Modern high speed machining of bulk-produced auto parts, typically gears for two-wheel vehicles, calls for a narrow (and higher) range of normalized hardness on hot-, warm- and cold-forged blanks. This requirement places new demands on the design of normalizing furnaces.
For example, the requirement for a lug gear (Fig. 1) made of SCM 420H (SAE 4120) is to be normalized in a hardness range of 180 to 220 BHN. For high-speed hobbing, the ideal microstructure would consist of decarburization-free fine pearlite + ferrite with a minimum hardness of 180 BHN to reduce burr formation, eliminate scoring and improve tool life.
Conventional normalizing (in this case, meaning heating up in a controlled atmosphere batch furnace to eliminate decarb and cooling in an attached water-jacketed chamber with atmosphere gas recirculation) yields a hardness spread of 145 to 200 BHN. Obtaining a hardness band between 180 and 220 BHN becomes difficult to achieve when normalizing parts in baskets due to the varying cooling rates of components in different parts of the basket.
Conditions that facilitate achieving the desired hardness range are:
- High, controlled heat transfer rates during the cooling phase to achieve (but not exceed) a hardness up to 220 BHN
- Uniform cooling rates of all components being processed
- Uniform heating rates of all components, not only to achieve homogenous austenite, but also so all components are at a similar temperature when cooling starts
These conditions are best achieved when components are heated and cooled while spread out in one to two layers as they would be in a continuous wire mesh-belt furnace (Fig.2) of the type used for sintering or brazing, where a jacketed cooling muffle follows the heating section. A variable rate cooling (VRC) module (Fig.3) that recirculates atmosphere gas through a heat exchanger prior to impingement on the belt is sandwiched between the two.
One drawback of such furnaces (apart from periodic belt replacement) is the high gas consumption, which is inevitable when both ends of a furnace are open. When components are to be coated with a rust preventive liquid after normalizing, the addition of a sealed quench tank can reduce atmosphere gas consumption by about 40%.
Design elements of an effective mesh belt normalizing furnace include (with respect to capacity) belt loading, belt speed and sizing of different furnace sections. Of primary importance is the design of the VRC module, which requires CFD modeling (Fig.4) to optimize power, flow velocity, flow uniformity, heat exchange and the control required when adjusting flow rates to suit different components.
The component parts of the VRC module include one or more variable-speed blower(s), heat exchanger(s), blast chamber(s) and duct work. The cooling rate required to obtain the desired hardness band in the reference component is illustrated in Fig.5.
The output quality of the plant illustrated in Figs. 2 and 3 when processing the reference lug gear with 90% blower speed is shown in Fig. 6, and the resultant microstructure is shown in Fig. 7. The microstructure and hardness obtained at 20% blower speed is shown in Fig. 8. A mean hardness of 200 BHN comes along with around 15% bainite. Practice shows that this level of bainite does not affect machinability of SCM 420H (SAE 4120H) components. An upper limit of 220 BHN is specified to avoid excessive bainite and consequent machinability problems.
A recently developed furnace design for effective "enhanced" normalizing is a shallow tray close-pitched roller hearth furnace. The drivers for this development were:
- Reducing gas consumption, made possible by the fully closing furnace doors
- Reducing down time, as the trays (that replace the belt) are repaired or changed without need to stop production
- Improving control over cooling, as the time taken for component travel is reduced compared with that of mesh belt furnaces and for the same reason
- Flexibility in furnace sizing for a given capacity.