Zhuojin Pipe Fitting Co., LtdOptimization Design of Wear-Resistant and Crack-Resistant Overlay Electrodes
Literature Overview
The paper by Pei Haixu, Wang Sheng, and Yi Yinghui (Ordnance Materials and Engineering Science, Vol. 26, Issue 4, 2003, pp. 25–28) presents the development of a new overlay welding electrode with excellent wear resistance and crack resistance using the WMCAD (Welding Material Composition and Application Design) method. The electrode is designed for overlay welding of rolling mill rolls and offers the advantage of requiring no preheating before welding. This work is significant because it addresses two of the most challenging aspects of overlay welding electrode design: achieving high wear resistance while maintaining sufficient toughness to prevent cracking, and reducing the need for preheating to simplify the welding process and improve productivity.
Core Technical Points
The paper describes a systematic approach to the design and optimization of overlay welding electrodes:
1. WMCAD Methodology
The WMCAD method is a systematic approach to welding material design that considers the composition, microstructure, and mechanical properties of the weld metal in relation to the application requirements. The method involves:
- Defining the application requirements (wear resistance, crack resistance, no preheating)
- Selecting the base alloy system (high-carbon, high-chromium martensitic steel)
- Optimizing the alloy composition to achieve the desired properties
- Designing the electrode coating to control the welding process and weld metal composition
- Testing and validating the electrode through welding trials and wear tests
2. Electrode Composition Design
The electrode is designed as a high-carbon, high-chromium martensitic steel with the following approximate composition:
| Element | Content (wt%) | Function |
|---|---|---|
| C | 1.5–2.0 | Carbide formation, hardness |
| Cr | 12–16 | Carbide formation, oxidation resistance |
| Mo | 2–3 | Hardening, thermal stability |
| V | 0.5–1.0 | Fine carbide formation, wear resistance |
| Mn | 1.0–1.5 | Deoxidation, austenite stabilization |
| Si | 0.5–1.0 | Deoxidation |
| Fe | Balance | Base metal |
The high carbon and chromium content promotes the formation of hard M7C3 and M23C6 carbides, which provide excellent wear resistance. The addition of molybdenum and vanadium refines the carbide structure and improves thermal stability. The manganese and silicon serve as deoxidizers and help control the weld metal microstructure.
3. Electrode Coating Design
The electrode coating plays a critical role in controlling the welding process and the weld metal composition. The coating is designed to:
- Provide a stable arc and good slag coverage
- Control the dilution ratio between the base metal and the weld metal
- Add alloying elements to the weld metal
- Control the cooling rate of the weld metal
- Provide protection against atmospheric contamination
The coating composition typically includes iron powder, alloy powder (to add Cr, Mo, V to the weld metal), flux (to control the slag composition), and binders (to hold the coating on the electrode wire). The coating thickness and composition are optimized to achieve a stable arc, good slag fluidity, and a weld metal composition that meets the wear resistance and crack resistance requirements.
4. Welding Process Parameters
The electrode is designed for use with shielded metal arc welding (SMAW) process. The recommended welding parameters are:
| Parameter | Value |
|---|---|
| Electrode diameter | 3.2–4.0 mm |
| Current | 100–200 A |
| Voltage | 22–28 V |
| Travel speed | 200–400 mm/min |
| Preheating | Not required |
| Interpass temperature | ≤ 200 °C |
| Post-weld treatment | Tempering at 550–600 °C (recommended) |
The fact that the electrode does not require preheating is a significant advantage for field applications where preheating equipment may not be available. This is achieved by carefully controlling the alloy composition to minimize the hardenability of the weld metal and by using a coating that promotes a slower cooling rate.
Defect Analysis and Prevention
The paper addresses several potential defects that can occur when using the new overlay electrode:
1. Crack Resistance
The primary challenge in designing a wear-resistant overlay electrode is to achieve high hardness (which promotes wear resistance) while maintaining sufficient toughness (which prevents cracking). The authors address this challenge through several design strategies:
- Using a balanced alloy composition that promotes a tempered martensite microstructure rather than a fully hardened martensite
- Adding elements such as manganese and silicon to reduce the hardenability of the weld metal
- Using a coating that promotes a slower cooling rate, which allows for the formation of a more ductile microstructure
- Designing the electrode for use with a multi-pass technique, where the first pass acts as a transition layer and subsequent builds achieve the desired hardness
The crack resistance of the electrode is evaluated through several tests:
| Test Method | Purpose |
|---|---|
| Bend Test | Evaluate weld metal ductility |
| Transverse Tensile Test | Evaluate bond strength and weld metal strength |
| Microhardness Mapping | Evaluate hardness profile across the weld |
| Metallographic Examination | Evaluate microstructure and look for cracks |
| Wear Test (pin-on-disk) | Evaluate wear resistance |
2. Wear Resistance
The wear resistance of the overlay is evaluated through pin-on-disk wear tests and field trials on rolling mill rolls. The wear resistance is primarily governed by the hardness and microstructure of the weld metal. The authors report that the overlay hardness is in the range of 50–60 HRC, which provides good wear resistance for rolling mill applications. The microstructure consists of tempered martensite with dispersed M7C3 and M23C6 carbides, which provide a combination of hardness and toughness.
3. Dilution Control
The dilution ratio between the base metal and the weld metal is a critical factor in determining the final properties of the overlay. The authors recommend using a multi-pass technique with alternating directions to minimize the dilution effect. The first pass is designed to have a composition that is compatible with both the base metal and the subsequent overlay passes. The dilution ratio is monitored through chemical analysis of the weld metal.
Engineering Practice Integration
The practical significance of this paper lies in the development of an electrode that simplifies the overlay welding process by eliminating the need for preheating. In field applications, such as rolling mill maintenance, preheating equipment may not be readily available, and the time required for preheating and post-weld heat treatment can be a significant constraint. The no-preheat requirement of this electrode makes it suitable for field repair applications where rapid turnaround is essential.
From a quality assurance perspective, the following steps are recommended when using this electrode for overlay welding:
- Surface preparation: Clean the base surface to remove scale, rust, and contaminants. A surface roughness of Ra 10–25 μm is recommended.
- Welding: Use the recommended welding parameters. Apply the overlay in multiple passes, alternating the welding direction between passes.
- Post-weld treatment: Tempering at 550–600 °C is recommended to relieve residual stresses and improve toughness.
- Inspection: Perform visual inspection, hardness testing, and metallographic examination to verify the quality of the overlay.
- Wear testing: If possible, perform a pin-on-disk wear test on a coupon to verify the wear resistance of the overlay.
Study Insights and Implications
This paper provides a valuable example of systematic welding material design using the WMCAD methodology. The approach of balancing wear resistance and crack resistance through careful alloy composition design is applicable to many other overlay welding applications. The elimination of the preheating requirement is a significant practical advantage that can improve productivity and reduce costs in field applications. Engineers should note that the no-preheat capability of the electrode is achieved through careful control of the alloy composition and coating design, and that this capability should be verified through welding trials before use in critical applications. The paper's emphasis on multi-pass welding with alternating directions is a practical technique that can be applied to many overlay welding applications to minimize residual stresses and improve the quality of the overlay. Overall, this work serves as a useful reference for engineers involved in the design and application of overlay welding electrodes for wear-resistant applications.