Machinery's Handbook, 31st Edition
462 Heat Treatment of Steel Snap Temper: A precautionary interim stress-relieving treatment applied to high hard enability steels immediately after quenching to prevent cracking because of delay in tem pering them at the prescribed higher temperature. Temper Brittleness: Brittleness that results when certain steels are held within, or are cooled slowly through, a certain range of temperatures below the transformation range. The brittleness is revealed by notched-bar impact tests at or below room temperature. Transformation Ranges or Transformation Temperature Ranges: Those ranges of tem perature within which austenite forms during heating and transforms during cooling. The two ranges are distinct, sometimes overlapping but never coinciding. The limiting tem peratures of the ranges depend on the composition of the alloy and on the rate of change of temperature, particularly during cooling. Transformation Temperature: The temperature at which a change in phase occurs. The term is sometimes used to denote the limiting temperature of a transformation range. The following symbols are used for iron and steels: Ac cm = In hypereutectoid steel, the temperature at which the solution of cementite in austenite is completed during heating Ac 1 = The temperature at which austenite begins to form during heating Ac 3 = The temperature at which transformation of ferrite to austenite is completed during heating Ac 4 = The temperature at which austenite transforms to delta ferrite during heating Ae 1 , Ae 3 , Ae cm , Ae 4 = The temperatures of phase changes at equilibrium Ar cm = In hypereutectoid steel, the temperature at which precipitation of cementite starts during cooling Ar 1 = The temperature at which transformation of austenite to ferrite or to ferrite plus cementite is completed during cooling Ar 3 = The temperature at which austenite begins to transform to ferrite during cooling Ar 4 = The temperature at which delta ferrite transforms to austenite during cooling M s = The temperature at which transformation of austenite to martensite starts during cooling M f = The temperature, during cooling, at which transformation of austenite to mar tensite is substantially completed All these changes, except the formation of martensite, occur at lower temperatures during cooling than during heating, and depend on the rate of change of temperature. Hardness and Hardenability.— Hardenability is the property of steel that determines the depth and distribution of hardness induced by quenching from the austenitizing temperature. Hardenability should not be confused with hardness as such or with max- imum hardness. Hardness is a measure of the ability of a metal to resist penetration as determined by any one of a number of standard tests (Brinell, Rockwell, Vickers, etc). The maximum attainable hardness of any steel depends solely on carbon content and is not significantly affected by alloy content. Maximum hardness is realized only when the cooling rate in quenching is rapid enough to ensure full transformation to martensite. The as-quenched surface hardness of a steel part is dependent on carbon content and cooling rate, but the depth to which a certain hardness level is maintained with given quenching conditions is a function of its hardenability. Hardenability is largely determined by the percentage of alloying elements in the steel; however, austenite grain size, time and temperature during austenitizing, and prior microstructure also significantly affect the hardness depth. The hardenability required for a particular part depends on size, design, and service stresses. For highly stressed parts, the best combination of strength and toughness is obtained by through hardening to a martensitic structure followed by adequate tempering. There are applications, however, where through hardening is not
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