What Is Spheroidizing Annealing
Spheroidizing annealing is a heat treatment used to soften steel, especially high carbon and alloy steels, so that it can be machined, cold formed or further processed. During the treatment, the cementite platelets in pearlite break up and grow into small spherical particles dispersed in a ferrite matrix. The resulting structure, called spheroidite, is the softest and most ductile condition that a steel can reach at a given hardness.
Why Spheroidizing Is Needed
High carbon steels in the normal rolled or normalized condition are hard and abrasive, causing rapid tool wear and poor surface finish in machining. Spheroidizing lowers the hardness by 20 to 30 Rockwell C points or more and improves the uniformity of the material. It also restores ductility after cold working so that wire drawing, stamping and bending can continue without cracking.
The Heat Treatment Cycle
The process heats the steel to a temperature just below the lower critical temperature, typically 680 to 720 degrees Celsius, and holds it for a long time, often several hours to more than a day. Slow cooling follows the hold. Alternatively, the steel can be cycled above and below the critical temperature to accelerate carbide spheroidization. The exact cycle depends on the composition and the required final hardness.
Effect on Microstructure
The lamellar cementite of pearlite is unstable at the holding temperature and gradually dissolves at the edges of the plates, allowing the carbide to redistribute as spheroids. The diffusion process is slow, which is why the treatment takes time. The spheroidal carbides are smaller in surface area per volume than plates, which lowers the hardness and improves formability.
Benefits for Manufacturing
Spheroidized steel machines faster with longer tool life and better chip control, cuts cleaner threads and keyways, and cold heads into fasteners without cracking. The structure also improves the response to subsequent hardening treatments because the dissolved carbon distribution is uniform. For these reasons, spheroidizing is a standard mill treatment for wire rod, strip and bar used in cold heading and machining.
Typical Applications
Typical applications include cold heading wire for bolts and screws, carbon steel strip for stamping, spring steel before forming, bearing steel before machining, and tool steel before shaping. The treatment is specified for steels with carbon content above about 0.4 percent when severe machining or cold forming is planned.
Frequently Asked Questions
How long does spheroidizing take? The cycle typically lasts from several hours to more than a day, depending on steel composition, section size and the hardness target.
Does spheroidizing change the chemical composition? No, only the distribution of the carbides changes; the overall composition of the steel remains the same.
Can low carbon steel be spheroidized? Low carbon steels are already soft and do not need spheroidizing; the treatment is used for medium and high carbon and alloy steels.
What hardness is achieved after spheroidizing? Hardness depends on composition but commonly falls in the range of 130 to 200 HBW for medium carbon steels.
Does spheroidizing improve machinability of all steels? For high carbon and alloy steels it generally does, but for some low alloy steels a normalizing treatment may give better machining behavior.
