MARAGING STEELSFOR AERO SPACE APPLICATION

210 April 2023

Introduction Applications Discussions Landing Gear Material Effect of alloying elements Physical metallurgy Strengthening Mechanism Conclusions

OUT LINE

310 April 2023

WHAT IS STEEL?

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten.

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WHAT IS MARAGING STEEL?

Maraging steels are primarily based on Fe-Ni system and derive their properties due to combination of two solid state reactions: MAR+AGING,

Meaning martensitic transformation from austenite and aging of the resulting martensite.

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APPLICATIONS

In Aerospace

• Aircraft forging (Eg. Undercarriage parts/landing gear, wing fittings)

• Solid-propellant missile cases• Jet-engine starter impellers• Aircraft arrestor hooks• Torque transmission shafts• Aircraft ejector release units

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In Tooling and Machinery

• Punches and die bolsters for cold forging• Extrusion press rams and mandrels• Aluminium die-casting and extrusion dies• Cold reducing mandrels in tube production• Zinc-base alloy die-casting dies• Machine components: gears

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Structural Engineering and Ordnance

• Lightweight portable military bridges• Ordnance components• Fasteners (a button, hook, or zipper etc)

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Lath Martensite Plate Martensite

Concentration Low C+ Alloying Elements High C+ Alloying Elements

Shape Strip Plates

Temperature High Transition Temperature Low Transition Temperature

MORPHOLOGIES OF MARTENSITE

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COMPARISON BETWEEN CONVENTIONAL TEMPERED MARTENSITE AND MARAGING STEEL

Conventional Tempered Martensite

Maraging steel

Fe-C MartensiteBCT Structure

Fe-Ni MartensiteBCC Structure

Hard & Brittle Soft

Tough and less ductile Soft & Ductile

Tempering increases ductility &Reduces Strength

Aging (Precipitates of Intermetallic compounds) increases ductility & Toughness

Shear component of strain 0.19 Shear component of strain 0.20

Normal component of strain 0.09 Normal component of strain 0.05

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SELECTION CRITERIA FOR AERONAUTICAL vs. GENERAL ENGINEERING MATERIALS

Sl. No.

Nomenclature Aeronautical General Engineering Materials

1. Design philosophySafe-life design

Fatigue Crack GrowthDamage Tolerant Design

Infinite Life DesignStatic Mech. Properties

2. Efficiency High > 60% Low < 40% for land based Turbines

3. Weight Lighter Heavy

4. Strength Specific strength (Y.S/Density)

UTS

5. Safety Factor Low

High

6. Performance Consistent Variable

7. Reliability Good Poor

8. Confidence level High Low

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LANDING GEAR MATERIAL

• Steel – Aermet

Maraging Steel• Titanium alloys• Magnesium alloys• Aluminium alloys

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PROPERTY REQUIREMENTS FOR LANDING GEAR

Requirements Property

Lower Aircraft weight Low density

Lower non damaging permanent set during take-off & landing

High Y.S.

Maximum stress levels in plastic range High Y.S./U.T.S

Better dynamic response as sliding member High Young’s Modulus

Deformation rather than fracture outside the design envelope & landing gear

Good ductility in both directions, longitudinal + transverse

Resistance to crack initiation & growth High Fatigue strength

Resistance from salt environments High Resistance to Stress Corrosion Cracking

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SPECIFIC CHARACTERISTICS OF MARAGING STEEL

• Ultra high strength at room temperature and retain strength up to 350 C⁰

• Combination of strength & Ductility• Superior Fracture Toughness• Simple Heat Treatment Cycle - Minimum Distortion• Low Carbon Martensite: Fe-Ni Martensite (BCC),

soft, workable /cold worked upto 80-90% without cracking

• Easily fabricated and Good Weldability• Carbonising & Nitriding

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1810 April 2023

PROCESS FLOW CHART (MDN 250A)VIM

CONDITIONING

VAR

FORGING

ULTRASONIC TESTING

HEAT TREATMENT

STRAIGHTENING

MACHINING

SAMPLE PREPARATION INSPECTION UPSETTING

HEAT TREATMENT

TESTING

PACKING&

DISPATCH

HEAT TREATMENT

TESTING- HCF- LCF

- IMPACT

Primary Melting 480 mmΦ ingot

Remelting 550 mmΦ

230 mmΦ

215 mmΦ

25 mmΦ

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PROPERTIES TEST RESULTS SPECIFIED

A. 215mm SIZE STAGE

Tensile Properties• UTS(MPa)• 0.2%PS (MPa)• % El (4D)

1760-18251688-1768

11-13

Record1655

6.0 min.

NTS/UTS 1.53 TO 1.58 1.50 Minimum

Shear Strength (MPa) 977 to 1063 -

Impact Strength at RT 19 to 25.8J 8J Minimum

Torsional Shear Strength (MPa)

1378 to 1411 -

Hardness Rc 51 to 54 48 Min.

Macrostructure Satisfactory -

MECHANICAL PROPERTIES OF MARAGING STEEL MDN 250A

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B. 25mm SIZE STAGE:

Impact Strength at RT 28.5 J 27 J

Low Temperature Tests• Impact strength at -70 C ⁰(J)

15-20 48 min.

High Cycle Fatigue at RT• Smooth 680 Mpa• Notch 350 MPa

Upto 1.83x 107

Upto 1.37x 107

1.0x 107 cycles min.

1.0x 107 cycles min.

Low Cycle Fatigue at RT• Smooth 1448 Mpa 1059 to 1642 1.0x 103 cycles

min.

Microstructure Prior austenite grain size 5±1 ASTM No. RA below 1% Non-

metallic inclusion

-

Forgeability Good

Physical properties α- 10.94x 106cm/cm/ C density ⁰7.96 gm/cm3, E= 185GPa

8

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QUALITY ASSURANCE STEPS DURING PROCESSING

PROCESS STEPS QUALITY ASSURANCE

Raw Material Control Material control /Approval of Sources• Comparison with Specification• Inspection & Analysis• Supplier Test Certificate

PROCESSING• Primary Melting (AAM/AIM/VIM)

• Secondary Melting (VAR/ESR)

• Crucible Condition• Melt Chemistry• Electrode Surface & soundness

• Leak Rate• Ultimate Vacuum level• Melt Rate• Ingot Surface

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PROCESS STEPS QUALITY ASSURANCE

• Forging • Furnace Atmosphere• Heating Cycle• Forge Finish Temperature• Macro & Ultrasonic Test

• Rolling • Temperature control• Inspection & Removal of Defects

• Heat Treatment Solution Annealing (H) 820 ± 10 C/9½ hrs/AC⁰ Aging (X) 480 ± 5 C/3 hrs/AC⁰

• Heating cycle• Furnace Atmosphere

• Testing & Evaluation

• Test Piece Dimension• Load Cell Calibration• Metallurgical Evaluation• Verification of Results• Consistency in Melt Batches

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Name of the

Element

Content Low

Content High

Effect on Properties

Nickel (Ni)

8-19%

< 8%Toughness

> 19% • Austenite in matrix (retained ‘γ’)• Austenite does not merge with matrix• Toughness

• Tough-Ductile Martensite matrix• Primary strengthener• Ni3Mo Precipitate

Cobalt (Co)

8-20%

< 8% Strength

> 20%Toughness

• Principal Strengthener• Raises Ms temp.• Go to matrix to free the Mo for ppt (Co-Mo combined effect)

Mo2-9%

<2%Strength

Inadequate

> 9%MicrosegragationToughness

• Principal Strengthener• Ni3Mo Precipitate• Fe2Mo Precipitate

EFFECT OF ALLOYING ELEMENTS

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Name of the Element

Content Low

Content High

Effect on Properties

Aluminium (Al)(0.15% or less)

• Deoxidizer• Charpy Impact • No Strengthening Effect

Boron (Bo) &

Zirconium (Zr)

• Little effect on Mechanical prop• Enhance resistance to strength corrosion

Si & Mn• Impact strength• Si above 0.1% reduce plasticity

Carbon (C)0.03% max

Good BadMore carbides inclusions

• Carbides, Carbo-nitrides• Inclusions at GB• Embrittlement TiC, TiCN ppt at GB• Reduce fatigue

Sulphur (S) 0.10% maxReduce properties in transverse directions

• Most determined element• Calcium is added to reduce sulphur effect

Phosphorous (P)

• Tolerated with no problem

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PHYSICAL METALLURGY

Iron-Nickel Binary Phase Diagram

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2910 April 2023

Norstrom Equation, 1976

σf = σO + σi + K1 d-1/2 + K2 l-1/2 +αGbe1/2 +βGb/L

Where,

σf = Flow stress σO = Friction stress

σi = Solid Solution Hardening component

d = Grain size l = lath packet size

G = Shear Modulus b = Burger Vector

L = Inter particle spacing

e = Dislocation density

α, β are constants

STRENGTHENING MECHANISM

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SOME RECENTLY DEVELOPED MARAGING STEEL

Alloy Nominal composition Yield strength (in MPa)

Ni Co Mo Ti Cr Al Others

Co-Free

Korean W250Inco T250

1818

--

-3

1.41.4

--

0.10.1

4.5W-

17801700

Ni-CrRussian 10 - 2 1.1 11 0.2 - 1600

Ni-Cr-SiIndian (DMR

1700)3 - - - 1 - 2.1 Si 1500

Ni-Co-Mo-CrRussianIndian

88

912

55

A0.1

1111

A-

--

-1700

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1. The programme has been satisfactorily completed and material supplied for end use and satisfactory performance feedback obtained from users.

2. The alloy has been successfully developed, produced commercially and type certified.

3. Indigenization and supply of this strategic alloy has helped self reliance in ISRO and DRDO programmes.

4. Consistency among melts and Integrity within each melt has been achieved and the material was found to have comparable properties with that of imported material.

CONCLUSIONS

3210 April 2023

REFERENCES

• ASM Hand Books • http://www.keytometals.com• http:/en.wikipedia .org • Presentation given by Mr. Ashok Kumar,

Additional Regional Director, DRDO, HYD.

3310 April 2023