CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of CMT Cladding 310 Stainless Steel on Q235 Steel Surface Study Notes
Literature Overview
This study investigates the microstructure evolution and mechanical properties of a 310 stainless steel overlay deposited onto a Q235 carbon steel substrate using Cold Metal Transfer (CMT) arc welding. The research addresses a common industrial challenge: achieving a high-quality metallurgical bond between dissimilar metals where the substrate is a low-carbon structural steel and the overlay is a high-nickel austenitic stainless steel. The work examines how CMT process parameters influence dilution rate, microstructural morphology, hardness distribution, and corrosion resistance of the cladding layer.
Core Technical Analysis
CMT welding represents a pulsed GMAW variant characterized by extremely low heat input, typically in the range of 0.5 to 1.5 kJ/mm. The process employs a mechanical wire-feed modulation combined with electrical current pulsing, resulting in wire deposition rates as low as 1 to 5 mm per minute. This minimal thermal input is critical when cladding high-alloy materials onto low-alloy substrates because it significantly reduces the dilution rate — the study reports dilution rates below 15 percent compared to 25 to 40 percent for conventional GMAW processes.
The microstructural analysis reveals a gradient from the fusion line through the overlay layer. At the fusion boundary, a narrow zone of martensitic transformation is observed due to rapid cooling rates exceeding 200 degrees Celsius per second. Moving into the overlay, the microstructure transitions through a mixed ferrite-austenite region into a fully austenitic structure with occasional delta-ferrite stringers. The high nickel and chromium content of 310 stainless steel (approximately 19 to 25 percent Ni, 24 to 26 percent Cr) stabilizes the austenite phase even at room temperature, provided the dilution from the carbon steel substrate remains controlled.
Key Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Cladding Quality |
|---|---|---|
| Wire feed speed | 0.5 to 3.0 m/min | Lower speeds reduce dilution but increase deposition time |
| Arc voltage | 16 to 20 V | Higher voltage increases arc length and wire melting rate |
| Travel speed | 50 to 200 mm/min | Slower speeds improve bond strength but risk excessive heat input |
| Wire stick-out | 10 to 15 mm | Critical for CMT stability and droplet transfer control |
| Shielding gas | 98% Ar + 2% CO2 | CO2 addition improves arc stability and penetration |
| Interpass temperature | Below 150 degrees C | Prevents grain coarsening in the overlay |
The hardness distribution across the cross-section shows a characteristic gradient: the Q235 substrate exhibits approximately 120 to 140 HV, while the fully austenitic 310 overlay reaches 180 to 210 HV. The heat-affected zone (HAZ) near the fusion line shows a hardness peak of 220 to 250 HV due to localized martensitic transformation, which can be a concern for cracking susceptibility.
Engineering Practice Integration
From a practical standpoint, the CMT process offers several advantages for industrial cladding applications. The low heat input minimizes distortion of thin-walled components, making it suitable for retrofitting existing carbon steel equipment with corrosion-resistant linings. However, the process is inherently slow, and deposition rates of 0.5 to 2 kg/h must be balanced against production requirements.
A critical practical consideration is the pre-heating requirement. Despite the low heat input of CMT, pre-heating the Q235 substrate to 100 to 150 degrees Celsius is recommended to prevent hydrogen-induced cracking in the martensitic transformation zone. Post-weld stress relief at 400 to 450 degrees Celsius for one hour per 25 mm of thickness is essential to relieve residual stresses and transform any retained martensite into tempered structures.
The study also highlights that multi-pass cladding is necessary to achieve adequate overlay thickness. A typical build-up strategy involves 3 to 5 passes, with the first pass serving as a transition layer. Some practitioners recommend depositing an intermediate alloy such as 309L stainless steel between the Q235 substrate and the 310 overlay to further reduce cracking susceptibility.
Key Reflections and Implications
The most significant insight from this study is that CMT provides a viable pathway for cladding high-nickel austenitic alloys onto carbon steel substrates with dilution rates low enough to maintain the required alloy composition in the overlay. This is particularly relevant for hydrogenation reactor internals, sulfuric acid storage tanks, and marine applications where 310 stainless steel's resistance to pitting and intergranular corrosion is required.
However, the presence of martensite at the fusion line remains a concern. In service environments subject to cyclic loading or thermal cycling, this brittle phase could initiate cracks that propagate through the overlay. Future work should explore the use of intermediate transition layers or post-weld heat treatment cycles specifically designed to eliminate martensite while preserving the overlay's corrosion resistance. The study reinforces the principle that cladding is not merely a surface treatment but a complex metallurgical interface problem requiring careful process design.