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GATE NM: Materials and Welding in Marine Construction

GATE NM · Work through your exam, one topic at a time.

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Materials and Welding in Marine Construction

Materials and Welding in Marine Construction covers hull steel grades, weldability, welding processes, defects and inspection. GATE NM tests why higher-strength steel needs controlled heat input and how weld discontinuities become fatigue or brittle-fracture sites.

Core rules

  • Hull structural steels are graded by toughness: A, B, D and E correspond to increasing impact-test severity; higher grades are used in thicker or more critical regions.
  • Carbon equivalent CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15, in mass %. Higher CE increases hardenability and hydrogen-cracking risk.
  • Common processes: SMAW, FCAW, SAW and GTAW. SAW gives high deposition and deep penetration; FCAW is productive for block assembly.
  • Preheat and controlled interpass temperature reduce cooling rate and hydrogen cracking. Low-hydrogen consumables and dry storage are essential.
  • Defects include porosity, slag inclusion, lack of fusion, undercut, incomplete penetration and solidification cracking.
  • NDT methods: visual, dye penetrant, magnetic particle, ultrasonic and radiographic; each has different detection capability.

Worked example

A steel has C = 0.18, Mn = 1.20, Cr = 0.10, Mo = 0.05, V = 0.02, Ni = 0.20 and Cu = 0.10, all in mass %. CE = 0.18 + 1.20/6 + (0.10 + 0.05 + 0.02)/5 + (0.20 + 0.10)/15 = 0.18 + 0.20 + 0.034 + 0.020 = 0.434. A value above about 0.40 signals a need for preheat, low-hydrogen practice and careful heat input, especially in thick sections. The calculation is a weighted sum, not a simple average.

Traps

Using the wrong CE formula or forgetting that alloying elements are in mass % changes the result. Undercut reduces section thickness and acts as a stress raiser. Radiography detects volumetric defects well but may miss tight lack-of-fusion cracks; ultrasonic testing is better for planar defects. Do not inspect before the weld has cooled to the correct temperature. High heat input can improve hydrogen escape but degrade toughness.