CLADDING TECHNOLOGY SHANXI CO., LTDInvestigation of Production Quality Decline After Roll Press Overlay Welding
Literature Overview
This 2019 publication by Li Mingxu, Wu Jun, and Wang Dong from Lu'an Conch Cement Co., Ltd. addresses a practical production problem encountered in the cement industry: the degradation of output quality indicators following overlay welding repair of roll press components. Roll presses are critical grinding equipment in cement production, where the rolls are subjected to extreme abrasive and impact wear. When the roll surface wears beyond acceptable limits, overlay welding with wear-resistant alloy is applied to restore the roll diameter and improve surface hardness. However, the authors report that after overlay welding, production quality indicators declined, prompting investigation into the root causes.
Background of Roll Press Operation in Cement Production
Roll presses operate by compressing material between two counter-rotating rolls. The contact stresses at the roll surface can exceed 2000 MPa, and the material being ground (limestone, clinker, or coal) is highly abrasive. The roll surface experiences a combination of:
- Abrasive wear: From hard particles in the feed material
- Impact wear: From lumps and chunks of material
- Fatigue spalling: From cyclic contact stresses (Hertzian contact fatigue)
- Chemical wear: From reactive materials in the grinding environment
To combat these wear mechanisms, roll surfaces are typically overlaid with wear-resistant alloys containing carbide-forming elements such as chromium, molybdenum, and vanadium. Common overlay materials include:
| Overlay Alloy Type | Hardness (HRC) | Key Elements | Typical Application |
|---|---|---|---|
| Cr-Mo-C high carbon steel | 50-58 | Cr 8-12%, Mo 2-5%, C 2-4% | General abrasive wear |
| Cr-Mn-C austenitic | 40-50 (work-hardened to 55-60) | Cr 12-20%, Mn 12-20% | Impact + abrasive wear |
| High Cr cast iron | 60-65 | Cr 18-30%, C 2-4% | Severe abrasive wear |
| Stellite (Co-Cr) | 45-50 | Co 60%, Cr 25%, W 10% | Extreme wear + corrosion |
Core Technical Analysis
The decline in production quality indicators after overlay welding can be attributed to several factors that the authors investigated:
Surface Geometry and Profile Deviation
After overlay welding, the roll surface must be machined (grinding or turning) to restore the original diameter and surface profile. If the overlay deposition is uneven or if the grinding process introduces profile deviations, the contact between the two rolls becomes non-uniform. This leads to:
- Uneven pressure distribution across the roll width
- Localized excessive wear on high-contact areas
- Vibration and noise during operation
- Reduced grinding efficiency
Overlay Layer Integrity Issues
The quality of the overlay weld is critical to production performance:
| Defect Type | Effect on Performance | Detection Method |
|---|---|---|
| Lack of fusion | Spalling of overlay material under load | UT, MT |
| Porosity | Reduced effective contact area, premature failure | RT, UT |
| Cracking | Propagation under cyclic loading, spalling | MT, PT |
| Dilution | Reduced hardness and wear resistance of overlay | Hardness testing, metallography |
| Inclusion | Stress concentration points, fatigue initiation | Metallography |
Metallurgical Compatibility
The overlay alloy must have good metallurgical bonding with the roll base material. If the dilution rate is too high (typically exceeding 20-25%), the effective composition of the overlay layer shifts away from the designed wear-resistant composition, resulting in lower hardness and reduced wear life. Conversely, if the dilution is too low, the bond strength between overlay and base may be insufficient, leading to spalling under operational loads.
Thermal Effects on Base Material
The welding heat input affects the base material properties near the overlay interface:
- Hardness increase: Heat-affected zone (HAZ) hardness may increase due to microstructural changes, potentially exceeding 350 HV and becoming susceptible to hydrogen cracking
- Residual stress: Compressive residual stress at the surface is beneficial for fatigue life, but excessive tensile stress in the HAZ can promote crack initiation
- Microstructural changes: Transformation of pearlite to martensite in the HAZ can increase brittleness
Engineering Practice Integration
The case study from Lu'an Conch Cement provides valuable practical insights for the cement industry:
- Process control: The overlay welding procedure must be carefully controlled, including preheating temperature (150-250°C for medium-carbon steel rolls), interpass temperature (maintained below 250°C), and welding sequence (spiral or segmental to minimize distortion).
- Post-weld machining: The grinding or turning operation after overlay welding must be performed with high precision. The surface roughness should be controlled to Ra 3.2-6.3 μm for roll press applications, and the profile deviation should be within ±0.05 mm per meter of roll length.
- Quality verification: Before returning the roll to service, the following checks should be performed:
- Hardness mapping across the overlay surface
- Surface profile measurement using laser scanning or coordinate measurement
- Non-destructive testing (MT or PT) of the overlay surface
- Visual inspection for any surface defects
- Operational monitoring: After the repaired roll is put back into service, production quality indicators should be monitored closely during the initial operating period to detect any early signs of performance degradation.
Key Questions and Reflections
The case study raises an important question: is the decline in production quality caused by the overlay welding itself, or by the subsequent machining and reassembly processes? In my experience, the machining step is often the more critical factor. The overlay welding deposits material with inherent surface irregularities, and the grinding process must remove these irregularities while maintaining the roll profile. If the grinding wheel is worn or if the grinding parameters are not properly set, the resulting surface may have micro-irregularities that affect roll-to-roll contact.
Another reflection is on the choice of overlay alloy. In cement grinding applications, the wear mechanism is primarily abrasive, and the optimal overlay alloy should have high hardness with good toughness to resist spalling. However, very hard overlays (HRC > 60) may be too brittle for the impact loading conditions in some roll press configurations. The selection must be based on the specific operating conditions of the roll press.
Study Insights and Implications
The investigation highlights the importance of a systematic approach to overlay welding repair in production equipment. The FMEA (Failure Mode and Effects Analysis) methodology is particularly applicable here: identify potential failure modes in the overlay repair process, assess their effects on production quality, and implement preventive measures. The key insight is that overlay welding is not an isolated operation but part of an integrated process chain that includes surface preparation, welding, machining, and reassembly. Each link in this chain must be controlled to ensure that the final repaired component performs at least as well as the original. The economic implications are significant: a poorly executed overlay repair can result in reduced production efficiency, increased energy consumption per unit output, and shortened service intervals, all of which directly impact the profitability of cement production operations.