CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of H3Cr5WMoV Submerged Arc Weld Overlay Alloy Layer
Literature Overview
This 2002 study by Liu Shuofeng from the Technical Center of Nanjing Iron and Steel Co., Ltd., published in the Journal of Anhui University of Technology (Natural Science Edition), investigates the microstructure and mechanical properties of H3Cr5WMoV submerged arc weld (SAW) overlay alloy layers. The H3Cr5WMoV steel is a martensitic heat-resistant steel containing approximately 3% Cr, 5% W, and 1% Mo, widely used in power plant applications including boiler tubes, steam pipes, and pressure vessel components operating at elevated temperatures. The study examines the weld overlay layer composition, microstructure evolution, hardness distribution, and high-temperature creep resistance of the deposited alloy.
Material System and Application Context
The H3Cr5WMoV steel system is specifically designed for high-temperature service in the range of 550–650 °C, where it provides adequate creep strength, thermal stability, and resistance to oxidation and corrosion. The key alloying elements and their roles are:
| Element | Content (wt%) | Primary Role |
|---|---|---|
| Cr | 3.0–3.5 | Solid solution strengthening, oxidation resistance |
| W | 4.5–5.5 | Carbide precipitation strengthening, creep resistance |
| Mo | 0.9–1.1 | Solid solution strengthening, creep resistance |
| C | 0.35–0.45 | Carbide formation, hardenability |
| V | 0.15–0.30 | Fine carbide precipitation, creep resistance |
| Fe | Balance | Base matrix |
The application of SAW overlay using H3Cr5WMoV filler metal is typically employed for:
- Repair of erosion damage on boiler tube surfaces
- Restoration of dimensional accuracy on worn components
- Addition of heat-resistant overlay layers on carbon steel pressure vessel components
- Manufacturing of bimetallic components where a heat-resistant surface is required on a structural steel base
Microstructural Characteristics of the Overlay Layer
The microstructure of the H3Cr5WMoV SAW overlay deposit is characterized by several key features:
- Martensitic matrix: The rapid solidification during welding produces a predominantly martensitic microstructure. The high carbon content (0.35–0.45%) and alloying elements suppress austenite stability, resulting in a fully martensitic structure upon air cooling.
- Carbide precipitation: Multiple carbide types are present:
- MC carbides (VC, WC): Extremely hard (HV > 2500), spherical, formed preferentially at grain boundaries
- M6C carbides (Fe3W3C): Plate-like, formed during tempering
- M23C6 carbides: Formed at grain boundaries during high-temperature exposure
- Grain structure: The overlay exhibits a columnar grain structure in the direction of heat flow, with grain sizes typically in the range of 20–50 µm. The grain structure is finer in the lower layers (near the base metal) due to the higher cooling rate.
- Segregation: Microsegregation of alloying elements (particularly C, Cr, W) occurs within the dendritic structure, creating local compositional variations that affect local hardness and corrosion resistance.
Mechanical Properties and Heat Treatment Effects
The mechanical properties of the H3Cr5WMoV overlay are strongly dependent on the heat treatment condition:
| Condition | Hardness (HRC) | Tensile Strength (MPa) | Impact Energy (J) | Grain Size |
|---|---|---|---|---|
| As-deposited (air cooled) | 45–52 | 800–950 | 15–25 | 25–50 µm |
| Tempered at 720 °C/2h | 38–42 | 750–850 | 35–50 | 25–50 µm |
| Tempered at 760 °C/2h | 35–38 | 700–800 | 45–60 | 30–55 µm |
| Normalized + Tempered | 40–44 | 800–900 | 30–45 | 15–30 µm |
The tempering treatment is essential for the following reasons:
- Residual stress relief: As-deposited martensitic overlay layers contain high residual stresses that can cause cracking during subsequent service or machining.
- Toughness improvement: Tempering transforms the brittle martensitic structure to tempered martensite with significantly improved fracture toughness.
- Creep resistance optimization: The tempering process promotes the formation of fine, stable M6C and MC carbides that provide creep strengthening at elevated temperatures.
- Dimensional stability: Tempered microstructures exhibit less dimensional change during subsequent thermal cycling.
Process Parameters for SAW Overlay
The submerged arc welding process parameters for H3Cr5WMoV overlay are critical for achieving the desired microstructure and properties:
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Current | 300–500 A | Adequate penetration and deposition rate |
| Voltage | 28–36 V | Arc stability and heat input control |
| Welding speed | 150–300 mm/min | Cooling rate control (faster = finer grains) |
| Wire feed speed | 15–25 m/min | Matched to current |
| Flux coverage | Continuous, >3 mm thickness | Ensure proper shielding and slag formation |
| Preheat | 200–300 °C | Reduce cracking tendency |
| Interpass temperature | 200–300 °C | Control cooling rate between passes |
| Number of passes | 2–5 | Depending on required overlay thickness |
| Post-weld treatment | Temper at 720–760 °C for 2–4 h | Essential for property optimization |
Engineering Practice and Quality Considerations
In practical applications of H3Cr5WMoV SAW overlay, the following quality considerations are paramount:
- Cracking susceptibility: The high carbon and alloy content of H3Cr5WMoV creates a high susceptibility to cold cracking (hydrogen-induced) and hot cracking. Preheating, controlled interpass temperatures, and post-weld heat treatment are mandatory.
- Bond strength: The metallurgical bond between the overlay layer and the base material (typically carbon or low-alloy steel) must be verified by mechanical testing. Bond strength should exceed 90% of the base material's minimum tensile strength.
- Intergranular corrosion: The microsegregation of chromium at grain boundaries in the overlay layer can create localized susceptibility to intergranular corrosion. Proper tempering and, if necessary, solution treatment can mitigate this risk.
- Creep life prediction: For applications at elevated temperatures, the creep life of the overlay layer should be evaluated through long-term creep testing or extrapolation from short-term data.
Study Reflections
This study provides valuable data on the microstructure-property relationships of H3Cr5WMoV SAW overlay deposits, which is directly applicable to power plant and petrochemical industry applications where high-temperature overlay repair is required. The emphasis on the tempering treatment as a critical step for optimizing both toughness and creep resistance reflects the fundamental metallurgical understanding that as-deposited martensitic structures, while hard, are inherently brittle and unsuitable for long-term high-temperature service without appropriate heat treatment. Engineers working with H3Cr5WMoV overlay systems should ensure that their process specifications include mandatory post-weld tempering and that the tempering parameters are validated through mechanical property testing on production weld coupons. The systematic approach of this research—examining the overlay in both as-deposited and heat-treated conditions—provides a comprehensive understanding of the material behavior that can guide practical process development and quality assurance procedures.