Chapter 7 Alloy Steels 3 - WordPress.com · Maraging Steels • Fe-Ni martensite alloys (which belong to the family of iron-base alloys)additionally alloyed with cobalt, ... Mechanical

Alloy SteelsAlloy Steels

77

SME 1613 : Materials Science 2007

High strength low alloy (HSLA).

� a type of steel alloy that provides many benefits over regular

steel alloys

� contain a very small percentage of carbon (less than one-tenth

of a percent) and only small amounts of very specific alloying

elements

� referred to as 'microalloyed', as they are indeed alloyed in

extremely small amounts by comparison to other main

commercial alloy steels

� group of low carbon steels that utilize small amounts of alloying

elements to attain yield strengths in excess of 275 MPa in the

as-rolled or normalized conditions


TYPICAL CHEMICAL COMPOSITION OF HSLA


MECHANICAL PROPERTIES: HSLA

• Much stronger and tougher than ordinary plain carbon

steels.

• alloying elements : alter the microstructure of plain carbon

steels, which is usually a ferrite-pearlite aggregate, to

produce a very fine dispersion of alloy carbides in an

almost pure ferrite.

• Increases the material's strength by precipitation

strengthening and by refining the grain size, which in the

case of ferrite increases yield strength by 50% for every

halving of the mean grain diameter.


Microstructure of HSLA

A517 Grade

7,quenched from

925°C,tempered at

650C = tempered

martensite.

A533 grade b, quenched

from 900C and tempered

at 620C . This is a heavy

section plate, the

microstructure consists of

ferrite and tempered

bainite.


Application of HSLA

• Applications: include oil and gas pipelines,

automotive beams, offshore structures and

shipbuilding.

• Also used in cars, trucks, cranes, bridges and other

structures that are designed to handle a lot of stress,

often at very low temperatures.



Maraging Steels

• Fe-Ni martensite alloys (which belong to the family of

iron-base alloys)additionally alloyed with cobalt,

molybdenum, titanium and some other elements.

• difference between carbon steels and maraging steels is

that the martensite that forms in carbon steels,when

quenched, transforms to a softer more ductile material

when tempered.

• maraging steels are not included in the AISI numbered

listings since they are not machine steels.


Type of Maraging Steels

Type of Maraging Steels

~very high resistance to stress corrosion

compared to carbon steels.

~these steels develop notch strenght higher

than carbon steels.

Mechanical properties

18% Ni, 8% Co, 5% Mo,

0.9% Ti

ease of welding.

~high quality to fracture toughness and

~the strenght is maintained with increase in

2. 23% Nickel

3. 12% Nickel

Composition

1. 18% Nickel

less amount of Al and Ti

12% Ni, 12% Cr, Mo,

23% Ni, 2.0% Al, 2.5% Ti

section thickness and also up to 350°C

~high hardness results from the intermetallic

compounds Ni3Mo and Ni3Ti.


• The maraging steels are strengthened by a process of martensitic transformation, followed by age or precipitation hardening. Precipitation hardenable stainless steels are also in this group.

(1) Annealing Fe-Ni alloys to 820 °C, cooling from the austenitic condition,

the alloy transforms to a fine lath type martensite.

(2)(3)(4)The iron-nickel martensite starts out ductile and become hard and tough with aging. Cooling in air for 1 hours and tempered at 480 °C for 3 hours

• The tempering results in strong precipitation hardening owing to the precipitation of intermetallics from the martensite, which is supersaturated with the alloying elements.

� By analogy with the precipitation hardening in aluminum, copper and other non-ferrous alloys, this process has been termed ageing, and since the initial structure is martensite, the steels have been called maraging.

Heat treatment: Maraging Steel


Microstructure of Maraging Steels

Fe-Ni Martensite Tempered Fe-Ni Martensite

• The maraging steels have high fracture toughness due to a

combination of the grain size of the martensite and the

dislocation density,leading to a fine

precipitation.(eg,Ni3TiAl,orthorhombic Ni,Mo)

• The steels can be nitrided. The corrosion resistance is only

slightly improved but the 12% Cr variety has been developed for

corrosion resistance.


Mechanical Properties: Maraging Steel

� Ultra-high strength at room temperature.(yield strength=1100MPa)

� Simple heat treatment, which results in minimum distortion.

� Superior fracture toughness compared to quenched and tempered

steel of similar strength level.

� Low carbon content, which precludes decarburisation problems.

� Section size is an important factor in the hardening process.

� Easily fabricated.

� Good weldability.


Application: Maraging Steel

• Applications : shafts, and substitute for long, thin, carburized or

nitrided parts, and components subject to impact fatigue, such as

print hammers or clutches.

• Also commonly used: air frame and engine components,injection

moulds and dies due to its high resistance of

decarburisation,distortion and cracking.

• Maraging steels work well in electro-mechanical components where

ultra-high strength is required, along with good dimensional stability

during heat treatment.


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Chapter 7 Alloy Steels 3 - WordPress.com · Maraging Steels • Fe-Ni martensite alloys (which belong to the family of iron-base alloys)additionally alloyed with cobalt, ... Mechanical