Zhuojin Pipe Fitting Co., LtdMeasures to Prevent Hot Cracking in Overlay Welding of Continuous Casting Rolls
Literature Overview
This paper by Miao Hailiang from the Tianjin Second Metallurgical Machinery Repair Center, published in the journal Welding (1995, Issue 7, pp. 24), addresses a critical practical problem in the maintenance of continuous casting (CC) rolls. Continuous casting has become the backbone of modern steel production in China, offering significant advantages in steel yield, energy consumption reduction, and product quality. The CC roll, as a key spare part, directly determines casting productivity through its high-temperature strength and service life. The paper documents the author's experience with imported overlay welding technology from P.E. Company (USA) and Lincoln Electric equipment, highlighting the challenges encountered with hot cracking and the countermeasures developed to overcome them.
Background and Technical Context
Continuous casting rolls operate under extreme thermal cycling conditions, experiencing repeated heating and cooling during the solidification of molten steel. The rolls are typically made from 15CrMo and 30CrMo alloy steels, which possess good high-temperature strength and thermal fatigue resistance. When the roll surface is worn or damaged, overlay welding is the standard repair method to restore the functional surface. The original overlay welding process employed a two-layer approach:
| Layer | Wire Type | Diameter | Flux | Purpose |
|---|---|---|---|---|
| Base layer | Lincore 8620 (flux-cored) | 3.2 mm | Lincoln 802 | Transition layer between substrate and cap layer |
| Surface layer | Lincore 423Cr (flux-cored) | 3.2 mm | Lincoln 802 | Functional surface with high-temperature resistance |
The original welding parameters were set at I = 400–450 A, U = 30–32 V, and wire feed speed v = 120–130 mm/min. These parameters were established during the initial technology transfer from Lincoln Electric and were assumed to be optimal for the application.
Core Problem: Hot Cracking Mechanism
Hot cracking in overlay welding of CC rolls is primarily a solidification cracking phenomenon associated with the following metallurgical factors:
- Sulfur and phosphorus segregation: The 15CrMo and 30CrMo substrates, while generally low in S and P, can still contain localized concentrations at grain boundaries that promote liquid film formation during solidification.
- Wide solidification temperature range: The Cr-Mo alloy system combined with the overlay weld metal composition can create a mushy zone with a broad temperature interval, increasing susceptibility to cracking.
- Thermal stress concentration: The high thermal conductivity of the steel substrate coupled with the constrained geometry of the cylindrical roll surface creates significant拘束 stresses during cooling.
- Intermetallic compound formation: The interaction between the base metal and the overlay metal can produce brittle intermetallic phases at the interface.
The author identified that the original parameters, while producing acceptable welds in the laboratory, led to unacceptable rates of hot cracking in production due to the cumulative effect of thermal cycles and the actual operating conditions on the shop floor.
Countermeasures and Process Optimization
1. Preheating Strategy
The author recommended implementing a controlled preheating procedure to reduce the cooling rate and minimize thermal stress:
| Substrate Material | Preheat Temperature | Interpass Temperature | Purpose |
|---|---|---|---|
| 15CrMo | 200–250 °C | ≥ 150 °C | Reduce thermal gradient |
| 30CrMo | 250–300 °C | ≥ 200 °C | Reduce thermal gradient |
2. Parameter Adjustment
The optimized welding parameters were modified as follows:
| Parameter | Original Value | Optimized Value | Rationale |
|---|---|---|---|
| Current (I) | 400–450 A | 350–400 A | Reduce heat input per pass |
| Voltage (U) | 30–32 V | 28–30 V | Reduce arc energy |
| Wire feed speed (v) | 120–130 mm/min | 100–110 mm/min | Improve penetration control |
| Travel speed | Not specified | Increased by 15–20% | Reduce residence time |
3. Welding Sequence Optimization
A systematic welding sequence was established to minimize residual stress concentration:
- Divide the circumferential weld into multiple segments (typically 6–8 segments per revolution).
- Weld in a staggered pattern to distribute heat input uniformly.
- Apply a backing technique to prevent root cracking.
- Implement post-weld stress relief at 550–600 °C for 2 hours per 25 mm of weld thickness.
4. Flux Modification
The Lincoln 802 flux was supplemented with additional deoxidizers and sulfur/phosphorus scavengers. The addition of approximately 2–3 wt% of CaF₂ and 1–2 wt% of Al₂O₃ to the flux improved the fluidity of the slag and reduced the susceptibility to cracking.
Engineering Practice Insights
From a practical standpoint, this paper highlights several lessons that remain relevant to modern welding operations:
- Technology transfer limitations: The direct adoption of manufacturer-recommended parameters without local adaptation is a common source of quality problems. The paper demonstrates that parameters validated in the manufacturer's laboratory must be re-validated under actual production conditions.
- Cumulative thermal effects: In overlay welding of large components like CC rolls, the cumulative heat input from multiple passes creates a thermal history that significantly differs from single-pass laboratory tests.
- Importance of preheating: For Cr-Mo steels, preheating is not merely a procedural requirement but a metallurgical necessity to prevent both hot and cold cracking.
- Post-weld heat treatment: Stress relief treatment is essential for CC rolls because the operational thermal cycling will otherwise cause crack initiation at residual stress concentrations.
Key Questions and Reflections
The paper raises several questions that warrant further investigation:
- What is the optimal number of overlay layers for different wear conditions? The paper does not address the relationship between layer thickness and service life.
- How does the microstructure of the overlay weld evolve during thermal cycling in service? Long-term microstructural stability is critical for reliable operation.
- Can modern welding consumables (e.g., low-sulfur, low-phosphorus flux-cored wires) eliminate the need for the extensive parameter optimization described in this paper?
Study Insights and Implications
This 1995 paper, while addressing a specific industrial problem, contains fundamental welding metallurgy principles that transcend its original context. The systematic approach to diagnosing and solving hot cracking problems—through parameter optimization, sequence design, and flux modification—represents a classic PDCA (Plan-Do-Check-Act) methodology in welding process development. For contemporary engineers working with modern welding technologies, the paper serves as a reminder that the fundamental metallurgical challenges of overlay welding have not changed, even as equipment and consumables have advanced. The emphasis on preheating, controlled cooling, and stress relief remains as relevant today as it was in 1995. This paper is particularly valuable for engineers in the steel industry who are responsible for the maintenance of critical casting equipment, as it provides a practical framework for troubleshooting and improving overlay welding processes under production constraints.