A quick catch-up:
In Part I we showed how Stelliting (hard-facing with cobalt alloys) keeps gate, globe and check valves leak-tight. In Part II we proved why a minimum 2 mm overlay isn’t a number on paper it’s durability in service.
This chapter is about the moment of truth: when base metal and Stellite fuse into a single, tougher surface and why that fusion, done right, saves customers time, money, and shutdowns.
A short story from the shop floor
Picture two metals meeting in a narrow band of heat: the valve body’s steel and the Stellite wire or powder. Too much heat and the Stellite dilutes, going soft. Too little and it doesn’t bond, risking micro-voids and eventual leaks. In that slim interval seconds, sometimes less fusion decides reliability. IPC’s job is to make that moment repeatable, every time.
What “fusion” means in Stelliting (and what it’s not)
- Fusion is a metallurgical bond: a controlled melt where a shallow layer of base metal mixes with the Stellite overlay to form a strong, continuous interface.
- It is not brazing, plating, or paint. There’s no glue layer to shear; no brittle interface to crack.
- The goal: just enough dilution for adhesion and toughness, not so much that the cobalt-carbide network loses hardness.
Common fusion processes at IPC:
- PTA (Plasma Transferred Arc) for consistent overlays and low dilution.
- GTAW/TIG hard-facing for geometry control on complex seats.
The science in five decisions
1. Surface preparation
Degrease → grit-blast → dry bake. A clean, activated surface gives the fusion line somewhere to “bite.”
2. Preheat & heat input
Preheat thick castings to avoid thermal shock. Then control current/voltage/travel speed to keep dilution typically with in the sweet spot for a tough, hard overlay.
3. Bead geometry & overlap
Oscillation and overlap angles remove pores and “valleys.” IPC programs weave patterns to achieve uniform 2 mm+ thickness with minimal waviness.
4. Interpass temperature
Keep it in a narrow band so carbides form correctly and residual stresses stay tame no hot cracks, no brittle zones.
5. Stress relief & finish
Post-weld heat treatment (when required) relaxes the structure. Finish machining restores seat geometry, parallelism, and surface finish so the overlay seals like glass.
How fusion quality shows up in the field
- Leak-tight integrity that lasts
A sound fusion line won’t unpeel under vibration, water hammer, or thermal cycles. Seats stay aligned; torque stays predictable. - Wear that slows to a crawl
Correct dilution preserves the cobalt-carbide skeleton; erosion grooves don’t form easily, so shut-off remains crisp. - Fewer re-touches, longer intervals
Packing and actuators aren’t fighting misalignment. You spend less time reworking seats and more time running. - Lower life-cycle cost
One solid overlay done right beats multiple quick fixes. Less rework, fewer leaks, safer operation.
IPC’s “no-surprises” way of fusing metals
- Recipe control, not guesswork
Weld procedures lock amps, volts, feed rate, travel speed, interpass temperature. Operators are qualified on the exact P-numbers and overlay chemistry. - Digital weld-logs
Every bead captured. If heat input drifts, alarms do not. - Proof beyond pretty
UT for fusion defects, DPT for surface cracks, thickness mapping to prove the ≥ 2 mm overlay, hardness windows to confirm the microstructure. - Seat geometry first
Fusion is only half the story; IPC finishes to tight concentricity and surface roughness, because a hard face without the right geometry still leaks.
The customer’s bottom line
When fusion is right, Stelliting turns a valve into a long-lived asset: tighter shut-off, fewer leaks, smoother operation, and fewer interventions. That’s the difference you feel quarter after quarter especially in high-pressure, high-temperature, erosive, or cyclic duties.