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Steels for special applications

EF420 lecture 7

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This lecture covers

Deep drawing steel Structural steel Pressure equipment steel Pipeline steel

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Sheet steel for deep drawing

Up to 3mm thick

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Formable sheet steel

Motor body panels, whitegoods, etc Ductility for deep drawing, work hardened Pure ferrite, Interstitials controlled

Carbon less than 0.10%

Age hardening Strain aging

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Age hardening

Quenched low carbon steel C, N and O retained in super saturated solid

solution Hardness raised from 130 to 180 HV on aging for

500 h at 20°C Intermediate carbides and nitrides (epsilon) are

precipitated during aging Aging is more rapid when paint is baked

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Strain aging

On aging, interstitial elements migrate to dislocations, where they form solute atmospheres

N, O and C in solution are responsible

This locks mobile dislocations, leading to an increase in yield strength (upper yield)

When the dislocations break away from the solute atmospheres, the flow strain is reduced (lower yield point)

The brake-away from solute atmospheres is localised giving ugly stretcher-strains or Lüders lines

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Yielding behaviour

StressMPa

Strain %

Elastic behaviour

Upper yieldpoint

Lower yieldpoint

Lüders lines

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Prevention of strain aging

Control interstitials Low carbon, aluminium killed Decarburised steels

• Anneal in wet hydrogen Expensive

Deep draw immediately after rolling 2% temper rolling just before drawing Keep steel in cold storage after rolling

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Structural and pressure equipment steels

Strength and toughness

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Composition and properties

For Weldability Carbon 0.25% maximum Manganese 0.3 to 1.5% Carbon equivalent (IIW) often 0.45 maximum Strength specified minimum not exceeding 450MPa

(normal grades) or 800MPa for quenched and tempered grades

Some grades suitable for low temperature (impact tested)

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Carbon equivalent formulae

A measure of hardenability and weldability

High CE, High risk of hydrogen induced cracking in HAZ.

CEIIW widely used. Pcm preferred for microalloyed steel

CEIIW =C+Mn6

+Cr+Mo+V

5+Ni+Cu15

Pcm=C+Si30

+Mn+Cu+Cr

20+Ni60

+Mo15

+V10

+5B

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Delivery conditions

Plates, bars, rolled sections (bulb flats, angles, tees, columns and beams), tubes, forgings

Hot rolled (as-rolled) Normalised Controlled rolled or TMCP

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TMCP and TMCR steels

Rolling carried out below recrystallisation temp Grain refined A + F rolling with accelerated cooling Microalloyed Rely on processing

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Coarse ferrite & pearliteGrow inside austenite grains

Prior austenite grains

As-rolled structural steel

Between Ar1 and Ar3 fine ferrite grains grow round austenite grain boundaries

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As-rolled steel

Mixed grain size Reasonable properties, not sufficient low

temperature toughness for critical uses

Between Ar1 and Ar3 fine ferrite grains grow round and within austenite grain boundaries

Deformed austenite grains

Controlled rolled steel

Fine ferrite & pearlitegrow inside austenite grainsBelow Ar3

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Controlled rolled steel

Finer and more even grain than as-rolled Higher strength, better toughness AS 1548-7-460T AS 3678 Grades 350, 400, 450 Offshore structural steels

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Next step

Work while between Ar1 and Ar3

Austenite + ferrite Even finer grains

Accelerated cool necessary Or toughness poor

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Examples

High strength line pipe steels X80, X90

High strength steels to replace QT structural steels

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Problems with TMCP steel

Effect of subsequent austenitisation Normalising, hot forming Destroys fine grain structure Loss of strength and toughness

High arc energy welding Low strength HAZ

CTOD inferior to normalised steel

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Structural steels

Buildings, bridges, etc designed to AS1538, AS3990 or AS4100

Carbon steel plates to AS3678 Grades 250, 300, 350, 400 Carbon steel hot rolled sections to AS 3679.1 same

grades Formability and weldability important Can select impact tested grades

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Pressure vessel steels

Closed vessels under pressure. Designed to AS1210. Covered by regulation.

Carbon steel plate to AS 1548. Better quality than structural. Killed steels Designation shows steelmaking practice Can select impact tested and normalised grades Weldability and formability important

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Shipbuilding and offshore

Classification authority specifications (Lloyds, DnV etc) Different quality than pressure vessel or structural Through-thickness properties can be important Weldability and formability Grades A through E, AH through EH

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Pipe and tube products

Pipe intended to carry fluid at pressure Tube - structural shape

May be cold formed

Boiler tubes are carrying pressure and are fired Pipe fittings - valves, flanges, tees, elbows,

reducers

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Pipe manufacture

Welded pipe and tube ERW, SAW long lengths Seamless pipe and tube. Piercing a billet.

Restricted to 12m lengths

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Pipe sizes

Standard throughout world Nominal bore and scheduled wall thickness

Standard and extra strong

ASTM A106 Grade B ASTM A333 Grade 6 “fine grain” impact tested at

-46°C

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Transmission line pipe

Cross country pressure pipe lines Strength is important as is weldability Sophisticated HSLA steels API (American Petroleum Institute) grades

API 5L Grade B, API 5L X42, X46, X52, X56, X60, X65 and X70

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Quenched and tempered plate

Structural (Bisalloy 60, 70, 80) AS1554 Part 4 (AS 3597, ASTM A514, A517)

Pressure Vessel (Bisalloy 80PV) AS1210 (AS3597, ASTM A517)

Military (BIS812EMA, HY80) Wear plates (Bisalloy 320, 360, 400, 500) HSLA micro alloyed with boron etc