Design Ultra High Strength Steel

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Metallurgical Design of Steels-The service to be performed by the components, determined by the required properties

- Production and fabrication ease of the finished or semi-finished components

- The surface based properties of the components

Control of Properties and Microstructure

- Properties of steels depends on the microstructure

- Microstructure is defined by not only the microscopically visible structure but alsosolute concentrations, crystallographic orientations and lattice imperfections.

- Composition controls the phase balance, morphology and lattice imperfections.

- Heat-treatment also determines the proportion, size and distribution of phases, grainsize, equilibrium or non equilibrium partitioning of phases, dislocation substructure andlattice imperfections.

- Metal Working also determine the texture along with other micro-features.


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Required Properties for Ultra High Strength Steels

- Very high strength- Adequate ductility- High impact resistance and toughness- Very good fatigue strength- Good weldability, particularly in sheet for rocket motors and missile casing

Methods to produce Ultra High Strength Steels

- Low temperature tempering up to 350C- High temperature tempering up to 550C-650C- Thermo-mechanical treatment- Rapid austenitising treatment

- Cold working or strain ageing of martensite- Cold drawing pearlite- Age hardening very low carbon martensite, i.e. the marageing steels.


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Marageing Steels

- The development of ultra-high strength Marageing steels for defence purpose withexcellent ductility, enhanced impact resistance and fracture toughness

- The controlled and precise processing of clean steel production by vacuum-meltedvery low carbon steels, which are highly alloyed with nickel, rendering the phasetransformation to the low carbon martensite even at slow cooling rates.

- Though this ductile low carbon martensite contains high dislocation density but due tolow carbon content softness is induced. Properties of ductile martensite are following

0.2 % Proof Strength (MPa)

Tensile Strength(MPa)

% Elongation % Cross SectionReduction

HardnessVickers (H.V)

650-800 1000-1050 17-20 70-75 300-320

- The strength of low carbon martensite is increased by age-hardening precipitatingintermetallic compounds. The alloying elements are cobalt and molybdenum along withthe association of trace titanium and aluminium or higher percentage of titanium andaluminium alone.


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18% Ni Steels

They are age-hardened by additions of high percentage of molybdenum and cobalt withnsignificant percentage of titanium and aluminium.

The impurity content is controlled by double vacuum melting and controlled additions of race elements such as 0.003% B, 0.02% Zr and 0.05% Ca to scavenge impurities and tonhance hot-workability.

Yield Strength(MPa)

Weight%

C Mn Si S P Ni Co Mo Al Ti

1350


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0

100

200

300

400

500

600

700

800

100

1 Hr soaking

artensite Start

3 Hr age-hardening andstress relieving

Martensite (30-32 Rc)Cold work as desired

AirCoolin

g

AirCooling

Marageing Steel (55 Rc)

Heat-treatment cycle of 18% Ni Steels

-The major contribution in strengthening comes out after ageing treatment due to

formation of dispersed plate like orthorhombic Ni 3Mo precipitates. The role of cobalt is toenhance the precipitation while the molybdenum is the major strengthening element. Theageing treatment can be manipulated over certain range to get the best strengtheningresults but over ageing should be get rid of due to formation of Fe 2Mo and austenitestability which can lower the martensite start temperature below room temperature.


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0

100

200

300

400

500

600

700

800

100

1 Hr soaking

artensite Start

Age-hardening andstress relieving

Martensite

(30-35 Rc)Cold work as desired

AirCooling

AirCoolin

g



1 Hr 3 Hr (alternate)


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0

100

200

300

400

500

600

700

800

100

1 Hr soaking

artensite Start

Age-hardening andstress relieving

Cold work25%

(alternate)

A

irCooling

AirC

ooling



1 Hr

3 Hr (alternate)

4 Hr Ausage

Refrigerate

Austenite(10 Rc)

AirCooling


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Properties of 25%NiSteels

Solution Treatment +Ausageing Treatment at 700 C- 80 C refrigeration treatmentAgeing treatment 500 C for 3 hours

Solution Treatment +Cold Working 60%- 80 C refrigeration treatmentAgeing treatment 500 C for 3hours

Tensile Strength (MPa) 1850 2000

Yield Strength (MPa) 1730 1950

% Elongation 12 13

% Cross Section Reduction 53 58


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Alternative Marageing Steels

These steels are developed to reduce the cost of marageing steels replacing Nipartially or wholly by the additions of chromium, manganese, copper etc.

Though very high strength level (as high as the Fe-Ni steels) can be obtained in thesesteels but ductility and toughness are reduced.

The corrosion resistance properties of 18-25 % Ni marageing steels are also enhancedby the addition of Cr in alternative marageing steels if delta-ferrite phase can be formedby proper compositional balance.

The additions of Si and Be (0.2%) in alternative marageing steels change the character of age-hardening precipitates. Globular and coherent bcc precipitates such as NiBe canprovide high strengthening.

Composition (wt %) Properties

C Ni Cr Co Mo Ti Al

0.02 10 10 - 2 0.2 0.3 Not heavily age-hardened and yieldstrength on 1150-1400 MPa. Toughnessvalues comparable with those 18% Ni-Co-Mo steels at similar strength levels.

0.02 7 10 10 5.5 - - Highly alloyed and expensive but showage-hardening. Yield Strength 1500-1600 MPa.


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Defence Application of Ultra High Strength Steels

- Armour plates of main battle tanks

- Gun barrels

- Military bridges

- Heavy weight vehicle springs

- High speed rotor

- High duty gears

- Bolts and fasteners

- Aircraft undercarriages

- Rocket motor cases

- Missile bodies

- Landing gears of helicopter and light weight naval aircraft

Documents

Design Ultra High Strength Steel