CLADDING TECHNOLOGY SHANXI CO., LTDTransition Coefficients of Alloy Elements in Manual Arc Surfacing Electrodes
Literature Overview and Background
This study note addresses the fundamental metallurgical behavior of alloy element transfer during manual metal arc (MMA) surfacing operations. The transition coefficient — defined as the ratio of the alloy element content in the weld metal (or surfacing deposit) to that in the electrode coating — is a critical parameter that governs the final composition and, consequently, the corrosion resistance, hardness, and wear resistance of the overlay layer. For engineers designing surfacing procedures, understanding how elements such as chromium, molybdenum, nickel, tungsten, vanadium, and carbon behave during the welding arc is essential for predicting deposit properties and ensuring specification compliance.
The literature reviewed provides experimental data on the transition coefficients of multiple alloying elements when using various types of surfacing electrodes (e.g., E309, E310, E309Mo, E309L, E515, E516, E309MoL, and specialized wear-resistant grades). The work typically involves depositing multiple layers on carbon steel or low-alloy steel substrates, followed by chemical analysis of the electrode coating, the weld metal, and the dilution-corrected deposit composition.
Core Technical Findings
General Transition Coefficient Ranges
The transition coefficients vary significantly depending on the element, the electrode type, the arc voltage, the welding current, and the number of surfacing layers. Based on the literature and my own engineering experience, the following representative ranges can be established:
| Alloy Element | Typical Transition Coefficient (MMA Surfacing) | Key Influencing Factors |
|---|---|---|
| Chromium (Cr) | 0.75 – 0.95 | Electrode coating flux composition, arc voltage |
| Molybdenum (Mo) | 0.80 – 0.98 | High boiling point favors good transfer |
| Nickel (Ni) | 0.85 – 1.00 | Low vaporization loss; nearly complete transfer |
| Tungsten (W) | 0.60 – 0.85 | Density and melting point affect transfer efficiency |
| Vanadium (V) | 0.70 – 0.90 | Moderate oxidation losses |
| Carbon (C) | 0.50 – 0.80 | Significant burn-off; depends on flux deoxidation |
| Silicon (Si) | 0.30 – 0.60 | High oxidation tendency; major losses to slag |
| Manganese (Mn) | 0.40 – 0.70 | Oxidation losses; partially restored by deoxidizers |
| Iron (Fe) | 0.90 – 1.00 | Base metal reference element |
Effect of Number of Surfacing Layers
One of the most important practical findings is the effect of the number of surfacing passes on the effective transition coefficient. The first layer suffers from significant dilution with the base metal, which lowers the alloy content of the deposit. As additional layers are deposited, the dilution effect diminishes because the previous surfacing layer becomes the effective "base metal." After typically three to five layers, the deposit composition stabilizes and closely approaches the composition predicted by the transition coefficient of the electrode coating alone.
In practice, for a 309-type electrode surfacing a carbon steel substrate, the first layer may show only 60–70% of the nominal chromium content, while the third and subsequent layers approach 90–95% of the coating composition. This dilution behavior has direct implications for the minimum number of surfacing layers required to achieve the specified corrosion resistance or hardness of the overlay.
Influence of Welding Parameters
The arc voltage and welding current have measurable effects on transition coefficients. Higher arc voltages generally increase the arc temperature and the arc length, which can lead to greater vaporization losses for elements with lower boiling points such as manganese and silicon. Conversely, higher current densities can improve the transfer efficiency of refractory elements like tungsten and molybdenum due to the more efficient melting of the electrode core.
Engineering Practice Implications
Electrode Selection and Specification Compliance
When specifying surfacing electrodes for critical applications — such as corrosion-resistant overlays on pressure vessels per GB/T 150 or NB/T 47002, or wear-resistant overlays on mining equipment — the transition coefficient data must be incorporated into the welding procedure qualification (WPS) development. A common engineering error is to assume that the deposit composition equals the electrode coating composition. In reality, the effective composition of the final surfacing layer is a function of:
- The transition coefficient of each alloy element.
- The dilution rate from the base metal and previous layers.
- The number of surfacing passes.
- The welding parameters (current, voltage, travel speed).
Practical Recommendations
- For the first surfacing layer on a dissimilar substrate, always perform a chemical analysis to verify that the deposit composition meets the minimum alloy content requirements.
- When the transition coefficient of a critical element (e.g., chromium in a 316L overlay) is below 0.85, consider increasing the number of surfacing layers to compensate for dilution.
- For high-alloy deposits (e.g., Inconel 625 or Hastelloy C276 overlays), the transition coefficient data should be cross-referenced with the dilution curve to ensure that the final deposit meets the ASTM or EN specification for chemistry.
- During welding procedure qualification per NB/T 47014 or ASME IX, include metallographic and chemical analysis of the stabilized deposit (third or fourth layer) rather than the first layer.
Key Questions and Reflections
The transition coefficient concept, while well-established, is often underutilized in day-to-day engineering practice. Many welding engineers rely solely on the electrode manufacturer's datasheet, which typically reports the deposit composition under ideal conditions (low dilution, multiple layers). However, in field conditions — with varying preheat temperatures, different joint geometries, and single-pass surfacing on thin sections — the actual transition behavior may deviate significantly.
A particularly interesting observation from the literature is that the transition coefficient is not a fixed constant for a given element and electrode type; it varies with welding position, electrode diameter, and the flux composition of the coating. For instance, a 6 mm diameter E309 electrode may exhibit a different chromium transition coefficient than a 3.2 mm diameter electrode of the same type, due to differences in the arc characteristics and the coating-to-core ratio.
Study Insights and Implications
Understanding transition coefficients transforms the surfacing engineer from a recipe-follower to a process designer. By incorporating transition coefficient data into the WPS development process, one can predict the final deposit composition with reasonable accuracy, minimize the risk of non-conformance, and optimize the number of surfacing layers to balance quality with productivity. In the context of bimetal pressure vessel fabrication, where the overlay layer thickness and composition are directly related to the vessel's corrosion allowance and service life, this knowledge is indispensable.
The literature also highlights the importance of standardized testing methods for determining transition coefficients. Different laboratories may report different values if the testing protocol — particularly regarding the number of layers sampled, the sampling location, and the analytical method — is not strictly controlled. Engineers should insist on consistent testing protocols when commissioning transition coefficient studies for new electrode types or when validating supplier data for critical applications.
In summary, the transition coefficient is a fundamental metallurgical parameter that bridges the gap between electrode chemistry and deposit performance. Its systematic application in welding procedure design, qualification, and field verification is essential for producing high-quality surfacing overlays that meet the demanding requirements of modern industrial equipment.