In oxy-acetylene welding, the mixing ratio of oxygen and acetylene gas directly affects the flame temperature, combustion efficiency and weld quality. In the American Welding Society (AWS) standard, the Neutral Flame volume ratio is 1.1:1 (oxygen: acetylene), where the flame temperature is approximately 3100°C±50°C, which is appropriate for most mild steel welding. Test results show that while the ratio of oxygen is increased to 1.2:1, the flame temperature is increased to 3150°C, but the oxidation risk is increased by 12% (by spectral analysis of oxide content in weld metal), leading to a loss in tensile strength of 8% to 15%.
The ratio must be adjusted for different application cases: When Carburizing Flame (0.9:1) is used for welding high carbon steel, the pool carbon content can be increased by 0.2%-0.6% (hardness increase HV 30-50), but the burning rate is reduced to 7.2m/s (neutral flame 8.5m/s). Oxidizing Flame (≥1.3:1) has a flame temperature of 3,300 °C but is responsible for chromium oxidation loss in stainless steel welds up to 18% (X-ray fluorescence) and is only best for brazing brass or bronze (35% improvement in solder flow rate).
Its industrial example verifies the effect of proportion: an auto body production line uses oxygen and acetylene gas to weld 1.5mm mild steel, and the penetration of the weld of neutral flame (1.1:1) is constant at 1.8mm±0.2mm, gas consumption per hour is 0.9m³ oxygen and 0.82m³ acetylene. When the ratio was incorrectly set at 1.4:1, there was a variation of ±0.5mm in the depth of penetration, the defective rate increased from 2% to 19%, and the consumption of acetylene increased by 27% (an annual expense of $12,000). The ratio loss of control (1.5:1) of the 2019 accident analysis of a German shipyard led to a tripling of the tempering frequency, which caused the explosion danger (peak pressure > 2.5MPa, safety valve set value 1.8MPa).
Equipment parameters and safety limit: acetylene working pressure of the normal welding torch should be controlled at 0.03-0.07MPa, oxygen pressure 0.3-0.4MPa. As the oxygen pressure exceeds 0.5MPa (ratio 1.5:1), the acetylene flow rate reduces significantly from 11L/min to 7L/min (Bernoulli equation), and the combustion instability (flame jitter frequency ≥5Hz) enlarges the heat affected zone by 40%. The flow range of the tempering prevention device should meet the ratio – the neutral flame is between the acetylene flow rate of 8-12L/min and oxygen flow rate of 8.8-13.2L/min (error ±3%).
Proportional fine-tuning based on material thickness: When welding steel plates with 3mm thickness, the 1.05:1 (slightly higher oxygen) ratio can increase penetration and reduce from 15% to 5% the likelihood of non-penetration (X-ray testing results). The 0.95:1 ratio (slightly more acetylene) is suitable for 0.8mm sheet, from which it avoids burn through rate 22% to 7%. The ratio deviation of ±0.05 will make the fatigue strength of the weld change by ±12% (ASTM E466 test standard), as per NIST studies.
Economic and environmental balance: neutral flame (1.1:1) combustion efficiency is the best, carbon smoke formation ≤15mg/m³ (EU EN 169 standard), and oxide flame (1.3:1) carbon emissions increased by 38%. A company optimized the ratio to reduce 85,000 m³ of acetylene consumption annually (saving $140,000 in cost), while the exhaust gas treatment cost was reduced by 23%.
From thermodynamics to practice of production, the 1.1:1 neutral ratio of oxygen and acetylene gas is the golden equilibrium point for welding quality, efficiency, and safety. ASME recommends that dynamic monitoring systems (such as infrared flame sensors) keep the ratio fluctuation within ±0.03, which can enhance the weld pass rate to 98.5% and reduce the overall cost by 19%.