Process Description :
Common long products and hot rolled sheet are generally shipped as rolled. However, cold rolling of strip, surface coating, deep drawing of pipe and wire, stainless and high alloy steels, and large specialty forgings require heat treating steps. Heat treating changes the properties of steel being treated by affecting the size and alignment of the crystalline structure of the metal, the carbon, and other elements in the steel. The specific changes to crystalline structures and the temperature-time ranges required to achieve the desired physical properties are very complicated and are not discussed here. However, heat treating involves the following factors: heating, temperature control, atmosphere control, and controlled cooling. Heat treating may also involve maintaining a specific temperature for a period of time and reheating after an initial period of cooling. Heat treating furnaces are one of the major uses of natural gas in the steel industry. There are several types of heat treating commonly used.




Annealing
Annealing consists of heating the steel to or near the critical temperature (temperature at which crystalline phase change occurs) to make it suitable for fabrication. Annealing is performed to soften steel after cold rolling, before surface coating and rolling, after drawing wired rod or cold drawing seamless tube. Stainless steels and high alloy steels generally require annealing because these steels are more resistant to rolling.

Normalizing
Normalizing consists of heating the steel above the critical temperature and cooling in air. This treatment refines the grain size and improves the uniformity of microstructure and properties of hot rolled steel. Normalizing is used in some plate mills, in the production of large forgings such as railroad wheels and axles, some bar products.

Stress Relieving
Stress Relieving consists of heating the steel to a temperature below the critical range to relieve the stresses resulting from cold working, shearing, or gas cutting. It is not intended to alter the microstructure or mechanical properties significantly.

Spheroidize Annealing
Spheroidize annealing is a prolonged heating of the steel in a controlled atmosphere furnace at or near the lower critical point, followed by a very slow cooling. This process provides improvement in the performance of the steel in cold forming.

Accelerated Cooling
Accelerated cooling improves the resistance to impact and refines the grain size of certain grades of steel. The cooling is provided by fans or by a water spray or dip.

Quenching
Quenching consists of heating the steel above the critical point and holding at that temperature for enough time to change the crystalline structure. This heat is followed by quenching in a water or oil bath to bring the steel back through the critical temperature range without further changes to the microstructure. Quenching produces a very hard, very brittle steel.

Tempering
Tempering is carried out by preheating previously quenched or normalized steel to a temperature below the critical range, holding, and then cooling to obtain the desired mechanical properties. Tempering is used to reduce the brittleness of quenched steel. Many products that require hardness and resistance to breakage are quenched and tempered.

Properties of Steel
Strength - the ability to withstand mechanical stress
Ductility - Ability to be formed without rupture
Hardness - Resistance to deformation, abrasion, cutting, crushing, etc.
Toughness - ability to absorb shock without breaking
Fatigue Resistance - ability to undergo cyclic forces without failure


Equipments :
There are several common types of heat treating furnaces. There are batch furnaces and continuous furnaces.

Batch Furnaces
Car Bottom Furnace
Car bottom furnaces are used for heat treating bar stock in bar mills, plate that is too thick for the continuous lines (more than 2" thick), and annealing of tube stock for multiple cold pass drawing. Direct top and bottom firing gives a temperature range up to 1940°F. For hardening furnaces, heavy insulation is desirable because cooling is achieved by quenching, but for certain other cycles that require rapid cooling, common practice calls for furnaces that have rapid heat loss after firing ceases. Slower cooling is achieved by partial firing.

Box Annealing Furnace
Box annealing furnaces (also referred to as bell furnaces) have a rectangular or circular base for coil annealing of cold reduced strip. The base is cast iron or refractory lined. The removable box furnace contains the burners and exhaust stack. One cover may be designed for use with more than one base. Inner covers are placed over the stacked coils; a protective atmosphere is maintained within these inner covers. A sealing material such as sand, oil, or a low melting allow is used to seal the top to the base to keep air out. Annealing cold reduced strip requires the furnace to be under fire for 30 to 90 hours with the hottest part of the coil reaching 1275°F to 1350°F for 20-50 hours and the coldest part reaching 1225°F to 1275°F for 10 to 20 hours.

Vertical Pit Furnace
The manufacture of heavy press forgings such as steam-turbine rotors, generator shafts, and similar shaft type forgings often utilize vertical pit type heat-treating furnaces. The shaft is lowered through the top of the furnace and rests on special hearth casting and is stabilized by pins inserted through the side of the furnace. Tangential burners provide the heat without direct flame impingement on the work. A typical heating cycle involves a heating period, a soaking period, a second higher temperature soaking period, and then a controlled cooling period. A complete cycle can last from one to eight days.



Continuous Furnaces:

Roller Hearth
Roller hearth furnaces are used in plate mills, tube mills, and bar mills for hardening, normalizing, and tempering operations. Heat resistant steel rollers move the plates (100" wide by 30' long, 3/4 to 2" thick) through the furnace. Temperature control (Hardening 1250-1750°F, Tempering 750-1250°F, stainless, 1250-2100°F) depends on the process. After exiting the hardware furnaces, the plates are rapidly water quenched. Plates treated in this fashion are typically returned to a similar furnace for tempering. Roller hearths can also be operated on a semi-continuous basis with several heating and soaking zones.

Continuous Strip Annealing Line
Cold reduced strip can also be annealed on large continuous processing lines in which the continuous strip makes several passes through the heating towers, under protective atmosphere. The continuous annealing lines use a combination of direct fired heating and radiant tube heating zones. Very similar configuration is used for galvanizing lines. The continuous heat treating furnace on a galvanizing line serves three functions: reduction of the surface oxides by heating in a controlled atmosphere, heating of the steel to just above the temperature of the of the molten zinc bath, and controlling the cooling of the sheet to the zinc-pot temperature and below so the required coating thickness is achieved.

Induction Heaters
Induction heating is done by passing a high-frequency alternating current through a coil surrounding the material to be heated. The resulting electrical field causes rapid heating of the steel due to eddy currents and hysteresis. Induction heating is more commonly used for heating bar stock in the forging industry. In the Steel industry, induction heating is used for rail hardening and certain strip heating applications. Induction heating is used to apply heating and hardening for the head of the rails only.

Combustion Technology :
Radiant tube and direct-fired burners are both used for heat treating furnaces. Radiant tube burners have historically been used in batch annealing furnaces. Direct fired burner technology is being applied more due to advancements in firing techniques that produce better air/fuel ratio and temperature control, improved gas circulation techniques and new materials for metal inner covers. Direct-fired reducing burners are being developed for continuous heat treating lines as discussed in the R&D section.





Energy Consumption :
Typical batch annealing furnace energy consumption is on the order or 1.2 MMBtu/ton. Direct fired recuperative burners can achieve 0.85 MMBtu/ton. Energy consumption for a continuous strip annealing line ranges from 1-1.5 MMBtu/ton.