Gas-fired pit furnaces with built-in retort available in single or two-stage nitriding systems. Cyclone gas-fired pit furnaces are capable of temperatures up to 1400°F. The top loading furnaces have a separate heating/fan chamber that provides heat to the work chamber. The workload is heated by a unique re-circulating convection heating system that forces pressurized air through the work chamber at a high velocity. A “suspended” fan shroud mates with either the basket or baffle to provide a closed path of circulation when the cover is closed. Atmosphere leaves the fan and travels downward through annular clearance between the retort and basket or shroud to below the load support, then upward through the work before again entering the fan inlet.