Effects of time and temperature

Knowledge Base > Effects of time and temperature

Proper treatment of heat requires precise-control over temperature, cooling rate and time at a particular temperature. Normally, heat treating process starts from heating alloys beyond upper transformation temperature. The exception to this is tempering, aging and stress relieving. This upper transformation temperature is known as “arrest” because at upper transformation temperature nothing noticeable happens. So, the alloy needs to be heated at more than transformation temperature. Alloy is usually held at such higher temperature for long enough time, so that heat completely penetrates into the alloy, resulting in a complete solid state solution for it.

Mechanical properties are usually enhanced by smaller grain-size, for example shear strength, tensile strength and toughness, such metals are commonly heated at more than upper-critical temperature, for prevention against solution grains from growing very large. Upon heating of steel at above upper-critical temperature austenite`s small grains are formed. By increasing the temperature, these grains increase in size. On quickly cooling, during martensite-transformation, the austenite grain size directly affects martensitic grain size. Large grain boundries come up with larger grains that serve as the weak spots of the structure. In order to reduce breakage`s probability, grain-size is normally controlled.

Diffusion transformation depends upon time. Cooling of metals usually suppresses precipitation towards lower temperature. For example, austenite normally exists at above upper critical-temperature. By quick cooling of the austenite, transformation is suppressed for lots of degrees below than the lower-critical temperature. Austenite is much stable and by providing enough time to it, it precipitates into microstructures of the cementite and ferrite. Cooling rate assists in controlling grain growth rate and also in producing partial martensitic transformation temperature before full production of other microstructures. Transformation usually occurs at under the sound`s speed. By slower cooling of the austenite, martensite-transformation doesn`t occur. Austenite grain-size affects nucleation rate but microstructure and grain size are generally controlled by cooling rate and temperature. Upon extremely slow cooling of austenite, it forms larger ferrite crystals. These crystals are filled with the spherical cementite inclusions. Microstructure is normally referedd as “sphereoidite”. Upon faster cooling, it forms coarse peartile. When cooled more quickly, fine peartile results and upon fastest cooling, it is bainite that forms. These microstructures are also formed upon cooling to specific temperature and holding for a specific time.

Majority of the non-ferrous alloys are heated for forming solution. These are quickly cooled down for producing martensite transformation, putting solution within the supersaturated solution. Respective alloy, becomes in a softer state and could then work while being cold.  Cold working enhances the hardness and strength of alloy. Defects resulted from plastic deformation speed-up the precipitation and increase hardness more than that of normal alloy`s hardness. If this alloy is not cold worked, even then solutes within these alloys usually precipitates, irrespective of the process taking much longer time. Such metals are sometimes heated at a temperature below lower critical temperature, to prevent recrystallization, for speeding up the precipitation.

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