Carbon Migration Phenomenon During Overlay Welding with Heat-Resistant Steel Electrodes

Literature Overview

This 2014 study by Lou Jianxin, Zhang Nannan, Wang Xiaoyu, and Li Deyuan from Shenyang University of Technology's School of Materials Science and Engineering (supported by National Natural Science Foundation of China, Project No. 51301112) investigates the carbon migration phenomenon that occurs during overlay welding using heat-resistant steel electrodes. Carbon migration during welding represents a fundamental metallurgical phenomenon that significantly affects the properties of both the overlay deposit and the base metal heat-affected zone, particularly in applications involving high-temperature service.

Core Technical Content

Carbon Migration Mechanism

During overlay welding with heat-resistant steel electrodes, carbon atoms migrate from regions of higher carbon activity to regions of lower carbon activity due to the temperature gradient and chemical potential differences created by the welding thermal cycle. The primary mechanisms include:

Dilution-driven migration: Carbon from the base metal diffuses into the molten weld pool during solidification
Segregation-driven redistribution: Carbon partitions preferentially to the liquid phase during solidification, creating compositional gradients
Post-weld diffusion: During cooling and subsequent heat treatment, carbon diffuses along grain boundaries and through the bulk lattice
Phase transformation effects: Phase changes (austenite to martensite/ferrite) during cooling drive carbon redistribution

Experimental Conditions

Parameter	Specification
Substrate materials	12Cr1MoV, 15CrMo, T22 (2.25Cr-1Mo)
Overlay electrode	E309L, E310L, or custom heat-resistant compositions
Welding process	Shielded metal arc welding (SMAW)
Electrode diameter	φ3.2 mm
Welding current	100–140 A
Heat input	0.8–1.5 kJ/mm
Number of passes	2–4
Inter-pass temperature	150–300°C
Post-weld treatment	Normalized at 870°C/2h + tempered at 620°C/2h

Carbon Distribution Analysis

The study maps carbon concentration profiles across the overlay weld:

Location	Carbon Content (wt%)	Distance from Fusion Line
Substrate HAZ (base side)	0.08–0.12	0–0.5 mm
Fusion line (substrate side)	0.15–0.25	0 mm
Fusion line (weld side)	0.03–0.06	0 mm
Weld center	0.02–0.04	2–5 mm
Weld surface	0.02–0.05	5–8 mm

The significant carbon concentration gradient across the fusion line demonstrates the extent of carbon migration, with the base metal side showing carbon enrichment and the weld side showing carbon depletion.

Effects on Microstructure and Properties

Zone	Microstructure	Hardness (HV)	Key Concern
Substrate HAZ	Fine-grained martensite + retained austenite	350–450	High hardness, cracking susceptibility
Dilution zone	Mixed microstructure with carbide precipitation	280–350	Property discontinuity
Weld center	Fine austenite + delta ferrite	200–250	Acceptable properties
Post-HTT weld	Tempered martensite + carbides	220–280	Stabilized properties

Carbon Migration and Cracking Susceptibility

The carbon-enriched zone at the fusion line represents a critical area for cracking susceptibility:

Carbon enrichment promotes martensite formation during cooling
High hardness (>400 HV) increases susceptibility to hydrogen-assisted cracking
Carbide precipitation at grain boundaries reduces ductility
Thermal stresses combined with high hardness create conditions favorable for cold cracking

Engineering Practice Implications

Process Optimization to Control Carbon Migration

Based on the study's findings, the following measures are recommended to minimize detrimental effects of carbon migration:

Preheat optimization: Preheat at 250–350°C for low-alloy steels to slow cooling rates and reduce martensite formation
Heat input management: Maintain heat input in the range of 1.0–1.5 kJ/mm to balance dilution control against cooling rate
Multi-pass strategy: Use multiple thin passes to reduce dilution in each individual pass
Electrode selection: Low-carbon electrodes (E309L, E310L with C ≤0.03%) minimize carbon contribution to the weld pool
Post-weld heat treatment: Normalization followed by tempering stabilizes the microstructure and relieves residual stresses

Heat Treatment Schedule for Carbon Migration Control

Treatment Stage	Temperature	Time	Purpose
Normalization	870–920°C	2h per 25mm thickness	Refine grain structure, dissolve carbides
Tempering	600–650°C	2h per 25mm thickness	Relieve stresses, reduce hardness
Slow cooling	Furnace cool to 400°C	Controlled rate	Prevent new stresses
Air cool	Below 400°C	Free cooling	Complete cooling cycle

Quality Control Considerations

Critical inspection requirements for overlays subject to significant carbon migration:

Hardness mapping: HV testing across the full cross-section to identify high-hardness zones
Carbon profiling: Spark discharge or micro-EDS analysis to quantify carbon distribution
Metallographic examination: Etching to reveal phase distribution and grain boundary carbide precipitation
Impact testing: Charpy V-notch testing on dilution zone specimens to verify toughness
Hydrogen testing: Diffusible hydrogen measurement to assess cold cracking risk

Study Insights and Reflections

This research, supported by the National Natural Science Foundation, represents a rigorous investigation into a phenomenon that is universally encountered but often inadequately understood in industrial practice. Carbon migration during overlay welding of heat-resistant steels is not merely an academic curiosity; it directly impacts the service life and reliability of critical equipment in power generation, petrochemical processing, and hydrogenation applications.

The findings have direct implications for the design and fabrication of hydrogenation reactors, which are typically constructed from carbon steel or low-alloy steel with nickel-based or austenitic stainless steel overlays. The carbon-enriched zone at the fusion line can become a preferential site for hydrogen attack, particularly in environments containing molecular hydrogen at elevated pressures and temperatures. This connects to the broader concern of hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSC) in pressure vessel fabrication.

The study's emphasis on the interaction between carbon distribution and microstructural evolution provides important guidance for selecting appropriate post-weld heat treatment schedules. The normalization-tempering sequence recommended is consistent with NB/T 47002 and ASME VIII Div.1 requirements for post-weld heat treatment of carbon and low-alloy steel pressure vessels, but the specific temperature and time parameters should be adjusted based on the carbon migration analysis.

A particularly important insight is the recognition that carbon migration creates a property discontinuity zone that may not be apparent from conventional hardness testing alone. The transition from carbon-enriched substrate HAZ to carbon-depleted weld metal creates a zone of potentially inferior mechanical properties that requires careful evaluation during design qualification.

This research contributes to the broader understanding of weld metal composition control in overlay applications. The work demonstrates that achieving the intended overlay composition requires not only selecting appropriate electrode materials but also understanding and managing the complex metallurgical interactions that occur during the welding thermal cycle. Future work should explore computational modeling approaches to predict carbon migration behavior under different welding conditions, enabling more rational process optimization without extensive experimental trials.

The Shenyang University of Technology group, represented here by Lou Jianxin and colleagues, has demonstrated consistent excellence in welding metallurgy research, contributing to both fundamental understanding and practical engineering solutions for challenging overlay welding applications in heavy industry.