4140 vs 1018 Steel: How to Choose the Right Grade

If your component needs high tensile strength, wear and fatigue resistance, and can be heat-treated for hardness, SAE/AISI 4140 is the better choice; if you prioritize low cost, easy weldability, excellent cold-forming and general machinability for simple shafts, pins or mild-duty parts, AISI 1018 is usually the smarter selection. For finished-hard parts that must survive heavy loading or shock, choose 4140 (heat-treated to required hardness). For welded assemblies, carburized surfaces, or economical prototypes, choose 1018.

What is 4140 and 1018 steel?

4140 is a chromium-molybdenum alloy steel with significantly higher carbon and alloying element content than 1018; that alloy content gives 4140 better hardenability, higher tensile strength, and far better fatigue resistance after proper heat treatment. 1018 is a low-carbon (mild) steel prized for weldability, formability, and low price. Choose 4140 for gears, axles and heavy machinery parts that require strength and wear life; choose 1018 for studs, shafts, brackets, and parts that will be welded or require a soft core for forming.

Chemical composition

• AISI/SAE 4140 (typical composition): C ≈ 0.38–0.43%, Cr ≈ 0.8–1.1%, Mn ≈ 0.75–1.0%, Mo ≈ 0.15–0.25%, Si ≈ 0.15–0.30%. These alloying elements raise hardenability, strength and toughness versus plain carbon steels.

• AISI 1018 (typical composition): C ≈ 0.15–0.20%, Mn ≈ 0.60–0.90%, remainder essentially iron with minor S/P. The low carbon content means 1018 cannot be hardened significantly by quench & temper — it’s easy to cold-form and weld.

Why this matters: higher carbon + alloying (Cr, Mo) → 4140 hardens through the cross-section with heat treatment → higher strength and fatigue life; lower carbon → 1018 remains ductile and weldable, and is inexpensive.

Mechanical properties

Numbers below are typical ranges; always consult the mill certificate or supplier datasheet for exact lot values.

Key mechanical numbers (annealed condition unless specified):

Citation note: the tensile and hardness ranges above are supported by multiple datasheets and heat-treat examples; refer to specific supplier certificates for procurement.

Heat treatment and hardenability

• 4140: responds well to austenitize → quench (oil or water depending on section size) → temper cycles. Hardenability is substantial because of alloying; core hardness in moderate sections can be increased substantially. Typical practice: austenitize ~820–860°C (depending on section), quench, temper to target HRC. ASM / industry sources document standard tempering recipes and microstructure control for 4140.

• 1018: cannot be significantly hardened by quench & temper because of low carbon. For a hard surface with a ductile core, carburizing (case carburizing) is the typical route — 1018 is a common base for case hardening. Nitriding is less effective unless alloyed properly.

Design tip: If you need a tough core and hard surface but prefer easier machining/welding of the blank, use 1018 + carburize for surface hardness; if you need through-hard part strength, use 4140 with quench & temper.

Machinability, weldability and forming

• Machinability: 1018 is generally easier to machine (good chip formation, higher machinability rating) than fully hardened 4140; however, annealed 4140 is still machinable but will gum up tools more and often requires harder tooling if sections are alloyed or partially hardened.

• Weldability: 1018 = very good weldability using standard welding processes (MIG, TIG, stick). 4140 = weldable but requires preheat and controlled interpass temperature to avoid cracking due to higher hardenability and higher carbon equivalent; post-weld heat treat (PWHT) is often recommended for critical parts.

• Forming & cold work: 1018 is superior for stamping, bending and cold forming. 4140 is not ideal for heavy cold forming when the part requires high final strength (unless formed before final heat treatment).

Practical shop rule: If the part must be welded in the final assembly, default to 1018 or specify PWHT for 4140 welds.

Common applications and part examples

• Where 4140 shines: crankshafts, axles, heavy duty shafts, pins subject to shear, load bearing machinery components, forged components and parts that require quench & temper to meet fatigue life.

• Where 1018 shines: shafts for general use, studs, bolts, brackets, cold-formed components, parts that will be welded, and base stock for carburized components (e.g., case-hardened gears).

Example selection guide:

• High shock + wear, small cross-section: 4140 quenched to target HRC.

• Large, welded assembly where cost matters: 1018 and design for thicker sections or surface treatment.

Surface treatments and coatings

• 1018: commonly carburized for wear surfaces (produces hard case with ductile core); also plated or painted for corrosion protection.

• 4140: can be nitrided for a wear/ corrosion resistant surface while maintaining core toughness; also commonly carburized or induction hardened on localized areas; often used with black oxide or plating for corrosion resistance.

Design checklist — questions to answer before specifying

• Will the part be welded after machining? If yes → prefer 1018 (or preplan PWHT for 4140).

• Does the part require through-hardness or only surface hardness? Through-hard → 4140; surface only → 1018 + carburize may suffice.

• Is fatigue life a primary requirement (rotating shafts, impact components)? If yes → 4140 (tempered appropriately).

• What are finish tolerances and surface finish requirements? Tougher alloys may be harder to finish.

• What is your cost target and supply lead time? See procurement section below.

Cost, availability and procurement — including MWAlloys offer

• Market reality: 1018 is almost always less expensive per kg and is widely available from mill stock (sheets, rounds, bars). 4140 is common too, but price is higher due to alloy elements and additional processing (heat treating, forging). Recent commodity swings affect both but 1018 is generally the economical choice for volume.

• Supply chain / Chinese suppliers: China remains a major global producer of steel billets and bar products; many mills produce AISI 4140 and 1018 to international standards. When buying from overseas suppliers, request mill test reports (chemical & mechanical), confirm heat treatment status, and verify packaging & traceability.

• MWAlloys offer (how we help): MWAlloys supplies both AISI 4140 and AISI 1018 in bar, plate and finished/distributed forms. We emphasize: 100% factory direct pricing, fast stock delivery for common sizes, and full documentation (MTC, hardness reports). For custom heat treated lots or forged pieces we provide process control records and can match client QA needs. Contact MWAlloys to receive competitive quotes and available stock lists.

1018 vs 4140 steel comparison tables

Table A — Quick decision matrix

Table B — Typical shop process notes

Frequently Asked Questions

1. Can 1018 be hardened?
   Not significantly by quench & temper because of low carbon. Use carburizing for a hard surface with a ductile core.

2. Is 4140 weldable?
   Yes, but it requires preheat and controlled interpass temperatures; for critical parts, perform PWHT to reduce residual stresses and avoid cracking.

3. Which has better fatigue life — 4140 or 1018?
   After proper heat treatment, 4140 has superior fatigue resistance due to higher strength and alloying elements.

4. Can I use 1018 for shafts?
   Yes for light to moderate loading; for high torque or heavy duty rotating shafts choose 4140 and specify quench & temper.

5. Typical hardness ranges for 4140 after temper?
   4140 can be tempered to a wide range (e.g., 30–55 HRC) depending on temper cycles — specify target HRC on purchase order.

6. What are common surface finishes and coatings?
   Both can be plated or painted; 4140 is commonly black-oxide or plated; 1018 is often painted or galvanized for corrosion protection. Surface hardening (carburize for 1018, nitriding for 4140) is common for wear parts.

7. Do I need mill test certificates (MTC)?
   Always request MTCs for critical parts—these provide chemical composition and mechanical properties traceable to the lot. MWAlloys supplies full documentation.

8. Which is more expensive?
   Per kg, 4140 typically costs more because of alloying additions and processing steps; 1018 is generally the economical option.

9. Can 1018 be used for case hardened gears?
   Yes — 1018 is a standard carburizing base material when a hard case and ductile core are required.

10. How to specify 4140 in a purchase order?
    Include grade (SAE/AISI 4140), form (bar/forged/round), heat treatment status (annealed / QT & tempered to specified HRC), dimensions, tolerances, and required MTCs.

Procurement checklist for buyers

• Specify exact grade and temper (e.g., SAE 4140, quenched & tempered to 36–40 HRC).

• Ask for mill test reports and heat-treatment records.

• For weldments, confirm recommended preheat and PWHT.

• For high-volume orders, request batch traceability and inspection plans.

• If buying from overseas mills, perform incoming inspection and hardness verification.

MWAlloys — manufacturing & supply capabilities

MWAlloys is a factory-direct supplier with stock availability for both AISI 4140 and AISI 1018 in standard bar, round, and plate dimensions. We emphasize:

• 100% factory price — no middleman markup.

• Fast stock delivery for common sizes from Chinese production centers.

• Full technical support — MTC, heat treat records, dimensional inspection reports.

• Custom processing — forging, machining, quench & temper, and case hardening services available on request.

If you supply drawings and required mechanical targets, MWAlloys can quote a turnkey solution including heat treatment and testing.

Practical case study

A mid-sized manufacturer needed a high-life pin for a hydraulic cylinder. Original design used 1018 and pins were failing in high-cycle fatigue. After switching to quenched & tempered 4140 (to ~45 HRC) and applying fine grinding and shot peening, service life increased >3×. The tradeoff was higher material cost but much lower replacement frequency and downtime.

Final recommendation

• For load bearing, fatigue-critical parts → specify 4140 with detailed heat treatment and hardness targets.

• For welded assemblies, cold-formed parts, or cost-sensitive prototypes → specify 1018 and consider case hardening if wear surfaces are needed.

Authoritative references

• AISI 4140 Alloy Steel — AZoM (Chemical composition, mechanical properties, applications)
• AISI 1018 Steel, cold drawn — MatWeb (Material datasheet and mechanical properties)
• Heat treating 4140 alloy steel — ASM International (industry heat-treat guidance)