Page 11 - Stainless Steel Solutions

JOINING

Welding guidelines

High helium blends are not usually used in spray transfer because of the considerably higher voltages and
currents required to obtain it. The high heat input could lead to greater distortion and possible sensitization.

3. Use the correct filler metal
Selection of the correct filler metal is critical to the successful long-term performance of a stainless steel
weldment. Fillers are selected based on chemistry of the materials to be joined, the corrosive media to which
they will be exposed, and the microstructure required in the final deposit.

For ferritic, martensitic, and duplex stainless steels, the consumables selected generally have a composition
nearly identical to that of the base material. The selection for austenitic alloys is not as simple.
The microstructure of austenitic stainless steel weld metal varies depending upon the alloys involved. To
assure a strong, tough weld metal, a balance between the predominant austenitic material and the ferritic
microstructural constituent must be maintained; so, selection of the proper filler metal alloy is important. To
minimize microfissuring and cracking which can occur when low melting point constituents in the stainless
steel segregate to grain boundaries, ferrite is used to “absorb” these impurities. The amount of ferrite is
controlled by the composition of the weld metal. By selecting a consumable with more ferrite than austenite
stabilizers, a proper balance between the two microstructures is obtained.
A special “selection diagram” for austenitic filler materials has been developed and modified by several
researchers. First developed as the “Schaeffler Diagram” and then modified to become the “Delong
Constitution Diagram” and the “WRC Ferrite Number Selection Guide”, these tools can be used to select the
appropriate filler material, depending upon the type of base material to be joined and the expected amount
of mixing (or dilution) between the base and filler metals.
Copies of these diagrams can be found in documents published by the American Welding Society and a
number of the filler metal manufacturers.
Some applications can also benefit from the use of low carbon and stabilized filler materials that help control
sensitization. High silicon wires are specified to improve metal transfer and make the puddle more fluid for
improved bead shape. JOINING

4. Fill the crater upon weld completion
A very good welding practice is to spend an extra fraction of a second at the weld crater to ensure that it fills
properly. The shrinkage stresses that occur as the weld metal solidifies can produce strains great enough
to pull the metal in the crater apart as it freezes. These cracks are sometimes not visible to the naked eye.
Filling the crater provides enough metal to resist these strains, while the increased heat also helps to slow
the weld cooling rate to reduce any strain produced.
5. Avoid sensitization (overheating the base material)
Sensitization is the formation of chromium carbides in the Heat-Affected Zone (HAZ), the area directly
adjacent to the weld. The heat-affected zone has been heated to just below its melting temperature followed
by rapid cooling. As figure 1 shows, the metal atoms occupy the regular sites in the matrix.
These metals are the iron, chromium, nickel, molybdenum Figure 1
and manganese atoms. The carbon atoms occupy the
small spaces between the metal atoms called interstitial
spaces. In the temperature range of 800-1500 ºF CrC
(427-815 ºC), the carbon atoms actually move through
the metal matrix and combine with the chromium atoms
to form chromium carbide (Cr23C6). CrC
Stainless steels begin to lose corrosion resistance when
the free chromium in the matrix falls below about 10.5%.
When carbide precipitation occurs, some chromium is tied Base Metal HAZ Weld Metal
up as carbides (lowering the level to <10.5%), and the
corrosion resistance of the material is reduced. Iron Chromium Carbon

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