Материаловедение: свойства металлов. Матросова Т.А - 42 стр.

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42
прочностных свойств металла. 5. Дайте определение упрочнения. 6. Дайте
определение операции закалки. 7. Какие среды могут использоваться для
охлаждения нагретого металла? 8. В чем состоит суть операции отпуска и
каково изменение свойств обрабатываемого материала? 9. Каково назначение
операции отжига? 10. Как охлаждают металл в процессе нормализации? 11.
Какие части детали упрочняются преимущественно при цементации и
азотировании? 12. Опишите операции
пламенной закалки. 13. Как обычно
снимают остаточные напряжения? 14. Укажите сферу приложения операций
снятия остаточных напряжений. 15. Как изменяется микроструктура вследствие
операций снятия остаточных напряжений?
Topic 15. Annealing
Annealing involves heating to and holding at a suitable temperature, followed
by cooling at an appropriate rate, primarily for softening of metals. When annealing
iron-based alloys, the work is 50 to 100°F above the critical temperature range. The
alloy is held for a period of time to insure uniform temperature throughout the part,
and then allowed to cool slowly by keeping the parts in the furnace and allowing
both to cool (see fig. 9.1). The cooling method should not permit one portion of the
part to cool more rapidly than another portion. Its purpose is to remove stresses;
induce softness; alter ductility, toughness, and magnetic properties; change grain
size; remove gases; and produce a definitie microstructure.
Steel is annealed for one or more of the following reasons:
To soften it for machining or fabrication operations;
To relieve stresses in the material (often necessary after casting or welding
operations);
To alter its properties;
To condition the steel for subsequent heat treatments or cold work;
To refine grain size, improve ductility and promote dimensional stability.
Perhaps the principal reason for annealing is to improve machinability
characteristics of the steel. It is not possible to present a general rule for annealing
that can be used on all types of steel subjected to diversified metal-removal
operations. For heavy roughing cuts, the material should be as soft as possible
(Rockwell B80). This is especially true when accuracy and finish are not important;
however, if a close tolerance is required, as on a broaching operation, then a finer
lamellar
microstructure with a hardness of approximately Rockwell B100 is more
desirable. The principal point for the engineer to keep in mind when using medium-
carbon steel is to specify the condition of the steel as purchased.
As the hardness of steel increases during cold working, ductility decreases and
additional cold working becomes so difficult that the steel must be annealed to
restore its ductility.

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