Zhuojin Pipe Fitting Co., LtdHardness Distribution and Microstructure Evolution in Hardox400 Surfacing Welds
Literature Overview
This paper by Liu Xixue, He Dingyong, Jiang Jianmin, and colleagues from Beijing University of Technology (published in Welding journal, 2013, Issue 4, pp. 41-43) investigates the hardness distribution and microstructural evolution in the weld joint formed during single-pass surfacing of Hardox400 wear-resistant steel using CO2 gas shielded arc welding. The study focuses on understanding how the surfacing process affects the base metal properties in the heat-affected zone (HAZ) and the fusion zone, which is critical for ensuring that the repair does not compromise the structural integrity of the base material.
Hardness Distribution Analysis
The authors measured macro Rockwell hardness values from the HAZ to the base metal after single-pass surfacing with CO2 gas shielded welding. The key findings regarding hardness distribution are:
| Distance from Fusion Line | Hardness Change | Notes |
|---|---|---|
| 0-1 mm (fusion zone) | Maximum decrease (18-26% of base metal) | Softest region |
| 1-2 mm (near fusion zone) | Significant decrease | Transition region |
| 2-5 mm (HAZ) | Moderate decrease | Gradual recovery |
| Beyond 5 mm | No significant change | Base metal properties retained |
The hardness decrease region is confined to within 5 mm from the fusion line, with the maximum decrease occurring in the 1-2 mm range from the fusion boundary. The maximum hardness reduction is approximately 18-26% of the base metal value. For Hardox400 with a base hardness of approximately HRC 40-42 (400 HV), this translates to a minimum hardness of approximately HRC 30-32 (300-320 HV) in the most affected region.
Microstructural Analysis
The microstructural investigation revealed several important features:
- Fusion zone microstructure: The weld metal microstructure consists primarily of martensite as the wear skeleton, with fine Fe2B hard phases. This microstructure is compatible with the base martensitic structure of Hardox400, ensuring good metallurgical bonding.
- HAZ microstructure: No significant grain coarsening was observed in the fusion zone. This is a critical finding because grain coarsening in the HAZ of high-carbon martensitic steels can lead to reduced toughness and increased susceptibility to cracking. The absence of grain coarsening indicates that the CO2 gas shielded welding parameters used were appropriate for this material.
- Metallurgical compatibility: The weld microstructure (martensite + Fe2B) matches well with the base martensitic microstructure, preventing the formation of a brittle interfacial layer that could lead to spalling of the surfacing layer.
Process Parameters and Their Effects
The study implicitly addresses the relationship between welding parameters and HAZ hardness distribution. The CO2 gas shielded welding process was selected for its widespread availability and cost-effectiveness in field repair applications. The key parameters that influence the HAZ hardness profile include:
- Heat input: Higher heat input increases the HAZ width and reduces the peak cooling rate, which can lead to softer microstructures in the HAZ. The confined 5 mm hardness decrease zone suggests moderate heat input was used.
- Welding current and voltage: These determine the arc energy and penetration depth, directly affecting the dilution rate and HAZ thermal cycle.
- Travel speed: Affects the heat input per unit length and the cooling rate of the HAZ.
- Preheat temperature: While not explicitly discussed in this paper, preheat is essential for Hardox400 to prevent cold cracking, and it also affects the HAZ cooling rate and hardness distribution.
Engineering Practice Implications
For pipeline and fitting repair engineers, the findings of this study have several practical implications:
- Acceptable HAZ softening: The 18-26% hardness reduction in the HAZ is generally acceptable for most service applications, as the base metal retains sufficient hardness for wear resistance. However, for critical applications where the HAZ hardness must be maintained, additional measures such as post-weld heat treatment may be necessary.
- Single-pass vs. multi-pass: The study focused on single-pass surfacing. Multi-pass surfacing will produce a different HAZ thermal cycle, potentially reducing the HAZ softening through tempering of the HAZ by subsequent passes. However, multi-pass also increases the total heat input, which could widen the affected zone.
- Application to pipe geometry: The hardness distribution data obtained from flat plate specimens can be extrapolated to pipe geometry with appropriate adjustments for curvature effects. The thermal mass of a pipe wall is different from a thick plate, which can affect the cooling rate and HAZ properties.
- Quality assurance: The hardness profile measurement should be included in the quality assurance plan for surfacing repairs on Hardox400 components. A hardness survey across the weld and HAZ can verify that the process parameters were appropriate and that the repair maintains structural integrity.
Key Reflections and Study Insights
The most significant finding of this study is the confirmation that CO2 gas shielded welding of Hardox400 does not cause grain coarsening in the HAZ. This is a reassuring result for field repair engineers, as it indicates that the process is metallurgically compatible with the base material. The absence of grain coarsening is likely due to the relatively low heat input of the CO2 gas shielded process and the short time at elevated temperatures in the HAZ.
The hardness distribution data provides a quantitative basis for evaluating the impact of surfacing repair on base metal properties. The 5 mm maximum affected zone width is manageable for most pipe and fitting applications, where wall thicknesses are typically 6-25 mm. For thin-walled pipes (6-10 mm), the affected zone may represent a significant fraction of the wall thickness, and additional care in parameter selection is warranted.
The compatibility between the weld microstructure and the base microstructure is another critical finding. The martensite + Fe2B weld structure integrates well with the base martensitic structure, preventing the formation of brittle phases at the interface. This metallurgical compatibility is essential for preventing spalling of the surfacing layer under service loading.
This study, together with the companion paper on flux-cored wire development (Topic 2), provides a comprehensive technical basis for Hardox400 surfacing repair. Together, they address both the consumable selection and the process effects on base metal properties, forming a complete engineering knowledge package for field application.