The Role of Carbon Equivalent in the Performance of Steel Pipes

Carbon Equivalent (CE) plays a core role as an assessment indicator in the performance of steel pipes, mainly used to predict and evaluate the weldability, hardenability, and related mechanical properties (especially the balance between strength and toughness) of steel pipes.

Its core functions are reflected in the following aspects:

Weldability assessment (the most important function):

Predicting cold cracking tendency: Carbon equivalent is the most critical indicator for assessing the risk of hydrogen-induced cold cracking in steel pipe welding. Carbon and alloy elements (such as Mn, Cr, Mo, V, Ni, Cu, etc.) increase the hardening tendency of steel. During the welding thermal cycle, the rapid cooling of the heat-affected zone (HAZ) forms hard and brittle martensitic structure. Hydrogen atoms (from welding materials, environment or contaminants) tend to accumulate in this area and induce cracks under the action of welding stress.

Guide the formulation of welding procedures: Based on the calculated carbon equivalent value:

Determine whether preheating is required and the preheating temperature: The higher the CE value, the greater the risk of cold cracking, and the higher the preheating temperature usually required. For example, when CE (IIW) > 0.4 – 0.45%, preheating is usually considered necessary; when it exceeds 0.6%, strict preheating and strict welding process control are required.

Select appropriate welding materials and methods: For high CE steel pipes, low hydrogen electrodes, fluxes or shielding gases should be used, and in some cases, special welding methods may be required.

Determine interpass temperature control requirements: Prevent excessive heat input from causing rapid cooling and increasing quenching.

Evaluate the need for post-weld heat treatment: High CE value steel pipes often require hydrogen removal treatment or stress relief annealing after welding to reduce the risk of cold cracking.

Impact on the toughness of the heat-affected zone: A high CE value usually means that the heat-affected zone is more prone to form coarse-grained structures and brittle phases, leading to a decrease in toughness in this area.

Assessment of hardenability:

Carbon equivalent (especially certain formulas such as CET) can well reflect the hardenability of steel. Hardenability refers to the ability of steel to obtain a martensitic structure during quenching.

Impact on heat treatment effects: For steel pipes that require quenching and tempering treatment (such as high-strength seamless pipes), a higher CE value means:

Easier to achieve high strength and hardness: Good hardenability allows martensite to be obtained in a larger depth even with a larger cross-section or slightly slower cooling rate.

More uniform cross-sectional properties: Thick-walled steel pipes require higher hardenability to ensure that the core also achieves the required properties.

Increased sensitivity to quenching cracking: More strict control of quenching cooling processes (such as cooling medium, stirring speed) is required.

Impact on as-rolled or normalized microstructure: Even in non-quenched and tempered conditions, a higher CE value increases the tendency to form bainite or martensite during cooling after rolling or normalizing, affecting the final microstructure and properties.

Balance between strength and toughness:

Carbon and alloy elements are the main means to increase the strength of steel pipes. Therefore, to achieve higher strength grades (such as X80, X100 pipeline steel), it is usually necessary to increase the alloy content, resulting in a higher CE value.

Key contradiction: A high CE value, while enhancing strength, often compromises toughness (especially low-temperature toughness) and weldability.

Objective of modern metallurgical technology: Modern high-performance steel pipes (such as high-grade pipeline steel) achieve extremely high strength by using low-carbon microalloying design and advanced TMCP processes, maintaining a relatively low CE value (to ensure good weldability and toughness). This is achieved through mechanisms such as grain refinement, precipitation strengthening, and dislocation strengthening. This makes carbon equivalent a key constraint in designing the composition of steel pipes to balance strength, toughness, and weldability.

Prediction of the properties of the heat-affected zone:

As mentioned earlier, a high CE value indicates that the heat-affected zone of the weld is more prone to hardening and a reduction in toughness, which directly affects the integrity and resistance to brittle fracture of the overall structure of the steel pipe.

Steel pipe manufacturing process design:

When a manufacturing plant formulates the production process of steel pipes (especially welding processes such as ERW, SAW, and LSAW) and heat treatment processes, it must take carbon equivalent as one of the core input parameters. It directly affects production efficiency and product quality control strategies.

Commonly used carbon equivalent calculation formulas:

International Institute of Welding (IIW) CE(IIW) / CEN: CE(IIW) = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15 (Most commonly used, focuses on cold cracking tendency in welding)

Japanese Industrial Standard (JIS) Ceq(JIS): Ceq(JIS) = C + Si/24 + Mn/6 + Ni/40 + Cr/5 + Mo/4 + V/14 (Similar to IIW, with slightly different coefficients)

Weldability carbon equivalent CET (CE for Hardenability): CET = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40 (More focused on hardenability and prediction of heat-affected zone hardness, more sensitive to low-alloy high-strength steels)

Pcm (Crack Sensitivity Index): Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B (Developed in Japan, specifically for predicting cold cracking in welding of low-carbon micro-alloyed high-strength steels)

Summary:

Carbon equivalent serves as a crucial link between the chemical composition of steel pipes and their key properties, especially weld ability and harden-ability. As a comprehensive quantitative indicator, it is of vital importance in the following aspects:

1. Predicting and avoiding the risk of cold cracks during welding to ensure the safety of welded structures.
2. Guiding the formulation and optimization of welding processes.
3. Assessing the harden-ability of steel pipes, predicting the effect of heat treatment and the uniformity of cross-sectional properties.
4. Balancing the contradiction between high strength and good toughness and weld-ability, which is the core consideration in the composition design of high-performance steel pipes.
5. Affecting the properties of the heat-affected zone.
6. Guiding the design of steel pipe manufacturing processes.

Therefore, in the production of steel pipes, material selection (especially in cases involving welding), standard formulation, and quality control, carbon equivalent is an indispensable and crucial parameter. Steel pipe standards (such as API 5L, ASTM/ASME specifications, etc.) typically stipulate the upper limit of CE values for steel pipes of different strength grades to ensure their weld-ability and usage safety.