CLADDING TECHNOLOGY SHANXI CO., LTDComparison of Laser Cladding and TIG Welding Microstructure and Properties on H13 Steel
Literature Overview
H13 steel (equivalent to AISI H13, 4Cr5MoSiV1) is a hot-work tool steel widely used in hot forging dies, extrusion dies, and casting molds. It combines high hardness (up to 50 HRC after heat treatment), excellent hot hardness, thermal fatigue resistance, and good toughness. When H13 components wear or crack in service, repair welding is essential to restore functionality. However, the repair of H13 is challenging due to its high carbon and alloy content, which leads to hard and brittle HAZ structures and a high susceptibility to cracking.
This study compares two repair welding methods: laser cladding and gas tungsten arc welding (GTAW/TIG). The comparison covers microstructure, hardness, mechanical properties, and service performance, providing guidance for method selection in engineering practice.
Core Technical Content
Process Fundamentals
Laser Cladding uses a high-power laser (typically 2–10 kW) to melt a thin layer of powder or wire onto the substrate. The process characteristics include:
- Extremely high energy density (10^6–10^8 W/cm²)
- Rapid heating and cooling rates (10^3–10^6 K/s)
- Minimal heat-affected zone (typically <0.5 mm)
- Low dilution with base metal (5–20%)
- Excellent metallurgical bonding
GTAW/TIG uses a non-consumable tungsten electrode and inert gas shielding to deposit filler metal. The process characteristics include:
- Moderate energy density (10^3–10^4 W/cm²)
- Lower cooling rates (10^2–10^3 K/s)
- Larger HAZ (1–3 mm)
- Higher dilution (15–40%)
- Good process control with experienced operators
Filler Material Selection
| Parameter | Laser Cladding (Powder) | GTAW (Wire) |
|---|---|---|
| Composition | H13 equivalent or Ni-based alloy | H13 equivalent or Ni-based alloy |
| C (%) | 0.35–0.45 | 0.35–0.45 |
| Cr (%) | 4.5–5.5 | 4.5–5.5 |
| Mo (%) | 1.0–1.5 | 1.0–1.5 |
| Si (%) | 0.20–0.40 | 0.20–0.40 |
| V (%) | 0.8–1.5 | 0.8–1.5 |
| Form | Powder (spherical, 75–150 μm) | Solid wire (1.0–1.6 mm diameter) |
| Cost | High (powder + laser system) | Moderate (wire + TIG equipment) |
Microstructure Comparison
Laser Cladding Microstructure
The laser cladding layer on H13 exhibits a distinctive microstructure due to the rapid solidification:
- Matrix — Fine martensite with some retained austenite. The rapid cooling suppresses pearlite and bainite formation, resulting in a predominantly martensitic structure.
- Carbides — Fine, uniformly distributed M2C and MC carbides (Cr, Mo, V carbides). The high cooling rate refines the carbide size to 0.1–0.5 μm, compared to 1–5 μm in the heat-treated base metal.
- Grain structure — Columnar dendrites perpendicular to the substrate surface, with grain size of 10–30 μm. The rapid solidification produces a fine, equiaxed structure in the first layer and columnar growth in subsequent layers.
- HAZ — Extremely narrow (<0.3 mm), with minimal grain coarsening. The laser's rapid heating and cooling limit thermal diffusion, preserving the base metal microstructure.
GTAW Microstructure
The GTAW weld overlay on H13 shows a more thermally affected structure:
- Matrix — Mixed martensite, bainite, and some retained austenite. The slower cooling rate allows bainite formation in addition to martensite.
- Carbides — Coarser M2C and MC carbides, with some M7C3 formation. The cooling rate is insufficient to fully suppress pearlite-type carbides.
- Grain structure — Coarse columnar dendrites with grain size of 50–150 μm. The slower cooling allows grain growth.
- HAZ — Wide (1–3 mm), with distinct sub-zones:
- Recrystallized zone (grain size 50–100 μm)
- Partial recrystallization zone (grain size 20–50 μm)
- Sub-critical zone (minimal grain growth)
Microstructural Comparison Summary
| Feature | Laser Cladding | GTAW |
|---|---|---|
| Matrix structure | Fine martensite + retained austenite | Martensite + bainite + retained austenite |
| Carbide size | 0.1–0.5 μm | 1–5 μm |
| Carbide type | M2C, MC (fine, uniform) | M2C, MC, M7C3 (coarser) |
| Grain size | 10–30 μm | 50–150 μm |
| HAZ width | <0.3 mm | 1–3 mm |
| Dilution | 5–20% | 15–40% |
| Cooling rate | 10^3–10^6 K/s | 10^2–10^3 K/s |
Mechanical Properties Comparison
Hardness
| Location | Laser Cladding | GTAW | Base Metal (Heat Treated) |
|---|---|---|---|
| Surface | 48–55 HRC | 42–48 HRC | 45–50 HRC |
| Mid-layer | 45–52 HRC | 38–45 HRC | 45–50 HRC |
| Near-BAZ | 42–48 HRC | 35–42 HRC | 45–50 HRC |
| HAZ | 40–45 HRC | 25–35 HRC | 45–50 HRC |
The laser cladding layer maintains hardness closer to the base metal throughout the overlay thickness, with minimal softening in the HAZ. The GTAW overlay shows significant HAZ softening (25–35 HRC), which is a critical concern for hot work applications where the HAZ is the weakest link.
Mechanical Properties
| Property | Laser Cladding | GTAW | Base Metal (Tempered) |
|---|---|---|---|
| Tensile strength (MPa) | 1200–1500 | 900–1200 | 1100–1300 |
| Yield strength (MPa) | 1000–1300 | 800–1000 | 950–1200 |
| Elongation (%) | 3–8 | 5–12 | 8–15 |
| Impact energy (CVN, 23°C) | 15–30 J | 25–50 J | 30–60 J |
| Hot hardness (500°C) | 42–48 HRC | 35–42 HRC | 40–45 HRC |
| Thermal fatigue life (cycles) | 1500–2500 | 800–1500 | 2000–3000 |
The laser cladding layer exhibits higher hardness and strength but lower ductility and impact toughness compared to GTAW. This is a direct consequence of the fine martensitic structure and high carbide density. The trade-off is that the laser cladding layer offers superior wear resistance and hot hardness but may be more susceptible to cracking under impact loading.
Defect Analysis
Laser Cladding Defects
- Cracking — Can occur due to high residual stress from rapid cooling. Countermeasures: use of multiple thin layers (0.3–0.5 mm per pass), preheating to 200–300°C, and post-weld tempering at 540–580°C.
- Porosity — Rare with laser cladding due to the inert gas atmosphere and rapid solidification. When present, it is typically caused by powder contamination or inadequate gas shielding.
- Delamination — Can occur if the base metal is not properly prepared. Countermeasures: thorough surface cleaning (grinding to bare metal), removal of oxide scale, and preheating.
GTAW Defects
- Cracking — More common than in laser cladding due to the wider HAZ and higher hydrogen absorption. Countermeasures: thorough preheating (300–400°C), low-hydrogen filler metal, and controlled cooling.
- Porosity — More frequent due to the larger weld pool and longer exposure to atmospheric contamination. Countermeasures: proper gas shielding (high-purity argon, 99.99%), proper gas flow rate (15–25 L/min), and back-gassing.
- Undercut — Common at the weld edges due to excessive current or travel speed. Countermeasures: reduce current by 10–15% or increase travel speed.
Process Parameter Comparison
| Parameter | Laser Cladding | GTAW |
|---|---|---|
| Power/Current | 2–10 kW (laser) | 100–200 A |
| Voltage | — | 15–22 V |
| Travel speed | 200–800 mm/min | 50–200 mm/min |
| Wire/powder feed | 50–200 g/min | 0.5–2.0 kg/h |
| Shielding gas | Ar (99.99%) | Ar (99.99%) |
| Gas flow rate | 5–10 L/min | 15–25 L/min |
| Preheat | 200–300°C | 300–400°C |
| Layer thickness | 0.3 |