8-10 HEAT TREATMENT OF CARBON STEELS

8-10 HEAT TREATMENT OF CARBON STEELS

Carbon steels are those ferrous alloys in which carbon is the main element used to control their mechanical properties. Other elements, such as aluminum, boron, chromium, cobalt, nickel, titanium, tungsten, vanadium, and zirconium, are also commonly found in carbon steels, but their content is not specified and their presence is due to manufacturing processes. In addition, carbon steels contain up to 1.65% manganese, 0.60% silicon, and 0.4% copper.

Carbon steels are divided into three classes, according to their carbon content. Low- carbon steels contain from 0.08 to 0.35% carbon, medium-carbon steels contain from

0.35 to 0.50% carbon, and high-carbon steels have a carbon content greater than 0.5%. Annealing and Normalizing. Steel, whether wrought or cast, is in a more or less

coarsely granular condition. Heat treatment is necessary, particularly for cast steels, to break up the coarse structure and bring about refinement of grain size as well as uniform distribution of the constituents present. This is accomplished by annealing and normalizing. Full annealing consists of heating the steel to a temperature above its critical range, holding it there for a sufficient period of time, and slowly cooling it (usually in the furnace) to obtain an equilibrium structure. Heating above the upper critical temperature causes pearlite and any excess of cementite or ferrite to transform to austenite. This is known as austenitizing. Hypoeutectoíd steels are heated above the

upper critical temperature (A 3 line), whereas hypereutectoid steels are heated only above the lower critical temperature (A 1 line), as seen in Fig. 8-21. Annealed hypereutectoid steels contain cementite and can never really be soft, but they are in the best possible condition for machining. Other heat treatment processes in which the steel is heated below the lower critical temperature are called stress relief and spheroidizing, in contrast to full annealing, which always consists of heating above the critical temperature.

Fine grain temperature range

A cm 850 − Normalizing o C A 3 Hardening

Fine grain

em 750 − T Austenite

FIGURE 8-21 Heat treatment temperatures for carbon steels. (Metals Handbook, 1948 edition, American Society for Metals, Metals Park, Ohio.)

Spheroidizing is the formation of spheroids, which are embedded in a matrix of ferrite, from cementite. Particles are large enough to be readily visible under the light microscope. Martensite, bainite, and even pearlite can be changed to this structure, which results in considerable softening of the steel. Normalizing consists of heating the steel above its upper critical temperature, and then cooling it in still air. It differs from full annealing in that the rate of cooling is more rapid and there is no extended soaking period. Normalizing produces a uniform structure and refines the grain size of the steel, which may have been unduly coarsened at the forging temperature. If a steel is too soft Spheroidizing is the formation of spheroids, which are embedded in a matrix of ferrite, from cementite. Particles are large enough to be readily visible under the light microscope. Martensite, bainite, and even pearlite can be changed to this structure, which results in considerable softening of the steel. Normalizing consists of heating the steel above its upper critical temperature, and then cooling it in still air. It differs from full annealing in that the rate of cooling is more rapid and there is no extended soaking period. Normalizing produces a uniform structure and refines the grain size of the steel, which may have been unduly coarsened at the forging temperature. If a steel is too soft

Quenching. The hardening of steel requires the formation of martensite. This is accomplished by heating to a temperature high enough for steel to become austenitic, then cooling fast enough, usually by quenching in water or oil, to secure complete transformation to martensite. The composition of the steel to be hardened, the quenching technique used, and the design for heat treatment are all very important factors affecting the properties of the final product. Sharp corners, reentrant angles, and drastic changes in thickness should be avoided whenever possible in order to keep temperature gradients at

a minimum throughout the specimen. In the fully quenched state steel containing more than 0.2% carbon has such low ductility as to render the material useless for engineering applications, and it must be softened to some desired degree by tempering before use.

Tempering. Tempering is a controlled heat treatment consisting of reheating martensite, which is extremely brittle when produced during quenching of carbon steel. To improve its properties and to impart greater toughness, the quenched steel is subject to tempering

by heating to various temperatures below the critical temperature A 1 . Three stages of tempering are distinguished. In the first stage the quenched steel is heated to a temperature of 80 to 160°C (176 to 320°F), during which martensite loses some ε - carbide. This latter separates into very small particles and becomes less tetragonal. During the second stage. 230 to 280°C (446 to 536°F), any retained austenite transforms to bainite, and large dimensional changes occur. Finally, in the third stage, 260 to 360°C (500 to 680°F), ε -carbide changes to cementite platelets, producing a structure of ferrite and cementite. This is accompanied by a marked softening of the steel. The ε -carbide particles are extremely small in size, but they can be identified with the aid of the electron microscope. The ε -carbide is a closely packed hexagonal structure of

composition of about Fe 5 C 2 . The tempered steel becomes much more ductile, but at the same time, there is an unavoidable loss of hardness.

In the process called martempering the steel is quenched to a temperature just above the martensite formation M s and held there long enough to obtain the uniform temperature, but not long enough for bainite to form (see Fig.8-19). Then the steel is cooled in air. The resulting microstructure is martensitic, but the steel shows an improved ductility, and no tempering is necessary because martensite has been formed without the production of high internal stresses.

Hardenablllty. Hardenability is an index of the depth to which a given steel will harden when quenched in a particular manner, or it is the ease with which martensite can be produced in the steel. Hardenability is determined by a highly standardized test such as the Jominy end-quench test, but it is best assessed by the diameter of the largest steel cylinder that can be hardened all the way to its center by water quenching.

During quenching, a piece of steel will cool more rapidly at the surface than in the interior. This may cause the interior of the piece to be hardened to a lesser degree than the surface since, in a less rapidly cooled portion, some pearlite may be formed before martensite. For a piece of steel to be hardenable throughout, the critical cooling rate should be such that martensite forms before any pearlite has a chance to appear. This can

be achieved by making the cross section very thin or by retarding the transformation of austenite to pearlite. This latter is equivalent to displacement of the S curve to the right on the TTT diagram so that the gate at the nose is considerably widened.

High ratios of surface to mass tend to produce greater depths of hardening because the rate of cooling depends on the speed with which heat leaves the specimen. Other factors are the surface conditions and the austenite grain size. Any scale formed on the quenched High ratios of surface to mass tend to produce greater depths of hardening because the rate of cooling depends on the speed with which heat leaves the specimen. Other factors are the surface conditions and the austenite grain size. Any scale formed on the quenched