CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Cr13 Stainless Steel CMT Overlay Combined with High-Pressure Rolling
Literature Overview
This research, published in the Chinese Journal of Mechanical Engineering in 2017 by Zhou Jianan, Zhang Guodong, Zheng Fei, Yang Hui, and Mei Qingsong from the Key Laboratory of Transition Process of Hydraulic Machinery, Ministry of Education, Wuhan University, investigates the microstructure evolution and mechanical properties of Cr13 martensitic stainless steel deposited by Cold Metal Transfer (CMT) welding followed by high-pressure rolling (HPR) surface treatment. The study was supported by the National Natural Science Foundation of China (Grant No. 51371128) and the Hubei Provincial Natural Science Foundation (Grant No. 2014CFB707), reflecting its academic significance in the field of advanced surface engineering.
Core Technical Content
Cold Metal Transfer (CMT) welding is a variant of the gas metal arc welding (GMAW) process that operates at significantly lower heat input than conventional GMAW by cyclically reducing the welding current during the wire feed cycle. This unique operating principle results in a narrow weld bead with reduced dilution, minimal distortion, and excellent control over the microstructure of the deposited layer. When combined with high-pressure rolling, which is a plastic deformation-based surface treatment, the resulting overlay layer exhibits enhanced mechanical properties due to grain refinement and work hardening.
The Cr13 stainless steel (equivalent to 410 or 420 grade) selected for this study is a martensitic stainless steel containing approximately 12–14% chromium, which provides moderate corrosion resistance combined with good strength and wear resistance. This material is particularly suitable for hydraulic machinery components that operate in water environments and require resistance to cavitation erosion and wear.
CMT Welding Parameters and Process Characteristics
The CMT process was operated with the following typical parameters:
| Parameter | Value |
|---|---|
| Base metal | Q235 carbon steel |
| Filler wire | Cr13 stainless steel (0.8 mm diameter) |
| Welding current (peak) | 80–120 A |
| Welding current (background) | 20–30 A |
| Travel speed | 100–200 mm/min |
| Shielding gas | Ar + 2% CO₂ |
| Wire feed speed | 3–5 m/min |
| Heat input | 0.3–0.6 kJ/mm |
The low heat input of CMT welding (typically 30–50% of conventional GMAW) is a critical advantage, as it minimizes the heat-affected zone (HAZ) in the base metal and reduces the risk of cracking in the martensitic overlay layer. The cyclic current modulation also promotes a more uniform solidification microstructure in the weld bead.
High-Pressure Rolling Parameters
The high-pressure rolling treatment was applied to the as-welded overlay layer using a hardened steel roller with a radius of 5–10 mm. The rolling was performed at normal loads of 5–15 kN with a rolling speed of 0.5–2 m/min, producing a plastic deformation depth of 0.1–0.3 mm on the overlay surface.
Microstructure Analysis
The as-welded CMT overlay layer exhibits a martensitic microstructure consisting of lenticular martensite and retained austenite. The low heat input and rapid cooling rate inherent to the CMT process promote the formation of fine martensite plates with a high dislocation density. Metallographic examination reveals that the overlay layer consists of multiple weld beads with a columnar-to-equiaxed transition in the grain morphology, with columnar grains near the weld interface and equiaxed grains in the upper portion of the overlay.
After high-pressure rolling, significant microstructural changes occur in the near-surface region:
- Grain refinement: The martensite lath width decreases from approximately 150–200 nm in the as-welded condition to 50–80 nm in the rolled condition.
- Work hardening: The dislocation density increases from approximately 10¹⁴ m⁻² to 10¹⁵ m⁻² in the deformed layer.
- Surface compressive residual stress: The HPR treatment introduces compressive residual stresses of 300–600 MPa in the near-surface region, which are beneficial for improving fatigue and cavitation erosion resistance.
Mechanical Property Comparison
| Property | As-Welded | After HPR | Improvement |
|---|---|---|---|
| Hardness (HV0.3) | 380–420 | 520–580 | 35–40% increase |
| Compressive residual stress (MPa) | Tensile (+50–100) | Compressive (-300 to -600) | Significant improvement |
| Cavitation erosion resistance | Baseline | 2–3× improvement | Substantial |
| Wear resistance | Baseline | 1.5–2× improvement | Moderate |
| Corrosion resistance (in water) | Good | Slightly reduced at surface | Minor degradation |
Engineering Practice and Application Scenarios
The combination of CMT overlay and high-pressure rolling is particularly relevant for hydraulic machinery components such as pump impellers, turbine runners, and hydraulic cylinder barrels that operate in water or aqueous environments. These components are subject to cavitation erosion, wear, and corrosion, and the Cr13 overlay layer provides a comprehensive solution by combining corrosion resistance with enhanced mechanical properties.
The study demonstrates that the CMT + HPR combination offers several advantages over conventional overlay methods:
- Lower heat input compared to SAW or ESW overlay, reducing distortion in thin-walled components.
- Better control over overlay composition due to reduced dilution.
- Enhanced surface properties through the combined effect of overlay and plastic deformation.
- Applicable to complex geometries where ESW overlay is not feasible.
Defect Analysis and Process Control
| Defect Type | Root Cause | Prevention Strategy |
|---|---|---|
| Cracking in overlay | Rapid cooling of martensitic structure | Preheat base metal to 150–200 °C; control interpass temperature below 250 °C |
| Poor bond strength | Insufficient interfacial melting | Increase peak current; ensure clean base metal surface |
| Porosity | Wire surface oxidation; gas entrapment | Use clean wire; optimize shielding gas flow rate |
| Incomplete rolling | Surface roughness of as-welded layer | Grind as-welded surface before rolling; use appropriate roller geometry |
Study Insights and Implications
This research demonstrates the synergistic effect of combining additive manufacturing (CMT welding) with surface mechanical attrition treatment (high-pressure rolling) to produce overlay layers with superior performance. The key insight is that post-weld surface treatment can significantly enhance the properties of the overlay layer without compromising its corrosion resistance, which is a common concern with aggressive surface treatments.
The work also highlights the importance of understanding the microstructure-property relationships in martensitic stainless steel overlays. The fine martensite structure produced by CMT welding provides an excellent base for further refinement through plastic deformation, resulting in a surface layer with exceptional cavitation erosion and wear resistance.
For engineering practice, this approach offers a viable alternative to more expensive overlay methods such as PTA cladding or laser cladding, particularly for large-area applications where productivity is important. The CMT process can be applied to a wide range of geometries and is compatible with automated welding systems, making it suitable for industrial-scale production.
This literature provides valuable guidance for engineers working on surface protection solutions for hydraulic machinery and other components operating in corrosive and erosive environments.