|
Welding of Ferritic Stainless Steels |
Welding of Ferritic Stainless Steels -
Actually, the terms ferritic and non-hardening are only approximate when applied to stainless steels, because many of the types normally included in these classifications do exhibit a degree of thermal hardenability.
Metallurgical structures are apt to be duplex, since several types develop a small amount of austenite during heating, which, upon cooling, produces martensite within the ferrite matrix. In general, the ferritic stainless steels are less weldable than the austenitic stainless steels.
When ferritic steels are heated above 954°C as during welding, it promotes the growth of enlarged ferrite grains. This tendency towards large grain size, coupled with the presence of a small amount of martensite, results in welds that are apt to be brittle and lacking in toughness and ductility at room temperature. Where these steels are used at elevated temperatures the welds may prove somewhat more ductile and shock resistant.
|
|
To minimize grain growth, preheat, inter pass temperatures and weld heat input are kept as low as possible. Grain coarsening can also be controlled by avoiding slow cooling from welding temperature.
Ferritic steels are subject to a type of intergranular precipitation when they are cooled too rapidly. Heat-treatment at 704 to 816°C followed by cooling at a moderate rate is the corrective treatment. Ferritic stainless steels are preheated, before welding, from 149 to 232°C, depending upon material thickness and joint restraint.
In post weld heat treating, temperatures are usually held below 843°C to prevent grain growth and austenite formation beyond the heat affected zone. (The austenite thus formed might, upon cooling, be transformed to brittle martensite). For most purposes, a temperature of 788°C is employed.
Normally, the weldment is furnace-cooled to 593°C, but slow cooling through 565 to 399°C range must be avoided to prevent embrittlement from precipitation or development of a brittle phase. Austenitic electrodes can be used to produce a more ductile weld when heat treating is not possible.
|
|
|
