CLADDING TECHNOLOGY SHANXI CO., LTDApplication of Pumice in Cr-Mo Series Medium-Hardness Weld Overlay Electrodes
Literature Overview
Pumice (海泡石, also known as sepiolite or attapulgite) is a hydrated magnesium silicate mineral with the chemical formula Mg₄Si₆O₁₂(OH)₂·8H₂O. Its application in weld overlay electrodes is an innovative approach to modifying the slag system and improving the mechanical properties of Cr-Mo series medium-hardness overlay weld metals. This literature explores the role of pumice as a slag-forming additive and its effects on weld metal microstructure, hardness, and overall performance.
Core Technical Content
Properties of Pumice and Its Role in Welding
Pumice has several unique properties that make it suitable for welding electrode applications:
- High specific surface area: Due to its fibrous structure, pumice provides a large surface area for chemical reactions during welding.
- Thermal stability: Pumice remains stable up to 1000 °C, decomposing gradually to release water vapor and magnesium oxide.
- Slag-forming capability: The silica and magnesium oxide content contributes to slag formation and modification.
- Desulfurization ability: The basic oxides in pumice can interact with sulfur in the weld metal.
The decomposition reactions of pumice during welding include:
- Mg₄Si₆O₁₂(OH)₂·8H₂O → 4MgO + 6SiO₂ + 9H₂O (at 800-1000 °C)
- The released MgO acts as a basic flux, while SiO₂ acts as an acidic flux.
Electrode Formulation and Slag System
The Cr-Mo series medium-hardness weld overlay electrode contains the following typical composition:
| Component | Content (wt%) | Function |
|---|---|---|
| Cr | 2.0-3.0 | Solid solution hardening, carbide formation |
| Mo | 0.5-1.0 | Precipitation hardening, high-temperature strength |
| C | 0.4-0.6 | Carbide formation, hardness |
| Mn | 1.0-1.5 | Deoxidation, grain refinement |
| Si | 0.3-0.5 | Deoxidation |
| Pumice | 3-8 (in flux) | Slag modification, desulfurization |
| Other flux components | Balance | Slag viscosity, protect molten pool |
The pumice is incorporated into the flux coating of the electrode, where it participates in slag formation during welding. The optimal pumice content in the flux is 5-6 wt%, which provides the best balance between slag fluidity, desulfurization, and weld metal cleanliness.
Effects on Weld Metal Properties
The addition of pumice to the flux system produces several beneficial effects on the weld metal:
Microstructure modification:
- Refinement of the grain structure from 80-100 μm to 50-70 μm
- Reduction of non-metallic inclusions from 2-3% to 0.5-1% area fraction
- Modification of inclusion morphology from angular to rounded
- Uniform distribution of Cr₇C₃ and Mo₂C carbides
Mechanical properties:
| Property | Without Pumice | With 5% Pumice | Improvement |
|---|---|---|---|
| Hardness (HV) | 380-400 | 410-430 | +8-10% |
| Impact energy (J/cm²) | 15-20 | 25-32 | +50-60% |
| Tensile strength (MPa) | 850-900 | 920-960 | +8-7% |
| Sulfur content (ppm) | 35-45 | 15-22 | -50% |
| Oxygen content (ppm) | 200-280 | 120-180 | -35% |
The improvement in impact toughness is particularly significant, as it addresses one of the major limitations of Cr-Mo hardfacing alloys. The reduction in sulfur and oxygen content is achieved through the desulfurization and deoxidation reactions facilitated by the MgO released from pumice decomposition.
Process Analysis and Quality Control
Welding Process Parameters
The overlay welds were deposited using shielded metal arc welding (SMAW) with the pumice-containing electrode. The process parameters were optimized to ensure proper slag coverage and adequate heat input.
| Parameter | Value |
|---|---|