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WELDING CARBON STEEL
The following information is not intended to be a guide to welding structures in
Industry. It is intended as a basic introduction to a complex subject known as
Metallurgy. In many Industrial settings the procedure to be followed when welding a
given type and grade of metal is established through testing to a specific Code or
Standard or through practical experience. However; it may be useful for the welder tounderstand the affects of welding on metal. One of the most widely welded
classifications of metal is the group of carbon steels.
WHY DOES THE WELDER NEED TO KNOW ANYTHING ABOUT THE STEEL
HE IS ASKED TO WELD?
In many cases the welder needs only to know the techniques of actual welding and
does not need to be concerned about the type or grade of steel being welded. This is
because a large amount of steel used in fabricating a metal structure is low Carbon
or plain carbon steel (also called mild steel). When welding these steels with any of
the common arc welding processes like Stick Mig or Tig there are generally few
precautions necessary to prevent changing the properties of the steel.Steels that have higher amounts of Carbon or other alloys added may require
special procedures such as preheating and slow cooling, to prevent cracking or
changing the strength characteristics of the steel. The welder may be involved in
following a specific welding procedure to ensure weld metal and base metal have the
desired strength characteristics.
WHAT ARE THE TERMS USED TO DESCRIBE THE CHARACTERISTICS OF
METAL?
Before reviewing the weldability of steel we need to understand the terms used to
describe the changes that may occur due to welding the steels.Review the definitions below as an introduction and refer back to them as necessary.
MECHANICAL PROPERTIES OF STEEL
Mechanical properties are the properties of the steel reacting to some load or
mechanical working such as bending, machining, or shaping. Mechanical properties
affect how the metal will react when fabricating a structure. While Iron is a
relatively soft metal that can be easily shaped or formed, other elements may be
added to the iron to give it a specific strength or enhanced mechanical properties.
The terms used to describe these properties are as follows:
STRESS
Stress is defined as the load per unit area and is measured in pounds per square
inch. Stress is pressure acting on a weld or metal to pull it apart, twist it, compress
it, or shear it, depending on the direction and type of load. In some cases one or
more of the above loads may be applied in varying degrees.
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STRAIN
Strain is the resulting deformation of the applied stress. For example: If a piece has
stress acting to bend it, the amount or degree to which the piece bends is the
measure of the strain. In other words stress and strain go together, for instance; if
you stress your back by lifting or carrying a heavy load the resulting pain or damage
is the strain.
ELASTICITY
Elasticity is the property of a material that when stressed or has a force applied
allows the shape to return to its original shape. In other word when the load is
removed there is no appreciable strain or deformation. Metals have a limit of
elasticity and when the load increases beyond the limit deformation or strain will
occur.
PLASTICITY
Plasticity is the ability of a metal to be deformed or shaped without rupture. For
example a piece of plain carbon steel can be shaped easier then a piece of tool steelwithout rupturing or breaking.
STRENGTH
Strength is the ability of a material to resist deformation. Plasticity and strength
work together since plasticity is the ability to take the applied load its strength is the
ability to withstand or resist deforming under the load. Metals with high strength
will deform less than metals with lower strength.
TENSILE STRENGTH
The tensile strength is the ability of a metal to withstand forces acting to pull it
apart and is measured in pounds per square inch. For example: the E-7018 electrode
produces a weld with a tensile strength of 70,000 pounds per square inch as shown
by the first two digits of its number.
DUCTILITY
Ductility is the ability of a metal to be easily shaped or elongated without failure or
rupture. Generally metals with high tensile strength are tougher but have lower
ductility and ductile metals are softer and have lower tensile strength. Ductility is
the property that allows metals such as aluminum and copper to be drawn into wire
forms.
HARDNESS
Hardness is defined as the ability of a material to resist indentation and is a function
of its elastic and plastic properties. The harder the metal the more it is able to resist
wear and tear.
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MALLEABILITY
Malleability is the ability of a metal to be shaped by compressive forces without
rupture. Metals with good malleability can be rolled into thin sheets. For example
gold has high malleability and can be rolled and shaped into thin sheets.
BRITTLENESS
Brittleness is basically a term used to describe the lack of plasticity or ductility. A
brittle metal cannot be easily deformed or shaped. For example: a hardened steel or
cast iron may be brittle and show very little resistance to impact or shock.
PHYSICAL PROPERTIES OF STEEL
Physical properties are related to the structure and nature of the steel or Alloy and
include density, electrical conductivity, heat conductivity, melting point, magneticproperties, reflectivity, and coefficient of thermal expansion.
Of the above properties, one of the most important is the coefficient of thermal
expansion. Steel when heated increases in length, width, and thickness. The increase
in unit length when a metal is heated one degree is called its coefficient of thermal
expansion. When welding takes place a localized area is heated to melting
temperature and begins cooling, steel that has a high coefficient of thermal
expansion such as Stainless Steel will warp or change dimensionally more than
regular steel. Distortion or warping due to welding will be covered in a later lesson.
CARBON STEELS
WHAT IS CARBON STEEL?
Carbon Steel is principally a mixture (or Alloy) of Iron and Carbon with small
amounts of silicon, sulfur, phosphorous, and manganese. Other elements may be
added to the steel to impart a specific quality to enhance its usefulness.
An Alloy may be thought of as a recipe, similar to a recipe for chicken soup that has
ingredients to enhance the flavor, Iron has other elements or ingredients to enhance
the properties of the Iron.
In plain carbon steels it is the Carbon additive that has the greatest effect on the
strength and weldability of the steel.
The carbon is added to the Iron in varying amounts to harden or strengthen the
steel. As carbon content increases the hardness and tensile strength increases and
the ductility, plasticity, and malleability will decrease.
The reason the carbon content or carbon recipe varies is to produce a family of
steels that exhibit the desired characteristics for a given application.
HOW DOES THE AMOUNT OF CARBON AFFECT WELDABILITY OF
STEELS?
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In general as the carbon content increases the weldability (how easily welded)
decreases. In other words the higher the carbon content the more likely special
procedures such as preheating, interpass temperature control and postheating are
necessary.
The following chart groups carbon content, typical uses and weldability.
Group Content % Typical usage Weldability
Low
carbon
steel
0.15
Maximum
Welding electrodes, rivets
and nails
softer easily formed shapes.
Excellent weldability with all processes usually no
preheat interpass or postheat necessary
Mild
steel
Plain
carbon
0.15 to 0.30 Plate, angle, and bar stock
for general fabrication.
Mild steel accounts for a
large segment of welded
parts of Industry where
good plasticity and ductility
is required.
Readily weldable with all processes without
preheat, interpass, or postheat except for very
thick sections.
Medium
carbon
steel
0.30 to 0.50 Used for Machine parts,
gears, and where parts may
be hardened by heat
treating.
Parts may be readily welded with all process if
preheat, interpass temperature controls, and post
heat recommendations are followed.
Use Low hydrogen Electrodes and appropriatefiller wire.
Heat treating after welding may be applied
High
carbon
steels
0.50 To 1.0
Springs, Dies, Railroad
Track, Many tools, Band
saws, and Knives. Also used
where a sharp edge is
required.
Usually require preheat interpass temperature
control and postheat. Special heating and cooling
procedures in a furnace such as normalizing may
be required to restore the properties of the metal
after welding. High carbon Electrodes designed
for welding tool steels or the specific alloy are
readily available from welding supply companies.
Note: As carbon increases steel toughness and welding precautions increase
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widely used 1020 steel, the first digit indicates carbon steel the second digit indicates
no predominant alloy other than carbon and the last two digits indicate .20 carbon
content.
The following shows some of the AISI SAE series designations of steel with xx
representing the range of carbon content for the group.
SERIES DESIGNATION TYPE OF STEEL
Carbon Steels
10xx Plain Carbon
11xx Machining Resulferized
12xx Machining Resulferized Phospherized
Manganese Steels
13xx Manganese 1.75Nickel Steels
31xx Nickel 3.5
25xx Nickel 5.0
Nickel Chromium Steels
31xx Nickel 1.25 Chromium 0.60
32xx Nickel 1.75 Chromium 1.00
33xx Nickel 3.5 Chromium 1.5
34xx Nickel 3.0 Chromium 0.77
Molybdenum Steels
40xx Molybdenum Carbon
41xx Molybdenum Chromium
43xx Molybdenum Nickel ChromiumNickel Chromium Molybdenum Steels
43xx Nickel Chromium Molybdenum
Nickel Molybdenum Steels
46xx Nickel Molybdenum
48xx Nickel Molybdenum
Chromium Steels
50xx Chromium
51xx Chromium
As shown by the above chart the first group of steels are the Carbon steels. The
other groups of steels have additional elements ( alloys) added to enhance their
properties in some specific way.
HOW DOES THE ADDITIONAL ELEMENTS IN THE ABOVE CHART AFFECT
THE STEEL AND WELDABILITY?
Low alloy steels are Carbon steels that have additional elements added (alloyed) to
produce a classification of steel that has a specific benefit for production use.
Although Carbon is the main alloy that affects hardenability and weldability other
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elements also harden steel and play a role in the weldability of steel. For example
Manganese and Molybdenum aid in hardening steels. For this reason a formula may
be applied to a classification of steel to roughly determine the hardenability and
hence the weldability and need for pre-heating. One example of a formula is shown
below.
Carbon equivalent for alloy steels
CE = % C + %Mn + Ni + Cr + Mo + V
6 15 6 4 5
CE = Carbon Equivalent
C = Carbon
Mn = Manganese
Ni = NickelCr = Chromium
Mo = Molybdenum
V = Vanadium
Manganese
Manganese is used to harden steels and increase its toughness and strength. High
manganese content coupled with increased carbon content lowers the ductility and
weldability. Consideration of preheat and or postheat techniques usually apply.
Molybdenum
May be used in conjunction with other elements to aid in hardening and provide
steel with good strength at elevated temperatures. Preheating may be required for
welding and they are often heat treated after welding.
Nickel
Nickel may be used to Increase toughness and impact strength and improve
corrosion resistance. Good strength and ductility may be obtained even with lower
carbon content. Depending on the amount added special procedures may be
necessary when welding.
Chromium
Chromium helps improve the hardenability of steels and improves wear resistance,
heat resistance, and corrosion resistance. Depending on the amount added special
procedures may be necessary when welding.
Chromium and Chromium Nickel are used in the production of Stainless Steel.
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The Strength and Mechanical properties of carbon and alloy steels may be changed
or shaped for a specific application by heat treating in furnaces or ovens. When two
pieces of metal are welded using any of the commonly used arc welding processes:
Stick, Mig, Or Tig the metals and filler are heated to the melting temperature underthe arc and allowed to solidify to form the weld.
HOW DOES THE WELDER KNOW HOW TO WELD A GIVEN STEEL
STRUCTURE?
The best way for a welder to know how to weld a particular steel or steel
classification is through the use of a Welding Procedure Specification (WPS). A
WPS is a written set of instructions (specifications) detailing the welding procedure,
joint preparation, filler metal, current type and range as well as any required
preheat, interpass temperature controls and postheat treatments. Whenever possible
welders request and use a Welding Procedure Specification for the type and grade of metal they are welding.
A welding Procedure Specification is developed by engineering or inspection
personnel using qualified welders to weld a specific type of metal and joint
configuration that will be used on the job, while recording the welding parameters
and variables. The completed joint is then tested in accordance with a specific Code
or Standard. The resulting information is written on a form called a Procedure
Qualification Record. The information from the Welding Procedure Qualification
Record is used to write the Welding specification and as long as the procedure is
carefully followed the resulting welded products will have the required strength
characteristics.
Some companies that do not have a formal Welding Specification have through
practical experience developed a set of instructions that the welder must follow to
successfully weld the given project.
If no Welding Procedure is provided at a minimum the welder MUST know what
the base metal is and find out if special precautions are necessary for welding.
There should always be some method of traceability for metals used to fabricate
parts. Metals sections and shapes should be stamped color coded or made from
known materials. There are ways to test unknown metals through appearance,
magnetic properties or spark testing; however, these test are subjective and may not
be reliable for all cases. When you can trace the material through purchase orders
or metal identification you know or can find out how to weld it.
WHAT IS THE HEAT AFFECTED ZONE AND HOW DOES IT AFFECT
WELDABILITY?
The heating and cooling action that occurs when welding is a form of heat treating
in the localized area of the puddle and weld joint that may result in changes to the
mechanical properties of the base metal and surrounding area. The area most
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affected by heating and cooling during welding is called the HEAT AFFECTED
ZONE (HAZ)
THE HEAT AFFECTED ZONE
The heating and cooling rate of welding directly under the arc is from the melting
temperature to normal temperatures and may occur relatively quickly or methods
may be used to slow the cooling rate of the joint. These methods include postheating
the weld area with an oxy-fuel torch, blanketing the weld area, or using a precise
heating and cooling method in a furnace or industrial setting.
The more expensive and precise method of using a furnace under controlled
conditions restores the mechanical properties of the weld joint and the surrounding
base metal.
The area surrounding the joint is heated to various temperatures depending on the
distance from the arc, the heat input of the process and the number of weld passes.
This area is referred to as the Heat Affected Zone.
The grains structure in the melted weld area may form a desirable size and shape,
while the grain structure of the surrounding heat affected area may change to a less
desirable shape and size and may cause cracking when welding on medium or high
carbon steels. Often when welding a hardenable steel the heat affected area can
harden to undesirable levels, while welding an already hardened steel may result in
a softened heat affected zone with loss of desired hardness.
The heat affected zone may also have locked in stresses that can lead to problems
when the welded structure is in service. Some industries employ a heat treating
process called stress relieving to relieve residual stresses due to working or welding
the structure.
It is imperative to use the correct electrode for the application so that weld metal is
compatible with the base metal and fewer changes occur due to the carbon or alloy
content of the filler wire. Electrodes are available for welding tool steels and Cast
iron.
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When welding thick sections, medium carbon, high carbon, and high alloy steels
check the recommended procedures for control of the heating and cooling rate
There are heat treating options such as annealing or normalizing that may be used
to restore the grain structure of the welded piece.
When welding low carbon, mild steels and most low alloy steels the heat affected
area does not change the properties of the metal enough to become a problem
regardless of the cooling rate.
The heating and cooling that occurs in the heat affected area and surrounding metal
may also lead to heat distortion of the parts being joined.
Procedures may be used before, during, and after welding to minimize distortion.
WHAT IS HEAT DISTORTION?
Steel when heated increases in length, width, and thickness. The increase in unit
length when a metal is heated one degree is called its coefficient of thermal
expansion. I f a small square block of steel were heated evenly under ideal
conditions it would expand with the heat and contract when cooling relatively
evenly. When welding a piece of steel only the joint and surrounding area is heated
and cooled. This cause uneven expansion and cooling and the piece begins to warp
or distort. Uncontrolled Distortion may lead to a serious dimensional defect or lead
to failure of the part. Steps may be taken before, during, and after welding to
minimize or control the effects of heat distortion.
WHAT ARE SOME OF THE THINGS A WELDER CAN DO BEFORE WELDING
TO LIMIT HEAT DISTORTION?
Joint Preparation
The joint should be planned and prepared to limit the amount of weld and weld
passes. For example: Wide angle V grooves welded from one side would distort more
than double V grooves welded from both sides.
Select the proper Equipment
Higher welding speeds using iron powder electrodes (E-7018) and larger diameters
may reduce the amount and effect of heat distortion. Semi-automatic and fully
automatic welding processes limit the heat input and distortion.
Use Clamps Jigs and Fixtures
Jigs and fixtures with clamps hold parts in alignment and reduce the free movement
of parts from heat expansion. The clamps are left in place until the parts are welded
and cooled. In addition to clamps pieces called stiffeners may be temporarily added
to areas that tend to distort and removed when the part cools.
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WHAT ARE SOME OF THE THINGS A WELDER CAN DO DURING WELDING
TO LIMIT HEAT DISTORTION?
Sequencing Welding
Use a skip or backstep method of welding to distribute the heat around the joint.
This involves making shorter welds at different locations of the joint then joining
them together.
Welding the joint
If possible two welders weld opposite sides of the joint at the same time.
Use the smallest size fillet welds practical to reduce heat input.
If solid welding is not necessary for strength use intermittent welding and stagger
the sequence.
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Weld flat or horizontal positions with larger size electrodes that allow for more weld
deposit at faster speeds whenever practical. Vertical positions and multiple pass
welds result in more heat input.
CONTROL OF DISTORTION AFTER WELDING
Distortion is more difficult to control after welding. Techniques like alternating
heating and cooling to remove warpage called straightening require a degree of skill
and practice. Postheating to remove stresses and warpage in controlled
environments such as normalizing and annealing, often involving the use of furnaces
is usually done by qualified personnel.
SUMMARY
Although the vast majority of carbon steels used in fabricating parts are mild steelor low carbon and presents little difficulty in welding, some carbon steels that have
more carbon such as tool steels, high alloy steels and cast Iron require special
procedures to prevent cracking and weld failure. The welder should know the type
of steel he is asked to weld to prevent problems that may lead to questions of his or
her ability. If you are unsure ask questions and research the type of steel and its
weldability.