Zhuojin Pipe Fitting Co., LtdMicrostructure and Property Analysis of SMAW Overlay Metal Deposited with CHR172 Electrode on 45 Steel Substrate
Literature Overview
This paper by Zhong Yu, Qu Jinshan, Chen Wenjing, Pan Quanxi, and Luo Chaoyu from Xihua University, published in Welding Technology (Vol. 36, No. 3, 2007, pp. 10–12), examines the microstructure and microhardness of overlay welds produced using CHR172 electrode via Shielded Metal Arc Welding (SMAW) on 45 steel (C45/45# steel) substrate. The study systematically investigates the influence of welding heat input and alloying elements on the deposited metal's microstructure, hard phase morphology, and mechanical properties.
Core Technical Content
Experimental Configuration
CHR172 is a cobalt-chromium alloy overlay electrode designed for high-temperature wear and corrosion resistance applications. The substrate 45 steel is a medium-carbon steel with approximately 0.42–0.50% C, commonly used in structural and mechanical applications. The SMAW process was conducted under varying welding parameters to produce different heat inputs.
| Welding Parameter | Low Heat Input | High Heat Input |
|---|---|---|
| Current (A) | Lower range | Higher range |
| Arc voltage (V) | Lower range | Higher range |
| Travel speed (mm/min) | Higher | Lower |
| Heat input (kJ/mm) | Low | High |
Microstructural Observations
The overlay metal microstructure consists of a matrix phase with dispersed hard particles. The specific morphology and distribution of hard phases vary with welding conditions:
- Low heat input: Finer grain structure with smaller, more uniformly distributed hard particles. The rapid cooling rate promotes finer microstructural features.
- High heat input: Coarser grain structure with larger hard particles that may exhibit agglomeration. The slower cooling allows more time for particle growth and coarsening.
Microhardness Distribution
Microhardness measurements across the overlay layer reveal a gradient from the fusion boundary to the surface:
- Near the fusion boundary: Lower hardness due to higher dilution and different solidification conditions.
- Mid-layer: Moderate hardness with well-dispersed hard phases.
- Surface layer: Highest hardness, attributable to the highest cooling rate and finest microstructure.
Alloy Element Effects on Microstructure
The alloy composition of CHR172 includes significant amounts of Cr, Co, Mo, and possibly W, which form carbide phases such as M7C3, M23C6, and possibly MC-type carbides. The study demonstrates that:
- Chromium content: Promotes the formation of Cr-rich carbides (M23C6, M7C3), which serve as wear-resistant hard phases.
- Cobalt content: Enhances the strength of the matrix phase through solid solution strengthening and improves high-temperature stability.
- Molybdenum content: Contributes to secondary hardening through Mo-rich carbides and increases the thermal stability of the microstructure.
Hard Phase-Matrix Interaction
The study highlights that the bonding quality between hard particles and the matrix phase is critical for overall wear resistance. Poorly bonded particles can act as crack initiation sites under load, leading to premature material loss. The welding heat input affects this bonding through its influence on solidification rate and grain boundary morphology.
Engineering Practice Integration
Heat Input Control for Optimal Performance
For field applications requiring high wear resistance from CHR172 overlay welds, the following heat input control strategies are recommended:
| Objective | Recommended Heat Input | Rationale |
|---|---|---|
| Maximum surface hardness | Low (1.0–1.5 kJ/mm) | Fine grains, uniform hard phases |
| Maximum fatigue resistance | Moderate (1.5–2.5 kJ/mm) | Balanced microstructure |
| Maximum dilution resistance | Low to moderate | Reduced substrate influence |
Multi-Pass Strategy
In practice, achieving consistent overlay properties requires multi-pass welding with careful interpass temperature control. Each subsequent pass re-therms the previous layer, potentially coarsening the microstructure. The optimal number of passes balances reinforcement requirements with microstructural degradation.
Critical Reflections
The paper provides valuable qualitative insights into the microstructure-property relationships of CHR172 overlay welds. However, several aspects merit further investigation:
- The absence of quantitative carbide size distribution data limits the ability to correlate specific carbide morphology with measured hardness values.
- The study does not address the effect of welding sequence and joint geometry on residual stress distribution, which is critical for fatigue applications.
- The dilution rate at the fusion boundary is not quantified, making it difficult to assess the transition zone properties in terms of crack susceptibility.
Reference Value and Outlook
This work reinforces the fundamental principle that welding heat input is a primary lever for controlling overlay microstructure and properties. For engineers specifying SMAW overlay procedures for wear-critical components, the findings support the use of controlled low heat input to achieve optimal hard phase distribution. The CHR172 electrode system, when properly applied, offers a reliable solution for high-temperature wear protection on carbon steel substrates. Future studies should incorporate in-service wear testing and long-term thermal stability evaluation to complete the performance characterization.