Baseline Grade
AISI 4140
0.38–0.43% C  ·  UNS G41400  ·  DIN 42CrMo4
VS
Higher-Carbon Grade
AISI 4142
0.40–0.45% C  ·  UNS G41420  ·  Near DIN 45CrMo4

Why This Comparison Matters

When engineers and procurement teams specify a chromium-molybdenum (Cr-Mo) forging grade, the choice between AISI 4142 and AISI 4140 often comes down to a single percentage point of carbon. Yet that narrow gap — roughly 0.02–0.05% more carbon in 4142 — produces measurable, consequential changes in hardenability, peak hardness ceiling, quench cracking susceptibility, and ultimately component service life in demanding industrial applications.

This technical guide is written for engineers, metallurgical buyers, and sourcing managers who need a defensible specification choice between these two grades. It draws on chromoly forging experience at Jiangsu Liangyi Co., Limited — ISO 9001:2015 certified open die forging manufacturer. If you have already confirmed AISI 4142 is your required grade, full dimensional range, chemical certifications, and ordering details are on our AISI 4142 forging specifications page.

Expert Recommendation

AISI 4142 is not simply "better than 4140." It is specifically superior where you need a higher ceiling on achievable hardness, through-hardening consistency in heavy cross-sections, and maximum wear resistance after quench-and-temper treatment. Where toughness, weldability, or international cross-certification (DIN 42CrMo4, JIS SCM440) takes priority, AISI 4140 is the more appropriate specification.

Chemical Composition: Where the Difference Lives

Both grades belong to the SAE 41xx chromium-molybdenum series, sharing identical chromium (0.80–1.10%) and molybdenum (0.15–0.25%) content. The distinction is confined entirely to the carbon range — but carbon governs hardenability and the strength ceiling more than any other alloying element in this steel family.

Chemical Composition — Weight Percent (ASTM / SAE Specification)
Element AISI 4140 AISI 4142 Engineering Significance
Carbon (C)0.38 – 0.43%0.40 – 0.45%Controls max hardness & hardenability depth — the key differentiator
Chromium (Cr)0.80 – 1.10%0.80 – 1.10%Surface hardenability, oxidation resistance, identical in both grades
Molybdenum (Mo)0.15 – 0.25%0.15 – 0.25%Deep hardenability, suppresses temper embrittlement, identical
Manganese (Mn)0.75 – 1.00%0.75 – 1.00%Hardenability contribution, deoxidation during steelmaking
Silicon (Si)0.15 – 0.35%0.15 – 0.35%Deoxidation, minor strength contribution
Phosphorus (P)≤ 0.035%≤ 0.035%Maximum limit — embrittlement risk above this threshold
Sulfur (S)≤ 0.040%≤ 0.040%Maximum limit — machinability vs. toughness trade-off

The overlap in carbon ranges is intentional and practically significant. A heat of 4140 produced at the top of its specification range (0.43% C) and a heat of 4142 produced at the bottom of its range (0.40% C) will have nearly identical properties after equivalent heat treatment. This is why the specification decision becomes especially important when end-use hardness targets are at or above HRC 38, where the separation between grades becomes consistent and predictable. Full heat analysis and product chemical certification data for our AISI 4142 alloy forgings are available on the product page.

Mechanical Properties After Quench and Temper

Both grades are specified in the quenched and tempered (Q+T) condition for virtually all structural forging applications. The comparison below reflects typical properties achieved on a 25mm reference diameter round bar after oil quenching from 845–870°C and tempering to standard industrial hardness targets. Properties in larger cross-sections will be lower due to hardenability limits.

Typical Q+T Properties — 25mm Round Bar Reference Section
Peak Achievable Hardness (as-quenched) 4140: ~HRC 544142: ~HRC 57
4140
4142
Ultimate Tensile Strength (Q+T) 4140: ~1,035 MPa4142: ~1,080 MPa
4140
4142
Yield Strength (Q+T) 4140: ~896 MPa4142: ~930 MPa
4140
4142
Charpy Impact Toughness — V-notch, room temp 4140: 55–68 J (higher)4142: 45–60 J
4140
4142
Fatigue Strength (rotating bending, 10⁷ cycles) 4140: ~470 MPa4142: ~510 MPa
4140
4142

The data confirms the consistent pattern: AISI 4142 delivers higher strength, a higher hardness ceiling, and approximately 8.5% better fatigue performance versus AISI 4140 at equivalent heat treatment conditions. The trade-off is a meaningful reduction in Charpy V-notch impact energy — typically 10–15 J lower — which matters significantly in shock-loaded, low-temperature, or dynamic loading applications.

Hardenability in Heavy Forging Sections: The Critical Difference

For small cross-sections under 40mm diameter, AISI 4140 and AISI 4142 perform nearly identically. The separation becomes significant in heavy sections — the very sections where forged industrial components most commonly appear.

Hardenability refers to the depth to which a steel can be hardened during quenching, governed by the Jominy end-quench test (ASTM A255). The higher carbon content of 4142 shifts its Jominy curve upward across all distances from the quench end. At equivalent depths within a large forging cross-section, AISI 4142 consistently achieves hardness values 2–4 HRC points higher than AISI 4140 after identical heat treatment.

Engineering Insight — Heavy Section Performance

For a 200mm (8-inch) diameter forged shaft, AISI 4142 achieves through-hardness targets of HRC 28–32 reliably after standard oil quench and temper. Achieving the same target with AISI 4140 in the same section size requires either more aggressive quench media (higher distortion risk) or relaxation of the hardness specification. This is why 4142 is the preferred specification for large-bore drill collars, heavy-duty gear blanks, and deep-section pressure vessel forgings where API 6A specifications require consistent through-section properties.

Heat Treatment Parameters: AISI 4142 vs AISI 4140

The heat treatment windows for both grades are closely aligned, making them often interchangeable at facilities with established Q+T procedures. However, AISI 4142 demands more careful process control — particularly during quenching — due to the elevated carbon content raising quench cracking sensitivity.

Step 01 — Pre-treatment
Normalize
870–900°C

Air cool. Homogenizes as-forged microstructure and reduces residual stress before hardening. More critical for 4142 than 4140 due to higher transformation stress during subsequent quench.

Step 02 — Hardening
Austenitize & Quench
845–870°C → Oil

Identical austenitizing range for both grades. 4142 requires more caution in components with geometric transitions — warm oil (60–80°C) or polymer quench media recommended for complex sections.

Step 03 — Toughening
Temper
400–650°C

Minimum 1 hour per 25mm of section. Higher tempering temperatures give lower hardness with improved toughness. Transfer to tempering furnace within 1–2 hours of quench reaching ambient.

Quench Cracking Risk: The Most Important Practical Difference

Elevated carbon in AISI 4142 increases the martensitic transformation stress gradient during oil quenching, raising cracking risk in:

  • Cross-sections with sharp geometric transitions — keyways, bores, threads, undercuts
  • Forgings with wall thickness variations exceeding a 3:1 ratio
  • Components where residual forging stresses were not relieved by pre-quench normalizing
  • Large-diameter rolled rings where the bore-to-OD temperature differential during quench is significant

This does not disqualify 4142 for complex geometries — it requires experienced heat treatment engineering. Jiangsu Liangyi's in-house facilities include programmable gas atmosphere furnaces, precision temperature-controlled oil quench tanks, and mandatory pre-quench normalizing for all 4142 components.

Application-by-Application Selection Guide

The correct grade specification is determined by the actual demands of the end-use application — not by a general preference for one grade. The following six scenarios represent the most common selection decisions encountered in chromoly forging practice.

Specify 4142
AISI 4142 — Best Choice

Heavy-Section Gear Blanks

When surface hardness after induction hardening must reach HRC 55+, 4142's higher carbon ceiling produces more consistent results in gear forgings over module 8 with section sizes above 100mm.

AISI 4140 — Best Choice

High-Toughness Structural Shafts

For shafts subject to frequent shock or impact — mining crusher shafts, pinion shafts with sudden load reversals — 4140's superior Charpy impact values (55–68 J vs 45–60 J) are the design priority.

Specify 4142
AISI 4142 — Best Choice

Downhole Drilling Tools (API 6A)

Drill collars, stabilizers, and crossovers in heavy-wall sections over 150mm OD achieve more consistent through-hardness with 4142 — critical for meeting API 6A and NACE MR0175 specification requirements.

AISI 4140 — Best Choice

Weld-Repaired Components

Where field or shop weld repairs are expected during service life, 4140's lower carbon equivalent reduces pre-heat requirements and post-weld heat treatment demands — meaningfully lower risk of HAZ cracking.

Specify 4142
AISI 4142 — Best Choice

Fatigue-Critical Rotating Shafts

With ~510 MPa fatigue strength at 10⁷ cycles versus ~470 MPa for 4140, AISI 4142 provides a measurable 8.5% advantage in high-cycle rotating service — gear shafts, pump shafts, crank assemblies.

AISI 4140 — Best Choice

General Transmission Shafts ≤100mm

For standard shafts in the 80–150mm range targeting HRC 28–38, 4140 achieves this reliably with lower quench cracking risk, lower material cost, and simpler workshop heat treatment process control.

Grade Selection Decision Matrix

11 Specification Criteria — Which Grade Wins and Why
Specification Criterion Preferred Grade Engineering Rationale
Hardness target > HRC 504142Higher carbon ceiling consistently reaches HRC 54–57 in standard Q+T
Section size > 150mm diameter4142Superior hardenability ensures consistent through-hardening in heavy forgings
Charpy impact > 60 J required4140Lower carbon = less martensite embrittlement risk; 55–68 J typical for 4140
Complex geometry / weld repairs anticipated4140Lower CE (carbon equivalent) reduces quench cracking risk and simplifies PWHT
API 6A / NACE MR0175 oilfield service4142Heavy-wall downhole tools benefit from 4142 through-hardening in thick sections
High-cycle fatigue-critical component4142~510 MPa vs ~470 MPa fatigue strength at 10⁷ cycles — measurable 8.5% advantage
Standard shaft ≤100mm diameter, HRC 28–38 target4140Both grades achieve this reliably; 4140 is lower risk and typically lower cost
DIN / EN / JIS cross-certification required41404140 = DIN 42CrMo4/1.7225/JIS SCM440; 4142 has no direct equivalent
High-volume cost-sensitive program41404140 is more widely stocked globally and often carries a lower material premium
Heavy gear blank > module 8, section >120mm4142Through-hardness consistency supports tooth root fatigue and flank contact load
Wind energy main shaft / large bearing ringEitherVerify OEM specification — both grades are used; choice depends on fatigue vs. toughness driver

Forgeability and Manufacturing Considerations

From a hot forging process standpoint, both grades behave similarly in the 1,050–1,200°C forging range. Both require a controlled finishing temperature above the Ar3 transformation line (approximately 820°C) to prevent forging into the two-phase austenite-ferrite region, which produces banded microstructures detrimental to mechanical property uniformity.

Key Process Differences for AISI 4142

  • Post-forge cooling: Components with significant cross-section variation must be cooled more slowly after forging — controlled furnace or pit cooling recommended for sections over 100mm — to prevent thermal transformation stresses before heat treatment.
  • Pre-machining annealing: AISI 4142 at as-forged hardness (HB 220–260 typical) is harder to rough-machine than 4140 (HB 197–235). Full anneal to HB 187–229 is recommended before heavy machining where tight dimensional tolerances are required.
  • Mandatory normalizing before Q+T: More critical for 4142 than 4140, particularly in open-die forgings with inhomogeneous deformation history, to ensure microstructure uniformity before the final quench-and-temper cycle.
Quality Assurance — Jiangsu Liangyi Process

All AISI 4142 forgings produced by Jiangsu Liangyi undergo mandatory normalizing before final Q+T heat treatment. Our in-house facilities include programmable gas atmosphere furnaces, precision temperature-controlled oil quench tanks, Brinell and Rockwell hardness testing, and ultrasonic testing. Full EN 10204 3.1 material test certificates are issued with every shipment. Contact us to discuss your project requirements and section size range.

International Standards and Equivalents

Cross-Reference: AISI 4140 vs AISI 4142 by Standard System
Standard System AISI 4140 Designation AISI 4142 Designation
AISI / SAE (USA)SAE 4140 / AISI 4140SAE 4142 / AISI 4142
UNS (USA)UNS G41400UNS G41420
DIN / EN (Europe)42CrMo4 / EN 1.7225No direct equiv. — nearest 45CrMo4
BS (United Kingdom)EN19 / 708M40No direct BS equivalent
JIS (Japan)SCM440No direct JIS equivalent
GB / YB (China)42CrMo (GB/T 3077)45CrMo (approximate)
ASTM (forging product)A322, A505, A519A322, A505, A519
API (oilfield)API 6A, NACE MR0175API 6A, NACE MR0175

A critical procurement note: AISI 4142 has significantly fewer direct international equivalents than AISI 4140. For projects requiring European CE-marked equipment or JIS-specified industrial machinery, AISI 4140 with its well-established DIN 42CrMo4 / EN 1.7225 equivalence and JIS SCM440 mapping is typically the more practical specification for multi-market supply chains.

Summary: Making the Right Specification Decision

The decision between AISI 4142 and AISI 4140 for industrial forgings is not a question of which grade is inherently superior. It is a question of which grade's property profile best matches your specific application requirements, cross-section size, heat treatment process capabilities, and supply chain constraints.

Specify AISI 4142 when your application requires the highest achievable hardness from the Cr-Mo family (above HRC 50), when through-hardening consistency in sections over 150mm is critical, when fatigue performance is the primary design driver, or when producing heavy-section oilfield components to API 6A specifications. To request a quote or review available section sizes and material certifications, visit the AISI 4142 open die forgings product page.

Specify AISI 4140 when Charpy impact toughness and ductility are the primary requirements, when the component geometry is complex or weld repairs are anticipated, when your hardness target is within HRC 28–42 in sections under 100mm, or when international standard cross-certification to DIN/EN/JIS is essential for downstream market access.

Further Technical Resources

For full chemical composition tables, material test certificates, and product enquiries, our engineering team provides grade selection support and competitive quotations within 24 hours.

Frequently Asked Questions

AISI 4142 has a carbon range of 0.40–0.45% compared to AISI 4140's 0.38–0.43%. This higher carbon content gives 4142 a greater hardenability depth, a higher peak hardness ceiling (~HRC 57 vs ~HRC 54), and superior fatigue strength (~510 MPa vs ~470 MPa). The trade-off is reduced Charpy impact toughness (45–60 J vs 55–68 J) and increased quench cracking risk in complex geometries. Both grades share identical chromium (0.80–1.10%) and molybdenum (0.15–0.25%) content.
Specify AISI 4142 when: (1) your hardness target exceeds HRC 50 after quench and temper; (2) the forging cross-section exceeds 150mm diameter and consistent through-hardening is critical; (3) the application involves high-cycle fatigue — gear shafts, pump shafts, rotating components; (4) you are producing downhole drilling tools such as drill collars or stabilizers to API 6A specifications. Use AISI 4140 when impact toughness is paramount, when weld repairs are expected, or when DIN/JIS international cross-certification is required.
AISI 4142 forgings are heat treated as follows: (1) Normalize at 870–900°C, air cool to ambient; (2) Austenitize at 845–870°C with adequate soak time; (3) Oil quench — warm oil or polymer quench recommended for complex sections; (4) Temper immediately at 400–650°C for a minimum of 1 hour per 25mm of section thickness. Higher tempering temperatures reduce hardness but significantly improve toughness. Due to the elevated carbon content, 4142 requires careful quench management to minimize cracking risk in components with geometric transitions.
AISI 4142 (UNS G41420) does not have a single direct international equivalent. The closest European designation is 45CrMo4 under DIN/EN standards, though this is approximate. By contrast, AISI 4140 maps directly to DIN 42CrMo4 / EN 1.7225, JIS SCM440, and GB/T 3077 42CrMo. For projects requiring strict international cross-certification — CE marking, JIS compliance — AISI 4140 is typically the more practical specification.
Yes. The elevated carbon in AISI 4142 (0.40–0.45% vs 0.38–0.43% in 4140) increases the martensitic transformation stress during oil quenching, raising quench cracking risk in components with sharp geometric transitions (keyways, bores, threads), significant wall-thickness variations, or large section sizes. Mitigation requires mandatory pre-quench normalizing, warm-oil or polymer quench media, and careful geometry design. For components where quench cracking risk is a concern, AISI 4140 is the lower-risk specification.
AISI 4142 is generally preferred for heavy-wall downhole drilling tools — drill collars, stabilizers, crossovers — with outside diameters exceeding 150mm. The superior hardenability of 4142 ensures more consistent through-hardness across the heavy cross-section, critical for meeting API 6A and NACE MR0175 specification requirements in H2S-containing environments. For smaller-bore wellhead components where 4140 can achieve the required hardness in the section size, 4140 may be equally acceptable and offers lower quench cracking risk.
AISI 4142 forgings in the quenched and tempered condition typically achieve an ultimate tensile strength (UTS) of approximately 1,080 MPa and a yield strength of approximately 930 MPa on a 25mm reference diameter round bar. Properties in larger cross-sections will be lower due to reduced through-hardening efficiency. AISI 4140 under identical conditions achieves approximately 1,035 MPa UTS and 896 MPa yield strength. Both values comply with minimum ASTM A322 / A505 requirements.
Yes. Jiangsu Liangyi Co., Limited, based in Jiangyin, Jiangsu Province, China, is an ISO 9001:2015 certified manufacturer of both AISI 4142 and AISI 4140 open die forgings and seamless rolled rings. Material test certificates are issued per EN 10204 3.1. ASTM, DIN, and EN standard compliance is available. Enquiries receive a quotation within 24 hours.