CLADDING TECHNOLOGY SHANXI CO., LTDManual Arc Weld Overlay of 75CrMo Rolls
Literature Overview
This technical report by Hao Huimin (1996) from Taiyuan Mining Machinery Factory documents the application of manual shielded metal arc welding (SMAW) for the weld overlay repair of 75CrMo alloy steel rolls used in mining and metal processing applications. The study addresses the practical challenges of overlay welding on high-carbon, high-chromium alloy steel rolls that are subject to severe abrasive and impact wear in mineral processing environments.
Technical Background
75CrMo is a high-carbon chromium alloy steel with the following approximate composition:
- Carbon: 0.70-0.80%
- Chromium: 0.80-1.10%
- Molybdenum: 0.15-0.30%
- Manganese: 0.50-0.80%
This steel is characterized by high hardness (typically 300-400 HB as supplied), excellent wear resistance, and good toughness. However, these same characteristics make it extremely challenging to weld due to high hardenability, susceptibility to cracking, and the formation of hard martensitic structures in the heat affected zone.
Rolls in mining applications experience:
- Continuous abrasive contact with ore particles
- Impact loading from material flow
- Thermal cycling from material temperature variations
- Corrosive environments in wet processing conditions
When the roll surface is worn beyond acceptable limits, weld overlay is used to restore the surface dimensions and improve wear resistance.
Welding Challenges
The overlay of 75CrMo rolls presents several unique challenges:
| Challenge | Mechanism | Consequence |
|---|---|---|
| High carbon equivalent | CE ≈ 0.65-0.75 | Extreme hardenability |
| High cooling rate | Thick section, alloy content | Hard martensite in HAZ |
| Hydrogen sensitivity | High carbon, high alloy | Delayed cracking |
| Thermal stress | Restrained geometry | Cracking under restraint |
| Dilution | Base metal melting into weld | Increased hardness, reduced toughness |
Process Development
The study developed a manual SMAW overlay process using the following approach:
Consumable Selection
The author selected a high-toughness, low-hydrogen electrode (E5015 or equivalent) with the following characteristics:
- Low hydrogen content (< 5 ml/100g) to prevent delayed cracking
- Moderate alloy content to provide dilution control
- Rutile or basic coating for arc stability and slag fluidity
- Suitable for all-position welding on roll geometry
Process Parameters
| Parameter | Value | Rationale |
|---|---|---|
| Preheat temperature | 250-300°C | Reduce cooling rate, prevent cracking |
| Interpass temperature | 250-300°C | Maintain preheat effect |
| Current | 120-160 A | Adequate penetration without excessive heat |
| Arc voltage | 22-28 V | Stable arc, good wetting |
| Travel speed | Moderate | Balance penetration and deposition |
| Bead width | 1.5-2.5× electrode diameter | Controlled heat input |
| Overlay thickness | 3-5 mm | Restore worn dimensions |
Critical Process Controls
- Preheating: Uniform preheating of the entire roll to 250-300°C using gas torch or induction heating, maintained throughout the welding operation.
- Post-weld heat treatment: Slow cooling in insulated blankets or furnace cooling to 200°C before air cooling, to reduce residual stresses and prevent delayed cracking.
- Welding sequence: Use of a staggered or balanced sequence to minimize distortion and residual stress buildup.
- Bevel preparation: V-groove or U-groove preparation at 60° included angle to ensure adequate root penetration.
- Surface preparation: Grinding to remove scale, rust, and worn material down to bright metal, followed by immediate welding to prevent re-oxidation.
Defect Analysis and Results
The study documented the following defects encountered during initial trials and the corrective measures taken:
- Cracking: Cold cracking in the HAZ due to excessive cooling rate. Corrected by increasing preheat to 300°C and implementing post-weld heat treatment.
- Porosity: Gas porosity from electrode moisture. Corrected by ensuring proper electrode storage and baking at 300-350°C for 1 hour before use.
- Lack of fusion: Inadequate root penetration. Corrected by increasing current and using a proper bevel preparation.
- Excessive hardness: Hardness exceeding 500 HV in the weld metal due to high dilution. Corrected by using multiple thin passes and increasing the number of overlay layers.
After optimization, the overlay deposits achieved:
- Hardness: 350-420 HV (acceptable for wear resistance)
- No cracking in bend tests
- Bond strength exceeding 95% of base metal tensile strength
- Service life improvement of 2-3× compared to the original unwelded roll surface
Study Insights and Reflections
This 1996 study, while predating many modern welding technologies, demonstrates fundamental principles that remain valid today: the importance of preheat control, the critical role of hydrogen management, and the necessity of post-weld heat treatment for high-carbon alloy steels. The manual SMAW approach, while less efficient than modern automated processes, offers the flexibility needed for roll geometry and the ability to adapt to varying wear patterns. For engineers working with high-carbon alloy steels, this study reinforces the principle that welding procedure development must be driven by metallurgical understanding rather than empirical trial and error. The systematic approach to defect identification and correction serves as a model for quality improvement in any weld overlay operation, regardless of the specific process or material combination involved.