Abe / Kern / Viswanathan Creep-Resistant Steels

1. Auflage 2008
ISBN: 978-1-84569-401-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 700 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Creep-resistant steels are widely used in the petroleum, chemical and power generation industries. Creep-resistant steels must be reliable over very long periods of time at high temperatures and in severe environments. Understanding and improving long-term creep strength is essential for safe operation of plant and equipment. This book provides an authoritative summary of key research in this important area.The first part of the book describes the specifications and manufacture of creep-resistant steels. Part two covers the behaviour of creep-resistant steels and methods for strengthening them. The final group of chapters analyses applications in such areas as turbines and nuclear reactors.With its distinguished editors and international team of contributors, Creep-resistant steels is a valuable reference for the power generation, petrochemical and other industries which use high strength steels at elevated temperatures. - Describes the specifications and manufacture of creep-resistant steels - Strengthening methods are discussed in detail - Different applications are analysed including turbines and nuclear reactors

Autoren/Hrsg.

Abe, Fujio

Kern, Torsten-Ulf

Viswanathan, R.

Leseproben

1 Introduction
F. Abe    National Institute for Materials Science (NIMS), Japan 1.1 Definition of creep
Plastic deformation is irreversible and it consists of time-dependent and time-independent components. In general, creep refers to the time-dependent component of plastic deformation. This means that creep is a slow and continuous plastic deformation of materials over extended periods under load. Although creep can take place at all temperatures above absolute zero Kelvin, traditionally creep has been associated with time-dependent plastic deformation at elevated temperatures, often higher than roughly 0.4Tm, where Tm is the absolute melting temperature, because diffusion can assist creep at elevated temperatures. For detailed description of mechanical equation of state, creep behavior of metals and alloys, dislocation motion during creep, mechanisms of creep, creep damage and fracture, the reader is referred to standard text books on creep.1-6 1.2 Creep and creep rate curves
Creep tests can be conducted either at constant load or at constant stress. For experimental convenience, most frequently the creep tests of engineering steels are conducted at constant tensile load and at constant temperature. The test results can be plotted as creep curves, which represent graphically the time dependence of strain measured over a reference or gauge length. Figure 1.1 shows schematically three types of creep curves under constant tensile load and constant temperature conditions and also their creep rates ? = de/dt, where e is the strain and t the time, as a function of time. Textbooks on creep of metals and alloys generally describe that three stages of creep, consisting of primary or transient, secondary or steady-state and tertiary or acceleration creep that appear after instantaneous strain e0 upon loading as shown in Fig. 1.1(a), when the test temperature is high enough or at a high homologous temperature. The homologous temperature is defined as the ratio T/Tm, where T is the test temperature in absolute Kelvin and Tm the absolute melting temperature. The instantaneous strain e0 contains elastic strain and possibly plastic strain depending on the stress level. 1.1 (a), (b) and (c) Creep curves of engineering steels under constant tensile load and constant temperature and (d), (e) and (f) their creep rate curves as a function of time. In the primary creep stage between e0 and e1, the creep rate, ?, decreases with time, as shown in Fig. 1.1(d). The decreasing creep rate in the primary creep stage has been attributed to strain hardening or to a decrease in free or mobile dislocations. In the secondary creep stage between e1 and e2, the creep rate remains constant. This creep rate is designated as a steady-state creep rate, ?s, which is given by ?s = (e2 – e1)/(t2 – t1) and is commonly attributed to a state of balance between the rate of generation of dislocations contributing to hardening and the rate of recovery contributing to softening. At high homologous temperatures, creep mainly involves diffusion and hence the recovery rate is high enough to balance the strain hardening and results in the appearance of secondary or steady-state creep. In the tertiary creep stage, the creep rate increases with time until rupture at rupture time tr and rupture strain, er. It should be remembered that under the constant tensile load, the stress continuously increases as creep proceeds or as cross-section decreases and a pronounced effect of increase in stress on the creep rate appears in the tertiary creep stage. Necking of the specimens before rupture causes a significant increase in stress. The increase in creep rate with time in the tertiary creep stage can follow from increasing stress or from microstructure evolution including damage evolution taking place during creep. Microstructure evolution usually consists of dynamic recovery, dynamic recrystallization, coarsening of precipitates and other phenomena, which cause softening and result in a decrease in resistance to creep. Damage evolution includes the development of creep voids and cracks, often along grain boundaries. The extent and shape of the three creep stages described above can vary markedly depending on test conditions of stress and temperature, as shown schematically in Fig. 1.2, where the final point in each curve represents creep rupture. With increasing stress and temperature, the time to rupture and the extent of secondary creep usually decrease but the total elongation increases. 1.2 Schematic creep curves varying with stress and temperature. Under certain conditions, the secondary or steady-state creep stage may be absent, so that immediately after the primary creep stage the tertiary creep stage begins at tm, as shown in Fig. 1.1(b) and 1.1(e). In this case, the minimum creep rate ?min, can be defined instead of the steady-stage creep rate, ?s. Similar to the steady-stage creep rate, ?s, the minimum creep rate, ?min, can be explained by the process where hardening in the primary stage is balanced by softening in the tertiary stage. In many cases, there is substantially no steady-state stage in engineering creep-resistant steels and alloys. Many researchers have shown that there is an ever-evolving microstructure during creep for engineering creep-resistant steels and alloys. This suggests that there is no dynamic microstructural equilibrium in engineering creep-resistant steels and other alloys during creep, which characterizes steady-state creep of simple metals and alloys. Therefore, the term ‘minimum creep rate’ has been favored by engineers and researchers who are concerned with engineering creep-resistant steels and alloys. The stress dependence of minimum or steady-state creep rate is usually expressed by a power law as: ?minore?s=Asn   [1.1] =A'exp-Qc/RT   [1.2] where n is the stress exponent, Qc the activation energy for creep, R the gas constant and T the absolute temperature. The parameter A' includes microstructure parameters such as grain size and so on. Equation [1.1] is often referred to Norton’s law. It is well known that the minimum or steady-state creep rate is inversely proportional to the time to rupture tr as: ?minore?s=C/trm=A'snexp-Qc/RT   [1.3] where C is a constant depending on total elongation during creep and m is a constant often nearly equal to 1. Equation [1.3] is often referred to as the Monkman–Grant relationship, which has been experimentally confirmed not only for simple metals and alloys but also for a number of engineering creep-resistant steels and alloys. Equation [1.3] suggests that the minimum or steady-state creep rate and the time to rupture vary in a similar manner to stress and temperature. At low homologous temperatures, with T/Tm often less than roughly 0.3, where diffusion is not important, only the primary stage appears. Usually only limited strains well below 1% occur that do not lead to final rupture, as shown in Fig. 1.1(c) and 1.1(f). This deformation process is designated as logarithmic creep. Considerable efforts have been made to describe the creep curves, namely, the time dependence of creep strain. There are several model equations available for characterizing the primary, secondary and tertiary creep stage characteristics, ranging in complexity from simple phenomenological to physically based constitutive. Recent progress on the suitability of some of these to specific materials classes and analytical applications is reviewed by Holdsworth et al. [7]. Although Fig. 1.1 shows the idealized creep and creep rate curves, engineering creep-resistant steels sometimes exhibit complicated behavior, especially under low stress and long time conditions, reflecting complex microstructural evolution during creep. Complicated behavior is clearly demonstrated by creep rate curves rather than creep curves. Figure 1.3 shows an example of complicated creep rate curves of 1Cr–0.5Mo steel at 550 °C.8 At high stresses above 108 MPa, the creep rate curves are relatively simple and consist of the primary and tertiary stages but there is no substantial steady-state stage, similar to Fig. 1.1(e). The shape of creep rate curve becomes gradually complicated with decreasing stress. At low stresses below 88 MPa, two minima appear in the creep rate curves. This suggests that new strengthening effects such as the precipitation of new phases seem to operate after an extended period, causing a decrease in creep rate again after an growing the previous acceleration creep. The subsequent loss of the...