MaterialsReading time 13 min

S45C or SCM440: the logic behind choosing a material

Material is the shortest entry on a drawing, and it decides the tooling, the speeds, the heat treatment, the surface treatment and the delivery date.

01Carbon and alloy steels

Grade	Carbon	Hardness after treatment	Properties and uses
S15C / SS400	0.15% /,	—	Structural; cheap and weldable, not hardened
S45C	0.45%	HRC 45 ～ 55	The most general-purpose medium-carbon steel: shafts, gears, fixtures
SCM440	0.40% + Cr-Mo	HRC 30 to 35 (quenched and tempered)	High strength, good hardenability; drive shafts
SNCM439	Ni-Cr-Mo	HRC 32 ～ 38	Large-section high-strength parts
SCM415	0.15% + Cr-Mo	Surface HRC 58 to 62	For carburising: gears, cams

Carbon content decides whether it can be hardened: below 0.3% it essentially cannot, and carburising is the only route.

Grade	Series	Magnetic	Properties
SUS304	Austenitic	None	The most general-purpose; good corrosion resistance, work-hardens badly
SUS303	Austenitic	None	Sulphur added for machinability; slightly less corrosion resistant
SUS316	Austenitic	None	Molybdenum added; resists chlorides, for food and marine use
SUS420J2	Martensitic	Yes	Hardenable to HRC 50+; tooling, dies
SUS440C	Martensitic	Yes	HRC 58+; bearings, blades
SUS630 (17-4PH)	Precipitation hardening	Yes	Precipitation hardened; high strength with corrosion resistance

The austenitic family cannot be quench hardened, only work hardened. If it has to be hard, choose martensitic or precipitation hardening.

Work hardening. A cut surface can go from HB 180 to above HB 350.
Poor thermal conductivity. The heat stays in the edge, about a third that of carbon steel.
High toughness. Chips do not break easily and wrap around the tool.
What to do. Cut deep enough to get under the hardened layer, do not feed too lightly, keep the edge sharp, flood with coolant, and prefer inserts with good chip breaking.

Grade	Series	Heat treatable	Properties
A1050	1000 pure aluminium	No	Excellent conductivity, soft, poor for machining
A5052	5000 Al-Mg	No	The workhorse for sheet metal; bends well, corrosion resistant
A6061-T6	6000 Al-Mg-Si	Yes	The most machined grade; balanced strength and machinability
A6063	6000	Yes	For extrusion; good surface, anodises beautifully
A7075-T6	7000 Al-Zn	Yes	Strength approaching steel, but poor corrosion resistance and weldability
ADC12	Die casting	No	Die-cast parts; high silicon content, abrasive on tooling

T6 means solution treated then artificially aged. Heat treating again after machining will move the dimensions.

Grade	Type	Hardness	Uses
SKD11	Cold work tool steel	HRC 58 ～ 62	Blanking dies, shear blades, wear parts
SKD61	Hot work tool steel	HRC 45 ～ 52	Die-casting dies, forging dies
SKH51	High-speed steel	HRC 62 ～ 65	Cutting tools, punches
FC250 / FCD450	Grey cast iron / ductile iron	HB 180 ～ 250	Machine bases, housings
C3604	Free-cutting brass	—	Turned parts, fittings
POM / PEEK	Engineering plastics	—	Wear bushings, insulating parts

SKD11 distorts considerably after heat treatment; leave grinding stock on precision parts.

What does it have to withstand? Tension, bending, impact, wear, fatigue, each maps to different properties.
What is the environment? Dry indoors, outdoors, chlorides, high temperature, food contact, these set the corrosion class.
Does it have to be hard? Hard means hardenable, so check the carbon content or the option of carburising first.
Can you buy it, can you make it? Lead time and minimum order on a special grade often affect a project more than the properties do.