Zhuojin Pipe Fitting Co., LtdHardfacing Process and Application of Cone-Shaped Carbide Deposits
Literature Overview
This technical paper by Zhou Chuanfeng from China Fifth Construction Engineering Corporation, published in Fertilizer Design in 2015 (Vol. 53, No. 4, pp. 14-17), addresses a practical and challenging hardfacing problem encountered in the coal gasification industry. The study focuses on the hardfacing of the conical bottom section of a slag discharge tank in a coal gasification unit, where previous attempts to apply carbide hardfacing resulted in excessive surface cracking. The author systematically analyzes the root causes of the cracking problem and proposes a comprehensive solution involving process selection, parameter optimization, fixture design, and enhanced process control.
Problem Analysis and Root Cause Identification
The conical bottom of a slag discharge tank in a coal gasification unit is subjected to extremely severe operating conditions. Molten slag, which can reach temperatures exceeding 1400°C, flows continuously over the conical surface, causing rapid erosive and abrasive wear. The material must therefore possess exceptional wear resistance, which is typically achieved through carbide hardfacing with cobalt-based or nickel-based carbide alloys. However, the conical geometry presents unique challenges that are not encountered in flat or cylindrical hardfacing applications.
The primary problem identified is excessive surface cracking after carbide hardfacing. Through systematic investigation, several contributing factors were identified:
- Geometric constraints: The conical shape creates non-uniform heat distribution during hardfacing, with higher heat accumulation at the apex and lower heat input at the wider base.
- Thermal stress concentration: The steep conical angle amplifies thermal gradients, leading to localized stress concentrations that exceed the cracking threshold of the hardfacing deposit.
- Inadequate process parameters: The original hardfacing parameters were likely optimized for flat surfaces and were not adjusted for the conical geometry.
- Insufficient fixture design: The workpiece fixture did not adequately support the conical geometry, allowing thermal distortion that exacerbated cracking.
- Process control deficiencies: Inadequate monitoring of interpass temperature and welding sequence contributed to uneven heat input and stress accumulation.
Solution Approach and Process Optimization
The author adopted a multi-faceted approach to resolve the cracking problem, addressing each root cause systematically:
Process Selection
The selection of the appropriate hardfacing process is the first critical decision. For conical surfaces with steep angles, the choice between manual arc hardfacing, submerged arc hardfacing, and plasma arc hardfacing must be evaluated based on accessibility, heat input control, and deposit quality requirements. The study implies that a process capable of precise heat input control was selected, likely manual GTAW or plasma arc hardfacing, which allows the welder to adjust parameters in real-time based on the local geometry.
Parameter Adjustment
Key process parameters were optimized for the conical geometry:
| Parameter | Adjustment Strategy | Rationale |
|---|---|---|
| Heat input | Reduced and varied with cone angle | Minimize thermal stress at apex |
| Welding sequence | Spiral or ring pattern from base to apex | Ensure uniform heat distribution |
| Interpass temperature | Monitored and controlled | Prevent excessive thermal cycling |
| Wire/feed rate | Adjusted for geometry | Maintain consistent bead profile |
| Travel speed | Increased at apex, decreased at base | Compensate for geometry-induced heat variation |
Fixture Design
The design of a specialized fixture was identified as a critical success factor. The fixture must:
- Rigidly support the conical workpiece to prevent thermal distortion.
- Allow access to all areas of the conical surface for hardfacing.
- Incorporate thermal expansion accommodation to prevent constraint-induced cracking.
- Provide clamping points that do not interfere with the hardfacing process.
- Be designed to facilitate welding from multiple positions, including overhead and vertical positions.
Process Control Enhancement
Strengthened process control measures included:
- Mandatory interpass temperature monitoring using infrared thermometers or embedded thermocouples.
- Welding sequence documentation and adherence to approved procedures.
- Visual inspection of each bead before proceeding to the next layer.
- Post-hardfacing inspection using magnetic particle testing (MT) or dye penetrant testing (PT) to detect any residual cracks.
- Implementation of a quality hold point after each major section is completed.
Metallurgical Considerations
The metallurgical behavior of carbide hardfacing deposits on conical surfaces differs from that on flat surfaces in several important ways. The thermal cycling experienced during hardfacing a conical surface is more complex because the heat input varies with position, and the cooling rate is influenced by the local geometry. At the apex of the cone, where the cross-sectional area is smallest, heat accumulates more rapidly, leading to higher peak temperatures and slower cooling rates. This can result in coarser grain structures and increased carbide size, which may reduce wear resistance in localized areas.
Conversely, at the wider base of the cone, the larger cross-sectional area acts as a heat sink, resulting in faster cooling rates and potentially finer grain structures. However, the faster cooling can also increase the risk of martensitic transformation in the heat-affected zone, leading to increased hardness and brittleness. The optimal hardfacing procedure must account for these variations by adjusting parameters throughout the welding sequence.
Engineering Practice Integration
From the perspective of piping and pressure vessel engineering, the hardfacing of conical sections is not uncommon.reducers, tees, and other fittings with conical geometries often require hardfacing in abrasive service. The lessons from this study are directly applicable to the hardfacing of such components. The emphasis on fixture design is particularly relevant, as standard fixtures designed for flat or cylindrical surfaces are often inadequate for conical geometries.
The systematic approach to problem resolution—identifying root causes, implementing targeted countermeasures, and verifying results through inspection—reflects sound engineering practice. This methodology is consistent with the PDCA (Plan-Do-Check-Act) cycle and can be applied to any hardfacing challenge encountered in piping or pressure equipment manufacturing.
Study Insights and Implications
This study underscores the importance of adapting hardfacing procedures to the specific geometry of the workpiece. A hardfacing procedure that works well on a flat surface may fail catastrophically on a conical surface if the process parameters and welding sequence are not adjusted accordingly. The fixture design, often overlooked in hardfacing procedure development, can be the difference between a successful and a failed hardfacing operation. For engineers responsible for specifying and overseeing hardfacing operations on complex geometries, this study provides a practical framework for systematic problem resolution. The key insight is that hardfacing is not merely a matter of selecting the right alloy and welding parameters—it is a holistic process that must integrate metallurgy, geometry, fixture design, and process control into a cohesive solution.