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The steel change when heat treatment heats
From;    Author:Stand originally
One, the A when steel heats changes a course

1.Eutectoid steel P is heating temperature is big beyond when be equal to Ac1, translate into A.

Its change can divide for the following 4 phase: A of deliquescent ④ of grown ③ leftover Fe3C changes nucleus of A of ② of nucleus of ① A form equably

A forms the T of △ of degree of superheat with must want to have certain, offer change G of driving force △

Nucleus of ① A form, nucleate position is normally on F and Fe3C two-phase interface.
② A nucleus is grown, mix to F at the same time after form nucleus Fe3C elapse of face of two phase boundary, f crystal lattice forms face heart cubic metre again, fe3C is ceaseless and deliquescent, offer C cent to A.

Reframe rate is rapidder than Fe3C deliquescent speed, so F dissolves first, odd Fe3C diffuses through C atom, make A is changed equably thereby.

Inferior eutectoid steel and cross eutectoid steel to want to heat to transform completely to Ac3 or Accm above ability respectively for A.

2, A grain size

Grain size reachs the organization that its acquire and function to all have very big effect to cooling transition process. Accordingly, the regularity that masters A grain to be brought up reachs the method of granuality of control A brilliant, to heat treatment practice has very important sense

1.The grain after A is changed is grown

After A is changed, lift as temperature or heat preservation time is lengthened, a grain can be brought up ceaselessly

2.Index of A grain size

① grain spends: The average of grain diameter.

According to GB6394-86, a brilliant granuality divides 10 level commonly (standard photograph contrasts) , the number is larger, grain is finer. 1-4 class, thick grain; 5-10 class fine grain; 10 class above exceeds fine, also have thicker than 1 class 0 class, - 1 class.

② initiative grain spends: A forms firm end, the grain size when its grain border just is contacted each other. With A grown tendentiousness is concerned, still concern with chemical composition.

3.Affect A grown factor:

① internal cause: The composition of steel, organization decides it has certain A grown tendentiousness.

Grown tendentiousness: Below similar requirement, some grain are brought up easily, what plant because of steel is different and different, right even same kind of steel, because smelt method is different, heating likewise different grain also can be shown below the condition grown tendentiousness.
A. ↑ of steel C% , inferior eutectoid steel is easy grown, cross eutectoid steel to be brought up not easily, eutectoid steel is brought up the most easily. B. Alloying element
Outside dividing Mn, P, common element adding up to brilliant all can prevent A grain to be brought up, if cloth of divide into equal parts of V, Ti, Nb, Al is in,crystal boundary forms difficult dissolve compound to all can prevent A grain strongly to be brought up. C. Grain of steel of steel of high grade structure, carbolic element tool, A is brought up not easily
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