For many general-purpose engineering parts that require a balanced combination of toughness, machinability, and fatigue resistance, AISI 4140 is the more versatile and widely used choice. When higher strength, greater hardenability, and slightly higher as-quenched or tempered hardness are priorities. for example in large-diameter shafts, downhole oilfield components, and heavy-duty forged parts. AISI 4145 (and its modified/higher-carbon variants) frequently becomes the preferred option. The two alloys are closely related chromium-molybdenum steels, but small differences in carbon and processing lead to distinct performance tradeoffs that should guide material selection.
Quick technical snapshot and executive comparison
Both AISI 4140 and AISI 4145 belong to the AISI/SAE 4000 series of chromium-molybdenum low alloy steels. They share the same principal alloying elements (Cr, Mo, Mn, Si) so the baseline metallurgy is similar, but 4145 typically carries a higher nominal carbon content than common 4140 compositions. That higher carbon increases strength and hardenability at the expense of a small reduction in toughness and sometimes machinability. In practice, this means 4145 can reach higher through-hardening levels for a given quench and temper schedule, which is valuable for large cross sections and high-stress parts.
Chemical composition (typical ranges and what they mean)
Below are commonly reported nominal compositions for the two grades. Note that producers can supply slightly different analyses, and standards or “modified” variants may exist (for example 4145H, 4145M). Consult mill certificates for exact chemistry for any purchase lot.
Typical chemical composition (nominal ranges, wt%)
| Element | AISI 4140 (typical) | AISI 4145 (typical) |
|---|---|---|
| Carbon (C) | 0.38 – 0.43 | 0.43 – 0.48 |
| Chromium (Cr) | 0.80 – 1.10 | 0.80 – 1.10 |
| Molybdenum (Mo) | 0.15 – 0.25 | 0.15 – 0.25 |
| Manganese (Mn) | 0.60 – 1.00 | 0.75 – 1.00 |
| Silicon (Si) | 0.15 – 0.35 | 0.15 – 0.30 |
| Sulfur (S) | ≤ 0.04 | ≤ 0.04 |
| Phosphorus (P) | ≤ 0.035 | ≤ 0.035 |
| Iron (Fe) | balance | balance |
The most meaningful difference is the carbon window. Higher carbon increases tensile strength and potential hardness after quench and temper; it also increases hardenability slightly and reduces the section size needed to achieve a target hardness. This characteristic is why 4145 variants are often chosen for larger diameter parts and heavy forged components.
Mechanical properties and microstructure after heat treatment
Mechanical values vary with heat treatment, section size, and supplier processing. Typical ranges below should be used only as guidance; always require mill test reports and, for critical parts, perform acceptance testing.
Representative mechanical properties (quenched and tempered condition, indicative ranges)
| Property | AISI 4140 (QT typical) | AISI 4145 (QT typical) |
|---|---|---|
| Tensile strength (MPa) | 750 – 1,350 (depends on temper) | 800 – 1,500 |
| Yield strength (0.2% offset, MPa) | ~500 – 1,200 | ~550 – 1,250 |
| Elongation (%) | 10 – 20 | 8 – 18 |
| Reduction of area (%) | 30 – 60 | 25 – 55 |
| Hardness (HRC after hardening/tempering) | 20 – 60 (wide range) | 25 – 62 (can achieve higher HRC in large sections) |
Microstructure: after austenitizing and rapid quench, both steels form martensite with retained carbides depending on cooling rate. Tempering reduces hardness and refines toughness through carbide precipitation and tempering transformations. Because 4145 tends to have higher carbon, martensitic hardness for a given tempering temperature will be higher than 4140.
Hardenability, heat treatment practice, and processing notes
Hardenability
Hardenability describes the steel’s capacity to form martensite through a section during quenching. 4145’s slightly greater carbon content increases hardenability and the achievable hardness at depth. For large sections where a deep hard case is required (shafts, downhole tool components), that property is often decisive.
Typical heat treatment windows
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Austenitize (common): 800 – 860°C (1475 – 1580°F) depending on section size and supplier recommendation.
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Quench medium: oil quench is common for both alloys; for thin sections or where distortion must be minimized, polymer quenchant or controlled gas cooling is sometimes used.
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Temper: tempering temperature selected to reach the target hardness/toughness tradeoff (e.g., 200–650°C). Higher tempering reduces strength but increases toughness. 4145 often requires careful tempering to avoid temper embrittlement in certain temperature ranges for critical service.
Practical note: Because 4145 is used heavily in oil & gas and heavy equipment, suppliers often deliver it in quenched & tempered condition to the hardness specified. Typical supplied hardness ranges for 4145 may be 30–36 HRC for some downhole steels, though higher hardness levels can be processed where needed.
Product forms, supply conditions, and standards
Common supply forms:
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Round bars (ground, turned, or forged)
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Forgings and billets for shafts and couplings
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Tubing and casing for certain specialized uses
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Flat plate and bar stock in some mills
Common standards and specifications that reference these grades include SAE/AISI listings, various ASTM references for tubing and casing, and oilfield material specs for downhole components. Some fields also reference modified forms (4145H, 4145M) that adjust chemistry or processing for enhanced performance. Suppliers will often quote UNS numbers: 4140 → UNS G41400, 4145 → UNS G41450.
Welding, machining, and surface treatment considerations
Welding
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Preheat is recommended before welding to reduce thermal gradients and prevent cracking, especially in 4145 due to higher carbon and hardenability. Typical preheat: 150–300°C depending on thickness and joint design. Post-weld heat treatment (PWHT) may be mandated for high-strength or critical components.
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Use low-hydrogen consumables and follow approved welding procedures. When welding in the as-quenched and tempered condition, a PWHT may restore toughness.
Machining
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Both steels machine reasonably well in the annealed condition; machinability decreases with higher hardness. 4145’s slightly higher carbon can make high-hardness stock more demanding to machine. Proper tooling, coolant, and speeds are essential.
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When tight dimensional control is required after quench and temper, rough machine to allowance, heat treat, then perform final finish machining.
Surface treatments
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Case carburizing is generally not applied to these chromium-molybdenum bulk alloys; the alloys are usually hardened via through-hardening treatments. Typical surface treatments include nitriding (for some service conditions), shot peening, induction hardening for local wear surfaces, and plating/coatings for corrosion protection when needed.
Application-driven selection — when to choose 4140 or 4145
Choose AISI 4140 when:
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The part requires a balanced combination of toughness and ductility with moderate-to-high strength.
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Machinability and finishing cost-efficiency are priorities.
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Components are subject to dynamic loading, fatigue, or torsional stresses (shafts, gears, spindles, fasteners in many industries).
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You prefer a broadly standardized, widely available grade with many mill supply forms and established machining/heat treatment practices.
Choose AISI 4145 when:
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Higher hardenability and slightly higher achievable hardness are required, particularly in large cross-sections.
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Service involves sustained high static loads or wear where a tougher, higher-strength microstructure after quench/temper is useful (heavy-duty shafts, oilfield drilling components, downhole tools).
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A supplier offers 4145 in matched heat-treatment and certification for an oil and gas specification or forging application.
Quality control, testing, and inspection
For critical components, require and review:
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Mill certificates with actual chemical analysis.
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Hardness maps across sections after heat treatment (verify target HRC or HB).
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Tensile and impact tests (Charpy V-notch) at relevant temperatures for dynamic service.
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Non-destructive testing (UT, MPI) for forgings and welded assemblies.
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Metallographic checks to confirm grain size, tempered martensite distribution, and the absence of critical defects such as excessive inclusions or segregation.
For oil and gas downhole use, additional fatigue or fracture mechanics testing may be required by the project specification.
Side-by-side comparison tables
Summary comparison (decision matrix)
| Criterion | AISI 4140 | AISI 4145 |
|---|---|---|
| Typical carbon | Moderate (0.38–0.43) | Slightly higher (0.43–0.48) |
| Hardenability | Good | Better (useful for large sections) |
| Toughness | Slightly higher | Slightly lower at comparable hardness |
| Machinability (annealed) | Good | Good (but harder grades harder to machine) |
| Typical uses | Shafts, gears, pins, general engineering | Heavy shafts, drilling tools, large forgings |
| Heat treatment control | Well established | Requires careful tempering for some uses |
| Availability | Very widely available | Widely available; many mills offer variants |
Typical heat treatment processing notes (quick reference)
| Process step | 4140 typical | 4145 typical |
|---|---|---|
| Austenitize | 800–860°C | 800–860°C |
| Quench | Oil (commonly) | Oil (commonly); larger sections require careful quench) |
| Temper | 200–650°C depending on target hardness | 200–650°C; choose higher temper to recover toughness if needed |
Practical selection checklist
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Confirm required hardness and whether that hardness must be achieved through thickness.
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Review design cross-section: large sections favor higher hardenability.
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Define service load type: dynamic fatigue and impact favor 4140’s toughness; static or wear-heavy loads may favor 4145’s strength.
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Ask supplier for mill cert with chemistry and heat treatment records.
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If welding is involved, include preheat/PWHT requirements in drawings.
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For critical parts, require Charpy impact tests at expected service temperature.
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For oil & gas, confirm compliance to relevant industry specifications and traceability.
FAQs
Q1: Are 4140 and 4145 interchangeable?
Short answer: they are similar and sometimes interchangeable for non-critical parts, but they are not identical. For critical components, check chemistry, mechanical properties, and heat-treatment certifications before substituting.
Q2: Which grade is stronger in tensile tests?
At comparable tempering conditions, 4145 tends to reach slightly higher tensile strength due to higher carbon, though the actual strength depends greatly on tempering, quench, and section size.
Q3: Which grade is better for large-diameter shafts?
4145 is frequently preferred when high hardness and strength are required through large sections because it offers improved hardenability.
Q4: Is one steel easier to weld?
4140 is generally considered slightly easier to weld because of its marginally lower carbon. With preheat and proper consumables, both can be welded, but PWHT might be necessary in service-critical cases.
Q5: Can both grades be heat treated in the same furnace cycle?
Yes, they have similar austenitizing temperatures, but tempering and quench control should be adjusted for the specific grade and section size to reach target properties.
Q6: Which grade is more common in automotive parts?
4140 is widely used in automotive components like gears, shafts, and fasteners due to its good combination of properties and cost-effectiveness.
Q7: Is 4145 used in oil and gas applications?
Yes, 4145 and its modified variants are commonly used for downhole and drilling tools because of their higher strength and hardenability; many suppliers produce oilfield-certified 4145 variants.
Q8: What are common failure modes to watch for?
For high-strength, high-hardness parts: brittle fracture, temper embrittlement, hydrogen-assisted cracking in welded areas, and surface or subsurface fatigue are the main concerns. Appropriate testing and design margins are essential.
Q9: Are there standard equivalents in EN or other systems?
Yes, 4140 maps closely to EN 42CrMo4/1.7225. Exact equivalents require cross-referencing based on composition and required mechanical properties.
Q10: How should I specify the material on a drawing?
Specify the exact grade (AISI 4140 or AISI 4145), required heat treatment condition (e.g., QT to 40–45 HRC), required tests (UT, hardness mapping, Charpy, tensile), and traceability/ mill-certificate requirements. Include welding instructions if applicable.
Testing and inspection checklist for purchase orders
When writing the P.O. or technical specification, require:
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Mill certificate with full chemical analysis and heat treatment record.
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Hardness verification across section (at least three points for shafts).
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Specified mechanical tests (tensile at ambient, Charpy V-notch if dynamic service).
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NDT for large forgings and critical parts.
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Traceability back to heat and batch.
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Any industry-specific certificates for oil & gas (if applicable).
Practical case examples
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Heavy-duty transmission shaft (industrial crane): choose 4145 when the design requires deeper through-hardening to resist wear at bearings and high contact stresses. Balance final temper to retain sufficient toughness.
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High-speed spindle (machine tool): choose 4140 when high toughness, dimensional stability after heat treatment, and dynamic fatigue resistance are essential.
Closing recommendations
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Treat the selection as a system decision: material chemistry, section size, heat treatment capability, and planned QC form a single decision chain.
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Ask the mill for representative mechanical property data for your specified heat treatment and section. Don’t rely only on nominal published values.
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For mission-critical components, require full NDT and mechanical testing and consider finite element analysis of residual stresses and expected fatigue life.
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If heat treatment capability is limited locally, procure material already delivered in the specified quenched and tempered condition with verified certificates.
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