AISI 4135 (SAE 4135 / UNS G41350) Alloy Steel Forging Parts

AISI 4135 Forged Products Gallery

AISI 4135 (UNS G41350) Alloy Steel Chemical Composition

The composition of AISI 4135 alloy steel is precisely defined by the AISI/SAE standard to achieve a specific balance of hardenability, strength, and toughness in forged and heat-treated condition. Unlike commodity carbon steels, every element in 4135 performs a specific metallurgical function. Knowing the function of each alloying element allows engineers to predict the behavior of the material in service and choose the heat treatment parameters according to their section size and application needs. Following is the full chemical composition per AISI/SAE specification :

Every incoming heat of AISI 4135 billet is verified against the steel mill's chemical test certificate using our in-house optical emission spectrometer (OES), and a re-analysis is performed on a sample from the finished forging to confirm composition integrity through the forging process. Both sets of chemical data are reported on our EN 10204 3.1 Mill Test Certificate. For customers with tighter compositional requirements — for example, P ≤ 0.020% and S ≤ 0.020% for subsea or low-temperature service — we can specify a narrowed chemistry range with our qualified steel mill suppliers at the time of material ordering.

AISI 4135 International Grade Equivalents & Cross-Reference

AISI 4135 (SAE 4135, UNS G41350) is a globally recognized chromium-molybdenum alloy steel grade. The following table shows the closest equivalent designations across the main international standards, to help engineers worldwide to specify the right material for their projects:

* Grade equivalents are approximate based on chemical composition similarity. Always verify final suitability against your project specification, relevant standard, and our MTC data before ordering.

Heat Treatment Process for AISI 4135 Forging Parts

Heat treatment is not a post-processing step for AISI 4135 forgings — it is an integral part of the manufacturing engineering that determines the final mechanical properties of your component. The same forged piece of 4135 can be delivered as-forged (soft, easily machinable), annealed (fully recrystallized, ~180 HB), normalized (fine-grained, ~200 HB), or quenched-and-tempered (high strength, 28–40 HRC depending on tempering temperature). Selecting the correct heat treatment condition for your application — and executing it with tight process control — is where our 28 years of experience translates directly into parts that perform consistently in service.

All heat treatment at Jiangsu Liangyi is performed in our in-house, calibrated furnaces equipped with programmable multi-zone temperature controllers, chart recorders, and thermocouple validation. Every heat treatment batch is accompanied by a heat treatment record (HTR) document that forms part of our EN 10204 3.1 MTC package. Our standard AISI 4135 heat treatment process parameters are as follows:

Stage 1: Hot Forging Temperature Control

Hot forging of AISI 4135 begins with billet heating to a maximum furnace setpoint of 1205°C (2200°F) — the upper safe limit before incipient melting of low-melting-point phases at prior austenite grain boundaries. Forging is completed above the austenite-to-ferrite transformation temperature (approximately 900°C / 1650°F for 4135) to avoid deformation-induced ferrite formation, which would create a duplex, non-uniform microstructure. The minimum forging finish temperature is controlled to ensure that any phase transformation occurring during final forging passes takes place in the full austenite field, producing the equiaxed, fine-grained structure that responds predictably to subsequent heat treatment. Forging reduction ratios of at least 3:1 (and typically 4:1 to 6:1) are maintained to ensure adequate grain refinement and closure of any residual solidification porosity from the original ingot.

Stage 2: Post-Forge Annealing or Normalizing

After forging and initial air cooling, most AISI 4135 forgings receive either a full anneal (790–845°C / 1454–1553°F, furnace-cooled at a controlled rate of ≤15°C/hour through the eutectoid range to prevent pearlite coarsening) or a normalizing treatment (870–900°C / 1598–1652°F, air-cooled). Annealing produces the softest, most machinable condition (typically 163–229 HB) and is the standard delivery condition for customers who perform their own final machining and heat treatment. Normalizing produces a finer, more uniform pearlitic microstructure (typically 170–220 HB) with higher strength than fully annealed, and is often specified as the delivery condition for rough-machined blanks that will receive final Q&T by the customer. Both treatments also relieve forging residual stresses, preventing dimensional movement during subsequent machining operations.

Stage 3: Hardening (Austenitizing + Quenching)

For customers who need quenched-and-tempered (Q&T) forgings, the hardening process begins with austenitizing at 845–870°C (1553–1598°F) for a soak time calculated as 1 hour minimum plus approximately 1 hour per 25mm of ruling section — guaranteeing full, uniform austenite formation throughout the cross-section before quenching. Quenching medium for AISI 4135 is typically oil (or polymer quenchant at controlled concentration and temperature), selected to provide sufficient cooling rate to achieve martensite transformation throughout the ruling section while avoiding the thermal shock of water quenching that would risk quench cracking in heavier sections. The Grossman hardenability of 4135 (DI ≈ 100–140mm) means that through-hardening to a martensitic microstructure can be reliably achieved in sections up to approximately 75–100mm diameter after oil quenching — a significant advantage over plain carbon steels.

Stage 4: Tempering

Quenched 4135 in the as-quenched condition has very high hardness (typically 54–58 HRC) but dangerously low toughness — it is too brittle for engineering use. Tempering is the critical final step that converts the as-quenched martensite into tempered martensite, simultaneously reducing hardness (and thus brittleness) and increasing ductility and impact energy to engineering-useful levels. The tempering temperature is the primary variable controlling final mechanical properties:

• High tempering temperature (580–680°C): Produces a lower tensile strength (750–900 N/mm²) with high ductility and impact energy — the standard specification for heavily loaded structural components, shaft forgings, and components requiring low-temperature Charpy impact certification (such as API 6A PSL 2/3 wellhead forgings tested at −60°F)
• Medium tempering temperature (450–580°C): Achieves intermediate tensile strength (900–1100 N/mm²) with good toughness — typical for general industrial gear shaft forgings, gearbox components, and mining equipment parts
• Lower tempering temperature (250–400°C): Produces maximum hardness and strength (1100–1200 N/mm²) with acceptable but reduced toughness — used for components requiring high surface hardness before final machining, or where the application load is predominantly compressive rather than impact-tensile

A critical process control point: AISI 4135 contains sufficient molybdenum (0.15–0.25%) to suppress temper embrittlement, but all tempering cycles are designed to either avoid the embrittlement range (375–575°C) or cool rapidly through it after tempering at higher temperatures. This is a process discipline that distinguishes properly manufactured 4135 forgings from lower-quality alternatives where tempering protocols are not rigorously controlled. All tempering is performed in calibrated furnaces with chart-recorded temperature verification and documented soak times calculated from part ruling section.

Supplementary Heat Treatments Available

• Stress relieving: 550–600°C furnace soak followed by slow cooling — performed after rough machining on precision components to eliminate machining-induced stresses before final cuts
• Induction hardening: Selective surface hardening of gear tooth flanks, journal surfaces, or cam profiles to 55–62 HRC while maintaining the tough Q&T core structure — available as a combined service with our machining operations for pinion shafts and gear shaft blanks
• Nitriding (gas or plasma): Available for AISI 4135 components requiring case depths of 0.2–0.6mm to improve surface hardness (650–750 HV) and fatigue performance without the quenching distortion of induction hardening

Mechanical Properties of AISI 4135 Forged Steel

The mechanical properties of quench-and-tempered AISI 4135 forgings are section-size dependent — a fundamental characteristic of any hardenable alloy steel that reflects the effect of cooling rate on microstructure throughout large cross-sections. In smaller sections (≤ 16mm), the surface and core cool at similar rates, producing a uniformly martensitic microstructure throughout the section and the highest achievable strength values. In larger sections (100–250mm), the core cooling rate is significantly slower than the surface, producing mixed microstructures (martensite + bainite + ferrite-pearlite at the core in very large sections) with progressively lower strength and higher ductility. This section-size effect is why AISI 4135 forgings used in large structural components — for example, a 500mm diameter crusher eccentric shaft — are designed with safety factors that reflect core, not surface, mechanical properties.

The following table presents typical mechanical properties for AISI 4135 forged parts after standard quench and temper heat treatment, per DIN EN 10083-3 (34CrMo4 / 1.7220) as the reference standard. Actual guaranteed values are confirmed on the EN 10204 3.1 MTC accompanying each delivery. ASTM A322 / SAE J1397 equivalent properties are also available on request for North American procurement specifications:

Engineering notes on using this data:

• These values are for standard Q&T condition with a high tempering temperature (approx. 580–650°C). Lower tempering temperatures can achieve higher tensile strength (up to 1,300 N/mm² in small sections) at reduced impact energy — we will specify the optimal tempering condition for your application on request.
• Test specimens for mechanical property verification are taken from the sacrificial test extension forged integrally with the part, at 1/4 T depth from the surface for ring and disc forgings, or from the center of a separately forged representative coupon for shaft sections larger than 250mm ruling section.
• For applications requiring low-temperature impact certification (e.g., API 6A PSL 3 at −60°F / −51°C, or Charpy testing at −20°C for subsea structural components), we can design a heat treatment cycle and test protocol to achieve 27 J minimum CVN at the specified test temperature — please specify this requirement at the time of enquiry.
• Hardness values (HB) listed above are approximate conversions from tensile strength using the Brinell hardness correlation in EN ISO 18265. Actual hardness is verified on each forging by Brinell hardness testing at a minimum of 3 points per piece, reported on the MTC.

Physical Properties of AISI 4135 Steel

The following table lists the key physical properties of AISI 4135 chromium-molybdenum alloy steel in the annealed or normalized condition. These properties are important for thermal analysis, stress modeling, and design calculations:

AISI 4135 vs. AISI 4140 vs. AISI 4130 — Grade Comparison

The choice of the proper grade of chromium-molybdenum steel is important for performance and cost. The table below shows how AISI 4135 compares with closely related AISI 4140 and AISI 4130 grades in main parameters:

Summary: AISI 4135 is the optimal balance between the lighter 4130 (better weldability, lower strength) and the heavier 4140 (higher strength, less toughness). It is the best choice material when both high fatigue resistance and good impact toughness are required in thick-section forgings — for example, gear shafts, drilling components, and heavy machinery drive parts.

Strict Quality Control for AISI 4135 Forged Parts

Quality at Jiangsu Liangyi is not a department — it is a manufacturing discipline built into every step of the production sequence, from raw material choice through final dimension test before packing. For AISI 4135 forged parts, where the mechanical property specification is entirely dependent on the quality of the forging process and heat treatment execution, this integrated quality approach is essential. The following describes our actual quality control sequence, the specific tests performed, the equipment used, and the documentation generated at each stage:

Stage 1 — Raw Material Incoming Inspection

All AISI 4135 steel billet and ingot material is sourced from qualified steel mill suppliers that operate EAF + LF + VD (electric arc furnace + ladle refining + vacuum degassing) production routes. We review and retain the steel mill's original chemical test certificate for every incoming heat. Each heat is then re-verified using our in-house optical emission spectrometer (OES) — any heat with chemical composition outside AISI/SAE specification is rejected before entering the forging process. Incoming billet is checked visually and dimensionally for surface condition, straightness and cross section size conformance. For new supplier qualification, macro-etch tests on billet cross-sections are carried out to confirm the absence of centre segregation, pipe and excessive inclusions. 

Stage 2 — Forging Process Monitoring

Every furnace batch is monitored with calibrated digital pyrometers to track forging temperatures in real time. For multi-pass open-die forging processes, press load and stroke data are fully recorded. The forging ratio (cross-section reduction) of each part is calculated and documented to meet the minimum reduction requirement, generally 3:1.During ring rolling, laser measuring systems continuously monitor outer diameter, inner diameter and height dimensions. This ensures the dimensional accuracy of ring blanks right after rolling and before cooling down.

Stage 3 — Heat Treatment Records

All heat treatment cycles are executed in programmed, calibrated furnaces with continuous chart recording. The heat treatment record (HTR) document captures: furnace ID number, batch identification, loading date/time, temperature setpoints vs. actual temperature (with deviation alerts), soak time, quench medium temperature, quench duration, and tempering parameters. Furnace calibration certificates (traceable to national standards) are kept and renewed annually. This HTR document is a mandatory attachment to our EN 10204 3.1 MTC for all Q&T deliveries.

Stage 4 — Chemical Composition Re-verification

A sample cut from each finished forging (or from a test ring forged from the same heat simultaneously) is re-analyzed by OES spectrometry to confirm that no composition drift occurred during the forging process (possible if, for example, decarburization is suspected). Results are compared to the steelmaker's mill cert and both sets of chemical data are reported on the MTC, providing dual-source chemical traceability.

Stage 5 — Mechanical Properties Testing

Tensile testing (yield strength Rp0.2, ultimate tensile strength Rm, elongation A5, reduction of area Z) is performed on standard round test specimens per ISO 6892-1 (equivalent to ASTM E8), machined from test material taken at ¼T depth from the forging surface (for discs and rings) or from the center of the forging (for solid bars and shafts in the ruling section range). Charpy V-notch impact testing (ISO 148-1 / ASTM E23) at 20°C is standard; low-temperature testing (−20°C, −40°C, −60°F / −51°C) is available on request. Hardness testing (Brinell, HBW per ISO 6506 / ASTM E10) is performed on each forging piece at a minimum of three surface locations, with results reported per piece on the MTC.

Stage 6 — Ultrasonic Testing (UT)

All AISI 4135 forgings above a defined size threshold (typically > 50kg, or as specified by the customer) undergo full volumetric ultrasonic testing per EN 10228-3 (Class 3 or 4 as standard; Class 5 for critical applications) or ASTM A388 using calibrated UT equipment with straight-beam and angle-beam probes. Forgings are scanned on 100% of accessible surfaces with a scan index ≤ half the transducer diameter. Any indication exceeding the specified rejection level (typically equivalent to a 2mm flat-bottom hole reflector at the test frequency and gain used) results in rejection or rework of the affected area. UT operators hold valid qualification certificates to the relevant NDT personnel certification standard.

Stage 7 — Surface NDT (MT / PT)

For forgings in the as-machined or rough-turned condition, magnetic particle testing (MT, per EN ISO 17638 or ASTM E709) is performed on all accessible surfaces to detect surface and near-surface discontinuities — forging laps, seams, quench cracks, or grinding burns. For non-magnetic or austenitic applications (rare for 4135 but applicable to cladded components), liquid penetrant testing (PT, per EN ISO 3452-1 or ASTM E165) is used as an alternative. MT results are typically to acceptance level ASME VIII Div. 1 App. 6, or the customer's specification as applicable.

Stage 8 — Dimensional Inspection

Full dimensional verification is performed by our metrology team using calibrated instruments: vernier calipers, micrometers, bore gauges, digital height gauges, and coordinate measuring machine (CMM) for complex geometries. All important dimensions (bearing seat diameters, bore diameters, shoulder positions, runout/concentricity, surface finish Ra values) are measured and recorded on the dimensional inspection report (DIR), which accompanies the MTC in the final documentation package.

Final Documentation Package

Each shipment of AISI 4135 forgings from Jiangsu Liangyi includes: EN 10204 3.1 Mill Test Certificate (or 3.2 with third-party witness inspection by SGS, BV, TÜV, or Intertek at additional cost), heat treatment records, dimensional inspection report, UT test report, MT/PT test report (if applicable), packing list with piece marking cross-reference, and a copy of our ISO 9001:2015 certification. All documentation is available in English, and in Chinese upon request. Electronic document sets (PDF) are transmitted by email prior to or with shipment.

Learn more about our advanced production and inspection equipment — including our hydraulic press capacities, ring rolling equipment, heat treatment furnace inventory, and NDT instrumentation.

Contact Us for Custom AISI 4135 Forging Solutions

As a professional AISI 4135 forging manufacturer in China with 28 years of focused experience, Jiangsu Liangyi is equipped to handle your enquiry from first contact through to delivery — with technical depth, transparent pricing, and documented quality at every step. Whether you are an engineering procurement manager sourcing a single prototype forging, a maintenance team needing an urgent replacement part, or an OEM requiring a stable long-term supply of AISI 4135 alloy steel forged components, our sales and engineering team will respond with specificity and speed.

How to Get the Most Accurate Quotation Quickly

To provide you with the most accurate quotation within 24 working hours, please include as much of the following information as possible in your initial enquiry:

• Drawing or sketch: PDF, DWG, STEP, or IGES preferred. If no drawing is available, please provide approximate dimensions (OD × ID × Length or OD × Height for rings, OD × Length for bars/shafts)
• Material specification: Confirm AISI 4135 / SAE 4135 / DIN 34CrMo4 / EN 1.7220, or specify UNS G41350. If you have a customer specification document, please attach it.
• Heat treatment condition: As-forged, annealed, normalized, or Q&T — and if Q&T, your required hardness range (HB or HRC) or tensile strength / yield strength minimum values
• Delivery condition: Rough forged, rough-turned (with machining allowance), semi-machined, or fully machined to drawing. If fully machined, specify surface finish requirements (Ra value) for critical surfaces
• Inspection requirements: Standard EN 10204 3.1 MTC, or EN 10204 3.2 with specific third-party inspection body (SGS, BV, TÜV, Intertek, or your representative). UT class, MT/PT scope if required. Charpy impact temperature if other than 20°C.
• Quantity: Number of pieces per order, and whether this is a one-time prototype or a repeat production requirement
• Required delivery date: Your target delivery date or project schedule milestone. If urgent, flag this clearly — we will assess press schedule availability and confirm the earliest possible delivery date honestly.

You do not need to have all of this information ready before contacting us. Even a rough sketch and a basic description of your application is enough for us to start a technical conversation. Our engineering team is accustomed to working from preliminary information and refining specifications collaboratively.