Zhuojin Pipe Fitting Co., LtdSurface High-Pressure Rolling of Cr13 Stainless Steel CMT Overlay Layer: Structure and Properties
Literature Overview and Research Context
This paper by Zhou Jianan, Zhang Guodong, Zheng Fei, Yang Hui, and Mei Qingsong from the Key Laboratory of Hydrodynamic Transition Processes, Ministry of Education, Wuhan University, published in the Chinese Journal of Mechanical Engineering in 2017 (Vol. 53, No. 8, pp. 68–74), investigates a novel approach to improving the properties of CMT (Cold Metal Transfer) overlay welds on Cr13 stainless steel. The research addresses a common challenge in overlay welding: the inherent microstructural and property inhomogeneity within the deposited metal, which can lead to inconsistent performance in service.
The authors employed surface high-pressure rolling (SHPR) as a post-weld mechanical treatment to refine the grain structure and improve the uniformity of the overlay layer. This approach is particularly relevant for hydraulic machinery components made of Cr13 stainless steel (such as 1Cr13, 2Cr13), which are commonly used in pump impellers, turbine blades, and other components subjected to erosion-corrosion and wear.
Experimental Methodology and Process Parameters
The experimental work involved two main stages: CMT overlay welding and subsequent surface high-pressure rolling.
CMT Overlay Welding Parameters
The CMT welding process was conducted using ER309L stainless steel wire (φ2.5 mm) as the filler metal, with argon as the shielding gas. CMT is characterized by a low heat input and a controlled wire feed, which results in a fine microstructure with minimal dilution. The selection of ER309L (a high-austenite stainless steel) as the filler metal is appropriate for Cr13 stainless steel substrates, as it provides good compatibility and crack resistance.
High-Pressure Rolling Parameters
| Parameter | Value |
|---|---|
| Rolling pressure | 12 kN |
| Rolling time | 15 min |
| Number of rollers | 6 |
| Roller material | GCr15 |
| Roller dimensions | φ8 mm × 10 mm |
| Roller rotation speed | 2 r/min |
The SHPR process involves pressing multiple hardened rollers against the surface of the overlay weld under high pressure, inducing plastic deformation that refines the grain structure and introduces compressive residual stresses. The specific parameters selected in this study represent a balance between achieving sufficient deformation for grain refinement and avoiding excessive strain that could lead to cracking.
Microstructural and Property Analysis
The authors employed OM (Optical Microscopy), SEM, microhardness testing, friction-wear testing, and electrochemical testing to evaluate the effects of SHPR on the overlay layer.
Microstructural Changes
The most significant microstructural change observed was the refinement of grain size in the rolled layer. The SHPR process introduces severe plastic deformation, which promotes dynamic recrystallization and grain refinement. The rolled layer was divided into two zones: a high-pressure rolling layer (near-surface) and a transition layer (deeper).
| Zone | Average Microhardness (HV) | Grain Structure |
|---|---|---|
| High-pressure rolling layer | 403.62 | Fine, refined grains |
| Original overlay metal | 287.83 | Coarser, as-welded grains |
The increase in microhardness from 287.83 HV to 403.62 HV represents a significant improvement (approximately 40%), which is attributed to the grain refinement and work hardening effects of the SHPR process.
Wear Resistance
The friction-wear tests demonstrated that the SHPR-treated overlay layer exhibited more stable friction characteristics and lower wear volume compared to the untreated overlay. The grain refinement and compressive residual stresses introduced by rolling contribute to improved wear resistance by increasing the surface hardness and reducing crack initiation.
Corrosion Resistance
Interestingly, the SHPR treatment resulted in a decrease in corrosion resistance. This is attributed to the introduction of residual stresses and the disruption of the passive film on the rolled surface. The increased surface energy and the presence of dislocations and other defects can promote localized corrosion. This finding highlights the trade-off between wear resistance and corrosion resistance that must be considered when applying SHPR to stainless steel components.
Engineering Practice Implications
The research provides several important insights for engineers working with Cr13 stainless steel components:
- Post-weld treatment: SHPR is an effective method for improving the uniformity and properties of CMT overlay layers, particularly for wear resistance.
- Process optimization: The rolling parameters (pressure, time, roller configuration) must be carefully optimized to achieve the desired balance between wear resistance and corrosion resistance.
- Application selection: SHPR is most beneficial for components where wear resistance is the primary concern, such as pump impellers and turbine blades. For components where corrosion resistance is critical, alternative surface treatments may be more appropriate.
- Quality control: The microhardness profile and surface roughness should be monitored after SHPR to ensure consistent treatment quality.
The study demonstrates that mechanical surface treatments can be used to enhance the performance of overlay welds, providing an additional tool in the engineer's toolkit for optimizing component properties.
Key Reflections and Study Insights
One of the most important findings of this research is the trade-off between wear resistance and corrosion resistance introduced by the SHPR process. This is a critical consideration for engineers selecting surface treatment methods for Cr13 stainless steel components. The decision to apply SHPR should be based on a thorough analysis of the service environment and the relative importance of wear and corrosion resistance.
The use of CMT welding as the base process is also noteworthy. CMT offers several advantages over conventional GMAW (Gas Metal Arc Welding), including lower heat input, reduced spatter, and better control over the deposition rate. These advantages contribute to a more uniform overlay layer, which is further improved by the SHPR treatment.
This research exemplifies the integration of welding technology with mechanical surface treatment to achieve optimized component properties. The systematic evaluation of microstructure, hardness, wear resistance, and corrosion resistance provides a comprehensive understanding of the effects of SHPR, which is essential for rational process design and quality control in industrial applications.