This document discusses metallurgical difficulties in welding ferritic, martensitic, and duplex stainless steels. For ferritic steels, welding can cause loss of ductility through small amounts of martensite formation or rapid grain growth. Precautions like limiting heat input are recommended. Martensitic steels are more weldable but prone to cold cracking; preheating and post-weld heat treatment may be needed. Duplex steels can experience precipitation or secondary austenite formation with improper welding parameters. Selection of the correctly matched filler metal composition is also important to avoid undesirable microstructures in the weld metal.

1. METALLURGICAL DIFFICULTIES
IN WELDING OF FERRITIC
MARTENSITIC AND DUPLEX
STAINLESS STEEL
Prepared by
E,SARATHKUMAR
1763013`
GCE SALEM

2. STAINLESS STEEL
Stainless steels are stainless and corrosion resistant, due to the presence of
chromium in amounts greater than 12%, where it forms a passive film on the surface
of the steel.
FERRITIC ss
Chromium is an alloying element that promotes the formation of ferrite in steel; in the
case of the ferritic stainless steels, this ferrite is the high temperature form known as
delta-ferrite.
The ferritic stainless steels contain up to 27% chromium and are used in applications
where good corrosion/oxidation resistance is required but in service loads are not
excessive, e.g. flue gas ducting, vehicle exhausts, road and rail vehicles.
ferritics possess slightly higher yield strength and slightly lower tensile strength ,
although the difference is not great

3. Difficulties in ferritic ss
 At very low temperatures, the ferrites may become brittle. This effect is seen
as a sudden decrease in notch toughness
 There are a number of welding problems with the ferritic steels. Although
they are not regarded as hardenable, small amounts of martensite can form,
resulting in a loss of ductility.
 In addition, if the steel is heated to a sufficiently high temperature, very
rapid grain growth can occur, also resulting in a loss of ductility and
toughness.
 Although the ferritic steels contain only small amounts of carbon, on rapid
cooling carbide precipitation at the grain boundaries can 'sensitise' the steel
making it susceptible to inter-crystalline corrosion. When this is associated
with a weld it is often known as weld decay.
 The ferritic stainless steels are generally welded in thin sections. Most are
less than 6mm in thickness where any loss of toughness is less significant

4.  Most of the common arc welding processes are used although it is regarded as
good practice to limit heat input with these steels to minimise grain growth
(1kj/mm heat input and a maximum interpass temperature of 100-120°C is
recommended) implying that the high deposition rate processes are inadvisable.
 Welding consumables for the ferritic steels are generally of the austenitic type;
type 309L (low carbon grade) is the most commonly used.
 This is to ensure that any dilution that occurs does not result in a low ductility
austenitic / ferritic / martensitic weld metal micro-structure.
 However, provided care is taken to control dilution, types 308 and 316 may be
used. Nickel based consumables may also be used and will result in better
service performance where the component is thermally cycled.
 Post weld heat treatment (PWHT) at around 620°C is rarely carried out
although a reduction in residual stress will give an improved fatigue performance:

5.  The martensitic grades contain up to 18% chromium and have better
weldability and higher strengths than the ferritic grades. They are often
found in creep service and in the oil and gas industries where they have
good erosion and corrosion resistance.
 A hydrogen release treatment from the preheat temperature, say 350°C
for four hours, is unlikely to reduce the risk of cold cracking. If the steel is
not allowed to cool to a sufficiently low temperature so that full
transformation to martensite takes place then there will be austenite
present during the hydrogen release treatment.
 This austenite will retain hydrogen and may generate cracks when it
transforms to martensite as the joint is cooled to ambient. If cold cracking is
a real issue, even with good hydrogen control, then it may be necessary to
PWHT directly from the preheat temperature, cool to ambient and repeat
the PWHT to temper any martensite that was formed following the first
cycle of PWHT.
 Welding consumables matching the base metal composition are available
for most of the martensitic stainless steels, often with small additions of
nickel to ensure that no ferrite is formed in the weld.
MARTENSITIC ss

6. DUPLEX STAINLESS STEEL
• At elevated temperatures duplex stainless steels are vulnerable to
DISTILLATION COLUMN corrosion. This makes thorough failure analysis
and corrosion testing imperative.
• The current condition of the materials used is not only important for safety
and availability but also crucial for the profitability and the service life of
existing plants.
• Inappropriate base metal specification often leads to unsuitable heat
affected zone (HAZ) properties. Autogenous fusion zones are also of
concern. This issue centers around nitrogen limits. The most frequently
encountered is applying the UNS S31803 composition for 2205 DSS,
instead of the S32205 composition.
• Inappropriate welding heat input arises most frequently with SDSS. While
0.5 to 1.5 kJ/mm is a normal heat input recommendation for SDSS, either a
root pass or many small beads towards the low end of this heat input range
tends to result in precipitation and/or secondary austenite formation in weld
metal subjected to repeated thermal cycles from multiple weld passes.

7. • Inappropriate PWHT occurs when the enhanced nickel filler metals (typically
9% Ni) are used. DSS are not normally given PWHT, but extensive forming
of heads, for example, or repair welding of castings, may require a postweld
anneal.
• Specifications such as ASTM A790 and A890 call for annealing at 1040ºC
minimum, and the fabricator tends to use temperatures close to that
minimum. However, the enhanced nickel filler metals require higher
temperatures to dissolve sigma phase that forms during heating to the
annealing temperature.

8. SELECTION OF FILLER MATERIAL
TYPES
1. OVER MATCHED
2. UNDER MATCHED
3. EXACTLY MATCHED

9. Ni eqi = %Ni + 30%C + 0.5%Mn+ 30%N
Cr eqi = %Cr + 1.5%Si + %Mo+ 0.5nb
Based on the above formulas the microstructure of weld metal was predetermined
Base material composition are provided in ASME sec IX
Standardised filler metal composition are selected form ASME sec IIC

10. The following AWS standards2 are referenced in the mandatory
sections of this document:
AWS A5.01M/A5.01 (ISO 14344 MOD), Procurement Guidelines
for Consumables—Welding and Allied Processes—
Flux and Gas Shielded Electrical Welding Processes
AWS A5.02/A5.02M:2007, Specification for Filler Metal Standard
Sizes, Packaging, and Physical Attributes

11. FERRITIC SS
Base material composition :
c-.1% Mn-2% S-.045%
Ni-14% Si-1% p-.03%
Cr-18% Mo-3%
Filler material composition:

12. Base material composition :
c-.03% Mn-2% S-.03%
Ni-10-15% Si-1% p-.045%
Cr-16-18% Mo-2-3%
AUSTINITIC SS
Filler material composition:

13. Base material composition :
c-.15% Mn-1% S-.02%
Ni-1-2% Si-1% p-.02%
Cr-11-13% Mo-.4-.6%
MARTENSITIC SS
Filler material composition:

15. THANK YOU