Gas Carburizing of Steels,Gas Carburizing of Steels,Gas Carburizing of Steels,Gas Carburizing of Steels
CARBURIZING is a case-hardening process in which carbon is dissolved in the surface layers of a low-carbon steel part
at a temperature sufficient to render the steel austenitic, followed by quenching and tempering to form a martensitic
microstructure. The resulting gradient in carbon content below the surface of the part causes a gradient in hardness,
producing a strong, wear-resistant surface layer on a material, usually low-carbon steel, which is readily fabricated into
parts. In gas carburizing, commercially the most important variant of carburizing, the source of carbon is a carbon-rich
furnace atmosphere produced either from gaseous hydrocarbons, for example, methane (CH4), propane (C3H3), and
butane (C4H10), or from vaporized hydro-carbon liquids.
Acknowledgements
Preparation of this article benefited from the suggestions and criticisms of Gary D. Keil, Caterpillar, Inc. and Gregory A.
Fuller and his colleagues at the Timken Company.
Carbon Sources
Low-carbon steel parts exposed to carbon-rich atmospheres derived from a wide variety of sources will carburize at
temperatures of 850 °C (1560 °F) and above. In the most primitive form of this process, the carbon source is so rich that
the solubility limit of carbon in austenite is reached at the surface of the steel and some carbides may form at the surface.
(In earlier editions of Metals Handbook, the carbon gradient produced by maintaining saturated austenite at the surface of
the steel is referred to as the normal carbon gradient.) Such atmospheres will also deposit soot on surfaces within the
furnace, including the parts. While this mode of carburizing is still practiced in parts of the world in which resources are
limited, the goal of current practice in modern manufacturing plants is to control the carbon content of furnace
atmospheres so that:
· The final carbon concentration at the surface of the parts is below the solubility limit in austenite
· Sooting of the furnace atmosphere is minimized
Controlled carburizing atmospheres are produced by blending a carrier gas with an enriching gas, which serves as the
source of carbon. The usual carrier, endothermic gas, is not merely a diluent, but plays a role, described below, in
accelerating the carburizing reaction at the surface of the parts. The amount of enriching gas required by the process
depends primarily on the carbon demand, that is, the rate at which carbon is absorbed by the work load.
Endothermic gas (Endogas) is a blend of carbon monoxide, hydrogen, and nitrogen (with smaller amounts of carbon
dioxide, water vapor, and methane) produced by reacting a hydrocarbon gas such as natural gas (primarily methane),
propane, or butane with air. Endogas is usually produced in a separately fired retort furnace (Endogas generator) using an
air-to-hydrocarbon feed ratio that will produce an oxygen-to-carbon atom ratio of about 1.05 in the Endogas. For Endogas
produced from pure methane, the air-to-methane ratio is about 2.5; for Endogas produced from pure propane, the air-topropane
ratio is about 7.5. These ratios will change depending on the composition of the hydrocarbon feed gases and the
water vapor content of the ambient air. Table 1 lists typical compositions of natural gas. Propane for atmosphere
generation should contain less than 5% propylene (CH3CH CH2) and less than 2.5% butane or heavier hydrocarbons,
satisfying the requirements in ASTM D 1835 for so-called special-duty propane or the Gas Producers Association
specification 2140, grade HD 5.

Additional comments :