6140
Chromium-Vanadium Tool Steel
Overview
6140 is a medium-carbon chromium-vanadium forging steel widely used in Taiwan's hand tool industry, with carbon 0.38–0.43%, chromium 0.50–0.90%, and vanadium 0.10–0.15%. The designation follows the SAE 61XX chromium-vanadium series numbering convention, but the composition is a mill-specific specification tailored for hand tool manufacturing — sulfur, phosphorus, and copper impurities are controlled far more strictly than standard structural steel (S ≤0.004%, P ≤0.035%). This cleanliness level directly serves the forging process: low sulfur reduces hot-shortness and extends fatigue life; vanadium microalloying pins austenite grain boundaries during hot forging, producing a fine-grained, tough microstructure. For wrenches, sockets, ratchets, and torque tools, 6140 is the first-choice material.
Double Steel has specialized in bar steel for over 30 years. 6140 is a permanent stock item sourced from Fengxing, CSC, and Weizhi mills, with a mill certificate (MTC) provided for every shipment.
Chemical Composition (Mill Specification)
| Element | Spec Range (%) |
|---|---|
| C (Carbon) | 0.38 – 0.43 |
| Si (Silicon) | 0.15 – 0.35 |
| Mn (Manganese) | 0.50 – 0.90 |
| P (Phosphorus) | ≤ 0.035 |
| S (Sulfur) | ≤ 0.004 |
| Ni (Nickel) | ≤ 0.25 |
| Cr (Chromium) | 0.50 – 0.90 |
| V (Vanadium) | 0.10 – 0.15 |
| Cu (Copper) | ≤ 0.035 |
6140 is not a JIS G 4053 or SAE J404 standard grade — it is a mill specification developed for the hand tool industry. Compared to SAE 6140 (Cr 0.70–0.90, V ≥0.15), the Cr and V ranges differ slightly, but sulfur and phosphorus impurity control is more stringent, reflecting the cleanliness requirements of hand tool forging.
Mechanical Properties After Heat Treatment (Reference)
| Condition | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|
| Normalizing | ≈ 650–750 | ≈ 400–500 | ≥ 17 | ≈ HB 180–220 |
| Q+T (Tempered at 450–550°C) | ≈ 950–1100 | ≈ 800–950 | ≥ 12 | ≈ HRC 30–36 |
| Q+T (Tempered at 350–450°C) | ≈ 1100–1300 | ≈ 950–1150 | ≥ 10 | ≈ HRC 38–44 |
| Q+T (Tempered at 200–300°C) | ≈ 1300–1500 | ≈ 1150–1350 | ≥ 8 | ≈ HRC 44–50 |
Values above are reference figures. Actual hardness is influenced by section size, post-forge microstructure, and tempering conditions. Finished hand tools typically target HRC 40–48, adjusted by tempering temperature to meet individual brand hardness specifications.
Forging & Heat Treatment Characteristics
Forging Temperature Window
Recommended hot forging temperature for 6140 is 1050–1200°C, with a finish-forging temperature no lower than 850°C. Vanadium suppresses austenite grain growth at forging temperatures — even with extended soak times, grain size remains fine. This is the core advantage of 6140 over plain carbon steel forging: post-forge microstructure uniformity directly determines fatigue life under repeated torque loading.
Post-Forge Heat Treatment Path
Hand tool forgings typically follow a "forge → air cool → quench and temper" sequence. Air cooling after forging produces a normalized structure (HB 180–220) with good machinability for subsequent turning, milling, and thread rolling. Final Q+T is performed after machining: austenitize at 830–860°C, oil quench; temper to target hardness — wrench heads typically HRC 42–46, socket walls HRC 38–44, ratchet teeth HRC 44–48.
Vanadium Strengthening Mechanism
V (0.10–0.15%) performs three functions in 6140: at forging temperature, VC precipitates pin grain boundaries and prevent grain growth; during Q+T, secondary carbides refine the martensitic structure; during tempering, vanadium retards softening and enhances secondary hardening. Collectively, this gives 6140 superior fatigue life at equivalent hardness compared to vanadium-free medium-carbon chromium steels — critical for tools subject to repeated torque-impact loading.
Low Sulfur and Tool Longevity
S ≤0.004% drastically reduces MnS inclusions — which act as fatigue crack initiation sites along the forging flow lines. For wrenches and ratchets subjected to tens of thousands of torque cycles daily, a clean matrix directly extends service life and reduces fracture risk. This is the key differentiator of 6140 over general structural steels like SAE 4140 or SCM440 for hand tool applications.
Machining Notes
Machinability
6140 in the normalized condition has hardness approximately HB 180–220 — similar to S45C but slightly harder than plain carbon steel. Forgings are typically near-net-shape, requiring only finish machining such as thread turning, mating surface milling, and drilling. Tool life is normal. Hand tool production lines typically complete all machining in the annealed-after-forge condition, with Q+T tempering as the final step.
Utilizing Forging Flow Lines
Hand tool design deliberately orients the forging flow lines to reinforce critical load-bearing surfaces — e.g., ratchet pawl bearing direction aligned with flow lines, wrench jaw contact surfaces extending along the forging direction. When selecting hot-rolled bar stock, diameter should be chosen to match the forging reduction ratio (typically 2–4×) to ensure continuous, complete flow line structure in the finished part.
Surface Treatment Compatibility
Common hand tool surface treatments — chrome plating, nickel plating, black oxide, and shot blasting — are all compatible with 6140. Acid pickling and post-plating hydrogen bake-out must be performed properly to prevent hydrogen embrittlement in high-hardness Q+T parts. For surface hardening, use induction hardening (localized to HRC 50+) or nitriding (CrV nitrides are stable, surface HV 600+) rather than carburizing.
International Standard Equivalents
| Standard | Equivalent Grade | Correspondence |
|---|---|---|
| Mill specification | 6140 | Reference standard |
| AISI/SAE (USA) | 6140 (J404) | Near equivalent (lower Cr minimum; narrower V range; stricter S control) |
| JIS (Japan) | SCr440 / SCM440 | Reference only (Cr-series; no vanadium) |
| DIN / EN (Germany/Europe) | 41CrS4 (1.7039) / 42CrMo4 (1.7225) | Reference only (Cr Q+T steel; no V microalloying) |
| GB (China) | 40CrV | Near equivalent (same Cr-V design concept; composition range varies) |
6140 has no single exact equivalent in any international standard system. Its combination of medium carbon + medium Cr + trace V + ultra-low sulfur is a hand-tool-industry-specific design. SAE 6140 is the closest by name but has different composition ranges; GB 40CrV is the closest in design philosophy. When specifying substitutes, Cr content, V content, and sulfur/phosphorus cleanliness level are the three mandatory verification points.
Typical Applications
Wrench-Type Hand Tools
Box-end wrenches, open-end wrenches, adjustable wrenches, and pipe wrenches — high-torque tool bodies — rely on 6140's Q+T hardness of HRC 42–46 combined with vanadium-enhanced fatigue life. This is the standard material specification for Taiwan's export hand tool brands.
Sockets & Driver Bits
Hex sockets, hex key bits, and power tool adapters — these tools experience torque impact combined with high-RPM power tool operation, placing extreme demands on torsional fatigue strength. The 6140 Cr-V combination provides ample performance margin.
Ratchet Mechanisms
Ratchet pawls, ratchet teeth, and drive squares — precision load-bearing components — are typically hardened to HRC 46–50 by induction hardening or full Q+T. Fine grain structure is the prerequisite for sustained engagement reliability.
Automotive & Engineering Forgings
Trailer hooks, connecting pins, high-strength bolts, forged claws, and chain link heads subject to repeated tension and torque. 6140's fatigue performance exceeds vanadium-free alloy steels, making it the recommended upgrade from SCM440 when fatigue life is the critical constraint.
Supply Specifications
| Form | Size Range | Source |
|---|---|---|
| Hot-rolled round bar | Ø 13 – 100 mm | Fengxing, Weizhi |
| Hot-rolled round bar | Ø 13 – 42 mm | CSC |
| Hot-rolled coil (wire rod) | Ø 5.5 – 42 mm | Common for tool forging |
| Ground bar | Ø 6 – 100 mm | — |
Length 5–12 m; cut-to-length available on request. Hand tool forging facilities typically use coil/wire rod for automatic cut-and-forge lines; straight bar is used for machined components. Every shipment includes a mill certificate (MTC) with full chemistry and heat number traceability.
Material Selection Guide
6140 is the standard material for hand tool forgings — when parts must withstand repeated torque cycling with defined fatigue life requirements, it offers the optimal balance between cost and performance. Vanadium microalloying is the key differentiator: at equivalent hardness, 6140 delivers longer fatigue life than vanadium-free Cr-Mo steels like SCM440.
6135
Higher toughness required; heavy impact loading
6135 has lower carbon (0.32–0.38%), giving a lower hardness ceiling but better toughness after Q+T. Suitable for heavy-duty tools subject to impact loading or larger cross-section forgings where core toughness is the primary concern.
SCM440
General structural parts; not hand tool specific
SCM440 is more broadly used in mechanical structural applications — Mo enhances hardenability. For hand tool forgings specifically, 6140's low sulfur control and vanadium microalloying are better matched to the application.
SCr440
More cost-sensitive; non-tool applications
SCr440 contains no vanadium and no ultra-low sulfur control, making it less expensive but with inferior fatigue life. Not recommended for hand tools subjected to sustained torque cycling.
For technical consultation, sample quotations, or mill certificate review, contact the Double Steel sales team via the inquiry form below or LINE customer service.