Weldability of materials
Stainless steel
Stainless steels are chosen because of their enhanced corrosion resistance,
high temperature oxidation resistance or their strength. The various types of
stainless steel are identified and guidance given on welding processes and
techniques which can be employed in fabricating stainless steel components
without impairing the corrosion, oxidation and mechanical properties of the
material or introducing defects into the weld.
Material types
The unique properties of the stainless steels are derived from the addition
of alloying elements, principally chromium and nickel, to steel. Typically, more
than 10% chromium is required to produce a stainless iron. The four grades of
stainless steel have been classified according to their material properties and
welding requirements:
- Austenitic
- Ferritic
- Martensitic
- Austenitic-ferritic (duplex)
The alloy groups are designated largely according to their microstructure.
The first three consist of a single phase but the fourth group contains both
ferrite and austenite in the microstructure.
As nickel (plus carbon, manganese and nitrogen) promotes austenite and
chromium (plus silicon, molybdenum and niobium) encourages ferrite formation,
the structure of welds in commercially available stainless steels can be largely
predicted on the basis of their chemical composition. The predicted weld metal
structure is shown in the Schaeffler diagram in which austenite and ferrite
promoting elements are plotted in terms of the nickel and chromium equivalents.
Because of the different microstructures, the alloy groups have both
different welding characteristics and susceptibility to defects.
Austenitic stainless steel
Austenitic stainless steels typically have a composition within the range
16-26% chromium (Cr) and 8-22% nickel (Ni). A commonly used alloy for welded
fabrications is Type 304 which contains approximately 18%Cr and 10%Ni. These
alloys can be readily welded using any of the arc welding processes (TIG, MIG,
MMA and SA). As they are non-hardenable on cooling, they exhibit good toughness
and there is no need for pre- or post-weld heat treatment.
Avoiding weld imperfections
Although austenitic stainless steel is readily welded, weld metal and HAZ
cracking can occur. Weld metal solidification cracking is more likely in fully
austenitic structures which are more crack sensitive than those containing a
small amount of ferrite. The beneficial effect of ferrite has been attributed
largely to its capacity to dissolve harmful impurities which would otherwise
form low melting point segregates and interdendritic cracks.
As the presence of 5-10% ferrite in the microstructure is extremely
beneficial, the choice of filler material composition is crucial in suppressing
the risk of cracking. An indication of the ferrite-austenite balance for
different compositions is provided by the Schaeffler diagram. For example, when
welding Type 304 stainless steel, a Type 308 filler material which has a
slightly different alloy content, is used.
Ferritic stainless steel
Ferritic stainless steels have a Cr content typically within the range
11-28%. Commonly used alloys include the 430 grade, having 16-18% Cr and 407
grade having 10-12% Cr. As these alloys can be considered to be predominantly
single phase and non-hardenable, they can be readily fusion welded. However, a
coarse grained HAZ will have poor toughness.
Avoiding weld imperfections
The main problem when welding this type of stainless steel is poor HAZ
toughness. Excessive grain coarsening can lead to cracking in highly restrained
joints and thick section material. When welding thin section material, (less
than 6mm) no special precautions are necessary.
In thicker material, it is necessary to employ a low heat input to minimise
the width of the grain coarsened zone and an austenitic filler to produce a
tougher weld metal. Although preheating will not reduce the grain size, it will
reduce the HAZ cooling rate, maintain the weld metal above the ductile-brittle
transition temperature and may reduce residual stresses. Preheat temperature
should be within the range 50-250 deg.C depending on material composition.
Martensitic stainless steel
The most common martensitic alloys e.g. type 410, have a moderate chromium
content, 12-18% Cr, with low Ni but more importantly have a relatively high
carbon content. The principal difference compared with welding the austenitic
and ferritic grades of stainless steel is the potentially hard HAZ martensitic
structure and the matching composition weld metal. The material can be
successfully welded providing precautions are taken to avoid cracking in the
HAZ, especially in thick section components and highly restrained joints.
Avoiding weld imperfections
High hardness in the HAZ makes this type of stainless steel very prone to
hydrogen cracking. The risk of cracking generally increases with the carbon
content. Precautions which must be taken to minimise the risk, include:
- using low hydrogen process (TIG or MIG) and ensure the flux or flux coated
consumable are dried (MMA and SAW) according to the manufacturer's
instructions;
- preheating to around 200 to 300 deg.C. Actual temperature will depend on
welding procedure, chemical composition (especially Cr and C content), section
thickness and the amount of hydrogen entering the weld metal;
- maintaining the recommended minimum interpass temperature.
- carrying out post-weld heat treatment, e.g. at 650-750 deg.C. The time and
temperature will be determined by chemical composition.
Thin section, low carbon material, typically less than 3mm, can often be
welded without preheat, providing that a low hydrogen process is used, the
joints have low restraint and attention is paid to cleaning the joint area.
Thicker section and higher carbon (> O.1%) material will probably need
preheat and post-weld heat treatment. The post-weld heat treatment should be
carried out immediately after welding not only to temper (toughen) the structure
but also to enable the hydrogen to diffuse away from the weld metal and HAZ.
Duplex stainless steels
Duplex stainless steels have a two phase structure of almost equal
proportions of austenite and ferrite. The composition of the most common duplex
steels lies within the range 22-26% Cr, 4-7% Ni and 0-3% Mo normally with a
small amount of nitrogen (0.1-0.3%) to stabilise the austenite. Modern duplex
steels are readily weldable but the procedure, especially maintaining the heat
input range, must be strictly followed to obtain the correct weld metal
structure.
Avoiding weld imperfections
Although most welding processes can be used, low heat input welding
procedures are usually avoided. Preheat is not normally required and the maximum
interpass temperature must be controlled. Choice of filler is important as it is
designed to produce a weld metal structure with a ferrite-austenite balance to
match the parent metal. To compensate for nitrogen loss, the filler may be
overalloyed with nitrogen or the shielding gas itself may contain a small amount
of nitrogen.
Copyright by TWI, 1999
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