Basic Welding Metallurgy of Structural Steels Explained in Everyday Terms. Prepared by J. Manning September 27, 2017 Modern controlled rolled carbon manganese or micro alloyed steels derive their strength and mechanical properties from the interaction of two very different forms or microstructures that the iron takes on through the thickness or across the dimensions of the part. The shape and distribution of these microstructures is created by the mechanical action of the rolling or forming process and recrystallization that occurs because of the elevated rolling temperature and the degree of cold work. When these steels are welded the weld and the areas of the steel heated by the welding arc experience heating and cooling cycles that are totally different from anything experience by the steel plate or member as it was formed by the controlled rolling process. It is understandable then that the microstructures of the deposited weld metal, admixed base metal(base metal melted and mixed into the weld) and the base metal heated by welding arc but not melted should be totally different that the microstructures formed in the base metal by controlled rolling. What is more important is that the mechanical properties of these areas of the welded connection could be very different from that of the unheated base metal. Luckily the structures that form in the different regions of a welded connection on carbon manganese steels follow predicable patterns. Certain microstructures can be anticipated and actions may be taken to prevent the formation of the more severe forms of these microstructures. Some of these actions include cleaning, preheat, post-heat, temper-bead, heat Input control, use of basic electrode. It is the very particular mixture of two microstructures that interact when an external stress is applied that give the steel the desired combination of strength and toughness, toughness is usually considered the ability to absorb an impact load without fracturing. There are other important design requirements for a structural quality steel such as formability by cold bending in a press brake, fatigue resistance, corrosion resistance, and weldability. The metallograph show below shows those two microstructures. The white areas are ferrite in this case about 75% of the grains are ferrite. They are not particularly strong but have excellent ductility and formability. They are for all practical purposes pure iron they contain almost none of the carbon < 0.008% by weight at room temperature. The strength of carbon manganese steels comes from the dark areas these are pearlite grains. These are much stronger than ferrite. Without these dark grains even very light loading would cause the ferrite grains to stretch or change shape and deform the steel structure, pipe or vessel. However the pearlite resist stress and resist deformation and when long banded pearlite grains are evenly distributed through the ferrite and a load is applied the ferrite is unable to deform because of a locking effect caused by the pearlite. If an impact load is applied as in an earth quake or collision at sea the ferrite allows the structure to flex and deform rather than fracture and fall down around the people inside or split open to allow in sea water, and the pearlite grains put a check on how much flexing or deformation that can actually occur.
Fig 1: 100X, Micrograph showing the shape and distribution of ferrite and pearlite in a CSA G40.21 Grade 44W(330W) carbon manganese steel. The white areas are ferrite grains which here are shown in a blocky recrystallized form. The darker areas are elongated pearlite grains. This microstructure having a matrix of blocky ferrite with long bands of elongated pearlite evenly dispersed through is formed from the controlled rolling process used for most wrought steels. When the steel cooled from liquid austentite(a very high temperature microstructure in carbon steels) grains formed but quickly changed to rounded grains of ferrite with rounded grains pearlite evenly distributed throughout. At first both the ferrite and the pearlite grains were flattened out by the rolling process as the steel was being formed but the controlled rolling temperature is high enough for the highly strained ferrite grains to recrystallize from flattened elongated back to round or blocky but the rolling temperature is not high enough for the flattened pearlite grains to recrystallize. Pearlite being much stronger require a much higher recrystallization temperature. The result is this excellent combination of a tough impact resistant matrix or background of ferrite through which are passing the very strong bands of ferrite. For all practical purposes this microstructure behaves as a composite material and the combined reaction of the two microstructures to stress or loading gives far better results that either of them alone.
Fig 2: 100X, Another micrograph of rolled steel showing white ferrite grains in blocky form and elongated bands of pearlite. When controlled rolled steel is joined by welding the microstructures that develop in the deposited weld metal and across the welded connection will inevitably be different than the microstructures in the base metal because the heating and cooling cycles of the the welding process are completely different from the cooling and mechanical action of the rolling which formed the steel at the steel mill. When we cut through the cross section of a welded connection on controlled rolled steel and polish the surface to a mirror like finish we can etch that surface by applying a reagent or mild acid such as Nital a 2% solution of Nitric Acid in Methanol. The reagent react more with highly strained areas such as grain boundaries and carbides, this creates microscopic hills and valleys that produces the image we see when we view the surface as a marco with less than 10X magnification or as a micrograph the image we get when we view the surface through a metallurgical microscope at up to 500X magnification. The following is a series of Marcos of weld cross sections from an assortment of welding process. Some of the things to note about these would include the number of weld passes, the width of the apparent Heat Affected Zone(HAZ), the presence of weld flaws. The scale at the bottom of each of the welds is divided in millimeters and can be used a reference for the width of the HAZ and the size of the weld passes. Fig 3: Submerged Arc weld on 20mm plate.
The weld macro shown above in Fig 3 is of a Submerged Arc weld on 20mm steel plate CSA G40.41 grade 44W(300W). The width of the visible HAZ is up to 5mm wide. The number of weld passes that can be counted on this macro is 7. The crystalline pattern that make up each weld pass is created by the outlines of what had been long columnar grains that grew as the weld solidified. This austenite then transformed to Bainite a microstructure that consist of ferrite in various forms and carbides. Fig 4: Flux Core weld on 20mm plate The weld macro shown above in Fig 4 is of a Flux Core Arc weld on 20mm steel plate CSA G40.21 grade 44W(300W). From this macro we can count the number of weld passes to be 19. The width of the visible HAZ here is only about 2mm wide. This weld macro displays porosity in the root pass, weld spatter on the cap side just to the right of the weld toe, and the height of the weld reinforcement may be excessive. The line running horizontal at mid thickness through the steel plate(base metal) is indicating chemical segregation at the centre of this plate. This chemical segregation should not be a problem with this weld stress being transferred across the connection, but may lead to laminar tearing if joint design were such as to cause stresses through the thickness of this plate. Fig 5: Shielded Metal Arc weld
The weld macro shown above in Fig 5 is of a Shielded Arc weld connecting AISI SAE 4340 steel pipe section(on left to CSA G40.21 grade 44W(300W) steel bar. The width of the visible HAZ here is only about 3mm wide. This HAZ on the left on the 4340 steel shows very dark, viewing this region under higher magnification would reveal very hard brittle martensitic microstructures. And would be reason to revise the welding procedure, perhaps to include control on the cooling rate through 800-500 C range or control on the heat input. It is obvious that the weld cap pass was completed as a wide weave >18mm wide. The increase in heat input cause by the weave may have been the reason for the martensitic microstructures in the HAZ. Fig 6: Shielded metal Arc weld double vee The weld shown above in Figure 6 is a Shielded Metal Arc weld on to 20mm thick G40.21 grade 44W(300W) steel plate. The chemical segregation band at the mid thickness of the plate on the right is very pronounced. This region contains much more pearlite that the area outside this band. If this steel plate was welded such as to create stress across or through the thickness of this plate and a poor weld geometry was select lamellar tearing could result.
Fig 7: FCAW weld in single vee groove. Fig 8: SAW weld in single vee groove
Fig 9: SMAW weld double vee note the roots are misaligned almost to the point of creating incomplete penetration. Fig 10: FCAW weld double vee groove. Fig 11: Single vee groove 2mm HAZ, weld flaw at centre
Fig 12: SAW fillet weld in tee joint on micro-alloyed steel. Visible are the outlines of what had been coarse columnar austentie grains when this weld was at a temperature of 723 C or slightly higher. These grains growing into the center is what is used to compare to the grains growing into the center of castings. As is evident for the macro we have seen the welded connection is made up of several very different areas. Areas which reveal very different microstructures and very different mechanical properties. There are different ways to break down the areas of the welded connection some of the terms for the different areas include Base Metal Deposited weld metal or sometimes Weld Nugget Un-reheated or As deposited Weld Reheated weld Heat Affected Zone(basemetal) Deposited weld metal or sometimes Weld Nugget Fusion Line or sometimes Fusion Zone Root Pass or Root Zone Hot Pass Weld Cap
Earlier on in this paper we looked at the microstructures of the base metal and the affect that these microstructures have on the mechanical properties of the steel. Fig 13: Basemetal 100X We concluded that discussion by saying that this microstructure of not so strong but tough blocky ferrite grains interspersed by long bands of very strong pearlite grains makes for an ideal microstructure strong yet able to flex and withstand impact loading and still formable through cold forming with a press brake. When a weld in deposited on structural quality carbon manganese steel the resultant weld microstructure is the result of a mixing of welding rod(core and flux coating), melted base metal, any contaminants on the surface and contaminants from the atmosphere. The weld microstructures of SMAW welds are usually very similar to figure 14 below. Fig 14: 100X As deposited Weld metal The patterns we see running diagonally from lower left to upper right in the micrograph shown above Figure 14 are the outlines of what had been Austenite grains when this weld metal first solidified. However as the weld cooled through 800-500 C this unstable Austenite transformed first to pro-eutectoid or grain boundary ferrite at the Austenitic grain boundaries, the white outlines. Then side plates formed, the comb like pattern at centre. Ferritic needle and more round ferrite grains more to the center and last to form are the carbides or the very dark squiggly lines that make the centre area appear darker. The white ferrite here are not strong and it s pattern is problematic. Near the ferrite are remnants of the Austentitic grain boundaries, which are collect sites for hydrogen and easy paths for fracture propagation. Compare to the base metal and it s ideal microstructure enabling it to behave as a composite material, strong tough, formable this weld metal microstructure is more erratic could show poor fracture resistance and is susceptible to hydrogen embrittlement.
If however we run a second weld pass over this more unfavorable microstructure the heating cycle of the second pass will again cause most of this weld microstructure to transform back again to Austenite and as this cools for the second time it transforms this time to very fine rounded grains of ferrite interspersed with fine carbides. Fig 15: 100X Grain refined weld metal The Microstructure shown above in Figure 15 is of weld metal that was fist deposited as above but the heating of a second pass over this weld metal converted the microstructure to the very fine grains of ferrite and fine carbides shown here. These fine grains are much stronger than what had existed before, the remnants of the grain boundaries are gone and the large elongated rather weak ferrite grains are no longer there. The Reheating by running a second pass over the originally deposited weld metal has refined the grain structure, improved the strength, improved the toughness, relieved microscopic strain, improved the fracture resistance. This is the reason that many Welding procedures will require Temper Bead weld pass sequencing. We know now that the base metal and the deposited weld have some good and some not so good features about their microstructures, but it is when we look at the areas of the base metal that were heated by the weld pass but not heated enough to melt and fuse or incorporate in with the weld nugget that we find the area that can be most compromised by the welding process. This is the region is referred to as the Heat Affected Zone(HAZ). The heat affected zone is not a single homogenous zone but rather can be divided into four or sometimes five distinct regions. The region closest to the weld nugget is the area that had temperatures approach the melting temperature for steel about 1500 C. These peak temperatures did not last for very long but because the temperatures were so high reaching a area of the Austenite range called the grain growth region this part of the HAZ is referred to as the Grain Coarsened HAZ. In this region the grain size can be twice or perhaps four times larger than the unaffected base metal. Large grains are not as strong as fine grains but what can be far worse is that the carbides that form in this region can be very long brittle structures and form significant areas of Martensitic structures. Appling this metallurgical knowledge to the steel making process many steels now have trace amounts of aluminum or titanium to help resist the tendency for grain growth in this region, and to reduce the size of this region to a minimum.
Fig 16:100X Grain Coarsened HAZ, G40.21 grade 44W(300W) steel. The micrograph shown above is of the Grain Coarsened HAZ of a SMAW weld on to G40.21 grade 44W(300W) steel. The dark region in the top right hand corner is the As Deposited Weld and the region at the lower left corner is the next region out form the Grain Coarsened HAZ. Some of the Grains shown here are four times larger than the grain size in the base metal. The fact that Proeutectoid Ferrite has formed along the grain boundaries produces a weaken area at these grain boundaries where remnant of the Austenitic grain boundaries are now collection sites for hydrogen and initiation sites for fractures. The carbides here tend to from Martensitic structures which can be brittle and can contribute to failure of the welded connection. Fig 17: 100X Grain Coarsened HAZ, AISI 4340 steel The microstructure shown above is of a Grain Coarsened HAZ on a AISI 4340 steel. The upper right hand corner shows the As Deposited weld. The very dark spikey microstructure immediately next to the As deposited weld and extending away from it is untempered Martensite. This region would be very hard and brittle. And would likely fail in this welded connection was subjected to shock or impact loading. The next part of the Heat Affected Zone out form the weld is the where peak temperatures reached into the austenizing range but not in to the grain growth region. Here we find a microstructure that consist of very fine rounded grains of Ferrite and fine rounded carbides.
Fig 18: 100X Grain Refined HAZ The microstructure above shows the Grain Refined HAZ. In the lower left corner is the next region out or the Intercritical HAZ. The very fine rounded grains and the fine rounded carbides means that this region would be strong and would show good toughness. After the Grain refined HAZ the next region is that part of the HAZ where the peak temperatures reached between the A 3 and the A 1 /A cm lines or between the upper critical and the lower critical lines on the Iron-Iron Carbide phase diagram. Here the Pearlite grains transformed to Austenite on heating but the ferrite grains did not transform. On cooling what had been Pearlite transformed back to fine distributed carbides in a matrix of mostly unaffected ferrite grains. This region is referred to as the Inter-critical HAZ. Fig 19: 100X Inter-critical HAZ The microstructure shown above in figure 19 is that of the Inter-critical HAZ, the upper right hand corner is showing the Grain refined HAZ and the lower left hand corner is showing the microstructure of the base metal. There is a region referred to as the Subcritical HAZ because peak temperature are below the lower critical or A 1 /A cm lines on the Iron-Iron Carbide phase diagram but above 600 C and changes happen here to the microstructure that should not be ignored. This part of the base metal is heated into a region often referred to as the Stress Relieving range. This region seems particularly prone to certain failure mechanism such a stress corrosion cracking. However the changes to the microstructure cannot be observed on an optical microscope. The micrograph for this region would look the same as the micrograph for the base metal. Some sources consider the Base metal which was heated above 100 C up to 600 C, or heated to below the Stress Relieving temperature as being a separate region of the HAZ. Although any affect her would be very slight and should not compromise the mechanical properties of this region. Having some knowledge of how steel is affected by welding is important to all personnel who work in the steel fabrication industry. Welding Engineers, Welders, Welding Inspectors, and Nondestructive Testing Personnel all should consider the entire welded connection not just the deposited weld metal. They should appreciate that the welded connection on carbon manganese or structural quality steel is not a homogeneous unit but is complex and consist many differing microstructures with varying strengths and differing mechanical properties.