A Study on Transverse Weld Cracks in Thick Steel Plate with the FCAW Process

The transverse crack in thick plate welding is investigated under simulated construction conditions

BY H. W. LEE, S. W. KANG AND D. S. UM

ABSTRACT. The transverse crack in thick plate welding is discussed with respect to deposited metal. In recent years, many of the new steel developments such as thermo-mechanical controlled process (TMCP) have been intended to improve weldability. When TMCP steel is used to achieve high strength with lean composition, the weld metal is more likely to suffer hydrogen cracking than the heat-affected zone (HAZ) of the base steel. Weld metal hydrogen cracking is even more likely if alloying is necessary to match the strength and toughness of the base metal. This is primarily due to the more highly alloyed weld metal's increased susceptibility to hydrogen cracking (Ref. 1).

One type of cold crack, referred to as a transverse crack, is caused by the com- plex interaction of the diffusible hydrogen supply, tensile residual stress and suscep- tible microstructure. This form of cracking generally is not encountered when weld- ing plate sections less than 10 mm thick. However, when thicker sections (50 mm or more) are welded, welds are subjected to more rapid cooling accompanied by more severe cooling stresses (Ref. 2).

Introduction

The various cracks that can occur in weld joints according to welding condi- tions and processes are classified as "cold crack" and "hot crack" according to occurrence temperatures.

Hot cracking, such as solidification cracks and liquation cracks, are the most severe problems associated with

H. W. LEE is with the Welding Research Team of Samsung Heavy Industries, Koje City, Korea. S. W. KANG and D. S. UM are with the Research Institute of Mechanical Tech- nology, Pusan National University, Korea.

the partially melted zone. The cause of hot cracking in the partially melted zone is the combination of grain bound- ary liquation and stresses induced by both solidification shrinkage and ther- mal contraction during welding (Refs. 3, 4).

The transverse crack, a type of cold crack, occurs perpendicular to the axis of the weld interface. It generally occurs at temperatures below 200C (392F), ei- ther immediately upon cooling or after a period of several hours. The time delay depends upon the type of steel, the mag- nitude of the welding stresses and the hy- drogen content of the weld (Refs. 5-7). However, most of the literature on trans- verse cracks published thus far differs when compared to the appearance of transverse cracks in actual construction.

In this study, two EH 32 steel panels were welded to resemble actual con- struction conditions. The appearance of transverse cracks, hardness, impact, mi- crostructure and residual stresses were then determined for two different weld- ing conditions.

KEY WORDS

Diffusible Hydrogen Intergranular (IG) Magnetic Particle

Inspection Microvoid Coalescence

(MVC) Quasi Cleavage (QC) Residual Stresses Stress Intensity Factor Transverse Crack

Experimental Procedures

Test Panel

The size of the test panel was 2000 mm long x 1800 mm wide x 50 mm thick. The panel was fabricated from EH32 TMCP higher-strength hull steel (as shown in Table 2), to provide test conditions similar to actual construction conditions - - Fig. 1. To magnify fabrica- tion-related weld residual stresses, the welding jig and test panel were fillet- welded together.

Test Weldments

The specimen sections were welded in layers as shown in Fig. 2. To compare the residual stresses and the position of occurrence of the transverse cracks, the sections were welded under the follow- ing conditions:

1) Below 30C (86F) of preheating and interpass temperatures.

2) Preheating and interpass tempera- tures of 100-120C (212-248F).

The preheating temperature of 100C was obtained from the Yurioka (Ref. 8) report shown in Fig. 3 (using Table 2, 50-mm-thick steel plate, Ceq 0.34). The test specimens were welded at 100-120C in consideration of ambient temperatures.

The panel was welded according to AWS A5.29 E8OT1-K2 specifications, using the flux cored arc welding (FCAW) process (1.2 ~ diameter, electrode exten- sion of 25-30 mm); welding parameters are shown in Table 1.

Chemical Composition/Strength

A spectroanalyzer was used to deter- mine the chemical composition of the base and weld metal. Mean values of the three specimens were then recorded in Table 2.

WELDING RESEARCH SUPPLEMENT [ 503-s

Fig. I - - Schematic diagram of weld panel.

I-

un i t : mm C A B 4 .

300 ~- 300 ~ 1,400

D

WELDING DIRECTION

4

A' ~B' C'

~,,~,,~,.---~ 3o" ~ /

----~l root opening: 10

(A - A')

I I

(B - B')

4#___ 3_~_

I I

(c - c )

Section A-A" : 1/3-specimen thickness weld deposit metal Section B-B" : 2/3-specimen thickness weld deposit metal Section C-C" : full thickness weld deposit metal

Fig. 2 - - Schematic diagram of weld deposit metal.

504-S J DECEMBER 1998

Hardness Traverses

Hardness was measured using the macro Vickers hardness test, with a load of 5 kg and 10 s of loading time. Mea- surements were made on transverse sections, 10 mm from the top surface.

Impact Test

The impact test was performed at 0, -20, -40 and -60C (32, -4, -40, and -76F) using Charpy V-notch for deposited metal. The test specimen loca- tion in the weldment is shown in Fig. 4.

Measurement of Residual Stresses

The surface residual stresses along the weld metal centerline (~ direction) were measured using the Rosette gauge hole- drilling method after the specimen was cooled completely.

Diffusible Hydrogen Test

The diffusible hydrogen was mea- sured by glycerin method per JIS Z3118. Before the hydrogen test, the steel plate was kept in the furnace at 500C (932F) for 1 h and air-cooled to remove dif- fusible hydrogen.

Distinction of Crack Position

To check the position and length of transverse cracks according to changing preheating, interpass temperatures and welding layer, the specimens were in- spected by ultrasonic testing. The surface of the weld bead was then cut at 0.5-mm- depth intervals using a milling machine and checked for accurate position and length of transverse cracks using mag- netic particle inspection after each ma- chining step.

Results and Discussion

Macro/Microstructure

The macrostructures of the weldments are shown in Fig. 5A and B.

Some significant differences can be noted between the HAZ that formed due to the welding pass and the HAZ located near the weld interface. The grain size was very coarse in the HAZ near the weld interface, resulting in the most brittle sec- tion of the weld joints. Impact values im- proved in the reheated zone because the grain boundary ferrite and Widmanstat- ten side plates were transformed into pearlite and ferrite, and the grain size was refined (Ref. 3).

Figures 6 and 7 are weldment mi- crostructures. Figure 6 is weld joint "A" (preheating and interpass temperature

below 30C) and Fig. 7 is weld joint "B" (preheating and interpass temperature 100-120C). Figures 6A and 7A show the microstructure of the base metal, consisting of ferrite (white area), pearlite (dark area) and bainite (slightly gray area). The fine grain size results in ex- cellent strength and toughness (Ref. 9). The refined- and coarsened-grain re-

gions are shown in Figs. 6B and C and 7B and C, respectively.

The refined-grain region was sub- jected to a peak temperature just above the effective upper critical temperatures, Ac3, thus al lowing austenite grains to nucleate. Such austenite grains decom- posed into small pearlite and ferrite grains during subsequent cooling. As

seen in Figs. 6B and 7B, the distribution of pearlite and ferrite is not exactly uni- form because insufficient time was al- lowed for the diffusion of carbon atoms due to the rapid heating rate during weld- ing. The coarsened-grain region was sub- jected to a peak temperature well above the Ac3 temperatures, thus promoting the coarsening of austenite grains.

Table 1--Welding Parameters

Identification Welding Condition

A Preheating/interpass temperature below 30C

B preheating/interpass temperature 100-120C

Current Voltage Speed Heat Input Pass (A) (V) (cm/min) (kJ/cm)

1 240-250 30 16 28 2-27 340-350 35 3741 26

1 240-250 30 15 29 2-27 340-350 35 38-42 25

Table 2--Chemical Composition of Base/Weld Metal

(%) C Si Mn P S Ni

0.18 0.10- 0.90- 0.040 0.040 0.40 EH32 TMCP max. 0.50 1.60 max. max. max.

Base metal 0.09 0.38 1.35 0.015 0.005 0.03

Weld A 0.04 0.29 1.05 0.012 0.017 1.32 metal B 0.04 0.29 1.03 0.013 0.016 1.31

Mo V Ti TS YS El (kgf/mm 2) (kgf/mm 2) (%)

0.08 0.10 0.02 45-60 32.0 20.0 max. max. max.

0.02 0.002 0.02 52.8 38.0 31.0

0.02 0.017 0.01 69.4 63.7 22.8 0.02 0.018 0.01 66.3 61.4 23.4

250 (~c)

~ 150

~ 100

H.~ = 5=~/100~ WM -- H.I. = 1.7Klknm

Ambient ~ = 10"(2

, / 0 ~'75 75 60 50 40 30 25 20 15 S. lOrnm

0.2 0.3 0.4 0.5

Carbon equivalent, Ceq

0.6

1 1 1! ,2 ,3 4 ,5,6 7 8

Fig. 3 - - Diagram of preheating temperature for Ceq and steel plate thickness.

5O

10mm

Fig. 4 - - The position of Charpy V-notch impact test specimen.

Fig. 5 - - Macrostructure of weld joint near section C - C'. A - - Preheating/interpass temperature below 30C; B - - preheat- ing/interpass temperature 100-120C.

WELDING RESEARCH SUPPLEMENT I 505-$

w

Hi @'j isu

m q

~L ,G I W

W

Wil l

0

Fig. 6 - - Microstructure of weld jo int A (preheating and interpass temperature below 300 . A - - Base metal; B - - grain-refined zone; C - - grain-coarsened zone; D - - weld metal.

Fig. 7 - Microstructure of weld jo int B (preheating and interpass temperature 100-1200 . A - - Base metal; B - - grain-refined zone; C - - grain-coarsened zone; D - - weld metal.

Because of the relatively high cooling rate and the large grain size in this region, acicular ferrite rather than blocky ferrite formed at grain boundaries-- Figs. 6C and 7C. Figure 7D is an optical micrograph taken from the deposited weld metal area revealing grain boundary ferrite, Wid- manstatten ferrite and acicular ferrite. To improve mechanical properties such as tensile and toughness, acicular ferrite has

to form fully instead of grain boundary fer- rite and Widmanstatten ferrite.

More amounts of acicular ferrite can be observed in Fig. 7D when compared to Fig. 6D.

Residual Stresses of Weld Joints

Welding induces high residual stresses in the vicinity of the weld. The

residual stress is caused by restraining the free contraction of the thermoplastically deformed weld zone during weld cool- ing. Therefore, the welding residual stresses are sometimes referred to as re- straint stresses. A geometrical notch in the weld joint further induces local stress concentration. In most cases, hydrogen- assisted cracking is initiated at a notch of the weld made under restraint.

The residual stresses measured at the surface of a deposited metal in a longitu- dinal direction of weld interface are shown in Fig. 8. In all measured points, the residual stress values for a specimen- welded preheating and interpass temper- ature below 30C was higher than the preheating and interpass temperature of 100-120C. Transverse crack occurrences are caused by the hardness of deposited metal, diffusible hydrogen contents and tensile residual stresses in the longitudinal direction of the weld interface.

Diffusible Hydrogen Contents

Hydrogen-assisted cracking is a severe problem in the welding of thick steel plate that occurs when the follow- ing three factors are simultaneously pre- sent: diffusible hydrogen in weld metal, high stress and susceptible microstruc- tures. The hydrogen dissolved in a weld metal is proportional to the square root of the partial pressure of the hydrogen gas.

The following sources of weld metal hydrogen are considered in FCAW (Ref. 10):

1) Moisture in flux 2) Moisture in CO 2 gas 3) Organic substance in flux 4) Hydrogen in wire steel and steel

plate 5) Moisture in atmosphere 6) Extraneous hydrogenous material,

e.g., moisture, grease and paint Hydrogen dissolved in a steel matrix

is diffusible, thereby causing hydrogen embrittlement. The weld metal hydrogen content is generally expressed by the content of diffusible hydrogen. The three methods of measuring diffusible hydro- gen contents are:

1) Glycerin method (Hjl s) 2) Mercury method (Hi] w) 3) Gas chromatograph method (HG_ c) The test results of these three methods

are related as follows (Ref. 11):

HHw = 1.27Hji s + 2.19

HG_ C = 2HjI s + 0.3

where Hil w, HG_ C and HjI S are the weld metal diffusible hydrogen content per 100 g of deposited weld metal.

The hydrogen contents, which de-

506-s I DECEMBER 1998

(A )

7O

50

~ 10 i

ol

---O-- preheating/interpass temp. below 30"C ] - " "0"" preheat ing / in terpass temp. 100-120"C J

I I I I [ I I A B C D E F G

(B)

Fig. 8 - - Distr ibutions o f surface residual stress for (~ direction in deposited metal. A - - Position of attached Rosette gauge; B - - results of surface residual stresses.

800

s fl,O0

m 0 O. at "U 400

g Q

~ 200

'I"

o,oo

240A X 30V, 25CPM

(~ 350A X 35V, 25CPM

1 0 ~) (D

/ [ I I I I 2 4 18 48 72

Exposure Time (Hour)

Fig. 9 - - Hydrogen content profiles depending on welding conditions.

v

-

UJ

0 u~

400 --

300

200

100

preheat/interpass temperature 100~120"C

preheat/interpass temperature below 300C

-80

!

t

I I I I I -60 -40 -20 0 20

Temperature ( C)

Fig. I 0 - - Results of Charpy V-notch impact tests for weld metal.

24000- ~ prebeating/interpass temp. 100-120"(." preheating/interpass temp. below 30"C

220.00

(~ 200.00

180.00

'10

~ 16000

W

14000

-soo -4oo ooo 4.oo 8oo ~2oo Distance from the weld interface (ram)

Fig. 11 Hardness traverses 10 mm from top surface.

pend on welding conditions, were mea- sured by the glycerin method and are shown in Fig. 9. These data indicate that welds made with the FCAW electrode have hydrogen contents of approx- imately 3-4 mL. Most diffusible hydrogen escaped within 2 h after weld- ing as shown in Fig. 9. However, when welding conditions were changed, there were no significant hydrogen contents.

Impact Properties

Figure 10 shows the Charpy V-notch impact test results for weld metal. Ab-

sorbed energy of preheating and inter- pass temperatures of 100-120C in weld joint B are higher than preheating and interpass temperatures of 30C in weld joint A, due to higher cooling rate.

Hardness Traverses

Figure 11 shows the hardness tra- verses 10 mm away from the weld sur- face. When the preheating/interpass temperature is below 30C, the value of hardness (HV) in deposited weld metal is 10-15 higher due to the rapid cooling rate. The hardness of the weld metal

depends on the preheating/interpass temperature, and when the preheating/ interpass temperature is low, the weld metal becomes more susceptible to transverse crack.

Distinction of Crack Position

In weld joint B, when preheating and interpass temperatures were 100-120C, no transverse cracks were detected. However, transverse cracks were de- tected for the specimen welded with pre- heating and interpass temperatures below 30C in weld joint A.

WELDING RESEARCH SUPPLEMENT I 507-s

(A) I(C) 16.0 mm

Fig. 12 - - MPI results of transverse cracks for 35-mm weld joint A (depth below weld top surface is shown at top right-hand corner).

(A) 9.0 mm

| lJi~ I I I lliIIIII [I }l[,l'il[!ll}flllLllIi[llillIItillIl!iiill ~ ' ~a ~ 9o INn

, ; 2

(B) 9.5mm I(C)

I

11.0 mm

(r- B I I

~ ^ nm

(G (H) 28.0 mm

Fig. 13 - - MPI results of transverse cracks for 50-mm weld joint A (depth below weld top surface is shown at top right-hand corner).

goR-~ I IOFCFMRER 1998

It was also noted that the number of transverse cracks increased as the weld- ing layers were increased. No cracks were observed in the one-third complete sample. However, transverse cracks were formed in specimens welded in two- thirds of their thickness (35 mm) and the full thickness of the weld joint (50 mm).

Figure 12 shows the morphology of detected transverse cracks for specimens welded in two-thirds of their thickness (35 mm) at various depths from the weld surface. Figure 12A shows the morphol- ogy of the transverse cracks that ap- peared for the first time, located 9.5 mm in depth from the weld surface. Some of these cracks can also be seen in Fig. 12B, which also shows the formation of new cracks at a depth of 10.0 mm from the weld surface. These cracks completely disappear and two new sets of cracks are visible in Fig.12C at a depth of 16.0 mm from the weld surface. The cracks disap- pear as the distance from the weld sur- face is increased and a new set of cracks is formed - - Fig. 12D.

Figure 12E shows the continuation of cracks detected in Fig. 12D; however, their number has decreased consider- ably. Transverse cracks completely dis- appear at a depth of 20.0 mm from the weld surface-- Fig. 12F.

It can be seen from the macrographs of Fig. 12 that transverse cracks have a pattern of appearing and disappearing in locations at depths 9.5-17.0 mm away from the weld surface.

The morphology of transverse cracks in the full-thickness joint is shown in Fig. 13. Cracks at a depth of 9.0 and 9.5 mm from the weld surface are shown in Fig. 13A and B. A comparison of these two macrographs shows that transverse cracks have a clearer morphology in Fig. 13B. Additionally, the morphology of cracks in Fig. 13E and F is more delicate, with a number of small-size cracks located in the surrounding area of the larger cracks than the cracks detected in Fig. 13C and D.

In the full-thickness weld, the trans- verse cracks also have a pattern of appearing and disappearing at a depth between 9.0-27.0 mm away from the weld surface and completely disappear at a depth of 28.0 mm - - Fig. 13H.

The position of transverse cracks rela- tive to the weld layer is shown in Fig. 14. For the specimen welded at two-thirds thickness (35 mm), transverse cracks were located between weld layers 4-6, as shown in Fig.14A. For the specimens welded at full-thickness (50 mm), trans- verse cracks were located between weld layers 5-8, as shown in Fig. 14B.

Takahashi, etal . (Ref. 12), have shown that transverse cracks are initiated in the

weld metal just / below the final layer of welds and 50 are gradually prop- agated toward both the top and bottom surface. This oc- curs because the largest residual stresses and the highest concentra- / tion of diffusible / hydrogen contents t5,.0 can be found in these locations.

In this study, the position of trans- verse cracks differed from that of the Takahashi report, since cracks were detected at a con- stant distance from the top of the weld surface (e.g., 9.5-10.0 mm, and not just below the final layer as reported by Takahashi). This is due to a large restraint stress under actual construction condi- tions, as compared to a small test piece.

Figures 15A and B show an optical micrograph of the middle and edge of transverse cracks. The formation of these cracks did not follow the grain boundary ferrite; rather, they propagated across the grains.

From fracture morphology, it is noted that transverse cracks occur in high stresses. Microscopic fracture modes from the Beachem (Ref. 13) report are shown in Fig. 16. These illustrations show the tip of cracks growing from left to right under four different K (stress intensity fac- tor) conditions, with the K decreasing from Fig. 16A through D. This represents a suggested explanation of the changes in the observed fracture modes.

Conclusions Macrostructure appearance of the

6 layer~~ t 4 layer'---~ ~ --3.- 9.5-20.0 I I

(A )

I ~ I unit :mm

(B)

Fig. 14 - - The transverse crack position according to changing welding layer. A - - 35-rnm weld joint; B - - 50-ram weld joint.

transverse crack and mechanical proper- ties such as hardness, impact and resid- ual stress measurement was studied for EH 32 TMCP 50-mm-thick plate welded with FCAW under the condition of changing preheat and interpass tempera- ture. The results of this study can be sum- marized as follows:

1) Transverse cracks were detected in the specimen welded with preheat- ing and interpass temperatures below 30C, but cracks were not detected for the specimen welded with preheat and interpass temperatures of 100-120C.

2) Two different locations of crack formation were detected in this experi- ment as follows:

a) In the specimen welded at two- thirds thickness of the joint, cracks were initiated at a distance of 9.5-10 mm away from the top of the welded surface, be- tween layers 4-6.

b) In the specimen welded at full thickness, cracks were initiated at a dis-

5 0 ~ : ;4~2i~; ' .~ " aO~m i

Fig. 15 - - Optical microstructure o f transverse cracks. A - - Middle of crack; B - - crack edge.

WELDING RESEARCH SUPPLEMENT I 509-s

(A)

(c)

(e)

/ . . . . . . . . . . . \

(D)

Fig. 16 - - Microscropic fracture modes. A - - MVC with high stress intensity factor; B - - QC with intermediate stress intensity factor; C - - IG cracking with low stress intensity factor; D - - IG cracking with assisted hydrogen pressure.

tance of 9.5-10.0 mm away from the top of the welded surface, between layers 5-8.

3) Hardness values of preheating and interpass temperatures below 30C were higher than preheating and interpass temperatures below 100-120C in de- posited weld metal.

4) The residual stress values for the specimen welded with a preheating and

interpass temperature below 30C was higher than the specimen welded with a preheating and interpass temperature at 100-120C.

5) The result of weld metal impact test shows higher impact values for the weld condition with preheating and interpass temperature at 100-120C, compared to the specimen welded with

510-s I DECEMBER 1998

a preheating and interpass temperature below 30C.

References

1. Bailey, N., and Wright, M. D. 1993. Weldability of high strength steels. Welding and Metal Fabrication, pp. 389-396.

2. Metals Handbook. 1973.9th ed., Vol. 6 ASM International, Materials Park, Ohio, pp. 129-130.

3. Kou, S. 1987. Welding Metallurgy, John Wiley and Sons, New York, N.Y., pp. 249, 326.

4. Welding Handbook, 1987. 8th ed., Vol. 1. American Welding Society, Miami, Fla., pp. 230-231.

5. Signes, E. G., and Howe, P. 1988. Hydrogen-assisted cracking in high-strength pipeline steel. Welding Journal 67(8): 163-s to 170-s.

6. Suzuki, H. 1978. Cold cracking and its prevention in steel welding. Transactions of the Japan Welding Society, pp. 82-86.

7. Hart, P. H. M. 1986. Resistance to hydrogen cracking in steel weld metals, weld- ing Journal 65(1 ): 14-s to 22-s.

8. Yurioka, N. 1995. A chart method to determine necessary preheat temperature in steel welding. Journal of Japan Welding Society, pp. 347-350.

9. Lee, H. W., and Kang, S. W. 1996. A study on microstructure and thoroughness of electrogas weldments. Journal of the Korea Welding Society, pp. 68-74.

10. Yurioka, N., and Suzuki, H. 1990. Hydrogen assisted cracking in C-Mn and low alloy steel weldments. International Materials Review, pp. 217-249.

11. JlS Z3118 Method of Measurement for Hydrogen Evolved from Steel. 1986. Japanese Standard Association.

12. Takahashi, E. 1979. Relations be- tween occurrence of the transverse. Journal of Japan Welding Society, pp. 855-872.

13. Beachem, C. D. 1972. A new model for hydrogen-assisted cracking. Metallurgical Transactions 3(2): 437-451.