals disappear is basically the same as that of soap bubbles in a semi-evacuated container.3. Burke's model for the formation of new grain boundaries at the expense of old ones.4. Grain growth is a discontinuous process.

The structural changes in certain quenched ferritic, austenitic and duplex stainless steels during subsequent reheating are described and discussed in detail.In 16-6 Cr-Mo stainless steel, austenite formed at certain solution treatment temperature is metastable and decomposes during subsequent reheating. New ferrite α′ forms at the γ/α interphase boundaries, the rejected carbon atoms then diffuse into the matrix forming alloyed carbides surrounding the γ/α phase boundarries.Fine carbides precipitated from ferrite in 16-6 Cr-Mo, 16-2.5-6 Cr-Ni-Mo, and 16-5-6 Cr-Ni-Mo steels in the early stages of ageing are metastable, during prolonged heating they redissolve and are gradually replaced by χ-phase. Some residual carbides serve as the nuclei of the growing χ-phase. The structure of quenched 16-15-6 Cr-Ni-Mo steel is austenitic, during subsequent heating only M_6C and M_(23)C_6 mixed carbides are precipitated.Both the alloyed carbides and χ-phase may markedly affect the hardness of the steels studied. In the duplex steels χ-phase is a good strengthener, manifesting pronounced hardening effect at elevated temperature.

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A series of creep tests has been made on four types of heat-resistant steels under different temperatures and under the same stress for each type of steel. Creep and rupture date of five types of Nimonic alloys and four types of heat-resistant steels obtained from the literature have been analyzed. The temperatures of our creep test and those given in the literature are all above 0.45 T_m (absolute melting temperature) for each type of steel or alloy. From our results of analyses, it is shown that, for the temperatures investigated, in the process of high temperature creep, the existance of activation energy is a general phenomenon for different types of heat-resistant steels and alloys given above. To accelerate creep tests by an elevation of temperature through activation energy is shown to be possible, the temperature range for the application of this extrapolation method and the errors involved are discussed.