RX® Endothermic Atmosphere Gas Generators for Heat Treating

Updated August 2026

Surface® Combustion RX® Endothermic Atmosphere Gas Generators provide onsite generation of endothermic gas for controlled atmosphere furnaces performing metallurgical heat treatments, such as carburizing, carbonitriding, ferritic nitrocarburizing (FNC), hardening, and annealing. Industries using RX gas include Automotive, Energy, Mining, Agricultural, and Commercial Heat Treating.

Surface Combustion pioneered the development of RX gas to be used as the atmosphere for modern gas carburizing. RX gas is produced by heating a mixture of natural gas[1] and air as it moves through catalyst and converts to a composition of 40% N2, 40% H2, and 20% CO. The RX gas generator’s reliable production of consistent atmosphere chemistry with low oxidation, decarburization, and soot potential revolutionized the heat-treating industry by enabling boost / diffuse carburizing, as well as atmosphere carbon control.

Our standard modular RX generator designs range from one (1) to five (5) tubes. Surface offers short and tall tube designs. Depending on the height of the tube, maximum flow rates of 2000 cfh or 3000 cfh can be produced in each tube. As furnace demand for RX gas changes, our generators can produce quality gas through a 6:1 automatic turndown range.

Our standard RX generators are 100% assembled and pre-tested at our manufacturing facility and are supported by a large inventory of critical and consumable spare parts like RX alloy retorts, catalyst, and refractory. Surface Combustion has the largest installed base of RX endothermic atmosphere gas generators in North America. Many have been in operation for decades, proving the ruggedness of the product.

Our customers find Surface Combustion RX generators so reliable, and the need to shutdown the generator so infrequent, that they consider them a large utility. To improve uptime, we offer on-line burnout. A tube can be taken offline and reconditioned by passing air through the retort while at temperature. A backup mixture pump allows for routine maintenance on the primary pump without impacting production.

Control features include our new continuous dew point sensor system which measures the dew point of the RX Gas continuously. The new system replaces the timed pump sample / burnout system to create a smoother, more reactive control. Our proprietary flow control system with our reliable air / gas mixer and positive displacement mixture pumps, allows consistent product gas chemistry and sufficient outlet pressure through the entire turndown range. Temperature controls maintain proper retort temperature for efficient RX production. ENDOQUNCH® air cooling locks the atmosphere chemistry and prevents carbon drop-out in piping headers.

We now offer our AP Diagnostics series of tools for predictive maintenance of RX generators. These expanding online diagnostic capabilities monitor each retort for equalized flow, pressure, temperatures, and utilization. When potential problems are identified, the system alarms to allow for maintenance attention before gas quality is affected.

Typical enriching gases added to RX gas at the furnace are natural gas, air, and ammonia. Furnace types include Allcase® Batch Integral Quench Furnace, Radiant Tube Box (RTB) furnaces, belt type furnaces, pusher tray, roller hearths, rotary hearts, rotary retort, and pit furnaces which are all products designed and built by Surface Combustion.

Surface supports our products with dedicated customer service, field service, and aftermarket parts personnel. From equipment and process troubleshooting to equipment rebuilds, Surface is ready to assist. Contact Surface for our expertise, and proven performance providing consistent and reliable protective atmospheres.

[1] Other hydrocarbon feedstock gases (e.g. propane, LPG, or acetone) can be used.  Slightly different gas compositions would result.