Due to their low carbon content, low carbon steels are neither too ductile nor too brittle. They have low tensile strength and hardness. Because of their low hardness, wear and tear is high during sliding and rolling contact with harder materials. Increasing the carbon content of the steel increases the strength as well as the hardness. It also becomes less ductile (a little bit more brittle with low fatigue strength), and weldability and machinability suffers. One way to address these issues is the carburization process.
What is Carburization?
Carburization is the process of hardening the exterior of ferrous workpieces (component parts) by Facilitating diffusion of carbon atoms into the surface up to a certain specified depth. It is one of the widely used methods harden the outer layer of metallic parts and components.
The surface to be processed with this type of heat treatment must first be thoroughly cleaned to remove contaminants before the treatment can begin. During carburization, the metal workpiece to be case hardened is heated in an environment of carbon-rich gases, liquids or solids. During heating, the carbon sources,(agents of the carburization process) decompose, liberating carbon atoms, which are diffused into the ferrous metal's surface. Low carbon steels and some alloy steels are normally heat treated with the carburization process. The rate of carbon diffusion depends upon the temperature and the carbon potential of the metal being heat treated. The depth of the carburized case depends upon the carbon diffusion temperature, the carbon potential of the metallic surface and the duration of carburization.
Metals consist of atoms and molecules tightly bound in a crystalline lattice structure. (Learn more in The Crystalline Structure of Metals.) Free carbon atoms from carbon-bearing materials (solid, liquid, plasma or gases) diffuse within the crystalline structure, thus increasing the carbon content and consequently the hardness of the workpiece's surface up to a certain depth.
Purpose of Carburization
The case carburization process is used to improve the following attributes of alloy steel and low carbon steel workpieces:
- Mechanical strength
- Wear resistance
- Corrosion resistance
- Fatigue strength
- Surface hardness
- Relieve internal stresses
During carburization, the temperature of the workpiece is maintained between 850°C to 950°C (1,560°F to 1,740°F), which is above steel material's critical temperature, for the intended duration based upon the intended depth of the hardened casing. After the carburization process is complete, the work piece is quenched, causing the carbon atoms to remain locked inside the metallic structure. (Quenching is discussed in the article How Quenching Improves the Performance of Metals.)
Due to these processes, the carbon percentage can go up to 1.2% on the carburized surface. This surface can be further subjected to induction hardening on selected portions of workpieces, such as the teeth of automotive and industrial gears.
Because carburization is performed at temperatures above the metal's critical temperature, during subsequent fast cooling (quenching) the outer surface becomes locked into a martensitic structure (high carbon), while the low carbon core retains a softer pearlite-ferritic structure.
The service requirements of many component parts such as piston pins, cams, gearboxes, roller bearings and pinions require an abrasion-resistant hard outer case alongside a shock-resistant and tough inner core. Such a complex requirement can be met by case hardened low carbon steel and some alloy steels. Carburizing is the most popular method employed for case hardening of heavy duty component parts made up of low carbon steels.
Classification of Carburization Processes
The carburization processes are classified into the following types, as discussed below:
- Liquid carburization
- Vacuum carburization
- Plasma carburization
- Gas carburization
- Pack (Solid) carburization
In the liquid carburization process, the metallic workpieces are immersed in a liquid bath of silicon carbide, sodium chloride and sodium carbonate. The temperature is maintained around 900°C (1,650°F). The workpieces are quenched after the carburization process. The surfaces are more uniformly hardened while soot build up and oxide generation is avoided in this process.
During vacuum carburization, the workpiece is heated to around 1,000°C (1,830°F) in a sealed chamber that is maintained at a very low pressure. After the heating cycle is complete, a carbon-rich gas (propane, ethylene or acetylene) is introduced into the chamber. The carbon produced by the decomposition of the carbon-rich gas becomes diffused into the surface of the workpiece in a uniform manner. The risk of oxidation on the workpiece surface is minimized. The time required for the carburization is lower for vacuum carburization than for most other types.
In a plasma carburization system, the workpiece is processed in a vacuum furnace operating around 900°C to 950°C (1,650°F to 1,740°F), with an attached oil quenching tank integral to it. The workpiece is held at a negative voltage of -800 volts to -400 volts. Propane or methane gas is diluted with argon, nitrogen and oxygen and introduced into the chamber at a controlled flow rate.
Plasma surrounds the workpiece due to presence of the negative voltage and the gas. Any carbon present in the glow discharge in the free state is diffused into the workpiece's surface. The required heat treated depth (normally below 2 mm) is achieved in this method after quenching in the oil bath.
Because this system is capital intensive, it is used only for carburizing critical high-value components. It is mainly used to improve the metal's surface parameters such as load capacity, case hardness and corrosion resistance. Workpieces with complex geometric shapes can be uniformly heat treated using this system.
In gas carburization, natural gas (a gaseous mixture of propane, ethane and methane) or carbon monoxide is used as the agent of the carburization process. Initially, parts to be carburized are heated in a furnace at 950°C (1,740°F) and then the gas is introduced into it. Carburization is achieved in around 5 to 10 hours using this process. Parts with complex geometry can be heat treated in this process and accurate surface hardness can be achieved.
In the case of solid carburization, the workpieces to be carburized are bundled with solids such as barium carbonate or bone charcoal, which are capable of releasing carbon when heated. When the workpiece is maintained at around 900°C (1,650°F) in a sealed container with the solid carburization agent, carbon is diffused into the surface of the workpiece and hardening of the case is accomplished.
In general, workpieces made of steel with low carbon content are case carburized using this method. This process, however, entails excessively high labor costs and processing time, and the accurate control of process parameters such as case depth and carbon gradient along the depth cannot be ensured.
Benefits of Carburization
Carburization in general provides the following advantages to the component parts:
- Higher mechanical strength
- Retained toughness and fatigue strength
- Enhanced wear resistance and durability
- Higher corrosion resistance
- Improved ductility
- Improved reliable surface hardness
The carburization process primarily produces a hard case (cover) over the softer ductile inner core, which can then be subjected to greater impact loads without damage. The corrosion resistance, wear resistance and fatigue strength are all improved.
The depth of the hardened case can be controlled depending on the durability required for the component. The depth can be shallow for parts that are frequently replaced, while higher case depths are recommended for parts that are subject to shock loads or crushing loads. Heavily loaded mill gears and bearings belong in this category.
Superior mechanical properties can be achieved by maintaining high temperature uniformly, which ensures a high carbon diffusion rate, and consequently a more cost effective process is ensured.
When compared to carbonitriding, carburizing has a thicker hardened layer that enhances the durability of the component parts. Compared to nitrocarburizing, the carburizing produces nonporous surface, which is needed for high contact stress applications.
Limitations of Carburization
Workpieces consisting of a large variety of geometrical shapes and sizes can be carburized for different applications. If the sections contain nonuniform material composition or if the sections are asymmetrical, the cooling rate differential can sometimes cause stress buildup and subsequent cracking.
Carburization necessarily results in some changes in dimensions, based on the process temperatures and agent of carbon diffusion used. Often, these changes in dimensions, shapes and distortions are small. However, these deviations and angle points can result in expensive subsequent machining costs. Changes in volume and grain growth could occur, which could be a concern in some applications. Case formation rate is around 0.02 mm to 0.035 mm per hour and dimensional deviation is expected to be around +0.2%.
Research studies indicate that use of plasma carburization on low alloy steel has a negative impact on durability, in the case of low cycle fatigue condition, unlike the case of high cycle fatigue where the durability and fatigue life is significantly extended.
Another study has confirmed the correlation between distortion versus hardness as well as case hardening depth. (The deviation factors, such as out-of-roundness, teething deviations in transverse and longitudinal direction, and out of flatness, were studied.)
Comparing Carburization with Nitriding
Both carburization and nitriding are used to increase the hardness of outer surfaces (cases) of steel components. Nitriding, however, uses lower subcritical temperatures of around 450°C to 570°C (840°F to 1,060°F). As no distortion is expected due to nitriding, fully finished parts can be nitrided with no further corrective machining requirement. (Related reading: Nitriding for Corrosion and Wear Fatigue Resistance.)
The microstructure of the core won’t be altered during this low temperature processing. While carburization is used mainly for low carbon steels and low alloy steels, nitriding is used for low carbon steels, alloy steels, tool steels and stainless steels. Nitrided steels can have superior wear resistance and hardness.
Nitrocarburizing, which involves diffusion of nitrogen and carbon, forms a wear-resistant hardened case with increased porosity at the surface, which is detrimental in applications involving intense contact stresses.
Process Optimization and Other Recent Developments in Carburization
To achieve high quality output with minimum distortion, the heating controls should ensure uniform temperature in the furnace and uniform carbon diffusion (through gas flow control in the case of vacuum and gas type carburization). When quenching, uniform heat removal must be ensured by optimizing the quenching speed. Studies report that modern technology solutions are available for high performance process control.
Research studies recount the latest developments in which diffusion rates of carbon are increased by enhancing the temperature of carburization from the limit of 950°C to 1,030°C (1,740°F to 1,890°F), thus reducing heat treatment cycle time around 25% to 40%, consequently also enhancing the economic advantage.
New steel alloying systems have been established for medium and high hardenability to meet the ever increasing challenges (such as high instant torque applied by turbocharged internal combustion engines and truck gear- boxes being subjected to prolonged periods of higher temperatures along with higher loads) posed by the automotive industry and others. For example, for very heavily loaded vehicle components and machinery component parts requiring high hardenability by carburization, steels with nickel, molybdenum and chromium alloys are now being recommended.
Applications for Carburization
For high fatigue strength and durable wear resistance, carburization is primarily used for:
- Automotive transmission gears, shafts, industrial gears, windmill gears
- Structures and drive trains
- Axles, machine parts (working at no more than 200°C (392°F)
- EOT crane wheels, shackles, crane rope drums, cable drums, flywheels, rolling bearings
- Railway equipment parts such as railway wheels, rolling bearings and gear boxes
- Cutting tools and blades
Durable wear-resistant parts can be manufactured from low-cost low carbon steels and low alloy steels by creating an outer casing of superior wear-resistance and adequate hardness.
To ensure quality output from carburization, the process controls must aim to ensure a uniform temperature and efficient carbon flow to the metal's surface (gas flow in the case of vacuum and gas carburization) as well as a uniform rate of heat removal during quenching.
The latest developments in the field of carburization include accelerated diffusion rates of carbon by enhancing the temperature of carburization from the limit of 950°C to 1,030°C (1,740°F to 1,890°F), thus reducing heat treatment cycle time. New steel alloying systems have been established for medium and high hardenability through carburization to meet the ever-increasing challenges faced by various industries.