Heat treatment of steel: heating and cooling

Heat treatment operations of steel, which are based on heating to a certain temperature and holding at it were considered earlier. These include: vacation, solution treatment and annealing. Next, consider the operations of heat treatment of steel, which include both heating, and regulated cooling with various intensities - from cooling in calm air to rapid cooling with water.

Steel normalization

Steel normalization is called steel solution treatment at a temperature not less, than on 55 ºС above the upper critical temperature with subsequent cooling in air. The normalization temperature depends on the carbon content in the steel as shown in the figure.. The goal of normalization is usually to improve grain structure and, Besides, harden steel a little.

nagrev-stali-pod-zakalkuFigure - Simplified iron-carbon diagram.
The hatched strip is the heating interval of the steel for normalizing and quenching

When normalizing, the products are positioned as follows, so that when the charge is cooled after heating, air can circulate freely around each item. If the air flow around heated items is obstructed, then the heat treatment operation will be closer to annealing, than normalization. Accelerated cooling by fans or compressed air can produce a result more like quenching.

Microstructure, which occurs during normalization, is a mixture ferrite and perlite, usually with low residual stresses and almost no warpage. Some products are tempered after normalization., to obtain a slight additional softening of the steel, as well as additional reduction of residual stresses. Homogeneous normalized structure is usually well cut.

Steel hardening

To get higher strength and hardness, than with normalization, it is necessary to apply such a cooling rate of steel from the austenitizing temperature, which would ensure the transformation of austenite into bainite and martensite, and not into a ferrite-pearlite mixture. This operation is called quenching.. Quenching consists in austenitizing the steel at temperatures, shown in the figure 1, and then cool sufficiently quickly to, so that ferrite and pearlite do not have time to form.

Quenching media

The maximum achievable hardness of hardened steel the hardness depends practically only on the carbon content. It is achieved by cooling at a rate equal to or higher than the critical cooling rate for a given alloy.. Water is used as a quenching medium, salt solutions, oil, water-polymer solutions and, in some cases, inert gases.

Quenching of steel in water and oil

Typically, water and salt solutions are used when quenching steel.. Where possible, cheaper water is used. However hardening, eg, high carbon steels, requires oil. When hardening complex steel products, oil is also often used to minimize warpage and cracking.. Oil cooling of steels is almost always slower, than water.

Hardening of steel in organic polymer solutions

Some organic polymers, when added to water, give it quenching properties., similar to those, possessed by oil. The main advantage of these solutions is, that they take away heat more slowly, than water, but without the danger of fire, which is characteristic of oil. The disadvantage of polymer solutions is that, that they require strict concentration control, temperature and stirring to achieve consistent hardening results. The severity of hardening in salt baths can vary widely and depends on the type of polymer, his concentration, bath temperature and the intensity of stirring the solution during quenching.

Interrupted hardening of steel

In some cases, it is required to quench steel in water or saline solution to obtain high surface hardness of the product. However, cooling with water or brine until the steel is fully hardened can lead to warpage or quench cracks.. If there is no need to harden the steel over the entire cross section, then the so-called interrupted quenching is often used. In English it is also called "slack quenching", weak or weakened hardening. Interrupted quenching usually involves quenching in water for a specified time, and then the product is transferred to an oil bath to complete the transformation.

Hardening of carbon and low alloy steels is always accompanied by tempering.

Tempering hardened steel

Vacation is the process of heating hardened steel to a temperature below the lower critical temperature, followed by cooling to room temperature. The purpose of tempering is to reduce internal stresses and reduce hardness and thereby obtain higher ductility., than in the case of hardened products without tempering. Tempering slightly modifies the structure of martensite and this change is used to "adjust" the strength, hardness, viscosity and other mechanical properties up to specified values.

Carbon and low-alloy steels are tempered in the temperature range from 175 to 700 ° C. The exposure time may vary from 30 minutes to hours. A longer tempering time at a given temperature or a higher temperature at a given holding time increases the softening of the steel.. At the same temperature, martensite decreases its strength and hardness more significantly, than perlite, and the rate of change in the properties of steel during tempering depends on its chemical composition.

Tempering brittleness of steel

Some alloy steels when tempered at temperatures below 595 ° C may suffer from holiday embrittlement. For such steels, they try to avoid or pass through the temper brittleness temperature range at a high speed..