22-Mec-A4 Design and Manufacture of Machine Elements · December 2017

Question 2 of 6: Weld Zones, Solidification Cracking and Hydrogen Cracking

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

Notes on this paper

Paper format. National Examinations, December 2017 — 16-Mec-A4, Design and Manufacture of Machine Elements. Three hours, open book, non-communicating calculator permitted. Six questions divided into Part A (Q1–Q3, manufacturing processes) and Part B (Q4–Q6, machine-element statics); candidates answer two from each part, and all questions carry equal value (25 %). All six questions are solved here.

Reference texts.

S. Kalpakjian and S. R. Schmid, Manufacturing Engineering and Technology — sheet-metal forming, welding metallurgy, polymer processing.
M. P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes and Systems — drawing, moulding and tooling design rules.
R. G. Budynas and J. K. Nisbett, Shigley's Mechanical Engineering Design — columns, welded joints, static failure theories.
R. C. Hibbeler, Mechanics of Materials — free-body diagrams, buckling.

Note on units and material data. The paper is metric throughout. Where a property is needed but not printed on the exam (the modulus of elasticity of steel, the reduction of area of 1015 steel, the minimum yield strength of E60 filler metal), the standard handbook value is used and flagged at the point of use, as the paper's own Note 1 invites.

Question 2: Weld Zones, Solidification Cracking and Hydrogen Cracking (25 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 not reproduced: Fig. w2 redrawn. Arrow i lands in the deposited weld metal, arrow ii in the narrow transformed band beside the fusion boundary, and arrow iii in plate that never exceeded the transformation temperature. See the official exam paper.]

(a) The three major areas

Arrow i — the weld metal, or fusion zone. This is the region that was fully molten and has solidified from the liquid: filler metal from the electrode diluted with melted parent plate. Its structure is a cast structure, with columnar grains that nucleate on the partly melted grains at the fusion boundary and grow inward along the steepest thermal gradient, meeting at the bead centreline. Its composition is not that of either the plate or the electrode but a weighted mixture of the two.

Arrow ii — the heat-affected zone. This is solid parent metal that was never melted but was taken high enough into the austenite field to be transformed, then cooled rapidly by conduction into the surrounding cold plate. It is a narrow band, typically a few millimetres wide, and it is not homogeneous: immediately against the fusion line it is coarse-grained, having spent longest at peak temperature, and it grades outward through a refined-grain region and a partly transformed region into unaffected plate. In a hardenable steel the coarse-grained sub-zone is the hardest and least tough material in the whole joint, because it combines a large prior-austenite grain size with a fast quench.

Arrow iii — the unaffected base metal. Plate far enough from the arc that its peak temperature stayed below the lower transformation temperature. Its microstructure and properties are those of the plate as supplied; the only welding effect it carries is residual stress and, where the joint is restrained, elastic strain.

(b) Solidification cracking: site and prevention

Site: area i, the weld metal — specifically along the bead centreline and at the root. Solidification cracking, also called hot cracking, is a phenomenon of the liquid-to-solid transition and can therefore only occur where there was liquid. As the columnar grains grow in from both fusion boundaries they reject sulphur, phosphorus and carbon ahead of the advancing front, so the last liquid to freeze — the film trapped where the two growth fronts meet at the centreline — is a low-melting-point iron sulphide eutectic. That film has almost no strength, while the solidified metal on either side of it is contracting as it cools and the joint is restrained. The film is pulled apart, and the crack appears as a longitudinal fissure on the bead centreline, often opening at the crater at the end of the run.

Prevention attacks either the segregated film or the strain applied to it. Control the consumable and the parent chemistry first: keep sulphur and phosphorus low, and maintain a manganese-to-sulphur ratio of about 30 or more so that sulphur is tied up as high-melting manganese sulphide instead of the iron sulphide eutectic. Then control the bead shape, because the depth-to-width ratio of the weld pool decides whether the growth fronts meet head-on at a plane or converge gently: keep the ratio below about one by reducing current or increasing travel speed, and prefer a slightly convex bead to a deep narrow one. Reduce the heat input generally, since a large pool segregates more and contracts more. Finally reduce restraint — sequence the welds so the joint can move, use a smaller root gap, and fill the crater at the end of every run rather than breaking the arc abruptly.

(c) Hydrogen cracking: site and prevention

Site: area ii, the heat-affected zone — specifically the coarse-grained band immediately alongside the fusion boundary. Hydrogen cracking, also called cold cracking, delayed cracking or underbead cracking, needs three things at once: dissolved atomic hydrogen, a crack-sensitive hard microstructure, and tensile stress. All three coincide in the coarse-grained HAZ. Hydrogen enters the arc from moisture in the electrode coating, from damp or rusty plate and from grease, dissolves readily in the austenitic weld pool, and then diffuses out into the HAZ as the weld metal transforms and its solubility collapses. The HAZ, having been austenitised and then quenched by the cold surrounding plate, transforms to untempered martensite in any steel with appreciable carbon equivalent. Hydrogen embrittles that martensite, and the residual tensile stress from contraction of a restrained joint supplies the driving force. Because hydrogen has to diffuse to the site, the crack appears hours or even days after welding — which is precisely why it is dangerous, and why inspection immediately after welding can miss it.

Prevention removes any one of the three ingredients, and good practice removes all three. Eliminate the hydrogen: use low-hydrogen basic-coated electrodes such as E7018, store them in a heated oven and re-bake any that have been exposed, and clean the joint of rust, scale, moisture, paint and grease before striking the arc. Eliminate the hard microstructure: preheat the plate and hold a minimum interpass temperature so the cooling rate through the transformation range is slowed and martensite is avoided or at least self-tempered, choosing the preheat from the carbon equivalent, the thickness and the restraint. Eliminate the stress and the residual hydrogen together with a post-weld heat treatment, or at minimum a post-weld hydrogen soak at a couple of hundred degrees, which lets the hydrogen diffuse harmlessly out before it can accumulate at the crack tip. Selecting a plate of lower carbon equivalent and reducing joint restraint complete the list.

Arrow	Area	Crack type located here	Primary control
i	Weld metal (fusion zone)	Solidification (hot) crack, on the centreline	Low S and P, Mn:S ratio above about 30, depth-to-width ratio below 1, fill the crater
ii	Heat-affected zone	Hydrogen (cold, underbead) crack	Low-hydrogen electrodes, preheat and interpass control, post-weld hydrogen soak
iii	Unaffected base metal	Neither — reference condition	Not applicable

Topic. Weld-zone metallurgy and the two classical fusion-welding cracking mechanisms.

Key relations. Carbon equivalent, $CE = C + \dfrac{Mn}{6} + \dfrac{Cr+Mo+V}{5} + \dfrac{Ni+Cu}{15}$, which sets HAZ hardenability and hence preheat; manganese-to-sulphur ratio above about 30 for hot-cracking resistance; weld-pool depth-to-width ratio below unity; cooling time from 800 to 500 degrees Celsius as the governing HAZ parameter.

Why this works. The three zones are defined by peak temperature: above the liquidus gives weld metal, between the liquidus and the lower transformation temperature gives the HAZ, and below that gives unaffected plate. Once that map is drawn, each defect can be placed by asking what physical state it requires. Hot cracking needs a liquid film, so it can only be in the fusion zone. Hydrogen cracking needs martensite plus diffused hydrogen plus tensile stress, and the only place all three are simultaneously maximised is the coarse-grained HAZ next to the fusion line — the weld metal itself is usually lower in carbon and more tolerant. Temperature, timing and location together identify the mechanism: a crack found immediately, on the centreline, is hot; a crack found next morning, parallel to and just outside the fusion line, is hydrogen.

Common pitfall. Two recur. The first is naming the weld crown, the weld root and the HAZ as the three areas — crown and root are both weld metal, and the answer the examiner wants is the three metallurgical zones, the third being unaffected base metal. The second is putting hydrogen cracking in the weld metal because that is where the hydrogen enters; hydrogen enters there but diffuses to and cracks the HAZ, which is the harder and less tough of the two.

Source. Kalpakjian and Schmid, Manufacturing Engineering and Technology, Ch. 30 (fusion-welding processes) and Ch. 31 (the metallurgy of welded joints, weldability and weld defects). Groover, Fundamentals of Modern Manufacturing, Ch. 31 covers the same weld-zone map.

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