Welding procedures for different steel types

Chapter 4 Welding procedures for different steel types This Book will make a Traveller of thee. If by its Counsel thou wilt ruled be; Yea, it will make the slothful active be. The blind also delightful things to see.

So far, the factors responsible for hydrogen-induced cracking have been described, the principles upon which safe welding procedures can be based have been outlined, and the information required before using a welding diagram has been listed. It is now possible to explain in this chapter the selection of safe welding procedures for different steel types. The welding procedures derived in this book were intended to give a high degree of protection against the risk of hydrogen cracking in weld HAZs. In 1974, when incorporation of the procedures into the British Standard for welding structural steels (BS 5135*) was discussed, comparison of the TWI data with the experience of the British structural steel fabrication industry showed that the TWI procedures were more conservative than had been used successfully up to that time. It appeared that the discrepancy could be removed if the TWI procedures were made appropriate to steels of 0.02 CE higher, and this difference was put into the standard, so that the procedures for a steel of, say, 0.45 CE in the present text were equivalent to those of a steel of 0.47 CE in BS 5135: 1974. Although experience since BS 5135 was introduced in 1974 has generally been satisfactory (and the shift from TWI data has been maintained in BS * BS 5135 was superseded by BS EN 1011-1 and 2 when these were published in

1998 and 2001 respectively. Recommendations on the derivation of procedures to avoid hydrogen cracking are presented by two methods, method A and method B. Method A is essentially that in BS 5135. with the following principal modifications and additions. (1) Modifications to welding procedures for steels of low CE, as in the present text. (2) A new CE scale (E), based on a maximum diffusible hydrogen level of 3ml/l00g deposited metal. (3) Procedures for steels having CEs up to 0.70 for use with scale C, 0 and E. (4) Heat input (as defined in chapter 3 and in BS EN 1011-1) is now used rather than arc energy as an entry point into the diagrams.

47

Welding procedures for different steel types EN 1011-2: 2001), the original TWI approach has been retained in the present text. This is because the more conservative TWI procedures give less risk of small hydrogen cracks occurring, which may be detected only when a high level of quality is needed and inspection standards are high. For mild steels, C-Mn steels and low alloy steels with yield strengths up to about 600N/mm2 , HAZ cracking is generally the major form induced by hydrogen. Possible exceptions involve welded joints where the design imposes a particularly high stress concentration; for example in a partial penetration weld or a weld made on to a permanent backing bar. Under such conditions the symptoms may be those of weld metal hydrogen cracking, but close examination often reveals that cracking has begun in the HAZ, even though it has propagated through the weld metal. Hence, the information given in Tables 4.1 and 4.2 and Fig. 4.1-4.3 is based on avoidance of hydrogen cracking in HAZs, and all experience shows that, for most fabrications, procedures based on these will also be adequate for avoiding hydrogen cracking in the weld metal. However, there are circumstances where cracking is more likely in weld metal and specific precautions may then be necessary to avoid that problem. This is particularly the case when little or no preheat is indicated by the diagrams and tables, either because a steel of fairly low CE «~0.45) is being welded or because low heat inputs are not being used. The risk of weld metal hydrogen cracking further increases when welding thick section steel (>~50 mm thick) and when using alloyed weld metal (especially if the CE of the weld metal exceeds that of the parent steel) or if a C-Mn weld metal is used containing >~1.5%Mn. As yet there are few comprehensive data to determine the preheat levels necessary to avoid weld metal hydrogen cracking. Recent work indicates that for non-alloyed C-Mn weld metal, in contrast to the situation in the HAZ, the risk of weld metal hydrogen cracking can increase, particularly in multipass welds, with increasing heat input. The normally envisaged relaxation of preheat at higher heat inputs should therefore be reviewed carefully in such situations. For steels containing <-0.2%C and of yield strengths greater than 600N/mm 2 , welded with weld metal of matching or over-matching yield strength, weld metal hydrogen cracking appears to be the predominant form. The precautions needed to prevent weld metal cracking are generally more than sufficient to prevent HAZ hydrogen cracking. It has already been stated (Chapter 1) that the factors responsible for weld metal cracking are essentially the same as those

48

Welding steels without hydrogen cracking for HAZ cracking. The types of microstructure which appear susceptible in the weld metal are, of course, different from those in the HAZ and for a given susceptibility have lower hardness values. There are, as yet, few systematic data which can be used to select safe welding procedures for steels of these higher strengths and in general an empirical approach, supported by joint simulation testing, is employed. A welding procedure can be defined by specifying a heat input calculated directly from welding parameters (Chapter 3). In manual welding it is convenient to maintain a specified heat input by controlling the length of weld produced by a single electrode. Tables 2.1-2.3 give the relationships between heat input and run length for electrodes containing different amounts of iron powder in their coatings. If, for strength purposes, a minimum fillet leg length is specified, the heat input values given in Table 2.4 should be used. It is advisable to check the run lengths of such welds, particularly when conditions begin to involve a small risk of cracking, because the relationship between heat input and leg length is affected by several factors.

Mild steel Approximate limits of composition: Carbon not more than Manganese not more than Silicon not more than

0.25% 1.0% 0.5%

Mild steel is the first, and generally the most weldable class of steel. It has very low hardenability and any suitable electrode may be used. For thin section welds preheat is not necessary, and with CE values less than 0.30 no control of bead size is needed for any combined thickness. Overhead and vertical-down welding can be performed successfully at 1.0 kj/rnm heat input on any thickness up to a CE of 0.32. Horizontal-vertical welding at 1.4kJ/mm can be undertaken up to a CE of 0.34. For mild steels generally, composition may not be known and a maximum CE of 0.38 must be assumed. Table 4.1 summarises the heat inputs to be used to control bead size when welding without preheat mild steels of assumed maximum CE 0.38.

For sections thicker than about 70 mm preheat may be necessary and Table 4.2 allows the appropriate level to be selected. Low hydrogen electrodes are required only when plate thickness exceeds about 25 mm, or if a specific toughness level is required. However, the use

49

Welding procedures for different steel types Table 4.1. Maximum thicknesses for welding mild steel without preheat (suitable for a maximum CE of 0.38)

Heat input, kJ/mm

Maximum combined thickness, mm Hydrogen potential:

0.6 0.8 1.0 1.4 1.8

High (A)

Medium (B)

Low (C)

33 42 51 69 87

37 48 60 83

44 58 73

Very Low (D)

~2.2

Notes:

A dash indicates no limit. 2 Lower values of combined thickness may be required for highly restrained joints.

1

Table 4.2 Maximum thicknesses for welding mild steel with preheat (suitable for a maximum CE of 0.38)

Preheat temperature,

°C

75

100

Notes:

Heat input, kl/mm

0.6 0.8 1.0 1.4 1.8 0.6 0.8 1.0 1.4 1.8

Maximum combined thickness, mm Hydrogen potential: High (A)

Medium (B)

Low (C)

41 51 62 82

45 60 74

58 79

48 63 78

59 76

Very Low (D)

1 A dash indicates no limit. 2 Lower values of combined thickness may be required for highly restrained joints.

of such electrodes or other low hydrogen processes may enable preheat temperatures and weld bead sizes to be reduced. Although a CE of 0.38 is generally the maximum for mild steel in relatively thin section, for thicknesses above about 100mm a higher level of CE may be encountered because of higher carbon and manganese contents. When the manganese content exceeds 1.2% the

50

Welding steels without hydrogen cracking steel should be treated as a C-Mn steel and welding procedures deduced as described below. When only the carbon content of thick section is known the procedures referring to fully hardened mild steel may be used as on pp. 61 to 62.

Carbon-manganese steels Approximate limits of composition: Carbon not greater than 0.25% Manganese 1.0-1.7% 4.1 Welding procedures for CMn steels without preheat: select electrode size and run length from Tables 2.1-2.4.

0.4.1

0.35 0.3

0.4lJ 0.41 0.38 0.33

Carbon equivalent levels 0.4 .)l 0.43 0.45 0.47 0.41 0.43 0.45 0.43 0.38 0.41

CE axis from Table 3.2

0.49 - - C I 0.47 _ Ii ...... 0.45 A ...

.

180 +---+t----ll-l-I---+-+---f---++----f

160 +---+t---1H---,I---+-+-+--+-+-....--l--L....-L........L-I /

140 t--1t--r-t-11111-r----tif-7~~~~

I

11 U

/

OA7OAY 0.510.57

OAY 0.51 0.53 O.5Y

/ / /[1'"' ' ' ' ' ' . /;//J~

//v~~~/ I /~V/)~ 1~1Ia~

80+---+-hf----HL-h,t-t,h~A-:.~-+----1

O+---I---+---+---t-----+---I o 2 3 4 5 6 Heat input. kJ/mm

O.m.550.57 -

Welding procedures for different steel types

51

Silicon not greater than 0.7% Niobium or vanadium not greater than 0.1% The second group of weldable steels comprises the weldable CMn steels, with or without microalloying additions of Nb, V and/or Ti.

These steels are of relatively low hardenability and preheat is not usually necessary for thin sections. Those thicknesses which can be welded without preheat however depend on composition, hydrogen level, and bead size; Fig. 4.1 relates these factors to define those thicknesses which can be handled without preheat. In this diagram the appropriate CE axis, A, B, C, or D, selected from Table 3.2,

e-

4.2 Welding procedures for Mn steels with preheat.

Combined thickness = t. +t2 +1 3 II = average thickness

t,

t

2

~

over a length of 75mm ... 75mm'For directly opposed twin fillel welds combined thickness = '/2 (I. +1 2 + (3) I,

Combined thickness, mm

"-1j+-t R ~o

/

3

./" V

///// V

1--7~/,y h-¥ /'/ /."...q /i--7"q-----!o,L. /"=-----::>.L+--""~ 1$// /

CE Scale A

o

./.........

0.3 0.35 0.40 1",,111" 1

B C

~ //. h/.

0.300.35 0.40

'""I"",

0.30 0.35 0.40 I

I

0.40

I

I

I

I

0.45

I

I

I

I

I

I

I

!

0.45

I

I

I

0.45

I

I

I

0.50

I

0.50 ,

I

I

0.55

Carbon Equivalent

I

I

'"

0.50

I

I

I

I

I

I

I

0.55

15 20

/

/

/

25

/

30

/

40

,/

50

/

60

/

75

1/

100

/

/ / / / 1/ / / / / / 1/ 125 II/II 4~ 11/ / /

'!OW.-/.:'··V···

'/>--:'......

50

~gv,,'

0.50

"!

I

0.45

~

:..----

10

.

2

I

!

0.55 I

I

Ma,ximUm{nOprehe 100°C

combined thickness

valu1elo1beused'jith

Heal input (kJ/mm)

2~oC

4

6

52

Welding steels without hydrogen cracking Minimum preheat temperature, "C 75 20 0

-

140

";

i

! !

120

!

,

100

!

§

Ii
:s ~

o

80

"'c::"

Scale (see Table 3.2)

A

B

0.30 0.33 0.38 0.38 0.41 0.41 0.43 0.43 0.45 0.45 0.47 0.47 0.49 0.49 0.51 0.51 0.53 0.53 0.55

C

D

0.35 0.41 0.43 0.45 0.47 0.49 0.51 0.53 0.55 0.57

0.43 0.46 0.48 0.50 0.53 0.55 0.57 0.59 -

-

~

see (b)

~

see (c) see (d) see (e) see (f) see (g) see (h) see (i) see (j)

~ ~

~ ~ ~

~ ~


:E 60 E 0

U

40

II II

, 20

(a)

4.3a-j Welding procedures for C-Mn steels with selected CE values for MMA, select electrode sizes and run lengths from Tables 2.1-2.4, for other processes use factors from p.

42.

o o

0.5 Heat input, kJ/mm

1.5

together with the maximum CE value for the steel in question, are used to identify the line relating to the heat inputs for different combined thicknesses. Figure 4.2 is a comprehensive diagram showing the weld sizes and preheat levels necessary when heat input must be restricted, as in positional welding, and for thicker plates. It is used in the manner described in Chapter 2 and links all the variables into one diagram. The same information is given again in Fig. 4.3a-j, but subdivided, so that the preheat temperature, combined thickness, and heat input values at specific CE levels can be emphasised. This method of presentation was also used in Fig. 4.1. Provided strength and toughness requirements can be met, low hydrogen processes are not essential, although their use allows bead sizes and preheat temperatures to be reduced. It is always advantageous to use the lowest strength consumable consistent with achieving the required strength and toughness. The likelihood of hydrogen cracking occurring in this type of steel increases as carbon and man-

53 4.3

Welding procedures for different steel types

Continued

Minimum preheat temperature, °C

Minimum preheat temperature, °C

125 100 75

100 7'5 200

,

14v

;

120

12v

s

S 100

]'"

,

80

u

:£

20 0

s

S 100

]''"" u

-e ~ 60

I ;

~o

u 40

I

,

20

:£

]

-

~

80 60

40 20

;

0 (b)

o

200

1 Heat input, kJ/mm

150

12'5

100

2

0.5

75

20 0

160

140

,

;i 120

~

100

~

80

,

60

,,

40 20

(d)

1

1.5

2

2.5

3

Minimum preheat temperature. °C -i

180

£ ]

1

Heat input, kJ/mm

2 3 Heat input. kJ/mm

4

Welding steels without hydrogen cracking

54

4.3

Continued

ISO

200

125

100

75

o

20

Minimum preheat temperature, °C

180 160

§ vi

'"

~ .~

-5 "0

ll)

140 120 100

c

80

u

60

~o

40 20

,

o

o

2

(e)

175

200

150

3 4 Heat input, kl/mm 125

100

5

75

6

20 Minimum preheat temperature. °C

180 160

S S

:i c

ll)

o

140

I

120

-""

.~ -5 100 "0

c

ll)

:E

S 0

u

80 60 40 20

o

o

(f)

~ 234 Heat input, kJ/mm

5

6

55 4.3 Continued

Welding procedures for different steel types 175

200

125

150

~

'" ~oS

I----

l-r-

~-

~

---r--

180

§

Minimum preheat temperature, °C

100

160

75

140

o

20

120 100

] ~

80 60

40

-l

-'

20

o

o

(g)

200

2

3

5

4

6

Heat input, kJ/mm

175

150

125

Minimum preheat temperature, °C

180 160

§

100

75

140

20

o

'" 120

'"

]

~ 100

l

~o

80

u 60 40 20 :

(h)

o o

234 Heat input, kJ/mm

5

6

56 4.3

Welding steels without hydrogen cracking

Continued

200

200

175

125 ,

ISO

Minimum preheat temperature, °C

180 160 E E V> V>

OJ

J2o

100

140

75

120

o

:s :E "E

100

U

60

20

"0

OJ

80

0

40 20 234

(i)

6

5

Heat input, kJ/mm

,.....

200

200

150

,

Minimum preheat temperature, °C

180 160

§ ~

]

, 125 100 75

140 120

20

o

~ 100

"0

~

:E 80

,

E

(3

60 40 20

o (j)

o

2

3 Heat input, kJ/mm

4

5

6

57

Tentative guidelines for avoiding weld metal hydrogen cracking when submerged-arc welding C: Mn steels with 83 (l.5%Mn) wire and basic flux. Multipass weld metal contains approximately 0.8%C. 1.5%Mn and has a CE of -0.36. Interpass time 20 minutes, hydrogen levels, thickness and preheat/interpass temperatures as shown. C - cracking found: NC - cracking not found. Interpass and preheat temperatures were the same for each test weld.

Welding procedures for different steel types 25 - w - - - - - r - - - - - - , - - - - - - - - - r - - - - - r - - - - - . ,

4.4

20 t - - - - - + - - - - + - - - I - - - I I - - - - + - - - - - i

8

15

t-----I-------+-+----+-+---I---+-------1

::J

E

NC 5 kJ/mm

IO.-----II----¥--+----+---.--I-----I

5t-----t----+-----I1----+-----; NC

o

50

100

150

Preheat/interpass temperature,

200

250

°c

ganese contents are raised, and a watch should be kept for occasional 'rogue' plates of high composition. It is important to ensure that tack welds are either made using the same procedure as for the main welds, or that the procedures for them are chosen from Fig. 4.2 or 4.3.

When C-Mn steels of very thick section are to be welded and fully hardened HAZs are likely to be formed, the procedures des-

58

Welding steels without hydrogen cracking cribed under 'Medium carbon and carbon-manganese steels' in this chapter can be used as an alternative to those summarised in Fig. 4.2. Also, with thick sections, and in the other cases mentioned in p. 47, the risk of weld metal hydrogen cracking can be minimised by taking account of the data given in Fig. 4.4. This figure is relevant to multipass submerged-arc weld metals (made from S3 (1.5%Mn) wire and a basic flux) containing about 1.5%Mn and having CE values of about 0.36. The welds were made with controlled interpass times of 20 minutes and preheat and interpass temperatures as shown. Up to weld metal hydrogen contents of 5 mIllOO g, hydrogen cracking in the weld metal was not a problem, but at higher levels, heat input was the major factor. Unlike HAZ cracking, increasing heat input increased the risk of cracking.

Lower carbon, lean alloy steels Approximate limits of composition: Carbon not greater than 0.20% Manganese not greater than 1.5% Other elements are present, but in amounts such that the CE does not greatly exceed 0.60, i.e. insufficient to confer high hardenability: in this group, hardenability is high if welding a combined thickness of 25 mm at a heat input of 1.4 k]/mm fully hardens the HAZ. The group is the third major group of weldable steels. These steels are generally of relatively low hardenability and can often be successfully welded with procedures similar to those used for C-Mn steels. Some of them, however, show different susceptibilities to hydrogen cracking at given HAZ hardness levels as compared with C-Mn steels. For example, a molybdenum-boron steel has a critical HAZ hardness of 375 HV instead of 350 HV when high and medium hydrogen processes are used. On the other hand, although differing in transformation behaviour, the Mn-Cr-Mo-V steel resembles C-Mn steel in critical hardness values. The use of Fig. 4.2 derived for C-Mn steels can therefore lead to either unsafe or uneconomic procedures in some instances. Of the very many individual steels in this group, only a few have been examined in detail and procedures for these are given, but others can be treated as C-Mn steels or as fully hardened alloy steels (see later in this chapter). In either case the presence of boron should be noted because it makes the steel more hardenable, even

59

Welding procedures for different steel types Minimumlocalpreheattemperature, "C

100

150

200 175

150

125

§

] u

:s -0

II

/V / / /

100

75

I /

~

~0

u

50

25

20

V

'/ /II J

l VI

V/ V

(b) .....

2

/ I /

~V /

(a)/

0

/

100

3

4

175

200

,)

J

20

V

/

I

/

I

J

II

/IJV/ II

150

10~'"

/V

If

rV /

V ,/

/ ~

125

100 20

~

(c) ....

2

3

4

2

3

4

Minimum heat input,kJ/mm

4.5 Welding procedures for steels to BS 1501: Part 2: 1988271, -281 (nearest current equivalent is BS EN 10028-6: 2003: P460Q) (Mn-Cr-Mo-V steels with maximum CE of 0.62); (a) very low hydrogen processes, Scale D, Table 3.2, (b) low hydrogen processes, Scale C, Table 3.2, and (c) medium and high hydrogen processes, Scales A and B, Table 3.2; select electrode size and run length from Tables 2.1-2.4.

in very small amounts. Although boron is not in the CE formula [1.1], it is in the P crn formula [1.2] with a multiplying factor of 5. Low hydrogen electrodes are normally required to produce the necessary strength and toughness, although sound welds can be made with rutile electrodes, especially in thin section. For steels not listed individually it is advisable to consult TWI to determine whether new information on weldability has become available. If much welding of an unlisted steel is envisaged, some experimental work to determine its transformation behaviour and susceptibility to cracking is recommended. The cost of this in most cases will be more than offset by savings either in unnecessary preheat or in repair. BS 1501: Part 2: 1988-271, -281 (Mn-Cr-Mo-V steels) (nearest current equivalent is BS EN 10028-6: 2003: P460Q)

These steels are less hardenable than would be expected from their CE levels. To reduce the HAZ hardness, therefore, lower preheats are needed than for C-Mn steels. Procedures for welding this steel, based on a maximum CE level of 0.62, may be obtained from Fig. 4.5. If fully hardened HAZs are expected an indication of suitable preheat levels may be gained from curve L of Fig. 4.6.

60

Welding steels without hydrogen cracking

!;J 300 r - - - - - - , - - - , - - - - - - - , - - - , - - - - , - - - - . , - - - , - - - - - ,

i

!

8'" 250 I - - - - - + - - - - t - - - - + - - - ! - - - - . . = . , - - - + - - - - - - j - - - - + - - - - - - j

~

I

8. 200 I - - - - - + - - - - t - - - g

.i~

150

1

~ 100 I - - - - - l -....

E

.~

~

01----+---f----I----+----+---t---+-----1 0.4

.----+---+----+----1r----+----t---+--~4

Ii< ~

0.31-----+----t----+---4r----+--""'7""'-t---,,----t---,,------I

.~

Ii 0.21-----+---+----+-"7"'~ ~

U

0.1 1-----+--:7""'--+-~"""'--+"7"''''---+_---+-----l----t------1

450

500

550

Expected HAZ hardness, HV

Diagram for selecting minimum preheat, interpass and postheat temperatures for alloy and lean alloy steels giving fully hardened HAZs. Note: relaxation of preheat temperatures may be possible with low hydrogen processes and thin sections, particularly at low C contents. Confirmation should be obtained by joint simulation tests. For class of steel refer to Table 4.3 4.6

600

650

700

ASTM A514 grade B (Tl type A steel] Experimental work with the controlled thermal severity (CTS) test indicates that a critical hardness of 400 HV can be accepted without cracking using medium hydrogen (normally dried basic) electrodes on this steel.' With matching strength electrodes of low hydrogen level, a similar critical hardness level was obtained. Studies of the hardenability of this steel suggest that the normal CE formula ([3.1], Chapter 3) overestimates the effect of composition when selecting the cooling rate to produce a 400 HV hardness level. It thus becomes possible to use Fig. 4.2 directly for predicting welding procedures for Tl type A steel as long as the CE scale, marked 'C', is taken as referring to medium hydrogen mild steel electrodes and low

61

Welding procedures for different steel types hydrogen matching strength electrodes (ef Table 3.2). The limited nature of available data must be borne in mind when using Fig. 4.2 in this way. It is also necessary to observe the steelmaker's limits on heat input and preheat to avoid reducing HAZ fracture toughness. If fully hardened HAZs are expected, the curve labelled 'C-Mn steels' in Fig. 4.7 may be used.

Medium carbon and carbon-manganese steels Approximate limits of composition: C steels

- Carbon 0.25-0.45% Manganese not more than 1.0% Silicon not more than 0.5% C-Mn steels - Carbon 0.25-0.45% Manganese 1.0-1.7% Silicon not more than 0.5%

The steels in this group are mainly the C and C-Mn steels used for general engineering purposes. In thin section, medium carbon and C-Mn steels can be welded using the procedures recommended for mild and C-Mn steels given already in this chapter. Such procedures ensure that sufficiently large weld beads are made so that the cooling rate is slow enough to avoid certain HAZ hardness levels. In thicker sections, however, fully hardened HAZs, which are very susceptible to hydrogen cracking, are formed readily, and welding procedures similar to those for alloy steels become necessary. Figure 4.7 enables a preheat, interpass, and postheat temperature to be selected for both carbon and C-Mn steels. Entering the lower part of the diagram at the appropriate carbon content and moving horizontally, the expected HAZ hardness level can be found from the bottom scale. Vertical movement into the top half of the diagram reveals a range of temperatures for different restraint conditions. The particular temperature selected for preheat, interpass, and postheat will depend on the particular welding problem and it may be possible to reduce this temperature by using low hydrogen processes, particularly if the carbon content is low and the section is thin. The suitability of the procedure chosen should be confirmed by joint simulation tests. With procedures of this type no control of weld bead size is normally necessary. However, when conditions are severe (for example, high carbon content, thick plate, or high restraint levels) there is some advantage in producing relatively small beads. These require little time at the interpass temperature for much of their

62

Welding steels without hydrogen cracking

4.7 Diagram for selecting minimum preheat, interpass, and postheat temperatures (carbon and C-Mn steels with fully hardened HAZs). Note: Relaxation of preheat temperatures may be possible with low hydrogen processes and thin sections, particularly at low C contents. Confirmation should be sought by joint simulation tests.

JJ

300

~

'High restraint"

==+----+----+----+----t-z""'"""-I----+---+----i 250

1

roo

3=+---+---+----l.
1

150

.1

j 100 5E .~" 0 ~

0.4

~~

-;O~.3;t---+---t---+----+~,...,:::...----P~---+----+----1

I

c -:0-:.2+---t----+---:::;,...,:::...----j,,~---+---+----+----+-----I

t

u

250

300

350

400

450 500 550 Expected HAZ hardness, HV

600

650

700

hydrogen to be lost by diffusion. If adequate time for such diffusion is not given, any remaining hydrogen will be trapped by succeeding runs and can give rise to cracking. Suitable inter-run times can be calculated by using the data and techniques given in Chapter 5, in which is given an example of such a calculation where each run is treated as an infinite cylinder. Steels of this type may also be welded using the isothermal transformation method described later in this chapter.

Alloy steels Approximate limits of composition: Carbon not greater than Total alloying elements greater than (excluding Mn)

0.45% 1.0%

63

Welding procedures for different steel types

The final group comprises the less weldable alloy steels, often used at high temperatures or for general engineering purposes. Most welding processes, with the possible exception of electroslag, produce hardened HAZs when applied to these steels. Preheat will therefore be required in nearly all circumstances. Basic coated, alloyed consumables are usually needed to achieve the required strength and toughness and sometimes to provide resistance to creep or hydrogen attack at elevated temperature during service. The weld metal is therefore more likely to be the site of hydrogen cracking than is the HAZ. As it is often necessary to restrict the weld bead size and even the preheat temperature in order to achieve desired properties, such as adequate HAZ toughness, the use of a low hydrogen process becomes doubly essential. Of the many varieties in this group the Cr-Mo steels show a high susceptibility to hydrogen cracking and are more likely than the others to require a postheat treatment for hydrogen removal before being allowed to cool out after welding. The same is also likely to be true for steels of high manganese content (>1.7%). Three methods of establishing welding procedures for highly hardenable steels were described in Chapter 2. These involved the use of: 1. 2. 3.

Temperature control The isothermal transformation characteristics of the material Austenitic or nickel alloy filler metal

It was explained that the first method is more applicable to steels with lower carbon contents «0.3%) and the second is suitable for

higher carbon steels. The third method is used where conditions prevent the use of high preheating temperatures. In this chapter each method is detailed separately.

The temperature control method As a first step the steel is placed in one of five grades by calculation (as shown below) or by means of Table 4.3. These grades are based on welding tests, on studies of the continuous cooling transformation behaviour of steels, and on an empirical formula" relating composition to maximum HAZ hardness: HV = 90+1050C + [47Si + 75Mn +30Ni +31Cr]

[4.1]

The chemical symbols refer to the percentage of the element in the steel, but this formua is not valid for low carbon low alloy steels containing boron, although it appears to work for higher carbon

ri Type A

Creuselso 47 D6AC Durabele 900, 950 Durabele 1050 FV 520(B) HY80 HY130 Ducol QT455 Maraging (18Ni-8Co) Maraging (12Ni-5Cr) Superelso 70

Dueol W30A, B

Commercial example of steel type

* Normally welded with Ni alloy electrodes.

lMn-j-Ni-j-Cr-Mo-B

l~r~lNi-V

1!,Mn-l,Ni-V lCr-j-Ni lj-Cr-l+Mo(-}V) 1Cr-1Mo-j-V 14Cr-5Ni-Mo-Cu-Nb 2+Ni-l.!;Cr-J,Mo 5Ni-Cr-Mo-V l lNi-1Cr...!.Mo-V 18Ni -8Co=5Mo-Ti-Al 12Ni-5Cr-3Mo-Ti-Al l1Mn-1Ni-1Cr l1Mn-1Cr-j-NHMo l.Cr-Mo-B 3Mn ll,Mn-2Ni(Co) 9Ni-4Co 12Cr-4Ni-Mo 5Cr-l,Mo

2.!;Cr-Mo

1 and liCr-+Mo

3Ni 9Ni

Mn(Ni)-Cr-Mo-V

l~Mn-Ni-Cr-Mo

l.l,cr-Al-Mo

3~r-Mo-V

3Ni-Cr lCr-Mo LCr-Mo 3Cr-Mo 1!,Mn-Ni-Mo l+Ni-Cr-Mo ljNi-Cr-Mo 2Ni-Cr-Mo 21,Ni-Cr-Mo 3Ni-Cr-Mo 4Ni-Cr-Mo

l~Ni-Cr

1.JMn-Mo 11,Mn-Mo

t Cr

lNi

Steel type 503-37,40,42 530-30, 32, 36, 40 605-30, 36, 37 608-37, 38 640-35,40 653 M31 708-37,40,42 709 M40 722 M24 785 M19 816 M40 817 M40 823 M30 826 M31, 40 830 M31 835 M30 897 M39 905 M31, 39 945-38, 40 BS 1501: Part 2: 1988-271, 281 BS EN 10083-6:2003 P460Q BS 1501-503 BS EN 10083-4:2003 12Ni14 BS 1501: Part 2: 1988-510 BS EN 10083-4:2003 X7Ni9, X8Ni9 BS 1501: Part 2: 1988-620,621 BS EN 10083-2:2003 13CrMo4-5, 13CrMoSi5-5 BS 1501: Part 2: 1988-622 BS EN 10083-2:2003 10CrMo9-10, 12CrMo9-10

Old BS 970. Pt 2, designation

Table 4.3 Grading alloy steels for use with Fig. 4.6 and 4.7

-

34CrNiMo6 30CrNiMo8 36NiCrMo16

K L L M L K L K M L L L C-Mn L L M M L L C-Mn

i.

L

-

-

Observed grading

42CrMo4

34Cr4, 37Cr4, 41Cr4

BS EN 100 83-1 designation

-

-

-

-

-

L L

K -

C-Mn K K K K K L L L L K K K K K L L L L -

Estimated grading

--

Q.

-=....~

..,

OC

il';'"

('l

('l

Ctl

= .., = .... =

OC

Q. 0

'<

I; 0

I~

I:Il

Ctl Ctl

I:Il

OC

I~

65

Welding procedures for different steel types steels. The portion of [4.1] used for grading steels is that given within the brackets and is described by a parameter F: F

= 47Si + 75Mn + 30Ni + 31Cr

[4.2]

For values ofthis parameter, F, up to 115, a steel is graded 'carbon steels'. For values from 116-145 the steel is graded 'carbonmanganese steels' and examples can be found in Table 4.3. For these two grades the relationship between carbon content and expected HAZ hardness is given in Fig. 4.7, and its subsequent use to establish minimum preheat, interpass, and postheat temperatures is exactly as described previously. For higher values of the parameter, F, from 146-180, steels are graded as 'K', from 181-225 as 'L', and for higher values as 'M'. Examples of all these grades are to be found among the steels listed in Table 4.3. The K, L, and M grades enable the appropriate lines on Fig. 4.6 to be used to relate, for a particular steel type, carbon content to the expected HAZ hardness. This figure is used exactly as Fig. 4.7 described previously. Using the carbon content of the steel (maximum specified, analysed level or the figure given on mill certificates) the appropriate line is intersected horizontally in the lower half of the figure. Vertical movement to the band in the upper half of the figure then defines a minimum temperature for preheat, interpass, and postheat. This temperature should not exceed the M, temperature of the steel in question, and this point should be checked (see 'High carbon, plain and alloy steels' later in this chapter). At low hardness values (below about 450 HV), low preheat and interpass temperatures are predicted and postweld heating may not be necessary. A further reduction of temperature may be possible if very low hydrogen processes can be used, but this should be confirmed by joint simulation tests. For example, in maraging steels graded 'M' and having a maximum carbon content of 0.02%, Fig. 4.6 would indicate a preheat temperature of 130°C for a highly restrained weld. However, this temperature would not be compatible with achieving adequate mechanical properties, so that a very low hydrogen process is normally used to avoid the use of any preheat. As the expected HAZ hardness increases, higher temperatures are predicted and it becomes necessary to select appropriate times for which postheat should be held to assist hydrogen removal. These times may be obtained from the hydrogen removal curves which are described and listed in Chapter 5. If the times involved appear unacceptably long, the possibility of tempering before the weld cools out can be considered. If this is not possible careful use of a

66

Welding steels without hydrogen cracking temper bead technique can sometimes give acceptable results. The Cr-Mo and Cr-Mo-V steels appear to be particularly susceptible to hydrogen cracking and, although temperatures higher than those predicted from Fig. 4.6 are not normally required when using a low hydrogen process, postheating and slow cooling after welding are commonly used for such steels, for example those to BS EN 100 83-2: 2003 13CrM04-5, 13CrMoSi5-5, 10CrM09-10, 12CrM09-10. With steels which give a hard HAZ two points should always be noted: (a)

An as-welded HAZ which contains appreciable amounts of retained austenite produces hard, brittle martensite after a single PWHT or tempering heat treatment. Hence, a second tempering treatment will be necessary. (b) A check should be made to ensure that the selected preheat temperature does not exceed the M, temperature of the steel; in making this check, the M, temperature and the various degrees of transformation may be estimated from formulae given later. In addition, it must be realised that significant hydrogen loss from the weld metal and HAZ will not take place until the temperature is sufficiently below the M, for substantial transformation of the austenite to have occurred. Highly alloyed materials such as the martensitic 12%Cr creep resisting steels display M, points well below 250°C and, to obtain hydrogen removal, must be cooled to fairly low temperatures. This can greatly increase the risk of cracking. Hydrogen levels must be minimised, and careful consideration should be given to the temperature to which the joint is cooled to achieve transformation and hydrogen removal during the preheat stage. To reduce postheat time, it may be advantageous to permit cooling to the selected transformation temperature followed by heating to a higher temperature giving faster hydrogen diffusion.

The use of isothermal transformation data In the description of the principle of this method in Chapter 2 it was explained that a knowledge of the transformation behaviour of a steel made it possible to control the cooling of the weld HAZ and so to produce certain preferred, i.e. less crack sensitive, microstructures. To use this technique it is necessary to know the isothermal transformation characteristics of the steel. These may be obtained from the steel-makers' data sheets or from one of the collections of such data."? As explained in Chapter 2, a temperature is selected

67

Welding procedures for different steel types which promotes transformation, usually to bainite, in a reasonable time and over the temperature range which can be controlled in practice. This selected temperature, which is, in fact, a preheat and interpass temperature, must be held long enough after welding to ensure complete transformation. It must also be recognised that adequate times will be twice as long as those indicated in diagrams which have been prepared from specimens austenitised at temperatures less than 1250°C.

The use of austenitic and nickel alloy consumables When the temperature control methods described above cannot be used because of limitations on the preheat temperature, or if they do not succeed in avoiding hydrogen cracking, the only alternative remaining is to use a consumable which is itself insensitive to hydrogen and which results in less hydrogen being left in the HAZ after it has cooled to temperatures at which hydrogen cracking can occur. Such consumables are provided by appropriate austenitic stainless steels and nickel alloys. However, both types require some preheat for the more difficult steels they cannot be effectively stress relieved thermally, and welds are difficult to inspect non-destructively for cracking. Austenitic weld metals can be selected which are of higher strength than the common suitable nickel alloys. When selecting austenitic stainless steel or nickel alloy fillers, it is necessary to ensure that dilution from the base steel can be satisfactorily accommodated. The normal choice of austenitic consumabIes for MMA welding is from the types 23Cr: 12Ni, 29Cr: 9Ni or 20Cr:9Ni:3Mo, e.g. from BS EN 1600: 1997. Grades 23 12L, 29 9 or 18 9 Mn Mo. The first named is most commonly used and is suitable for giving deposits containing sufficient ferrite to suppress solidification (hot) cracking, with little or no martensite in the bulk deposit. The 29Cr: 9Ni type may be preferred for high dilution runs to avoid a fully austenitic deposit. However, in low dilution situations, the weld metal will contain a high ferrite level, perhaps as high as 35%. Although this is of possible benefit in tolerating the pick-up of sulphur from the parent steel, and also in giving a high yield strength, this type of high ferrite weld deposit should not be subjected to PWHT, since it will show marked embrittlement as a result of the formation of the sigma phase during heat treatment. Nickel alloy fillers have the advantage of lower coefficients of thermal expansion than stainless steels, and this can reduce shrinkage strains and thus the risk of cracking in highly restrained

68 4.8 Guide to preheat temperatures when using austenitic manual metal-arc electrodes at about 0.81.6k]/mm a) low restrain, e.g. material thickness <30mm; b) high restraint, e.g. material thickness >30 mm.

Welding steels without hydrogen cracking 200

~--.,.---~-----.-----,----.--,

Increasing parent metal hardenability or weld metal hydrogen level

~

::l

~

~

100 1 - -__f---+---..........--f---+--+---1--t

~

Reducing parent metal

.

hardenability or weld metal hydrogen leve I

~

~ c..

50 I--_ _+-__-+-~_~~~

o

0.1

0.2

0.3

0.4

0.5

Carbon content, %

joints. A number of Ni: Cr :Fe alloys are suitable, among them MMA electrodes of the types covered by the American AWS specification ANSI/AWS A5.11/A5.11M-97-NiCrFe-2 and E-NiCrFe-3. However, they are more sensitive to solidification cracking and microfissuring than the stainless steels, and may not be usable for high sulphur steels. If PWHT is required, a nickel alloy filler may well become the preferred choice to avoid intermetallic formation during the heat treatment and consequent embrittlement. When austenitic electrodes are used, preheat is not normally required for steels containing up to O.2%C, although they are liable to give hard regions of crack-sensitive alloyed martensite at the fusion boundary. Such regions are prone to hydrogen cracking and are very difficult to detect. At O.4%C and above, a minimum temperature of 150°C is required to prevent such cracking, as well as normal HAZ cracking. Figure 4.8 gives guidance, showing schematically how hydrogen and restraint levels affect the degree of preheat needed. Buttering the surfaces to be welded may reduce the level of preheat necessary. Although the technique is generally successful, hydrogen cracking can still occur in severe situations, and it is always advantageous to reduce the hydrogen input to the joint by using covered electrodes dried at a high temperature (following manufacturers' recommendations), or to use a gas-shielded process with solid wires of high quality or cored wires known to give very low total hydrogen levels. Diffusible hydrogen measurements are not useful on austenitic or nickel alloy consumables. High heat input is often helpful, and techniques which give low dilution should be employed to minimise

69

Welding procedures for different steel types

the formation of martensite in the weld metal. Hard HAZs will normally be produced and it is usually advantageous to temper them, even if only by using a temper bead technique. Although it tempers the HAZ, PWHT is usually ineffective in giving a high degree of stress relief, because of the difference in coefficients of thermal expansion between the austenitic weld metal and the ferritic parent steel. In this respect, nickel alloys are likely to be advantageous. When welding 9%Ni steels for cryogenic applications, nickel alloy fillers are used almost exclusively and preheat is not required. Nickel alloys are also used for welding other steels without preheat in specialised applications. These include repairs to heavy power station plant in alloy steels of the Cr-Mo and Cr-Mo-V types and to a limited extent in underwater wet welding. Although solidification cracking can be a problem if the steel contains more than a trace of sulphur and dilution levels are high, nickel alloy fillers give rise to much less severe problems with hard martensite at the fusion boundary and therefore are unlikely to require preheat. Because it gives large contraction strains compared with ferritic steels, it is, however, often recommended that large repairs are filled with weld metal of the 18Cr: 10Ni type after buttering on to the ferritic steels with 29Cr: 9Ni. It must be emphasised that ferritic weld metal should never be deposited on to weld metals of the stainless steel or nickel alloy types.

High carbon, plain and alloy steels Approximate compositions: Carbon greater than 0.45% For steels of this type, welding procedures are similar to those already described for alloy steels. When using the temperature control method (see earlier in this chapter), the preheat temperature should be below that corresponding to the M, temperature of the material rather than that predicted from the expected HAZ hardness. It should not exceed the M, temperature since there would then be a risk of the HAZ remaining austenitic and retaining hydrogen during the PWHT. Little hydrogen would be lost during the PWHT, because of its low diffusion rate in austenite (see Fig. 5.17), and most would remain during the subsequent transformation to martensite on cooling. This would complete the conditions necessary for cracking. The same considerations apply when austenitic consumables are used.

70

Welding steels without hydrogen cracking Double tempering is usually necessary after welding: the first temper removes retained austenite but converts it to martensite which can be removed only by a second temper. It is an advantage to cool slowly after welding to a temperature as low as possible below the M, (usually 50-70°C minimum) before reheating for tempering. M, temperatures can be obtained from the steelmakers' literature, from the collections of isothermal transformation data,4- 9 or from formulae based on composition, as set out below (see Ref. 3). M,

= 539 - 432C - 30.4Mn - 17.7Ni - 12.1Cr - 7.5Mo

[4.3]

For steels containing between 2 and 5%Cr, the following empirical formula is more useful: M,

= 512 - 453C - 16.9Ni + 15Cr - 9.5Mo - 71.4(C x Mn) [4.4] - 67.6(C x Cr) + 217(C)z

For steels containing nominally 12% to 18%Cr (10), with carbon below about 0.3%, the following relationship may be used: M,

= 540 - 497C - 6.3Mn - 36.3Ni - 10.8Cr - 46.6Mo

[4.5]

It may be an advantage to allow the HAZ to partially transform

isothermally during the first tempering to minimise the subsequent formation of martensite from retained austenite, provided that the isothermal transformation product has adequate mechanical properties. In this respect, [4.6]-[4.8]3 are useful for estimating the temperatures which lead to different degrees of transformation in the HAZ during cooling prior to the initial tempering. M;

= k1

Mx M;

= 538

-

361C - 39Mn - 19.5Ni - 39Cr - 28Mo

[4.6]

k z - 474C - 33Mn - 17Ni - 17Cr - 21Mo

[4.7]

- k 3(361C + 39Mn + 19.5Ni + 39Cr + 28Mo)

[4.8]

=

where M; refers to the temperature leading to various degrees of transformation; the appropriate values for k., k z, and k 3 can be found in Table 4.4. In tool steels which are extremely notch sensitive, two possibilities exist. The first involves welding at a temperature as close as possible to the tempering temperature, slowly cooling below the Ms , and then double tempering. The second involves re-austenitising the component, quenching into" a lead bath at a temperature at which austenite is stable for a sufficient period of time, and then welding at that temperature. After welding, the component is slowly cooled to below the M, and then double tempered. Welding

71

Welding procedures for different steel types Table 4.4 Values of the constants k, k 2 , k, in [4.61-[4.81 which allow the calculation of temperatures for various degrees of transformation

x

Ms

M,o

MOll

Mm)

M""

k, k2 k"

538 561 1.0

513 551 1.084

488 514 1.18

452 458 1.29

416 1.45

Mr 346

steels with alloy contents high enough for them to be essentially austenitic, and welding cast irons" are outside the scope of this book.

Machinable grades of steel Such steels contain additions of sulphur within the range 0.10-0.50%, whilst lead and selenium may be present also. In PD 970: 2001 the free-cutting grades are numbered 212M36 216M44 214M15 and 606M36. Up to 0.12% of lead may be added to any of the PD 970: 2001 steels by agreement. It should also be noted that the high sulphur free-cutting carbon steels' contain manganese up to a maximum of 1.2-1. 7%, and should therefore be treated as C-Mn steels. The presence of sulphur renders these steels liable to liquation cracking in the HAZ, and such cracks, although usually short and relatively harmless in themselves, can act as nuclei for hydrogen cracks. For this reason, and because sulphur, lead, etc, is picked up by the weld pool, machinable grades of steel should not be used in joints where full strength is required. Basic consumables should be used and low dilution welding techniques employed to minimise sulphur pick-up. Bead sizes should be small to reduce the size of liquation cracks. Apart from these precautions, procedures for avoiding hydrogen cracking should be those appropriate to the normal grades of these steels. REFERENCES 1 Hart P H M: Weld Inst Members Report M/60172. 2 Brisson J et al: 'Study of underbead hardness in carbon and low alloy steels'. Soudages et Tech Conn exes 22 (11/12) 1968 437-55. 3 Woolman J and Mottram R A: 'The mechanical and physical properties of the British Standard En steels', Vol 1, 2, and 3. BISRA, London 1964/66/69. 4 Delbart G, Constant A, and Clerc A: 'Courbes de transformation des aciers de fabrication Francaise', Vol 1-4. Inst. de Recherches de la Siderurgie, St-Cermain-en-Laye, France.

72

Welding steels without hydrogen cracking 5 Wever F et al: 'Atlas zur Warmebehandlung der Stahle'. Max-PlanckInstitut fur Eisenforschung. Verlag Stahleisen, Dusseldorf 1954/56/58. 6 'Atlas of isothermal transformation diagrams'. US Steel Corp., Pittsburgh, USA 1951 and supplement 1953. 7 Atkins M: 'Atlas of continuous cooling transformation diagrams for engineering steels', British Steel Corporation, Sheffield, 1977. 8 Roberts G and Cory R: 'Tool steels', 4th ed, ASM, 1980. 9 Vander Voort G F (ed): 'Atlas of time-temperature diagrams for irons and steels', ASM, 1991. 10 Gooch T G: 'Welding murtensitic stainless steels'. TWI Res Bull 18 (12) 1977 343-9. 11 Cottrell C L M: 'Welding cast irons'. TWI, 1985,