1 Fabrication Technology Lecture 9 Welding Steel October 10-16, 2001 MIT.

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Fabrication TechnologyFabrication TechnologyLecture 9Lecture 9

Welding SteelWelding Steel

October 10-16 , 2001

MIT

Copyright @ 2001Thomas W. Eagar Lecture 16 2

Why Steel?Why Steel?

Best combination of strength, toughness, and cost

Achilles Heel: Weight– Sometimes you don’t care

Overcapacity in world steel market causes low prices, losses to steel industry


Steel IndustrySteel Industry

US has 10% of world marketNew integrated steel plant would

cost $15 billion-too high for private companies

All plants in last 20 years financed by governments

US steel industry faced with extinction, went from 16 person hours/ton to 0.6 person hours/ton


Chaparral Steel and Mini Chaparral Steel and Mini MillsMills

Mini mills recycle steel1990 one man-hour per ton (at time

most efficient in world)Originally could only make rebar

(trash steel)Chaparral improved it, also

developed net shaped casting– Reduced processing from 28 passes

through roller to 9


Iron-Carbon Phase DiagramIron-Carbon Phase Diagram


Welding SteelsWelding Steels

Face Centered CubicBody Centered CubicBody Centered Tetragonal

– Quenched and tempered– Incredibly tough, great grain size

(gives both strength and toughness)– Can get 300ksi– Welding a problem in higher strength

steels

Can get a 10 fold increase in strength (critical flaw size down)


Iron-Carbon Phase DiagramIron-Carbon Phase Diagram


Weldability of SteelsWeldability of Steels

See Phase diagram

Easiest to weld is low carbon

High carbon steels are a pain to weld

Cast irons are terrible to weld (critical flaw size down)


Types of Defects in WeldingTypes of Defects in Welding

Cracking, many typesWelding through primers (can get

phosphor) Cavities, several types Inclusions Lack of fusion/penetration Imperfections in shape

Useful Book:Weldability of Steels, good appendix,

80pages with welding parameters

Copyright @ 2001Thomas W. Eagar

Lecture 16 10

Hydrogen CrackingHydrogen Cracking

Venn diagram on board, need all three of the following items to have a problem– Hydrogen– Stress– Susceptible microstructure

Typically concerned with hardness >Rc30 approx 120ksi

Hydrogen dissolves more in liquid steel than solid steel– Can go to inclusions or other flaws, promotes other

mechanisms– Sometimes called delayed cracking, often hours after

the welding, up to days, shipyards wait a week then do NDT

Have to bake out hydrogen in oven, will diffuse out but takes time and can’t let it diffuse through


Lecture 16 11

Electrodes for Stick Electrodes for Stick WeldingWelding

Different types of flux coatings that generate more or less hydrogen

Need to keep moisture offBefore 1900, used bare steel wires

but had lots of nitrogen Wrapped piece of paper around

electrode, beginning of cellulosic electrodes

Stuck electrode in mud, beginning of mineral coated electrodes

Flux formulations still somewhat of a black art, one uses old tobacco stalks (concentration of rubidium)


Lecture 16 12

GMAW (MIG) and GTAW GMAW (MIG) and GTAW (TIG)(TIG)

GMAW– Stands for “Gas Metal Arc Welding”– Shielding gas around the nozzle– 60% arc time– Lower hydrogen, no flux to carry

moisture– Shipyards have gone to gas metal arc

welding

GTAW– Stands for “Gas Tungsten Arc

Welding”– Can’t get any lower hydrogen than

this


Lecture 16 13

Susceptibility to Hydrogen Susceptibility to Hydrogen CrackingCracking

Source: BA Granville, Cold Cracking in Welds in HSLA Steels, Welding of HSLA Steels, Proc. Int. Conf., ASM, NOV 1976


Lecture 16 14

Review: Welding SteelsReview: Welding SteelsBig problem is Hydrogen Cracking

– Need: Hydrogen+stress+microstructure

Types of Steel Welding– Arc (with electrode)– GMAW– GTAW– Submerged Arc (SMAW)

Large arc through sand of flux Sand melts, makes melted glass layer

around arc, protects arc and makes good welds

Often used on ship panel lines, high productivity process,


Lecture 16 15

Preheat Baseplates to Control Preheat Baseplates to Control Hydrogen CrackingHydrogen Cracking

Reduces hydrogenOne of largest single costs in a

military shipyardTypical 200degF

– HY100 on SeaWolf 400degF “blue jelly” suits

– Workers have to wear water cooled suits to weld submarines

Typically use electric resistance heaters, large % of the electricity usage

Susceptibility to H2 cracking maximizes at about room temp


Lecture 16 16

Postheat to Control Postheat to Control Hydrogen CrackingHydrogen Cracking

reduce hydrogenkeep the temperaturediffuses out (analogy CO2 bubbles

from ginger ale)time depending on thickness and

temperatureSeaWolf doing postheating as wellDiagram of weld prep, ¾” thick,

then do a postheat, if went to full4” then wouldn’t get the hydrogen

out for weeksCan help temper the weld metal


Lecture 16 17

Postheat Steel to Control Postheat Steel to Control Hydrogen CrackingHydrogen Cracking

Reduces hydrogenDiffuses out (like CO2 bubbles

from ginger ale)Time depending on thickness and

temperatureSeaWolf doing postheating as wellFull 4” postheat would take weeks

to diffuse all bubblesCan help temper the weld metal


Lecture 16 18

Susceptibility to Hydrogen Susceptibility to Hydrogen Cracking PlotCracking Plot


Lecture 16 19

Explanation of Hydrogen Explanation of Hydrogen Cracking PlotCracking Plot

Used in Welding of Steels Zone II, traditional steels HY130 is near middle, HY100, HY80 below Carbon equivalent is a measure of the hardenability of

the steel, Preheat required is a function of the carbon equivalent

(to avoid hydrogen

cracking)– Boiler and pressure vessel code: “shall not weld” below

50degF without preheating. – Things start to get many monolayers of water on surface


Lecture 16 20

Controlling Hydrogen Controlling Hydrogen CrackingCracking

PreheatPostheatShot PeenStress RelieveHydrogen usually shows itself

within 1 week, but in low temp conditions can take much longer

Heating can be done many ways:– Induction heating– Resistance heating– Local flame heating– Large flame furnace


Lecture 16 21

PeeningPeening

Very empiricalUse Almen gauge, about 1”x 4” of

the material, – Hit it, see how much it distorts– Measure height of the bow

Not much science to peeningOther choices have better controlWas the only way to weld heavy

armor plateNot used much anymore

1 Fabrication Technology Lecture 9 Welding Steel October 10-16, 2001 MIT.

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