Plasma Spray Welding Quality Defect Analysis – Cracks
Plasma Spray Welding Quality Defect Analysis: Why Cracks Happen and How to Prevent Them
Cracking is one of the most serious quality defects in plasma spray welding, PTA hardfacing, and weld overlay repair. It can appear as longitudinal cracks, transverse cracks, crater cracks, large opening cracks, or heat-affected zone cracks. To prevent crack failure, buyers and welding engineers must evaluate alloy hardness, base material rigidity, surface preparation, bead geometry, process parameters, preheating, cooling control, and post-weld treatment.
A Crack in the Overlay Is Not Just a Cosmetic Problem — It Can Lead to Peeling, Spalling, Leakage, Fatigue Failure, and Premature Shutdown.
The key is not simply welding again. The real solution is to identify the crack mechanism and correct the alloy, surface condition, heat input, bead shape, and cooling strategy.
Common Crack Types in Plasma Spray Welding and PTA Hardfacing
Cracks can be classified by location, direction, formation temperature, and time of occurrence. Correct classification helps identify whether the cause is alloy selection, base metal stress, bead shape, cooling rate, or surface preparation.
| Crack Type | Typical Appearance | Common Technical Meaning |
|---|---|---|
| Longitudinal Cracks | Cracks running along the welding or cladding direction. | Often related to shrinkage stress, bead shape, high-hardness alloy, or poor crack resistance. |
| Transverse Cracks | Cracks crossing the weld bead or overlay layer. | May indicate high tensile stress, rapid cooling, insufficient preheating, or base metal restraint. |
| Large Opening Cracks | Wide cracks visible on the overlay surface. | Often caused by excessive shrinkage stress, high hardness, rapid cooling, or improper current decay. |
| Arc Crater Cracks | Cracks located at the end of a weld bead or stop point. | Usually related to poor crater filling, fast current decay, or rapid solidification at the bead end. |
| Heat-Affected Zone Cracks | Cracks appearing near the fusion zone or base metal HAZ. | May be linked to base metal hardenability, high restraint, hydrogen risk, or unsuitable preheat and cooling control. |
Hot Cracks vs Cold Cracks: Why Timing Matters
Plasma spray welding cracks can also be divided into hot cracks and cold cracks. The prevention strategy is different, so the timing of crack formation should be checked during quality analysis.
Hot Cracks
Hot cracks usually form during solidification or at high temperature. They are often related to alloy composition, solidification shrinkage, bead shape, low ductility, and tensile stress during cooling.
- Often appear along bead centerline or weak solidification zones
- Common in high-hardness or low-toughness overlay alloys
- Can be promoted by poor bead shape or high restraint
Cold Cracks
Cold cracks appear after the overlay has cooled or during delayed stress release. They are often related to rapid cooling, high restraint, hard microstructure, hydrogen, and insufficient preheating or post-weld slow cooling.
- May appear hours after welding
- Often linked to high hardness and internal stress
- Preheating and slow cooling are critical prevention tools
Main Causes of Cracks in Plasma Spray Welding
The original defect analysis identifies multiple causes. In practical production, cracking is usually the result of several factors acting together: hard alloy, rigid workpiece, existing surface defects, poor bead geometry, improper parameters, fast cooling, and insufficient heat treatment.
1. High Alloy Hardness and Poor Crack Resistance
High-carbon, high-boron, high-silicon, or carbide-rich alloys provide high hardness, but they may have lower toughness and higher crack sensitivity if process control is poor.
2. Large Workpiece Rigidity and High Restraint
Rigid parts cannot freely deform during heating and cooling. When shrinkage stress exceeds the overlay strength, cracks can form in the cladding layer or heat-affected zone.
3. Surface Defects Before Welding
Cracks, notches, casting pores, trachoma, oil contamination, old repair defects, and sharp transitions can create stress concentration and trigger cracking.
4. Poor Weld Bead Width-to-Thickness Ratio
A narrow and thick bead may have poor crack resistance because solidification stress becomes concentrated and bead shape is not favorable for stress distribution.
5. Improper Process Parameters
Poor fusion, poor forming, lack of material, slag inclusion, excessive dilution, or unstable melt pool can all increase crack risk.
6. Current Decay Too Fast at the Ending Point
If the current drops too quickly at the weld end, the molten pool can contract rapidly, creating crater cracks or large opening cracks at the stopping point.
7. Cold Airflow During Welding
Direct cold airflow or workshop drafts can increase cooling speed, create thermal shock, and increase crack tendency in hard overlay layers.
8. Incorrect Preheating or Slow Cooling
Insufficient preheat, uncontrolled interpass temperature, or fast post-weld cooling can increase residual stress and cause delayed cracking.
Crack Cause and Prevention Matrix
Use this matrix as a practical checklist when troubleshooting crack defects in plasma spray welding, PTA cladding, and hardfacing overlay.
| Potential Cause | Risk Mechanism | Prevention Direction |
|---|---|---|
| High-hardness alloy | Hard and brittle overlay has low crack tolerance. | Select alloy powder with better crack resistance or adjust hardness target according to application. |
| High workpiece rigidity | Thermal stress cannot be released during cooling. | Use suitable preheating, slow cooling, stress relief, or structural design measures. |
| Surface defects before welding | Defects create stress concentration and crack initiation points. | Remove cracks, pores, notches, oil, rust, and old unstable overlay before cladding. |
| Poor bead geometry | Unfavorable bead shape concentrates solidification stress. | Control bead width, thickness, overlap, travel speed, and powder feed rate. |
| Improper process parameters | Poor fusion, slag, underfill, excessive dilution, or unstable melt pool. | Optimize current, voltage, plasma arc, powder feed, travel speed, shielding gas, and torch angle. |
| Fast current decay at crater | Rapid solidification and shrinkage at bead end. | Use proper crater filling, current downslope, overlap sequence, and ending control. |
| Cold airflow | Cooling rate becomes too fast, increasing thermal stress. | Avoid direct drafts and use insulation or controlled cooling for sensitive parts. |
| Insufficient preheat or slow cooling | Residual stress and hard microstructure increase crack risk. | Apply correct preheat, interpass temperature control, insulation, slow cooling, and post-weld heat treatment if required. |
Main Measures to Prevent Plasma Spray Welding Cracks
Crack prevention should be built into the full process: material selection, workpiece preparation, bead forming, welding parameters, temperature control, and post-weld treatment.
Improve Alloy Crack Resistance
Choose alloy powder according to wear mode, hardness requirement, toughness, base metal compatibility, and expected thermal stress.
Remove Surface Defects
Eliminate cracks, notches, casting defects, pores, oil, rust, old failed overlay, and sharp transitions before spray welding.
Control Bead Forming
Use reasonable bead width, thickness, overlap, melt pool control, and process parameters to avoid underfill, slag, poor fusion, and stress concentration.
Use Preheating and Slow Cooling
Select preheating, interpass temperature, insulation, slow cooling, and post-weld heat treatment according to workpiece rigidity and alloy material.
Process Control Points for Crack-Sensitive Plasma Hardfacing
For crack-sensitive workpieces, quality control should start before welding and continue through overlay, cooling, inspection, and final machining.
Before Welding
Check base material, surface defects, hardness target, drawing, workpiece rigidity, and preheating requirement.
During Welding
Control current, voltage, powder feed, travel speed, bead overlap, shielding gas, interpass temperature, and crater filling.
After Welding
Avoid rapid cooling, direct cold airflow, sudden temperature drop, and uncontrolled stress release.
Final Inspection
Inspect surface cracks, crater areas, overlap zones, hardness, dimensions, fusion condition, and final machining allowance.
How to Diagnose a Crack Defect Before Repair
Do not repair cracks blindly. A crack must be inspected first to decide whether the part can be reworked, needs local grinding and re-cladding, or should be rejected.
| Inspection Question | What to Check | Repair Decision Impact |
|---|---|---|
| Where is the crack? | Overlay surface, bead center, overlap zone, crater, fusion line, or HAZ. | Location helps identify whether the cause is bead shape, crater control, fusion, or base metal stress. |
| How deep is the crack? | Surface-only, through overlay, into fusion line, or into base metal. | Deep cracks may require complete removal and re-cladding or part rejection. |
| When did it appear? | During welding, immediately after cooling, or delayed after several hours. | Timing helps separate hot cracking from cold cracking or delayed stress cracking. |
| Is the crack isolated or repeated? | Single crack, repeated cracks on every bead, or concentrated at starts/stops. | Repeated cracks suggest systematic parameter, alloy, or cooling problems. |
| Is there an original defect underneath? | Casting pore, notch, old crack, oil contamination, old overlay, or machining groove. | Underlying defects must be removed before any repair weld is applied. |
Important Note: Not Every Crack Has the Same Meaning
Some hardfacing systems may show fine stress-relief checking that does not affect service, while deep cracks, crater cracks, HAZ cracks, and cracks connected to poor bonding can be serious defects. Crack acceptance must be judged by material system, part function, operating load, crack depth, and service risk.
What Information Should You Send for Crack Defect Analysis?
To help review crack defects in plasma spray welding, PTA hardfacing, or weld overlay, please provide part photos, process records, material information, and crack location details.
Part Information
- Part name and function
- Base material and hardness
- Drawing or photo
- Part size and rigidity
- Repair area and final machining requirement
Welding Process Information
- Powder alloy grade
- Current, voltage, plasma arc, and travel speed
- Powder feed rate and shielding gas
- Preheat and interpass temperature
- Cooling method and post-weld treatment
Crack Information
- Crack photos from different angles
- Crack location and direction
- When the crack appeared
- Crack depth if inspected
- Whether cracks repeat across multiple parts
Frequently Asked Questions
What causes cracks in plasma spray welding?
Common causes include high-hardness alloy with poor crack resistance, high workpiece rigidity, surface defects, poor bead geometry, improper process parameters, fast crater cooling, cold airflow, insufficient preheating, and poor slow-cooling control.
How can plasma spray welding cracks be prevented?
Prevention measures include selecting crack-resistant alloy powder, removing surface defects, controlling bead shape, optimizing welding parameters, using correct preheating, controlling interpass temperature, avoiding cold airflow, slow cooling, and applying post-weld heat treatment when required.
Are all cracks in hardfacing unacceptable?
Not always. Some hardfacing overlays may show fine stress-relief checking. However, deep cracks, crater cracks, HAZ cracks, fusion-line cracks, or cracks causing peeling and spalling must be treated as serious defects.
Why does high hardness increase crack risk?
High-hardness alloys often contain hard carbides or hard phases that improve wear resistance but reduce toughness. If residual stress is high or cooling is too fast, the overlay may crack.
What should I provide for crack defect review?
Please provide part photos, crack close-ups, base material, powder alloy, process parameters, preheating and cooling method, crack timing, crack location, and whether the defect appears repeatedly across multiple parts.
Need Help Analyzing Plasma Spray Welding Cracks?
Send HALDEN your crack photos, part material, powder alloy, process parameters, preheat record, cooling method, and service condition. We can help review possible causes and recommend process adjustments for plasma hardfacing, PTA welding, or weld overlay production.
Please include these details:
- Part name, base material, drawing, photos, and application
- Crack photos, crack location, crack direction, and crack depth if available
- Powder alloy grade, target hardness, and overlay thickness
- Current, voltage, travel speed, powder feed rate, shielding gas, and welding sequence
- Preheating temperature, interpass temperature, slow cooling method, and post-weld heat treatment
- Whether the defect is occasional or repeated across multiple parts
