Welding-metallurgy:

The Most Important Ingredient.

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Welding-metallurgy is the subject of this page.

An introductory treatise is found in Chapter 4 of the Welding Handbook, Ninth Edition, Volume 1, Welding Science and Technology.

AWS WHB-1.9
Welding Science and Technology,
Welding Handbook, 9th Ed., Vol. 1
American Welding Society, 01-Jan-2001

For those who deal with welding it is a necessary sub-set of all the metallurgical knowledge.

Metallurgy in general would also encompass arguments widely remote from the worries of average welders, like extractive technologies or base metal shape production.

In the normal welding practice however, Welding-metallurgy principles are often called in cause.

There is a need to understand and explain the behavior of weldments.

Also to look for suitable procedures necessary to surmount the difficulties arising sometimes from certain local conditions.

Designers of welded structures should be well advised to check their projects with welding metallurgists before releasing their drawings to manufacturing.

It is easy to avoid pitfalls by adding of a simple note to the drawing.

And by specifying a special material whose properties had been overlooked.

Much more economical than looking for expensive solutions of welding problems detected in service failures.

Among the subjects covered by Welding-metallurgy are those regarding:

  • the structure of metals and alloys,
  • melting,
  • fusion,
  • solidification,
  • phase transformation,
  • mechanical properties as influenced by cold working and heat treatment,
  • thermal expansion,
  • conductivity
  • and corrosion resistance.

Welding-metallurgy to understand what happens

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Metals in solid form are materials displaying crystalline structure, meaning that their atoms are arranged in definite geometric patterns.

Common metals can be classified according to the type of crystalline structures they exhibit.

Some metals change their crystal structure from one type to another at specific temperatures.

Welding-metallurgy studies how some of the properties are structure dependent.

Pure metals are seldom used.

More common are alloys or complex mixtures.

Alloys are identified by indicating the base metal, that of the largest proportion in the detailed composition.

Specific alloying elements are added to the base metal, to modify the properties in favorable ways.

Residual elements in tiny proportions may remain in the composition, either innocuously or for influencing properties.

Alloys displaying useful engineering properties are exploited to withstand service conditions at the least possible cost.

The Welding-metallurgy examination of micro-structure is performed on ground, polished and etched metallic sections.

It reveals, under optical microscope, the presence of small bodies, called grains, each with its own crystallographic orientation, and touching at their boundaries.

The crystal structure is far from perfect, including different types of defects that contribute to limit the strength of metallic bodies.

Melting, fusion and solidification describe the transformation from solid to liquid and vice versa as the temperature increases and decreases.

The melting temperature is definite and exact only for pure metals and for certain alloys called eutectics.

For all other alloys one can speak only of melting ranges, temperatures at which solid and liquid coexist in varying proportions.

In Welding-metallurgy phase changes describe the crystallographic "allotropic" structure transformations.

These are incurred by distinct, describable portions, called phases, under driving forces caused by temperature changes, when crossing certain critical temperatures.

Phase diagrams are useful graphic representations, possibly simplified to include only two elements.

They describe the phases present, both in solid and liquid form, at different temperatures as a function of concentration.

The percentage of one element decreases from left to right from 100% to 0, while the percentage of the other element increases from 0 to 100%.

Any feature representing special behavior results clearly identified by the composition and the temperature where occurring.

Iron and steel are among the most popular materials due to their relatively limited cost and to their versatility.

This means that subtle changes in composition and/or heat treatment make them suitable for widely different applications.

In particular phase transformations occurring while cooling down certain steels at well defined cooling rates provide useful mechanical properties, sought for particular services.

Thorough understanding of these transformations as explained by Welding-metallurgy is essential to the successful application of welding processes to steel structures.

Molten pure iron solidifies as a body-centered cubic structure called delta ferrite. This means the idealized figure of a cube with an atom at each corner and one more at its center.

Further cooling drives its transformation into a face-centered cubic structure called gamma iron or austenite.

Here the atoms are found at different positions than those described above.

Additional cooling causes a further structural transformation to a different body-centered cubic structure called alpha ferrite.

Steel is an alloy of iron and carbon (with a low percentage of this element) that can contain also other alloying elements to enhance certain properties.

The presence and the amount of carbon modifies the temperatures at which phase transformations occur.

Other common structures, called cementite, pearlite, bainite and martensite, cannot be described in a short exposition.

The versatility of steel depends on careful control of composition and on suitable heat treatments to generate the required structures.

Heat Treatment consists in the application of specific cycles of heating and cooling at given rates of temperature change vs. time.

HT causes metallurgical phase transformations that change the structure, developing the mechanical properties needed to meet definite service conditions.

Cold rolling and cold forging introduce in the material plastic deformations that increase strength while decreasing ductility.

In other words, Cold work changes the mechanical properties in such ways as to be favorable for given applications.

Successive application of heat (e.g. by welding) may reduce the strength to unacceptable low levels for given applications.

This is an example of the importance of Welding-metallurgy for understanding the effects of applied treatments.

Welding-metallurgy studies also the influence of thermal expansion and conductivity.

These may have a direct bearing on the possibility to provide the required heat at the joint.

And also on the deformations likely to be caused by joining processes.

Corrosion resistance too must be studied by Welding-metallurgy to find the most favorable applications of suitable materials in given service conditions.

The most cost effective solutions must be singled out for every application.

Sometimes the use of clad metals could be preferred depending on the total cost calculated for the entire duration of the useful life of the project.

An introduction on a new Website Page titled Metallurgical Expertise was published (8) in issue 82 of Practical Welding Letter for June 2010.
Click on PWL#082 to see it.

An Article on Precautions when welding Superferritic [stainless steels] was published (2) in Issue 140 of Practical Welding Letter for April 2015.
Click on PWL#140 to see it.

An Article on new materials and new metallurgy still in the earliest stages of its formation introduces a new class of materials being studied and developed.
See the note and the links to a website page on High Entropy Alloys and to a Resource Page on the same argument, presented in (8), Issue 141 of Practical Welding Letter for May 2015.
Click on PWL#141 to see them.

An Article on Metallurgical call up was published (7) in Issue 143 of Practical Welding Letter for July 2015.
Click on PWL#143 to see it.

An Article introducing the Last Volume of the Welding Handbook, volume 5 Edition 9 (WHB-5.9) was published (2) in Issue 143 of Practical Welding Letter for August 2015.
Click on PWL#144 to see it.

An Article on Furnace Brazing Carbon Steel was published (3)
in Issue 147 of Practical Welding Letter for November 2015.
Click on PWL#147.

An Article introducing the new Bulletin_117 was published (8) in Issue 151 of Practical Welding Letter for March 2016. It contains Resources on Metals, Part 1 with instructions on how to build a no cost Welding Encyclopedia as explained in our page on Metals Knowledge.
Click on PWL#151.

In conclusion Welding-metallurgy is the sum of cumulative vast knowledge.

It considers all aspects of physical transformations occurring locally at the joint location.

Therefore it is the most important tool for assuring welding performance.

Any welding shop could profit by securing access to the services of knowledgeable professionals.

Coming from a welding engineer this comment can be carelessly disregarded as self-serving.

It would be a mistake. Sometimes a useful hint could save lots of time and of money.

Watch the following Video

Hobart Institute - Basic Metallurgy

http://www.youtube.com/watch?v=K8TEPwCE1Ow

* * *

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