Salt Pot Application in Heat Treatment Processes
Salt pots are used mostly in the heat treatment of ferrous and non-ferrous materials in a myriad of application in various industries. They are used for heat treatment activities such as annealing, stress relieving, hardening, and case-hardening, etc.
Typically, they are made from high alloyed stainless steel containing up to 20% or more Nickel. Incessant failures are common salt pot application leading to concerns for various operators. Since these assets are made with exotic materials, there is a requirement for an iterative process for material selection to maximize cost effectiveness. Issues revolving around mechanical properties, corrosion resistance properties, high temperature strength and life cycle time guides in this process.
Salt pot maintenance
Many of the heat treatment experts deciding on material selection usually require the expertise of material consultants or metallurgist to make informed decisions.
At the end of the day, design life cycle cost is the most important factor due to the high turnaround asset life. Design life cycle cost includes both initial cost and operating cost. Initial cost – highlighting material properties, corrosion properties, fabrication and availability – is usually a fixed cost. Operating cost – incorporates initial cost with other cost such as lost productivity, maintenance, environmental damage, efficiency and product quality – are variable and hidden costs. This cost can be managed considerably well, but when it affects the longevity of the salt pot, maintenance is the great and most crucial issue. The issue of maintenance becomes important if an operator is seeking to improve on the life of a pot.
The mechanism of salt pot
Let us look at the operating mechanism of salt pot. A salt pot can be viewed as a ‘cooker vessel’ filled with salt.
The salt is heated up to a molten state via an external or internal heat source, usually a burner or electrode set up. Parts to be heat treated are lowered into the molten salt for the heat treatment to be carried out. Most popular salt used are chloride and cyanide salts depending on the heat treatment being carried out. Today, chloride salts are the most prevalent, and are very corrosive by nature. This raises a big concern as per integrity of the salt pot since corrosion mechanism is very rapid and usually localised. The type of failure typically seen is molten salt corrosion.
Molten salt corrosion
Plainly speaking, two things do happen; the molten salt attacks the alloy material and its protective chromium oxide layer and fluxes the alloy chromium content.
Alloy grain boundaries are exposed and salt penetrates and eventually creates a hole, causing leakage in a rapid manner. Eventually, some salt escapes to the burner area. Since the burner area is hotter, vaporisation of salt occurs and the chlorides which are more corrosive as gaseous species drive the corrosion mechanism faster. Also, oxidizing power of these salts increase in the presence of oxygen in the atmosphere; thus increasing its corrosiveness. The oxygen is present as metal oxides and chromates. As this is inevitable, the salts need to be rectified to reduce oxygen levels by introducing some additives such as silica, ferrosilicon, methyl chlorides etc., which converts the metal oxides back to chlorides. This leads to sludge build up in the pot.
Strict maintenance can improve life cycle time and reduce cost of the asset
In order to manage these issues, strict maintenance regime needs to be adhered and frequent sludge removal so as to avoid overheating in the salt pot.
These can improve life cycle time and reduce cost of the asset to operator significantly. For best material selection, high nickel grades with chromium, aluminium, and silicon elements, providing corrosion resistance to molten salt corrosion and oxidation resistance required are best candidates. Depending on the heat treatment operation and on the salt used, different grades respond differently at various temperatures and conditions. In our experience, alloy salt pots for heat treatment may last for anytime from 2 days to 12 months. Life time largely depends on maintenance and operating procedure.
Improving the life of salt pot
Alloy material is seldom a culprit. Until the below have been proven not to help should a change of material be considered. To improve life, attention should be given to the following;
- Scheduled maintenance is carried out.
- Removal of impurities like oil and salt residue in the burner areas.
- Rectification of salt.
- Removal of sludge, sweepings on a daily basis, at the least.
- Keep salt in pot molten rather than complete shutdown and solidification.
- Good pot fabrication- full penetration of welds.
Material consultants at NeoNickel are able to advise and investigate on issues around salt pot application. We suggest the utilization of this expertise to improve the design life time.