Carburization, or case hardening, is a fundamental thermochemical treatment that modifies the surface composition of steel by adding carbon. The main goal is to create a steel component with a hard, wear-resistant outer layer, called the case, while preserving a tough and ductile core. This structural combination is desired for parts subjected to impact and friction, such as gears, pins, and shafts. The concept has been practiced for millennia, dating back to ancient blacksmiths who used charcoal fires to improve the strength of their iron tools.
The process works by diffusing carbon atoms into the steel’s surface when heated in a carbon-rich atmosphere, increasing the carbon concentration of the outer layer. This allows the surface to be hardened by subsequent heat treatment, while the low-carbon core remains relatively soft. This method is a form of surface hardening, distinct from processes that alter the entire cross-section of the steel.
Selecting Material and Calculating Case Depth
Carburization is specifically applied to low-carbon or mild steels, which typically contain less than 0.30% carbon. Common examples include AISI 1018, 1026, and alloy steels like 8620. Their low carbon content prevents them from achieving sufficient surface hardness through simple quenching alone, making them ideal candidates. High-carbon steels are unsuitable because they already contain enough carbon to harden throughout, resulting in a brittle material without the desired tough core.
A primary consideration is determining the required “case depth,” which is the distance the carbon needs to penetrate to achieve a specific hardness. This depth dictates the total duration of the heat treatment, as carbon diffusion depends on time and temperature. A rough estimate for case depth can be calculated using a formula where the depth is proportional to the square root of the total time at the carburizing temperature. Target case depth for most applications ranges from 0.4 mm to 2.5 mm, depending on the part’s size and intended use. Since this process involves extreme temperatures, the steel container must be structurally sound to withstand the high heat of the furnace.
Detailed Steps for Pack Carburizing
Pack carburizing is one of the oldest and most accessible methods for adding carbon to steel, making it popular for small-scale applications. This method involves enclosing the low-carbon steel parts in a sealed container alongside a solid carburizing compound. The typical compound is a mixture of wood charcoal and a carbonate “energizer,” such as barium carbonate or sodium carbonate, which helps activate the process.
The steel parts are packed inside a steel box, ensuring they are fully surrounded by the carbonaceous mixture. The container must be sealed, often using fireclay, to contain the process gases that form inside. This sealed container is then heated in a furnace to the carburizing temperature, typically ranging from 900°C to 950°C (1650°F to 1740°F).
When the box is heated, the air inside reacts with the carbon source to generate carbon monoxide (CO). The carbonate activators promote the dissociation of this carbon monoxide at the steel’s surface, releasing atomic carbon. This carbon is then absorbed by the hot steel and begins to diffuse inward through the metal’s crystalline structure.
The duration of the soak time is directly proportional to the desired case depth, with longer times resulting in deeper penetration. Once the required time has elapsed, the entire container is removed from the furnace. The parts are often allowed to air cool before being removed from the compound, which prepares them for the final hardening treatment.
Ensuring Final Hardness: Quenching and Tempering
The carbon introduced during carburizing is not sufficient by itself to produce the final hard surface; the steel must undergo a rapid cooling process known as quenching. When the carburized part is quenched from high heat, the high-carbon surface layer transforms into martensite. This martensitic structure is exceptionally hard and wear-resistant, effectively locking the diffused carbon atoms into the iron lattice.
The choice of quenching medium, such as oil, water, or polymer solutions, depends on the specific steel alloy and the desired cooling rate. Oil is frequently used because it provides a cooling rate fast enough to form martensite without causing excessive distortion or cracking. Immediately following the quench, the steel’s surface is in a state of high stress and is extremely brittle.
To relieve this internal stress and restore ductility without sacrificing significant hardness, the part must be subjected to tempering. Tempering involves reheating the quenched steel to a lower temperature, typically between 150°C and 250°C, for a specified period. This process slightly reduces the hardness but improves the steel’s toughness and prevents premature failure.