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With the
ever-increasing demand for duplex stainless steel process
equipment fabricators have developed procedures for the welding
and fabrication of these grades. A lot of data on these procedures
as well as practical experiences have become available. When
fabricating duplex stainless steels special attention should
be paid to heat treatment and welding. Unsuitable heat treatment
can result in precipitation of intermetallic phase and deterioration
of toughness and corrosion resistance. Although most welding
methods can be used to weld duplex steels, they require special
procedures for the retention of properties after welding.
Below you will find some general guidelines for welding duplex
stainless steels and two practical papers on welding and fabrication
respectively.
General
Guidelines, Practical Aspects for production Welding, A Fabricators
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General
Guidelines: Differences Between Duplex and Austenitic Stainless
Steels, Selection of Starting Material, Cleaning Before Welding,
Joint Design, Preheating, Heat Input and Interpass Temperature,
Postweld Heat Treatment, Phase Balance in the Weld, Dissimilar
Metal Welds, Applicable Welding Methods, Welding Procedure
Qualification.
By Ralph
Davison, Technical Marketing Resources, USA
Originally published by TAPPI journal 2000, volume 83, no.9.
Introduction
It is
assumed that the reader already has experience in welding
of austenitic stainless steels such as Type 316L.This section
addresses some to commonly discussed welding characteristics
and procedures of the duplex stainless steels in terms of
how they differ from austenitic stainless steels. Addressing
each of these features is essential for the design of technically
and economically effective welding procedures to be qualified.
Differences
between Duplex and Austenitic Stainless Steels
Duplex
stainless steels are typically twice as strong as common austenitic
stainless steels. The thermal expansion of the duplex grades
is intermediate to that of carbon steel and the austenitic
stainless steels. The thermal conductivity of the duplex stainless
steels is also intermediate to that of carbon steels and the
austenitic stainless steels.
When there
are problems with welding of austenitic stainless steels,
those problems are most frequently associated with hot cracking
of the weld metal itself. This hot cracking tendency is aggravated
by fully or predominantly austenitic solidification, and by
the combination of high thermal expansion and low thermal
conductivity. For the more common austenitic stainless steels,
hot cracking is minimised by adjusting the composition of
the filler metal to provide a significant ferrite content.
For the more highly alloyed austenitic stainless steels where
the use of a nickel-base filler metal is necessary, austenitic
solidification is unavoidable. In these cases these problems
must be managed by minimising joint constraint and by low
heat input, often requiring many passes to build up the weld.
Duplex
stainless steels have good hot cracking resistance. Hot cracking
of the duplex weld metal is seldom a concern. The problems
most typical of duplex stainless steels are associated with
the heat-affected zone (HAZ), not with the weld metal. The
HAZ problems are not hot cracking but rather a loss of corrosion
resistance and toughness, or of post-weld cracking. To avoid
these problems, the welding procedure should focus on minimising
total time at temperature in the "red hot" range for the whole
procedure rather than managing the heat input for any one
pass. Experience has shown that this approach can lead to
procedures that are both technically and economically optimal.
The data
shown in the appendix of ASTM A 923 suggest how rapidly intermetallic
phases can precipitate to the extent that corrosion resistance
and toughness are significantly affected.
With this
introduction in mind, it is possible to give some general
guidelines for welding of duplex stainless steels and then
to apply this background and those guidelines to specific
welding methods.
The welding
characteristics of duplex stainless steels are much more sensitive
to minor within-grade variations in chemistry or processing
than are austenitic stainless steels. For example, the importance
of having sufficient nitrogen in the duplex stainless steel
base metal has been repeatedly emphasised. Air cooling of
a plate, even when rapid, through the 705 to 980°C (1300 to
1800°F) range will use up some of the "time on the clock"
for the welder to complete the weld before detrimental reactions
occur. Similarly, if a plate is allowed to air cool into this
range during transfer to water quenching, that time is no
longer available to the welder. The metallurgical condition
of the material used in actual fabrication should be the same
quality with regard to composition and production practice,
as the material used to qualify the welding procedure.
Cleaning
Before Welding
The need
to clean prior to welding applies to all stainless steels.
But the duplex stainless steels are more sensitive to contamination,
and especially to moisture, than the austenitic stainless
steels. The chemistries of the base metal and the filler metal
have been developed assuming no additional sources of contamination.
Dirt, grease, oil, paint, and sources of moisture of any sort
will interfere with welding operations and adversely affect
the corrosion resistance and mechanical properties of the
weldment. No amount of procedure qualification is effective
if the material is not thoroughly clean before welding.
Joint
Design
Duplex
stainless steels require good joint preparation. For duplex
stainless steels, a weld joint design must facilitate full
penetration and avoid autogenous regions in the weld solidification.
It is best to machine rather than grind the weld edge preparation
to provide uniformity of the land thickness or gap. When grinding
must be done, special attention should be given to uniformity
of the weld preparation and the fit-up. Any grinding burr
should be removed to maintain complete fusion and penetration.
For an austenitic stainless steel, a skilled welder can overcome
some deficiencies in joint preparation by manipulation of
the torch. For a duplex stainless steel, these techniques
can cause a longer than expected exposure in the harmful temperature
range, leading to results outside of those of the qualified
procedure.
Examples
of joint designs used with duplex stainless steels are shown
in Figure 1.4 Other designs are possible provided that they
assure full penetration welds and minimise the risk of burn-through.
Examples
of joint designs applied to 2205 duplex stainless steel:
Fig.
1a)
2 mm (0.08 in) < t < 4 mm (0.16 in)
A = 1-2 mm (0.04-0.08 in)
A. Square Butt Joint - Suitable for single-sided SMAW or double-sided
SMAW or GMAW.
Fig. 1b)
t < 2.5 mm (0.1 in)
A = 1-2 mm (0.04-0.08 in)
B. Square Butt Joint - Suitable for GTAW from one side. Backing
gas required.
Fig. 1c)
4 mm (0.16 in) < t < 12 mm (0.5 in)
A = 2 mm (0.08 in)
B = 2 mm (0.08 in)
C. Suitable for heavier sections with SMAW or GMAW. Increase
A to 3 mm (0.12 in) for vertical-up SMAW.
Fig. 1d)
12 mm (0.5 in) < t < 60 mm (2.5 in)
A = 3 mm (0.06 in)
B = 2 mm (0.08 in)
Radius = 6 mm (0.25 in)
D. Suitable for very thick base metal with SMAW or GMAW.
Fig. 1e)
9 mm (0.36 in) < t < 12 mm (0.5 in)
B = 5 mm (0.2 in)
E. Suitable for SAW. Grinding after first pass facilitates full
penetration.
Fig. 1f)
4 mm (0.16 in) < t < 12 mm (0.5 in)
A = 2.5 mm (0.1 in)
B = 5 mm (0.2 in)
F. Full penetration Fillet. Suitable for SMAW. Tack weld with
SMAW or GMAW.
Fig. 1g)
4 mm (0.16 in) < t < 12 mm (0.5 in)
A = 2.5 mm (0.1 in)
B = 2.5 mm (0.1 in)
G. Single V Joint. Pipe welding. Suitable with SMAW
Fig. 1h)
3 mm (0.12 in) < t < 12 mm (0.5 in)
A = 1-2 mm (0.04-0.08 in)
B = 2 mm (0.08 in)
H. Single U Joint. Pipe Welding. Suitable with GTAW.
Preheating
As a general
rule, preheating of duplex stainless steel is not recommended
because it slows the cooling of the heat-affected zone. Preheating
should not be a part of a procedure unless there is a specific
justification.
Preheating
may be beneficial when used to eliminate moisture from the
steel as may occur in cold ambient conditions or from overnight
condensation. When preheating to remove moisture, the steel
should be heated to about 95°C (200°F) uniformly and only
after the weld preparation has been cleaned.
Preheating
may also be beneficial in those exceptional cases where there
is a risk for forming a highly ferritic HAZ because of very
rapid quenching. Examples include welding a thin sheet to
a plate, as with a liner to a vessel or a tube to a tubesheet,
or any very low heat input weld where there is exceedingly
rapid cooling. (See FAQ)
Heat
Input and Interpass Temperature
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