Spheroidizing annealing of steel

Spheroidizing annealing is usually used to improve the cold formability of steel.. Its other application is to improve the machinability by cutting hypereutectoid steels..

Spheroidized steel structure

Spheroidized ferrite-pearlite structure has a low yield strength. The yield point level is determined by the ratio and distribution of ferrite and carbides. The strength of ferrite depends on the size of its grain. The ability of steel to form depends significantly on, in what form are carbides - in lamellar or globular.

By heating and cooling steels, it is possible to obtain its structure in the form of globular carbides in a ferritic matrix.. On the picture 1 shown steel 40 fully spheroidized.

structura40Picture 1 - Spheroidized microstructure of steel 40 after 21 hours at temperature 700 ° C

Methods for spheroidizing steel

To obtain a spheroidized structure in steel, the following methods are used:
1) Long exposure at temperatures just below Ac1.
2) Alternating heating and cooling between temperatures slightly above Ac1 and slightly below Ar1.
3) Heating to a temperature slightly above Ac1 and then or very slow cooling with an oven, or holding at a temperature slightly higher than Ar1.
4) Cooling at a suitable speed from the minimum temperature, at which all carbides dissolve, to prevent re-formation of the carbide mesh. Then reheating according to the methods 1) or 2). Suitable for hypereutectoid steels with carbide mesh.

Temperature range for spheroidizing steel

The temperature range for spheroidization of hypereutectoid and hypereutectoid steels is shown in the figure 2.

temperature-spheroidization-stalsPicture 2 - Simplified iron-carbon state diagram.
Temperature range for spheroidizing steel treatment

Steel spheroidizing rate

The spheroidization rate depends on the initial microstructure and is maximum for quenched structures., in which the carbide phase is finely dispersed (picture 3). Cold pre-deformation also increases the spheroidization rate..

dve-structuryPicture 3 – Influence of initial microstructure on steel spheroidization 40
at a temperature 700 ° C for 21 o'clock. Left – initial martensitic microstructure without tempering, case – original ferrite-pearlite microstructure

For complete spheroidization, processing temperatures must be or slightly higher than Ac1 or in the middle between Ace1 and As3. Low carbon steels are rarely spheroidized to improve cutting performance, so in this state they become soft and viscous and give a long, viscous shavings. On the other hand, spheroidized low carbon steel is able to withstand large plastic deformations.

Hardness of spheroidized steel

The hardness of spheroidized steel depends on its chemical composition: with an increase in the content of carbon and alloying elements, it can increase from 165 to 210 NV.

Sources:
1. Gulyaev A. P. Metallurgy, 1986.
2. The Heater’s Guide: Practices and Procedures for Irons and Steels, AMS International, 1995.