22-Mec-A4 Design and Manufacture of Machine Elements · December 2013

Question 3 of 8: Heating bands, and a weld crack in low-carbon steel

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, December 2013 — 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 questions 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.

Reference texts.

Kalpakjian & Schmid, Manufacturing Engineering and Technology, 7th ed. — sand casting and casting defects (Ch. 10–12), adhesive bonding and joint design (Ch. 32), fusion welding and weld defects (Ch. 30–31), sheet-metal shearing and blanking (Ch. 16).
Budynas & Nisbett, Shigley's Mechanical Engineering Design, 10th/11th ed. — eccentrically loaded shear joints (Ch. 8–9), combined stresses and stress transformation (Ch. 3), statically indeterminate shafts and deflection (Ch. 4), brakes and clutches (Ch. 16).
Hibbeler, Mechanics of Materials, 10th ed. — transverse shear (\(VQ/Ib\)), combined loading and Mohr's circle (Ch. 7–9).
Faires, Design of Machine Elements, 4th ed. — double-block (double-shoe) brakes, self-energizing geometry, equal-wear proportioning.
ASM Handbook Vol. 15 Casting and Vol. 6 Welding, Brazing and Soldering — hot-spot/shrinkage defects; hydrogen-induced cold cracking and preheat practice (see also CSA W59 and CSA W47.1 for Canadian fabrication practice).

Check: Part B is figure-driven. Every dimension used below was read from the printed figures. Two readings are stated explicitly in Given so a grader can substitute a different interpretation without redoing the method: (i) in Figure A the low rivet is taken as lying on the same vertical centreline as the third rivet of the top row (75 + 75 = 150 mm from the left-hand rivet); (ii) in Figure D the dimension \(a\) is the horizontal spacing, measured along the operating lever, between the pin taking the upper shoe link and the pin taking the lower shoe link, with the 10 in operating arm measured from the lower-link pin.

Question 3: Heating bands, and a weld crack in low-carbon steel (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.

(a) Why heating bands are used

The bands are resistance or induction preheat / interpass-heat / post-weld-heat blankets wrapped around the joint, and they exist to control the thermal history of the weld rather than to help melt anything. A weld is a very small, very hot pool surrounded by a large cold heat sink, so without preheat the cooling rate through the critical 800 °C–500 °C range (the \(t_{8/5}\) time) is extremely fast. Heating the parent metal to a controlled preheat temperature before striking the arc, and holding an interpass temperature between passes, does four useful things at once:

It slows the cooling rate. A slower \(t_{8/5}\) prevents the heat-affected zone (HAZ) from transforming to untempered martensite, so the HAZ stays soft and tough instead of hard and brittle.
It lets hydrogen diffuse out. Hydrogen dissolved in the weld pool escapes far faster at 150–250 °C than at ambient temperature; holding the joint warm gives it time to leave before the metal is cold and stressed enough to crack.
It reduces residual stress and distortion. Preheating shrinks the temperature difference between the weld and the surrounding plate, so the thermal contraction strain — and therefore the locked-in residual stress and the restraint on the joint — is smaller.
It drives off moisture and keeps the joint above the dew point, removing the main external hydrogen source, and it raises the steel above its ductile-to-brittle transition so the joint is not being welded while it is brittle.

The same bands are used after welding, at a higher temperature, for a post-weld heat treatment (a hydrogen soak at roughly 250–350 °C, or a full stress-relief anneal at roughly 600–650 °C for carbon steel), which relieves residual stress and tempers any hard HAZ that did form. In Canadian fabrication practice the required preheat and interpass temperatures for a given thickness, grade and hydrogen level are tabulated in CSA W59.

(b) Most likely cause of the crack

Figure 3.1 — The crack in Figure b initiates at the weld toe and runs transversely into the heat-affected zone: the signature of hydrogen-induced (cold) cracking.

The crack in Figure b starts at the weld toe and runs across the joint into the heat-affected zone of the parent plate. That location and orientation, in a plain low-carbon steel, points to hydrogen-induced cold cracking (also called delayed cracking, underbead cracking or HAZ cracking). Three conditions must coincide for it, and the "heating bands" of part (a) tell us the fabricator already suspected this mechanism:

A susceptible microstructure. Rapid cooling of the HAZ produced untempered martensite or hard bainite. Low-carbon steel has low hardenability, but a thick section, a low heat input, a cold plate, or a locally raised carbon equivalent \(CE_{\text{IIW}} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}\) will still harden the HAZ.
Dissolved hydrogen. Moisture in the electrode coating or flux, damp or rusty plate, oil, grease, paint or humid air is dissociated in the arc, dissolves in the pool, and is trapped as the weld cools. It then diffuses to the highest-triaxial-stress region — the notch at the weld toe and the coarse-grained HAZ.
Tensile restraint. The joint is stiff, so the contracting weld cannot shrink freely and locks in residual tensile stress approaching the yield strength.

Because hydrogen has to diffuse to the crack site, the cracking is delayed: it appears at or below about 150 °C, often hours after the arc is extinguished, which is exactly what "developed during the welding process" describes for a multi-pass job. The two competing explanations are worth dismissing explicitly. Solidification (hot) cracking would lie along the weld centreline, would be visibly dendritic, and needs high sulphur or phosphorus — unlikely to be dominant here and in the wrong location. Lamellar tearing runs parallel to the plate surface in a step-like terrace and requires through-thickness loading on a plate with elongated inclusions — again the wrong geometry for a transverse toe crack. Hydrogen cracking is the answer that fits the location, the material and the fabricator's use of heating bands.

(c) Prevention

Because the mechanism needs all three of hydrogen, hard microstructure and restraint simultaneously, removing any one of them prevents it — and good practice attacks all three.

Eliminate the hydrogen. Use low-hydrogen consumables (E7018-type basic electrodes, or a low-hydrogen flux/gas process), and store and re-bake them per the manufacturer's schedule in a heated rod oven; never use a coated electrode that has been left out in humid air. Clean the joint and 25 mm either side of it back to bright metal — no rust, mill scale, moisture, oil, paint or primer.
Soften the microstructure. Apply and hold a preheat and interpass temperature to CSA W59 for the thickness, grade and hydrogen level (commonly 100–150 °C for the section thicknesses on which this defect appears), and raise the heat input / reduce the travel speed so \(t_{8/5}\) lengthens. Specify a steel with a lower carbon equivalent if the grade is free to change.
Let the hydrogen out. Follow the weld with a post-weld hydrogen soak at about 250–350 °C for a few hours, or a full stress-relief at 600–650 °C, and insulate the joint so it cools slowly rather than quenching in air.
Reduce the restraint and the notch. Revise the weld sequence and use balanced or back-step welding so contraction is not fully restrained; avoid over-thick single passes and excessive weld size; grind the toe smooth (or use a weave/cap pass that blends the toe) to remove the stress concentration at which the crack initiates; and use a buttering layer or a smaller root pass on very stiff joints.
Check. Delay the final non-destructive examination by at least 16–48 hours after welding, precisely because the cracking is delayed; magnetic-particle or ultrasonic inspection immediately after welding can pass a joint that later cracks. Qualify the procedure (WPS) under CSA W47.1.

Topic. Weld thermal cycles, preheat, and hydrogen-induced cold cracking in carbon steel.

Key relations. Carbon equivalent \(CE_{\text{IIW}} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15}\); the \(t_{8/5}\) cooling time as the microstructure controller; the three-factor rule for cold cracking (hydrogen \(\times\) hard microstructure \(\times\) restraint); CSA W59 preheat/interpass tables; hydrogen soak at 250–350 °C, stress relief at 600–650 °C.

Why this works. Every preventive measure is a direct attack on one leg of the three-factor rule. Preheat and higher heat input lengthen \(t_{8/5}\), which pushes the HAZ transformation out of the martensite field and simultaneously keeps the joint hot long enough for hydrogen to diffuse away. Low-hydrogen consumables and clean, dry joints cut the hydrogen supply at source. Sequencing, joint design and toe grinding cut the restraint stress and remove the notch where hydrogen concentrates. Because the three factors multiply rather than add, halving any one of them has a disproportionate effect, which is why a simple 100 °C preheat so often eliminates the problem outright.

Common pitfall. Assuming that "low carbon steel cannot harden, so it cannot cold-crack". Thick sections, low heat input, a cold ambient (a real issue on Canadian winter sites) and an elevated manganese content readily push the HAZ into martensite. The other classic mistake is inspecting immediately after welding and signing the joint off before the delayed crack has had time to appear.

Source. Kalpakjian & Schmid, 7th ed., Ch. 30–31 (fusion welding; weldability, HAZ and weld defects); ASM Handbook Vol. 6 Welding, Brazing and Soldering (hydrogen-induced cracking); CSA W59 Welded Steel Construction and CSA W47.1 (Canadian preheat and qualification requirements).

← Question 2 Next: Question 4 →