Welding-metallurgy:The Most Important Ingredient.SOLUTIONS with Effective, Practical AdviceWelding-metallurgy is the subject of Chapter 4 of the Welding Handbook, Ninth Edition, Volume 1, Welding Science and Technology. AWS WHB-1.9 For those who deal with welding it is a necessary sub-set of all the metallurgical knowledge. That encompasses also 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 to understand and explain the behavior of weldments or to look for suitable procedures necessary to surmount the difficulties arising sometimes from certain local conditions. (Sponsored Links) Designers of welded structures should be well advised to check their projects with welding metallurgists before releasing their drawings to manufacturing, because it is much easier avoiding pitfalls by the addition of a simple note to the drawing, or by the specification of a certain material capable of withstanding service conditions, than looking for expensive solutions of welding problems when all materials are already cut for being assembled, or worse, when they fail in service. 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. 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 the 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 may remain in the composition, either innocuous or for influencing properties. Alloys displaying useful engineering properties are exploited to withstand certain service conditions at the least possible cost. The Welding-metallurgy examination of ground, polished and etched metallic sections 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 limiting the strength. Melting, fusion and solidification describe the transformation from solid to liquid and vice versa when the temperature increases and decreases. The melting temperature is definite and exact 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 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, describing 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, meaning that subtle changes in composition and/or treatment make them suitable for widely different applications. In particular phase transformations occurring while cooling down certain steels at given 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. Further cooling drives its transformation in a face-centered cubic structure called gamma iron or austenite. Additional cooling causes a further structural transformation to a different body-centered cubic structure called alpha ferrite. Steel is an alloy of iron containing a low percentage of carbon that can contain other alloying elements to enhance certain properties. The presence 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, or the application of specific cycles of heating and cooling at given rates of temperature change vs. time, causes metallurgical phase transformations 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. 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 levels for given applications.This is an example of the importance of Welding-metallurgy for understanding the effects ofapplied treatments. Welding-metallurgy studies also such properties as thermal expansion and conductivity that have a direct bearing on the possibility to provide the required heat at the joint, and on the deformations likely to be caused by joining processes. Also corrosion resistance must be studied by Welding-metallurgy to find the most favorable applications of suitable materials in given service conditions, and to study the most cost effective solutions for every application. Sometimes the use of clad metals could be preferred depending on the total cost projected 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. In conclusion Welding-metallurgy, the sum of cumulative knowledge that considers all aspects of the physical transformations occurring locally at the joint location, is the most important tool for assuring welding performance. 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