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.
The heated specimen is removed from the heating oven and quickly set in a water-quenching fixture, operating under prescribed conditions of water temperature and flow rate, quenching only the cylinder end face. Upon cooling, the cylinder is parted longitudinally (axially) down the center, and a series of Rockwell hardness readings are taken from the quenched edge. A hardness scan for several alloy steels is shown in Fig. A3.17. The Jominy test procedure is defined in ASTM A 255.
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.
The heated specimen is removed from the heating oven and quickly set in a water-quenching fixture, operating under prescribed conditions of water temperature and flow rate, quenching only the cylinder end face. Upon cooling, the cylinder is parted longitudinally (axially) down the center, and a series of Rockwell hardness readings are taken from the quenched edge. A hardness scan for several alloy steels is shown in Fig. A3.17. The Jominy test procedure is defined in ASTM A 255.
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| FIGURE A3.16 ASTMgrain size charts for classification of steels. 100 magnifications |
Many other metal alloys harden or strengthen with special aging or tempering heat treatments. However, this trait is normally not referred to as harden ability. These will be discussed in more detail later in the chapter.
Property Stability
The mechanical properties of materials may degrade with service time. In particular, alloys that depend on heat treatment or cold working to develop their strength may weaken if operated for long times at elevated temperatures. The actual exposure to service temperatures acts as a continuation of the heattreating process, albeit at a significantly reduced rate of effect. In many engineering metals, this effect is actually a property degradation due to overtempering of the material.
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| FIGURE A3.17 Jominy end-quench hardenability curves for various 0.40 percent carbon steels |
A number of thermodynamic relationships exist that relate material strength and time and temperature of exposure for carbon and alloy steels. The most famous and widely used of these is the Hollomon-Jaffe Parameter (HJP). It is defined as the following:
where T is temperature in degrees Celsius, t is time in hours, and C is a constant, usually around 20 for carbon steels.4 Using this equation and solving for HJP for a given set of time and temperature conditions, the engineer can determine the time at a different temperature of interest that can result in an equivalent metallurgical effect.
A limitation exists on the range of temperatures over which the predictive capability of the Holloman-Jaffe equation can be considered reliable. Phenominologically, the same metallurgical processes must be in effect over the range of temperatures under consideration. For example, if a phase change occurs, or if other important micro structural constituents, such as carbides, are not stable at the two temperatures being compared, the correlation is not valid.
Design codes using allowable design stresses based on creep properties of the metals, by the nature of the long-term rupture tests involved, take these degrading tendencies into account. However, it is not always appreciated that the time dependent properties, such as ultimate tensile strength and yield strength, can be decreased significantly below the starting property level by the same long-term service. This fact would be important to an engineer concerned with designing a high-temperature structure that must tolerate shock loads, such as seismic effects, that can occur near the end of life of the component.
More on degradation of properties and the mechanisms involved is discussed later in this chapter.
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