Carbon Equivalents in Steel Components

Steel’s weldability is primarily influenced by its carbon content. Additionally, the contribution of other elements, such as silicon (Si), nickel (Ni), vanadium (V), molybdenum (Mo), copper (Cu), manganese (Mn), and chromium (Cr), can also affect its carbon equivalence (CE).

These additional elements add up in scrap fed electric arc furnace steels that currently predominate the market and carry over into completed goods.

Originally, carbon equivalence was developed to allocate a numerical value for a given steel composition, indicating what carbon content would contribute to an equivalent level of hardenability for that steel.

Taking this further, carbon equivalence represents the material composition’s contribution to the cold-cracking (hydrogen cracking) susceptibility of the steel.

Carbon equivalent calculations are used in welding to predict the heat-affected zone (HAZ) hardenability. By using the carbon equivalence calculation to understand any differences in chemistry, one can determine whether two materials being joined together via a filler metal component have compatible properties for the process.

If the components are too different, or if the carbon equivalent advances towards a higher, undesirable value (Table 1), then special precautions could be needed before and throughout the welding process.

Carbon equivalence (CE) is an essential calculation prior to performing welding.

Carbon equivalence (CE) is an essential calculation prior to performing welding. Image Credit: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Welding precautions may include controlling heat input, using low hydrogen electrodes, and prescriptive heat treatment. Several of these guidelines are published in the NACE (National Association of Corrosion Engineers) standards (NACE MR0175/ISO 15156 and NACE MR0103/ISO 17945).

These standards were developed for natural gas, petrochemical, and offshore applications where carbon steels are susceptible to sulfide stress cracking (SSC) or hydrogen stress cracking (HSC) when in the presence of hydrogen sulfide (H2S, sour service).

The International Institute of Welding (IIW) and American Welding Society (AWS) developed two commonly used equations for the expression of carbon equivalence.

International Institute of Welding (IIW)

CE = C+Mn/6+(Cr+Mo+V)/5+(Cu+Ni)/15

American Welding Society (AWS)

CE = C+(Mn+Si)/6+(Cr+Mo+V)/5+(Cu+Ni)/15

Niton Apollo Handheld LIBS Analyzer

The Thermo Scientific™ Niton™ Apollo™ handheld LIBS analyzer is a new tool available to professionals for carrying out material analysis. It utilizes Laser-Induced Breakdown Spectroscopy (LIBS) to implement carbon equivalent calculations and inspection of critical assets.

The Niton Apollo weighs just 6.4 lbs. (2.9 kg.) and specializes in measuring carbon content in a portable, convenient form factor. This eliminates the demand for bulky Optical Emission Spectroscopy (OES) carts in several circumstances.

The Niton Apollo features a high purity argon purge and effective laser, producing lab-quality results in around 10 seconds. As well as calculating carbon equivalents, users can program pseudo-elements, perform advanced averaging, and identify alloy grades. Data is shown in real-time to allow efficient and fast decision making.

Test Method and Results

As with other elemental technologies, sample preparation is necessary when utilizing the Niton Apollo in order to ensure the most reliable results and performance. Additional information on sample preparation procedures can be accessed at thermofisher.com/niton.

Many samples of carbon steels used in a variety of applications were selected for testing with the Niton Apollo. Chemistry and subsequent carbon equivalent calculations are shown employing both IIW and AWS formulae as repeatability testing measures (Table 2).

Table 1. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Common CE Value Classifications
Carbon Equivalent (CE) Weldability
Up to 0.35 Excellent
0.36-0.40 Very Good
0.41-0.45 Good
0.46-0.50 Fair
Over 0.50 Poor

 

Table 2. Source: Thermo Fisher Scientific - Elemental Analyzers and Phase Analyzers

Sample CE-
IIW
CE-
AWS
C Mn Cu Si Ni Cr Mo V Ti Al
X65 0.287 0.335 0.049 1.061 0.118 0.286 0.117 0.090 0.056 0.081 0.005 0.038
X65 0.306 0.352 0.042 1.209 0.118 0.279 0.095 0.087 0.064 0.087 0.005 0.024
X65 0.291 0.338 0.040 1.134 0.125 0.277 0.105 0.091 0.061 0.085 0.005 0.030
X65 0.290 0.338 0.042 1.116 0.118 0.284 0.112 0.096 0.065 0.077 0.005 0.031
X65 0.300 0.346 0.039 1.196 0.116 0.276 0.094 0.080 0.068 0.088 0.005 0.028
X65 0.294 0.341 0.034 1.171 0.113 0.280 0.102 0.092 0.078 0.083 0.005 0.030
X65 0.278 0.326 0.039 1.091 0.124 0.289 0.106 0.090 0.049 0.069 0.006 0.032
X65 0.295 0.340 0.046 1.127 0.116 0.273 0.109 0.087 0.063 0.081 0.005 0.034
X65 0.282 0.333 0.041 1.078 0.117 0.310 0.107 0.095 0.059 0.075 0.004 0.038
X65 0.280 0.325 0.036 1.115 0.110 0.275 0.098 0.089 0.053 0.079 0.005 0.034
AVG 0.290 0.337 0.041 1.130 0.118 0.283 0.105 0.090 0.062 0.080 0.005 0.032
STDEV 0.009 0.008 0.004 0.049 0.004 0.011 0.007 0.005 0.008 0.006 0.000 0.004
RSD 3.1% 2.4% 10.7% 4.3% 3.8% 3.8% 7.1% 5.1% 13.3% 7.2% 9.1% 13.7%

 

Conclusion

The Niton Apollo handheld LIBS analyzer’s performance demonstrates the ability to repeatedly and accurately capture carbon content and to calculate carbon equivalence automatically using a prescribed formula via a pseudo-element feature.

Whether working offshore on an oil platform, in a pipeline trench, or at heights in a refinery, the Niton Apollo is the perfect tool for analysis in several different industries offering the greatest productivity and safety.

Acknowledgments

Produced from materials originally authored by Brian Wilson from Thermo Fisher Scientific.

 

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Handheld Elemental & Radiation Detection.

For more information on this source, please visit Thermo Fisher Scientific – Handheld Elemental & Radiation Detection

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