100+ Free Module 6 Materials & Hardware Practice Questions

Explanation: AISI 4130 steel is a chromium-molybdenum alloy steel. The '41' designation denotes chromium (approx. 0.8–1.1%) and molybdenum (approx. 0.15–0.25%), while '30' indicates approximately 0.30% carbon content. It provides excellent strength-to-weight ratio, toughness, and weldability for aircraft tubular structures.

Explanation: Full annealing involves heating steel above its critical temperature followed by very slow cooling inside the furnace, resulting in maximum softness and ductility. Normalizing involves heating above the critical temperature and cooling in still air, which produces a finer grain structure with slightly higher strength and hardness than full annealing.

Explanation: Nitriding is a surface hardening process in which steel parts are heated in an atmosphere of ammonia gas (NH3) at approximately 500°C (950°F). Nitrogen reacts with alloying elements (like aluminum, chromium, molybdenum) to form extremely hard nitrides. Because no liquid quench is required, dimensional distortion is virtually eliminated.

Explanation: Rockwell C scale (HRC) tests hard materials (like heat-treated alloy steels) using a 120° diamond cone (Brale indenter) with a 150 kg major load (preceded by a 10 kg minor load). Rockwell B (HRB) uses a 1/16-inch steel ball indenter with a 100 kg major load for softer metals like aluminum.

Explanation: Austenitic stainless steels (such as 302, 304, and 316, often called 18-8 stainless steel) have a Face-Centered Cubic (FCC) austenite structure stabilized by nickel. They are non-magnetic in the annealed condition and can only be hardened by cold working (work hardening), not by heat treatment.

Explanation: 17-4PH stainless steel achieves its maximum ultimate tensile strength (approx. 1310 MPa / 190,000 psi) and peak hardness when precipitation aged at the lowest standard temperature, Condition H900 (900°F for 1 hour). Higher aging temperatures (like H1150) over-age the alloy, increasing toughness but reducing ultimate strength.

Explanation: Magnetic particle inspection detects flaws that run perpendicular to the lines of magnetic force. To detect a longitudinal flaw (parallel to the component axis), a circular magnetic field is required. Passing current directly through the part (head shot or central conductor) produces magnetic lines of force wrapped in circles around the axis, cutting across longitudinal cracks.

Explanation: After MPI, residual magnetism must be removed by placing the part in a reversing, gradually decreasing AC magnetic field. Residual magnetism attracts ferrous metal chips/dust that can contaminate oil systems and destroy bearings, and causes deviation in aircraft magnetic compasses and sensitive instruments.

Explanation: Quenched martensitic steel is extremely hard, brittle, and contains severe internal stresses. Tempering involves reheating the hardened steel to a temperature below its lower critical limit (e.g., 200°C–650°C), which relieves internal stresses, decreases brittleness, and improves impact toughness and ductility at the cost of a slight reduction in peak hardness.

Explanation: Carburizing is a case-hardening process where low-carbon steel parts are heated in contact with a carbon-rich environment (gas, liquid salt, or solid charcoal/pack). Carbon diffuses into the outer surface, producing a high-carbon outer layer ('case') that can subsequently be quenched to form hard martensite while maintaining a tough low-carbon core.