For Modulpool in Sweden, stainless was the material of choice since the first hot tub was built. After testing the Supra 316plus, the company shifts to the new material in 2016.
In a workshop in Rättvik, about 300 kilometers northwest of Stockholm, Modulpool builds the stainless steel hot tubs they sell throughout the country.At the moment, only a few are exported, but the interest from abroad is growing.
Hygienic material that is easy to bend and weld
Quality down to the smallest screw is required to live up to Modulpool´s promise that their hot tubs will last one hundred years. Therefore, they also have high demands on the stainless steel.
"We wanted a material that is corrosion and UV resistant, without the risk of organic growth, and which is easy to maintain and is durable,"
Anders Olsson, the president and owner of Modulpool explains.
Modulpool has used 316L/4404 for several years now. "The quality of this steel meets our demands,” he continues. "The plates are flat, and even after cutting, they are easy to bend and weld."
Recently Modulpool tried Outokumpu's new steel grade: Supra 316plus.
Petri Kujala, Managing Director of Outokumpu Nordic, initiated the test.
"Supra 316plus delivers high strength, good shape variability, and excellent weldability. All these properties make Supra 316plus ideal in a variety of applications," he explains. "And due to its high chromium content, it offers similar or better corrosion resistance than austenitic standard grades with molybdenum in many corrosive environments."
While observing a hot tub built from the new stainless steel grade, Olsson and welder
Jan Forsberg note that it works as well as the steel previously used.
"Supra 316plus is a bit stiffer," says Forsberg, who also bends the laser cut plates. "But it was not a problem; I just increased the pressure in the bending machine."
Welding also works well. "I did not notice any difference to the steel grade that I am used to," says Forsberg.
"The Supra 316plus works very well with both MIG welding and TIG welding. In my opinion, these steel qualities are very easy to work with."
Work as usual
The Supra 316 plus is suited for a full range of conventional welding methods, like MMA, MIG/MAG, TIG, SAW, LBW or RSW. To ensure that the weld metal properties, such as strength and corrosion resistance, are equivalent to those of the parent metal, matching or slightly overalloyed fillers should preferably be used. Austenitic 316L filler metals can be used to get matching corrosion resistance and 22 09 NL type duplex welding consumable to get matching strength. The shielding gases should be Ar/He based or contain up to three percent nitrogen to minimize nitrogen drop. Typical heat input values for austenitic grades can be utilized.
Generally post weld heat treatment is not required on Supra 316plus. In special cases with high risks of stress corrosion cracking or fatigue, stress relief treatment may be considered. In order to fully restore the corrosion resistance of the weld seam, the weld discoloration should be removed by pickling and passivation.
To Olsson, the advantage lies in the fact that there is no need to change the welding routines for the new steel. He is also very satisfied with the test and has decided to use Supra 316plus in 2016.
"This way we can keep working as usual, and not lose any time in adapting to a new welding routine."