The TIG welders know that different strategies could be used when welding carbon and low alloy steels versus when welding stainless steels. Any ac TIG welder will tell you that when welding carbon and low alloy high strength steels, the main consideration is in most cases with a minimum preheat temp along and comes with a minimum inter pass temperature.

When the commercial welderis performing code welding of carbon and low alloy steels, if the carbon content or carbon equivalent is high enough, then a minimum preheat temperature is specified on the welding procedure. This low, even minimum preheat temperature slows the cooling rate and prevents the formation of brittle areas.

The minimum interpass temerature is pretty much using the same concept as with the minimum preheat temp. The minimum preheat means that the part has to reach a certain level of temperature before the TIG weder ever starts the welding. The minimum interpass means the part needs to maintain a certain temp. The carbon equivalent number for the high strength steel in this video is approximately 60%, and that is why it is important to adhere to preheat and interpass the temperatures.

The TIG welding is different when the stainless steel is concerned. Actually, the 300 series stainless steels will not harden by the quick cooling.

The commercial welders say that the welding stainless steels is to try to keep them below the maximum interpass temperature. The process of keeping 300 series stainless at a temperature that is elevated, for example like over 800 F for a long period of time, results sometimes in the so called carbide precipitation.

In other words, with the stainless steel, the carbon and chromium combine to form chromium carbides and that causes chromium depleted areas. The chromium, as the commercial welders say, is in general the element responsible for the corrosion resistance, for chromium depleted areas that will corrode, rust, and could also even crack. Of course, this will not happen immediately, but once the part is being used and placed in service. And this is the thing that causes problem.

In general, for carbon and low alloy steels, the commercial welder should make sure to keep the part hot and slow the cooling rate. With the 300 series stainless, the metal should not get too hot.