Oxydizing – Neutral – Reducing Flames

The temperature at which it burns is a function of the amount of oxygen present in the gas mixture. The figure demonstrates three types of flames that can be produced with oxyacetylene mixtures. Welding is generally carried out using the neutral flame setting, which has equal quantities of oxygen and acetylene.

Reducing flame. The reducing flame is the flame with low oxygen and an excess of acetylene. Flame has a secondary feather extending from the inner cone. This secondary feather is caused by excess acetylene in the flame mixture, which alters the chemical composition of the weld pool by reducing iron oxide (reducing effect) and adding carbon (carburizing effect). It has a yellow or yellowish color due to carbon or hydrocarbons.
Neutral flame. The neutral flame is the flame in which the amount of oxygen is precisely enough for burning, and neither oxidation nor reduction occurs. The flame is considered neutral because it neither significantly adds nor subtracts any elements from the weld pool. A flame with a good balance of oxygen is clear blue.
Oxidizing flame. The oxidizing flame is the flame produced with an excessive amount of oxygen. When the amount of oxygen increases, the flame shortens, its color darkens, and it hisses and roars. Since, as its name suggests, it oxidizes the metal’s surface, this flame has a harmful effect on the properties of ferrous alloys. With some exceptions (e.g., platinum soldering in the jewelry), the oxidizing flame is usually undesirable for welding and soldering.

References:

Materials Science:

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
Eberhart, Mark (2003). Why Things Break: Understanding the World, by the Way, It Comes Apart. Harmony. ISBN 978-1-4000-4760-4.
Gaskell, David R. (1995). Introduction to the Thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
González-Viñas, W. & Mancini, H.L. (2004). An Introduction to Materials Science. Princeton University Press. ISBN 978-0-691-07097-1.
Ashby, Michael; Hugh Shercliff; David Cebon (2007). Materials: engineering, science, processing, and design (1st ed.). Butterworth-Heinemann. ISBN 978-0-7506-8391-3.
J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:
Welding