Zhuojin Pipe Fitting Co., LtdMicrostructure and Hardness Gradient Analysis in Gradient Surfacing on 45 Steel Substrate
Literature Overview
The paper by Zhu Chengjun and Li Sicheng (Hot Working Technology, Vol. 44, No. 9, 2015, pp. 246–248) presents a systematic study of gradient surfacing on 45 steel substrates using two different alloy systems. Funded by the Henan Industrial Vocational Technical College President's Fund (Project No. GYYJ20140021), this work addresses the critical engineering challenge of achieving a smooth property transition between a moderate-strength base metal and a high-hardness wear-resistant overlay. The study compares three configurations: substrate + Material 1, substrate + Material 2, and substrate + Material 1 + Material 2 (gradient approach).
Welding Material Characterization
| Material | Composition System | Typical Hardness (HV) | Primary Carbides | Matrix Phase |
|---|---|---|---|---|
| Material 1 | C-Cr-Mo-Ni | 300–350 | Cr₇C₃, Mo₂C | Martensite + retained austenite |
| Material 2 | Cr-Mo-Ni-W-V | 450–550 | WC, VC, Cr₇C₃ | Martensite + carbides |
| 45 Steel (substrate) | 0.42–0.50% C | 180–220 | Cementite (Fe₃C) | Pearlite + ferrite |
Material 1 serves as a transition layer with moderate hardness and good toughness, while Material 2 provides the high-hardness wear-resistant surface. The gradient approach combines both to create a multi-layer structure with progressively increasing hardness from substrate to surface.
Microstructural Analysis at the Fusion Zone
The fusion zone microstructure is the critical determinant of bonding quality and crack resistance:
| Configuration | Fusion Zone Microstructure | Dilution Effect | Hardness Gradient |
|---|---|---|---|
| Substrate + Material 1 | Fine martensite + small carbides | Moderate (15–25%) | Smooth transition (220→350 HV) |
| Substrate + Material 2 | Coarse martensite + large carbides + decarburized zone | High (30–40%) | Abrupt transition (220→500 HV) |
| Substrate + Material 1 + Material 2 | Progressive refinement from base to surface | Controlled (10–20% per interface) | Gradual transition (220→350→500 HV) |
The key finding is that Material 1 (C-Cr-Mo-Ni system) produces a more favorable fusion zone than Material 2 (Cr-Mo-Ni-W-V system) due to:
- Lower dilution — The C-Cr-Mo-Ni composition is closer to 45 steel in terms of carbon equivalent, reducing the thermal shock at the fusion line.
- Reduced decarburization — Material 1 does not aggressively extract carbon from the base metal, preventing the formation of a soft, decarburized zone that would act as a crack initiation site.
- Lower carbon enrichment — Material 2 tends to deposit excess carbon in the heat-affected zone, creating a brittle, over-hard region susceptible to cracking.
- Reduced coarse grain embrittlement — The lower heat input associated with Material 1 deposition minimizes grain growth in the HAZ.
Hardness Gradient Distribution
The hardness profile from substrate surface to overlay surface is critical for determining the resistance to spalling and delamination:
- Substrate + Material 1 alone: Hardness increases from 220 HV (base) to 350 HV (surface) over approximately 3–5 mm. The gradient is approximately 30–40 HV/mm, which is mechanically acceptable for most applications.
- Substrate + Material 2 alone: Hardness jumps from 220 HV to 500 HV within 1–2 mm of the fusion line. The gradient exceeds 300 HV/mm, creating severe internal residual stresses that promote cracking and delamination.
- Substrate + Material 1 + Material 2 (gradient): Hardness transitions from 220 HV to 350 HV over the first 3–5 mm (Material 1 layer), then from 350 HV to 500 HV over the next 2–3 mm (Material 2 layer). The maximum gradient is limited to 50–70 HV/mm, well within acceptable limits for preventing mechanical failure.
Engineering Applications
This gradient surfacing approach is directly applicable to:
- Mold repair — Plastic injection molds and die-casting molds where 45 steel or similar substrates require surface hardening without compromising substrate toughness.
- Roll and shaft restoration — Rolling mill work rolls and machine tool spindles that need localized hardening of bearing surfaces.
- Mining equipment components — Bucket teeth, shovel edges, and conveyor rollers where a tough core with a hard surface is required.
- Pump and valve components — Wear surfaces that must resist both erosion and cavitation damage.
Process Recommendations
Based on the study findings, the following process parameters are recommended for gradient surfacing on 45 steel:
| Parameter | Material 1 (Transition Layer) | Material 2 (Wear Layer) |
|---|---|---|
| Process | GTAW or FCAW | FCAW or GMAW |
| Heat input | Low (≤ 1.5 kJ/mm) | Moderate (1.0–2.0 kJ/mm) |
| Wire diameter | 1.2–1.6 mm | 1.6–2.0 mm |
| Travel speed | 4–6 mm/s | 3–5 mm/s |
| Number of passes | 1–2 | 2–3 |
| Inter-pass temperature | ≤ 200 °C | ≤ 150 °C |
| Shielding gas | Ar + 5% CO₂ | Ar + 2% CO₂ |
Study Insights and Implications
This research validates the fundamental principle that property gradients must be managed in multi-layer surfacing systems. The abrupt hardness transition created by directly depositing a high-alloy wear material on a low-carbon substrate is a common cause of field failures in surfacing applications. The two-step gradient approach using a C-Cr-Mo-Ni transition layer followed by a Cr-Mo-Ni-W-V wear layer provides a practical and economical solution that significantly improves the service life of repaired components. The approach can be extended to three or more layers for applications requiring even more gradual transitions, such as cryogenic service or cyclic loading conditions. Engineers should note that the specific composition of the transition layer must be selected based on the base metal composition; for higher-alloy substrates (e.g., 42CrMo4), a different transition alloy may be more appropriate than the C-Cr-Mo-Ni system studied here.