Study on Submerged Arc Welding by Bonded Fluxes (4th Report)

Properties of Weld Metal for Ferrite Stainless Steel (18% Cr steel)

Abstract

High Chromium steels have been used for chemical plants as heat-and corrosion resistance steel. As one of ferrite stainless steele of this kind' we can mention an 18 % Cr steel used at the temperature of 550°C to 700°C.
This steel has such drawbacks as the cold notch shortness, coarse crystal grain, blittleness at 475°C, sigma phase formation etc., and cannot easily be welded. Almost this most of these steels have been, therefore, manually welded.
In order to achieve the automatic welding, we have recently made on trial the bonded fluxes which are added with specific alloy components, investigated various weldability and jointing prope, rties of the submerged arc weld metal by means of the above fluxes, and also examined several brittleness phenomena, which seemed to be problematic in the use of welded apparatus.
The test results can be summarized as follows.
(1) As for the bonded flux, H 430 for 18 Cr steel, DC reverse polarity should be used in welding.As this flux is characterized by low hydrogen, the removal of slag is somewhat difficult.
(2) Crystal grains of the submerged arc welded metal could be microrefined according to JIS 6-7 by virtue of Al, Ti and Cb effects.
(3) The most important problematic point in welding the 18 Cr steel is that the transition temperature is high, and the impact value at room temperature is low. When all the deposit metal was tempered at 750 °C, its impact value at 15 °C was very good, i.e., approximately 8kg·m/cm².
(4) The weld metal shall be preheated at 200 °C to 250 °C to abate the stress caused by rapid heating and quenching in time of welding, and prevent the crack. Air Cooling at 750 °C 2h is desirable for the heat treatment after welding.
(5) The 475 °C brittleness of the deposit metal becomes remarkable, when age-treatment is carried out for 1800h. At 300 °C, the impact value reduces to 3 kg-m/cm².