Q. Use of Stainless Steels in welding?

Considerable care must be exercised to obtain sound, crack-free welds. These difficulties are pronounced when welding very high-chromium ferritic alloys and, to a lesser extent, some of the fully austenitic stainless steels.

To achieve sound, crack-free welds, a controlled quantity of ferrite is usually incorporated into the otherwise austenitic weld metal, to aid in reducing the incidence of cracking during welding.  If the ferrite content is too high, the weldment is susceptible to embrittlement over long periods of time.

Welding causes a slight decrease in the rupture and creep strengths at higher temperatures, but with good welding practices, reliable values can be obtained.  Weld efficiency may decrease with increasing temperature and time.  The reduction in long-term rupture strength of weldments is associated with the lack of long-term ductility in the weld deposit.

HK-40 and other similar high carbon stainless steel castings are used by the petroleum and chemical industries for many high temperature applications.  While they have good creep resistance, they are difficult to repair by welding due to their low ductility (i.e. typical elongation is 10% for castings vs. 25% for wrought steels).  Elongation may be further reduced by brief exposure to temperatures of 1200° -1850°F (650° - 1000°C) during welding or in the aged condition, due to the susceptibility of sigma phase formation. 

Minimum joint restraint during welding is highly recommended to aid in achieving crack-free welds in cast materials intended for high temperature applications.