What Is The Phenomenon Of Carbonization On The Surface Of Steel Pipes?

The decarburization of steel pipe surface refers to the phenomenon where, during the high-temperature heat treatment process of the steel pipe, the carbon elements on its surface undergo chemical reactions with the medium in the heating environment (such as oxygen, water vapor, carbon dioxide, etc.) or diffuse into the surrounding environment, resulting in a significant reduction of the carbon content on the surface of the steel pipe.

Understanding decarbonization involves grasping the following key points:

• The essence of decarburization:

At high temperatures, carbon atoms react chemically with the atmosphere inside the furnace (such as oxygen, water vapor, carbon dioxide, hydrogen) to produce gases like carbon monoxide or methane, thereby being lost from the surface of the steel. This process is the result of diffusion. On one hand, atoms such as oxygen in the furnace gas diffuse into the steel; on the other hand, carbon atoms in the steel diffuse outwards.

• Composition of the decarburized layer:

The decarburized layer of steel typically consists of a full decarburized layer and a partial decarburized layer (also known as the transition layer). The full decarburized layer refers to the outermost surface layer where the carbon content has dropped to an extremely low level or even zero, and there is no pearlite in the metallographic structure; the partial decarburized layer is located within the full decarburized layer, with its carbon content being lower than the original value of the material but not completely removed, reaching the normal carbon content structure of the steel. In cases where decarburization is not severe, sometimes only the partial decarburized layer can be observed without the full decarburized layer.

• The competitive relationship between decarburization and oxidation:

Decarburization and oxidation often occur simultaneously, and there is a competitive relationship between them. The decarburization layer can only form and be observed when the decarburization rate exceeds the oxidation rate. If the oxidation rate is very fast, a layer of iron oxide will quickly form on the surface of the steel, and this layer of oxide may act like a "shield" to prevent further carbon loss. At this time, a clear decarburization layer may not be observable macroscopically, but the material is still being damaged due to oxidation.

Surface decarburization can have a significant negative impact on the performance of steel pipes:

• Decrease in mechanical properties:

A reduction in the carbon content at the surface directly leads to a significant decrease in the hardness, strength, wear resistance, and fatigue strength of the steel pipe surface.

• Causes quenching defects:

For steel pipes that require quenching, surface decarburization will prevent the formation of a high-hardness martensite structure after quenching, resulting in insufficient surface hardness, creating soft spots, and even potentially causing cracks due to uneven microstructure transformation.

• Service performance deterioration:

For components such as bearing steel, spring steel, and tool steel that have high requirements for surface performance, decarburization will significantly reduce their key service performance indicators such as wear resistance, contact fatigue resistance, and red hardness, leading to early failure of the components.

The factors influencing the decarburization of steel mainly include:

• Heating temperature and time:

The higher the heating temperature and the longer the time spent at high temperatures, the more severe the decarburization tendency. However, for some steel grades (such as 60Si2Mn spring steel), there may be a decarburization sensitive zone within a specific temperature range (such as 1100-1250℃), and at higher temperatures, the depth of the decarburization layer may actually decrease.

• Furnace atmosphere:

The oxidizing property of the furnace atmosphere is crucial. Water vapor, carbon dioxide, and oxygen all have strong decarburization capabilities. While carbon monoxide and methane, among others, have a certain carbon-enriching effect.

• Chemical composition of steel:

The higher the carbon content in steel, the greater the tendency for decarburization. The alloy elements present in steel also affect decarburization to varying degrees. Elements such as tungsten, aluminum, silicon, and cobalt can promote decarburization, while elements like chromium and manganese help to inhibit decarburization.

In actual production, the following measures are usually adopted to prevent and mitigate decarbonization:

• Optimize the heating process:

Minimize the heating temperature as much as possible and shorten the holding time at high temperatures, especially avoiding prolonged stays in the decarburization-sensitive temperature range of the steel type.

• Controlling the heating atmosphere:

This is the most crucial measure. Try to heat in a neutral or protective atmosphere (such as a controllable nitrogen-based atmosphere or inert gas), avoiding direct contact between the steel and deoxidation and decarburization gases. For certain situations, rapidly heating in a strongly oxidizing atmosphere to make the oxidation rate much higher than the decarburization rate, and using the generated oxide layer to protect the inner layer of carbon from significant loss, is also an optional process strategy.

• Implement physical protection measures:

For instance, apply protective coatings to the steel surface, or adopt vacuum heat treatment, etc.

• Reserve machining allowance:

During the design of the part, sufficient machining allowance should be reserved to ensure that the decarburized layer can be completely removed during subsequent mechanical processing.