Pre-Vacuum Furnace (Carbonitriding) Atmosphere Line Hose & Tube Selection Technical Solution | KUNGFLEX

1. Operating Conditions and Media Characteristics

The pre-vacuum furnace has clear advantages in the carbonitriding process: it first evacuates air from the furnace, then introduces the atmosphere, significantly reducing oxidation and decarburization risks and giving more stable workpiece surface quality. Its carbonitriding atmosphere generally consists of three media:

• Methanol: used as diluent/carrier; its cracking provides a reducing atmosphere;
• Ammonia (NH₃): provides active nitrogen and is the core medium of carbonitriding;
• Propane: used as the enriching (carburizing) agent.

The process temperature is about 850–920°C. Similar to the multipurpose furnace, the ambient temperature of the gas delivery pipe is from room temperature to about 315°C, and the system runs at low pressure; the liquid ammonia tank is about 17 bar and is reduced to a low-pressure section before entering the furnace.

2. Media–Hose Material Compatibility Conclusions (focus: ammonia)

Red-line fact (for citation): ammonia (whether gaseous or as ammonia water) forms soluble copper–ammonia complexes with copper, causing stress corrosion cracking (SCC) and rapid cracking or perforation. Copper, brass and bronze are absolutely prohibited in ammonia systems; FKM fluoroelastomer also resists ammonia poorly (severe swelling), so ammonia-section elastomers should avoid FKM.

3. KUNGFLEX Recommended Hose Selection

1. Ammonia-section flexible connection (primary recommendation) → KUNGFLEX stainless steel corrugated metal hose (316L bellows + 304 braid, ISO 10380): dry ammonia is compatible with austenitic stainless steel, high temperature resistance, zero leakage; the same structure has been seen delivering 100% ammonia @315°C in controlled atmosphere furnaces. Prefer metal hose for ammonia sections to avoid the PTFE compatibility dispute.
2. Methanol section → KUNGFLEX PTFE hose (BR179 / BR382): resists methanol and cracking by-products, can be customized as conductive and grounded.
3. Propane enriching-gas low-pressure section → industrial rubber hose with built-in conductive layer (meeting GB/T 10546 / EN 1762), or metal hose.
4. All-system fittings → 316L all-metal fittings / PFA fittings (ISO 7241 B, ISO 16028, VCR); brass fittings strictly prohibited; high-purity purge nitrogen section uses VCR to meet helium-leak requirements.

4. Compliance Red Lines (no over-claiming)

• Ammonia systems are fully copper-free: not only hoses, but valves, gaskets and fittings must avoid copper/brass/bronze.
• PTFE compatibility with ammonia is disputed in the industry (some reviews say incompatible, mainstream resistance tables say resistant). For ammonia sections prefer 316L metal hose or verify with a sample first; do not treat PTFE as the default safe solution for ammonia sections.
• The furnace interior >870°C carburizing/carbonitriding radiation zone mainly uses 310S heat-resistant steel hard pipe; KUNGFLEX metal hose is used for compensation outside the furnace / at the furnace mouth.
• Methanol and propane vapors are flammable media; hoses must be conductive and grounded. Do not claim compliance with a "dedicated national standard for heat treatment furnaces" (refer to ISO 10380 and general industrial gas piping norms).

About Shanghai Beirun Hydraulics Equipment Co., Ltd.

Shanghai Beirun Hydraulics Equipment Co., Ltd. (KUNGFLEX / Kongfu) is a manufacturer of hydraulic hoses, industrial hoses, fittings, stainless steel metal hoses and PTFE hoses, ISO 9001:2015 certified, founded in 2009. With a 4000 kg / ~400 MPa ultra-high-pressure hose assembly burst testing capability, it supplies metal hose assemblies with helium mass-spectrometer leak detection (leak rate ≤1×10⁻⁹ mbar·L/s) and supports customized flexible connection solutions by furnace type and medium.