Special steel semifinished products are heat treated using a variety of processes to improve cold-working properties. Traditionally, such heat treatment is done in a continuous furnace (which is economically advantageous at high production rates) and in a batch furnace, such as a bell- or car-type furnace, which produces a product of limited quality and is characterized by complicated operation and poor productivity.

A new type of furnace, called the STC furnace, developed by Daido Steel Ltd. (Japan) and engineered and supplied by Techint Technologies, is capable of annealing, normalizing, carbon restoration and other types of heat treatment of wire and rod coils, bars, pipes, and cold-forging parts at higher productivity rates and higher quality than those obtained using traditional furnaces.

Fig 1 STC roller hearth batch furnace for spheroidize annealing of steel wire and rod coils

Furnace specifics

The STC furnace is a 20 t (44,000 lb) capacity batch roller-hearth type (figure 1) used to spheroidize anneal steel rod and wire coil having dimensions ranging from 1,300 to 1,700 mm (51 to 67 in.) outside diameter by 1,100 to 1,200 mm (43 to 47 in.) high. Furnace operating temperature range is 700 to 760C (1290 to 1400F), with a maximum operating temperature of 800C (1470F). The heating system is a natural gas-fired recuperative radiant tube type. The purge atmosphere is nitrogen gas supplied from a storage tank, and the process atmosphere is endothermic gas supplied by an external gas generator.

The equipment line consists of a charge/discharge table and the furnace, or heating chamber. Each component of the line is sized to accommodate five loaded trays and is equipped with mechanically driven rolls to move the loaded trays in and out of the furnace from the charge/discharge table. The table also functions as a transfer car, allowing material to cool down outside of the furnace while new product is charged into the furnace. The entire operation, except for the manual load and unload requirements, is computer controlled and performed automatically; no operator attention is required. Three separate control zones in the heating chamber use radiant tube recuperative burners located above the load and below the roller hearth. The recuperator is installed at the exhaust side of the radiant tubes to preheat the combustion air to 420C (790F).

Five recirculating fans placed at an equal distance on the chamber roof recirculate the atmosphere and improve coil-temperature uniformity. Lightweight ceramic fiber insulation material reduces heat loss in the furnace and reduces the time to raise or lower the furnace temperature. A locking mechanism on the charge/discharge door minimizes furnace-atmosphere leakage and ensures a gas-tight design. All operating parameters of the computer-controlled system are displayed on a PC monitor.

Fig 2 Equilibrium constant, K, versus temperature for different steel carbon contents

Furnace gas atmosphere

A protective atmosphere mainly is used in steel heat treatment to control oxidation, decarburization and carburization of the product. The STC furnace atmosphere consists of nitrogen and endothermic gas (20% CO, 40% H2, 0.05% CO2, traces of CH4, and balance N2).

The extent of steel carburization and decarburization depends on furnace temperature and atmosphere gas composition. Experiments show that iron and ferrous oxide heated in a closed vessel at constant temperature in contact with any mixture of carbon monoxide and carbon dioxide changes the composition of the gas. The change continues until a certain definite composition (known as the equilibrium composition) characteristic of that temperature is attained.

Knowing the equilibrium composition of the gas as a function of temperature allows you to predict the way in which any mixture containing carbon monoxide and carbon dioxide reacts with iron, iron oxide and iron carbide at any temperature. The reaction of great concern in the heat treatment of steel is:

Fe3C + CO2 = 3Fe + 2CO (Eq 1)

The equilibrium constant for this reaction can be determined based on the free energy formation of iron carbide and the gas partial pressure:

K = [(PCO)2/PCO2] c 1/ac (Eq 2)

where ac = Ac/As, Ac = quantity of dissolved carbon in austenite (%), and As = quantity of saturated carbon in austenite (%). The carbon potential factor (PF) is given by:

PF = (%CO)2/(%CO2)
= 100 K c Ac/As (Eq 3)

log K = [-15966/1.8 c t (C) + 492] + 9.06 (Eq 4)

Thus, the atmosphere can be kept in a noncarburizing or nondecarburizing condition by controlling the PF value. Figure 2 shows the relationship between the equilibrium constant and temperature for different carbon contents steel. At a given temperature, any gas mixture for which the ratio of P2co/PCO2 is greater than that indicated for a particular temperature and carbon content tends to carburize steel; if the ratio is less, it tends to decarburize.

Fig 3 Typical profile for temperature and carbon potential factor, PF

Typical temperature and PF recipe

Figure 3 shows a typical recipe consisting of six steps for spheroidize annealing in a batch type roller hearth furnace. After manually loading the trays on the charging table, the operator selects the designed recipe and Level 2 commands to begin the automatic furnace heating and nitrogen gas purging cycle.

Fig 4 Spheroidized microstructure for AISI Type 52100 steel consisting essentially of spheroidized cementite particles in a ferrite matrix

The advantages and benefits of spheroidize annealing using the STC roller-hearth batch furnace include:

  • Up to 100% spheroidization (figure 4)

    Fig 5 Uniform product temperature is maintained during the heat treatment soak time
  • Uniform product temperature (I 3C, or I 5F) at the end of the soak (figure 5)
  • Uniform furnace temperature (I 1C, or I 3F) during soaking
  • Zero decarburization
  • Less than 2% product variability per load
  • A 20 to 30% reduction in time cycle compared with traditional furnaces
  • Low labor cost (furnace-operator requirement of 0.5 to 1 man-hour/day); no operator during night shift
  • No material has been reprocessed during an entire year of operation

Furnace performance considerations

STC batch furnace performance is best evaluated by comparing it with a bell type furnace. Which type of furnace is the best varies considerably and depends on the user's product mix. For example, the authors' experience shows a bell type furnace has the best economy of operation based on monthly heat-treating requirements of less than 500 ton/month, while an STC annealing furnace is best for 300 to 1,800 ton/month. Table 1 compares an STC with endothermic atmosphere and a bell type furnace with 100% hydrogen (H2) gas atmosphere based on the utility cost.

The main advantage of the STC furnace is higher product quality with reduced production cost in a shorter time cycle. The furnace is extremely flexible and can be configured to accommodate production and layout requirements. The many different options for equipment layout allow excellent flexibility for installation in either a new or existing building. After preassembling the furnace in the workshop, it is installed on the ground floor without the need of any extensive foundation work, which achieves easier, faster and less-expensive installation.

Compared with a bell type furnace, no tall building is required to lift equipment over other equipment, and less floor area is used inside the building because no cooling hoods, inner cover or multiple bases are required. The material handling system requires a small overhead crane (4 ton capacity) or a forklift for moving the products.

Maintenance is reduced by limiting the use of water-cooled components, and it also is facilitated by means of a design that provides easy accessibility to components, such as locating the burners on a sidewall and the roll mechanism on the opposite wall, for example. Furthermore, working site conditions are improved by installing a collective waste-gas manifold to discharge the products of combustion outside the building.