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X12CrNiCoMoWV12-10-3 (Ultra-High-Temp Turbine Spring)

X12CrNiCoMoWV12-10-3 -- martensitic precipitation-hardened turbine-grade spring/fastener steel. Rm 1350 MPa retained to 550 C. For jet engine and gas turbine springs, retaining rings and fasteners operating hot where standard spring steels lose their set resistance.

Spring Steels Heat-Resistant Spring Steel AMS 5731 (nearest family)12Cr precipitation-hardening turbine spring steel UNSK66286 (nearest -- Refractaloy 26 family) Trade NameSimilar to Refractaloy 26 / A-286 spring-hardness variant JUSNema JUS; turbinski federni celik
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Description

X12CrNiCoMoWV12-10-3 (similar to Refractaloy 26 / AISI 660-type class) per AMS 5731 family. Martensitic Cr-Ni-Co-Mo-W-V steel age-hardened to retain spring force (low relaxation) at continuous service to 550 C -- standard carbon/Si/Cr spring steels lose 30-50% of their preload by that temperature. Standard for turbine engine belleville washers, retaining rings, hot-section springs and high-temperature fasteners.

Key Properties — Solution Treated + Aged
Rm (Tensile Strength)
1,300 – 1,450
MPa
Re / Rp0.2 (Yield)
1,100
MPa
A₅ (Elongation)
≥ 8.0
%
E (Young's Modulus)
210
GPa
Density
7.80
g/cm³
Main Designations (4)
Standard BodyDesignationCountry / RegionNote
AMS 5731 (nearest family) 12Cr precipitation-hardening turbine spring steel USA (Aerospace)
UNS K66286 (nearest -- Refractaloy 26 family) USA (UNS)
Trade Name Similar to Refractaloy 26 / A-286 spring-hardness variant Global
JUS Nema JUS; turbinski federni celik Yugoslavia/BiH Visokotemp. federni celik za turbine
Standards (4)
#Standard BodyDesignationCountry / RegionNote
1 AMS 5731 (nearest family) 12Cr precipitation-hardening turbine spring steel USA (Aerospace) —
2 UNS K66286 (nearest -- Refractaloy 26 family) USA (UNS) —
3 Trade Name Similar to Refractaloy 26 / A-286 spring-hardness variant Global —
4 JUS Nema JUS; turbinski federni celik Yugoslavia/BiH Visokotemp. federni celik za turbine
All values in wt.%
C
Carbon
0.1000 – 0.1500
Cr
Chromium
11.5000 – 13.0000
Ni
Nickel
9.0000 – 11.0000
Co
Cobalt
2.5000 – 3.5000
Mo
Molybdenum
1.0000 – 1.5000
W
Tungsten
1.0000 – 1.5000
V
Vanadium
0.1500 – 0.3500
ElementSymbolMin %Max %Typical %
Carbon C 0.1000 0.1500 0.1250
Chromium Cr 11.5000 13.0000 12.2500
Nickel Ni 9.0000 11.0000 10.0000
Cobalt Co 2.5000 3.5000 3.0000
Molybdenum Mo 1.0000 1.5000 1.2500
Tungsten W 1.0000 1.5000 1.2500
Vanadium V 0.1500 0.3500 0.2500
Mechanical Properties
ConditionCode Rm min
MPa		Rm max
MPa		 Re min
MPa		Re max
MPa		 A₅ min
%		 Z
%		 KV
J		 HB minHB max HRC minHRC max E
GPa
Solution Treated + Aged +STA 1,300 1,450 1,100 — 8.0 25.0 — — — 38.0 42.0 210.0
At 550 C service (retained) +HT550 1,000 1,150 850 — 10.0 — — — — — — 195.0
Physical Properties
Density
7.800
g/cm³
Melting Point
1400–1440
°C
Young's Modulus
210.0
GPa
Poisson's Ratio
0.290
—
Thermal Conductivity
18.0
W/m·K
Thermal Expansion
11.00
×10⁻⁶ /K
Specific Heat
460
J/kg·K
Electrical Resistivity
0.7000
µΩ·m
Magnetic
Yes
 
Heat Treatment
Solution Treatment
Temperature:1000°C – 1040°C
Medium:Oil quench or air cool (section dependent)
Duration:30-60 min
Cooling:Rapid quench
Hardness After:HRC 30-35 -- machinable
Aging (precipitation hardening)
Temperature:550°C – 600°C
Medium:Air oven
Duration:4-16 h
Cooling:Air cool
Hardness After:HRC 38-42 / Rm 1300-1450 MPa
Precipitation of carbides and intermetallics gives high-temperature spring stability.
Stress Relief (after coiling)
Temperature:450°C – 480°C
Medium:Air oven
Duration:1-2 h
Cooling:Air cool
Hardness After:Sets final spring dimension
Critical for minimising relaxation in service.
Relaxation testing (quality control)
Temperature:550°C – 550°C
Medium:Sustained load test at service temp
Duration:100-1000 h
Cooling:N/A
Hardness After:Force retention >90% typical spec
Distinguishes turbine-grade spring steel from standard spring steels which relax rapidly above 250 C.
Machinability
Machinability Rating
relative to AISI B1112 = 100%
15.0%
Very Difficult
Turning Speed (HSS)
7
m/min
Turning Speed (Carbide)
42
m/min
Feed Rate (Turning)
0.060
mm/rev
Drilling Speed (HSS)
4
m/min
Milling Speed (Carbide)
40
m/min
CoolantSoluble oil 8-12% in solution-treated condition
Chip FormationMedium-short chips (solution treated); grinding after aging
Tool MaterialCarbide P20/K20 (ST condition); CBN grinding (aged)
Surface FinishRa 0.4-1.6 um
Machine in solution-treated condition, age afterward. Difficult in aged condition -- grinding preferred.
Applications
Turbine Springs
Belleville washers, retaining rings and preload springs in the hot section of gas turbines.
Aerospace Fasteners
High-temperature bolts and fasteners in engine casings.
Hot Section Hardware
Any spring-loaded hardware operating continuously above 300 C where standard spring steel relaxes.
Technical Notes
✅
Why standard spring steel fails hot
51CrV4 or 54SiCr6 valve springs lose spring force permanently (stress relaxation) within hours at 300 C+ -- the martensite tempers further and dislocations rearrange under sustained load. Turbine-grade precipitation-hardened steels are specifically designed so the aging treatment ALREADY represents the stable high-temperature condition, giving minimal further relaxation in service.