Zhuojin Pipe Fitting Co., LtdEffect of Preheating on Cement Horizontal Roller Grinding Roller Overlay Hardness and Cracking Tendency
Research Background and Problem Statement
This study by Zheng Shiqing et al. from Tsinghua University and Jiangsu Kehang Environmental Engineering Technology Co., Ltd., published in Welding (2010, Issue 7, pp. 46-49), examines the influence of preheating temperature on the hardness and cracking susceptibility of overlay weld layers applied to cement horizontal roller grinding rollers using QG-series electrodes. Cement grinding rollers are critical components in the cement industry that undergo severe abrasive wear from grinding cement clinker and raw materials. When these rollers wear beyond acceptable limits, overlay welding provides an economical and effective repair method. However, both the base material and the welding electrodes have high carbon equivalents, creating significant challenges related to weldability, cracking, and hardness maintenance.
Experimental Design and Methodology
The researchers selected three preheating temperatures—200°C, 300°C, and 400°C—and two types of wear-resistant overlay electrodes (QG-3 and QG-4) to systematically evaluate the effects of preheating on overlay weld quality. The experimental methodology included:
- Cracking Assessment: Penetrant testing (PT) was used to detect surface and near-surface cracks in the overlay layers.
- Hardness Measurement: Macroscopic hardness was measured across the overlay weld layers.
- Microstructural Analysis: Optical microscopy was employed to examine the microstructure evolution at different preheating temperatures.
| Preheating Temperature | QG-3 Cracking | QG-4 Cracking | QG-3 Hardness Trend | QG-4 Hardness Trend |
|---|---|---|---|---|
| 200°C | Cracks present | No cracks | Higher hardness | Higher hardness |
| 300°C | No cracks | No cracks | Moderate hardness | Moderate hardness |
| 400°C | No cracks | No cracks | Lower hardness | Lower hardness |
Core Findings and Metallurgical Interpretation
The study revealed a clear and important relationship between preheating temperature and overlay weld quality. For QG-3 working layers, preheating to 300°C or above eliminated cracking, while QG-4 working layers required only 200°C preheating to prevent cracks. This difference in cracking susceptibility between the two electrode types can be attributed to their different chemical compositions and carbon equivalent values. QG-3 electrodes likely have a higher carbon equivalent than QG-4, resulting in greater cold cracking susceptibility and requiring higher preheating temperatures.
The hardness measurements showed that increasing preheating temperature from 200°C to 400°C resulted in a consistent decrease in macroscopic hardness for both QG-3 and QG-4 overlay layers. The metallographic analysis provided the metallurgical explanation: higher preheating temperatures reduce the cooling rate of the weld metal, which leads to a reduction in the volume fraction of hard phases. Specifically, for QG-3 layers, the amount of (Fe,Cr)₇C₃ carbides decreased, while for QG-4 layers, the amount of martensite was reduced. Both of these phases are responsible for the high hardness of the overlay layers.
Preheating Temperature Optimization Framework
The challenge for engineers is to find the optimal preheating temperature that balances crack prevention against hardness maintenance. The following decision framework can be applied:
- Calculate Carbon Equivalent: Determine CE values for both the base material and the welding electrode using the appropriate formula (e.g., CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15).
- Establish Minimum Preheating Temperature: Based on CE values and material thickness, determine the minimum preheating temperature required to prevent cold cracking. For high-CE materials, preheating of 200-400°C is typically required.
- Verify Through Penetrant Testing: Always confirm crack-free welds through NDT (PT or MT) before proceeding with production.
- Measure Hardness: Verify that the achieved hardness meets the minimum specification for the intended service condition.
Engineering Practice Integration and Recommendations
For cement grinding roller repair applications, this research provides several practical recommendations. First, the choice between QG-3 and QG-4 electrodes should be based on the specific service conditions—QG-3 with its higher carbide content may be more suitable for severe abrasive wear, while QG-4 may be preferred for applications where toughness is more critical. Second, the post-weld cooling rate should be carefully controlled, preferably by wrapping the weld area with insulating blankets or by allowing slow air cooling in a draft-free environment. Third, multi-layer welding strategies should be considered, where a lower-carbon transition layer is deposited first, followed by the functional overlay layer, to reduce residual stress and improve crack resistance.
The interplay between preheating temperature, cooling rate, and microstructure evolution represents a fundamental challenge in overlay welding of high-carbon materials. Engineers must adopt a systematic approach that considers the entire thermal cycle rather than focusing on individual parameters in isolation. The findings from this study reinforce the importance of process qualification testing before implementing overlay welding procedures in production environments, particularly for critical components where failure could result in significant downtime and economic losses.