Welding:
Stainless steels can be welded by standard welding procedures.
The response to welding differs across the different families
of stainless steels and is quite different from carbon steel.
Austenitic:
Austenitic stainless steels are easily welded by all standard
procedures without preheat using near matching consumables.
Welds are very tough and corrosion resistant. Low carbon or
stabilized
grades are preferred to avoid sensitization.
Duplex:
Duplex austenitic-ferritic stainless steels require care with
heat input and consumable composition to prevent formation
of excessive
ferrite and precipitates that can reduce toughness and corrosion
resistance.
Ferritic:
Ferritic stainless steels weldments have comparatively low
toughness, low ductility microstructures, etc. Use of austenitic
consumables, reannealing and stabilized grades can give some
improvements.
Martensitic:
Martensitic and precipitation-hardened martensitic stainless
steels present varying degress of difficulty. Risk of hydrogen
embrittlement and hard zone cracking, the need for high temperature
preheat and immediate postweld tempering increases when carbon
content exceeds about 0.07%.
Precipitation hardened steels usually require postweld ageing.
Comparision between the Welding of Stainless
& Carbon Steel Significant physical
property differences compared with carbon
structural steels means stainless steel welding will:
Require less energy input
a) lower thermal conductivity and melting point
b) greater electrical resistance giving faster consumable
melt off at the same current.
Suffer greater local dilation
a) lower thermal conductivity, sharper gradients
b) higher expansion and contraction, requiring closer tacking
of sheet and thin plate.
Will not be subject to the uneven expansion stresses associated
with formation of hydrogen cracking
sensitive martensite, excepting martensitic and P-H grades.
Brazing:
Stainless steel assemblies may be brazed for water and corrosive
fluid services and for high temperature applications. For
water and fluids, various silver-based fillers (40-92%Ag-Cu
alloys, some with Zn, Cd, Ni, or Li, eg AWS B Ag series) are
used. Their melting and brazing temperatures range from 620-900°C
and account should be taken of the effect of this thermal
cycle on the corrosion resistance of the steel being brazed.
Strengths of Ag-based fillers decrease above 250°C therefore
Ni-based fillers (AWS B Ni series), involving brazing at 900-1200°C,
are used for high temperature service.
Brazing alloys are available in a wide variety of
forms: rod, wire, powder, sheet, strip and preformed shapes.
Joint design, gap and diametrical clearance are important.
In general, the tighter the fit, the stronger the joint and
clearances between 0.02 and 0.06 mm are normal.
Brazing
fluxes are necessary, unless brazing is done in special furnaces
with protective atmospheres.
By their nature, fluxes are corrosive and may contain fluorides
and chlorides. Therefore the flux residue must be removed
from the joint
immediately after brazing in order to prevent corrosion of
the stainless steel joint.
Soldering:
Most stainless steel is now joined by welding, but soldering
is still used, for example, in architecture, plumbing and
processing equipment. Principal fillers are 50-60% Sn 40-50%
Pb for soldering above the 180-220°C range. Tin-rich solders
(95-100% Sn,Sb,Ag) are used as non-toxic fillers and where
colour match to the stainless steel is important. These require
slightly higher soldering temperatures -
above 220-250°C.
Less reflective surface finishes are more easily soldered.
The lower thermal conductivity compared with copper requires
lower heat input to avoid overheating the joint. Fluxes are
usually phosphoric-acid based and should be neutralized and
removed thoroughly.
