What the 4130, 4140 and 4145H Designations Mean
The four-digit designations 4130, 4140 and 4145H come from the AISI/SAE alloy numbering system, while the purchase specification behind them is normally ASTM A29/A29M for hot-wrought carbon and alloy bars or ASTM A322 for alloy steel bars, with hardenability limits taken from ASTM A304. All three are chromium-molybdenum steels, and that combination of roughly 1% chromium with about 0.2% molybdenum is what gives them deep hardenability, good fatigue resistance and a predictable response to quenching and tempering. The suffix H in 4145H marks a hardenability-controlled variant supplied quenched and tempered to a hardness band rather than to a single tensile level, which is how downhole tooling is bought and sold.
Bars are produced from 5 mm to 250 mm diameter and can be delivered as rolled, normalized, annealed or quenched and tempered, so an order line should always state the grade, the heat treatment condition, the hardness or tensile requirement, the diameter and the straightness class: two bars of identical 4140 chemistry can differ by nearly a factor of two in yield strength depending on how they were cooled and tempered.
Chemical Composition and Mechanical Properties
| Grade | C (%) | Mn (%) | Si (%) | Cr (%) | Mo (%) |
|---|---|---|---|---|---|
| 4130 | 0.28-0.33 | 0.40-0.60 | 0.15-0.35 | 0.80-1.10 | 0.15-0.25 |
| 4140 | 0.38-0.43 | 0.75-1.00 | 0.15-0.35 | 0.80-1.10 | 0.15-0.25 |
| 4145H | 0.42-0.49 | 0.75-1.00 | 0.15-0.35 | 0.75-1.25 | 0.15-0.25 |
| Grade | Condition | Yield, min (MPa) | Tensile (MPa) | Elongation (%) | Hardness |
|---|---|---|---|---|---|
| 4130 | Normalized | 435 | 670 min | 25 | about 197 HBW |
| 4140 | Quenched and tempered | 655 | 1020 min | 18 | about 300 HBW |
| 4145H | Quenched and tempered | hardness governed | hardness governed | 15-18 typical | 285-341 HBW |
Chemistry values are heat-analysis limits, and phosphorus and sulphur can be held to 0.035% or lower for improved impact toughness. The rising carbon level from 4130 through 4145H explains why the grades behave differently at the weld and in the quench tank: 4130 tolerates welding with modest precautions, 4140 needs preheat and controlled cooling, and 4145H in the hardened and tempered condition is normally machined rather than welded.
For 4145H the hardness band is the controlling requirement, and converting that band at roughly 3.4 MPa per HBW point gives an approximate tensile strength of 970-1160 MPa. Typical cycles are normalizing at 870-900 C for 4130, austenitizing at 840-860 C followed by oil quenching and tempering at 550-650 C for 4140, and a comparable route for 4145H with the tempering temperature selected to land inside the specified hardness window.
Size Range, Manufacturing Route and Straightness
| Route | Diameter range | Surface condition | Straightness |
|---|---|---|---|
| Cold drawn | 5-120 mm | Bright, h9-h11 | 1 mm/m or better |
| Peeled or turned | 16-180 mm | Bright, decarburization removed | about 1 mm/m |
| Hot rolled | 16-250 mm | As-rolled or descaled | up to 3 mm/m |
| Forged | 180-400 mm | As-forged or rough turned | to order |
Standard mill lengths are 3 m to 6 m with saw-cut ends, and bars can be cut to a fixed length with a tolerance of 0 to +25 mm. Cold-drawn and peeled stock is chosen when the part goes straight onto a CNC lathe, while hot-rolled and forged bars suit larger diameter shafts, rings and rolled products that will be machined after heat treatment.
Where Each Grade Is Used
4130 - normalized or quenched and tempered bar and tube for aircraft and motorsport structures, bicycle frames, shafting and machine parts where strength has to combine with weldability.
4140 - gears, pinions, shafts, axles, connecting rods, bolts and studs, hydraulic piston rods, mould bases and tool holders; the workhorse of the three grades.
4145H - drill collars, stabilizers, subs and other downhole tools that operate under high torque, vibration and shock loading.
Equivalent designations in other systems are the chromium-molybdenum grades of GB/T 3077 such as 30CrMo, 35CrMo and 42CrMo, and the quenched and tempered alloy steel classes of ISO 683-1. Those cross references matter when a drawing quotes a European or Chinese designation while the mill is rolling to an ASTM chemistry, because the specified carbon and manganese windows still have to be reconciled before the heat is released.
Welding, Quality Control and Delivery
Preheat 200-300 C for 4130 and 4140 sections above roughly 25 mm, and use low-hydrogen electrodes or a matching low-alloy filler.
Control interpass temperature and stress relieve at 600-650 C after welding to restore toughness in the heat-affected zone.
4145H at 285-341 HBW is machined with carbide tooling; welding is not recommended in the hardened and tempered condition.
Machinability is best in the annealed or normalized condition, with 4140 around 200-230 HBW giving good chip control on automatic lathes.
Each heat is verified by spectrographic analysis before rolling, and finished bars are released with tensile testing to ASTM A370, end-quench hardenability data where the H designation applies, grain size determination to ASTM E112, and ultrasonic inspection to ASTM A388 on forged product. Dimensional checks cover diameter, ovality, length and straightness, and the mill test certificate is traceable to the heat number stamped on every bundle. Bars ship in 2-3 tonne bundles secured with steel strip, ends protected against handling damage and waterproof paper applied under the strapping, with a standard lead time of 7-15 days depending on diameter, condition and quantity.
Frequently Asked Questions
Q: What is the difference between 4130 and 4140 round bar?
4130 carries 0.28-0.33% carbon and 4140 carries 0.38-0.43%, so 4140 hardens more deeply and reaches higher strength after quenching and tempering, while 4130 is easier to weld and is common in tubular frames and motorsport structures.
Q: Is 4145H the same as 4145?
The base chemistry is the same family, but the H suffix means the hardenability limits are guaranteed and the bar is supplied quenched and tempered to a hardness band, typically 285-341 HBW for drill collars and similar downhole tools.
Q: What diameter range can be supplied?
Cold drawn bar covers 5-120 mm, peeled bar 16-180 mm, hot-rolled bar 16-250 mm and forged bar up to 400 mm, all in lengths from 3 m to 6 m or cut to a fixed length.
Q: How are the bars tested before shipment?
Each heat is chemically analysed, then sampled for tensile properties to ASTM A370, checked for hardenability where the H suffix applies, measured for grain size to ASTM E112, and inspected ultrasonically to ASTM A388 on forged material.
Q: Can these grades be welded?
4130 and 4140 can be welded with preheat of 200-300 C, low-hydrogen consumables, controlled interpass temperature and post-weld stress relief; 4145H in the hardened and tempered condition is not intended for welding.
Q: What hardness can be achieved after heat treatment?
4140 can reach roughly 55 HRC as quenched, but service hardness is normally specified between 28 and 40 HRC so that a balance of strength, ductility and impact toughness is retained.