ASTRALPOOL explain how we maintain the highest quality workmanship during the construction and installation of stainless steel pools, designed so that the starting requirements are always minimal. Our exclusive construction method precludes the builders from performing any preliminary installation with the exception of an initial cleaning of the area where the pool is to be installed so that there is no remaining construction work.
Astralpool specify the grades of steel used to ensure consistent quality through their operations.
For stainless steel pools the grades of steel most frequently used are:
1.4301 (AISI-304): used on structural parts, which are not in contact with water.
1.4404 (AISI-316L): used on floor, walls and overflow, all these parts, which are in direct contact with water. Higher grades are used in exceptional cases, which require them.
Welders must be approved according to EN 287-1: Qualification test for welders. Fusion welding. Steels. These norms are summarized in the following points for all welding procedures:
Verification of the welding method, detailing what the final outcome should obtain and tests on welding procedure.
Control of welding operations during, preparation, welding and subsequent treatment.
The degree of inspection and non-destructive testing techniques to be applied.
The acceptance criteria of permitted defects in welding.
Just like in other metals, stainless steel is affected by the distortion caused by welding. Different types of distortions (angular, curvature, shrinkage, etc.) are similar in nature to those experienced in carbon steel structures. Nevertheless, distortion in stainless steel structures, in particular for austenitic grade steel is greater than in carbon steel due to the higher coefficients of thermal expansion and lower thermal conductivity which results in greater temperature gradients. Distortion due to welding cannot be eliminated entirely, but it can be controlled.
Deformations are accentuated the thinner the piece to be welded. For this reason, ASTRALPOOL stainless steel pools never use sheets less than 2mm thick and in some cases it is necessary to use thicker pieces. Depending on the client request, a determined thickness will be required which is evaluated for each pool we build taking into account the mechanical characteristics of the steel used.
The stress-strain behaviour of stainless steel differs from that of carbon steel in various aspects. The most important difference resides in the stress-strain curve. While carbon steel exhibits a linear yield until it reaches its yield strength and levels out before strain hardening, stainless steel exhibits a stress-strain curve with a rounded form with no defined yield strength.
It is this behaviour which makes stainless steel an ideal material for the manufacture of outdoor pools in extreme climates, because it reacts to freezing water it contains with an elastic deformation and recovers its initial state after the ice has melted, without permanently damaging the stainless steel structure.
Key to maintaining quality are different methods by which welds can be inspected, and for each situation there is a specific method depending on the degree of structural integrity and the corrosion required for its environment. Nevertheless a visual inspection must be carried out during all the stages of the welding process to prevent problems, which could cause contamination.
With stainless steel the surface inspection is more important than it is for carbon steel because it is used primarily to prevent corrosion. Any small defect on its surface would make this material susceptible to corrosion.
Ultrasound methods have limited use in welding as it produces unreliable interpretation of results in welds; nevertheless they can be used as a base material. Radiography with gamma rays is not sufficient to detect fissures or failures to fuse in stainless steel materials thinner than 10mm. For this reason stainless steel pools must be visually examined and a liquid penetrate inspection can be applied where it is not clear if the weld was correctly performed.
All these methods are regulated by norm UNE-EN 12062: Non-destructive testing of welds. General rules for metallic materials.
After welding, the stainless steel surface should recover its corrosion resistance by eliminating any contamination, scaling and traces of thermal colouration from the welding. De-scaling is the process used to achieve both of these objectives. This can be done by immersion in an acid bath or by using de-scaling solutions in accordance with the manufacturer’s recommendations. De-scaling removes incrustations with a bristle brush while dissolving any particle imbedded in the iron or carbon steel.
Surface treatments after de-scaling such as brushing, grinding, reaming, milling, buffing, and polishing produce unidirectional finishes, and as a result, the combination of welds on sheets/plates with normal laminated surfaces can be difficult. A certain amount of experience is required to determine the necessary procedures to obtain an adequate finish. The finishing should be minimized on the welding and, if possible, wire brushing should be limited because the heat it produces during the grinding could affect the material’s corrosion resistance.
Electro-polishing removes a thin surface layer. A large range of finishes can be obtained with this method, including a matte finish or brilliant lustre, depending largely on the initial conditions of the surface of the material. The limitations of this method come from the size of the piece to be treated and the available tank as it is an immersion process. Once the pieces have been welded, they must be completely submerged in various solutions of acid combinations.
ASTRALPOOL stainless steel pools have been designed such that the starting requirements are always minimal. Our exclusive construction method precludes the builders from performing any preliminary installation with the exception of an initial cleaning of the area where the pool is to be installed so that there is no remaining construction work.
In both the case of a cement-lined basin and in the case of a self-supporting pool, it is only necessary that the basin, slab or lining technique (with cement or a grid of beams for weight support) are finalized and calculated to support the weight and water pressure. It should be added that in the case of the lined basin, the stainless steel does not have a structure to support the water pressure and therefore must be supported by constructed walls.
In ASTRALPOOL stainless steel pools, all surfaces that are visible or in contact with the water will be done in 1.4404 (AISI-316L) stainless steel with a satin finish. This includes the floor, walls and overflow. Other surfaces will be done in 1.4301 (AISI-304) steel, like the support structure, and will have a 2B finish.
As we previously commented, there are other types of steel of higher quality for extremely corrosive environments or where the client requires it. We also offer the client the possibility of other finishes (shine polish, painted, tinted, rough surface).