CLADDING TECHNOLOGY SHANXI CO., LTDHardness and Microstructure of Hardox400 Wear-Resistant Plate Cladding Weld Joint
Literature Overview
This companion study, also published in Welding in 2013 by the same research group from Beijing University of Technology, focuses specifically on the hardness distribution and microstructural characteristics of the weld joint formed during cladding of Hardox400 wear-resistant plate. While the previous study (Topic 3) addressed the development of the flux-cored wire, this paper provides a detailed analysis of the weld joint microstructure and hardness profile, which is critical for understanding the mechanical behavior and service performance of the cladded component.
Core Technical Content
The study investigates the weld joint formed between the Hardox400 substrate and the cladding layer deposited using the previously developed flux-cored wire. The focus is on the hardness distribution across the weld joint, including the substrate, heat-affected zone (HAZ), fusion zone, and cladding layer, as well as the microstructural evolution in each region.
Hardness Distribution
The hardness profile across the weld joint was measured using Vickers microhardness testing with a 1 kgf load and 100 μm indentation spacing. The results are summarized as follows:
| Region | Hardness (HV) | Microstructure |
|---|---|---|
| Hardox400 substrate | 380–400 HV | Bainite + retained austenite |
| HAZ (near fusion line) | 420–450 HV | Tempered martensite + fine grains |
| Fusion zone | 390–420 HV | Martensite + retained austenite + carbides |
| Cladding layer (near fusion line) | 380–410 HV | Martensite + retained austenite + M7C3 carbides |
| Cladding layer (surface) | 370–400 HV | Martensite + retained austenite + M23C6 carbides |
The hardness distribution reveals a peak hardness in the HAZ, which is approximately 10–15% higher than the substrate hardness. This is attributed to the tempering of the martensite in the HAZ, which results in a finer microstructure and higher hardness. The fusion zone and cladding layer exhibit hardness values that closely match the substrate, ensuring uniform wear resistance across the component.
Microstructural Analysis
Substrate (Hardox400)
The Hardox400 substrate exhibits a typical microstructure of high-strength wear-resistant steel, consisting of:
- Bainite: The primary phase, with a hardness of 350–380 HV.
- Retained austenite: Present in quantities of 10–20%, contributing to toughness and crack resistance.
- Carbides: M7C3 and M23C6 carbides, dispersed in the bainitic matrix.
Heat-Affected Zone (HAZ)
The HAZ exhibits a gradient of microstructures depending on the peak temperature experienced during welding:
- Coarse grain zone (CGZ): Peak temperatures above 1200 °C result in grain coarsening and the formation of tempered martensite with a hardness of 420–450 HV.
- Fine grain zone (FGZ): Peak temperatures of 900–1200 °C result in grain refinement and the formation of tempered martensite with a hardness of 400–430 HV.
- Intercritical zone: Peak temperatures of 700–900 °C result in partial recrystallization and the formation of a mixed microstructure with a hardness of 380–410 HV.
Fusion Zone
The fusion zone exhibits a microstructure similar to the cladding layer, consisting of:
- Martensite: The primary phase, with a hardness of 380–410 HV.
- Retained austenite: Present in quantities of 5–15%, contributing to toughness.
- Carbides: M7C3 carbides, dispersed in the martensitic matrix.
Cladding Layer
The cladding layer exhibits a microstructure similar to the fusion zone, with a slight variation in carbide distribution:
- Near the fusion line: M7C3 carbides are more prevalent, resulting in a slightly higher hardness of 390–410 HV.
- Surface: M23C6 carbides are more prevalent, resulting in a slightly lower hardness of 370–400 HV.
Standards and Specification Considerations
The evaluation of the weld joint hardness and microstructure must comply with the following standards:
| Standard | Applicability |
|---|---|
| GB/T 150 | Pressure vessel design and fabrication |
| NB/T 47014 | Welding procedure qualification |
| ASME IX | Welding procedure qualification |
| ASTM E92 | Rockwell hardness testing |
| ASTM E384 | Vickers microhardness testing |
| GB/T 229 | Charpy impact testing |
The hardness distribution across the weld joint must be measured in accordance with ASTM E384 or GB/T 230.2, with indentations spaced at intervals of 3–5 times the indentation diagonal length to avoid interaction effects.
Defect Analysis and Countermeasures
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Cold cracking in HAZ | High carbon equivalent; hydrogen absorption; high cooling rate | Preheat to 200 °C; use low-hydrogen consumables; control interpass temperature |
| Excessive HAZ hardness | High cooling rate; inadequate tempering | Increase preheat temperature; apply post-weld heat treatment |
| Hardness mismatch | Inappropriate consumable composition | Optimize consumable composition to match substrate hardness |
| Poor fusion | Insufficient heat input; inadequate technique | Increase arc current; reduce travel speed; improve technique |
| Cracking in cladding layer | Excessive carbon content; brittle carbide networks | Control cooling rate; optimize consumable composition |
The most critical defect to prevent is cold cracking in the HAZ, which is facilitated by the high carbon equivalent of Hardox400 and the high cooling rates associated with welding. The study recommends preheating to 200 °C and applying a post-weld heat treatment (PWHT) at 550–600 °C for 1–2 hours per 25 mm of thickness to reduce the HAZ hardness and improve toughness.
Engineering Practice Integration
The understanding of the weld joint hardness and microstructure is critical for the following engineering applications:
- Wear-resistant components: The uniform hardness distribution across the weld joint ensures uniform wear behavior, preventing premature failure due to differential wear.
- Pressure vessel components: The weld joint must be qualified in accordance with NB/T 47014, including mechanical testing and metallographic examination, to ensure the integrity of the cladded component.
- Repair and maintenance: The knowledge of the weld joint microstructure allows for the selection of appropriate repair procedures and consumables to restore the wear resistance of the component.
- Functionally graded structures: The understanding of the microstructural gradient across the weld joint allows for the design of functionally graded structures with tailored properties in different regions.
Study Insights and Implications
This study provides valuable insights into the weld joint characteristics of Hardox400 cladding applications. The hardness distribution reveals a peak hardness in the HAZ, which is a common phenomenon in welding of high-carbon equivalent steels. The peak hardness in the HAZ can be beneficial for wear resistance but may compromise the toughness and crack resistance of the weld joint.
The study demonstrates that the cladding layer hardness can be closely matched to the substrate hardness through careful consumable design and process control. This hardness matching is critical for ensuring uniform wear behavior across the composite component and preventing premature failure due to differential wear.
However, the study also highlights the challenge of controlling the HAZ hardness. The peak hardness in the HAZ can be reduced through preheating and post-weld heat treatment, but these measures may compromise the wear resistance of the component. Engineers must carefully balance the requirements for wear resistance and toughness when designing and fabricating cladded Hardox400 components.
In conclusion, the study provides a comprehensive understanding of the weld joint hardness and microstructure in Hardox400 cladding applications, offering practical guidance for the design and fabrication of composite wear-resistant components with uniform hardness and adequate toughness.