Introduction:
In the field of piping materials engineering, the assessment of material suitability and performance is crucial. One commonly used parameter for evaluating the weldability and susceptibility of materials to various issues is the carbon equivalent (CE). Carbon equivalent provides valuable insights into the behavior of steel alloys, enabling engineers to make informed decisions regarding material selection and welding procedures. This article aims to shed light on CE and its significance in piping materials engineering.
What is Carbon Equivalent (CE) ?
Carbon equivalent (CE) is a numerical value used to assess the weldability and hardenability of carbon and low-alloy steels. It is a calculated value that takes into account the combined effect of various alloying elements on the properties of steel. Carbon equivalent is primarily used to estimate the weldability of a given material, as it provides an indication of its potential for cold cracking during welding.
Calculating CE
CE values are calculated based on the chemical composition of a material, primarily focusing on the concentrations of carbon and other alloying elements. Although several methods exist, the most commonly used formula for carbon equivalent calculation is the IIW (International Institute of Welding) formula:
CE = C + (Mn/6) + (Cr+Mo+V)/5 + (Ni+Cu)/15
Where:
CE = Carbon Equivalent
C = Carbon content (%)
Mn = Manganese content (%)
Cr = Chromium content (%)
Mo = Molybdenum content (%)
V = Vanadium content (%)
Ni = Nickel content (%)
Cu = Copper content (%)
The calculated value is dimensionless and provides a relative measure of the carbon content along with other alloying elements. A higher carbon equivalent indicates an increased risk of weld cracking.
Application in Piping Materials Engineering
1. Weldability Assessment:
CE is extensively used to assess the weldability of piping materials. It helps engineers determine the risk of cold cracking, such as hydrogen-induced cracking and lamellar tearing, during the welding process. By understanding the CE, engineers can select suitable materials with lower CE values for critical applications to minimize the risk of weld defects and ensure the integrity of the welded joint.
2. Material Selection:
CE is an essential factor when selecting materials for specific applications. Different industries and standards have specific carbon equivalent limits to ensure the desired mechanical properties and weldability. For instance, in the oil and gas industry, materials with lower carbon equivalents are often preferred due to their improved resistance to sour environments, which can cause hydrogen-induced cracking.
3. Welding Procedure (WPS) Development:
CE plays a vital role in developing welding procedures. It helps establish preheat and interpass temperature requirements to control the cooling rate during welding, minimizing the risk of cold cracking. Welding procedures Specifications (WPS) are tailored based on the material’s carbon equivalent to ensure optimum weld quality and integrity.
4. Material Certification and Compliance:
CE values are often required for material certification and compliance with industry standards and codes. Manufacturers provide carbon equivalent values alongside other mechanical and chemical properties of materials, enabling engineers to verify compliance with specific requirements.
Example of CE for Various Materials
Higher Carbon Equivalent Values and Heat Treatments
As can be seen from above table, the material A335-P91 is having highest carbon equivalent values equal to 2.38. The welding of such material is really difficult and susceptible to cracking during welding due to higher CE values. This material is used normally for High pressure steam service in power plants applications. In order to provide control environment for welding this material requires Pre as well as post heat treatment for all the pipe thicknesses. Sample WPS is shown in below snapshot where in pre-heat and post heat treatments are required for such materials irrespective of wall thickness.
Conclusion
Carbon equivalent (CE) is a valuable parameter in piping materials engineering, providing critical insights into the weldability and susceptibility of carbon and low-alloy steels. By considering the combined effect of alloying elements, the CE helps engineers make informed decisions regarding material selection, welding procedures, and ensuring the integrity of welded joints. Understanding and applying the concept of CE enables piping professionals to mitigate welding-related issues and enhance the reliability and performance of piping systems in various industries.
