Thermomechanical Treatment of Metals

a combination of deformation, heating, and cooling operations performed in various cycles. The final microstructure and, consequently, the resultant properties of the metal are determined under conditions of high density and optimum distribution of the structural imperfections caused by plastic deformation. Thus, this type of treatment is distinguished by the combination of heat treatment and pressure shaping (plastic deformation). Thermomechanical treatment is made possible by the fact that the presence in real alloys of such structural imperfections as dislocations, stacking faults, and vacancies has an essential effect on the processes of structural transformations. On the other hand, certain structural changes produce new imperfections and redistribute existing ones. Hence, the mechanism and kinetics of structural changes caused by thermomechanical treatment depend on the nature and density of the imperfections and, in turn, affect the number and distribution of such imperfections.

Thermomechanical treatment cycles are best classified according to the order in which plastic deformation and heat treatment are carried out (Figure 1).

The combination of plastic deformation with phase transformations was first put into practical use in the early 20th century, with the introduction of patenting in the production of steel wire. A unique combination of plastic deformation and heat treatment produced excellent mechanical properties that could not be achieved by any other strengthening treatment. In the 1930’s, the mechanical properties of beryllium bronze were greatly improved by another type of thermomechanical treatment using a cycle of hardening, cold deformation, and aging.

The development of thermomechanical treatment and the establishment of its main principles were made possible only by dislocation theory, particularly in areas in which a connection was established between structural imperfections and structure formation during transformations. The process was first tested in 1954 in the USA, where low-temperature thermomechanical treatments were used to strengthen structural steels. The supercooling of austenite in the low-temperature process is intended to allow deformation to take place at a temperature below the recrystallization temperature. In this, the low-temperature process differs from high-temperature thermomechanical treatment, which was developed somewhat later in the USSR. The latter process became widely used for increasing the mechanical strength of mass-produced steels in modern machine building.

Since the deformation temperature for high-temperature thermomechanical treatment is usually above the upper critical point for polymorphic transformation, an analogy is made between high-temperature thermomechanical treatment and heat treatment at rolling or forging temperatures. The main difference between these treatments is that the high-temperature thermomechanical treatment results in such conditions of hot working and subsequent quenching that recrystallization processes are suppressed. This creates a unique structural state characterized by increased density of imperfections and their special distribution as a polygonization structure is formed. This is what produces the experimentally observed well-developed mosaic configuration after high-temperature thermomechanical treatment of steel and the increase in the fine submicroscopic nonuniformity of the martensitic structure and composition, which provides for a unique combination of properties, in which ductility, toughness, and resistance to brittle fracture are increased along with strength.

Table 1 compares the properties of a typical medium-carbon alloy structural steel after high-temperature and low-temperature thermomechanical treatments. Thermomechanical treatment produces better fatigue characteristics; after high-temperature treatment, there is a particularly large increase in fatigue life in the zone of limited endurance in the S-N diagram. The treatment raises the steel’s impact resistance, lowers the transition temperature,

Figure 1. Classification of types of thermomechanical treatment: (1) preliminary, (2) high-temperature, (3) high-temperature surface treatment, (4) high-temperature isothermal, (5) low-temperature, (6) low-temperature isothermal, (7) high-temperature-low-temperature, (8) low-temperature-high-temperature, (9) deformation of martensite with subsequent tempering, (10) deformation of martensite after high-temperature thermomechanical treatment with subsequent tempering, (11) deformation of nonmartensitic structures on creep region, sometimes repeated in several cycles, (12) deformation at room temperature with aging, (13) deformation at elevated temperatures with aging, (14) hereditary high-temperature thermomechanical hardening; (A₁) and (A₃) lower and upper critical temperatures, (M₁) temperature at which martensite is first formed. Thermomechanical treatment of classes I and IV is based on the phenomenon of inheritance of strengthening retained after appropriate thermal treatment.

and virtually eliminates a dangerous tendency to temper-brittleness, which is not observed after low-temperature treatment. Development of high-temperature thermomechanical treatment led to the creation of a new process, high-temperature thermomechanical isothermal treatment, in which hot working is combined with isothermal transformation (austempering). Articles treated by this process (in particular, springs) have increased service lives. All the thermomechanical strengthening treatments shown in Figure 1 are used to a greater or lesser degree. The choice of a process depends on the nature and purpose of the alloy and the specific workpiece. The efficiency of a specific thermomechanical strengthening method is determined by a group of mechanical properties. The engineering concept of increased strength involves a rise in resistance to deformation and fracture at various stresses, including those that can cause brittle cracks and premature failure. Therefore, in addition to traditional tensile, impact, and fatigue tests, modern high-strength steels, including thermomechanically strengthened steels, must be evaluated according to such mechanical failure criteria as crack propagation energy.

An understanding of the physical basis of thermomechanical strengthening is possible only after the basic laws governing structural changes during hot deformation have been made clear. The old concept that hot deformation is always accompanied by recrystallization was found to be incorrect.

In thermomechanical treatments, the metal is rapidly quenched as soon as hot deformation is completed. The final structure of the strengthened steel then inherits the fine structure of hot-deformed austenite. The structure of austenite after completion of hot deformation can vary considerably, depending on the stress, temperature, and deformation rate. The structure corresponds to any of the following states.

(1) Hot work-hardening with a disordered arrangement of dislocations, when subsequent quenching increases strength and at the same time reduces resistance to brittle fracture.

(2) Formation of a substructure caused by dynamic recovery and a particularly well-defined and stable subgrain structure through dynamic polygonization. In this case, quenching produces an excellent combination of high strength and resistance to brittle fracture.

(3) Dynamic recrystallization, when increased dislocation density is preserved in some sections and the density is considerably reduced in others. In such a case, quenching can produce a combination of excellent mechanical properties, but they are not reliable because of nonuniformity and instability of the fine structure.

Consequently, a hot deformation cycle for thermomechanical treatment must be so chosen as to produce a stable, well-developed substructure through dynamic polygonization. Because of the shear mechanism of the martensitic transformation, its structure inherits the substructure of the deformed austenite produced during dynamic polygonization. If another type of thermomechanical treatment—specifically, the high-temperature thermomechanical isothermal treatment shown in Figure 1—is used, then the bainite produced as a result of the displacement reaction also inherits the substructure of hot-deformed austenite. In all cases, the presence of this stable substructure in final phases, such as martensite, produces a finely dispersed and well-developed mosaic configuration, as well as a fine distribution of impurities, which leads to an improvement in all mechanical properties, characterized by an increase in resistance to both plastic deformation and fracture. This effect is observed not only in initial thermomechanical treatments but also in heat treatments that follow thermomechanical treatments. The phenomenon of “inheritance” of thermomechanical strengthening was discovered in the USSR and is widely used in domestic and foreign industry. The basis of the effect is that hot deformation creates a highly developed and stable substructure that remains stable during subsequent recrystallization. During subsequent heat treatment, recrystallization occurs through a shear mechanism that preserves the substructure and, consequently, the entire group of excellent mechanical properties produced by the initial thermomechanical treatment. Development of the inheritance concept has resulted in the production of a new process, thermomechanical pretreatment, which is widely used in the USSR and the USA. Inheritance also explains the excellent properties produced by patenting, which is essentially a variety of thermomechanical treatment.

The following cycles of thermomechanical treatment are used for precipitation hardening of alloys: (1) heating to the hardening temperature, deformation, immediate quenching, and aging (high-temperature treatment), and (2) hardening, deformation, and aging (low-temperature treatment). The first cycle is easily accomplished, but its disadvantage is the risk of excessive recrystallization during deformation at the high hardening temperature. It is extensively used in the production of pressed work-pieces made from numerous aluminum alloys containing small additives of Mn, Cr, and other elements that impede recrystallization. The second cycle may cause difficulties associated with the high resistance to deformation of a solid solution at room temperature. It has, however, a number of advantages, including highly dispersed phases that accompany aging even during cold or warm deformation and creation of more uniform distribution of precipitation-hardening phases at dislocations throughout the grains. This second cycle has been successfully used for strengthening age-hardenable copper and aluminum alloys.