CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding Repair and Heat Treatment of Channel Steel Rollers
Literature Overview and Technical Background
Channel steel rollers, used extensively in steel rolling mills as guide rollers and backup rollers, are subjected to extreme contact stresses, sliding friction, and thermal cycling. When the working surface of these rollers is worn or damaged, complete replacement is often economically unjustifiable, and overlay welding repair followed by appropriate heat treatment becomes the preferred restoration method. The literature under study examines the overlay welding repair of channel steel rollers, focusing on the selection of overlay materials, welding procedure design, and the critical role of post-weld heat treatment in achieving the required mechanical properties and service life.
The base material of the channel steel rollers is typically 45 steel or 50CrMo, quenched and tempered to a hardness of 28–35 HRC. The overlay layer must achieve a hardness exceeding 45 HRC to resist wear while maintaining sufficient toughness to avoid brittle fracture under impact loading. The challenge lies in balancing hardness and toughness in the overlay, controlling the heat-affected zone (HAZ) to prevent cracking, and ensuring that the post-weld heat treatment does not soften the overlay layer excessively.
Overlay Material Selection and Welding Procedure
The literature evaluates several overlay material options for the channel steel rollers:
| Overlay Material | Hardness (as-welded) | Hardness (after HTT) | Dilution Sensitivity | Cost |
|---|---|---|---|---|
| D2 tool steel (solid wire) | 58–62 HRC | 50–55 HRC | Low | Moderate |
| Cr12MoV (solid wire) | 55–60 HRC | 48–52 HRC | Moderate | Moderate |
| M2 high-speed steel (strip) | 62–65 HRC | 55–58 HRC | Low | High |
| Custom high-Cr alloy (FCAW) | 55–60 HRC | 48–53 HRC | Moderate | Low |
The selected approach in the literature was a two-layer overlay system: a transition layer of D2 tool steel deposited by GTAW to ensure a crack-free bond to the base metal, followed by a wear layer of M2 high-speed steel deposited by SAW or FCAW for high deposition efficiency. This two-layer approach reduces the dilution effect on the wear layer while providing a compatible metallurgical transition zone.
The welding procedure parameters were optimized as follows:
| Parameter | Transition Layer (GTAW) | Wear Layer (SAW/FCAW) |
|---|---|---|
| Current | 80–120 A | 350–450 A |
| Voltage | 12–16 V | 28–34 V |
| Travel speed | 100–150 mm/min | 150–250 mm/min |
| Preheat | 200–250°C | 200–250°C |
| Interpass temperature | ≤300°C | ≤300°C |
| Shielding gas | Ar (99.99%) | Flux (SAW) or CO₂ (FCAW) |
Post-Weld Heat Treatment Design
The post-weld heat treatment (PWHT) is the most critical aspect of the repair process, as it determines the final hardness, toughness, and residual stress state of the overlay and HAZ. The literature describes a two-stage PWHT cycle:
| Stage | Temperature | Holding Time | Cooling Method |
|---|---|---|---|
| Stress relief | 600°C | 2 h per 25 mm thickness | Furnace cool to 400°C, then air cool |
| Tempering | 550°C | 2 h per 25 mm thickness | Furnace cool to 300°C, then air cool |
The stress relief stage at 600°C reduces residual stresses from welding below 100 MPa, which is critical for preventing delayed cracking in the HAZ. The tempering stage at 550°C achieves a tempered martensite structure in the overlay layer with a hardness of 48–53 HRC, while simultaneously tempering the HAZ to a consistent hardness profile that avoids hard, brittle regions adjacent to the weld.
A key insight from the literature is that the PWHT temperature must be carefully controlled to avoid softening the M2 high-speed steel wear layer below 45 HRC. M2 steel retains its hardness well up to 600°C due to the presence of vanadium and tungsten carbides, but excessive holding time at high temperatures can lead to coarsening of secondary carbides and a drop in hardness. The recommended maximum PWHT temperature for M2 overlay is 620°C, with a holding time not exceeding 3 hours per 25 mm of total component thickness.
Defect Analysis and Quality Control
The following defects were identified during the repair process:
| Defect | Detection Method | Root Cause | Prevention |
|---|---|---|---|
| HAZ cracking | MT + UT | High carbon equivalent of base metal + excessive cooling rate | Increase preheat; reduce interpass temperature |
| Overlay cracking | MT | High dilution in single-pass welding | Use transition layer; limit single-pass width |
| Insufficient bond | UT (contact method) | Incomplete melting of base metal | Increase current; add edge pass |
| Hardness below specification | Hardness test | Over-tempering during PWHT | Limit PWHT temperature and time |
The quality control plan included 100% magnetic particle inspection (MT) of the overlay surface, 10% ultrasonic testing (UT) of the weld zone for bond defects, and hardness profiling across the overlay thickness at three locations per roller. The acceptance criteria required overlay hardness ≥45 HRC, HAZ hardness ≤35 HRC, and no cracks or lack of bond detected by MT or UT.
Engineering Practice Insights
The literature highlights several practical considerations that are often overlooked in theoretical discussions of overlay welding repair. First, the geometry of the channel steel roller creates a confined weld access, which limits the number of passes that can be deposited and increases the risk of incomplete melting at the root of the overlay. The use of a GTAW transition layer with a narrow, deep penetration profile was found to be essential for achieving full bond in the confined geometry.
Second, the PWHT of large rollers (typically 500–800 mm in diameter) presents thermal gradient challenges. The literature recommends using a furnace with a temperature uniformity of ±10°C and a maximum heating rate of 150°C/h for the first 200°C, followed by 100°C/h for the remainder of the heating cycle. This controlled heating rate prevents thermal shock cracking in the base metal.
Third, the literature emphasizes the importance of dimensional control during the repair process. The overlay welding adds material to the roller surface, and the subsequent grinding to restore the original diameter must be planned carefully to avoid removing more than 2 mm of the wear layer. A minimum wear layer thickness of 3 mm after grinding is recommended to ensure adequate service life.
Summary and Conclusions
The overlay welding repair and heat treatment of channel steel rollers is a technically demanding process that requires careful coordination of consumable selection, welding procedure design, and post-weld heat treatment. The two-layer overlay system with a D2 transition layer and M2 wear layer provides an effective solution for achieving high hardness with good bond integrity in a confined geometry. The PWHT cycle at 550°C is critical for achieving the target hardness of 48–53 HRC while controlling residual stresses and preventing cracking. The quality control plan, combining MT, UT, and hardness profiling, ensures that the repaired rollers meet the required performance criteria. This work demonstrates that overlay welding repair can extend the service life of channel steel rollers by 2 to 3 cycles compared to the original roller, providing significant economic benefits in rolling mill operations.