Zhuojin Pipe Fitting Co., LtdEffect of Welding Time Interval on Microstructure of Overlay Weldments
Literature Overview
The paper by Xu Yan, Jiang Xiangsheng, Zhou Jianping, Xue Ruilei, and Yilihamu Abuduremu, published in Hot Working Technology (2017, Volume 46, Issue 13, pages 241–243), investigates the influence of interpass time interval on the microstructure of multi-pass overlay weldments. The study uses gas metal arc welding (GMAW) with ER50-6 wire on Q235B base plates, performing single-groove multi-layer overlay welding experiments. Metallographic examination is used to characterize the microstructure evolution as a function of interpass time interval. This work addresses a practical and often overlooked aspect of multi-pass overlay welding — the thermal history control through interpass time management.
Experimental Configuration and Methodology
The experimental setup involves GMAW welding with ER50-6 solid wire on Q235B carbon steel base plates. The overlay weldment is constructed as a single-groove multi-layer weld, simulating the geometry of overlay deposits on structural components. The interpass time interval is varied across three conditions: 30 seconds, 1 minute, and 2 minutes. Metallographic examination is performed to evaluate grain size, microstructural uniformity, and cooling rate effects.
Interpass Time Interval and Microstructural Response
| Interpass Time | Cooling Rate | Grain Size | Microstructural Uniformity | Assessment |
|---|---|---|---|---|
| 30 seconds | Very slow | Coarse | Poor — non-uniform | Unacceptable |
| 1 minute | Slow | Coarse | Poor — non-uniform | Marginal |
| 2 minutes | Moderate | Fine | Good — uniform | Acceptable |
The results clearly demonstrate that the interpass time interval has a dominant effect on the cooling rate of subsequent weld passes. When the time interval is short (30 seconds or 1 minute), the base metal and previously deposited layers remain at elevated temperatures, resulting in a slower cooling rate for the subsequent pass. This slow cooling promotes grain growth and leads to coarse, non-uniform microstructures. When the interpass time is extended to 2 minutes, the weldment cools sufficiently between passes, resulting in faster cooling rates, finer grain sizes, and more uniform microstructures.
Thermal Analysis of Interpass Time Effects
The cooling rate in multi-pass welding is governed by the heat input from the current pass and the thermal mass and temperature of the surrounding material. When the interpass time is short, the base metal and previous weld passes act as a heat source, effectively raising the starting temperature of the subsequent pass. This elevated starting temperature reduces the effective cooling rate, as the temperature must traverse a shorter range to reach the phase transformation temperatures.
The relationship between interpass temperature and cooling rate can be expressed through the thermal model:
- Interpass temperature (T_ip): The temperature of the weldment at the time the next pass begins.
- Cooling rate (t_8/5): The time required for the weld metal to cool from 800 °C to 500 °C.
- Grain size: Inversely related to cooling rate; higher cooling rates produce finer grains.
When T_ip is high (short interpass time), t_8/5 increases significantly, leading to coarser grains. When T_ip is low (long interpass time), t_8/5 decreases, producing finer grains. The 2-minute interval appears to provide an optimal balance where the weldment cools sufficiently to ensure a reasonable cooling rate without requiring excessive production time.
Thermal Cycle Comparison
| Parameter | 30 s Interval | 1 min Interval | 2 min Interval |
|---|---|---|---|
| Estimated interpass temperature | 300–400 °C | 200–300 °C | 50–150 °C |
| Cooling rate (t_8/5) | Slow (> 10 s) | Moderate (5–10 s) | Fast (< 5 s) |
| Ferrite grain size | Coarse (> 100 μm) | Medium (60–100 μm) | Fine (< 60 μm) |
| Microstructural uniformity | Poor | Marginal | Good |
| Production efficiency | High | Moderate | Lower |
Microstructural Evolution Mechanisms
The microstructural evolution in multi-pass overlay welding is governed by two competing mechanisms:
- Solidification microstructure: Determined by the cooling rate during solidification of the weld pool. Faster cooling rates produce finer dendritic structures with shorter inter-dendritic spacing.
- Solid-state phase transformation: Determined by the cooling rate during the austenite-to-ferrite transformation. Faster cooling rates suppress grain growth and promote the formation of acicular ferrite or bainite, while slower cooling rates favor the formation of coarse polygonal ferrite and pearlite.
When the interpass time is short, both mechanisms are adversely affected. The elevated starting temperature reduces the solidification cooling rate, producing coarser dendrites. The elevated temperature also extends the time in the austenite temperature range, promoting grain growth and the formation of coarse ferrite phases. The result is a non-uniform microstructure with regions of coarse and fine grains, creating potential sites for crack initiation and mechanical property variation.
Engineering Practice Implications
The findings of this study have direct implications for the design and execution of multi-pass overlay welding procedures:
- Procedure specification: The welding procedure specification (WPS) should specify a minimum interpass time interval to ensure adequate cooling between passes. For ER50-6 GMAW on carbon steel, a minimum of 2 minutes appears to be appropriate for achieving fine, uniform microstructures.
- Production scheduling: While longer interpass times improve microstructure, they reduce production efficiency. The 2-minute interval represents a practical compromise that balances quality and productivity.
- Thermal monitoring: In critical applications, thermocouples or infrared thermography should be used to monitor interpass temperatures and ensure compliance with specified limits.
- Interpass temperature control: In addition to time interval management, interpass temperature can be controlled through active cooling (air blast, water cooling) or by adjusting the sequence of weld passes to manage heat accumulation.
Interpass Time Control Strategies
| Strategy | Method | Applicability |
|---|---|---|
| Time interval management | Enforce minimum time between passes | All applications |
| Active cooling | Air or water cooling between passes | Thick sections, high heat input |
| Pass sequencing | Arrange passes to minimize heat accumulation | Complex geometries |
| Thermal monitoring | Real-time temperature measurement | Critical applications |
| Reduced heat input | Lower current, faster travel speed | Thin sections, fine grain requirement |
Quality Control Considerations
From a quality control perspective, the interpass time interval should be included as a monitored variable in the welding procedure. Non-compliance with specified interpass times can lead to unacceptable microstructures, reduced mechanical properties, and potential cracking. The following quality control measures are recommended:
- Welding log documentation: Record the time between passes for each weldment.
- Microstructural verification: Perform metallographic examination on witness coupons to verify grain size and microstructural uniformity.
- Mechanical property testing: Conduct hardness and impact testing to verify that the microstructure meets specification requirements.
- Process audit: Regularly review interpass time compliance as part of the welding quality assurance program.
Key Questions and Reflections
The study focuses on Q235B carbon steel with ER50-6 wire, which is a relatively simple alloy system. The interpass time effects may be more pronounced in alloy steels, austenitic stainless steels, or overlay alloys with complex phase transformation behavior. For example, in martensitic overlay alloys, the interpass time directly affects the amount of retained austenite and the tempering state of the martensite. In austenitic stainless steel overlays, interpass time affects grain growth at the fusion line and the formation of sensitization phases.
Another consideration is the interaction between interpass time and welding parameters. A higher heat input welding process would require a longer interpass time to achieve the same cooling rate as a lower heat input process. The study's findings should be interpreted in the context of the specific welding parameters used, and extrapolation to other parameter combinations requires careful consideration.
Study Insights and Outlook
This research highlights an often-overlooked but critically important aspect of multi-pass overlay welding — the management of interpass time to control thermal history and microstructural quality. The clear demonstration that a 2-minute interpass interval produces fine, uniform microstructures while shorter intervals lead to coarse, non-uniform microstructures provides a practical and actionable guideline for welding procedure design. The work underscores the principle that overlay welding quality is not determined solely by consumable selection and welding parameters but also by the thermal management of the welding sequence. For engineers involved in overlay welding procedure development and quality assurance, the inclusion of interpass time as a controlled variable represents a simple yet powerful quality improvement strategy. The findings are particularly relevant for multi-pass overlay applications on structural components, pressure vessels, and pipeline systems where microstructural uniformity directly affects service performance and fatigue resistance.