Scholarly article on topic 'Study on hydrogen assisted cracking susceptibility of HSLA steel by implant test'

Study on hydrogen assisted cracking susceptibility of HSLA steel by implant test Academic research paper on "Materials engineering"

CC BY-NC-ND
0
0
Share paper
Academic journal
Defence Technology
OECD Field of science
Keywords
{"HSLA steel" / "Hydrogen assisted cracking" / "Diffusible hydrogen" / "Implant test" / "Lower critical stress"}

Abstract of research paper on Materials engineering, author of scientific article — Gopa Chakraborty, R. Rejeesh, S.K. Albert

Abstract DMR-249A is an indigenously developed high strength low alloy steel for Indian ship building industry for making ship-hull and is extensively used in the construction of war ships and submarines. Welding electrodes conforming to SFA 5.5 AWS E8018 C1 has been indigenously developed for welding of this steel using shielded metal arc welding process. In the present study, susceptibility to hydrogen assisted cracking of DMR-249A steel welds made using this electrode has been assessed using implant test. Implant tests were conducted using this electrode at two different levels of diffusible hydrogen, measured using gas chromatography technique. It is observed that both the steel and the welding consumable are not susceptible to hydrogen assisted cracking even with a high diffusible hydrogen level of 9 mL/100g of weld metal. In implant tests, specimen did not fracture even after loading to stress levels higher than the yield strength of the base metal. The good resistance of this steel and the welding consumable, even with high levels of diffusible hydrogen, is attributed to absence of a susceptible microstructure in both the weld metal and heat affected zone. Hence, this study shows that, in the absence of a susceptible microstructure, hydrogen assisted cracking is unlikely to occur even if hydrogen level is high. It also confirms that in welding of DMR-249A with indigenously developed E8018 C1 electrode, hydrogen assisted cracking is not a concern and no preheating is required to avoid it during welding.

Academic research paper on topic "Study on hydrogen assisted cracking susceptibility of HSLA steel by implant test"

Available online at www.sciencedirect.com

ScienceDirect

Defence Technology ■■ (2016) ■■-■■

www.elsevier.com/locate/dt

Study on hydrogen assisted cracking susceptibility of HSLA steel

by implant test

Gopa CHAKRABORTY a *, R. REJEESH b, S.K. ALBERT a

a Materials Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, 603102, India b National Institute of Technology, Surathkal, India Received 30 August 2016; revised 21 September 2016; accepted 22 September 2016

Available online

Abstract

DMR-249A is an indigenously developed high strength low alloy steel for Indian ship building industry for making ship-hull and is extensively used in the construction of war ships and submarines. Welding electrodes conforming to SFA 5.5 AWS E8018 C1 has been indigenously developed for welding of this steel using shielded metal arc welding process. In the present study, susceptibility to hydrogen assisted cracking of DMR-249A steel welds made using this electrode has been assessed using implant test. Implant tests were conducted using this electrode at two different levels of diffusible hydrogen, measured using gas chromatography technique. It is observed that both the steel and the welding consumable are not susceptible to hydrogen assisted cracking even with a high diffusible hydrogen level of 9 mL/100g of weld metal. In implant tests, specimen did not fracture even after loading to stress levels higher than the yield strength of the base metal. The good resistance of this steel and the welding consumable, even with high levels of diffusible hydrogen, is attributed to absence of a susceptible microstructure in both the weld metal and heat affected zone. Hence, this study shows that, in the absence of a susceptible microstructure, hydrogen assisted cracking is unlikely to occur even if hydrogen level is high. It also confirms that in welding of DMR-249A with indigenously developed E8018 C1 electrode, hydrogen assisted cracking is not a concern and no preheating is required to avoid it during welding.

© 2016 Production and hosting by Elsevier B.V on behalf of China Ordnance Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: HSLA steel; Hydrogen assisted cracking; Diffusible hydrogen; Implant test; Lower critical stress

1. Introduction

Microalloyed high strength low alloy (HSLA) steels containing low carbon and small additions of Nb, V, Ti exhibit an outstanding combination of high strength, resistance to brittle fracture and good weldability [1]. DMR-249A is a low carbon HSLA steel with micro additions of Nb, V and Ti, indigenously developed for Indian ship-building industries and is being used in the construction of war ships and submarines [2]. Shielded metal arc welding (SMAW) is one of the major welding processes employed by shipping industries. Complex dynamic loading, extreme temperature conditions during service together with residual stresses generated in the weld due to fit up and fabrication can make these weld joints susceptible to brittle fracture in service [3]. The presence of undetected cracks caused by hydrogen assisted cracking (HAC) during fabrication

Peer review under responsibility of China Ordnance Society.

* Corresponding author.

E-mail address: gopa_mjs@igcar.gov.in (G. Chakraborty).

can further assist brittle fracture in service. Hence, there is a need to assess the susceptibility of these welds to HAC and ensure that there is no risk of HAC during fabrication of naval structures using this steel and consumables [4].

Earlier studies [5] have shown that the conditions for HAC to occur in steel welds are: presence of diffusible hydrogen (Hd), residual stress and susceptible microstructure in the weld and temperature in the range of ambient to 200 °C. In this regard, martensitic microstructure with high hardness is most susceptible and ferritic microstructure with low hardness is least susceptible. Hence, during welding, efforts are made to reduce risk of HAC by avoiding development of a susceptible microstructure and minimizing the hydrogen levels in welding. The probability of having a susceptible microstructure in the HAZ or weld is assessed from the composition of the base metal and weld metal, heat input and preheating (which will reduce the cooling rate of the weld) chosen for welding [6]. In order to reduce hydrogen level, low hydrogen welding consumables, proper baking of the consumables to remove moisture content in the consumables and appropriate preheating or preheat-

http://dx.doi.org/10.1016/j.dt.2016.09.003

2214-9147/© 2016 Production and hosting by Elsevier B.V on behalf of China Ordnance Society. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

G. Chakraborty et al. /Defence Technology ■■ (2016) I

ing + post heating conditions that would provide more time for hydrogen to diffuse out at high temperature are chosen. For HSLA steels like DMR-249A, the hardenability is very low and the as-welded microstructure is ferritic and hence susceptibility to HAC is expected to be low. However, susceptibility of a weld to HAC can be quantified from implant test in terms of lower critical stress (LCS), the stress below which the weld does not fracture during the test, and in the present study this is attempted for welds of DMR-249A steel made with indigenously developed consumables.

In order to achieve two different levels of hydrogen in the weld metals, welds for implant tests were made both with baked electrodes and unbaked electrodes. Baking brings down the moisture content in the flux coating which in turn reduces the diffusible hydrogen content in the welds. Hence, during fabrication using low hydrogen welding consumables, they are baked prior to use as per the instruction provided by manufactures. In the present study, implant tests were conducted using electrodes with and without baking to produce welds with different levels of diffusible hydrogen during implant testing. Diffusible hydrogen levels in electrodes were measured using thermal conductivity based gas chromatography.

2. Experimental

DMR-249A is a low carbon (C:0.09, Mn:1.14, Si:0.18, Ni:0.62, Al:0.026, Nb: 0.039, V:0.02, Ti:0.02, S:0.006, P:0.14, N:56 ppm) HSLA steel with minimum yield strength and tensile strength of 390 MPa and 510 Mpa, respectively. AWS E8018 C1 is a basic coated low hydrogen electrode. Nominal chemical composition of the electrode (Ys: 483 MPa, UTS: 552 MPa) is given in Table 1.

2.1. Measurement of diffusible hydrogen content in the weld

For diffusible hydrogen measurement, DMR-249A steel samples are fabricated as per ISO 3690 specification. The specimen of size 30 mm x 15 mm x 10 mm is fixed in a copper jig with run-on and run-off pieces each of size 40 mm x 15 mm x 10 mm. Bead on plate welding was carried out with 3.15 mm diameter grade SFA 5.5 AWS E8018 C1 electrode. Welding parameters used for the welding are: current - 110 A, voltage - 23 V welding time — 30 s, welding length

~ 70 mm. Approximate heat input corresponding to the welding parameters is ~1100 J/mm. Diffusible hydrogen measurements were carried out for three different conditions of the electrodes: (1) baking the electrode at 450 °C for 4hr and maintaining the electrode temperature at 150 °C after baking prior to welding; (2) baking the electrode at 150 °C for 4 hr prior to welding; and (3) without any baking. It is to be noted here that the normal practice of welding is to bake the electrode prior to welding, which gives a minimum level of

All dimensions are in mm

Fig. 1. Schematic diagram of implant test specimen.

diffusible hydrogen. However, in the present study diffusible hydrogen in the electrodes in the as-received condition (no baking) as well as in the electrodes baked at lower temperature was also measured. The objective was to prepare implant test specimens with different levels of diffusible hydrogen in the welds using these electrodes. Immediately after completion of the welding, the specimens for diffusible hydrogen measurement were immersed in ice cold water for 5 s and kept inside liquid nitrogen to cool to subzero temperature until they are taken out for hydrogen extraction and measurement.

The HE_GCTCD set up used for diffusible hydrogen measurement consists of a diffusible hydrogen collection chamber, a heater to heat the chamber and a gas chromatograph (GC). The detail of HE_GCTCD set up and diffusible hydrogen measurement technique is provided elsewhere [7]. The specimen is kept inside the chamber at 400 °C for 30 minutes for extraction of diffusible hydrogen from the test specimen. Hydrogen collected in the chamber is transported to a GC with a thermal conductivity detector using Ar as carrier gas and the signal is recorded. Prior to measurement, GC is calibrated using known volumes of hydrogen injected into the GC and from this, the volume of hydrogen evolved from the weld specimen and collected in the chamber is estimated. Using the weight of the deposited metal in the weld, the volume of diffusible hydrogen is calculated in milliliters per 100 g of deposited weld metal. For each condition, three tests were performed and average of the data is reported.

2.2. Implant test

Fig. 1 shows the schematic diagram of implant specimen and base plate, prepared as per Doc. IIW-802 guidelines [8]. The implant testing machine is a computer controlled and mechanically operated machine along with a load-cell attached to it to display the load and time duration during the testing [8]. The specimen assembly consists of a base plate with a hole, into

Table 1

Chemical composition (Wt%) of weld metal.

C Mn Si Nb V Ti Al Ni S P Fe

0.02-0.04 1.1-1.5 0.1-0.25 <0.002 0.02 0.02 0.01-0.02 2.2-2.7 <0.01 <0.01 Bal

G. Chakraborty et al./Defence Technology ■■ (2016) I

Fig. 2. Schematic diagram of (a) notch tensile test and (b) tensile test specimen.

which implant specimen is inserted in such a way that the top surface of the implant specimen and base plate are at the same level. Single pass bead on plate welding was made on this specimen assembly using the test electrode and employing the same welding parameters used for making the specimens for diffusible hydrogen measurement in such a way that the weld bead passes over the implant specimen fusing its top surface completely with the base plate. Two separate sets of test were conducted using the specimens prepared with baked (at 450 °C for 4 hr) and unbaked electrodes. A thermocouple was attached to the base plate to monitor the temperature and loading was done when the assembly cools down to 100 °C. A series of tests with first specimen loaded at 1000 kg were conducted. Subsequently, loading was increased to 2000 kg in steps of 200 kg and later on in steps of 100 kg until failure of the sample. Two tests were repeated for each loading condition. After implant test, selected samples were sectioned to observe for micro cracks. The samples were sliced, metallographically polished and etched with 2% nital solution to study under optical microscope. Microhardness measurements were performed on base metal, HAZ and weld metal at 500 g load.

2.3. Notch tensile test and impact tests of simulated HAZ specimens

Implant test specimens contain notch in the HAZ produced by the weld. Hence, in order to compare the LCS with notch tensile strength of the HAZ, notch tensile tests of the HAZ were conducted. For this purpose, HAZ was simulated on a plate of size 100 mm x 150 mm x 10 mm by heating up to 1080 °C and then cooling in air, based on HAZ microstructure observed from implant samples and available literature reference for this steel [9]. Notch tensile samples were fabricated (Fig. 2(a)) from the simulated HAZ and tested in tensile testing machine under 10-4/s strain rate. For comparison purpose, tensile testing of the base metal (Fig. 2(b)) was also done at similar strain rate. Standard Charpy "V" notch impact testing samples were fabricated from the simulated HAZ material and impact testing was done at room temperature. Fracture surface of the failed samples after tensile and Charpy testing was observed under scanning electron microscope (SEM).

3. Results

The HE_GCTCD data indicate that diffusible hydrogen (Hd) content in the welding consumable, after baking at 450 °C/4h, is 3.1 mL/100 g of weld metal. This is certainly a low value of diffusible hydrogen, and as per IIW and AWS classifications,

this electrode comes under very low hydrogen category of electrode [10-12]. Results of the Hd measurement for the electrode without any baking is 9.6 mL/100 g of weld metal and the same for baking at 150 °C is 8.3 mL/100 g of weld metal. Thus, by altering the baking conditions of the electrodes, one can get different levels of Hd contents for the same batch of electrodes. This enables carrying out implant tests using electrodes differing only in their Hd content and determining LCS at these levels of Hd contents for the same consumables and base materials.

As mentioned earlier, implant test was carried out for two sets of welding prepared with baked (450 °C) and unbaked electrode. The tests were conducted in the range of 1000 kg (equivalent stress: 205 MPa) to 2000 kg (410 MPa) load in steps of 200 kg and above 2000 kg; load was increased up to 2400 kg (490 MPa) in a step of 100 kg. For samples welded with baked electrode, no failure occurred within 24 h (Fig. 3(a)) up to a loading of 2400 kg (490 MPa). No further test was carried out above 2400 kg (490 MPa) since it is above the yield strength of the base metal [9]. To estimate the stress level at which specimen fracture, for one specimen load was increased

Welded with unbaked electrode

J**» < m . I ify,

Welded with baked electrode

Fig. 3. Implant sample tested at (a) 2400 kg and (b) 2700 kg loads.

G. Chakraborty et al. /Defence Technology ■■ (2016) ■■-■■

229 Fig. 4. Micrograph of implant sample welded with (a) baked and (b) unbaked

230 electrodes.

231II ^H

232 until fracture and this occurred at the base metal (Fig. 3(b)) far

233 away from the HAZ and the corresponding fracture stress is

234 5 54 MPa (load = 2700 kg), which is nearly equivalent to the

235 tensile strength of the weld joint [9]. As LCS could not be

236 determined for the properly baked electrode, implant tests were

237 conducted for specimens prepared using electrodes without

238 baking (Hd levels = 9.6 mL/100 g of weld metal). These speci-

239 mens also did not fracture even after loading up to 2400 kg

240 (490 MPa). The implant specimens tested at the highest stress

241 levels were sliced along the length, polished, etched and then

242 examined for microcracks under optical microscope. No cracks

243 were found as shown in Fig. 4. Thus, results clearly confirm that

244 the steel is not susceptible for HAC even at high levels of

245 diffusible hydrogen.

246 The optical micrographs of DMR-249A steel base metal,

247 weld metal and HAZ are shown in Fig. 5(a)-(c). The base metal

248 consists of fine grained equiaxed ferrite and some percentage of

249 pearlite as a banded structure (Fig. 5(a)). Micrograph of weld

Fig. 5. Optical micrographs of (a) base metal, (b) weld metal and (c) HAZ.

260 261 262

280 281 282

G. Chakraborty et al. /Defence Technology

(2016) I

Fig. 6. (a) Hardness profile of the weld joint and (b) stress-strain diagram for base metal and HAZ (notch-tensile specimen).

metal shows fine bainitic structure along with acicular ferrite (Fig. 5(b)). The HAZ microstructure consists of acicular ferrite with some polygonal ferrite (Fig. 5(c)). No martensitic phase could be identified in the HAZ or in the weld metal. Hardness of the HAZ (275 VHN) is found to be marginally higher than the base metal (235 VHN). Weld metal hardness (315 VHN) is higher than that of HAZ (Fig. 6(a)).

Mechanical properties of the simulated HAZ are given in Table 2 and Fig. 6(b). Since notch effect is experienced by the HAZ of the implant sample due to presence of the helical notch, the notch tensile strength of the simulated HAZ was determined and the value obtained for the same is 660 MPa, which is higher than the tensile strength of the base metal (575 MPa), revealing the strengthening effect of the notch. Toughness of the HAZ simulated structure is appreciably high (170J), although much lesser than base metal (350J) [13]. Fractographs (Fig. 7a-b) also show ductile cup-cone fracture for tensile tested samples of both base metal and HAZ. Dimple size in case of base metal is much smaller as compared to HAZ, indicating high ductility of the material. Fibrous ductile fracture (Fig. 7(c)) is also noted for the impact tested specimen of simulated HAZ.

4. Discussion

In the present investigation, the results indicate that HSLA steel of grade DMR-249A is not susceptible to HAC irrespective of Hd content of the welding consumable. The reason for the same is investigated in the following section.

It is clear from the results presented above that without baking, Hd content in the flux of the electrode is quite high, and by baking at 150 °C only the moisture absorbed by the flux coating is driven off, whereas by baking at a temperature

Table 2

Mechanical test results of DMR-249A steel.

287 DMR-249A steel Test results

288 Yield strength (base metal) 480 MPa

289 Tensile strength (base metal) 575 Mpa

290 Elongation 30%

291 Notch tensile strength (HAZ) 660 MPa

292 Charpy impact toughness (HAZ) 170 J

recommended by the manufacturer (450 °C), chemically 293

bonded water in some of the flux constituents is also removed. 294

During arc welding processes, hydrogen gets introduced into 295

the weld from the moisture of the atmosphere as well as flux, 296

and from hydrogenous materials such as oil, grease, paint, etc. 297

[14]. The moisture can be removed by drying at moderately 298

higher temperature whereas elevated temperature (above 299

400 °C) is required to drive the chemically associated water of 300

the flux [14]. 301

It can be further seen from the results that LCS could not be 302

determined for weldments produced from either properly baked 303

or unbaked electrode by implant tests as the LCS values are 304

above the yield strength of the material in both the conditions. 305

It can be assumed that fully ferritic microstructure of the HAZ, 306

similar to base metal, and predominantly ferritic structure of the 307

weld metal are the major reasons for good resistance of the weld 308

joint to HAC. It has also been seen for other grades of HSLA 309

steel that ferritic microstructures are resistant to HAC irrespec- 310

tive of Hd content of the weld [15]. Another factor contributing 311

to HAC resistance could be the presence of carbides, especially 312

TiC in the weld metal, which are known to be strong traps for 313

hydrogen [16]. Though Rishi et al. [13] have characterized in 314

detail the inclusion content of SMAW weld metal of DMR- 315

249A steel with similar electrode, detailed study on effective- 316

ness of various precipitates and inclusion as hydrogen traps has 317

not been attempted. Such a study may reveal more information 318

about high resistance to HAC of these welds. 319

The tendency of HAC is much higher in weldments with a 320

stringent variation in hardness from base metal to weld or HAZ 321

[8]. However, in this case variation in hardness between HAZ 322

(275 VHN) and base metal (235 VHN) is only marginal due to 323

the presence of predominantly ferritic microstructure. Weld 324

metal hardness (315 VHN) is slightly higher probably because 325

of the presence of bainite in the weld metal. However, adverse 326

effect of high diffusible hydrogen content is much nullified by 327

the almost uniform hardness distribution across the weldment 328

[7]. Mechanical properties of the simulated HAZ also indicate 329

that fine acicular ferritic structure of HAZ contributes to its 330

good mechanical properties. From the notch tensile strength of 331

the HAZ and fractographs of the same, it can be concluded that 332

a ferritic microstructure of low hardness and high toughness is 333

G. Chakraborty et al. /Defence Technology ■■ (2016) I

334 Fig. 7. Fractograph of (a) impact tested base material, (b) tensile tested base

335 material and (c) notch tensile tested HAZ.

resistant to HAC, irrespective of the hydrogen level in the weld. 336 In this context, susceptibility to HAC of DMR-249A can be 337 compared to that of modified 9Cr-1Mo steel. For modified 9Cr-1Mo steel, the LCS is reported to be 185 MPa corresponding to Hd content of 3.7 mL/100 g of weld metal, which is considerably lower than the yield strength of the material (1000 MPa) [8]. With preheat to a temperature of 250 °C before 342 welding, Hd content comes down to 1.8 mL/100 g of weld metal; however, LCS increases only to 267 Mpa, which is still much below the yield strength of the steel [8]. In contrast to this, in spite of high hydrogen levels LCS could not be deter- 346 mined for DMR-249A grade of steel because of the good resistance of the steel to HAC. Results emphasize the point that in the absence of a susceptible microstructure, high hydrogen 349 content may not cause HAC [15]. This comparison clearly indicates that even very low hydrogen level can cause HAC in a martensitic microstructure as in case of modified 9Cr-1Mo steel; but if it is a ferritic microstructure, even high hydrogen level does not cause HAC. 354

5. Conclusions 356

The following conclusions can be made from the present study: 358

1) HSLA steel grade DMR-249A is not susceptible to HAC.

2) The fine acicular ferritic/bainitic microstructure pro- 360 duced in HAZ and weld can adequately resist HAC in this 361 steel. 362

3) In absence of a susceptible microstructure, HAC is 363 unlikely to occur even if hydrogen levels in the welding 364 consumables are high. 365

References 367

[1] Show BK, Veerababu R, Balamuralikrishnan R, Malakondaiah G. Effect of vanadium and niobium modification on the microstructure and mechanical properties of a microalloyed HSLA steel. Mater Sci Eng A 2010;527:1595-604. 371

[2] Rodrigues PCM, Pereloma EV, Santos DB. Mater Sci Eng A 372 2000;283:136-43. 373

[3] Yue X, Lippold JC. Evaluation of heat affected zone hydrogen induced cracking in navy steels. Welding Res Suppl 2013;92:20s-28s.

[4] Devletian JH, Fichtelberg ND. Controlling hydrogen cracking in ship building. Welding J 2011;80:46s-52s. 377

[5] Dickinsons DW, Ries GD. Implant testing of medium to high strength steel-a model for predicting delayed cracking susceptibility. Welding Res

Suppl 1979;205s-211s. 380

[6] Padhy GK, Komizo Y. Diffusible hydrogen in steel weldments- a status

review. Trans JWRI 2013;42:39-62. 382

[7] Padhy GK, Ramasubbu V, Albert SK, Murugesan N, Ramesh C. Hot extraction of diffusible hydrogen and its measurement using a hydrogen

sensor. Welding World 2012;56:7-8. 385

[8] Albert SK, Ramasubbu V, Sunder Raj SI, Bhaduri AK. Hydrogen assisted 386 cracking susceptibility of modified 9Cr-1Mo steel and its weld metal. Welding World 2011;7-8. 388

[9] Pamnani R, Vasudevan M, Jayakumar T, Vasantharaja P, Ganesh KC. Numerical simulation and experimental validation of arc welding of DMR-249A steel. Def Technol 2016;12:305-15. 391

[10] ISO 2560: 2009, Welding consumables- covered electrodes for manual arc 392 welding og non-alloy and fine grain steels-Classification.

[11] ANSI/AWS A5.1-91: Specification for carbon steel electrodes for shielded metal arc welding, Miami, Florida, American welding society.

G. Chakraborty et al. /Defence Technology ■■ (2016) ■■-■■ 7

[12] Specifications for low alloy steel electrodes for shielded metal arc quenched and tempered steel weldments. Int J Hydrogen Energy 404 welding, American Welding Society for Mechanical Engineer's boiler 2008;33:1897-908. 405

398 pressure vessel code. II C SFA 5.5; 2007;105-7. [15] Madhusudhan Reddy G, Mohandas T, Sarma DS. Cold cracking studies 406

[13] Pamnani R, Jayakumar T, Vasudevan M, Sakthivel T. Investigation on the on low alloy steel weldments: effect of filler metal composition. Sci 407 impact toughness of HSLA steel arc welded joints. J Manuf Process Technol Welding Joining 2003;8:407-14.

401 2016;21:75-86. [16] Depover T, Monbaliu O, Wallaert E, Verbeken K. Effect of Ti, Mo and Cr 409

402 [14] Magudeeswaranan G, Balasubramaniana V, Madhusudhan Reddy G. based precipitates on the hydrogen trapping and embrittlement of Fe-C-X 410

Hydrogen induce cold cracking studies on armour grade high strength, Q&T alloys. Int J Hydrogen Energy 2015;40:47.