Material
Steel Machined Parts
Strong, hard-wearing, and a good fit when rust can be managed.
Steel is the usual choice for machined parts that carry load, take wear, or hold an edge when weight is not the deciding factor: mild grades for structure, alloy grades like 4140 and 4340 for loaded shafts, and tool steels like A2, D2, and O1 for wear and cutting edges. The one tradeoff is rust, and a finish called out on the print, such as powder coating, handles it. Send a STEP file and a PDF print through the quote form and a quote comes back in 24-48 hours.
Updated September 2026
Steel
Mild, alloy, and tool grades. It rusts bare, so name a finish on the print.
When is steel the right material?
Steel suits a part that has to carry load, take wear, or hold an edge when weight is not the deciding factor. A36 steel is about 7.8 g/cm³ (source 1) and 6061 aluminum about 2.70 g/cm³ (source 2), so a steel part weighs roughly three times what the same part weighs in aluminum. What steel returns is stiffness, 200 GPa for A36 against 69 GPa for 6061 (sources 1, 2), and, in the alloy and tool grades, hardness and wear resistance.
Free-machining mild grades like 12L14 cut fast and finish cleanly. Lead acts as an internal lubricant in the cutting zone and sulfur forms low-shear-strength inclusions (source 3), which allows higher machining rates and longer tool life (source 4). The alloy grades cut more slowly: AZoM rates 4140 at 65 against AISI 1212 at 100 (source 5), and hardened tool steels are hard enough that conventional machining struggles with them (source 6). That difference shows in the quote.
Mild, alloy, and tool steel
Mild (low-carbon) steel holds about 0.05-0.30% carbon and is easily machined and welded (source 7). 1018 and A36 are the general-purpose pair: A36 is readily welded by all welding processes (source 1), and AZoM gives 1018 an ultimate tensile strength of 440 MPa (63,800 psi) and lists fixtures, mounting plates and spacers among its uses (source 8). 12L14 is the leaded, free-machining member of the 12xx series (source 9). The lead and sulfur that help it cut also cause hot cracking in a weld, and TWI advises eliminating free-machining steels at the material selection stage when welding is part of the fabrication (source 10), so keep 12L14 to parts that will not be welded.
Alloy steels add chromium and molybdenum, as in 4140, and nickel as well in 4340 (source 9). AZoM describes 4140 as having high fatigue strength, abrasion and impact resistance, toughness and torsional strength (source 5), and 4340 is used for heavy shafting (source 11), so these two are the usual picks for loaded shafts. Fixture bases and details in these grades are covered on steel fixtures.
Tool steels such as A2, D2, and O1 are bought for hardness and wear resistance (source 12): dies, punches, cutting edges, gauges, and fixture details. Their hardest features are often cut by wire EDM, which can machine hardened tool steel (source 6), or ground, which until recent decades was the only practical way to machine hardened steel (source 13), rather than milled. If you are unsure which family fits, describe the load and wear in the notes and the grade gets settled before anything is priced. Other alloys and plastics quoted on request. Name the grade on the print.
What gets made from it
Steel turns up wherever strength and wear matter more than weight: shafts and axles, dies and punches, jigs and fixtures, structural brackets, and bushings. Most of it is milled or turned, and it suits one-offs and small runs equally, with no minimum order.
Rust is the catch, and finishing handles it
Bare carbon steel is susceptible to rust and corrosion, especially with moisture or salt, and a coating of paint or another protective material shields it (source 7). Rust is iron oxide formed when iron reacts with oxygen in the presence of water or moisture, and a thin layer of oil gives temporary protection in storage or transport (source 14). Powder coating is a thick, tough finish, more durable than conventional paint, used mainly on metal objects that see rough use (source 15).
If a part lives outdoors, in washdown, or anywhere continuously wet, it is worth stepping up to stainless steel. Stainless carries its corrosion resistance in the base metal: 10.5% or more chromium forms a passive film that protects the metal and can repair itself when scratched (source 16), rather than relying on a coating.
Sources
All accessed 30 September 2026. Where a figure here differs from your print, the print governs.
1. Wikipedia, "A36 steel": en.wikipedia.org (density 7.8 g/cm³; Young’s modulus 200 GPa; at least 36 ksi yield; readily welded by all welding processes).
2. Wikipedia, "6061 aluminium alloy": en.wikipedia.org (density 2.70 g/cm³; Young’s modulus 69 GPa; 6061-T6 yield at least 35 ksi, typically 39 ksi).
3. Wikipedia, "Machinability": en.wikipedia.org (lead acts as an internal lubricant in the cutting zone; sulfur forms low-shear-strength inclusions that improve machinability).
4. Wikipedia, "Free-machining steel": en.wikipedia.org (higher machining speeds, larger cuts and longer tool life).
5. AZoM, "AISI 4140 Alloy Steel", updated 18 December 2019: azom.com/article.aspx?ArticleID=6769 (chromium, molybdenum and manganese low alloy steel with high fatigue strength, abrasion and impact resistance, toughness and torsional strength; tensile strength 655 MPa (95,000 psi); machinability rating 65 against AISI 1212 at 100).
6. Wikipedia, "Electrical discharge machining": en.wikipedia.org (extremely hard materials such as hardened tool steels, very difficult to machine conventionally, can be machined precisely by EDM).
7. Wikipedia, "Carbon steel": en.wikipedia.org (mild steel about 0.05-0.30% carbon, easily machined and welded; carbon steel susceptible to rust and corrosion, shielded by paint, varnish or another protective coating).
8. AZoM, "AISI 1018 Mild/Low Carbon Steel", updated 7 August 2019: azom.com/article.aspx?ArticleID=6115 (ultimate tensile strength 440 MPa, 63,800 psi; uses include fixtures, mounting plates and spacers).
9. Wikipedia, "SAE steel grades": en.wikipedia.org (12xx resulfurized and rephosphorized, 12L14 with lead added; 41xx chromium-molybdenum; 43xx nickel-chromium-molybdenum).
10. TWI, "Can I arc weld free-machining steels?": twi-global.com (sulphur and lead cause cracking and porosity in welds; free-machining steels should be eliminated at the material selection stage when welding is part of a fabrication procedure).
11. Wikipedia, "4340 steel": en.wikipedia.org (nickel-chromium-molybdenum, medium-carbon low-alloy steel; tensile strength 860-1,980 MPa and 24-53 HRC across conditions; used for heavy shafting).
12. Wikipedia, "Tool steel": en.wikipedia.org (hardness and abrasion resistance; A2 the most commonly used air-hardening grade, used for blanking and forming punches and trimming dies; D2 11-13% chromium, very wear resistant but not as tough as lower-alloyed steels; O1 used for gauges, cutting tools and knives, up to 66 HRC).
13. Wikipedia, "Grinding (abrasive cutting)": en.wikipedia.org (better suited to very hard materials than regular machining; until recent decades the only practical way to machine hardened steels).
14. Wikipedia, "Rust": en.wikipedia.org (iron oxide formed by iron and oxygen in the presence of water or moisture; a thin layer of oil or grease as temporary protection for storage or transport).
15. Wikipedia, "Powder coating": en.wikipedia.org (a thick, tough finish more durable than conventional paint; mainly used on metal objects, particularly those subject to rough use).
16. Wikipedia, "Stainless steel": en.wikipedia.org (chromium content of 10.5% or more forms a passive film that protects the material and can self-heal).
17. Justin Sims, "Stress evolution during quench hardening of steel", Thermal Processing, 15 July 2022: thermalprocessing.com (size change is inevitable and shape change can occur; the dimensional change on transforming to martensite is significant).
Questions
Before you send a job.
01 Which steel should I pick: mild, alloy, or tool?
Mild steel (1018, A36) for general, weldable parts, with free-machining 12L14 when cutting speed matters and the part will not be welded. Alloy steel (4140, 4340) when the part carries load, like shafts. Tool steel (A2, D2, O1) for hardness and wear, like dies, punches, and cutting edges. If you know the grade, put it on the print. If not, type or pick “Not sure - please advise” in the material field and describe the load and wear, and the right grade gets flagged before pricing.
02 Does steel rust, and what stops it?
Bare carbon steel rusts, yes, and a coating shields it (source 7). Powder coating is the durable option, and a thin film of oil is only temporary protection for storage or transport (source 14). A finish called out on the print is quoted with the part. If the part lives wet or outdoors all the time, stainless steel is worth a look before you coat carbon steel.
03 What changes when a steel part is hardened?
Hardness depends on the grade and the condition: 4340 spans about 24 to 53 HRC across conditions (source 11), and O1 tool steel can reach 66 HRC (source 12). Hardening changes a part’s size, and can change its shape, as the steel transforms to martensite (source 17), and the hardest features can be cut by wire EDM or ground instead of milled (sources 6, 13). Rush 1-2 weeks applies to machined parts with no secondary operations. Heat treat, grinding, EDM and finishing add time, and the quote gives the real date.
04 Steel or aluminum?
A36 steel and 6061-T6 aluminum are close in minimum yield strength: A36 specifies at least 36 ksi (source 1) and 6061-T6 at least 35 ksi (source 2). The differences are weight and stiffness. Steel is about 7.8 g/cm³ against 2.70, and its modulus is 200 GPa against 69 GPa (sources 1, 2), so a steel part is roughly three times heavier and stiffer. Pick steel when stiffness, wear, or hardness drives the part, and aluminum when weight does. Both quote the same way.
05 Should I use A2, D2, or O1 tool steel?
A2 is a common general-purpose tool steel, the most used air-hardening grade, and goes into blanking and forming punches and trimming dies. D2 carries 11-13% chromium and is very wear resistant but not as tough as lower-alloyed steels, so weigh it where a part sees shock. O1 is a cold-work steel used for gauges, cutting tools and knives (source 12). Name the grade on the print; custom punches has more on matching the grade to the work.
06 What is the difference between 4140 and 4340?
Both are low-alloy steels. 4140 is a chromium-molybdenum steel and 4340 adds nickel (source 9). AZoM credits 4140 with high fatigue strength, toughness and torsional strength (source 5), and Wikipedia describes 4340 as an ultra-high strength steel with tensile strength from 860 to 1,980 MPa depending on condition, used for heavy shafting (source 11). The grade your print names is the grade quoted. If the choice is open, describe the load and the quote flags a grade before pricing.
07 What tolerances can steel parts hold?
Typical machined tolerance is ±0.005 in, with ±0.001 in on critical turned diameters. Milled features are quoted from the print, and ground features are quoted to ±0.0001 in. Call out the critical dimensions on the print. Without a drawing, a STEP file is quoted to a ±0.005 in general tolerance or the one your notes name; what file to send has the full rule. Every part inspected against your print before it ships.
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