Do we, as Inspection or Technical personnel, have to know the properties of base materials and its weldability characteristics before welding?
Definitely! Even if you are using a qualified a procedure.
During the fabrication and repair of equipment, a wide variety of base metals may be joined by welding. The major principles of metallurgy are important to the inspector, and the impact on the metals from the welding process is significant. The welding process can affect the welding material’s properties.
Meanwhile, in the design point of view, the proper material selection technique starts with carefully defining the application requirements in terms of mechanical, thermal, environmental, electrical, and chemical properties. The properties determine how a material behaves when it is melted or when it cools down.
On the other hand, the properties of base materials are mainly classified as physical, chemical, mechanical, and dimensional, focusing on our welding terms.
What is a Physical Property in welding materials?
In general, the physical properties in welds are those that can be observed without changing the identity of the substance, things that are measurable. They are relatively insensitive to structure and can be measured without the application of force.
Although not an essential characteristic, it is used as a measure of quality. The mechanical properties of a material can be used to resist stress. Moreover, the most important physical properties are yield and tensile strength. A material with high yield strength can crack during loading. Furthermore, the percentage yield strength is the ratio of stress to strength. Stress, as we know, is the force per unit area applied on the material.
The properties of matter such as melting point, thermal conductivity, electrical conductivity, coefficient of thermal expansion, and density are examples of physical properties.
What is a Chemical Property in welding materials?
Meanwhile, chemical properties describe how a substance changes into a completely different substance. In other words, the chemistry of elements is related to the material’s chemical properties. Chemical properties include composition, chemical bonding, chemical reactions, and chemical properties of compounds. For example, flammability and corrosion/oxidation resistance as some of the properties.
What is a Mechanical Property?
The mechanical properties of materials simply imply, its behavior when different forces are applied to them. Their strength, ductility, wear resistance, for instance.
This is why we look at the operating temperatures and design pressures as well, on the construction drawings. The design and integrity of welded structures and components are dependent on the mechanical properties of base metals, filler metals, and completed welds.
Therefore, the Microstructure, that signifies the grain structure in welds and metals, is mostly responsible for the physical and mechanical properties of the metal.
The function and manufacturability of a specific product is determined by its material properties.
Difference Between Physical and Chemical Properties
Until the phase of the material is considered, the difference between a physical and chemical properties of base materials is simple. It seems like a different substance when a material changes from a solid to a liquid to a vapor (chemical).
Whereas, only the state of the substance changes when it is melted, solidified, vaporized, or condenses. Ice, liquid water, and water Vapor are all H2O (physical).
What is Metallurgy and Weldability for Welds?
Metallurgy is the science of heating or manipulating metals to produce desired properties.
Due to the rapid changes in temperature caused by the welding process, reactions like melting, solidification, gas-metal reactions, slag-metal reactions, surface phenomena, and base metal reactions occur at the base metal and joint.
As a result, the metallurgy at the joint, that is to say, the weld metal, heat affected zone (HAZ), base metal micro structure and chemical composition differs.
Especially, the composition of base metal is the most important factor. All solid metals have a structure in which the atoms of each crystal are arranged in a specific pattern. There is no orderly arrangement of atoms in molten or liquid metals. As they cool, clusters of atoms bond with each other and start to solidify into solid crystals. This is the basic reaction that happens in a joint welding process.
What Is Weldability?
Weldability is the ease with which a metal can be fused, in other words, the ability of the steel to withstand the stresses imposed by the welding operation.
The American Welding Society defines weldability as, “the capacity of a metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended service.”AWS.ORG
What Are the Factors Affecting Weldability of Steels?
In fact, several factors influence the weldability of metals, like metallurgy, welding process, joint Design, weld preparation, melting point, and electrical resistance.
How to Improve Weldability?
Understandably, the changes to metal can make it prone to poor weldability by controlling or monitoring various factors like proper filler metal selection, the correct shielding gases, the right welding process, heat treatment requirements as per code and mainly following the approved welding procedure.
Common Metallic Materials
Besides, a few of the common metals for a general understanding:
- Iron/Steel – Strength critical applications.
- Aluminum – It is easy to form, available and inexpensive.
- Copper – High electrical and thermal Conductivity, high ductility, and good corrosion resistance.
- Titanium – When component weight is a concern, titanium is used for strength in higher temperature(~1000° F).
- Nickel – For very higher temperatures(~1500-2000° F), and corrosion resistance nickel alloys are used.
Physical, and the Mechanical Properties Of An Alloy
The mechanical and physical properties of base materials are determined by both their chemical composition and their internal structure. Alterations of the internal structure may affect the mechanical properties.
Metalworking processes or heat treatment might play a role in affecting some physical properties like density and electrical conductivity, but those effects are usually insignificant.
A key determinant for which alloy is suitable for a given application is the mechanical and physical properties. Most of the time, the engineer designs the part to perform in a given range of properties. High performance in one category can be coupled with lower performance in another.
Certainly, we are to discuss some of the physical and chemical properties of alloys that are related to welds in brief.
Types of Physical and Chemical Properties in Welding materials
- Strength of Material
- Conductivity – Thermal / Electrical
- Corrosion Resistance
- Wear Resistance
Strength Properties in Material
A weld made by monitoring factors like neither with too little heat, or too much, or a previously heat treated history, a proper metal will meet or exceed the strength requirements of the joint.
Types of strength properties, especially, the six fundamental loading conditions in material are listed below:
- Tensile – Metal’s ability to resist forces, the load causes the metal to ultimately fracture.
- Yield – The point at which the material under load will no longer be the same, moves from elastic to plastic.
- Fatigue – Maximum stress the metal can endure in a given number of cycles
- Shear – Occurs when the metal’s internal structure slides against itself at the granular level.
- Torsional strength – The ability of a metal to resist any twisting forces.
- Compressive – The metal has the ability to resist being crushed.
Thermal and Electrical Conductivity of Materials
Thermal conductivity is a measure of the rate of heat flow through a material. All substances conduct heat, but heat moves more readily through some substances than others.
The quantity of electricity that is transferred through a material of known cross-section and length is called Electrical Conductivity.
Density of Material
The density of the alloy is what determines the weight of a component. Different materials are preferred according to their specific density targets. For example, the weight of an engine may be reduced by making the engine parts from a lighter alloy. SI units for density are kg/m³.
Corrosion Resistance of Material
A material’s ability to prevent a natural chemical or electrical attack by atmosphere or other agents is referred to as corrosion resistance. Corrosion involves two chemical processes the Oxidation and Reduction Reaction. Expressed as the maximum depth in mils to which corrosion would penetrate in one year.
Elasticity of a Material
Specifically, elasticity of material is when a force is removed, tends to return to its original size and shape.
Ductility in Metals
Ductility is the ability of a material to bend without fracturing and retain its shape when removed. In other words, the capability of a metal to be permanently bent, twisted, or otherwise manipulated without breaking or cracking.
Malleability of Material
Malleability is a metal’s ability to be formed without breaking. A material that has high malleability will be able to cope with higher pressure.
Plasticity in Material
Plasticity is the tendency of a certain solid material to hold its shape when subjected to forming forces. The quality of the material allows it to be bent or made into a new shape.
Understandably, hardness is the material’s ability to resist a permanent change in shape. Moreover, mostly used to identify its strength and quality of heat treatment. Hardness measurements have been used to check the quality of materials by Mohs Hardness Test, Brinell Hardness Test, Rockwell Hardness Test, Rockwell Superficial Hardness Test, Vickers, and Knoop Micro hardness Tests, Scleroscope and Rebound Hardness Tests, Durometer Hardness Test, Barcol Hardness Test.
The plastic deformation of the surface is considered in metals, ceramics, and most other polymers. Hardness is the resistance to elastic deformation of the surface of some polymers and elastomers.
Toughness is a material’s ability to absorb impact without fracturing at a given temperature, measured using the Charpy impact test.
Wear resistance, in contrast, is a measure of a material’s ability to resist rubbing against one another. Adhesion, abrasion, scratching, rolling and others are some of the ways.
The likelihood that a material will fail or fracture under a relatively small shock, force, or impact mostly due to the increase in hardness is termed as brittleness of material.
Material Testing in terms of Properties
While considering the properties of base materials, the strength and ductility of the weld, for instance, are evaluated in the simplest weldability tests.
- Strength – Weld tension tests, shear strength, and hardness.
- Ductility and fracture toughness – Bend tests and impact tests.
A weldment is usually acceptable for service if it has adequate strength and ductility.
Hope, that I have fairly covered the topics related to the properties of base materials, the metallurgy and weldability characteristics.
So, for you to have the best outcome, avoiding the defects as much as possible by understanding on Properties of Base Materials and their role in welds is very much recommended.
Of course, I will see you in my next post. Do comment your thoughts.