15.1 Material Identification: Ferrous & Non-Ferrous Metals, Plastics

Sub-task A-3.06 (Performs material identification) sits inside Task A-3, which carries 7 of the 25 questions in Major Work Activity A. Selecting the wrong material for a shaft, a wear plate or a repair weld is a failure that shows up weeks later, so the exam tests whether a journeyperson can classify what is in front of them.

The AISI / SAE Numbering System

Plain carbon and alloy steels use a four-digit designation:

Carbon Ranges and Their Consequence

The rule that follows is worth memorizing: weldability falls and hardenability rises as carbon content rises. A shaft that hardens under a torch flame is a medium- or high-carbon steel, and welding it without preheat will crack it.

Cast Irons

Grey iron is identified by a dull grey fracture face that leaves a grey mark on your hand from the graphite, whereas steel fractures bright.

Stainless Steels

The magnet is the fastest field test: a shaft that a magnet will not stick to, in a corrosive service, is almost certainly 304 or 316. That matters immediately because austenitic grades cannot be hardened by heat treatment and are prone to galling on threads unless anti-seize is used.

Non-Ferrous Metals and Bearing Alloys

Aluminum temper designations appear constantly on machine parts: -O annealed, -Hxx strain hardened (non-heat-treatable alloys), -T4 solution treated and naturally aged, -T6 solution treated and artificially aged (the common structural condition).

Engineering Plastics and Composites

Millwrights increasingly install plastic wear components, chain guides and bearings.

Plastic bearing selection trap: nylon absorbs moisture and can swell by 2–3%, so a nylon bushing machined to a tight clearance in a dry shop will seize once it is wet. Acetal or UHMW is used where dimensional stability matters.

Field Identification Tests

Reading a Spark Test

   WROUGHT IRON      Long straight straw-yellow streaks, forked ends, no bursts
   LOW-CARBON STEEL  Long straw streaks with a few small forks near the end
   MEDIUM CARBON     More numerous forks and small starbursts along the stream
   HIGH-CARBON STEEL Dense, bright, repeating multi-branch bursts near the wheel
   CAST IRON         Short, dull RED streaks that brighten and burst near the end
   STAINLESS (300)   Short, straw-coloured stream, few forks, less brilliant
   HIGH-SPEED STEEL  Faint dark red stream with distinctive orange spear points

Spark testing is a comparison method: keep known reference samples at the grinder and compare. It gives a family, not a grade.

Mechanical Properties the Exam Uses

Hardness scales: Rockwell C (HRC) for hardened steel above about 20 HRC, Rockwell B (HRB) for softer steels and non-ferrous, and Brinell (HB) for castings and large sections using a 10 mm ball at 3,000 kg. A shaft specified at 28–32 HRC is a quenched-and-tempered medium-carbon or alloy steel, not mild steel.

Low-temperature toughness is a Canadian issue. Ordinary carbon steel undergoes a ductile-to-brittle transition as temperature falls; structural and lifting components used outdoors in winter must be specified with notch-toughness requirements. Hoisting a load with a shackle or lifting lug made from an unrated steel at -30 C risks brittle fracture with no warning.