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 to understand 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.
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 steel without 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.
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, magnetic properties, 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?
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 appropriate filler 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
WHAT TOOLS DO WELDERS TYPICALLY USE WHEN PREHEATING, INTERPASS TEMPERATURE CONTROL AND POSTHEATING RECOMMENDATIONS ARE MADE?
When sophisticated inspection tools, ovens, and furnaces are not required or used a welder may use an oxy-fuel torch and tempelsticks (tempilstik) to control preheating, interpass temperatures, and postheat. Tempilstiks are crayon like indicators made of materials that melt within 1% of their rated temperature. Mark the workpiece before heating begins. The dry opaque Tempilstik mark will change to a distinct melted mark when the temperature rating of the selected Tempilstik has been reached. Tempilstiks are available in a range of temperatures making them useful for controlling temperature fluctuations between multiple pass welds (interpass temperature) and for cooling parts.
To slow the cooling rate of small parts welders use methods to keep the part warm and exclude air such as; burying in sand or some other medium, or wrapping in a fire retardant blanket.
HOW ARE STEELS CLASSIFIED?
The American Iron and Steel Institute (AISI) and the Society Of Automotive Engineers (SAE) use a four or five digit numbering system to classify steels by chemical composition. The first digit indicates the type of steel, the second digit indicates the approximate percentage of the main alloying element, and the last two or three digits indicate the average carbon content. For example; in the case of the 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 1.75 |
| Nickel 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 Chromium |
| Nickel 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 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 = Nickel
Cr = 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.
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 under the 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 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.
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.
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.
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 steel or 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.
Source: http://deltaschooloftrades.com
