Lesson 5 - Welding Filler Metals for Stainless Steels

the base metal. This allows the weld area to cool more slowly and as a result, the steel in the heat affected zone will not be hardened as severely. 5.7.0.3 The preheating temperature used is in the range of 350°F to 500°F and should be maintained during the entire welding operation. Upon completion of welding, the weldment should be cooled slowly, preferably furnace cooled, allowing gradual temperature change. 5.7.0.4 The mechanical properties of martensitic stainless steels are affected by welding since they harden intensely, even on relatively slow cooling from high temperatures. The weld deposit and the steel that surrounds the weld deposit is hard and brittle. Heat treatment of the weldment is necessary to improve these physical properties. 5.7.0.5 If preheating or postweld heat treatment is not practical, it may be necessary to use a higher alloy austenitic stainless steel electrode (such as 309) that deposits tough, ductile weld metal without cracking. This solution would depend on the required properties of the weldment and is not recommended in all cases. Martensitic stainless steels make up about 15% of the stainless steels that are welded. 5.8 FERRITIC STAINLESS STEELS Ferritic stainless steels are straight chrome alloys in the AISI 400 series. They are magnetic and have varying ranges of chromium content as shown in Figure 12. 5.8.0.1 All ferritic stainless steels have the room temperature crystal structure of ferrite stabilized to all temperatures. The higher chromium content provides good resistance to high temperature scaling. For this reason, the ferritic stainless steels are used to make heat treat- ing containers, jigs, and fixtures. 5.8.0.2 Welding the ferritic high chromium stainless steels, however, is difficult. The steels have rapid rates of grain growth at temperatures over 1700°F. The large grains absorb the smaller grains and grow larger. The resultant coarse grain structures are very crack sensitive. Grain growth is a time and temperature function. To keep the time of high welding temperature as short as possible, these steels should be mildly preheated to about 300°F, welded with small diameter electrodes and with the lowest possible welding current, thereby limiting the heat input. About 5% of the stainless steels welded are of the ferritic category. AISI No. Carbon %* Chromium %* Other %* 405 0.08 11.5 - 14.5 Aluminum 0.10 - 0.30 430 0.12 16.0 - 18.0 -- 446 0.20 23.0 - 27.0 Nitrogen 0.25 * Maximum unless otherwise noted. NOMINAL COMPOSITION-FERRITIC STAINLESS STEELS FIGURE 12

Current
Chapter
Table of Contents