Technical guide · 2026-08-03
Nickel-Chromium-Molybdenum Steel
Nickel-chromium-molybdenum steels use nickel for toughness, chromium and molybdenum for hardenability and temper resistance, enabling high strength through larger sections. Final performance depends on cleanliness, forging reduction, quench severity, temper and component geometry.
01
Definition and boundary
They are chosen for deep hardening and toughness, not simply as stronger 4140; machining, welding and cost consequences must be justified.
Reference information only: final acceptance is controlled by the ordered product specification, condition, dimensions and certified test results.
02
How the material system works
Alloy chemistry changes hardenability, tempering response, toughness or elevated-temperature stability.
Heat treatment develops the usable microstructure and must be tied to section size.
Cleanliness, segregation and surface condition influence fatigue and reliability.
03
Representative grades and systems
| Grade or system | Selection focus | Qualification |
|---|---|---|
| 4340 | SAE Ni-Cr-Mo grade for high-strength machinery and aerospace-type components. | Verify the ordered product specification, condition and dimensions before substitution. |
| 34CrNiMo6 | EN grade commonly compared with 4340 but controlled by different requirements. | Verify the ordered product specification, condition and dimensions before substitution. |
| 300M | Modified ultra-high-strength system requiring aerospace process control. | Verify the ordered product specification, condition and dimensions before substitution. |
04
Properties and trade-offs
Properties are condition- and section-specific rather than chemistry-only.
Higher hardenability or strength can reduce welding and machining margin.
Fatigue performance requires cleanliness and surface-integrity control.
05
Product forms and supply conditions
06
Applications and selection
Applications and selection
- Large shafts, gears and landing-gear-type parts
- High-strength fasteners and critical forgings
- Fatigue-critical heavy machinery components
Properties and trade-offs
- Define final component loads, life, temperature and failure mode.
- Select grade and hardenability for section and heat-treatment capability.
- Set inspection and process controls around the critical property.
07
Fabrication, durability and inspection
Match austenitizing, quench and temper or case-hardening procedure to section size and required microstructure.
Control decarburization, grain size, residual stress, straightness and distortion through the full route.
Qualify welding and repair separately; preheat, consumables and post-weld treatment depend on grade and restraint.
08
Procurement checklist
- Specify exact designation, product standard, melt practice, size and supply/heat-treatment condition.
- Define hardenability, cleanliness, grain size, decarburization, UT, hardness and mechanical tests as applicable.
- State machining allowance, surface class, straightness, traceability and final component heat treatment.
09
Standards, sources and review
Standards and designation context
- SAE designation/composition standards for SAE grades
- ASTM A29/A322 for applicable bar products
- EN/ISO/AMS or component standards for alternate systems and final acceptance