MATS Welding 445 - Midterm III Study set
Mitigation of solidification cracking
Buttering, different filler metal, slow down weld speed, closer arc length, oscillate arc, convex beads over concave, widen the groove since narrow grooves crack more often, decrease joint constraints, preheat, lower depth to width ratio
Metallurgical factors in solidification cracking
Impurities, distribution of liquid, primary solidifcation phase, ductility of solidifying weld metal, surface tension of grain boundary liquid, grain structure of weld metal
direction of liquation vs. solidification
Solidification is in a direction upward and towards the weld, liquation is downward
Mechanical factors in solidification cracking
contraction stresses/degree of constraint
solidification cracking
cracking from phase transformation causing density and thermal density change, gets worse if you constrain weld, usually in center but sometimes originates on edge, solidification shrinkage causes high tensile stress at centerline and mush zone, backfill from pool runs back into crack but doesnt "fix" because its a different composition and doesn't go whole length of crack
solidification cracking and grain size
finer, equiaxed grains means less cracking
issues macrosegregation can lead to:
hydrogen cracking, corrosion, martensite formation in low alloy/SS welds (use Ni-based alloy filler metal + preheating to fix)
Difficulties with PMZ
loss of ductility, hydrogen cracking, liquation cracking
liquation
occurs along grain boundary, gives a segregation profile like in the fusion zone, basically the diffusion of impurities to GBs, lowering melting point, then liquating
macrosegregation
segregation across many dendrites or structure as a whole, from not using same filler metal or welding dissimilar materials, minimized by weld pool mixing, LBW and EBW pool can solidify to fast for enough mixing, region near fusion boundary is very different
microsegregation
segregation between dendrites/cors, affects solidification cracking, reduced with solid state diffusion, more diffusion in delta ferrite (bcc) than gamma austenite (fcc) so there is less microsegregation from increased diffusion and less cracking
Mitigation of liquation cracking
select proper filler metal, (Tsolidus of weld metal should be higher than base metal but this may cause solidifcation cracking) decrease degree of constraint (good for liquation and H cracking) lower heat input (reduces crack length) Decrease grain size (good for both liquation and solidification, finer grains mean more GB area, so lower concentration of liquation inducing material at GB, and more convoluted path for cracking) Control impurities: less sulfur, phosphorous which lower eutectic and change phase diagram
microsegregation when k = 1
solid and liquidus line are on top of each other, so no seperation
hydrogen cracking in PMZ
solubility of H is much larger in liq. so H goes thru liq GB then when solidifcation occurs theres super brittle areas that crack
micro segregation when k<1
solute enriched @ boundary and depleted @ core
microsegregation when k > 1
solute enriched @core and depleted at boundary
Liquation cracking tests
varestraint test circular patch test hot ductility test (gleeble)
tests for solidification cracking
weld mockup(make weld then slice and count cracks) verastraint test(plate is bent so theres lots of stress on weld, bead on plate weld) circular patch test (welded in a circle, best w robot, thru penetration weld) houldcrot test or fishbone test(weld along plate with cracks) cast pin tear test differential thermal analysis (two samples are heated and cooled, temp difference is used to determine start and completion of phase transitions)
