CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of MAG Cladding Layer on E-Grade Steel
Literature Overview
The study under review investigates the microstructural evolution and mechanical properties of a Metal Active Gas (MAG) cladding layer deposited on E-grade steel, which is commonly used in heavy industrial machinery, shipbuilding, and pressure vessel fabrication. E-grade steel typically refers to a carbon or low-alloy steel with a minimum yield strength of approximately 235 MPa, widely specified in Chinese national standards such as GB/T 700 and GB/T 1591. The research focuses on understanding how the MAG welding process parameters influence the cladding layer's microstructure, hardness distribution, and bonding characteristics, which are critical for ensuring long-term service integrity in corrosive or erosive environments.
Core Technical Findings
The study employed varying MAG welding parameters including welding current, voltage, travel speed, and shielding gas composition to deposit stainless steel overlay layers on E-grade steel substrates. The resulting microstructure of the cladding layer was examined using optical microscopy and scanning electron microscopy (SEM), while hardness was measured via Vickers microhardness testing across the weld cross-section.
Key findings from the literature include the following observations regarding the microstructure of the MAG cladding layer:
| Parameter | Typical Range Studied | Effect on Microstructure |
|---|---|---|
| Welding Current (I) | 180–260 A | Higher current increases grain size and promotes columnar grain growth |
| Arc Voltage (U) | 22–28 V | Higher voltage widens the weld bead and reduces dilution ratio |
| Travel Speed (V) | 250–450 mm/min | Faster speed reduces heat input, producing finer grains |
| Shielding Gas | Ar + 2% CO2 | CO2 addition promotes martensitic transformation and increases hardness |
The microstructure of the cladding layer predominantly consists of martensite and acicular ferrite, with the proportion of martensite increasing as heat input decreases. The dilution ratio between the base metal and the cladding layer was found to range from 15% to 35% depending on the welding parameters. Lower dilution ratios yield cladding layers with higher hardness and better corrosion resistance, but excessively low dilution can compromise bond strength at the interface.
Analysis of Mechanical Properties
The hardness profile across the cladding layer, heat-affected zone (HAZ), and base metal was systematically measured. The cladding layer exhibited Vickers hardness values ranging from 350 HV to 520 HV, depending on the cooling rate and alloy composition. The HAZ showed a hardness increase of approximately 80–120 HV compared to the base metal, attributed to martensitic transformation during rapid cooling. The base metal hardness remained relatively unchanged at approximately 180–200 HV.
Tensile testing of transverse specimens revealed that the cladding layer maintained adequate ductility with elongation values of 12–18%, indicating that the MAG process can produce cladding layers with a favorable balance of hardness and toughness. The bond strength between the cladding layer and the base metal was evaluated through shear testing, with results consistently exceeding 250 MPa, well above the minimum requirements specified in NB/T 47014.
Engineering Practice Implications
In engineering practice, the selection of MAG welding parameters for E-grade steel cladding must account for several practical considerations. The preheating temperature is typically maintained at 100–150°C to reduce the risk of cold cracking in the HAZ, particularly when the carbon equivalent of the base metal exceeds 0.45%. Post-weld heat treatment (PWHT) is generally recommended for thick sections exceeding 25 mm to relieve residual stresses and temper the martensitic structure in the HAZ.
From a quality control perspective, the following inspection procedures are essential:
- Visual inspection (VT) for surface defects such as porosity, undercut, and weld spatter.
- Magnetic particle testing (MT) for detecting surface and near-surface cracks in the cladding layer and HAZ.
- Ultrasonic testing (UT) for assessing internal defects including lack of fusion and delamination at the bond line.
- Hardness mapping across the full cross-section to verify compliance with specification requirements.
Key Reflections
A critical insight from this study is the interplay between dilution ratio and service performance. In aggressive environments such as acidic chemical processing or offshore marine applications, the dilution ratio must be minimized to ensure that the cladding layer retains sufficient alloying elements for corrosion resistance. However, reducing dilution often requires multiple passes with careful management of interpass temperature, which increases production costs and cycle time.
Another important consideration is the effect of welding sequence on residual stress distribution. In large-scale cladding operations, a back-step welding sequence or a symmetric multi-pass strategy is recommended to minimize warpage and reduce the risk of cracking. The literature confirms that careful thermal management during multi-pass cladding is as important as the selection of individual welding parameters.
This study provides valuable guidance for engineers tasked with designing MAG cladding procedures for E-grade steel components. The systematic correlation between process parameters, microstructure, and mechanical properties offers a solid foundation for procedure qualification under NB/T 47014 and ASME IX, enabling confident extrapolation to production welding with appropriate essential variable controls.