When an engineering procurement team arrives at the material specification stage for a forged shaft — whether a gearbox output shaft in a wind turbine nacelle, a drive shaft in mining mill equipment, or a rotor shaft in an industrial compressor — two grades appear on almost every shortlist: AISI 4137 custom forgings and AISI 4140.
Both are chromium-molybdenum (CrMo) low-alloy steels. Both are open-die forged in cross-sections from a few kilograms to 30-tonne main shafts. Both respond well to quench-and-temper heat treatment and achieve high strength-to-toughness ratios no equivalent carbon steel can match. The datasheets look nearly identical — yet specifying the wrong grade can lead to inadequate depth of hardness in large-section forgings, avoidable cracking during welded repair, or unnecessary machining cost.
This guide is written from the perspective of Jiangsu Liangyi Co., Limited, a manufacturer that has produced open-die forgings in both grades across a weight range of 30 kg to 30,000 kg for more than 26 years. All data comes from production records and international standards including ASTM A29/A29M, EN 10083-3, DIN 17200, and JIS G4053.
Chemical Composition Side by Side
The two grades share identical chromium and molybdenum additions. The sole compositional difference is carbon content — and that single variable produces cascading effects on hardenability, weldability, machinability, and fatigue behavior.
| Element | AISI 4137 | AISI 4140 | Engineering Significance |
|---|---|---|---|
| Carbon (C) | 0.35–0.40% | 0.38–0.43% | Primary difference — controls max hardness and hardenability depth |
| Manganese (Mn) | 0.70–0.90% | 0.75–1.00% | Secondary hardenability supplement; 4140 slightly wider range |
| Chromium (Cr) | 0.80–1.10% | 0.80–1.10% | Identical — improves hardenability and mild corrosion resistance |
| Molybdenum (Mo) | 0.15–0.25% | 0.15–0.25% | Identical — temper embrittlement resistance and creep strength |
| Silicon (Si) | 0.15–0.35% | 0.15–0.35% | Identical — deoxidation |
| Phosphorus (P) max | 0.035% | 0.035% | Identical ASTM max — our production targets ≤ 0.020% |
| Sulfur (S) max | 0.040% | 0.040% | Identical ASTM max — our production targets ≤ 0.015% |
| Carbon Equivalent (CE) | ~0.68–0.75 | ~0.72–0.80 | 4137 more weldable; 4140 requires higher preheat temperatures |
Production note
At Jiangsu Liangyi Co., Limited, all billet is sourced via EAF + LF + VD production routes, achieving sulphur ≤ 0.015% and phosphorus ≤ 0.020% — tighter than ASTM A29 minimums. We perform incoming OES verification for every heat before the first press stroke.
Mechanical Properties After Quench & Temper
Values below assume quenched-and-tempered (Q&T) condition — the standard delivery condition for critical forged shafts. Data is based on ASTM A29/A29M, EN 10083-3, and production test certificate records from Jiangsu Liangyi Co., Limited for forgings in the 100–300 mm diameter range.
| Property | AISI 4137 (Q&T) | AISI 4140 (Q&T) | Practical Impact |
|---|---|---|---|
| Tensile Strength (Rm) | 980–1,180 MPa | 1,000–1,250 MPaHigher | 4140 achieves slightly higher peak strength at equivalent temper temperature |
| Yield Strength (Rp0.2) | 835–1,050 MPa | 860–1,100 MPaHigher | Useful where stress-proof loading at yield is design-limiting |
| Elongation (A5) | ≥ 12% | ≥ 11% | 4137 marginally more ductile — relevant for variable-amplitude loading |
| Reduction of Area (Z) | ≥ 50% | ≥ 45% | 4137 slightly better toughness reserve in the transverse direction |
| Charpy Impact (KV at 20°C, ≤100mm) | ≥ 55 JBetter | ≥ 40 J | Meaningful for rail, crane, offshore applications requiring impact certification |
| Hardness (HB, Q&T) | 280–320 HB | 295–341 HBHigher | 4140's harder floor can slow turning and milling cycles on large machined shafts |
| Fatigue Limit (σ-1, rotating bending) | ~490–530 MPa | ~510–560 MPaHigher | 4140 marginally better under constant-amplitude rotating bending fatigue |
Important limitation
These are coupon values — not through-thickness values in a large-diameter forging. The gap between coupon properties and center-section properties widens dramatically with section size. This is where hardenability, not the tabulated strength values, becomes the dominant variable.
Hardenability: The Critical Difference in Heavy Forgings
Hardenability — the ability of a steel to harden to depth when quenched — is the most important performance discriminator between these two grades in large forging applications, and it is systematically underweighted by teams that rely on datasheets rather than Jominy end-quench data.
Because AISI 4140 has 0.03–0.05% more carbon on average, its as-quenched hardness at the quenched end (J1) is approximately HRC 60–62 vs HRC 57–60 for 4137, and it maintains elevated hardness at greater distance from the quench end.
Jominy end-quench — hardenability curve (schematic, oil quench)
Hardenability by shaft diameter — practical guide
| Shaft Diameter | AISI 4137 Center HRC | AISI 4140 Center HRC | Specification Guidance |
|---|---|---|---|
| ≤ 75 mm | 54–58 HRC | 56–60 HRC | Both fully harden through-section. Functionally interchangeable. |
| 75–150 mm | 46–54 HRC | 50–57 HRC | 4140 maintains higher center hardness. Check minimum hardness specification. |
| 150–300 mm | 35–46 HRC | 40–50 HRC+5 HRC | Gap is significant. 4140 preferred if center tensile strength is specified. |
| > 300 mm | 28–38 HRC | 32–43 HRC+5 HRC | Specify H-band grades (4137H / 4140H) with Jominy band requirements on MTC. |
H-band grade recommendation
Both AISI 4137H and 4140H are hardenability-controlled variants with tighter Jominy band limits. For large shaft forgings where through-section mechanical properties are specified and audited, we recommend the H-band grade with Jominy data documented on the EN 10204 3.1 MTC.
Heat Treatment Parameters Compared
Both grades respond to the same heat treatment family — normalizing, annealing, quench-and-temper, and stress relieving — but the specific temperature windows and process cautions differ.
| Treatment Step | AISI 4137 | AISI 4140 | Notes |
|---|---|---|---|
| Austenitizing (hardening) | 840–870°C | 845–870°C | Nearly identical. Full solution of carbides required before quench. |
| Quench medium | Oil or polymer | Oil or polymer | Water quench: acceptable < 25 mm section; increases cracking risk above that. |
| Tempering range | 540–680°C | 540–680°C | Avoid 260–370°C temper embrittlement zone — both grades affected. |
| Temper to 1,000 MPa Rm | ~595–620°C | ~610–635°C | 4140 requires slightly higher temper temperature for equivalent softening. |
| Normalizing | 860–900°C, air cool | 860–900°C, air cool | Used for grain refinement after forging, before Q&T. |
| Max section for through-hardening | ~200 mm (oil quench) | ~250 mm (oil quench)Advantage | 4140 holds full martensite deeper — critical for large sections. |
Temper embrittlement: the field failure risk
Both grades are susceptible to temper embrittlement if tempered in the 260–370°C (500–700°F) range. In this zone, phosphorus and trace elements segregate to prior austenite grain boundaries, causing brittle intergranular fracture under impact or cyclic loading — without any visible change in hardness. Our standard practice avoids this temperature window entirely and holds for no less than 2 hours per 25 mm of cross-section to ensure thermal uniformity to the core.
Weldability & Carbon Equivalent
For forged shafts that will be welded — during assembly or repair — the carbon equivalent (CE) is the governing parameter. Higher CE means higher preheat requirement and greater susceptibility to hydrogen-induced cold cracking in the heat-affected zone (HAZ).
| Parameter | AISI 4137 | AISI 4140 |
|---|---|---|
| Carbon Equivalent (IIW) | ~0.68–0.75Lower | ~0.72–0.80 |
| Preheat (section < 25 mm) | 150–175°C | 175–200°C |
| Preheat (section 25–50 mm) | 200–230°C | 230–260°C |
| Post-weld heat treatment (PWHT) | Recommended ≥ 25 mm | Required ≥ 20 mmStricter |
| HAZ cold cracking susceptibility | ModerateLower risk | Moderate–High |
| Distortion risk on long shafts | Lower | Higher — elevated preheat adds thermal distortion |
Machinability Comparison
For large forged shafts requiring heavy stock removal — multiple journal diameters, keyways, spline ends, bore features — machinability is a measurable cost driver. The difference between 4137 and 4140 is subtle per pass but adds up over 30+ hours of CNC turning on a large main shaft.
| Machinability Factor | AISI 4137 | AISI 4140 |
|---|---|---|
| Relative machinability (AISI 1212 = 100%) | ~66–70%Better | ~62–66% |
| Cutting speed — turning (HB 280) | 140–165 m/min | 125–150 m/min |
| Tool wear rate | Lower | Higher |
| Achievable surface finish (Ra, turned) | 0.8–1.6 μm | 0.8–1.6 μm |
| Cost impact on shaft > 5,000 kg | Baseline | +5–12% machining cost |
Application-by-Application Decision Guide
The recommendations below are based on 26+ years of production and customer feedback from oil & gas, mining, wind, rail, and heavy machinery OEMs across 50+ countries served by Jiangsu Liangyi Co., Limited.
Large section (400–700 mm). Fatigue-critical under variable loading. 4137 toughness and weld-repair flexibility outweigh 4140's marginal strength ceiling.
Sections 80–200 mm. Both harden adequately. Specify 4140 if surface hardness floor is stated; 4137 if weld assembly is involved.
Very large section, abrasion + impact duty. 4140's superior hardenability depth justifies higher preheat during any site welding.
Manufacturable to API 6A material requirements. Shaft dia. usually < 150 mm. Choose 4137 for better low-temperature Charpy performance.
Variable-amplitude fatigue governs. 4137 toughness and Charpy impact are advantageous. Compliant with EN 13262.
High tensile strength at operating temperature. 4140's higher Rm ceiling and creep resistance to ~400°C are the standard choice.
High Rp0.2 for pressure containment. Surface hardness for chrome plating adhesion. 4140 is standard in hydraulic rod specifications.
Torsional fatigue governs. Section usually 100–250 mm. 4137's toughness and better machinability make it cost-optimal.
Seawater corrosion + fatigue. 4137's lower carbon equivalent preferred where classification society surveys require HAZ toughness certification.
Decision matrix at a glance
AISI 4137 — specify when:
Toughness & weldability are primary
- Variable-amplitude fatigue loading (rail, crane, marine)
- Shaft will be welded in manufacturing or field repair
- Low-temperature Charpy impact certification required
- Large section (> 400 mm) where center toughness matters
- Machining budget is tight; large stock removal needed
- Wind turbine main shafts and generator shafts
AISI 4140 — specify when:
Strength & hardenability are primary
- Maximum tensile or yield strength is design-limiting
- Heavy section (> 200 mm) with minimum center hardness specified
- Abrasion resistance at the surface is a service requirement
- Compressor, turbine, or elevated-temperature rotor
- Hydraulic rod or pressure-retaining component with Rp0.2 threshold
- Through-hardening reliability across thick flanges or hub sections
The Forging Manufacturer's Perspective
From Jiangsu Liangyi's press floor: both grades are forged under identical parameters. Forging temperature (1,100–1,250°C), reduction ratio targets (minimum 5:1), and grain refinement protocols are the same for both grades. The quality differentiation happens downstream, in heat treatment and inspection. For full production scope, weight range, and available shapes in this grade, see our AISI 4137 forging parts page.
Why forging outperforms bar stock for both grades
Substituting hot-rolled or sawn bar for shaft applications in the 100–300 mm diameter range eliminates the grain flow alignment that forging produces, the closed-porosity guarantee that only forging provides, and the ultrasonic testability that ASTM A388 requires. When AISI 4137 or 4140 is open-die forged under our 6,300-tonne hydraulic press, fatigue cracks must propagate through — rather than along — the grain boundaries. Forged shaft fatigue life consistently exceeds bar-stock equivalents by 30–80% in controlled testing.
Our quality inspection sequence (both grades)
- →Incoming billet chemistry verified by OES (optical emission spectrometry) for every heat
- →Controlled-atmosphere forging at 1,100–1,250°C to preserve grain structure
- →Heat treatment documented to ±5°C in calibrated, chart-recorded furnaces
- →Tensile, yield, elongation, reduction of area, and Charpy impact on heat-representative coupons
- →Full-body ultrasonic testing per ASTM A388 (EN 10228-3 for European clients)
- →Magnetic particle inspection on all machined surfaces
- →CMM dimensional verification and EN 10204 3.1 MTC (3.2 with third-party witness on request)
Global Standard Cross-References
Procurement teams sourcing from China, Europe, and North America simultaneously often need to cross-reference AISI designations with DIN, EN, JIS, and BS equivalents. Jiangsu Liangyi Co., Limited can supply MTCs referencing any of these equivalent grades on request, subject to chemistry compliance with the specified standard.
| Standard System | AISI 4137 Nearest Equivalent | AISI 4140 Nearest Equivalent | Key Difference vs AISI |
|---|---|---|---|
| DIN / EN (Germany) | 34CrMo4 · 1.7220 | 42CrMo4 · 1.7225 | Slightly narrower Mn range; P/S sometimes tighter in premium quality |
| JIS (Japan) | SCM435 | SCM440 | Very close composition; P & S max often ≤ 0.030%; functionally equivalent in most applications |
| BS (UK) | 708M40 (En19) | 708M40 (En19C) | Older BS designations cover the 4137–4140 range; confirm C content when cross-referencing |
| GB (China) | 35CrMo (GB/T 3077) | 42CrMo (GB/T 3077) | We manufacture to AISI/EN/JIS on client request; GB grades also available |
| GOST (Russia/CIS) | 35KhM | 38KhM | Slightly wider composition ranges; broadly equivalent for most applications |
| UNS | G41370 | G41400 | Direct AISI equivalents in UNS system — no compositional difference |
Substitution caution
"Nearest equivalent" does not mean "direct substitute" in all applications. For pressure-retaining components or parts governed by design codes (ASME, EN 13480), always verify that the equivalent grade satisfies the original specification's mechanical property minimums — not just the chemical composition range.
Frequently Asked Questions
What is the key difference between AISI 4137 and AISI 4140 steel?
The primary difference is carbon content: AISI 4137 contains 0.35–0.40% carbon, while AISI 4140 contains 0.38–0.43% carbon. All other alloy additions — chromium 0.80–1.10% and molybdenum 0.15–0.25% — are identical. This carbon difference gives 4140 slightly higher tensile strength and hardenability depth, while 4137 offers better toughness, weldability, and machinability.
Can AISI 4137 be substituted for AISI 4140 without engineering review?
For sections under 75 mm in Q&T condition, the grades are functionally interchangeable in most non-code applications. For sections above 150 mm, or for components governed by ASME or other design codes, a formal material substitution review is required. The hardenability difference at large cross-sections is real and measurable in Material Test Certificate data.
Which grade is better for welded shaft assemblies?
AISI 4137 is better for welding applications. Its lower carbon equivalent (CE ~0.68–0.75 vs ~0.72–0.80 for 4140) means lower preheat temperatures (150–175°C vs 175–200°C for sections under 25 mm), reduced cold cracking risk, and less stringent post-weld heat treatment requirements.
What are the international equivalents of AISI 4137 and AISI 4140?
AISI 4137 is equivalent to DIN/EN 34CrMo4 (1.7220), JIS SCM435, BS 708M40 (En19), GB 35CrMo, and UNS G41370. AISI 4140 is equivalent to DIN/EN 42CrMo4 (1.7225), JIS SCM440, BS 708M40 (En19C), GB 42CrMo, and UNS G41400. These are nearest equivalents — verify mechanical property minimums for code-governed applications.
Which alloy is preferred for wind turbine main shaft forgings?
AISI 4137 (or DIN 34CrMo4) is preferred for wind turbine main shaft forgings. At 400–700 mm diameter, 4137's Charpy toughness and variable-amplitude fatigue resistance outweigh 4140's marginal strength advantage. Wind turbine shafts may also require weld repair in service, where 4137's lower carbon equivalent provides a more forgiving process window.
What lead time does Jiangsu Liangyi Co., Limited offer for custom AISI 4137 or 4140 forged shafts?
Standard lead time at Jiangsu Liangyi Co., Limited is 15–30 working days for most custom AISI 4137 or 4140 forged shafts including forging, heat treatment, and rough machining. Large main shafts of 3,000–30,000 kg with finish machining and full NDT typically require 45–70 days. EN 10204 3.2 MTC with third-party witness inspection adds 3–5 days.
What certifications does Jiangsu Liangyi Co., Limited hold?
Jiangsu Liangyi Co., Limited holds ISO 9001:2015 quality management system certification. We issue EN 10204 3.1 Mill Test Certificates as standard for all forgings, signed by our own authorised inspection representative. EN 10204 3.2 MTC (third-party witness certification) is available on request through approved inspection agencies including BV, SGS, TUV, DNV, and Lloyd's Register. We manufacture AISI 4137 and 4140 forgings to meet the material and dimensional requirements of API 6A — the API product licence is held by the OEM customer who incorporates our forgings into their licensed equipment.
Conclusion: Three Questions That Decide the Grade
After 26+ years of forging both grades across thousands of shaft geometries and dozens of industries, the choice between AISI 4137 and AISI 4140 reduces to three questions asked in order:
- What is the finished shaft diameter? Below 75 mm: both are interchangeable. Above 200 mm: 4140's superior hardenability depth produces measurably higher center-section strength. Above 400 mm: specify H-band grades with Jominy band requirements documented on the MTC.
- Will the shaft be welded — in manufacturing or in service? If yes: 4137's lower carbon equivalent reduces preheat requirements, narrows the HAZ, and makes post-weld heat treatment more manageable. If welding is never part of the lifecycle: this advantage is irrelevant.
- Is the design limited by maximum strength, or by toughness? If maximum tensile or yield strength governs: specify 4140. If fatigue life, Charpy impact, low-temperature service, or machining cost efficiency governs: specify 4137.
Map your answers to the decision matrix in Section 07 and the right grade becomes clear. If uncertainty remains, contact Jiangsu Liangyi Co., Limited with your shaft diameter, mechanical property requirements, and welding procedure — and request Jominy data from production heats in the grade you are considering. For chemical composition tables, section-specific mechanical property guarantees, available shapes, and lead times, the AISI 4137 material specification and ordering guide covers everything needed to complete your RFQ.