Weldability of materials
Steels
In arc welding, as the weld metal needs mechanical properties to match the
parent metal, the welder must avoid forming defects in the weld. Imperfections
are principally caused by:
- poor welder technique;
- insufficient measures to accommodate the material or welding process;
- high stress in the component.
Techniques to avoid imperfections such as lack of fusion and slag inclusions,
which result from poor welder techniques, are relatively well known. However,
the welder should be aware that the material itself may be susceptible to
formation of imperfections caused by the welding process. In the materials
section of the Job Knowledge for Welders, guidelines are given on material
weldability and precautions to be taken to avoid defects.
Material types
In terms of weldability, commonly used materials can be divided into the
following types:
- Steels
- Stainless steels
- Aluminium and its alloys
- Nickel and its alloys
- Copper and its alloys
- Titanium and its alloys
- Cast iron
Fusion welding processes can be used to weld most alloys of these materials,
in a wide range of thickness. When imperfections are formed, they will be
located in either the weld metal or the parent material immediately adjacent to
the weld, called the heat affected zone (HAZ). As chemical composition of the
weld metal determines the risk of imperfections, the choice of filler metal may
be crucial not only in achieving adequate mechanical properties and corrosion
resistance but also in producing a sound weld. However, HAZ imperfections are
caused by the adverse effect of the heat generated during welding and can only
be avoided by strict adherence to the welding procedure.
This part of the materials section of Job Knowledge for Welders considers the
weldability of carbon-manganese (C-Mn) steels and low alloy steels.
Imperfections in welds
Commonly used steels are considered to be readily welded. However, these
materials can be at risk from the following types of imperfection:
- porosity;
- solidification cracking;
- hydrogen cracking;
- reheat cracking.
Other fabrication imperfections are lamellar tearing and liquation cracking
but using modern steels and consumables, these types of defects are less likely
to arise.
In discussing the main causes of imperfections, guidance is given on
procedure and welder techniques for reducing the risk in arc welding.
Porosity
Porosity is formed by entrapment of discrete pockets of gas in the
solidifying weld pool. The gas may originate from poor gas shielding, surface
contaminants such as rust or grease, or insufficient deoxidants in the parent
metal (autogenous weld), electrode or filler wire. A particularly severe form of
porosity is 'wormholes', caused by gross surface contamination or welding with
damp electrodes.
The presence of manganese and silicon in the parent metal, electrode and
filler wire is beneficial as they act as deoxidants combining with entrapped air
in the weld pool to form slag. Rimming steels with a high oxygen content, can
only be welded satisfactorily with a consumable which adds aluminium to the weld
pool.
To obtain sound porosity-free welds, the joint area should be cleaned and
degreased before welding. Primer coatings should be removed unless considered
suitable for welding by that particular process and procedure. When using gas
shielded processes, the material surface demands more rigorous cleaning, such as
by degreasing, grinding or machining, followed by final degreasing, and the arc
must be protected from draughts.
Solidification cracking
Solidification cracks occur longitudinally as a result of the weld bead
having insufficient strength to withstand the contraction stresses within the
weld metal. Sulphur, phosphorus, and carbon pick up from the parent metal at
high dilution increase the risk of weld metal (solidification) cracking
especially in thick section and highly restrained joints. When welding high
carbon and sulphur content steels, thin weld beads will be more susceptible to
solidification cracking. However, a weld with a large depth to width ratio can
also be susceptible. In this case, the centre of the weld, the last part to
solidify, will have a high concentration of impurities increasing the risk of
cracking.
Solidification cracking is best avoided by careful attention to the choice of
consumable, welding parameters and welder technique. To minimise the risk,
consumables with low carbon and impurity levels and relatively high manganese
and silicon contents are preferred. High current density processes such as
submerged-arc and CO2, are more likely to induce cracking. The welding
parameters must produce an adequate depth to width ratio in butt welds, or
throat thickness in fillet welds. High welding speeds also increase the risk as
the amount of segregation and weld stresses will increase. The welder should
ensure that there is a good joint fit-up so as to avoid bridging wide gaps.
Surface contaminants, such as cutting oils, should be removed before welding.
Hydrogen cracking
A characteristic feature of high carbon and low alloy steels is that the HAZ
immediately adjacent to the weld hardens on welding with an attendant risk of
cold (hydrogen) cracking. Although the risk of cracking is determined by the
level of hydrogen produced by the welding process, susceptibility will also
depend upon several contributory factors:
- material composition (carbon equivalent);
- section thickness;
- arc energy (heat) input;
- degree of restraint.
The amount of hydrogen generated is determined by the electrode type and the
process. Basic electrodes generate less hydrogen than rutile electrodes (MMA)
and the gas shielded processes (MIG and TIG) produce only a small amount of
hydrogen in the weld pool. Steel composition and cooling rate determines the HAZ
hardness. Chemical composition determines material hardenability, and the higher
the carbon and alloy content of the material, the greater the HAZ hardness.
Section thickness and arc energy influences the cooling rate and hence, the
hardness of the HAZ.
For a given situation therefore, material composition, thickness, joint type,
electrode composition and arc energy input, HAZ cracking is prevented by heating
the material. Using preheat which reduces the cooling rate, promotes escape of
hydrogen and reduces HAZ hardness so preventing a crack-sensitive structure
being formed; the recommended levels of preheat for various practical situations
are detailed in the appropriate standards e.g. BS 5135:1984. As cracking only
occurs at temperatures slightly above ambient, maintaining the temperature of
the weld area above the recommended level during fabrication is especially
important. If the material is allowed to cool too quickly, cracking can occur up
to several hours after welding, often termed 'delayed hydrogen cracking'. After
welding, therefore, it is beneficial to maintain the heating for a given period
(hold time), depending on the steel thickness, to enable the hydrogen to diffuse
from the weld area.
When welding C-Mn structural and pressure vessel steels, the measures which
are taken to prevent HAZ cracking will also be adequate to avoid hydrogen
cracking in the weld metal. However, with increasing alloying of the weld metal
e.g. when welding alloyed or quenched and tempered steels, more stringent
precautions may be necessary.
The risk of HAZ cracking is reduced by using a low hydrogen process, low
hydrogen electrodes and high arc energy, and by reducing the level of restraint.
Practical precautions to avoid hydrogen cracking include drying the electrodes
and cleaning the joint faces. When using a gas shielded process, a significant
amount of hydrogen can be generated from contaminants on the surface of the
components and filler wire so preheat and arc energy requirements should be
maintained even for tack welds.
Reheat cracking
Reheat or stress relaxation cracking may occur in the HAZ of thick section
components, usually of greater than 50mm thickness, Fig. 4. The more likely
cause of cracking is embrittlement of the HAZ during high temperature service or
stress relief heat treatment.
As a coarse grained HAZ is more susceptible to cracking, low arc energy input
welding procedures reduce the risk. Although reheat cracking occurs in sensitive
materials, avoidance of high stresses during welding and elimination of local
points of stress concentration, e.g. by dressing the weld toes, can reduce the
risk.
Weldability of steel groups
European Standard EN 287 identifies a number of steels groups which have
similar metallurgical and welding characteristics. The main risks in welding
these groups are:
Group W 01 low carbon unalloyed (carbon-manganese) steels and/or low alloyed
steels
For thin section, unalloyed materials, these materials are normally readily
weldable. However, when welding thicker section material with a flux process
(MMA), there is a risk of HAZ cracking which will needs low hydrogen electrodes.
The more highly alloyed materials also require preheat, or a low hydrogen
welding process, to avoid HAZ cracking .
Group W 02 chromium-molybdenum (CrMo) and/or chromium-molybdenum-vanadium
(CrMoV) creep resisting steel
Thin section material may be welded without preheat but using a gas shielded
process (TIG and MIG); for thicker section material, and when using a flux
process, preheat with low hydrogen electrodes (MMA) is needed to avoid HAZ and
weld metal cracking. Post-weld heat treatment is used to improve HAZ toughness.
Group W 03 fine-grained structural steels and nickel steels (2% to 5%)
The weldability is similar to Group W 02 in that preheat is required for
welding thick section material with flux processes.
Group W 04 ferritic or martensitic stainless steel, with chromium (12% to
20%)
When using filler to produce matching weld metal strength, preheat is needed
to avoid HAZ cracking. Post-weld heat treatment is essential to restore HAZ
toughness.
An austenitic stainless steel filler can be used where it is not possible to
apply a preheat and post-weld treatment.
Copyright by TWI, 1999
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