Chapter 4 - Classification of Steels, Welding of Mild Steels

4.

Classification of Steels,

Welding of Mild Steels

4. Classification of Steels, Welding of Mild Steels 32

In the European Standard DIN EN

10020 (July 2000), the designations

(main symbols) for the classification of

steels are standardised. Figure 4.1

shows the definition of the term „steel“

and the classification of the steel

grades in accordance with their

chemical composition and the main

quality classes.

In accordance with the chemical com-

position the steel grades are classified

into unalloyed, stainless and other

alloyed steels. The mass fractions of

the individual elements in unalloyed

steels do not achieve the limit values

which are indicated in Figure 4.2.

Stainless steels are grades of steel

with a mass fraction of chromium of at

least 10,5 % and a maximum of 1,2 %

of carbon.

Other alloyed steels are steel grades

which do not comply with the definition

of stainless steels and where one

alloying element exceeds the limit

value indicated in Figure 4.2.

Figure 4.1

Definition for theclassification of steels

© ISF 2004br-er05-01.cdr

Classification in accordance with the chemical composition:

l

l

l

unalloyed steels

stainless steels

other, alloyed steels

Classification in accordance with the main quality class:

·

·

·

unalloyed steels - unalloyed quality steels- unalloyed special steels

stainless steels

other, alloyed steels - alloyed quality steels- alloyed special steels

Definition of the term “steel”

Steel is a material with a mass fraction if iron which is higherthan of every other element, ist carbon content is, in general,lower than 2% and steel contains, moreover, also otherelements. A limited number of chromium steels might contain acarbon content which is higher than 2%, but, however, 2% is thecommon boundary between steel and cast iron [DIN EN 10020(07.00)].

Figure 4.2


As far as the main quality classes are concerned, the steels are classified in accor-

dance with their main characteristics and main application properties into unalloyed,

stainless and other alloyed steels.

As regards unalloyed steels a distinction is made between unalloyed quality steels

and unalloyed high-grade steels.

Regarding unalloyed quality steels, prevailing demands apply, for example, to the

toughness, the grain size and / or the forming properties.

Unalloyed high-grade steels are characterised by a higher degree of purity than

unalloyed quality steels, particularly with regard to non-metal inclusions. A more

precise setting of the chemical composition and special diligence during the manufac-

turing and monitoring process guarantee better properties. In most cases these

steels are intended for tempering and surface hardening.

Stainless steels have a chromium mass fraction of at least 10,5 % and maximally

1,2 % of carbon. They are further classified in accordance with the nickel content and

the main characteristics (corrosion resistance, heat resistance and creep resistance).

Other alloyed steels are classified into alloyed quality steels and alloyed high-grade

steels.

Special demands are put on the alloyed quality steels, as, for example, to toughness,

grain size and / or forming properties. Those steels are generally not intended for

tempering or surface hardening.

The alloyed high-grade steels comprise steel grades which have improved properties

through precise setting of their chemical composition and also through special manu-

facturing and control conditions.


The European Standard DIN EN 10027-1 (September 1992) stipulates the rules for

the designation of the steels by means of code letters and identification numbers.

The code letters and identification numbers give information about the main applica-

tion field, about the mechanical or physical properties or about the composition.

The code designations of the steels are divided into two groups. The code designa-

tions of the first group refer to the application and to the mechanical or physical

properties of the steels. The code designations of the second group refer to the

chemical composition of the steels.

According to the utilization of the

steel and also to the mechanical or

physical properties, the steel grades

of the first group are designated with

different main symbols (Fig. 4.3).

Figure 4.3

Classification of steels in accordancewith their designated use


l

l

l

l

l

l

l

l

l

l

l

e.g. S235JR, S355J0

P =e.g. P265GH, P355M

L =e.g. L360A, L360QB

E =e.g. E295, E360

B =e.g. B500A, B500B

Y =e.g. Y1770C, Y1230H

R =e.g. R350GHT

H =

e.g. H400LA

D =e.g. DD14, DC04

T =

e.g. TH550, TS550

M =e.g. M400-50A, M660-50D

S = Steels for structural steel engineering

Steels for pressure vessel construction

Steels for pipeline construction

Engineering steels

Reinforcing steels

Prestressing steels

Steels for rails (or formed as rails)

Cold rolled flat-rolled steels with higher-strengthdrawing quality

Flat products made of soft steels for cold reforming

Black plate and tin plate and strips and also speciallychromium-plated plate and strip

Magnetic steel sheet and strip


An example of the code designation structure with reference to the usage and the

mechanical or physical properties for “steels in structural steel engineering“ is ex-

plained in Figure 4.4.

Figure 4.4


For designating special features of the steel or the steel product, additional symbols

are added to the code designation. A distinction is made between symbols for spe-

cial demands, symbols for the type of coating and symbols for the treatment con-

dition. These additional symbols are stipulated in the ECISS-note IC 10 and depicted

in Figures 4.5 and 4.6.

© ISF 2004br-er-05-06.cdr

1

2

))The symbols are separated from the preceding symbols by plus-signs (+)In order to avoid mix-ups with other symbols, the figure T may precede,for example +TA

Symbol ) )

+ A+ AC+ C

+ Cnnn+ CR+ HC+ LC+ Q+ S+ ST+ U

1 2 treatment condition

softenedannealed for the production of globular carbideswork-hardened (e.g., by rolling and drawing), also a distinguishingmark for cold-rolled narrow strips)cold-rolled to a minimum tensile strength of nnn MPa/mm²cold-rolledthermoformed/cold formedslightly cold-drawn or slightly rerolled (skin passed)quenched or hardenedtreatment for capacity for cold shearingsolution annealeduntreated

Symbols for the treatment condition

Figure 4.6


Symbol ) )

+ A+ AR+ AS+ AZ+ CE+ Cu+ IC+ OC+ S+ SE+ T+ TE+ Z+ ZA+ ZE+ ZF+ ZN

1 2 Coating

hot dippedaluminium, cladded by rollingcoated with Al-Si alloycoated with Al-Tn alloy (>50% Al)electrolytically chromium-platedcopper-coatedinorganically coatedorganically coatedhot-galvanised

upgraded by hot dipping with a lead-tin alloyelectrolytically coated with a lead-tin alloyhot-galvisedcoated with Al-Zn alloy (>50% Zn)electrolytically galvaniseddiffusion-annealed zinc coatings (galvannealed, with diffused Fe)nickel-zinc coating (electrolytically)

electrolytically galvanised

1

2

))The symbols are separated from the preceding symbols by plus-signs (+)In order to avoid mix-ups with other symbols, the figure S may precede,for example +SA

Symbols for the coating type

Figure 4.5


Figure 4.7 shows an example of the novel designation of a steel for structural steel

engineering which had formerly been labelled St37-2.

Figure 4.8 depicts the chemical composition and the mechanical parameters of dif-

ferent steel grades. The figure explains the influence of the chemical composition on

the mechanical properties.

Figure 4.7


S = steels for structural steel engineeringP = steels for pressure vessel constructionL = steels for pipeline constructionE = engineering steelsB = reinforcing steels

The steel St37-2 (DIN 17100) is, according to the new standard (DIN EN 10027-1),designated as follows:

S235 J 2 G3

Steel for structural steel engineering

R 235 MPa/mmeH

2³

further property(RR = normalised)

test temperature 20°C

impact energy ³ 27 J

Steel designation in accordance with DIN EN 10027-1

Stahl C Si Mn P S Cr Al Cu N Mo Ni Nb VS355J0(St 52-3)S500N(StE500)P295NH(HIV)S355J2G1W(WTSt510-3)S355G3S(EH36)

Stahl

S355J2G3(St 52-3)S500N(StE500)P295NH(HIV)S355J2G1W(WTSt510-3)S355G3S(EH36)

Kerbschlagarbeit AV

[J]

Zugfestigkeit Rm

[N/mm²]BruchdehnungA

[%]

StreckgrenzeReH

[N/mm²]0°C -20°C

27

610-780 500 16 31-47

27355510-680 20-22

285

355

355

>18

22

>22

49(bei +20°C)

76(bei -10°C)

21-39

460-550

510-610

400-490

£0,18

£0,55

£0,35

£0,1-0,35

£0,50

0,1- 0,6

£0,26

£0,15

0,21

£0,20 £1,60 0,040

1- 1,7 0,035

³0,6 £0,05

0,5- 1,3 0,035

0,7- 1,5 £0,05

0,040 /

0,030 0,30

£0,05 /

0,0350,40-0,80

£0,05 /

/ /

0,020 0,20

/ /

/0,25-0,5

/ /

£0,009 /

0,020 0,1

/ /

/ £0,30

/

/ /

1 0,05

/ /

£0,65 /

/

0,22

/

0,02-0,12

// / /

Chemical composition and mechanicalparameters of different steel sorts


impact energy AVelongation after fracture Ayield point ReHTensile strength RmSteel

Steel

Figure 4.8


The steel S355J2G2 represents the basic type of structural steels which are nowa-

days commonly used. Apart from a slightly increased Si content for desoxidisation it

this an unalloyed steel.

S500N is a typical fine-grained structural steel. A very fine-grained microstructure

with improved tensile strength values is provided by the addition of carbide forming

elements like Cr and Mo as well as by grain-refining elements like Nb and V.

The boiler steel P295NH is a heat-resistant steel which is applied up to a temperature

of 400°C. This steel shows a relatively low strength but very good toughness values

which are caused by the increased Mn content of 0,6%.

S355J2G1W is a weather-resistant structural steel with mechanical properties similar

to S355J2G2. By adding Cr, Cu and Ni, formed oxide layers stick firmly to the work-

piece surface. This oxide layer prevents further corrosion of the steel.

S355G3S belongs to the group of shipbuilding steels with properties similar to those

of usual structural steels. Due to special quality requirements of the classification

companies (in this case: impact energy) these steels are summarised under a special

group.


Figure 4.9

The steel grades are classified into four subgroups according to the chemical com-

position (Fig. 4.9):

● Unalloyed steels (except free-cutting steels) with a Mn content of < 1 %

● Unalloyed steels with a medium Mn content > 1 %, unalloyed free-cutting

steels and alloyed steels (except high-speed steels) with individual alloying

element contents of less than 5 percent in weight

● Alloyed steels (except high-speed steels), if, at least for one alloying element

the content is ≥ 5 percent in weight

● High-speed steels

The unalloyed steels with Mn con-

tents of < 1% are labelled with the

code letter C and a number which

complies with the hundredfold of the

mean value which is stipulated for the

carbon content.

Unalloyed steels with a medium Mn

content > 1 %, unalloyed free-

cutting steels and alloyed steels

(individual alloying element con-

tents < 5 %) are labelled with a num-

ber which also complies with a

hundredfold of the mean value which

is stipulated for the carbon content,

the chemical symbols for the alloying

elements, ordered according to the

decreasing contents of the alloying

elements and numbers, which in the

sequence of the designating alloying

elements give reference about their content. The individual numbers stand for the

medium content of the respective alloying element, the content had been multiplied


by the factor as indicated in Fig. 4.9 / Table 4.1 and rounded up to the next whole

number.

The alloyed steels are labelled with the code letter X, a number which again com-

plies with the hundredfold of the mean value of the range stipulated for the carbon

content, the chemical symbols of the alloying elements, ordered according to de-

creasing contents of the elements and numbers which in sequence of the designating

alloying elements refer to their content.

High-speed steels are designated with the code letter HS and numbers which, in the

following sequence, indicate the contents of elements:: tungsten (W), molybdenum

(Mo), vanadium (V) and cobalt (Co).

The European Standard DIN EN 10027-2 (September 1992) specifies a numbering

system for the designation of steel grades, which is also called material number

system..

The structure of the material number is as follows:

1. XX XX (XX)

Sequential number The digits inside the brackets are intended for possible future demands.

Steel group number (see Fig. 4.10)

Material main group number (1=steel)


Figure 4.10 specifies the material numbers for the material main group „steel“.

Figure 4.10


The influence of the austenite grain size on the transformation behaviour has been

explained in Chapter 2. Figure 4.11 shows the dependence between grain size of the

austenite which develops during the welding cycle, the distance from the fusion line

and the energy-per-unit length from the welding method. The higher the energy-per-

until length, the

bigger the austen-

ite grains in the

HAZ and the width

of the HAZ in-

creases. Such

coarsened austen-

ite grain decreases

the critical cooling

time, thus increas-

ing the tendency of

the steel to harden.

With fine-grained structural steels it is tried to suppress the grain growth with alloying

elements. Favourable are nitride and carbide forming alloys. They develop precipita-

tions which suppress undesired grain growth. There is, however, a limitation due to

the solubility of these precipitations, starting with a certain temperature, as shown in

Figure 4.12. Steel 1 does not contain any precipitations and shows therefore a con-

tinuous grain growth related to temperature. Steel 2 contains AIN precipitations which

are stable up to a temperature of approx. 1100°C, thus preventing a growth of the

austenite grain.

Influence of the energy-per-unitlength on the austenite grain size

13

11

9

7

5

30 0,2 0,4 0,6 0,8 1,0

Aust

enite

gra

in s

ize in

dex

acc

ord

ing

to D

IN 5

0601

Distance of the fusion linemm

Energy-per-unit length in kJ/cm

9 12 18 36


Figure 4.11


With higher temperatures, these

precipitations dissolve and cannot

suppress a grain growth any more.

Steel 3 contains mainly titanium car-

bonitrides of a much lower grain-

refining effect than that of AIN. Steel 4

is a combination of the most effective

properties of steels nos. 2 and 3.

The importance of grain refinement

for the mechanical properties of a

steel is shown in Figure 4.13. Pro-

vided the temperature keeps con-

stant, the yield strength of a steel

increases with decreasing grain size.

This influence on the yield point Rel is

specified in the Hall-Petch-law:

dKR

iel

1⋅+= σ

According to the

above-mentioned

law, the increase of

the yield point is

inversely propor-

tional to the root of

the medium grain

diameter d. σi

stands for the inter-

nal friction stress of

the material. The

grain boundary

resistance K is a

measure for the

influence of the grain size on the forming mechanisms. Apart from this increase of the

yield point, grain refinement also results in improved toughness values. As far as

Austenite grain size as a functionof the austenitization temperature

Steel % C % Mn % Al % N % Ti

1 0,21 1,16 0,004 0,010 /

2 0,17 1,35 0,047 0,017 /

3 0,18 1,43 0,004 0,024 0,067

4 0,19 1,34 0,060 0,018 0,140

900 1000 1100 1200 1300 1400°C

Austenitization temperature

18

6

4

2

10-1

8

6

4

2

10-2

6 10-3

8

mm

Me

diu

m f

ibre

len

gth

Gra

in s

ize

ind

ex

acc

ord

ing

to

DIN

50

60

1

-4

-2

0

2

4

6

8

10

12

Steel 1Steel 2Steel 3Steel 4


Figure 4.12

Connection betweenyield point and grain size

900

800

700

600

500

400

300

200

N/mm²

10 2 3 4 5 6 7 8 10mm-1/2

Yie

ld p

oin

t o

r 0

,2 b

ou

nd

ary

Grain size d-1/2

Temperature in °C:

-193

-185

-180

-155

+20

-40

-100

-170


Figure 4.13


structural steels are concerned, this means the improvement of the mechanical prop-

erties without any further alloying. Modern fine-grained structural steels show im-

proved mechanical properties with, at the same time, decreased content of alloying

elements. As a consequence of this chemical composition the carbon equivalent

decreases, the weldability is improved and processing of the steel is easier.

The major advan-

tages of microal-

loyed fine-grained

structural steels in

comparison with

conventional struc-

tural steels are

shown in Figure

4.14. Due to the

considerably better

mechanical proper-

ties of the fine-

grained structural

steel in comparison

with unalloyed structural steel, substantial savings of material are possible. This

leads also to reduced joint cross-sections and, in total, to lower costs when making

welded steel constructions.

Based on the

classification of

Figure 4.2, Fig-

ure 4.15 divides the

steels with regard

to their problematic

processes during

welding. When it

comes to unalloyed

steels, only ingot

Figure 4.14

Influence of the steel selection on theproducing costs of welded structures

S235JR S355J2G3 S690Q S890Q S960Q Verhältnis

(St37-2) (St52-3) (StE690) (StE890) (StE960) S235JR - S960Q

Streckgrenze N/mm2215 345 690 890 960 1 : 5

Blechdicke mm 50 31 14,4 11 10 5 : 1

Nahtquerschnitt mm2870 370 100 60 50 17 : 1

Schweißdraht ø 1.2 mm SG2 SG3 NiMoCr X 90 X 96 -

Schweißdrahtkosten Verhältnis 1 1 2,4 3,2 3,3 1 : 3,3

Stahlkosten Verhältnis 1 1,2 1,9 2,3 2,4 1 : 2,4

Schweißgutkosten Verhältnis 5,3 2,3 1,5 1,16 1 5,3 : 1

Spez. Schweißnahtkosten Verhältnis 12 5,1 1,8 1,18 1 12 : 1

Kostenverhältnis inklusiveGrundwerkstoffe

5 : 1

Randbedingungen: Schweißverfahren = MAG

Abschmelzleistung = 3 kg Schweißdraht / h, Nahtform X - 60°

Lohn- und Maschinenkosten = 60 DM / h

Spez. Schweißnahtkosten = Schweißzusatzwerkstoffe + Schweißen

Berechnungsgrundlage =szul = Re / 1.5

Stahlsorte


Yield point

Plate thickness

Weld cross-section

Welding wire Ø 1.2

Welding wire costs

Steel costs

Weld metal costs

Special weld costs

Costs ratio inclusive basematerials

Ratio

Ratio

Ratio

Ratio

Boundary condition: welding process = MAG

Deposition rate = 3 kg welding wire/h, weld shape X -60°

Costs of labour and equipment = 30€/h

Special weld costs = weld filler materials + welding

Calculation base = = Re/1.5szul

Steel type Ratio

Figure 4.15

Classification of steels withrespect to problems during welding

low-alloyed high-alloyed

hardeningspecial properties areachieved, for example:

heat resistance,tempering resistant,

high-pressure hydrogen resistance,toughness at low temperatures,

surface treeatment condition, etc.

corrosionresistant steels

tool steels

Hardening,special properties

are achieved

steels

unalloyed alloyed

mild steel higher-carbon steel

HardeningUnderbead cracking

rimmed steel killed steel duplex killed steel

cutting ofsegregation

zones

cold brittleness(coarse-grained recrystallization

after critical treatment)stress corrosion crackingsafety from brittle fracture

ferritic pearlitic-martensitic austenitic

grain desintegrationstress corrosion

cracking hot cracks(sigma phaseembrittlement)

hardeningembrittlement

formationof chromium

carbide

grain increase inthe weld interfaces

Post-weld treatment forhighest corrosion resistance



casts, rimmed and semi-killed steels are causing problems. “Killing” means the re-

moval of oxygen from the steel bath.

Figure 4.16 shows cross-sections of ingot blocks with different oxygen contents.

Rimming steels with increased oxygen content show, from the outside to the inside,

three different zones after solidification: 1.: a pronounced, very pure outer envelope,

2.: a typical blowhole formation (not critical, blowholes are forged together during

rolling), 3.: in the

centre a clearly

segregated zone

where unfavourable

elements like sul-

phur and phospho-

rus are enriched.

During rolling, such

zones are stretched

along the complete

length of the rolling

profile.

Figure 4.17 shows important points to be observed during welding such steels. Due

to their enrichment with alloy elements, the segregation zones are more transforma-

tion-inert than the

outer envelope

and are inclined to

hardening. In

addition, they are

sensitive to hot-

cracking, as, in

these zones, the

elements phospho-

rus and sulphur

are enriched.

Figure 4.16

Ingot cross-sectionsafter different casting methods

Figures: mass content of oxygen in %

fully killed steel semi-killed steel rimmed steel

0,003

0,012

0,025


Figure 4.17

Example of unfavourable (a) andfavourable (b) welds

a b

B CD

E



Therefore, “ touching” such segregation zones during welding must be avoided by all

means.

In the case of low-

alloy steels, the

problem of HAZ

hardening during

welding must be

observed. Figure

4.18 shows hard-

ness values of

various microstruc-

tures. The highest

hardness values

can be found with

martensite and

cementite. Hardness values of cementite are of minor importance for unalloyed and

low-alloy steels because its proportion in these steels remains low due to the low C-

content.

However, hardening because of martensite formation is of greatest importance as the

martensite proportion in the microstructure depends mainly on the cooling time.

Figure 4.19 shows

the essential influ-

ence of the mart-

ensite content in

the HAZ on the

crack formation of

welded joints.

Hardening through

martensite forma-

tion is not to be

expected with pure

carbon steels up to

about 0,22%,

Hardness of Several Microstructures

Microstructures Average Brinell Hardness (Approximately)

Ferrite 80

Austenite 250

Perlite (granular) 200

Perlite (lamellar) 300

Sorbite 350

Troostite 400

Cementite 600 - 650

Martensite 400 - 900

© ISF 2004Br-er-05-18.cdr

Figure 4.18

Influence of Martensite Content

strength,calculated at

max. hardness

with maximummartensite

contentHV HRC N/mm2 %

root crackingpresumable

400 41 1290 70

root crackingpossible

400 - 350 41 - 36 1290 - 1125 70 - 60

no root cracking 350 36 1125 60

sufficient operational safetywithout heat treatment

280 28 900 30

maximum hardness

If too much martensite develops in the heat affected zone during welding (below or next to the weld),a very hard zone will be formed which shows often cracks.


Figure 4.19


because the critical cooling rate with these low C-contents is so high that it normally

won’t be reached within the welding cycle. In general, such steels can be welded

without special problems (e.g., S. 235).

In addition to car-

bon, all other alloy

elements are im-

portant when it

comes to marten-

site formation in

the welding cycle,

as they have sub-

stantial influence

on the transforma-

tion behaviour of

steels (see

Fig. 2.12 ). It is not

appropriate just

to take the carbon content as a measure for the hardening tendency of such steels.

To estimate the weldability, several authors developed formulas for calculating the

so-called carbon equivalent, which include the contribution of the other alloy ele-

ments to hardening tendency, (Fig. 4.20). As these approximation formulas are em-

pirically determined

and as for the

hardening tendency

the general condi-

tions like plate

thickness, heat

input, etc., are also

of importance, the

carbon equivalent

cannot be a com-

mon limit value for

the weldability.

For the determina-

Figure 4.20

Definition of C - Equivalent

C-Äqu.= carbon equivalent (%) PLS = pipeline steels PCM = (%)cracking parameters

IIW

Stout

Ito and Bessyo

Mannesmann

Hoesch

Thyssen

15

NiCu

5

VMoCr

6

MnCÄqu.C

++

++++=-

40

Cu

20

Ni

10

MnCr

6

MnCÄqu.C ++

+++=-

5B10

V

15

Mo

60

Ni

20

CrCuMn

30

SiCPCM ++++

++++=

40

Ni

20

CuCr

10

MoMnCCET +

++

++=

20

VMoNiCrCuMnSiCÄqu.C

+++++++=-

15

V

40

Mo

60

Ni

20

Cr

16

CuMn

25

SiCÄqu.C PLS ++++

+++=-


Mo

Figure 4.21

Quelle: DIN EN 1011-2br-er05-21.cdr

Calculation of the preheating temperatures

Tp =697 CET + 160 tanh (d/35) + 62 HD + (53 CET - 32) Q - 3280,35

-100

-80

-60

-40

-20

0

20

40

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5

Wärmeeinbringen Q [kJ/mm]

delt

aT

p[°

C]

delta Tp = (53 CET - 32) Q - 53 CET + 32

d = 50 mmHD = 8

CET = 0,4 % CET = 0,2 % CET = 0,2 %

delta Tp = (53 CET - 32) Q - 53 CET + 32

CET = 0,4 % CET = 0,2 % CET = 0,2 %

d = 50 mmHD = 8

0

20

40

60

80

100

0 5 10 15 20 25

Wasserstoffgehalt HD des Schweißgutes [%]

de

lta

Tp

[°C

]

delta Tp = 62 HD 0,35 - 100

CET = 0,33 %d = 30 mmQ = 1 kJ/mm

delta Tp = 62 HD - 1000,35

CET = 0,33 %dQ = 1 kJ/mm

= 30 mm

0

50

100

150

200

250

0,2 0,3 0,4 0,5

Kohlenstoffäquivalent CET [%]

Tp

[°C

]

Tp = 750 CET - 150

d = 30 mmHD = 4Q = 1 kJ/mm

Tp = 750 CET - 150

d = 30 mmHD = 4Q = 1 kJ/mm

0

10

20

30

40

50

60

0 20 40 60 80 100

Blechdicke d [mm]

de

lta

Tp

[°C

]

delta Tp = 160 tanh (d/35) - 110

CET = 0,4 %HD 2Q = 1 kJ/mm

delta Tp = 160 tanh (d/35) - 110

CET = 0,4 %HD = 2Q = 1 kJ/mm

© ISF 2005

Heat input

Hydrogen content of the weld metalCarbon aquivalent

Plate thickness

Source:


tion of the preheating temperature Tp, the formula as shown in Figure 4.21 is used.

The effects of the chemical composition which is marked by the carbon equivalent

CET, the plate thickness d, the hydrogen content of the weld metal HD and the heat

input Q are considered.

The essential factor

to martensite forma-

tion in the welding

cycle is the cooling

time. As a measure

of cooling time, the

time of cooling from

800 to 500°C (t8/5) is

defined (Fig. 4.22).

The temperature

range was selected

in such a way that it

covered the most

important structural transformations and that the time can be easily transferred to the

TTT diagrams.

Figure 4.23 shows

measured time-

temperature distri-

butions in the vicin-

ity of a weld. Peak

values and dwell

times depend obvi-

ously on the loca-

tion of the

measurement and

are clearly strongly

determined by the

heat conduction

conditions.

Figure 4.22

Definition of t8/5

Tem

pera

ture

T

Time t

Tmax

°C

800

500

t t s800 500

DT

t8/5


Figure 4.23

Temperature-time curvesin the adjacence of a weld

2000

°C

1500

1000

500

00 50 100 150 200 250 s 300

Time t

Tem

pe

ratu

reT

A

B

C

10mm

A

B

C



With the use of thinner plates with complete heating of the cross-section during weld-

ing, the heat conductivity is only carried out in parallel to the plate surface, this is the

two-dimensional heat dissipation.

With thicker plates, e.g. during welding of a blind bead, heat dissipation can also be

carried out in direction of plate thickness, heat dissipation is three-dimensional.

These two cases

are covered by the

formulas given in

Figure 4.24, which

provide a method

of calculating the

cooling time t8/5 of

low-alloyed steels.

In the case of a

three-dimensional

heat dissipation,

t8/5 it independent

of plate thickness.

In the case of two-dimensional heat dissipation it is clear that t8/5 becomes the shorter

the thicker the plate thickness d is. Provided, the cooling times are equal, the plate

thickness can be calculated from these relations where a two-dimensional heat dissi-

pation changes to a three-dimensional heat dissipation.

Figure 4.25 shows

the influence of the

welding method on

the heat dissipa-

tion. With the same

heat input, the

energy which is

transferred to the

base material

depends on the

Figure 4.24

Calculation equation for two- andthree-dimensional heat dissipation

3 - dimensional:

2 - dimensional:


÷÷ø

öççè

æ

--

-×

××

××=

00

5/8800

1

500

1

2 TTv

IUt

lph

( ) 3

00

04

5/8800

1

500

110567,0 N

TTv

IUTt ×¢×÷÷

ø

öççè

æ

--

-×

×××-= - h

úúû

ù

êêë

é÷÷ø

öççè

æ

--÷÷

ø

öççè

æ

-××÷

ø

öçè

æ ××

××××=

2

0

2

02

22

5/8800

1

500

11

4 TTdv

IU

ct

rlph

( ) 22

2

0

2

02

2

05

5/8800

1

500

11103,4043,0 N

TTdv

IUTt ×¢×

úúû

ù

êêë

é÷÷ø

öççè

æ

--÷÷

ø

öççè

æ

-××÷

ø

öçè

æ ×××-= - h

÷÷ø

öççè

æ

-+

-×

××¢×

×-×-

=-

-

0004

05

800

1

500

1

10567,0

103,4043,0

TTv

IU

T

Td

üh

K3

universal formula:

extended formulaFor low-alloyed steel:

universal formula:

extended formulaFor low-alloyed steel:

K2

formula for the transitionthickness of low-alloyed steel:

Figure 4.25

Relative thermal efficiency degreeof different welding methods

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

SA welding

Manual arc welding

MAG-(CO )-2 welding

MIG-(Ar)-welding

TIG-(Ar)-welding

TIG-(He)-welding

welding methods

Relative thermal efficiency degree ‘h



welding method. This dependence is described by the relative thermal efficiency ŋ’.

The influence of

the groove ge-

ometry is covered

by seam factors

according to

Fig. 4.26. Empiri-

cally determined,

these factors were

introduced for an

easier calculation.

For other groove

geometries, tests

to measure the

cooling time are recommended.

Fig. 4.27 shows the transition of the two-dimensional to the three-dimensional heat

dissipation for two different preheating temperatures in form of a curve according to

the equation of Fig. 4.24. Above the curve, t8/5 depends only on the energy input, but

not on the plate thickness, heat dissipation is carried out three-dimensionally.

Figure 4.26

Weld factors for differentweld geometries

Type of weldweld factor

2-dimensionalheat dissipation

3-dimensionalheat dissipation

1

0,45 - 0,67

0,9

0,9

1

0,67

0,67

0,9


Figure 4.27

Transition From Two to ThreeDimensional Heat Flow

Heat input E. .N [kJ/cm]h n

0 10 20 30 40 50

5

cm

3

2

1

0

Pla

te t

hic

kne

ss

cooling time t [s]10 15 20 25

8/5

3040

60100

2-dimensional

3-dimensional

T =20°CA

0 10 20 30 40 50

cooling time t [s]10 20 30 40 50

8/5

2-dimensional

3-dimensional

T =200°CA

60

80

100

150



Fig. 4.28 shows the

possible range of

heat input depend-

ing on the elec-

trode diameter. It is

clear that a rela-

tively large working

range is available

for arc welding

procedures. A

variation of the

energy-per-unit

length can be

carried out by alteration of the welding current, the welding voltage and the welding

speed.

Fig. 4.29 depicts variations of the heat

input during manual metal arc weld-

ing. The shorter the fused electrode

distance, i.e., the shorter the ex-

tracted length, the higher the energy-

per-unit length.

Figure 4.28

br-er-05-28.cdr

Heat Inputs ofVarious Welding Methods

3,25 4 5 6 0,8 1,0 1,2 1,6 2,5 3,0 4,0 5,0

20

kJ/cm

12

8

4

Heat in

put

Manual metal arc welding MAGC-, MAGM-method

SA-welding

-short arc

-sprayarc

© ISF 2004

Figure 4.29


35

kJ/cm

25

20

15

10

5

0

En

erg

y-p

er-

un

it le

ng

th

0 50 100 150 200 250 300 350 400 450 500 mm 600

run-out length

Stick electrode(mm)

Current intensity (A)

Current intensity (A)

2,5

90

75

3,25

135

120

4,0

180

140

5,0

235

190

6,0

275

250

Æ6,0mm x 450mm

Æ5,0mm x 450mm

Æ4,0mm x 450mm

Æ3,25mm x 350mm

Æ2,5mm x 350mm

Energy-per-unit length as afunction of the run-out length


In order to minimize calculation efforts in practice, the specified relations were

transferred into nomograms from which permissible welding parameters can be read

out, provided some additional data are available. Fig. 4.30 shows diagrams for two-

dimensional heat dissipation, where a dependence between energy-per-unit length,

cooling time and preheating temperature is given, depending on the plate thickness. .

If a fine-grained structural steel is to be welded, the steel manufacturer presets a

certain interval of cooling times, where the steel characteristics are not too negatively

affected. The user lays down the plate thickness and, through the selection of a

welding method, a specified range of heat input E. Based on the data E and t8/5 the

diagram provides the required preheating temperature for welding the respective

plate thickness.

Figure 4.30

br-er05-30.cdr

Dependence of E, t andd During SA - Welding

8/5

Heat input E5 6 7 8 9 10 15 20 30 kJ/cm 50

504030

20

10

7

504030

20

10

7

504030

20

10

7

504030

20

10

7

Coolin

g tim

e t

in s

8/5

d = 7,5 mm

d = 10 mm

d = 15 mm

d = 20 mm

transition to3-dimensional

heat flow

T 200°C150°C100°C

20°C

0

T 200°C150°C100°C

20°C

0

T 200°C150°C100°C

20°C

0

T 200°C150°C100°C

20°C

0

© ISF 2004


With the transition to thicker plates,

the diagrams in Fig. 4.31 apply. The

upper part of the figure determines

whether a two-dimensional or a three-

dimensional heat dissipation is pre-

sent. For the three-dimensional heat

dissipation, the lower diagram applies

where the same information can be

determined, independent of plate

thickness, as with Fig. 4.30.

The relation be-

tween current and

voltage for MAG

welding is shown

in Fig. 4.32 and

the used shielding

gas is one of the

parameters. Weld-

ing voltage and

welding current, or

wire feed speed,

determine the type

of arc.

Figure 4.31

br-er05-31.cdr

Dependence ofE, T , t And d0 8/5 Ü

Heat input E

50s

40

30

20

15

109

87

Coolin

g tim

e t 8

/5

5 6 7 8 9 10 15 20 30 kJ/cm 50

T250

°C

200°C

150°C

100°C

20°C

0

Heat input E

50mm

40

30

20

15

109

87

Tra

nsi

tion thic

kness

dÜ

5 6 7 8 9 10 15 20 30 kJ/cm 50

aera of3-dimensional

heat flow

area of2-dimensional

heat flow

T250 °C 200 °C

150 °C 100 °C

20 °C

0

© ISF 2004

Figure 4.32

br-er-05-32.cdr

Dependence of Current And Voltage DuringMAG-Welding, Solid Wire, 1.2 mmÆ

35V

30

25

20

15

We

ldin

g v

olta

ge

Welding current

Wire feed

150 200 250 A 300

3,5 4,5 5,5 7,0 8,0 9,0 10,5 m/min

C1

M21

M23

gas composition:C1 100% COM21 82% Ar + 18% COM23 92% Ar + 8% O

2

2

2

short arc

contact tube distance ~15mm contact tube distance ~19mm

mixed arc spray arc

© ISF 2004


The diagram in Fig. 4.33 demon-

strates the dependence of plate thick-

ness, heat input E and cooling time

t8/5 for fillet welds at a preheating

temperature of T0 = 150°C. If d and

t8/5 are given, the acceptable range of

heat input can be determined with the

help of this diagram. The kinks of the

curves mark the transition between

two-dimensional and three-

dimensional heat dissipation.

Fig. 4.34 shows the same depend-

ence for butt welds with V groove

preparation.

Figure 4.33

br-er05-33.cdr

Permissible E-RangeDuring SA - And MAG - Welding

hh

' = 1' = 0,85

d = 32 mmd = 15 mm

UP

MAG

U max

U min

F = 0,67F = 0,67

3

2

t = 30 st = 6 s8/5 max

8/5 min

E = 66 kJ/cmE = 14 kJ/cm

max

min

60

kJ/cm

50

45

40

35

30

25

20

15

10

5

0

70

kJ/cm

59

53

47

41

35

29

23

18

12

6

0

He

at

inp

ut

ES

A-

we

ldin

g

He

at

inp

ut

EM

AG

- w

eld

ind

Plate thickness

0 5 10 15 20 25 30 mm 40

cracking tendency

toughness affection

fillet weldsT = 150 °C0 30s

25s

20s

15s

10s

6s

© ISF 2004

Figure 4.34

br-er05-34.cdr

Permissible E-RangeDuring SA - And MAG - Welding

hh

' = 1' = 0,85

d = 34 mmd = 15 mm

UP

MAG

U max

U min

F = 0,9F = 0,9

3

2

t = 30 st = 6 s8/5 max

8/5 min

E = 49 kJ/cmE = 10 kJ/cm

max

min

60

kJ/cm

50

45

40

35

30

25

20

15

10

5

0

70

kJ/cm

59

53

47

41

35

29

23

18

12

6

0

Heat

inp

ut

ES

A-

weld

ing

Heat

inp

ut

EM

AG

- w

eld

ing

Plate thickness

0 5 10 15 20 25 30 mm 40

cracking tendency

toughness affection

butt weldsT = 150 °C0

30s

25s

20s

15s

10s

6s

© ISF 2004


The curve family in Fig. 4.35 shows the dependence of the heat input from the weld-

ing speed as well as the acceptable working range. The parameters of the curves 1

to 8 in the table

have been taken

from Figures 4.32

and 4.34 and apply

only for related

conditions like wire

diameter, wire

feed, welding

voltage, etc.

Figure 4.36 shows

a reading example

for such diagrams

(according to DVS-

Reference Sheet

Nr. 0916).

In this example, a

plate thickness of

15 mm and a cool-

ing time t8/5 be-

tween 10 and 20 s

are given. In this

case, the maximum

cooling time for MAG welding is 15 s. A solid wire with a diameter of 1.2 mm at 29V

and 300A is used.

The left diagram provides heat input values between 13 and 16 kJ/cm, based on the

given data. Using these values, the acceptable range of welding speeds can be

taken from the diagram on the right.

Figure 4.35

br-er-05-35.cdr

E as a Function of Welding Speed,Solid Wire, 1.2mmÆ

MAG/ M21 (82% Ar, 18% CO)

25kJ/cm

20

15

10

5

010 15 20 25 30 35 40 45 50 cm/min 60

Welding speed vS

He

at

inp

ut

E

working range

12

34

56

7

8

curve

V

A

v (m/min)Z

29

300

10.5

27

275

9.0

24

250

8.0

22

225

7.0

20

200

5.5

19

175

4.5

18

150

3.5

17

125

3.0

1 2 3 4 5 6 7 8

© ISF 2004

Figure 4.36

br-er-05-36.cdr

Determination of Welding Speedfor MAG - Welding

curve

V

A

v (m/min)Z

29

300

10.5

27

275

9.0

24

250

8.0

22

225

7.0

20

200

5.5

19

175

4.5

18

150

3.5

17

125

3.0

1 2 3 4 5 6 7 860

kJ/cm

50

45

40

35

30

25

20

15

10

5

0

70

kJ/cm

59

53

47

41

35

29

23

18

12

6

0

He

at

inp

ut

E

SA

- w

eld

ing

He

at

inp

ut

E

MA

G -

we

ldin

g

Plate thickness0 5 10 15 20 25 30 mm 40

cracking tendency

toughness affection

butt weldsT = 150 °C0

30s

25s

20s

15s

10s

6s

30s

25s

20s

15s

10s

6s

1613

25kJ/cm

20

15

10

5

010 15 20 25 30 35 40 45 50 cm/min 60

Welding speed vS

he

at

inp

ut

E working range

12

34

56

7

8

16

13

33 41

© ISF 2004


Fig. 4.37 presents a simplification of

the determination of the microstruc-

tural composition and cooling time

subject to peak temperatures which

occur in the welding cycle. In the

lower diagram, the point of the plate

thickness at the top line is linked with

the point of heat input at the lower

line. The point of intersection of the

linking line with the middle scale

represents the cooling time t8/5 .

If the peak temperature of the welding

cycle is known, one can read from the

middle diagram in which transition

field the final microstructures are

formed. The advantage of the deter-

mination of microstructures compared

with the upper TTT diagram is that

a TTT diagram applies only for exactly one peak temperature, other peak tempera-

tures are disregarded. The disadvantage of the PTCT diagram (peak temperature

cooling time diagram) is the very expensive determination, therefore, due to the

measurement efforts a systematic application of this concept to all common steel

types is subject to failure.

Figure 4.37

© ISF 2004

Peak temperature/cooling time– diagram for the determination

of t and the structure8/5

bie5-37.cdr

1400

°C

1200

1000

800

600

800

°C

700

600

500

400

300

200

1 10 100 1000Te

mpera

ture

Peak

tem

pera

ture

B

M

M

Arc3

Arc1

B+M F+B

300 200HV30=400

F+P

F

P

s t8/5

40 30 25 20 15 10 9 8 7 6 5 mm 4plate thickness

300 100200three-dimensional

1 2 3 5 10 20 50 100 200 400 s 1000two-dimensional

0 100 °C 200preheating temperature

6 8 10 20 30 40 50 kJ/cm 70energy-per-unit length

t8/5

1000°C1400°C

Peak temperature

Documents

Chapter 4 - Classification of Steels, Welding of Mild Steels