Alloy Steel Selection for Critical Forgings

Alloy Steel Selection for Critical Forgings: EN24, 4340, 4130, 300M, and CrMoV

August 24, 2026
Alloy Steel Selection for Critical Forgings
Alloy Steel Selection for Critical Forgings

Selecting the correct alloy steel for a critical forging — where failure carries safety consequences in aerospace, defence, nuclear, marine, or oil and gas service — requires understanding the relationship between alloy composition, heat treatment, achievable mechanical properties, and the specific failure modes the component must resist. This technical guide compares the most widely used critical-industry alloy steels: EN24 (817M40), 4340, 4130, 4130M, CrMoV (30CrMoV9), P91, 300M, and 55NiCrMoV6 — explaining when each is appropriate and when substitution is acceptable or unacceptable.

Steel GradeUNS / DINTypical UTS (Q&T)Primary Application




EN24 (817M40)		UK BS970 / 817M40
850–1,050 MPa		Marine propulsion shafts, mining equipment shafts
4340 (AMS 6415)
G43400		980–1,250 MPaAerospace structural, defence components, open die shafts
4130 (AMS 6345)G41300550–860 MPaOil and gas valve bodies, general structural, API 20B
4130M (NACE sour)G41300 modified550–620 MPa (≤22 HRC)Sour service valve bodies, NACE MR0175 compliance
CrMoV (30CrMoV9)EN 10269 / 1.7701750–950 MPaSteam and gas turbine rotor discs and shafts
P91 (Grade 91)9Cr-1Mo-V-Nb585–760 MPaSupercritical boiler nozzles, main steam piping
300M (AMS 6257)K442201,930–2,100 MPaLanding gear, aircraft structural ultra-high-strength
55NiCrMoV6DIN 1.2745 VAR1,000–1,200 MPaArtillery barrel blanks, gun components

EN24 (British Standard 817M40, containing 1.5% Ni, 1.2% Cr, 0.25% Mo) is the most widely used alloy steel for open die shaft forgings in marine, mining, and general heavy industrial applications. EN24 achieves 850–1,050 MPa tensile strength after Q&T — adequate for most propulsion shaft, crusher shaft, and conveyor shaft applications. Its 1.5% nickel content provides adequate toughness at ambient and low temperatures (Charpy typically 40–80 J at -20°C after standard Q&T). EN24 is produced by Indian steel mills (SAIL, Tata Steel, Shyam Steel) in billet form at competitive cost — making it the most economical choice for large-section Indian open die shaft forgings where the higher strength of 4340 is not required.

4340 (SAE/AMS 6415, containing 1.8% Ni, 0.80% Cr, 0.25% Mo) is the primary aerospace and defence structural alloy steel — it achieves 980–1,250 MPa tensile strength after Q&T, with adequate toughness in the 1,100 MPa range (Charpy 40–60 J at -20°C). AMS 6415 specifies 4340 with cleanliness requirements (magnetic particle inspection of billet, restricted sulphur and phosphorus) for aerospace applications. 4340 can be substituted for EN24 in applications requiring higher tensile strength — but EN24 cannot be substituted for 4340 in aerospace structural applications where AMS 6415 is specified. The key distinctions: 4340 has tighter chemistry control and cleanliness requirements; EN24 is a commercial standard with wider chemistry bands and no cleanliness specification.

4130 (SAE 4130, containing 0.28–0.33% C, 0.80–1.10% Cr, 0.15–0.25% Mo) is the workhorse alloy steel for API 20B oil and gas valve body forgings — providing adequate strength (550–860 MPa Q&T depending on section size and temper temperature) with good weldability (the lower carbon content compared to 4340 makes PWHT after welding simpler). API 20B requires 4130 valve body forgings in the annealed or normalised-and-tempered condition with hardness ≤22 HRC (approximately 230 HBW) for NACE MR0175 sour service compliance — this is the 4130M (sour-service modified) condition, where the maximum hardness requirement constrains the heat treatment to a higher tempering temperature that reduces strength but ensures NACE compliance.

CrMoV (30CrMoV9, EN 10269 Grade 30CrMoV9) is the standard alloy steel for steam and gas turbine rotor disc and shaft forgings — its 9% chromium content provides oxidation resistance and creep resistance at temperatures up to 550°C, while molybdenum and vanadium precipitation hardening maintains strength in the elevated temperature creep range. CrMoV is not interchangeable with 4340 or EN24 for structural applications — it is a specific power generation alloy with the elevated temperature properties that general engineering alloys cannot provide. P91 (Grade 91, 9Cr-1Mo-V-Nb) extends the CrMoV approach to supercritical steam conditions above 550°C — with niobium addition for additional precipitation hardening.

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Vinir Capability

All standard critical-industry alloy steels in inventory and production capability: EN24 (817M40) open die shaft forgings to 15,000 kg; 4340 (AMS 6415) closed die and open die forgings 1–5,000 kg; 4130 and 4130M NACE MR0175-compliant valve body and structural forgings; CrMoV (30CrMoV9) power generation disc and shaft forgings with EN 10269 and ASTM A470 Class 8 dual certification; P91 (SA-182 F91) open die forgings for supercritical steam applications; 300M (AMS 6257/AMS 6417) closed die forgings for ultra-high-strength aerospace applications; 55NiCrMoV6 VAR open die gun barrel blanks. Material-specific NABL testing for all grades. AMS 2750-calibrated heat treatment for all aerospace and defence alloys.

Frequently Asked Questions

1.When should a buyer specify 4340 (AMS 6415) instead of EN24 (817M40) for a structural shaft forging?+–
Specify 4340 (AMS 6415) when: the component is in an aerospace or defence application where the material specification is mandated by the design authority (Boeing, Airbus, Lockheed); the component requires tensile strength above 1,050 MPa (EN24’s typical upper limit); cleanliness requirements are specified (AMS 6415 requires magnetic particle inspection of billet and restricted inclusion content); or the component will be fracture-mechanically analysed using material data generated from AMS 6415-compliant material. Specify EN24 (817M40) when: the application is marine, mining, or general industrial where commercial BS970 specification is acceptable; the tensile strength requirement is below 1,050 MPa; no cleanliness specification is mandated; and cost-effective domestic Indian billet sourcing is a priority (EN24 billet is available from Indian mills; AMS 6415 4340 billet must be imported from US or European mills).
2.What is the NACE MR0175 hardness restriction for 4130M and how does it affect the achievable mechanical properties?+–
NACE MR0175/ISO 15156 restricts the maximum hardness of carbon and low alloy steels in sour service (H₂S-containing environments) to 22 HRC maximum (approximately 237 HBW or 250 HV). For 4130 alloy steel, achieving ≤22 HRC requires tempering at 620–680°C — a high tempering temperature that reduces the strength. At 22 HRC maximum hardness, 4130 has a tensile strength of approximately 550–620 MPa and yield strength of approximately 420–500 MPa — significantly lower than its full Q&T capability (860 MPa tensile at 30 HRC). This is why sour service valve bodies and pressure fittings specified to NACE MR0175 use heavier wall sections than equivalent non-sour service components — the thicker walls compensate for the lower allowable design stress.
3.What is the difference between P91 and P22 (2.25Cr-1Mo) for supercritical boiler nozzle forgings?+–
P22 (2.25Cr-1Mo, SA-335 P22 / SA-182 F22) is the standard alloy steel for subcritical and early supercritical steam applications — adequate up to approximately 540°C. Above 540°C, P22’s creep resistance drops sharply and the component wall thickness required to provide adequate creep life becomes impractical. P91 (Grade 91, 9Cr-1Mo-V-Nb, SA-335 P91 / SA-182 F91) provides creep resistance adequate up to 620°C — the addition of niobium (precipitation hardening as NbC and NbN) provides the elevated temperature strength that P22 cannot achieve. For supercritical power plants operating at 580–620°C steam temperature (Indian NTPC supercritical fleet), P91 is the mandatory material for main steam piping, valve bodies, and boiler header nozzle forgings. P91 cannot be substituted with P22 for above-540°C applications — and P22 need not be upgraded to P91 for below-540°C applications where P22’s properties are adequate.
4.What is the CrMoV alloy steel family and which turbine OEMs specify it?+–
CrMoV (Chromium-Molybdenum-Vanadium) is a family of elevated-temperature alloy steels used for steam and gas turbine rotor discs and shafts. The primary grade in Europe is 30CrMoV9 (EN 10269 Grade 30CrMoV9 / DIN 1.7701) — approximately 0.30% C, 9% Cr, 0.6% Mo, 0.2% V. The US equivalent is ASTM A470 Class 8 (approximately equivalent chemistry). This alloy provides adequate creep resistance up to 540–550°C and is standard for turbine rotor shafts and compressor discs in the lower-temperature sections of power generation gas turbines and steam turbines. Siemens Energy specifies EN 10269 Grade 30CrMoV9; GE Vernova uses ASTM A470 Class 8; Mitsubishi Power specifies JIS G4107 (approximately equivalent). Indian BHEL specifies IS 2004 (Carbon and Low Alloy Steel Forgings for General Industrial Use) which covers an equivalent grade.
5.Can 4130 be welded without post-weld heat treatment in oil and gas valve body applications?+–
4130 alloy steel can be welded without post-weld heat treatment (PWHT) for valve body assembly when: the carbon equivalent (CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15) is below 0.40, the section thickness at the weld is below 25mm, and the service conditions do not require PWHT (no NACE MR0175 constraint that would make PWHT necessary for hardness reduction). At higher carbon equivalent values or thicker sections, hydrogen-induced cold cracking risk increases and PWHT becomes necessary. For sour service API 20B valve bodies in 4130M, PWHT is typically mandatory (to achieve ≤22 HRC maximum hardness through the weld heat-affected zone, not just in the base material). The weld qualification procedure for any 4130 valve body welding must include Charpy impact testing of the HAZ specimens — HAZ toughness is typically lower than base material toughness and must be verified to meet the service temperature requirements.