Feb 5 2009
SECO WARWICK Allied has recently commissioned a Spheroidize Annealing Furnace at Mukand Ltd. in Kalwa, India. The charge material consists of solid round bars, ranging from 25 – 160 mm (1 – 6.3 inches) in diameter. Lengths of the bars may vary from 6 – 6.5 m (20 to 21 feet).
Computer Model of Spheroidize Annealing System at Mukand Ltd. in Kalwa, India.
The Process
Spheroidize annealing is an annealing process used for high carbon steels (carbon > 0.6%) that will be machined or cold-formed. The purpose is to soften the high carbon steels and allow more formability, thus making the softest and most ductile form of steel.
This involves partial austenitizing of the charge material. Spheroidite is formed when carbon steel is heated to around 700 °C (1292°F) for over 30 hours.
This is then followed by rapid cooling, and subsequently, slow-cooling. The process parameters are critical due to the long cycle time (60 hours and above). The charge batch weight is 30 MT (66,139 pounds), and approx. 7 MT (15,432 pounds) of charge spacers are employed for proper spacing.
The Design
This furnace is unique in concept. It is actually a blend of a car furnace and a bell (lifting) furnace. The goal of this design is the need to maintain the furnace chamber pressure at 40 mm WG, for best results.
Circulation inside the furnace is achieved through 4 recirculation fans provided on top of the furnace. The furnace is electric-heated, divided into 4 zones controlled through thyristors. The process calls for limiting the maximum heating rate to 110 °C/hour, and controlled cooling with a cooling rate between 5 - 25 °C/hour.
This process is delicately-balanced through the interplay between the heaters and circulation of the protective gases through a sealed heat exchanger.
The system performance exceeds the original requirements with spheroidization of over 90% for SAE 52100 grade and over 80% for other grades. Temperature uniformity, oxygen percentage and decarb are all well within control.
Work is currently going on for the commissioning of the second furnace at the same plant.