GATE NM cheat sheet
GATE NM · Keep the key ideas within reach.
Materials and Welding in Marine Construction
Mild steel for shipbuilding typically has yield strength 235 MPa (Grade A); higher-strength grades AH32, AH36 have yield 315 MPa and 355 MPa respectively.
Carbon equivalent CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15; higher CE increases hardenability and cold-cracking risk, requiring preheat.
Weldability: hydrogen-induced cold cracking is controlled by low-hydrogen electrodes, preheat, and post-weld heat treatment; moisture on electrodes is a primary hydrogen source.
Common weld defects: porosity (gas entrapment), slag inclusion, lack of fusion, undercut, and lamellar tearing in thick restrained joints.
Non-destructive testing: ultrasonic testing for internal flaws, radiographic testing for volumetric flaws, magnetic particle and dye penetrant for surface flaws.
Corrosion protection: impressed current cathodic protection (ICCP) and sacrificial anodes; coating breakdown accelerates localized pitting and galvanic corrosion.
Heat-affected zone (HAZ) softening or hardening depends on cooling rate; high heat input coarsens grains and reduces toughness in the HAZ.
Fatigue strength of welded joints is governed by detail category, not base metal strength; weld toe grinding and peening improve fatigue life.