What is the difference between 1018 and 4140 steel?

1018 is a low-carbon steel (0.18% C) that machines easily, welds well, and case-hardens but cannot be through-hardened. 4140 is a chromium-molybdenum alloy steel that through-hardens to HRC 28–54, has 2–3× the yield strength, and offers superior fatigue resistance. Use 1018 for general brackets; use 4140 for high-load shafts, gears, and structural components.

What is the difference between 4140 and 4340 steel?

4340 adds nickel (1.65–2.00%) to the chromium-molybdenum base of 4140, giving it deeper hardenability and higher toughness at the same hardness. 4340 achieves higher UTS (280 ksi (1931 MPa) vs 235 ksi (1620 MPa) at full hardness) and is preferred for large cross-sections where 4140 may not through-harden. 4340 costs ~25% more than 4140.

What does "pre-hardened" mean for 4140?

Pre-hardened 4140 (typically 4140 PH or 4140HT) is supplied at HRC 28–32, which balances machinability with mechanical properties — yield strength ~130 ksi (896 MPa). It eliminates post-machining heat treatment and associated distortion risk. Use pre-hardened stock when you need moderate hardness without tight-tolerance post-heat-treat grinding.

Does 1018 steel rust?

Yes. 1018 is a plain carbon steel with no chromium content and will rust rapidly in humid or wet environments without surface protection. Common treatments include black oxide (minimal protection, cosmetic), zinc plating (moderate outdoor protection), and electroless nickel (excellent wear and corrosion resistance). Always specify a surface finish on carbon steel drawings.

What is 17-4 PH stainless steel used for?

17-4 PH is a precipitation-hardening martensitic stainless steel heat-treated to H900 condition (190 ksi / 1,310 MPa UTS, 170 ksi / 1,170 MPa yield). It combines high strength with good corrosion resistance — used for high-strength shafts, fasteners, surgical instruments, and structural parts that need stainless corrosion resistance at carbon steel strength levels.

How do I call out steel material on an engineering drawing?

Use the full AISI/SAE designation plus condition: "AISI 4140 Steel, Pre-Hardened to HRC 28–32 per ASTM A193" or "AISI 1018 Steel, Cold-Drawn, ASTM A108." For 17-4 PH: "17-4 PH Stainless Steel, Condition H900 per AMS 5604." Never write just "steel" — this forces the shop to make a substitution that may not meet your mechanical requirements.

Material Guide · 12 min read

Steel Grades for CNC Machining

1018, 1045, 4130, 4140, 4340, and 17-4 PH — mechanical properties, machinability, heat treatment effects, and exactly when to use each grade.

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Steel Grade Quick Select

Most common CNC steel grades

AISI 1018

General brackets, pins

58–64 ksi (400–441 MPa) UTS

AISI 4140 PH

Shafts, gears, tooling

140–160 ksi (965–1103 MPa) UTS

AISI 4340

High-load structural

175–195 ksi (1207–1344 MPa) UTS

AISI 17-4 PH

High-strength stainless

180–200 ksi (1241–1379 MPa) UTS

6 grades covered with full property data

Heat treatment effects explained

Drawing callout format included

Updated 2026

Steel Grades

Sections

HRC 28–54

Hardness Range

$0.40–7/lb ($0.9–15/kg)

Raw Material Cost

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Picking the Right Steel Grade Matters More Than You Think

Specifying "steel" without a grade is the most common material callout mistake in CNC machining. The yield strength difference between 1018 and 4340 is 4×. The machinability difference between 1018 and 316 stainless is 6×. This guide gives you the engineering data to specify the right grade the first time — with full property tables, heat treatment effects, and exact drawing callout format.

Section 1 of 5

The AISI/SAE Numbering System

When you see a four-digit steel code on a drawing, those digits encode the alloy family and carbon content directly — and knowing how to read them saves you from specifying the wrong grade.

The AISI/SAE numbering system is the universal shorthand in US job shops. International equivalents exist under SAE J403 (US), EN 10083 (Europe, e.g., 42CrMo4 = 4140), and JIS G4053 (Japan, e.g., SCM440 = 4140).

10xx

Plain Carbon Steel

No significant alloying elements. The last two digits are carbon content × 100: 1018 = 0.18% C, 1045 = 0.45% C. Higher carbon = higher hardness but lower weldability.

41xx

Chromium-Molybdenum (CrMo)

Cr (0.80–1.10%) and Mo (0.15–0.25%) provide through-hardenability and high fatigue strength. 4130 and 4140 are the workhorse alloy steels for CNC machined shafts and gears.

43xx

Nickel-Chromium-Molybdenum

4340 adds Ni (1.65–2.00%) to the CrMo base for deeper hardenability, higher toughness, and ~15–20% higher fatigue endurance limit vs. 4140 at equivalent hardness. Required when section thickness exceeds ~3 in. (76 mm) for through-hardening.

17-4 PH

Precipitation-Hardening Stainless

Not an AISI 4-digit code — 17-4 PH is a proprietary designation (17% Cr, 4% Ni, Cu addition). Hardened by precipitation aging, not quench + temper. Combines stainless corrosion resistance with high strength.

Condition

Temper / Condition Suffix

Condition (annealed, normalized, pre-hardened) determines delivered properties. 4140 "PH" = Pre-Hardened to HRC 28–32. Annealed 4140 machines differently than pre-hardened. Always specify condition on drawings.

ASTM

ASTM Bar Stock Standards

Common specs: ASTM A108 (carbon steel bars), ASTM A193 (alloy steel fastener stock), AMS 6349 (4340 premium quality). Specify the ASTM/AMS standard when the application requires certified mechanical properties.

Pro Tip

The last two digits of any 10xx grade tell you the carbon content in hundredths of a percent. 1018 = 0.18% C (easy to weld, cannot through-harden). 1045 = 0.45% C (harder, more brittle, limited weldability). The inflection point for weldability is roughly 0.30% C — above that, preheat is required to prevent heat-affected zone cracking.

Section 2 of 5

Steel Grade Comparison Table

Your grade selection determines every downstream decision — machinability, heat treatment, cost, and whether the part survives its loading environment.

The table below shows mechanical properties in the most common supplied condition. UTS and yield values are for the condition as-machined or pre-hardened — not max achievable after heat treatment. Machinability rating compares how easily a material cuts relative to AISI B1112 free-machining steel (rated 100%) — a 78% rating means the grade machines at roughly 78% the speed of the baseline, translating to proportionally longer cycle times and higher per-part cost.

Grade	Condition	UTS	Yield Strength	Elongation	Hardness	Machinability	Cost, $/lb ($/kg)
AISI 1018	Cold-drawn	400–480 MPa (58–70 ksi)	350–390 MPa (51–57 ksi (352–393 MPa))	15%	HRB 71	Excellent (78%)	$0.40–0.70 ($0.88–1.54)
AISI 1045	Cold-drawn	565–650 MPa (82–94 ksi)	490–560 MPa (71–81 ksi (490–558 MPa))	12%	HRB 96	Good (57%)	$0.50–0.80 ($1.10–1.76)
AISI 4130	Normalized	620–720 MPa (90–104 ksi)	415–475 MPa (60–69 ksi (414–476 MPa))	25%	HRB 92	Good (70%)	$0.80–1.20 ($1.76–2.65)
AISI 4140	Pre-hardened (HRC 28–32)	965–1,070 MPa (140–155 ksi)	860–930 MPa (125–135 ksi (862–931 MPa))	17%	HRC 28–32	Good (66%)	$0.90–1.40 ($2.00–3.09)
AISI 4340	Normalized	1,200–1,350 MPa (174–196 ksi)	830–900 MPa (120–130 ksi (827–896 MPa))	12%	HRC 38–40	Moderate (50%)	$1.20–1.80 ($2.65–3.97)
17-4 PH	Condition H900	1,240–1,380 MPa (180–200 ksi)	1,100–1,210 MPa (160–175 ksi (1103–1207 MPa))	10%	HRC 40	Moderate (45%)	$4.00–7.00 ($8.82–15.43)

* Machinability rating relative to AISI B1112 free-machining steel = 100%. All values from ASM Handbook Vol. 1 and supplier data sheets. Properties vary with bar diameter and heat.

Quote the Right Steel Grade for Your Part

MakerStage machines all six grades above — from 1018 cold-drawn bar to pre-hardened 4140 and 17-4 PH H900. Quote review can confirm material availability, hardness condition, and any drawing-callout questions before production starts.

Review Steel Grade Requirements

Section 3 of 5

Heat Treatment Effects on Mechanical Properties

If you specify a steel grade without specifying its heat treatment condition, the shop will deliver whatever is on the shelf — and the mechanical properties could be 50% lower than what your design requires. The same alloy can deliver wildly different properties depending on heat treatment. Before reading the table below, here are the key terms:

Hardenability — how deeply a steel can be hardened from the surface inward during quenching. High hardenability (4340) means the center of a 4 in. (101.6 mm) bar reaches full hardness; low hardenability (1018) means only the outer 0.030–0.060 in. (0.76–1.52 mm) hardens.
Through-hardening — heating the entire part above its critical temperature (~1,500°F / 1499°F (815°C) for 4140), then quenching in oil or water so the full cross-section transforms to martensite. Requires alloy steels (41xx, 43xx) with sufficient hardenability.
Case hardening (carburizing) — diffusing carbon into only the surface layer (typically 0.020–0.060 in. / 0.5–1.5 mm deep) to create a hard, wear-resistant shell around a tough, ductile core. Used on low-carbon steels like 1018 that cannot through-harden.
Q&T (Quench and Temper) — the two-step process: quench (rapid cool) to maximize hardness, then temper (reheat to 400–1,200°F / 392–1202°F (200–650°C)) to trade some hardness for ductility and toughness. Higher temper temperature = lower hardness but higher toughness.
Austenitizing — heating steel above its upper critical temperature so the crystal structure transforms to austenite (face-centered cubic). This is the starting point for all hardening — the subsequent cooling rate determines the final structure and hardness.
Normalizing — austenitizing followed by air cooling (not quenching). Produces a uniform, fine-grained structure with moderate strength — used to create a consistent starting point before further heat treatment or machining.

Here is what each condition means for 4140 — the most commonly heat-treated CNC steel.

Condition	Process	Hardness	UTS	Yield	Use When
Annealed	Slow furnace cool	HRB 85 max	655 MPa (95 ksi)	415 MPa (60 ksi (414 MPa))	Maximum machinability, forming, or welding required
Normalized	Air cool from austenitizing temp	HRB 96	1,020 MPa (148 ksi)	655 MPa (95 ksi (655 MPa))	Uniform grain structure, standard mechanical properties
Pre-Hardened (PH)	Q&T to HRC 28–32	HRC 28–32	965–1,070 MPa (140–155 ksi)	860–930 MPa (125–135 ksi (862–931 MPa))	Ready-to-machine; no post-machine heat treat needed
Q&T (Fully Hardened)	Quench + temper to spec	HRC 32–38	1,240 MPa (180 ksi)	1,103 MPa (160 ksi (1103 MPa))	Maximum mechanical properties; requires grinding after HT
Nitrided	Gas/plasma nitride surface	HRC 60+ surface, HRC 28 core	Core: 1,020 MPa	Core: 896 MPa	Wear-critical surfaces: gears, cams, sliding components

Machine Before or After Heat Treatment?

The standard sequence for hardened steel parts is: rough machine → heat treat → finish grind to final tolerance. Heat treatment causes distortion (typically 0.001–0.003 in/in for 4140), so tight-tolerance features (bores, bearing seats, ground surfaces) should be left with stock for post-HT grinding. Pre-hardened 4140 eliminates this by delivering the part already at HRC 28–32.

Common Heat Treatment Mistake

Specifying "4140, hardened to HRC 52+" for a machined part with threaded holes and bored features is almost always a mistake. At HRC 52+, thread tapping becomes impractical and bore distortion requires grinding. For most CNC parts, HRC 28–38 (pre-hardened or lightly tempered) is the sweet spot.

Pro Tip

If you need HRC 28–32 and do not need post-machining heat treat, order pre-hardened 4140 bar stock. It eliminates the heat treatment step, reduces distortion risk, and ships faster than processed raw material. Most CNC job shops stock it.

Section 4 of 5

When to Use Each Grade

Start with 1018 as your default — it machines the fastest, costs the least, and welds without preheat. Upgrade to a higher alloy only when your stress analysis, fatigue requirements, or corrosion environment demands it. Each step up costs more to machine and procure.

Application	Recommended Grade	Why	Avoid
Structural brackets, fixtures	1018 Cold-Drawn	Easy to machine, weld, and case-harden. Cost-effective for low-to-moderate loads.	4340 (overkill — costs 3× more, machines harder)
Shafts, axles, couplings	4140 Pre-Hardened	HRC 28–32 provides fatigue resistance and surface hardness without post-machining HT.	1018 (insufficient fatigue strength under cyclic bending)
Gears, pinions	4140 or 4340 + case harden	Through-hardenable core (tough) + carburized/nitrided surface (hard). Highest fatigue life and wear resistance among these grades.	1018 (case-hardens, but core is too soft for high gear loads)
High-load structural (>150 ksi (1034 MPa) yield)	4340 Q&T	Nickel addition enables deep hardenability in large cross-sections. Highest toughness at equivalent hardness among these grades.	4140 (may not through-harden in sections > 3 in diameter)
Corrosion-resistant + high-strength	17-4 PH H900	Stainless corrosion resistance at 170 ksi (1172 MPa) yield — no surface treatment needed.	4140 in corrosive environments (rusts without plating)
Weldable structural tubing	4130 Normalized	4130 welds without preheat at standard wall thicknesses. Stronger than mild steel, lighter wall possible.	4140 (requires preheat/postheat; over-engineered for tubing frames)

Decision Rule

Start with 1018. If yield strength > 54 ksi (372 MPa) is needed, switch to 4140 PH. If fatigue life is critical (cyclic bending or torsion), use 4140 or 4340 + heat treat. If corrosion resistance is also required, use 17-4 PH H900. Each step up costs more to machine and procure — justify the upgrade with analysis, not instinct.

Section 5 of 5

Drawing Callout Format

Your material callout on the drawing is a contractual specification — if it is ambiguous or incomplete, the shop will substitute whatever is available and you may not catch it until parts fail. Use these exact formats to eliminate substitution risk.

1018 Cold-Drawn

AISI 1018 Steel, Cold-Drawn, ASTM A108

Add "Carburize and Harden per AMS 2759/7" if surface hardening required. Specify case depth (e.g., 0.030–0.040 in. / 0.76–1.02 mm) and surface hardness (e.g., HRC 58–62).

4140 Pre-Hardened

AISI 4140 Steel, Pre-Hardened to HRC 28–32 per ASTM A193 Gr B7

If further hardening is required post-machining, remove "Pre-Hardened" and add "Quench & Temper to HRC 32–36 after machining."

4340 Normalized

AISI 4340 Steel, Normalized per AMS 6415, UTS 180 ksi (1241 MPa) min

For critical applications, add "Certified Mill Test Report (MTR) required." Specifying UTS min protects against under-strength heat.

17-4 PH H900

17-4 PH Stainless Steel, Condition H900 per AMS 5604, UTS 190 ksi (1310 MPa) min

H900 has highest strength but lowest ductility (10%). For applications needing higher toughness at lower strength, specify H1025 (155 ksi (1069 MPa) UTS, 14% elongation).

Pro Tip

Always include the minimum UTS or yield strength on your drawing notes block when heat treatment is specified. Heat lots of alloy steel can vary — a minimum UTS callout ensures the shop performs hardness verification and rejects under-strength material before machining your part.