AISI 4135 (0.33–0.38% C, DIN 34CrMo4, JIS SCM435) is preferred where toughness, weldability, and low-temperature performance are critical — sections under 100 mm, wind power, pressure vessels, API 6A valve forgings, and applications requiring welding.
AISI 4140 (0.38–0.43% C, DIN 42CrMo4, JIS SCM440) is preferred where maximum through-hardened strength is required — sections 100–250 mm, drill collars per API 7-1, gear shafts, mining equipment, and heavy machinery where welding is not needed.
The key difference is carbon content: 4140 delivers 8–10% higher tensile strength, but 4135 provides significantly better Charpy impact toughness (70–100 J vs 55–80 J at room temperature) and superior weldability (Carbon Equivalent 0.65–0.75 vs 0.70–0.80).
Why This Comparison Matters
When engineers and procurement teams specify chromium-molybdenum (Cr-Mo) alloy steel forgings, the choice between AISI 4135 and AISI 4140 appears straightforward — one carbon digit apart. In practice, that 0.05% carbon difference drives meaningful divergence in weldability, through-hardening depth, impact toughness, and suitability across different forging applications and industry standards.
Jiangsu Liangyi Co., Limited has produced open-die forgings and seamless rolled rings in both grades since 1997, shipping to oil & gas, wind power, mining, and heavy machinery customers in over 50 countries. If you are already specifying AISI 4135, our custom AISI 4135 forged parts page covers full size ranges, product forms, and heat treatment options. This guide focuses on the selection decision itself — distilling our process engineers' accumulated knowledge into a practical framework built from real production experience.
Chemical Composition: Where the Grades Actually Differ
Both AISI 4135 and 4140 belong to the SAE-AISI 41xx chromium-molybdenum family. Their alloying additions are identical in type — the only meaningful variable is carbon content.
| Element | AISI 4135 (UNS G41350) | AISI 4140 (UNS G41400) | Engineering Implication |
|---|---|---|---|
| Carbon (C) | 0.33 – 0.38% | 0.38 – 0.43% | Higher C in 4140 → greater as-quenched hardness, reduced weldability |
| Chromium (Cr) | 0.80 – 1.10% | 0.80 – 1.10% | Identical — both grades get same Cr hardenability boost |
| Molybdenum (Mo) | 0.15 – 0.25% | 0.15 – 0.25% | Identical — Mo eliminates temper embrittlement in both grades |
| Manganese (Mn) | 0.70 – 0.90% | 0.75 – 1.00% | Slightly higher Mn in 4140 supports hardenability |
| Silicon (Si) | 0.15 – 0.35% | 0.15 – 0.35% | Identical — deoxidation and minor strength contribution |
| Phosphorus (P) max | 0.035% | 0.035% | Identical — controlled as harmful impurity |
| Sulfur (S) max | 0.040% | 0.040% | Identical — controlled for toughness |
| Carbon Equivalent (CE) | ~0.65 – 0.75 | ~0.70 – 0.80 | 4135 closer to preheat-free welding threshold; 4140 requires mandatory preheat |
CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15. This explains why 4135 can be welded with lower preheat requirements on thinner sections, while 4140 requires 150–300 °C preheat to prevent heat-affected zone (HAZ) cracking in forgings above 25 mm section.
International Grade Equivalents
- Standard in European wind and pressure vessel specs as 34CrMo4 under EN 10083-3
- JIS SCM435 is the direct Japanese equivalent used in Asia-Pacific projects
- GB 35CrMo is the Chinese national standard near-equivalent
- 42CrMo4 is the European default gear and shaft alloy under DIN/EN standards
- JIS SCM440 widely specified in Japanese and Korean heavy equipment sectors
- Commonly referenced in API 7-1 drill string material specifications
Mechanical Properties After Quench and Temper
The following represents typical values for a 100 mm diameter forged round bar in the quenched and tempered (Q&T) condition, tested at center. Actual values depend on section size, forging reduction ratio, and the Q&T curve applied.
| Property | AISI 4135 (Q&T) | AISI 4140 (Q&T) | Verdict |
|---|---|---|---|
| Tensile Strength (UTS) | 930 – 1,100 MPa | 1,000 – 1,200 MPa | 4140 ~8–10% higher |
| Yield Strength (0.2% PS) | 780 – 950 MPa | 860 – 1,050 MPa | 4140 ~10% higher |
| Elongation (A₅) | 14 – 18% | 12 – 16% | 4135 better ductility |
| Reduction of Area (Z) | 55 – 65% | 50 – 60% | 4135 marginally better |
| Charpy Impact (Room Temp) | 70 – 100 J | 55 – 80 J | 4135 clearly tougher |
| Charpy Impact (−20 °C) | 40 – 65 J | 25 – 45 J | 4135 significantly tougher |
| Hardness (Q&T at 600 °C) | 270 – 310 HBW | 290 – 330 HBW | 4140 harder |
| Max As-Quenched Hardness | ~55 HRC | ~58 HRC | 4140 higher surface hardness |
The most consequential difference is Charpy impact toughness at sub-zero temperatures. For applications involving dynamic loading, shock, or operation below 0 °C — offshore equipment, arctic mining, sub-sea forgings — AISI 4135 carries a meaningful safety margin that AISI 4140 cannot match at equivalent tempering temperatures.
Through-Hardening Depth and Section Size Limits
Hardenability — the ability to achieve consistent hardness through the full cross-section — is arguably the most critical variable for selecting between these grades in a heavy forging context.
- Full through-hardening up to approximately 80 mm section after oil quench
- Acceptable mechanical uniformity to 150 mm section
- Above 200 mm, core properties soften significantly
- Ideal for flanges, rolled rings, sleeves, hollow forgings
- Full through-hardening up to approximately 120 mm section after oil quench
- Acceptable mechanical uniformity to 250 mm section
- Preferred for heavy shafts, large bars, and blocks
- Standard choice for drill collars above 150 mm OD per API 7-1
For forgings with a critical section below 100 mm, both grades perform comparably. Above 120 mm, specify AISI 4140. For sections above 250 mm requiring high core toughness, consider stepping up to AISI 4340 or 4330V rather than accepting the core softening inherent to 4140 at these sizes.
Heat Treatment Windows Compared
Both grades follow the same processing sequence — normalize, austenitize, quench, temper — but effective temperature windows differ, particularly for final tempering to achieve target properties.
| Process Step | AISI 4135 | AISI 4140 | Notes |
|---|---|---|---|
| Normalizing | 870 – 900 °C, air cool | 870 – 900 °C, air cool | Grain refinement after forging; identical for both grades |
| Annealing | 830 – 860 °C, furnace cool | 830 – 860 °C, furnace cool | Soften for machining; identical process |
| Austenitizing | 845 – 870 °C | 845 – 870 °C | Same window; soak time calculated from section thickness |
| Quench medium | Oil or polymer quench | Oil or polymer quench | Both are oil-quench grades |
| Temper — high toughness | 580 – 640 °C | 600 – 660 °C | 4135 reaches equivalent toughness at slightly lower temper temperature |
| Temper — high strength | 480 – 550 °C | 450 – 530 °C | 4140 holds higher strength at lower tempering temperatures |
| Stress relieving | 550 – 600 °C, 2–4 h | 550 – 600 °C, 2–4 h | Post-machining stress relief; identical for both grades |
Our heat treatment facility maintains ±5 °C furnace uniformity across all zones, documented via calibrated thermocouples at multiple positions within the forging. All heat treatment time-temperature records are attached as part of every EN 10204 3.1 Mill Test Certificate.
Weldability: The Most Underrated Selection Factor
For forged components, weldability matters in three scenarios: repair welding after machining damage, structural weldments where a forged hub integrates into a fabricated assembly, and cladding or hardfacing for wear protection.
Sections under 25 mm may weld with 100–150 °C preheat. Medium sections (25–75 mm) require 150–200 °C preheat. PWHT at 550–600 °C recommended for critical structural joints. Lower HAZ cracking risk than 4140.
Preheat of 150–300 °C is mandatory for all sections above 25 mm. PWHT strongly recommended for all structural welds. Elevated HAZ hardening and hydrogen-assisted cracking risk without strict procedure control. Not recommended for field repair welding without qualified procedures.
If your application involves welding during fabrication, WPS qualification under AWS D1.1, ASME IX, or EN ISO 15614, or field repair welding in service — AISI 4135 materially reduces procedure complexity, preheat requirements, and rework risk compared to AISI 4140.
Industry-Specific Application Guidance
These recommendations are based on engineering logic — section size, load type, and fabrication constraints — not grade preference. Where both grades appear viable, the final choice depends on your specific section geometry and downstream processing requirements.
Drill collars, stabilizer mandrels, crossover subs — large sections, extreme torque. Core strength and through-hardening depth take priority. Note: API 7-1 monogram is held by the equipment OEM, not the forging supplier.
Valve bodies, stems, flanges per API 6A material requirements and NACE MR0175. Better low-temperature toughness. Note: API 6A monogram is held by the wellhead equipment manufacturer, not the forging supplier.
Tower flanges, nacelle rings, pitch bearing races. Dynamic fatigue loading at low temperatures makes Charpy toughness the priority. Weldability also required for tower fabrication joints.
Crusher shafts, eccentric sleeves, mill trunnions — heavy sections and impact/abrasion loads. Higher hardness and through-hardening depth of 4140 serve better at these section sizes.
Specify AISI 4135 for turbine shafts and impellers under 100 mm. AISI 4140 for shafts above 150 mm. Both are proven under sustained elevated-temperature fatigue loading.
Gear shafts, crankshafts, large connecting rods. Higher surface hardness after Q&T or nitriding is required. 4140 maintains core strength better after surface treatment at these section sizes.
Forged nozzles, flanges, closures per ASME Section VIII or PED 2014/68/EU. Better weldability simplifies WPS qualification and reduces post-weld heat treatment thermal cycles.
Kiln tyre rings, support roller shafts, vertical mill rollers. Continuous wear and sustained high static loads favor the higher hardness profile of 4140 at these large section sizes.
Forging Process: Grade Behavior Differences
From a hot-working standpoint, both grades behave similarly, but our process engineers account for several practical differences in production:
Hot-Working Temperature Window
Both grades are hot-forged in the range of 1,100–1,205 °C, with finish forging above 900 °C to maintain full austenite. The slightly higher carbon content of 4140 narrows the effective working window before surface scaling becomes aggressive, but this is manageable with controlled descaling practice in our facility.
Forging Reduction Ratio
We target a minimum 3:1 forging reduction ratio for both grades, typically 4:1 to 6:1, to break down the as-cast ingot structure and align grain flow with the primary stress axis of the finished part. For AISI 4140 in sections above 200 mm, we typically target 5:1 or 6:1 to ensure adequate center deformation — insufficient center working is the most common root cause of core property shortfall in large 4140 forgings.
Post-Forging Cooling Practice
Uncontrolled fast cooling of AISI 4140 after forging can produce bainitic or martensitic transformation products at the surface, creating residual stresses that cause cracking during subsequent machining. We sand bury or pit-cool all forgings above 80 mm until below 400 °C before transferring to the heat treatment furnace. AISI 4135 has somewhat lower sensitivity to surface transformation cracking, but we apply the same controlled cooling practice for both grades.
NDT and Quality Control for Both Grades
Our standard NDT scope applies equally to AISI 4135 and 4140 forgings. Every production batch includes all of the following:
- Ultrasonic Testing (UT) per ASTM A388 or EN 10228-3 — performed by ASNT Level II certified technicians. Phased-array UT available for critical or complex geometries.
- Magnetic Particle Inspection (MT) per ASTM E709 or EN 10228-1 — wet fluorescent method under UV light for surface and near-surface defect detection.
- Brinell Hardness Survey — mapped at defined probe points per agreed drawing, documented on the MTC.
- Tensile and Charpy Impact Testing — on heat-representative coupons at room temperature; optional testing at −20 °C or −40 °C on request.
- Optical Emission Spectrometry (OES) — chemistry confirmed on every heat from our in-house laboratory, independent of the steel mill certificate.
All test results are consolidated in an EN 10204 Type 3.1 Mill Test Certificate as standard. Type 3.2 third-party witness inspection is available at additional cost via SGS, Bureau Veritas, TÜV Rheinland, or Intertek. Jiangsu Liangyi Co., Limited holds ISO 9001:2015 certification. Export crating and preservation are included as standard for ocean freight shipments. Full inspection scope, acceptance criteria, and documentation details are listed on our AISI 4135 forging specifications page.
The Decision Framework: 6 Engineering Questions
When specifying between AISI 4135 and 4140, our metallurgical engineers use this six-question checklist:
- What is the critical section — the thickest load-bearing dimension? Below 100 mm → either grade performs comparably. Above 120 mm → lean toward AISI 4140 for through-hardening depth.
- Is welding required at any stage? If yes → AISI 4135 simplifies procedure qualification and reduces PWHT requirements. AISI 4140 requires mandatory preheat and controlled PWHT.
- What is the lowest operating temperature? Below 0 °C → AISI 4135's superior Charpy impact toughness is a strong selection argument.
- Is maximum surface hardness or wear resistance the primary load driver? Yes → AISI 4140. Or consider case-hardened grades (4320, 8620) for surface-critical wear applications.
- Are there NACE MR0175 / ISO 15156 sour service requirements? Both grades can comply. AISI 4135 achieves the required hardness limit (≤22 HRC for most sour service) more easily without maximum tempering.
- Does an existing engineering standard mandate a grade? Follow the applicable standard first — API 7-1 for drill string, ASME for pressure components, API 6A for wellhead material requirements.
Final Verdict: Side-by-Side Comparison
Section under 100 mm · welding required · low-temp toughness critical · wind power · pressure vessels · API 6A valve material · ASME fabrication · moderate strength sufficient
Section 100–250 mm · no welding needed · maximum UTS required · API 7-1 drill string material · heavy gear shafts · mining and crushing · cement plant equipment
Weldability (lower CE) · Charpy impact toughness · sub-zero performance · elongation and ductility · PWHT simplicity · NACE compliance ease · thin-section uniformity
UTS and yield strength · through-hardening depth in large sections · maximum surface hardness · wear resistance · heavy-section hardenability · large drill string section applications
Our AISI 4135 Forging Capability
Jiangsu Liangyi Co., Limited is an ISO 9001:2015 certified open-die forging manufacturer based in Jiangyin, Jiangsu Province, China, with 28 years of specialization in chromium-molybdenum alloy forgings. Our AISI 4135 product range spans forged round bars (Ø80–1,200 mm), step shafts up to 12,000 mm length, seamless rolled rings to OD 5,000 mm, and custom machined components from 30 kg to 30,000 kg per piece — all processed entirely in-house from billet inspection to final NDT and MTC documentation.
We also supply AISI 4140 forgings to the same quality standard when the application calls for it. Our metallurgical engineering team can review your drawings, advise on the optimal forging envelope and heat treatment curve for your section geometry, and provide a full quotation within 24 hours.
Full size range (Ø80–1,200 mm bars, OD 5,000 mm rings, 12,000 mm shafts), product forms, heat treatment conditions, mechanical property tables, and quotation — all on our product page.
AISI 4135 Forging Parts — Sizes, Specs & Quote →