Zhuojin Pipe Fitting Co., LtdManual Arc Surfacing of 75CrMo Rolling Mill Roll with Surface Defects
Literature Overview
This paper by Hao Huimin (1996), published in Welding (No. 12), documents a practical repair case involving the manual arc surfacing (SMAW) of a 75CrMo rolling mill roll that suffered from significant surface scale defects. The work was conducted at Taiyuan Mining Machinery Factory, a major manufacturer of mining and metallurgical equipment. The case study provides valuable insights into the assessment, preparation, and execution of surfacing repairs on critical rolling mill components.
Background and Problem Description
The rolling mill roll in question was a bar rolling mill roll with a nominal diameter of 500 mm. During machining, it was discovered that the blank had insufficient dimensions, resulting in a section of 269 mm diameter that retained extensive continuous black scale (mill scale). The scale characteristics were:
| Defect Characteristic | Measurement |
|---|---|
| Scale shape | Willow-leaf pattern (elongated) |
| Scale length | Nearly full circumference |
| Maximum scale width | 115 mm |
| Maximum scale depth | 0.6 mm below surface |
| Affected diameter | 269 mm section |
The initial response was to proceed with machining the remainder of the roll, but this approach was recognized as inadequate because the scale defect would propagate through the finished surface, creating a critical quality issue for a component subjected to extreme contact stresses and thermal cycling during rolling operations.
Repair Strategy and Technical Assessment
The decision to attempt repair through manual arc surfacing was driven by economic considerations: the roll blank represented significant material and machining investment, and replacing it would cause unacceptable production delays. The repair strategy involved:
- Defect assessment: Determining that the scale depth (0.6 mm) was within acceptable limits for repair by surfacing, as the scale could be removed by grinding and the underlying base metal was sound.
- Process selection: SMAW was chosen over submerged arc welding due to the localized nature of the defect and the need for precise control over the repair area.
- Material selection: A matching 75CrMo surfacing electrode was selected to maintain the chemical composition and mechanical properties of the base roll material.
The key technical considerations for this repair included:
- Dilution control: The surfacing deposit must achieve sufficient build-up thickness to ensure the final machined surface is entirely weld metal, with no residual scale influence.
- Crack resistance: 75CrMo is a high-carbon, high-chromium alloy with significant hardenability, making it susceptible to cracking during welding.
- Hardness matching: The deposited layer must match the base material hardness (typically 35–45 HRC) to ensure uniform wear behavior.
Process Parameters and Execution
The surfacing procedure was designed as follows:
| Process Parameter | Specification |
|---|---|
| Base material | 75CrMo (0.75% C, 1.0% Cr, 0.5% Mo) |
| Surfacing electrode | E71CrMoV (matching composition) |
| Electrode diameter | 4.0 mm |
| Welding current | 160–200 A (DCEN) |
| Preheat temperature | 300–400°C |
| Interpass temperature | ≤ 250°C |
| Layers | 3–4 layers (minimum 3 mm build-up) |
| Post-weld heat treatment | 550–600°C, 2 hours, slow cool in furnace |
The preheating was critical to reduce the cooling rate and minimize the formation of martensite in the heat-affected zone. The interpass temperature control prevented excessive hardness buildup in the weld metal. The post-weld heat treatment (PWHT) served to temper any martensite formed during welding and relieve residual stresses.
Quality Verification
Post-repair quality verification included:
- Visual inspection: 100% examination for surface cracks, porosity, and incomplete fusion.
- Magnetic particle testing (MT): Applied to all surfacing layers and the HAZ to detect surface-breaking cracks.
- Hardness testing: Vickers hardness measurements across the deposit cross-section to verify hardness profile and dilution zone.
- Dimensional verification: Confirmation that the machined surface diameter met specification tolerances.
- Metallurgical examination: Transverse section to verify microstructure, dilution zone, and absence of subsurface defects.
Engineering Lessons and Reflections
This case study illustrates several important principles in welding repair practice:
First, the economic decision to repair rather than replace requires careful technical assessment. Not all defects are repairable, and the repair itself may introduce new risks. The decision to proceed was justified by the depth and nature of the defect—shallow scale that could be fully removed and covered by adequate build-up.
Second, the choice of SMAW for this repair reflects the importance of process flexibility. While submerged arc welding offers higher deposition rates, SMAW provides superior accessibility for localized repairs and allows the welder to visually monitor the repair area throughout the process.
Third, the case highlights the challenges of welding high-carbon alloy steels. The 75CrMo composition, with its high carbon and alloy content, requires careful thermal management to avoid cracking. The preheat, interpass temperature control, and PWHT are not optional—they are essential for producing a crack-free repair.
Fourth, the documentation of this repair case serves as a valuable reference for similar situations. In the absence of a formal repair procedure, engineers often face the challenge of developing a repair strategy from first principles. Having documented precedents accelerates the decision-making process and provides confidence in the proposed approach.
This literature, while dated, remains highly relevant as a practical example of welding repair engineering, demonstrating the integration of metallurgical knowledge, process selection, and quality assurance in solving real-world manufacturing challenges.