Some thoughts on welding and related microstructure

Part 1

Welding is arguably the most common, popular and successful method of joining metal components, and has virtually completely replaced older methods, such as riveting.  However, the welding process does affect the microstructure of the welded and adjacent material, which may have effects on local properties and physical performance, particularly as far as fatigue is concerned.  Welding effectively constitutes a ‘miniature casting’, which is often rapidly cooled with consequent effects on microstructure and residual stress.  These microstructure changes and the magnitude of the residual stresses are dependent on the cooling rates, nature and number of weld passes, inter-pass temperatures and chemical composition of the material. 

Within a single pass weld, in steels for example, one may obtain long columnar grains/crystals growing in from the side of the weld.  If the temperature is too high and is maintained for a suitable period the growing columnar grains may meet in the center of the weld, forming a plane of weakness within the weld, where crystalline cracking may occur.  However if the welding conditions and temperature are more correct, the grains structure will be more equi-axed, and there is less chance of columnar grain junction and altogether a stronger weld joint is achieved.  With multi run welds, the first run is ‘normalized/austenised’ by the second run, there is grain refinement, (beneficial) destruction of the columnar grains, and so on with each subsequent weld run, further normalization will occur.  Obviously no normalization of the material deposited in the last pass will occur and it will typically exhibits a more coarse, ‘as cast’ structure.

The weld can also exhibit defects like gas porosity or slag inclusions, although these latter defects can be avoided by using a suitable flux, careful preparation/inter-pass flux removal, or by using an inert gas shielding, such as in argon arc welding.  Modern flux coatings and electric arc electrodes yield high quality welds, free of significant defects.  One can, however, encounter ‘gas porosity’ which may be caused by many factors including contamination of the material being welded with hydrocarbons, the use of inappropriate shielding gas, improperly dried welding rods and even cracks in the water cooled nozzles used in some welding torches.  Such issues effectively introduce gas bubbles into the weld and cause porosity.  So called ‘weld metal cracking’ can also occur if the components are constrained and strains developed during welding are not readily released.

Welds also introduce geometric imperfections/changes in section.  In butt welds of two plates, lack of penetration to the full depth of the weld preparation can lead to defects, which from a stress concentration (SCF) perspective can readily lead to crack development under fatigue conditions.  Similarly undercut at the toe of a weld also introduces a stress concentration feature, from which fatigue cracking is not uncommon.  Indeed, experimental studies have shown that the re-entrant obtuse angle at the top of a butt weld results in a stress concentration that limits the fatigue life.  For example when the obtuse angle is 180º, (i.e. the weld has been ground flat) there is no SCF or reduction in fatigue life, but if the obtuse angle formed by the weld is 120º (which is not unusual) the fatigue life is reduce to approximately one third!  Similar effects are introduced when components being joined with butt welds are misaligned, where eccentricities of the order of the plate thickness result in a reduction in the tensile fatigue life to about 30% of the fatigue life of the flat plate (if there were no eccentricity).

Metallurgical discontinuities or ‘notches’ are also introduced by the changes in microstructure associated with the weld.  This is exacerbated when hardened/tempered or work hardened (cold rolled) materials/sections are welded and the heat input changes the hardness/tensile strength across welds, which can be reduced by as much as fifty percent in the heat affected (HAZ) regions adjacent to the weld.  Such reduction in hardness/tensile also decreases the fatigue performance of the material.

These issues can often be tolerated but can also cause premature failure in critical components and should therefore not be discounted.  Generally they can all be addressed by careful design and material choice, attention to detail, suitably skilled/qualified welding personnel, the use of the right welding techniques for the job and the use of appropriate welding procedures.

Our next tip will address the more detailed microstructural and residual stress issues that can be introduced by welding.