Effects of welding process variables on C-Mn metal microstructures

Weld metal microstructure is primarily controlled by two factors; the weld metal composition and cooling rate. The heat input controls the cooling rate and the rate weld pool heat liberation which is a function of base plate thickness, extent of preheat and weld joint geometry. Other influencing factors are interpass temperature, flux and heat input rate [75].

2.6.3.1 Heat Input Rate

One of the most important variables in fusion welding is the heat input rate, as this influences cooling rates, heating rates and weld pool size. As a general rule, the higher the heat input rate the slower the cooling rate and larger weld pool. It also controls the grain size in the AD

and the HAZ of the weld metal. Previous research [71, 73, 74] has shown that there is an inverse relationship between the cooling rate and the size of weld pool. This is an important relationship in the welding of steel since increased cooling rates increases the risk of hydrogen-induced cracking [76].

Previous research has shown that a high input rate process like SAW has less HAZ cracking in the welding of alloy steel, than manual metal arc welding, where the heat inputs are lower (n.b. there might be other variables that contribute to the difference in behaviour). As a result this leads to coarsening of the prior austenitic grain size and also a general coarsening of the as-deposited (AD) microstructure of the weld [14, 15], as well as a decrease in the degree of acicularity was observed. Slower cooling rates produce a decrease in such constituents as acicular ferrite as well as an increase in the volume fracture of the allotriomorphic ferrite at the expense of the acicular ferrite. Acicular ferrite volume fraction might be improved by an increase in the inclusion population density. Thus, as a result the weld pool remains molten for longer allowing manganese and silicon (key constituents for the formation of inclusions) to evaporate and allowing formed inclusions to diffuse into the slag. Also the way in which the heat input is introduced is important, if it is increased by raising the arc current, the width/

depth ratio of the weld pool decreases, thus giving a shorter cooling time, even though the heat input is increasing.

2.6.3.2 Flux

Flux is a coarse granular powder, made of minerals, that is dispensed onto the workpiece immediately ahead of the arc; the main role of the flux in the SAW process is to protect the weld pool and the arc from the atmosphere. As well as cleaning the surface of the weld pool it can also influence the surface profile of the weld. The granular flux can be chemically basic, natural or acid. Fluxes generally basically fall into two classifications, according to how they manufactured by bonding the dried ingredients with low-melting components, for example MnO with SiO², or CaO with SiO² [72]. Fused fluxes are manufactured by mixing and melting the ingredients, then casting them to form solid glassy particles, and then ground

to the required practicle size. Fused fluxes produce a stage arc and easily removable slag layer, whereas agglomerated fluxes produce slag, which is difficult to remove, and the welded surface is not as smooth, as it is with fused fluxes. Agglomerated fluxes are both alloying and deoxidising, the constituents are not affected in the melting process, as this generally takes place at lower temperature. Fused fluxes formed at higher temperatures have difficulty in retaining the deoxidising and alloying elements.

As the flux melts around the arc it forms a pool, which solidifies and reforms periodically, dependant on the movement of the welding torch. Metal may be transferred directly across the slag cavity so formed or occasionally around the edge of molten flux. The gases generated during the SAW process by vaporisation and chemical reaction at the electrode tip are also protective along with the flux, of the molten metal. The slag which is formed during the welding process protects the weld from the atmosphere and in particular from the absorption of moisture. Other compounds added in small quantities control the viscosity and help the flow of liquid filler metal into the weld pool.

2.6.3.3 Interpass Temperature

Whilst keeping other welding parameters constant, an increase in the interpass temperature will reduce the cooling rate. Evans [77] observed the following effects of increasing the interpass temperature; slight reduction in the amount of manganese and silicon; increased width of the recrystallised zones; coarsening of the AD weld metal region and a reduction of the volume fraction of the acicular ferrite phase.

2.6.3.4 Postweld Heat Treatment (PWHT)

Postweld heat treatment (PWHT) within the subcritical range has the influence of reducing both the hardness and strength of an alloy-steel weld deposit. In thickness over

approximately 30 mm, PWHT increases both the ductility and the fracture toughness of the welded joint, as a whole. It has been noted, in some cases, for Charpy impact and crack tip opening displacement (CTOD) values for the weld metals increase by PWHT, although there are certain circumstances were they might be reduced. To accurately determine the effect of PWHT, welding procedure testpieces must be subjected to the same thermal conditioning as that used in production.