What Is 30CrMo Structural Alloy Steel Plate?
30CrMo is a medium-carbon chromium-molybdenum alloy structural steel plate defined by GB/T 3077, the Chinese standard for alloy structural steels. The designation records a nominal carbon content of 0.30%, about 1% chromium and roughly 0.2% molybdenum. That combination produces high strength and deep hardenability after quenching and tempering, together with a level of toughness that plain carbon structural steels cannot reach at the same strength level.
The chemistry of 30CrMo is closely comparable to AISI/SAE 4130, one of the most familiar chromium-molybdenum structural steels in engineering practice. Chromium increases hardenability and wears resistance, while molybdenum refines the grain and suppresses temper embrittlement, so heavy sections can be quenched to a uniform through-thickness structure.
The source article for this grade carried a nickel-bearing chemistry that belongs to a different alloy family. The limits below are the accepted 30CrMo composition under GB/T 3077 and are the figures to quote on a purchase order.
Chemical Composition of 30CrMo Plate
| Element | Content (%) |
|---|---|
| C | 0.26-0.34 |
| Si | 0.17-0.37 |
| Mn | 0.40-0.70 |
| Cr | 0.80-1.10 |
| Mo | 0.15-0.25 |
| Ni | 0.30 max |
| Cu | 0.30 max |
| P | 0.030 max |
| S | 0.030 max |
Carbon above 0.26% is required to reach the strength level expected of 30CrMo after tempering, while the chromium and molybdenum ranges are balanced so that hardenability is sufficient for sections of moderate thickness without becoming sensitive to cracking during quenching.
Mechanical Properties
Values below compare typical results for quenched and tempered plate with the requirements of GB/T 3077.
| Property | Typical quenched and tempered plate | GB/T 3077 requirement |
|---|---|---|
| Tensile strength Rm (MPa) | 980 | 930 min |
| Yield strength ReL (MPa) | 835 | 785 min |
| Reduction of area Z (%) | 55 | 45 min |
| Brinell hardness (HBW) | 269 | Reported |
The strength and hardness achieved in practice depend on the tempering temperature rather than on chemistry alone. Raising the tempering temperature lowers tensile strength and hardness while increasing ductility, so a 30CrMo plate can be tuned to either a high-strength or a tougher condition from the same heat.
Heat Treatment
30CrMo plate is austenitised at 850-880 °C and quenched in oil, then tempered between 540 °C and 680 °C. Tempering near the lower end of that range produces strengths around 980 MPa and hardness in the region of 270 HBW, which suits shafts and fasteners. Moving the tempering temperature towards 650-680 °C gives a softer, more ductile structure for parts that must absorb impact or be machined after heat treatment.
Heavy sections should be quenched with controlled agitation and tempered immediately after quenching to avoid the risk of delayed cracking. Where distortion must be kept small, as in long shafts and spindles, straightening is carried out after tempering at a temperature below the tempering temperature used previously.
Applications of 30CrMo Plate
Shafts, axles, spindles and rotors machined from plate
High-strength bolts, studs, nuts and fasteners for pressure equipment
Gears, pinions and transmission components
Drill collars, tool joints and downhole hardware for oil and gas service
Connecting rods, links and load-bearing parts in machinery
Turbine and power plant fittings requiring strength with moderate heat resistance
In the petrochemical and power industries the grade is used for pressure-retaining parts and piping components where higher strength than carbon steel is needed without moving to a stainless grade. In aerospace and transport engineering it serves fasteners and structural fittings where strength-to-weight ratio matters.
Fabrication and Welding Notes
The relatively high carbon and alloy content of 30CrMo gives it a carbon equivalent of roughly 0.6, so welding demands care. A preheat of 200-250 °C is normally applied, low-hydrogen consumables are essential, and post-weld heat treatment at 600-650 °C is used on critical joints to reduce hardness in the heat-affected zone. Where welding is not essential, bolted or machined connections avoid the issue entirely.
Machining is best done in the quenched and tempered condition. The steel has good machinability for an alloy grade when carbide tooling and appropriate cutting speeds are used, and it can be ground or surface treated when wear resistance is needed.
Corrosion performance is that of a low-alloy structural steel, so exposed parts are protected by painting, plating or coating rather than relying on the base metal itself.
Frequently Asked Questions
Q: Which standard covers 30CrMo plate?
A: GB/T 3077, the Chinese standard for alloy structural steels, defines the composition and mechanical requirements of 30CrMo.
Q: What is the equivalent grade in other systems?
A: 30CrMo is closely comparable to AISI/SAE 4130, although individual element ranges differ slightly, so equivalence should always be confirmed against the governing specification of the project.
Q: What strength can be expected after heat treatment?
A: Typical quenched and tempered plate reaches about 980 MPa tensile strength and 835 MPa yield strength, while GB/T 3077 sets minimum values of 930 MPa and 785 MPa.
Q: Why does the source chemistry show a nickel content of about 1.5%?
A: That composition belongs to a nickel-chromium-molybdenum grade and not to 30CrMo. Correct 30CrMo chemistry carries about 1% chromium with only residual nickel.
Q: Is 30CrMo suitable for welding?
A: It can be welded with preheat of 200-250 °C, low-hydrogen filler metal and post-weld heat treatment, but the carbon equivalent is high enough that welded joints need qualified procedures.
Q: What is the maximum section thickness for through hardening?
A: The chromium and molybdenum content allows uniform hardening in moderate sections; very heavy plate and large bars should be assessed against hardenability data before the heat treatment cycle is fixed.