AISI/SAE 4145 is a chromium-molybdenum low-alloy steel that offers stronger hardenability and higher attainable strength than 4140, making it particularly suited to large-section and downhole oil and gas components where deep hardening and high yield are required; this performance is achieved through slightly higher carbon and controlled Cr and Mo additions, and it is normally supplied quenched and tempered to specific hardness or minimum yield targets.
Quick facts
| Item | Short description |
|---|---|
| Common names | AISI 4145, SAE 4145, 4145 MOD (modified) |
| Type | Cr-Mo low-alloy steel (through-hardening) |
| Typical uses | Drill collars, downhole components, heavy shafts, large diameter bars, quenched and tempered parts in oil and gas, high-strength mechanical parts |
| Typical heat treatment | Quench and temper to specified hardness (commonly 30–36 HRC or higher for certain applications) |
| Key features | Higher hardenability than 4140, higher attainable strength, good dimensional stability after Q&T, limited weldability unless pre/post-heated |
| Common form factors | Round bar, forged blanks, tubes, machined components |
| Typical delivered condition | Normalized, hot-rolled, or quenched and tempered to customer requirement. |
Chemical composition (typical specification ranges)
Below is a practical composition table assembled from industry datasheets and product specifications. Suppliers may publish slight variations, especially for 4145 Modified which may add manganese, chromium, or molybdenum to improve hardenability in large sections.
| Element | Typical range (wt %) | Notes |
|---|---|---|
| Carbon (C) | 0.43 – 0.48 | Higher than 4140; main contributor to increased strength and hardenability. |
| Manganese (Mn) | 0.75 – 1.00 | Deoxidizer and toughness contributor. |
| Phosphorus (P) | ≤ 0.035 | Impurity limit. |
| Sulfur (S) | ≤ 0.040 | Impurity limit. |
| Silicon (Si) | 0.15 – 0.35 | Strengthening and deoxidation. |
| Chromium (Cr) | 0.80 – 1.10 | Improves hardenability and temper resistance. |
| Molybdenum (Mo) | 0.15 – 0.35 | Key for hardenability; 0.20 typical; mod variants up to 0.35. |
| Vanadium, Ni, Cu | trace or controlled | Not standard; some modified grades include small additions. |
Practical note: 4145 MOD variants may intentionally increase Mo and/or Cr to meet specified minimum yield or hardness in larger cross sections; always confirm supplier mill certificates for exact chemistry.
Mechanical properties (typical ranges and design values)
Mechanical properties depend heavily on heat treatment. Below are industry-typical values drawn from supplier data and technical datasheets; use specific heat-treat certificates for final design.
Typical delivered (normalized or annealed) ranges
| Property | Typical range |
|---|---|
| Tensile strength (ultimate) | 650 – 850 MPa (varies with condition) |
| Yield strength (0.2%) | 420 – 600 MPa |
| Elongation (A75 mm or 50 mm) | 12 – 20 % |
| Reduction of area | 40 – 60 % |
| Hardness (HB) | 195 – 235 HB (depending on condition) |
Quenched and tempered target ranges (common practice)
| Tempering target | Typical tensile | Typical yield | Hardness |
|---|---|---|---|
| Medium strength temper | 800 – 1000 MPa | 600 – 800 MPa | 28 – 36 HRC |
| High strength temper | 900 – 1100 MPa | 700 – 900 MPa | 36 – 50 HRC |
| Very high hardening (special) | up to 1200 MPa | up to 1000 MPa | 55 – 62 HRC (special processing) |
Design practice: For safety-critical components, specify both a minimum yield (for example 110 ksi / 758 MPa) and a maximum tempering hardness, and require mill and heat-treatment certification. 4145 is often supplied to minimum yield levels for oilfield tubulars and drill collars.
Heat treatment and hardenability behaviour
Quenching
Typical quench media include oil or polymer quenches for bars and forgings. For large cross sections or very stringent hardenability needs, controlled quenching protocols are required to avoid cracking and to achieve uniform hardness.
Tempering
Tempering temperature selection balances strength and toughness. Lower tempering temps produce higher hardness and strength with reduced impact toughness. Common practice is to specify tempering temperature to achieve the target HRC or minimum yield.
Hardenability
4145, because of its slightly higher carbon and Mo content, exhibits greater hardenability than 4140. This makes it better suited for geometries that require deep hardening. For very large diameters, the 4145 MOD chemistry with higher Mo or Mn is sometimes used to ensure adequate core hardness.
Typical heat-treatment sequence (industrial example)
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Normalize at 850–900 °C, furnace cool to remove rolling stresses (optional).
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Austenitize at 820–870 °C (exact temperature supplier-specific), soak per section size.
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Quench in oil or polymer; for large sections consider controlled cooling to reduce residual stresses.
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Temper at the temperature required to reach specified HRC or mechanicals; tempering cycles are often 1–2 hours per inch of thickness.
Practical caution: 4145 can be susceptible to quench cracking if improperly preheated, quenched too severely, or if hydrogen embrittlement is introduced during processing. Post-weld heat treatment is often required after any welding.
Machinability, welding and forming notes
Machinability
In normalized condition, 4145 machines similarly to other Cr-Mo steels but often slightly less easily than 4140 because of the higher carbon content. In quenched and tempered conditions, carbide precipitation and higher hardness reduce machinability; select cutting speeds and carbide tooling accordingly.
Welding
Weldability is limited relative to plain carbon steels. Preheat, low hydrogen consumables, and controlled interpass temperatures are strongly recommended. When possible, weld in a lower-strength, lower-hardenability condition and apply appropriate post-weld heat treatment (PWHT) to restore toughness and relieve residual stress. 4145 is described as "poor weldability" by some suppliers but weldable with correct procedures.
Forming
Hot forming above the recrystallization temperature is routine. Cold forming should be limited unless material is in a relatively soft annealed condition; cold work increases the risk of cracking in high-carbon variants.
Design guidance and selection criteria
When choosing 4145 over alternatives, weigh these points:
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Choose 4145 when deep hardening and high yield in larger sections are required. The alloy is commonly selected for heavy shafts, mandrels, and downhole oilfield components for this reason.
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If maximum toughness at a given hardness is the priority, 4140 may be preferable because of the slightly lower carbon content. Comparative data often shows 4145 produces slightly better hardness but marginally less impact toughness at equivalent HRC.
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For critical components, specify both the hardness range and minimum impact energy (Charpy V notch) at the application temperature. Require full mill test reports and heat-treatment records.
Failure modes to watch for
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Quench cracking and temper embrittlement if tempering is incorrectly applied.
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Hydrogen-assisted cracking if hydrogen is introduced during pickling, electroplating, or welding.
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Fatigue cracks in cyclic loading if residual tensile stresses or poor surface finish exist.
Corrosion and protective measures
4145 is not corrosion resistant and requires coatings, plating, or corrosion-resistant cladding for aggressive environments. For downhole use, surface treatments and corrosion allowances must be included in design.
Common applications and industry examples
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Oil and gas: drill collars, sub-assemblies, downhole heavy components, large-diameter tubulars (4145 MOD often used).
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Heavy mechanical shafts and spindles where higher hardenability is needed for large cross sections.
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Industrial tools and components that require through-hardening to high strength.
Case snapshot: Many oilfield component suppliers specify 4145 MOD for drill collars because it provides required yield strength and core hardness in substantial diameters; mill certificates are supplied to prove minimum yield and hardness across section size.
International equivalents and standards
The exact equivalents vary by nation and by standardization body. The following table lists commonly used cross-references and comments for procurement.
| Designation / Standard | Equivalent or note |
|---|---|
| SAE/AISI 4145 | Base US designation |
| 4145 MOD | Modified variant with higher Mo or Mn for better hardenability |
| DIN/EN | No single exact EN equivalent; close analogues include 1.7225 family (e.g., 42CrMo4/4140) but confirm chemistry differences. For large-section hardening, check EN grade suitability. |
| JIS | No direct one-to-one JIS; use material property matching or consult conversion tables. |
| UNS G41450 | UNS designation for 4145 |
Procurement tip: Because small differences in carbon and molybdenum materially affect hardenability, do not substitute purely by name; require chemical and mechanical certificate matching if an alternate standard grade is proposed.
Side-by-side comparison: 4145 versus 4140 (practical summary)
| Feature | 4140 | 4145 |
|---|---|---|
| Carbon content | ~0.38 – 0.43 % | ~0.43 – 0.48 % (higher) |
| Hardenability | Good | Better, deeper hardening in larger sections |
| Typical use | Shafts, gears, general quenched and tempered parts | Larger sections, drill collars, heavy-duty components needing higher yield |
| Toughness at equal HRC | Slightly better | Slightly lower (tradeoff for higher hardness) |
| Weldability | Better than 4145 | Lower, requires stricter controls |
| Common choice when | Toughness with good machinability is required | Higher hardenability and strength in larger sections are required. |
Quality control, testing and specification wording
Request from supplier:
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Mill chemical certificate (C of C) with full element breakdown.
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Heat-treatment report with austenitizing temperature, quench medium, tempering temperature and time, and hardness profiles measured at specified points.
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Mechanical test report: tensile, yield, elongation, reduction of area, and Charpy V-notch impact values at specified temperature where applicable.
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Hardness traverse results for large sections to confirm core hardness and surface hardness.
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Non-destructive testing if required (UT for forgings, PMI if alloy verification needed).
Sample specification clause (concise)
"Material shall be SAE/AISI 4145 (or 4145 MOD where specified) with chemical composition and mechanical properties per supplier C of C. Parts shall be heat treated to a minimum yield strength of X MPa and hardness between Y and Z HRC; supplier to provide heat-treatment records and hardness traverse showing compliance."
Tables engineers use
Table A: Typical hardness vs temper temperature (illustrative; confirm with supplier)
| Tempering temperature (°C) | Typical hardness (HRC) after temper |
|---|---|
| 200 | 48 – 55 |
| 300 | 42 – 48 |
| 400 | 36 – 42 |
| 500 | 28 – 36 |
| 600 | 22 – 28 |
Note: these are indicative and vary with austenitizing temperature and exact chemistry; request a tempering chart from the mill for design-critical parts.
Table B: Typical CCT / hardenability observation
| Section size (mm) | Expected hardness penetration for 4145 (quenched) |
|---|---|
| ≤ 25 mm | Full martensitic through section at standard austenitizing and oil quench |
| 25 – 75 mm | High hardness in core possible; check 4145 MOD for very large sections |
| > 75 mm | Hardness gradient increases; specify 4145 MOD or adjust heat treat to achieve target. |
FAQs
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What is the main difference between 4145 and 4140?
4145 has slightly higher carbon and sometimes higher Mo which yields greater hardenability and higher attainable strength, making it preferable for large sections and downhole components; 4140 typically offers marginally better toughness at equal hardness. -
Can 4145 be welded?
Yes, but welding requires preheat, low hydrogen consumables, and often PWHT; avoid welding in high-strength condition without metallurgical control. -
What is 4145 MOD?
A modified chemistry intended to improve hardenability for large-section parts; often specified in oilfield components. -
What heat treatment is typical for 4145?
Austenitize in the 820–870 °C range, quench (oil/polymer), then temper to the selected HRC or strength target. Specific cycles depend on section size and final properties. -
Is 4145 suitable for high-temperature use?
4145 is not a high-temperature creep alloy; it retains strength at moderate elevated temperatures similar to other CrMo steels but is not intended for continuous service at very high temperatures. Evaluate specific alloy families for sustained high-temperature duty. -
What inspection certificates should I require?
Mill chemical certificate, heat-treatment report, mechanical test report (tensile, yield, impact if required), and hardness traverses for large sections. -
How does carbon content affect performance?
The slightly higher carbon in 4145 raises achievable hardness and strength after quench and temper but can reduce weldability and may slightly reduce impact toughness compared to lower-carbon relatives. -
Is 4145 used for bearings or gears?
Not typically for rolling-element bearings. It can be used for heavy-duty gear and shaft applications where through-hardening and high core strength are prioritized. -
What are common failure modes for 4145 parts?
Stress-corrosion or corrosion fatigue in aggressive environments if not protected, quench cracking from improper heat treatment, and hydrogen-related cracking if surface processes introduce hydrogen. -
Where can I find standard specification language?
Supplier datasheets, SAE/AISI references, and oilfield material specifications often provide the required wording; ask suppliers for specific AISI/SAE or API-based material sheets for inclusion in purchase orders.
Final selection checklist for engineers and buyers
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Confirm exact chemistry via mill certificate.
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Define required minimum yield and maximum hardness, plus required impact energy.
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Specify heat-treatment record and hardness traverse for large sections.
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For welded assemblies require preheat and PWHT procedures and welder qualifications.
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For corrosive service define coatings or cladding and verify compatibility.
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