A replacement shaft is ordered with the note “material: steel.” It is machined from the most readily available bar, fits perfectly, and is installed. A month later the keyway has hammered out and the bearing seat is worn through. The original shaft was a hardened alloy steel. The replacement was soft low-carbon bar. Both are “steel,” and both look the same on the workbench. Choosing the right steel grade for machined parts is a design decision, not a purchasing detail.
Why “Steel” Is Not a Specification
Steel grades differ mainly in carbon content and alloying elements such as chromium, molybdenum, and nickel. Those differences decide how strong the part is, whether it can be hardened, how deep that hardness goes, how well it machines, and how much it costs. Two parts with identical dimensions can have completely different service lives depending on the grade.
The Main Families, Simplified
| Family (examples) | Typical strengths | Limitations | Common uses |
|---|---|---|---|
| Low carbon steel (S10C, S20C, AISI 1018) | Easy to machine and weld, low cost | Cannot be through-hardened; low wear resistance unless case hardened | Brackets, spacers, lightly loaded pins |
| Medium carbon steel (S45C, AISI 1045) | Good strength; responds well to induction hardening | Limited hardenability in thick sections | Shafts, rods, gears with moderate loads |
| Chromium-molybdenum alloy steel (SCM440, AISI 4140) | High strength and toughness; hardens deeper | Higher cost; harder to machine when pre-hardened | Heavily loaded shafts, high-stress fasteners |
| Nickel-chromium-molybdenum steel (SNCM439, AISI 4340) | Excellent toughness in large sections | Expensive; longer lead time | Critical shafts, heavy-duty components |
| Tool steels (SKD11, D2) | Very high hardness and wear resistance | Brittle if misapplied; difficult to machine | Blades, dies, wear plates |
| Stainless steels (SUS304, SUS316) | Corrosion resistance, hygienic surfaces | Work hardens; slower to machine | Food, cosmetic, and process equipment parts |
Equivalent names vary between standards (JIS, AISI, DIN), so when in doubt, state the standard as well as the grade.
Four Questions That Narrow the Choice
- What load does it carry? Steady, cyclic, or shock loads point to different toughness requirements.
- Does it need to be hard, and where? Only the surface (induction or case hardening), or all the way through?
- What environment does it work in? Moisture, chemicals, or hygiene requirements may rule out carbon steels.
- How many will be made? For high volumes, machinability and bar availability affect price significantly.
Hardenability Is Not Hardness
Hardness is how hard the steel becomes. Hardenability is how deep that hardness can reach. A medium carbon steel can reach high surface hardness, but in a thick section the core may stay soft. An alloy steel with chromium and molybdenum hardens deeper in the same section. If a large shaft needs strength through its whole cross-section, grade selection matters more than the heat treatment recipe.
Copying an Old Part Without Knowing Its Material
When the original grade is unknown, a hardness test on the old part gives a strong hint. A part that reads well above 30 HRC was almost certainly heat treated, which means a plain low-carbon replacement will not survive. It is better to ask before cutting than to discover the answer in service.
How Merdeka Precision Approaches Material Selection
In our fabrication service, material can be supplied by us or by the customer. When the drawing does not specify a grade, we ask about load, hardness, and environment before quoting, and we can test the hardness of an existing part with our Rockwell and Microvickers testers to guide the choice.
For high-volume work, our precision component manufacturing line processes bar stock through cutting, induction hardening, straightening, machining, and thread rolling, so material choice and hardening are planned together from the start. Many of these parts go into vehicles and equipment for the automotive industry, where the wrong grade simply is not an option.
Put the Grade on the Drawing
Write the full grade and standard (for example, “S45C, JIS G4051”) along with hardness requirements. If you are not sure, write the load and environment instead, and let the supplier propose a grade for your approval.
How many parts in your spare parts list say only “steel” in the material column? Send us one and we will suggest a grade based on how it is actually used.