Australian Standard AS2028-1977 “Methods for the Measurement of Depth of Hardening in Flame and Induction Hardened Steel Products” provides the following basic but important definitions:
A surface layer having a higher hardness than the core (such differences arise from micrographic changes which occur as a result of heating, by flame or induction methods, and quenching).
The portion of steel product which is unaffected by the case hardening process.
Effective case depth
The distance measured along a line normal to the original surface to the point where the hardness first equals a specified value. Unless otherwise agreed the hardness criterion for effective case depth shall be 400HV (41HRc).
Total case depth
The distance measured along a line normal to the original surface to the point where differences in physical properties of the case and core cannot be distinguished.
There are other ways of specifying case depth. A common one is Case Depth of Flame or induction hardened components. This is defined as the depth at which the hardness is 10 Rockwell points below the minimum specified surface hardness.
Hardness and Carbon Content
The goal of heat treatment of steel is very often to achieve a satisfactory hardness. The important microstructural phase is normally martensite, the hardness of which primarily is dependent on its carbon content. The maximum hardness that can be produced in any given carbon steel is that associated with a fully martensitic microstructure.
The high hardness and associated high strength, fatigue resistance, and wear resistance are the prime reasons for applying the quenching heat treatments that produce martensite. These heat treatments generally are applied to steels containing more than 0.3% carbon, in which the gains in hardness are most substantial.
Hardenability is defined as the relative ability of steels to be deep hardened, and is unique to a given steel composition and grain size. A method to determine hardenability is the Jominy end-quench hardenability test in which a standard round bar specimen is cooled at one end by a column of water; thus, the entire specimen experiences a range of cooling rates.
Austenite transforms to microstructures containing increasingly greater amounts of pearlite (with associated lower hardness) with decreasing cooling rate or increasing distance from the quenched end of the specimen.
Tempering Effect on Steel Hardness
The maximum hardness associated with as-quenched martensite decreases with increasing tempering temperature. Lower carbon steels have a lower hardness in the as-quenched condition and throughout tempering. Therefore, if maximum hardness is required, a high-carbon steel should be selected and tempering should be carried out in the 150 to 200C (300 to 400F) temperature range.
Tempering in this range produces a modest increase in toughness that is adequate in applications requiring high strength and fatigue resistance (medium-carbon steels) and in applications requiring the high hardness and associated good wear resistance that high-carbon martensite and light tempers provide, such as high-carbon steel bearings and gears where loading is primarily compressive
Two types of physical changes that cause residual stresses produced during cooling of heat treated parts are thermal contraction that occurs during cooling of a single phase or microstructure consisting of a mix of phases in the absence of a phase transformation, and transformation of austenite to the more open, higher specific volume crystal structures of ferrite, cementite, and martensite.
Volume expansion due to austenite transformation is the dominant factor in any heat treatment that involves cooling from the austenite phase field, while thermal contraction is the dominant factor in subcritical heat treatments. Residual stresses and distortion arise because cooling rate is a function of section size or position in a part.
Carbon and low-alloy steels slowly cooled from tempering at temperatures higher than 575C (1070F), or tempered for long times between the critical temperature range of 375 and 575C (710 and 1070F) suffer a loss in toughness, called temper embrittlement (TE).
This manifests itself in reduced notch-bar impact strength (increased impact transition temperature) compared with that resulting from relatively fast cooling rates. TE can occur due to the presence of impurities in steel (antimony, phosphorus, tin, and arsenic) on the order of 100 ppm (0.01 percent) or less.
Temper embrittlement is due to precipitation of compounds containing trace amounts of these elements. Large amounts of silicon and manganese also can be detrimental. TE is reversible; that is, de-embrittlement can be achieved by heating to about 575C for only a few minutes.