In parts that are reheated for hardening and in heavy sectioned parts, however, both case and core hardenability requirements should be carefully evaluated.
The relationship between the thermal gradient and the carbon gradient during quenching of a carburized part can make a measurable difference in the case depth as measured by hardness. That is, an increase in base hardenability can produce a higher proportion of martensite for a given carbon level, yielding an increased measured case depth. Therefore, a shallower carbon profile and shorter carburizing time could be used to attain the desired result in properly chosen steel.
Carburizing Process and Carburizing Steels satyendra September 3, 0 Comments alloy steel , carbon steel , carburizing , case hardening , heat treatment , Carburizing Process and Carburizing Steels Carburizing is one of the most widely used surface hardening processes. Fig 1 Typical carburizing cycle including the quenching and tempering step Carburized steel consists of a composite material, where the carburized surface is hard but the unaffected core is softer and ductile.
Fig 2 Typical hardness, carbon content and residual stress gradients after carburizing, quenching and tempering Carburizing increases strength and wear resistance by diffusing carbon into the surface of the steel creating a case while retaining a substantially lesser hardness in the core. The carburizing process is characterized by the following key points. It is applied to low carbon steels Low carbon steel is in contact with a high-carbon gas, liquid or solid It produces a hard steel surface by increasing the carbon content of the surface The increase in surface hardness results in enhanced wear and fatigue resistance The steel cores largely retain their toughness and ductility It produces case hardened depths of up to 6 mm.
In some cases it serves as a remedy for undesired decarburization which has happened earlier in a manufacturing process. Carburizing processes While the fundamental carburizing principle has not changed much since its inception, the carbon introduction techniques have been improving.
Pack carburizing — In this process low carbon steel components are packed in an environment with high carbon content such as cast iron shavings or carbon powder. The components are heated with the production of carbon monoxide, which is a reducing agent. The reduction occurs on the steel surface with the release of carbon that is diffused into the surface because of high temperatures. With the carbon absorption inside the component, the steel components are hardened.
The surface carbon is in the range of 0. The case depth is approximately 0. Control of pack carburizing is difficult because it is difficult to maintain uniform temperatures. Pack carburizing is effective in introducing carbon but this method is exceedingly slow.
Gas carburizing — In gas carburizing, the parts are surrounded by a carbon bearing atmosphere that can be continuously replenished so that a high carbon potential can be maintained. While the rate of carburizing is substantially increased in the gaseous atmosphere, the method requires the use of a multi component atmosphere whose composition must be very closely controlled to avoid deleterious side effects, for example, surface and grain-boundary oxides. In addition, a separate piece of equipment is required to generate the atmosphere and control its composition.
The gas carburizing process is theoretically similar to pack carburizing process aside from the supply of carbon monoxide CO gas to the heated furnace and the carbon decomposition.
Many of the problems with pack carburizing are eliminated in this process. The CO gas needs to be contained safely. Despite this increased complexity, gas carburizing has become the most effective and widely used method for carburizing steel parts in large quantities.
The workpiece is held at a negative voltage of volts to 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.
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. 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. 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. 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.
Then, a gas such as hydrocarbon is pumped into the environment, allowing carbon molecules to attach to said alloys. Because the process is void of oxygen, it makes the oxidation of steel alloys a near impossibility. This allows for high heats to be injected into the contained atmosphere, greatly expediting the carburization process. This process is performed within a sealed furnace.
Unless the furnace can entirely seal off oxygen, it can not carry out the desired process. Liquid carburization is a form of carburization which takes place in a sort of liquid vat. This vat is filled with a mixture of substances, typically including cyanide and salt.
While metal alloy items are being submerged in this concoction, they come into contact with a collection of carbon molecules. Generally, these carbon molecules will diffuse into the alloyed items in a rapid manner, allowing for a hard case to form in just a short time. Steels which have been liquid carburized typically possess high levels of carbon and low levels of nitrogen. Pack carburization is a process which involves placing steel items into a furnace in close proximity to high-carbon items.
These high-carbon items include everything from carbon powder, to cast iron particles, and more. After you've inserted these items, they will be heated with the use of carbon monoxide.
This gas is a reducer of carbon, causing carbon to pull from the surface of the carbon-dense items which were placed in the furnace. After these carbon molecules are no longer attached, they will diffuse into the surfaces of the steel items which are to be carburized. This is the easiest of the carburization methods to pull off.
In fact, you might even be able to pull it off in a garage or home workshop. The problem with it is that it's unreliable and inconsistent. While it will allow for carbon diffusion, this diffusion typically won't occur uniformly across an entire steel item. The last carburization method we'll discuss is gas carburization.
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