Controlled atmosphere heat treatment refers to controlling the gas composition in the furnace within a set range so that the steel parts neither oxidize nor decarburize during heating, or transfer carbon and nitrogen to the surface as required. It consists of four parts: atmosphere source (generator, nitrogen, methanol, etc.), furnace seal, sensor (oxygen probe, analyzer) and control system. Compared with air heating, it can reduce oxidation loss and processing margin, and make the results repeatable.
- Controlled atmosphere = atmosphere source + sealed furnace + sensor + control system
- The goal is to get the "desired balance" between the furnace gases and the steel surface
- Main benefits: less oxidation, reduced machining allowance, stable surface quality, traceability
- The quality requirements of the automotive industry promote the upgrade of ordinary mesh belt furnaces to controlled atmosphere mesh belt furnaces
Definition: Not just "protect" but "control"
When ordinary box furnaces or mesh belt furnaces are heated in the air, the surface of the steel parts will react with oxygen, water vapor, and carbon dioxide, forming oxide scale and losing surface carbon (decarburization). The early method was to pass in inert gases such as nitrogen for Protect;and controlled atmosphere Going one step further: Actively adjust the proportions of CO, CO₂, H₂, H₂O, CH₄, NH₃ and other components in the furnace so that the effect of the atmosphere on steel (neutral, carburizing or nitriding) can be set and measured.
The system consists of four parts
- Source of atmosphere: Endothermic gas generator (RX/Endo gas), nitrogen + methanol, pure methanol cracking, dissociated ammonia, nitrogen-hydrogen mixed gas, etc.;
- Sealed furnace body: The seals of the furnace door, furnace curtain, and fan shaft determine the amount of air infiltration, and leakage will directly destroy the atmosphere balance;
- Sensors and Analytics: The oxygen probe measures oxygen partial pressure, the infrared analyzer measures CO/CO₂/CH₄, the dew point meter measures water content, and the hydrogen probe measures H₂;
- control system: Calculate the carbon potential or nitriding potential based on the measured value, and achieve closed-loop control by adjusting the flow of enriched gas, diluted gas or ammonia gas, and record the data at the same time.
Advantages compared to ordinary heating
- Less oxidation: Reduce oxide scale and material loss, which can reduce subsequent grinding allowance;
- Control surface carbon content: Avoid insufficient surface hardness and reduced fatigue life caused by decarburization;
- Realize chemical heat treatment: Carburizing, carbonitriding, and nitriding all rely on controlled atmosphere;
- Consistency and traceability: Continuous recording meets the special process requirements of systems such as CQI-9.
Domestic upgrade trends
In the automotive fasteners, bearings, gears and other industries, quality requirements and cost control have become stricter at the same time, and ordinary quenched and tempered mesh belt furnaces are gradually being replaced by controlled atmosphere mesh belt furnaces. This trend is also described in the CQI-9 Carbon Potential Management article published by Gauss Control: Less oxidation-free heating can simultaneously improve quality, reduce material loss and machining allowance.
The key to upgrading is not "what Qi is introduced", but whether it can Prepare the atmosphere stably, measure the atmosphere accurately, control and record the atmosphere reliably。
Frequently asked questions
What is the difference between protective atmosphere and controlled atmosphere?
The protective atmosphere only requires non-oxidation (such as pure nitrogen or nitrogen and hydrogen), and usually does not actively control the carbon activity; the controllable atmosphere measures and closes the loop of the carbon potential or nitriding potential, and can achieve neutral heating, recarburization or carburization.
Can the existing ordinary mesh belt furnace be transformed into a controlled atmosphere?
In most cases, it is possible. The key is the furnace seal, air intake method, atmosphere source and control system. Furnace leakage, gas consumption corresponding to output, and safety interlocks should be evaluated before modification.
Is it necessary to use a generator for controlled atmosphere heat treatment?
uncertain. Nitrogen-methanol or direct infusion of methanol can also be used. However, the composition of the RX gas produced by the endothermic gas generator is more stable, which is conducive to precise carbon potential control and can reduce the storage and management of hazardous chemicals such as methanol.