Most metals used in industry undergo heat treatment. Heat treatment involves heating a metal or alloy to a specific temperature, holding it there, and then cooling it in order to change its structure and obtain the required physical, mechanical, and other properties.
During heat treatment, the metal undergoes a transformation that improves its strength and wear resistance, gives it the required hardness, and makes it easier to machine on metal-cutting machine tools.
Transformations in steel during heating
Heating steel during heat treatment usually serves to obtain an austenitic structure. Steel with less than 0.8% carbon consists of pearlite and ferrite grains when heated up to the Ac1 temperature. At the Ac1 point, pearlite begins to recrystallize into fine-grained austenite. On further heating in the Ac1–Ac3 range, the excess ferrite dissolves in the austenite, and the transformation is complete on reaching Ac3 (line GS on the iron–carbon diagram). Above Ac3, the steel consists entirely of austenite.
In hypereutectoid steel, the excess cementite begins to dissolve above Ac1, and above the Acm point the structure consists of austenite only.
Once the transformations are complete and all the pearlite has turned into austenite, the steel acquires a fine-grained austenitic structure. The rate of austenite formation and the equalization of its composition depend on several factors, including temperature and the presence of alloying elements (chromium, tungsten, molybdenum, etc.), which slow the process down.
Austenite is chemically non-uniform: it is richer in carbon where the cementite plates used to be and poorer where the ferrite plates were. To equalize the chemical composition of the austenite grains, the steel is heated slightly above the upper critical point Ac3 and held at that temperature. As the temperature rises further, the fine austenite grains begin to coarsen, and the higher the temperature, the faster the grains grow.
Types of austenite grain
Austenite grains are classified as initial, actual, and inherent. The initial grain is the grain size at the moment the transformation of pearlite into austenite is complete. The actual grain is the grain obtained under specific heating conditions; its size depends on the steelmaking method and the type of the last heating. The actual grain size can be controlled through heat treatment modes.
The inherent (hereditary) grain describes the austenite’s tendency to grow. In steels of the same chemical composition, austenite grains can grow at different rates depending on the properties of the particular heat. Steels are divided into fine-grained and coarse-grained. Fine-grained steels keep a fine grain at 950–1000 °C, while in coarse-grained steels the grain starts to grow rapidly when heated only slightly above 800 °C.
Transformations in steel during cooling (austenite decomposition)
Austenite is stable only above 723 °C (the A1 point). When steel heated to the austenitic state is cooled below the Ar1 point, the austenite loses its stability and begins to transform.
When eutectoid carbon steel with 0.8% carbon is cooled slowly, at the temperature of line PSK the austenite transforms into pearlite, a mechanical mixture of ferrite and cementite. This process follows the laws of crystallization.
The transformation of austenite into pearlite is studied experimentally under isothermal conditions rather than during continuous cooling. Steel specimens are heated above Ac3 and, after holding long enough to heat them through, are quickly transferred into a bath at a temperature below Ac1. The transformation of austenite into pearlite then takes place in the bath.
With slow cooling, austenite transforms into pearlite. At a higher cooling rate, the undercooled austenite transforms into sorbite. At an even higher cooling rate, the steel becomes martensitic, which results in hardening.
The critical cooling rate is the minimum cooling rate at which austenite transforms completely into martensite. This value is of great importance in heat treatment. To harden steel, it must be cooled at a rate no lower than the critical rate.
The products of austenite decomposition are martensite, troostite, and sorbite.
Products of austenite decomposition
Martensite is a supersaturated solid solution of carbon in α-iron with a body-centered tetragonal crystal structure, in which carbon atoms occupy interstitial sites. Martensite has an acicular (needle-like) microstructure, high hardness (64–65 HRC), high strength, considerable brittleness, and magnetic properties. It is a metastable structure: on tempering, carbon precipitates from it as cementite particles, so tempered martensite turns into tempered troostite or tempered sorbite.
Troostite is a mixture of cementite and ferrite that is less brittle and less hard than martensite (40–45 HRC). Tempered troostite is valued for parts operating under variable loads, such as springs and leaf springs.
Sorbite is a mixture of ferrite and fine cementite particles. Tempered sorbite has a hardness of 30–40 HRC and combines a high elastic limit with sufficient impact toughness. This is why sorbite is used for parts subjected to variable loads.