Materials, heat treatment and surface finishing
Materials, heat treatment and surface finishing questions ask you to select a material for a part, name a treatment and its purpose, quote a working temperature or hardness range, and choose a finish for corrosion or wear resistance.
Designations
Steels: S235JR structural, C45 (1.0503) medium-carbon shaft material, 42CrMo4 (1.7225) alloy steel for stressed shafts and gears, 16MnCr5 case-hardening steel, 100Cr6 through-hardening bearing steel, X5CrNi18-10 austenitic stainless. Cast iron EN-GJL-250 (GG25) for housings and vibration-damped bases; AlMg3 and AlSi alloys for light parts; CuSn bronze for bushes and worm wheels; POM and PA6 for low-load components.
Heat treatments
Stress-relief annealing at 550–650 °C removes welding and machining stresses. Normalizing at 850–950 °C in air refines grain and gives a predictable structure. Soft annealing around 700 °C with slow cooling makes C45 machinable. Hardening: austenitize C45 at 830–860 °C, quench in water or oil to form martensite - always followed by tempering, otherwise the part is brittle. Low tempering at 150–200 °C keeps 58–62 HRC for tools and cams; high tempering at 500–650 °C gives 28–35 HRC with toughness for shafts. Case hardening (16MnCr5, 900–950 °C, 0.6–1.0 mm depth, quench and temper) produces a hard skin on a tough core; nitriding at 500–550 °C in ammonia, 0.2–0.5 mm deep, needs no quench and distorts little.
Surface finishing
Zinc electroplating and hot-dip galvanizing protect steel sacrificially; anodizing grows a 10–25 µm oxide layer on aluminium only; phosphating and black oxide aid paint adhesion and mild corrosion resistance; powder coating and painting give appearance and barrier protection; shot blasting raises fatigue life and keys paint.
Worked example
A gearbox shaft needs a 60 HRC bearing seat with a shock-resistant core. Choose 16MnCr5, case harden to 0.8 mm, quench and temper at 180 °C. The reasoning: the carbon-rich skin resists wear from the bearing, the low-carbon core stays tough under shock, and tempering removes quenching brittleness.
Traps
Specifying hardening without tempering, confusing case depth with surface hardness, quoting anodizing for steel parts, and selecting 304 stainless for a chloride-rich coolant circuit.