22-Mec-A4 Design and Manufacture of Machine Elements · May 2014

Question 3 of 8: Heating bands and a crack in a welded low-carbon steel joint

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. National Examination, 07-Mec-A4 Design and Manufacture of Machine Elements, May 2014 — 3 hours, open book, any non-communicating calculator permitted. Eight questions on six pages, divided into Part A (manufacturing processes, Q1–Q4) and Part B (machine-element design, Q5–Q8). The rubric asks for three from Part A and two from Part B, five questions constituting a complete paper, all of equal value (20 % each). All eight questions are solved here, because this document is a study resource rather than an examination script.

Reference texts.

Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed. — sand casting and casting defects (Ch. 10–12), sheet-metal shearing and fine blanking (Ch. 16), bulk deformation and hydrostatic extrusion (Ch. 15), fusion welding and weld defects (Ch. 30–31).
Budynas & Nisbett, Shigley's Mechanical Engineering Design, 10th/11th ed. — eccentrically loaded shear joints (Ch. 8), statically indeterminate beams and deflection superposition (Ch. 4), welded joints in torsion (Ch. 9).
Hibbeler, Mechanics of Materials, 10th ed. — combined loading, transverse shear and stress transformation / Mohr's circle (Ch. 7–9).
AWS D1.1 Structural Welding Code — Steel and CSA W59 — preheat, hydrogen control and minimum fillet-weld sizes (Canadian practice).
ASM Handbook Vol. 15 Casting and Vol. 6 Welding, Brazing and Soldering — hot tearing, solidification cracking and riser/chill practice.

Check: Part B is entirely figure-driven. Every number below was read from the printed figures (Figures A, B, C and S7). Two readings are worth stating explicitly so a grader can substitute a different interpretation without redoing the method: (i) in Figure A the rivet group is five rivets in the top row plus one rivet 200 mm below, the lower rivet lying on the same vertical line as the third top rivet; (ii) in Figure B the 67 500 N horizontal force acts on the centroidal axis of the section, so it produces pure tension and no additional bending.

PART A — Manufacturing Processes

Question 3: Heating bands and a crack in a welded low-carbon steel joint (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Figure 3.1 — (a) Preheat bands wrapped either side of a girth weld; (b) the reported defect, a longitudinal crack lying on the weld centreline.

(a) Why heating bands are used

The bands are resistance or induction preheat and interpass-heat elements, wrapped symmetrically either side of the joint and used to bring the parent metal to a controlled temperature before the arc is struck and to hold it there between passes. They serve four purposes at once, and all four are about controlling the thermal history rather than the deposited metal. First, they slow the cooling rate of the weld and the heat-affected zone through the critical 800–500 °C interval (the \(t_{8/5}\) time). A weld on cold, thick or highly conductive parent metal quenches itself, and in any steel with appreciable carbon equivalent the fast quench forms hard, brittle untempered martensite in the HAZ. Second, and closely coupled to the first, the extended time at temperature lets diffusible hydrogen escape from the weld and HAZ instead of being trapped; hydrogen, a hard microstructure and residual tensile stress are the three legs of hydrogen-induced (cold) cracking, and preheat removes two of them. Third, preheat flattens the thermal gradient between the fusion zone and the surrounding cold metal, which directly reduces the residual stresses and the distortion left after the joint contracts. Fourth, warming the surfaces above the dew point drives off condensed moisture, oil and frost before welding, removing the main site-borne source of hydrogen. On a heavy-wall pipe girth weld of the kind shown, CSA W59 and AWS D1.1 both prescribe a minimum preheat based on the material thickness and carbon equivalent, and the bands are the practical means of delivering and holding it uniformly around the circumference.

(b) The most likely cause of the crack

The parts are low-carbon steel, which is the decisive clue. Low-carbon steel has a low carbon equivalent, so it does not readily form the hard martensitic HAZ that hydrogen-induced cold cracking requires; and the crack is reported as having developed during the welding process, not hours afterward as delayed cold cracking does. The crack in the micrograph lies in the weld metal, on the centreline, running longitudinally along the bead. Those three facts together identify the defect as hot cracking — specifically solidification (centreline) cracking of the weld metal.

The mechanism is segregation during solidification. As the weld pool freezes, columnar grains grow inward from both fusion boundaries and meet on the centreline. Sulphur and phosphorus have very low solubility in solid iron, so they are rejected ahead of the advancing solid and end up concentrated in the last liquid to freeze, which is exactly the film left on the centreline. Iron sulphide forms low-melting eutectics (FeS melts near 988 °C, and Fe–FeS eutectics considerably lower) that remain liquid well below the solidus of the surrounding steel. Meanwhile the just-solidified weld is contracting and is restrained by the massive cold parent metal, so a tensile strain is applied across precisely that liquid film. A liquid film has no ductility, so it separates — the crack is a hot tear, opened at high temperature, and it commonly shows the oxidised, dendritic fracture surface characteristic of a solidification crack rather than the flat, bright surface of a cold crack. Two geometric factors make it far more likely and are almost always present when it occurs: a bead with a depth-to-width ratio greater than about one (deep and narrow, typical of high current with high travel speed, or of submerged-arc welding), which forces the two solidification fronts to meet in a long vertical plane and traps the liquid film there; and high joint restraint, which supplies the strain. Impure or unsuitable filler metal, contaminated (sulphurous) parent plate such as a resulphurised free-machining grade, and a large root gap all aggravate it.

(c) Preventing recurrence

Because the defect requires both a low-melting liquid film and a tensile strain across it while that film exists, every effective remedy attacks one or the other:

Change the bead shape. Weld so that the width exceeds the depth — reduce the current, reduce the travel speed, or use a slight weave — so the solidification fronts meet obliquely and the residual liquid is pushed to the surface instead of being buried on the centreline. This is the single most effective change and costs nothing.
Reduce sulphur and phosphorus. Specify low-S, low-P parent plate (never a resulphurised free-machining grade in a welded assembly) and a clean filler metal; use a filler with a controlled manganese-to-sulphur ratio — \(\mathrm{Mn/S} \gtrsim 30\) — because manganese ties up sulphur as high-melting MnS instead of low-melting FeS.
Reduce restraint and the strain rate on the joint. Improve fit-up so a large root gap need not be filled, revise the weld sequence and use balanced or back-step welding, avoid rigid tack-and-clamp arrangements, and where possible let one member move as the joint contracts.
Clean the joint. Remove oil, grease, cutting fluid, primer, paint, mill scale and rust from the groove faces — all are sulphur and hydrocarbon sources that end up in the weld pool.
Use preheat and controlled interpass temperature (the very bands of part (a)) to reduce the thermal gradient and hence the contraction strain rate across the freezing centreline, and to slow the cooling so that the last liquid solidifies under less strain.
Fill craters properly. Use crater-fill / downslope current or run-off tabs at stops, since the crater is the extreme case of a segregated final pool and is where centreline cracking most often starts.
Prove the fix. Re-qualify the procedure and inspect the first production joints by MT or UT per CSA W59 / AWS D1.1 before releasing the change.

Topic. Weld thermal cycles, preheat, and the distinction between hot (solidification) cracking and cold (hydrogen-induced) cracking.

Key relations. Carbon equivalent \(\mathrm{CE} = \mathrm{C} + \dfrac{\mathrm{Mn}}{6} + \dfrac{\mathrm{Cr}+\mathrm{Mo}+\mathrm{V}}{5} + \dfrac{\mathrm{Ni}+\mathrm{Cu}}{15}\); cooling time \(t_{8/5}\) between 800 and 500 °C as the governing HAZ parameter; hot-cracking susceptibility rises with the depth/width ratio of the bead and with \(\mathrm{S}\), \(\mathrm{P}\), and falls with \(\mathrm{Mn/S}\).

Why this works. The two crack families are separated by when and where they form. Hot cracks form above the solidus, in the weld metal, during welding, from segregated liquid films under contraction strain; they are cured by metallurgy and bead shape. Cold cracks form below about 200 °C, usually in the HAZ, minutes to days after welding, from the combination of hydrogen, a hard microstructure and residual stress; they are cured by preheat, low-hydrogen consumables and post-weld hold. Naming the family correctly is the whole of part (b), because low-carbon steel, a weld-metal location and an "during welding" timing rule out the cold-crack family entirely.

Common pitfall. Answering "hydrogen embrittlement" for any weld crack. In a genuinely low-carbon steel the HAZ does not harden enough for hydrogen cracking to be the leading candidate, and hydrogen cracks are delayed, not concurrent. A second pitfall is prescribing higher heat input as a cure — more current usually deepens and narrows the bead, which makes centreline cracking worse.

Source. Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed., §30.9 (Weld quality; weldability) and Ch. 31; ASM Handbook Vol. 6 Welding, Brazing and Soldering (solidification cracking); CSA W59 / AWS D1.1 preheat tables.

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