Technical guide · 2026-08-21
Chromium-Molybdenum Steel
Chromium-molybdenum steels include machinery grades selected for hardenability and temper response as well as separate pressure/high-temperature product families. The shared alloy elements do not create one interchangeable material class.
01
Definition and boundary
Separate 4130/4140 machinery steel from T/P and pressure-grade systems by product standard, service code, heat treatment and inspection.
Reference information only: final acceptance is controlled by the ordered product specification, condition, dimensions and certified test results.
02
How the material system works
Chromium raises hardenability while molybdenum supports temper resistance in machinery grades.
Carbon level separates more weldable 4130-type choices from higher-hardness 4140-type machinery duty.
Section diameter and quench severity determine the core structure that a chemistry can actually develop.
Creep-strengthened pressure grades use dedicated chemistry, product and code controls rather than a machinery-grade shortcut.
03
Representative grades and systems
| Grade or system | Selection focus | Qualification |
|---|---|---|
| 4130 | Lower-carbon Cr-Mo machinery and tube chemistry where weld procedure and section response are qualified. | Confirm machinery or pressure scope, exact standard, section, heat treatment, hardness location and weld/PWHT responsibility. |
| Quenched-and-tempered machinery choices commonly compared, but not accepted as automatic equivalents. | Confirm machinery or pressure scope, exact standard, section, heat treatment, hardness location and weld/PWHT responsibility. | |
| T/P11 and T/P22 systems | Dedicated elevated-temperature pressure-product families governed by their own specifications and codes. | Confirm machinery or pressure scope, exact standard, section, heat treatment, hardness location and weld/PWHT responsibility. |
04
Properties and trade-offs
Through-section hardness response changes with diameter and H-band control.
Tempered strength must be paired with toughness and residual-stress limits.
Weldability declines as carbon level, hardness and restraint increase.
Creep and oxidation claims apply only to the exact pressure-grade product and temperature regime.
05
Product forms and supply conditions
06
Applications and selection
Applications and selection
- Shafts, fasteners and moderately loaded gears
- Oilfield tools and machinery components
- Qualified welded tube and structural components
- Boiler, refinery and power piping under pressure codes
Properties and trade-offs
- Decide first whether the purchase is machinery steel or a code-controlled pressure product.
- Use section and target core response to choose 4130, 4140 or an H-band proposal.
- Review welding heat input, preheat and post-weld treatment at the ordered hardness.
- Require production-section verification when coupon properties cannot represent the component.
07
Fabrication, durability and inspection
Protect surfaces from decarburization before final machining allowance is removed.
Control austenitizing, transfer, quench agitation and tempering against the actual section.
Use qualified weld repair with hardness and delayed-cracking controls.
Keep pressure-grade PWHT records separate from machinery practice.
08
Procurement checklist
- State 4130/4140 chemistry or the exact T/P product grade; never order only 'Cr-Mo'.
- Specify bar, forging, tube or pressure product standard and delivery condition.
- Define section, H-band/Jominy requirement, final hardness and test location where applicable.
- Add weld/PWHT, NDT, surface, decarburization and traceability requirements by risk.
09
Standards, sources and review
Standards and designation context
- SAE J404_200901 for applicable SAE alloy chemistry designations
- ASTM A322-24 plus invoked A29/A29M requirements for applicable hot-wrought bars
- SAE J406_202402 or ASTM A304-20 where hardenability is ordered
- Dedicated ASTM/ASME product standards and construction code for T/P pressure grades
Authoritative research sources
SAE International · J404_200901, current verified revision
ASTM International · A322-24
SAE International · J406_202402
International Molybdenum Association · Technical reference, accessed 2026