Weldability
Martensitic stainless steels can be welded in the annealed,
hardened, and hardened-and-tempered conditions. Regardless of the
prior condition of the steel, welding produces a hardened
martensitic zone adjacent to the weld. In other words, the
high-temperature HAZ will be in the "as-quenched" condition after
welding, regardless of the prior condition of the material. In
addition, the HAZ hardness is very much independent of the cooling
rate over the temperature range experienced in common arc welding
practices.Because such high hardness values render the material
prone to cracking during fabrication, the selection of appropriate
preheating levels and welding procedures is critical to the success
of the welding process.
Hardenability of Ferrous AlloysThe capability of an alloy to be
hardened by heat treatment is called its hardenability. The term
hardenability should not be confused with hardness, which is the
resistance of a material to indentation or scratching.Hardenability
of ferrous alloys depends on the carbon content, the grain size of
the austenite, the alloying elements present in the material, and
the cooling rate.
The End-quench Hardenability Test- Lominy testA round test bar
100 mm long, made from the particular alloy, is austenitized-that
is, heated to the proper temperature to form 100% austenite then
quenched directly at one end with a stream of water at 24C. The
cooling rate thus varies throughout the length of the bar, the rate
being highest at the lower end, which is in direct contact with the
water. The hardness along the length of the bar is then measured at
various distances from the quenched end.
Hardness decreases away from the quenched end of the bar. The
greater the depth to which the hardness penetrates, the greater the
hardenability of the alloy. Each composition of an alloy has its
particular hardenability band. Note that the hardness at the
quenched end increases with increasing carbon content
AWS B4.0: Weldability TestingThe term weldability is the
capacity of material to be welded under the imposed fabrication
conditions into a specific, suitably designed structure and to
perform satisfactorily in the intended service. The tests
included:-the Controlled Thermal Severity (CTS) Test, -Cruciform
Test, -Implant Test, Lehigh Restraint Test, -Varestraint Test,
-Oblique YGroove Test, -Welding Institute of Canada (WIC)
Test,-Trough Test, and -the Gapped Bead On Plate (GBOP) Test.
Their applications are summarized below:
Welding Dissimilar MetalsSelecting the appropriate welding
process and thefiller metal requires careful consideration when
joining dissimilar metals. The choice of both should bebased on
metallurgical factors such as differences inthermal expansion
coefficients between the weld metaland base metal, the effects of
dilution on the weldmetal, and the possibility of changes in the
structureof the materials after extended service at
elevatedtemperatures.The shielded metal arc welding process has
theadvantage in making dissimilar metal welds in that theamount of
filler metal added is less influenced bywelder technique than the
GTAW or GMAW processes.In GTAW, the welder can vary filler
metaladdition to a very large degree.The gas tungsten arc welding
process permits morecontrol over dilution than most other
processes. Thegas metal arc welding (GMAW) process is sometimesused
for joining dissimilar metals, but the proceduremust be carefully
controlled to prevent excessive dilution.The submerged arc welding
(SAW) process canalso be used, but again, procedures must be
controlledto avoid excessive dilution from the joint sidewall.
Filler Metals.A variety of materials can be weldedusing nickel
alloy filler metals. Stainless and carbonsteels, low-alloy steels,
and high-nickel alloys areamong the possibilities.Either covered
electrodes or bare filler metals areavailable and can be specified
to suit equipment andskills.
Some of the most commonly used electrodes are listed in:
ANSI/AWS A5.14, Specification for Nickel and Nickel Alloy Bare
Welding Rods and Electrodes;and ANSI/AWS A5.11, Specification for
Nickel and Nickel AlloyWelding Electrodes for Shielded Metal Arc
Welding.
DILUTIONThe change in chemical composition of a welding filler
metal caused by the admixture of the base metal or previous weld
metal in the weld bead. It is measured by the percentage of base
metal or previous weld metal in the weld bead.
The percentage dilution can be determined by measuring areas
labeled A and B. Percentage dilution is then calculated as:
Dilution is usually considered as a percentage of the base metal
which has entered into the weld metal. When two pieces of base
metal are welded together, the final composition of the weld
deposit consists of a mixture of base metal and filler metal. The
portion of the base metal that has been melted in with the filler
material and has diluted it may be expressed in percent dilution.
This is determined by the following formula:
Typical values of dilution for various processes are:
Many factors affect dilution:The greatest dilution occurs when
no filler metal is added. In this instance, all of the weld deposit
is self-generated by the base metal. Similarly, a single-pass weld
will have a higher percentage of dilution than a multi-pass weld.
There is always considerable dilution in the root pass. The greater
the amount of weaving, the greater the dilution.Dilution as low as
2% has been achieved with the plasma arc hot wire cladding
operation utilizing two hot wires connected in series. The
application was welding copper rotating bands on artillery
shells.
What is the difference between the various Carbon Equivalent
Formulae used in relation to hydrogen cracking?Carbon is the most
important of all alloying additionsto steels because of the effects
it produces on themicrostructure as the welds cool from the very
hightemperatures associated with the deposition of weldmetal. This
applies as much to the heat-affected zone(HAZ) of the plate as it
does to the weld metal. Inaddition, when carbon equivalents are of
concern, theyare generally related to the HAZ.Two of the most
troublesome problems associatedwith fabricating steels are
hydrogen-induced crackingand poor toughness or ductility. Both are
aggravatedby a microstructure called martensite. Since martensiteis
very hard, its presence can be inferred by measuringthe hardness of
the HAZ, particularly in the coarsegrainedregions which are close
to the weld deposit.Carbon has a profound and direct effect on
hardness.Other alloying elements also affect hardness, althoughnot
to the same degree. In total, they affect the facilitywith which a
given hardness can be obtained in analloy steel. This is called
hardenability.However, the most important use of this concepthas
not been in predicting hardness, but predicting theminimumpreheat
temperature needed to avoid the formationof the hard martensite.
Since martensite is producedat higher cooling rates, anything that
can bedone to reduce cooling rates can be beneficial towardavoiding
that microstructure or a high hardness. Preheatisimportant because
it has a very strong effect on the rate at which welds cool.
Weldability, energy inputand cooling rates are important
variables.
Carbon equivalent formulae were originally developed to give a
numerical value for a steel composition which would give an
indication of a carbon content which would contribute to an
equivalent level of hardenability for that steel. These formulae
were later extended to represent the contribution of the
composition to the hydrogen cracking susceptibility of steel. They
are also used as compositional characterising parameters for other
properties that may be linked to hardness, such as toughness and
strength.These kind of relationships originate from about 1940 when
Dearden and O'Neill proposed a carbon equivalent formula to predict
steel strength, hardenability and HAZ hardness[1]. In 1967, the
International Institute for Welding (IIW) adopted a somewhat
simplified form of Dearden and O'Neill's formula for hardenability
which became a generally accepted measure of steel weldability -
CEIIW
Since its adoption by IIW, the equation has been incorporated
into a number of material standards and codes, including EN
1011-2:2001[2](replaces BS 5135-1984[3]) and in a modified form in
AWS D1.1[4], with a "+Si/6" term added to the equation.Further
development of Carbon equivalent formulae has taken place and
several can be found in technical literature today. Three of the
more common ones are Pcm, CEq and CEN. Ito and Bessyo[5]developed
Pcmin Japan based on a wider range of steels than the IIW
formula:
The CEq formula devised by Dren[6]has a similar appearance:
Both the Pcmand the CEq formulae were developed for low carbon
steels for which the CEIIWis less suitable. Pcmis generally used
for modern steels typically used for pipeline manufacture, where
carbon contents are no more than ~0.11 wt%[7].However, it should be
noted that the Pcmformula was derived largely from lower C low
alloy steels.The CEN formula was proposed to evaluate the
weldability of a wide variety of steels. For the higher C range,
the values of CEN correlate well with carbon equivalents such as
CEIIW, whereas for lower carbon steels the values are close to
those of the CEq formula. CEN is given by:
Yurioka[8]illustrated a good correlation between Pcmand CEN for
structural steels, low-alloy steels (Ni-Cr-Mo type) and carbon
steels, provided the carbon content was less than 0.17 wt%. From
this comparison the following relationship was derived:CEN = 2Pcm-
0.092 (C 0.17%)
For carbon steels, the values deviate from this relationship as
the Pcmoverestimates the cold cracking susceptibility. Where the
carbon content exceeds 0.17 wt%, there is a better correlation
between CEN and CEIIW:CEN = CEIIW+ 0.012 (C 0.17%)Yurioka[8]grouped
a number of carbon equivalents for the assessment of weldability,
as follows:
GroupFormula
A
B
C
Group A are characterised by 1/6 as the coefficient of
Manganese; Group B has Carbon as more important than the other
alloying elements and is more applicable to modern steels; Group C
includes interactions between Carbon and other elements.Other
existing formulae that could be classified as Group B are the CEw
developed by Cottrell[9], and the CET included in the standard EN
1011-2:2001[3]. Cottrell claimed that the CEw formula could improve
the predictions of cracking by a factor of three, compared to
CEIIW. The data on which the formula was based was collected from
results published in open literature, and covered a wide range of
composition and welding conditions. The resulting formula is as
follows:
The CET formula is based on similar elements to the CEIIWformula
with the exception of Vanadium, although carbon is considered to
have more significance than the other elements: