01Grade Overview
AISI 4150 is a medium-high carbon chromium-molybdenum (Cr-Mo) low-alloy engineering steel produced to AISI/SAE standards, with the Unified Numbering System designation UNS G41500. It is the highest-carbon member of the widely used 41XX Cr-Mo family -- sitting above 4140 (0.38-0.43% C) and 4145 (0.43-0.48% C) with a carbon range of 0.48-0.53%.
That elevated carbon content, combined with the same 0.80-1.10% chromium and 0.15-0.25% molybdenum used in the entire 41XX family, produces a steel with exceptional tensile strength (1,000-1,200 MPa in Q+T condition), deep hardenability in large cross-sections, and outstanding fatigue resistance. These properties make it the go-to specification for the most demanding forged components in oil and gas, mining, and power generation.
This guide covers everything you need to specify and heat-treat AISI 4150 -- from ASTM A322 composition limits to international equivalents and real-world applications. Engineers ready to source can go directly to our custom AISI 4150 forgings product page.
All mechanical property ranges refer to wrought forgings in the quench-and-temper (Q+T) condition unless otherwise stated. Cast properties differ significantly and are not covered here. For a full comparison with lower-carbon grades, see our article on AISI 4145 vs 4140 steel.
02Chemical Composition
The table below shows the AISI/SAE standard composition limits for AISI 4150 per ASTM A322 and SAE J404. These apply to standard bar stock and forgings; 4150H (hardenability-controlled) has slightly different limits.
| Element | Symbol | AISI 4150 Range (%) | Function in Steel |
|---|---|---|---|
| Carbon | C | 0.48 - 0.53 | Primary strength & hardness driver |
| Chromium | Cr | 0.80 - 1.10 | Hardenability, mild corrosion resistance |
| Molybdenum | Mo | 0.15 - 0.25 | High-temp strength, temper embrittlement resistance |
| Manganese | Mn | 0.75 - 1.00 | Hardenability, deoxidation |
| Silicon | Si | 0.15 - 0.35 | Deoxidation, strength |
| Phosphorus (max) | P | 0.035 | Controlled impurity -- reduces toughness |
| Sulfur (max) | S | 0.040 | Controlled impurity -- machinability vs. toughness trade-off |
| Iron | Fe | Balance | Base metal |
Why every element matters
Carbon (0.48-0.53%): The defining characteristic of 4150 over its lower-carbon siblings. Carbon raises the maximum achievable hardness after quenching -- for 4150, surface hardness can reach 52-55 HRC as-quenched, versus approximately 49-52 HRC for 4145 and 46-49 HRC for 4140. In the tempered condition, higher carbon translates directly to higher tensile and yield strength.
Chromium (0.80-1.10%): Chromium expands the hardenability band, allowing full martensite transformation deeper into the cross-section during quenching. It also provides mild resistance to surface oxidation and atmospheric corrosion, though 4150 should not be confused with stainless steels.
Molybdenum (0.15-0.25%): Molybdenum plays two critical roles -- it strengthens the steel at elevated temperatures (important for downhole and power generation applications) and suppresses temper embrittlement, making the tempering process more forgiving across a wider temperature range.
With a carbon range of 0.48-0.53%, any two heats of 4150 can differ by up to 0.05% in carbon -- the same difference that separates 4140 from 4145. Always review the actual material certificate (heat cert) and verify that mechanical properties are tested from a coupon representing the actual heat before accepting critical forgings.
03Mechanical Properties
The table below shows typical mechanical properties for AISI 4150 in the quenched-and-tempered (Q+T) condition. Values are representative of forgings with cross-section up to 150 mm. Properties decrease with increasing section size for both grades.
| Property | Condition | Typical Value | Note |
|---|---|---|---|
| Tensile Strength | Q+T | 1,000 - 1,200 MPa | Tunable via tempering temp. |
| Yield Strength (0.2%) | Q+T | 850 - 1,050 MPa | |
| Hardness (surface) | Q+T | 28 - 36 HRC | Up to 52 HRC as-quenched |
| Elongation | Q+T | ≥ 12% | Lower than 4140 due to higher C |
| Reduction of Area | Q+T | ≥ 50% | |
| Charpy Impact (RT) | Q+T | ≥ 40 J | Adequate for most applications |
| Fatigue Limit | Q+T | ~490 - 530 MPa | Rotating bending |
| Hardness (as-quenched) | Quenched only | 52 - 55 HRC | Before tempering |
| Density | -- | 7.85 g/cm³ | |
| Elastic Modulus (E) | -- | ~205 GPa |
How tempering temperature controls the property balance
One of the key design advantages of AISI 4150 is that engineers can adjust the tempering temperature to target a specific hardness/toughness balance for their application:
- Low temper (200-350°C): Maximum hardness (44-52 HRC), maximum wear resistance, tensile strength above 1,400 MPa -- used for wear plates, tooling, and surface-critical components.
- Mid temper (400-500°C): Balanced hardness (~35-44 HRC), good toughness -- used for gears, shafts, and moderate-impact components.
- High temper (500-650°C): Optimal toughness, tensile strength 1,000-1,200 MPa, hardness 28-36 HRC -- the standard Q+T condition for structural forgings, drill collars, and pressure vessel components.
Do not temper AISI 4150 between 260°C and 370°C. This range promotes tempered martensite embrittlement (TME), causing a sharp, often undetectable drop in impact toughness. The effect is more pronounced in 4150 than in 4140 due to the higher carbon martensite. Components tempered in this zone can fail catastrophically under shock loading with no visible warning.
Visualised properties -- relative comparison
Property bars normalised to 100 scale for visual comparison. Larger bar = higher performance in that category.
04AISI 4150 in the Cr-Mo Carbon Family
The 41XX family spans a wide carbon range. Understanding where 4150 sits -- and why -- helps engineers make the right grade selection for each application.
Carbon content across the family
The bars below show the mid-point carbon % for each major 41XX and adjacent grade. 4150 carries the highest carbon of the standard Cr-Mo group.
How 4150 compares to 4140 and 4145
| Property | AISI 4140 | AISI 4145 | AISI 4150 |
|---|---|---|---|
| Carbon (%) | 0.38-0.43 | 0.43-0.48 | 0.48-0.53 |
| Tensile Str. (Q+T) | 900-1,100 MPa | 950-1,200 MPa | 1,000-1,200 MPa |
| Hardness (Q+T) | 26-32 HRC | 28-36 HRC | 28-36 HRC |
| Hardenability | Good | Better | Best |
| Charpy (RT) | ≥ 47 J | ≥ 40 J | ≥ 40 J |
| Machinability | Good | Fair-Good | Fair |
| Best for sections | <= 100 mm | <= 200 mm | > 150 mm |
| EU equivalent | 42CrMo4 (1.7225) | 50CrMo4 (1.7228) | 50CrMo4 (1.7228) |
For cross-sections up to 150 mm, 4145 and 4150 deliver nearly identical through-hardened properties in practice. The real advantage of 4150 emerges above 200 mm diameter, where the additional carbon gives measurably better core hardness and more consistent martensitic microstructure. For detailed grade selection guidance, see our in-depth comparison: AISI 4145 vs 4140: Which Forging Grade Should You Choose?
05Hardenability
Hardenability -- the ability to achieve martensitic hardening throughout a cross-section -- is where AISI 4150 genuinely outperforms its lower-carbon siblings. For large forgings, this is the primary reason to specify 4150 over 4145 or 4140.
In a Jominy end-quench test (ASTM A255), 4150 consistently maintains higher hardness from J1 (1/16" from the quench surface) all the way to J16 and beyond, compared to 4145 and 4140. The higher carbon raises the martensite start temperature slightly, but more importantly it increases the carbon content of the martensite -- producing a harder, stronger microstructure throughout.
Through-hardening in large cross-sections
For a forged bar at 200 mm diameter, oil-quenched and tempered:
- AISI 4140: Core hardness drops to 18-24 HRC -- soft core with potential fatigue initiation sites.
- AISI 4145: Core hardness holds at 24-30 HRC -- significantly better, but still some gradient.
- AISI 4150: Core hardness reaches 28-34 HRC -- near-uniform through-hardened profile, most suitable for heavy-load rotating components.
This difference becomes the decisive factor for drill collars above 250 mm OD, large mill shafts, and power generation main shafts -- applications where the core must carry the same structural load as the surface.
"AISI 4150 is specified for our large-section drill collar forgings precisely because we consistently achieve a minimum core hardness that protects against downhole fatigue failures -- something the lighter grades cannot reliably deliver at those diameters."
Jiangsu Liangyi Co., Limited -- Technical Engineering TeamThe 4150H hardenability-controlled variant
For applications requiring the tightest consistency across production heats -- particularly for large-batch procurement -- AISI 4150H is available. Defined by ASTM A304, 4150H specifies minimum and maximum hardness values at each Jominy test distance, guaranteeing that every heat of 4150H will achieve the same through-hardening performance. This is valuable for automated production environments where heat-to-heat property variation cannot be accommodated by adjusting machining or heat treatment parameters.
06Heat Treatment
AISI 4150 responds excellently to heat treatment, but its higher carbon content demands more careful process control than 4140 -- particularly in quench selection and the avoidance of the embrittlement zone during tempering.
Refines the as-forged grain structure and homogenises the microstructure before hardening. Essential for large forgings where thermal gradients during the forging process can create mixed-grain microstructures. Hold for approximately 1 hour per 25 mm of maximum cross-section, then air-cool uniformly.
Lower austenitizing temperature than 4140 (840-870°C) to avoid carbide dissolution and grain growth associated with higher carbon steels at elevated temperatures. Uniform temperature throughout the section is critical -- a temperature gradient of more than 15°C across the cross-section will produce non-uniform hardening response.
Oil quench is strongly preferred. Water quenching above 50 mm section risks quench cracking due to 4150's higher carbon content creating more brittle as-quenched martensite. Polymer quench concentration must be precisely controlled. Agitate the quench medium uniformly and avoid partial immersion. Transfer from furnace to quench tank within 20 seconds to prevent surface transformation before quench.
Temper immediately after quench -- delayed tempering allows hydrogen embrittlement and internal stress cracking in high-carbon martensite. Target temperature depends on required hardness/toughness balance (see §3). Critical: never temper between 260°C and 370°C -- this is the tempered martensite embrittlement (TME) zone and produces a sharp, irreversible toughness loss that cannot be distinguished on hardness testing alone.
Recommended after rough machining, weld repair, or straightening operations. Always hold below the final tempering temperature to preserve the Q+T mechanical properties. A full stress relief at 600°C for large sections can reduce residual stress by 80-90% with negligible property change.
AISI 4150 can be welded but requires preheat (typically 250-350°C for sections above 25 mm), controlled interpass temperature, and post-weld heat treatment (PWHT) at 600-650°C. Given the high carbon content, welding of finished forgings is generally not recommended -- design to avoid weld joints in 4150 components where possible.
07Industry Applications
AISI 4150's combination of maximum tensile strength in the 41XX family, superior hardenability for large cross-sections, and proven fatigue performance makes it the specification choice for the most demanding forged components across several industries.
Large-section bodies requiring through-hardened microstructure and high-pressure integrity. These safety-critical components demand consistent mechanical properties throughout the full cross-section.
Large-diameter drill collars where 4145H may not consistently achieve the minimum core hardness required by the downhole fatigue life specification.
High-cycle fatigue life demands, large diameters (400-900 mm), strict ultrasonic testing requirements. 4150 delivers through-hardened microstructure at these dimensions.
Continuous cyclic loading at large cross-sections in abrasive, wet environments. Surface hardness and core strength both critical.
Gear blanks above 400 mm diameter where through-hardened core is required to prevent subsurface fatigue failure under heavy cyclic contact loading.
Components requiring verified tensile strength above 1,000 MPa with through-hardened cross-sections and full material traceability to ASTM/SAE standards.
Massive, slow-speed shafts that must maintain fatigue life over decades. 4150 Q+T provides the combination of strength and toughness at large diameters.
Large-diameter, continuously loaded rotating shafts subject to seawater splash -- 4150 provides through-hardening and fatigue resistance at large sections.
Jiangsu Liangyi Co., Limited manufactures all the component types listed above. For available sizes, material grades, and lead times, see our AISI 4150 forged parts product page.
Use 4150 when: (1) cross-section exceeds 200 mm and core hardness uniformity is critical, (2) tensile strength must reliably exceed 1,000 MPa across the entire section, or (3) fatigue life at the core is a design constraint. For smaller sections and general engineering duty, AISI 4145 or 4140 typically delivers equivalent performance at lower cost and with better machinability.
08International Equivalent Grades
AISI 4150 is a globally recognised grade with direct equivalents in all major international standards. These equivalencies are compositional -- always verify against actual material certificates before substitution in safety-critical components.
| Standard / Country | Designation | Material Number | Notes |
|---|---|---|---|
| AISI / SAE (USA) | AISI 4150 / SAE 4150 | -- | Origin standard |
| UNS (USA) | G41500 | -- | Unified Numbering System |
| DIN / EN (Europe) | 50CrMo4 | 1.7228 | Most common EU equivalent; also equals AISI 4145 |
| BS (United Kingdom) | EN 19C / 708A50 | -- | Legacy British Standard designation |
| JIS (Japan) | SCM451 | -- | Near-equivalent; slightly different Mn range |
| GB (China) | 50CrMo | -- | Chinese national standard |
| GOST (Russia) | 50ХМ | -- | Near-equivalent |
| IS (India) | 50Cr4Mo3 | -- | Approximate equivalent |
The EN material number 1.7228 (50CrMo4, equivalent to AISI 4145/4150) is not the same as 1.7225 (42CrMo4, equivalent to AISI 4140). These are distinct grades with different carbon content and hardenability -- a common source of procurement errors when reviewing European mill certificates. Always read the full 5-digit material number, not just the trade name.
09Sourcing AISI 4150 Forgings
Standard vs. hardenability-controlled (H) grade
Standard AISI 4150 specifies composition limits only (ASTM A322). For production environments or large cross-sections where consistent heat-to-heat hardenability is critical, specify AISI 4150H (ASTM A304) -- the H suffix adds a guaranteed Jominy hardenability band at each test distance, ensuring the same through-hardening performance across every heat.
What to include in your purchase specification
- Grade designation: AISI 4150 (or 4150H if hardenability control is required), with applicable standard (ASTM A322 or A304)
- Raw material chemical composition certificate (heat cert) -- verify carbon is mid-range or higher for the best hardenability
- Heat treatment condition and record: austenitizing temperature, soak time, quench medium, tempering temperature and time
- Mechanical test report: tensile, yield, elongation, reduction of area, Charpy impact, and hardness -- from a test coupon representing the heat
- Ultrasonic testing (UT) report per applicable acceptance criteria (e.g. ASTM A388, customer specification)
- Dimensional inspection report to drawing
- Certificate of conformance to applicable specification
Typical lead times
Standard AISI 4150 forgings in common section sizes are generally available with 6-10 weeks lead time from a qualified forge shop. Large custom forgings (above 500 mm diameter or 5,000 kg) in 4150 may require 12-18 weeks depending on current mill schedules and heat treatment capacity. 4150H and any components manufactured to API specification requirements involve additional testing and documentation time.
Jiangsu Liangyi Co., Limited produces custom forgings in AISI 4150 for global industrial clients. For product specifications, dimensions, and delivery lead times, browse our AISI 4150 forging range or contact sales@jnmtforgedparts.com.
10Frequently Asked Questions
AISI 4150 is a medium-high carbon chromium-molybdenum (Cr-Mo) low-alloy engineering steel with UNS designation G41500. It contains 0.48-0.53% carbon, 0.80-1.10% chromium, and 0.15-0.25% molybdenum, delivering tensile strength of 1,000-1,200 MPa and hardness of 28-36 HRC in the quench-and-temper condition. It is the highest-carbon member of the standard 41XX Cr-Mo family and is widely used for large-section forgings in oil and gas, mining, and power generation.
The European (DIN/EN) equivalent of AISI 4150 is 50CrMo4, material number 1.7228. Note that this same designation also covers AISI 4145 -- EN standards make a less fine carbon distinction than AISI. Other international equivalents: JIS SCM451 (Japan), GB 50CrMo (China), EN 19C / 708A50 (UK), 50ХМ (Russia GOST). Always verify actual composition ranges in the specific standard and material certificate before substitution.
AISI 4150 can achieve 52-55 HRC as-quenched (before tempering). In the standard quench-and-temper condition for structural forgings (tempered at 500-650°C), typical surface hardness is 28-36 HRC with tensile strength 1,000-1,200 MPa. Lower tempering temperatures (200-350°C) give 44-52 HRC with higher wear resistance but reduced toughness. Never temper between 260-370°C due to temper embrittlement risk.
AISI 4150 is used for demanding large-section forged components: wellhead bodies and BOP components (oil & gas), large-diameter drill collars above 250 mm OD, wind turbine main shafts, ball mill shafts, large gear blanks above 400 mm diameter, generator rotor shafts, ship propeller shafts, and defense structural forgings requiring verified tensile strength above 1,000 MPa. Its primary advantage over 4140 and 4145 is deeper through-hardening at large cross-sections.
All three are Cr-Mo steels with identical chromium (0.80-1.10%) and molybdenum (0.15-0.25%) content. The only difference is carbon: 4140 has 0.38-0.43%, 4145 has 0.43-0.48%, and 4150 has 0.48-0.53%. Higher carbon in 4150 gives better tensile strength, deeper hardenability, and superior through-hardening at large section sizes -- but at the cost of slightly reduced toughness, more difficult machinability, and greater sensitivity to quench cracking and temper embrittlement.
Standard Q+T sequence: (1) Normalize at 870-900°C, air cool; (2) Austenitize at 830-860°C for ~1 hr/25mm section thickness; (3) Oil quench -- water quench risks cracking above 50mm due to higher carbon; (4) Temper within 2 hours at 200-650°C based on required properties. Critical: never temper between 260-370°C (temper embrittlement zone). Optional: stress relieve at 550-600°C after heavy machining.
All technical data verified against ASTM A322, ASTM A304, and SAE J404. Mechanical property ranges reflect industry-standard quench-and-temper conditions for wrought forgings in cross-sections up to 150 mm. Last reviewed: by the Jiangsu Liangyi Co., Limited engineering team -- ISO 9001:2015 quality certified (subject to current certificate validity), over 20 years forging experience.
The Bottom Line on AISI 4150
AISI 4150 occupies a clear niche: it is the specification choice when tensile strength above 1,000 MPa and deep through-hardening in sections above 150-200 mm are both required. Its elevated carbon content (0.48-0.53%) gives it the deepest hardenability in the standard Cr-Mo family -- making it indispensable for large oil and gas components, power generation shafts, and heavy mining equipment. It demands more careful heat treatment than 4140 or 4145 -- oil quench, strict avoidance of the embrittlement zone, and prompt tempering after quench -- but managed correctly, it reliably delivers properties that no lower-carbon Cr-Mo grade can match at large cross-sections.