Hardenability and Property Stability is one of the important in Piping Materials and also Piping & Fabrication will expose this topics more deep and deep for our knowledge.
Hardenability
This is a property of certain steels that allows them to be strengthened, or hardened, by heat treating. In carbon and alloy steels, for example, this hardening is accomplished by heating the material to a temperature above about 1550_F (843_C), where the material completely changes its crystal structure from BCC to FCC. When this is followed by rapid cooling or quenching, usually in water or oil, the result is a crystal structure akin to the original BCC, but distorted along one of the unit cell directions. In the case of steels the result is amartensitic structure possessing a lattice, termed a body-centered tetragonal (BCT), with a larger volume per unit cell than the starting BCC.
The maximum hardness achieved in a quenched structure is primarily a function of the steel’s carbon content: the higher the carbon content, the greater the hardness. The depth into the material to which a high hardness is achieved for a given quenching operation is a function of the total alloy content within the steel. The substitutional alloying element nickel has perhaps the strongest effect on increasing the depth to which hardness extends.3 Other elements creating similar if less potent effects are manganese and boron, substitutional and interstitial alloying elements, respectively. Standard specimens and procedures have been adopted for testing the harden ability of steels. The test rates a combination of the highest hardness achievable and the depth to which significant elevation of hardness occurs. It is called the JominyEnd-Quench test and is performed using a 1-in-diameter cylindrical specimen machined from the metal in question and heated to a temperature in its austenitic phase (FCC) region.
This is a property of certain steels that allows them to be strengthened, or hardened, by heat treating. In carbon and alloy steels, for example, this hardening is accomplished by heating the material to a temperature above about 1550_F (843_C), where the material completely changes its crystal structure from BCC to FCC. When this is followed by rapid cooling or quenching, usually in water or oil, the result is a crystal structure akin to the original BCC, but distorted along one of the unit cell directions. In the case of steels the result is amartensitic structure possessing a lattice, termed a body-centered tetragonal (BCT), with a larger volume per unit cell than the starting BCC.
The maximum hardness achieved in a quenched structure is primarily a function of the steel’s carbon content: the higher the carbon content, the greater the hardness. The depth into the material to which a high hardness is achieved for a given quenching operation is a function of the total alloy content within the steel. The substitutional alloying element nickel has perhaps the strongest effect on increasing the depth to which hardness extends.3 Other elements creating similar if less potent effects are manganese and boron, substitutional and interstitial alloying elements, respectively. Standard specimens and procedures have been adopted for testing the harden ability of steels. The test rates a combination of the highest hardness achievable and the depth to which significant elevation of hardness occurs. It is called the JominyEnd-Quench test and is performed using a 1-in-diameter cylindrical specimen machined from the metal in question and heated to a temperature in its austenitic phase (FCC) region.
