4140 & 42CrMo Alloy Steel Bar Buying Guide: QT Condition, Sizes, and How to Specify Chrome-Moly Bars for Shafts and Machinery
4140 and 42CrMo are the chrome-moly alloy bars behind crankshafts, gears, spindles, hydraulic rods, and high-strength fasteners. They deliver roughly double the strength of 1045 carbon steel after quenching and tempering — but only if you order the right delivery condition, hardness, and test regime. Order annealed when you needed pre-hardened, or skip impact testing on a cold-climate shaft, and the part either costs an extra heat-treat cycle or fails in service.
This guide compares the grades, conditions, sizes, standards, and a complete RFQ checklist.
Grades Compared
| Grade | Standard | Key Alloy | Typical QT Strength | Use |
|---|---|---|---|---|
| 4140 | ASTM A29 / A322 | 1% Cr, 0.2% Mo | 850–1100 MPa | US shafts, gears, fasteners |
| 42CrMo / 42CrMo4 | GB 3077 / EN 10083 | 1% Cr, 0.2% Mo | 900–1200 MPa | EU/CN machinery, forgings |
| 4130 | ASTM A29 | Leaner Cr-Mo | 670–900 MPa | Welded structures, tubing |
| 4340 | ASTM A29 | Cr-Ni-Mo | 1100–1400 MPa | Heavy-duty, aerospace-grade duty |
4140 ≈ 42CrMo4 for most commercial purposes — state the standard your inspector will test against.
Delivery Condition: The Decision That Matters Most
| Condition | Hardness | Machines Like | Use |
|---|---|---|---|
| Annealed | ~200 HB | Easy | You heat-treat after machining |
| Normalized | ~250 HB | Moderate | Forgings, uniform grain |
| QT / pre-hardened (28–32 HRC) | 270–320 HB | Slower, stable | Finished shafts, no further HT |
| Peeled / turned + polished | Per condition | Ready stock | Hydraulic rods, close tolerance |
Sizes and Forms
- Round: 16–500 mm diameter; peeled 20–200 mm for shaft stock.
- Flat / square: tool and die blocks.
- Hollow / bored: hydraulic cylinder blanks.
- Forged rings / discs: gears and flanges.
Standards and Testing
- ASTM A29 / A322: US carbon/alloy bar grades and limits.
- EN 10083-3: European QT steels incl. 42CrMo4.
- GB/T 3077: Chinese alloy structural steel.
- Tests to request: UT ( flaws), Charpy impact (cold service), hardenability (Jominy) for critical shafts.
Common Applications
- Shafts, spindles, and axles.
- Gears, pinions, and couplings.
- Hydraulic piston rods (hard-chrome ready).
- High-strength bolts and studs (Class 10.9 base).
RFQ Checklist
- Grade + standard: 4140 / 42CrMo4 + ASTM / EN / GB
- Condition: annealed / normalized / QT + hardness
- Size: diameter / section × length
- Surface: black / peeled / polished / chrome-ready
- Tests: UT / Charpy / Jominy as needed
- Quantity, packing, MTC, destination / Incoterm
FAQ
4140 or 42CrMo? Same family — pick the standard your drawings and inspector use.
Pre-hardened or annealed? Pre-hardened QT for finished parts with no furnace; annealed when you heat-treat after machining.
Do I need UT? Yes for shafts over ~100 mm or any fatigue-critical part — internal flaws don’t show until the part breaks.
Why Source Alloy Bars from Yihang Metal
Yihang Metal supplies 4140 and 42CrMo/42CrMo4 bars in annealed, normalized, and QT conditions, black, peeled, or polished, to ASTM, EN, and GB standards with UT, impact, and hardenability testing on request, plus MTCs and third-party inspection. Standard sizes typically ship in 20–30 days.
Ready to specify your alloy bars? Send us the grade, condition, hardness, sizes, and tests, and we will confirm stock and lead time with a fixed-price quote and MTC. Email [email protected] or use the quote form on this page — our team replies within one business day.