What actually makes a salt bath dirty
Three things accumulate in a nitrite-nitrate quench bath: oxide scale shed from workpieces, sludge formed by the salt itself, and carbon black or oil coke carried in from upstream oil-quench operations. Most of it does not disappear on its own.
Crust on the surface can be skimmed. The real problem is the fine particles suspended in the melt — they circulate with the salt, adhere to workpieces, and settle on the tank floor and heat-exchange surfaces.
How it changes the quench result
Quench cooling is a heat-transfer process between the medium and the workpiece surface. Suspended particles cause three effects:
- Poor wetting — particles adhering to the surface create a local vapour film or insulating layer, lowering local cooling rate and producing soft spots.
- Drifting cooling rate — suspended solids change effective conductivity and flow, so identical process settings produce different cooling curves over time.
- Fouling — deposits reduce heat-exchange efficiency and temperature uniformity; local overheating in turn accelerates salt ageing.
Why replacing the salt is not the best answer
A full salt change is expensive, takes the line down for a long time, and the new salt still has to be dehydrated, heated and brought back to the target water content before the process is stable again.
On-line filtration is more economical: the melt is drawn from the tank, passed through a screen, and returned. The tank is never emptied and the process does not have to be re-established.
Choosing and running the filter
In practice we filter in stages: a coarser screen (for example 20 mesh) removes the bulk and establishes flow, then a finer screen (10 mesh) raises capture of fine particles. Finer is not always better — too fine and throughput collapses and the screen blinds, which lengthens the job.
The campaign needs attendance: sample regularly, watch colour and clarity, log flow and equipment condition, and change screens when they blind. On site we typically run three to four operators in shifts with sampling every four hours.
When to filter
Signals worth watching:
- the salt darkens, and a settled sample shows sediment;
- hardness scatter widens or soft spots become repeatable;
- floor deposit thickens and skimming frequency rises;
- the salt change interval is not due yet, but cooling behaviour already deviates.
Any one of these justifies a sampling assessment: look at the salt first, then decide between filtration, tank cleaning and a salt change.