IX H 625 Repair Welding Dissimilar Joints

7/13/2019 IX H 625 Repair Welding Dissimilar Joints

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IIW Doc. XI - H 625 05

CONTRIBUTION TO REPAIR WELDING OFDISSIMILAR JOINTS BETWEEN DUPLEX AND

LOW ALLOY STEEL

Peter Bernasovsk

INTERNATIONAL INSTITUTE S SLOVAK DELEGATION

OF WELDING


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CONTRIBUTION TO REPAIR WELDING OF DISSIMILAR JOINTS BETWEENDUPLEX AND LOW ALLOY STEEL

Peter Bernasovsk Welding Research Institute - Industrial Institute of SR, Raianska 71,Bratislava 832 59, SR

Abstract

Welding of dissimilar welded joints requires a special attention, mainly in the case when theproperties of such joints must meet stringent requirements laid upon their properties for examplehardness limit of 248 HV, regarding the sulphide stress corrosion cracking (SSCC). It was the caseof defective welded joints of tubes (duplex steel DIN 1.4462) with a tubeplate (low alloy steel

type 15Mo3) in the air cooler of hydrocracking plant. Due to unsuitable structure and too highhardness all 3240 original welded joints had to be repaired by a relatively complex technology,which is more in detail described in the presented paper.

1. SULPHIDE STRESS CORROSION CRACKING OF AIR COOLERS INHYDROCRACK PLANT

This chapter deals with a case study of cracking in the air cooler of a hydrocrack plant inBratislava. The air cooler was designed with a plug header (see the scheme of the header in Figs.1and 9a) with dissimilar welded joints, because the tubeplate (h = 40 mm), as an internal part of the

plug header is made of the 15Mo3 steel and the tubes 25/2 mm are made of duplex steel DIN1.4462. The welds were fabricated by TIG process using a special automatic welding machine andthe ER 309Mo wire 0,6 mm, which is generally recommended for welding the givencombination of steels. The chemical composition of the materials used is given in Table 1.

Table 1. Chemical composition (wt%)

Material C Mn Si S P Cr Ni Mo Nb N15Mo3 0,130 0,77 0,22 0,010 0,009 - - 0,12 -1.4462 0,025 1,69 0,31 0,003 0,033 22 5,45 3,04 0,53 0,135

ER 306Mo 0,034 1,49 0,40 0,012 - 24 14,2 2,74 0,13 -

The working medium of air cooler consisted of hydrogen (up to 70%), hydrocarbons (up to28%) and hydrogen sulphide, water and other admixtures. The working pressure was 13,48 MPaand working temperature 50 to 122oC.

After about three weeks of service a leakage of the working medium through the weld wasobserved.

The cross section of a specimen extracted from the defective weld is shown in Fig.2. It isevident that a single pass weld is concerned. The crack propagated to weld metal in normaldirection to surface (Fig.3). The arrows indicate the initiation points of short crack. The weldmetal exhibited martensitic structure (Fig.4). Chemical microanalysis (EDAX) of the weld metalrevealed its considerable dilution by the tubeplate material:

element Cr Ni Mowt% 10,03 6,42 1,55


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As it is evident also from Fig.2 about 60% of the tubeplate material and only about 15% of thetube material fused into the weld metal. Based upon the calculated chromium and nickelequivalents (the content of non-analyzed elements was assessed approximately by the dilutiondegree) ECr= 11,9 and ENi= 10,2 plotted in the Schaeffler diagram, the weld metal structure wasexpected to be martensitic with a small proportion of austenite (see Fig.5) which well agrees withthe morphological determination of the structure as well as with hardness measurements.

Fig. 1. A scheme of the plug header in the

aircooler.

2. Macrosection of the defected welded joint.

Weld metal hardness attained even the values of HV 435. The results of hardnessmeasurement obtained from all zones of the welded joint are given in Table 2.

Table 2. Hardness of the tube to tubeplate weld joint

Location HV5

weld metal 412 435tube BM 248 262tube HAZ 257 262tubeplate - BM 152 165tubeplate - HAZ 322 381

From the viewpoint of SSCC the hardness should not exceed the values of HV 248 accordingto NACE standard for low alloy steels (for duplex steels, the hardness limit is higher, namelyHV 285).

Unacceptable hardness was observed also in the heat affected zone of the tubeplate which wasmostly bainitic with some martensitic islands (Fig.6, bottom).

The performed analyses have shown, that the specific joint configuration and restrictedwelding condition (access though a hole 25 mm in diameter on the opposite wall) resulted inundesired weld metal dilution by the tubeplate material leading to weld metal with a martensiticstructure which is usually extremely susceptible to sulphide stress corrosion cracking. It wasactually the main reason for necessity to repair all tube and tubeplate welded joints, including the

joints where no corrosion cracking was observed yet.


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Fig. 3. Sulphide stress corrosion crack in the

WM.

Fig. 4. Martensitic microstructure of the WM.

During the repair of air cooler, besides the martensitic weld metal also the hardened heat affectedzones of the tubeplate and tube ends (made of a duplex steel incapable to withstand the repeated

heating) had to be removed. Thus the repair of all 3240 welded joints in tube tubeplate connections ofthe air cooler was a really demanding job.

Fig. 5. Determination of the WM microstructure from the Schaeffler diagram.

Fig. 6. Transition from the tubeplate to theweld metal. Fig. 7. The dependence of WM hardness ofaustenitic filler metals on the degree of dilution

with the unalloyed carbon steel.


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2. PROBLEMS ENCOUNTERED IN WELDING DISSIMILAR STEELS

Due to dilution in welding dissimilar materials, the zones with different chemical compositionare created what results in properties differing from those of unaffected base metal or undilutedweld metal. Because in arc welding the dilution cannot be avoided a special attention must bedevoted to welding procedure and suitable filler metal selection. The root passes are most affected

by base metal dilution. For example in TIG surfacing the root layer is diluted with the unalloyedbase metal by 5 to 15%, whereas it is even more in MIG welding 20 to 30% and in welding withcovered electrodes it is even 30 40%. Slightly lower dilution is observed in fillet welds. Thehighest dilution about 40 to 60% usually occurs in the zones containing sharp edges, corners, rimsetc.

Dilution degree can be substantially affected also by variable thickness of material welded andby restricted electrode accessibility, as was probably the case of tube to tubeplate welded joints ofthe air cooler in a hydrocrack plant.If the chemical composition of base and filler metal is known, and when the dilution degree is alsoknown, then it is possible to determine not only the chemical composition but also to predict

structure of the weld by means of the well known Schaeffler diagram. The dependence of weldmetal hardness on the value of dilution of the appropriate filler metal (in %) by the unalloyed steelis plotted in Fig.7. The obtained data agree quite well with the Schaeffler diagram. Fully austenitichigh alloy filler metals are usually applied for welds in CrNi austenitic steel with unalloyedsteels. Ni based alloy provides formation of a very narrow transition layer with a negligiblenarrow band of a martensitic interlayer on the fusion line, in addition to high acceptable dilution.Schematic representation of widths of such interlayer, related to filler metals used is shown inFig.8. The benefits of NiCr alloy consist in their value of thermal expansivity coefficient, which isclose to that of unalloyed and low alloy steels. Great difference in thermal expansivitycoefficients of austenitic filler metal type Cr25Ni13 and similar ones, compared to those ofunalloyed and low alloy steels are considered unfavorable and result in formation of highresidual stresses which cannot be so easy eliminated by heat treatment.

Another advantage offered by the NiCralloys is that their fusion lines with theunalloyed steel are substantially moreresistant to diffusion processes, especiallyas far as the weld metal carburization isconcerned. In other words, they preventdecarburization of a certain portion of theunalloyed base metal. The above mentioned properties of NiCr alloys areimportant mainly in the cases whenstructures require heat treatment and are,during service, subjected to variable

thermal loading or to medium inducingthe stress corrosion. Having taken allmentioned facts into consideration, forthe repair we selected the filler metal ofInconel type, offering sufficientguarantee for attaining a suitable weldmetal structure even at unfavorabledilution rate by the unalloyed tubeplatematerial, while providing alsosatisfactory resistance against corrosioncrack in the given aggressiveenvironment used in the air cooler.


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3. REPAIR PROCEDURE OF THE AIR COOLER

In order to carry out a fast repair of the air cooler and to reduce the cost losses resulting fromthe production shut - down the possibility of on site repair by welding was firstly considered bymanufacturer.


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In addition to the problems connected with filler metal selection and with technological masteringof welding the tubes to trough small unplugged holes 25 mm in diameter and 160 mm deep it wasnecessary to propose and verify the welding procedure suitable for fabrication of joints exhibitingsatisfactory HAZ hardness on the medium side of welds. To calculate the desired preheattemperature ensuring the attained hardness level below HV 248 in the heat affected zone of thetube sheet, the well known Suzuki equation [1] was applied, giving the value of min. 200 oC.However, such a preheat or post weld heat treatment of the repaired welds was impractical due totechnical reasons (cooling down effect of the air cooler) and the urgency of a fast repair madeapplication of some automated process impossible and therefore only manual arc welding withcovered electrodes could be employed. Welding was performed in vertical up hill position withInconel electrodes 2,5 mm in diameter through the holes 25 mm in diameter, as shown in Fig.9a,right. To attain sufficient fusion of the edges of the hole and tube ends root pass was firstfabricated. In this stage of welding the welder could see well the edges and was able to guideaccurately the arc around the joints. In spite of the fact that different variants aimed at utilizationof the self heating (the beads 1 + 2, Fig.9b) as well as the self tempering (the bead No.3)effects were employed, the welds with guaranteed HAZ hardness on the working medium side

below HV 240 were not attained in any case. The techniques applied for tempering beads require

extreme accuracy of bead deposition, which could not be assured under the given restricted on side conditions.It also should be noted that the service of air cooler resulted in a partial hydrogenation of materialssubjected to aggressive medium what consequently increased the risk of cracking and pores in therepaired welded joints. The following values of hydrogen content were measured:WM ......................... 8,18 ppm Htube ...........................7,16 ppm Htubeplate ...................6,91 ppm H

Fig. 10. Macrostructure of repaired

welded joint.

Dehydrogenization by tempering the air cooler header on site was impossible that was anotherreason why a plant repair of the dismounted air cooler, which was proposed by our institute [2]was adopted.

The plan repair procedure of air cooler was carried out in the following steps:

a) removal of all original welds (Fig.9c) by use of a special tool,b) removal of all tubes from the air cooler plug headers,c) dehydrogenization of the plug headers by tempering,d) machining of grooves in the tubeplate bores, serving for tube end expanding by rolling with a

special roller,e) buttering beads were deposited on the outs around the bore ends by use of a special welding

procedure,f) all butters were machined to the required shape,g) HAZ on the tubes were cut off (shortened by 10 mm),h) the buttering welds were tempered,i) the air cooler plugs were assembled with the tubes into the sections,

j) tube to tubeplate welds were fabricated by use of Inconel electrodes (Fig.9b) and the leakagetest was performed,

k) rolling expansions of tube to tubeplate by a special roller,


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l) pressure test of tube sections was performed

Macrostructure of the repaired welded joint is shown in Fig.10. The employed repair procedurehas assured the austenitic structure of weld metal and a tempered bainitic structure of the tubeplateheat affected zone under the buttered beads providing thus a sufficient hardness limit below thecritical HV 248 hardness limit. Only a slight excess of tube base metal hardness as a results ofrolling (expanding) the tube ends (HV 299 compared to NACE hardness limit being HV 285) wasmeasured. However, the results obtained in the corrosion tests according to NACE TM 01 77did not revel any difference in the sensitivity to sulphide stress corrosion cracking in the rolled(expanded) tubes (5%) compared to the non rolled tubes.

Good repair quality was proved also by the metallographical examination of trial specimensand by non destructive testing of welded joints in one air cooler section during the regular shut downs and by long term service without any problems.

4. REFERENCES

[1] Suzuki, H.: Carbon Equivalent and Maximum Hardness, Trans. of the Japan Welding Society,No.1, April 1984

[2] Bernasovsk, P. - inl, J. et al.: Welding procedure of tube tube plate joints for shop repairof air cooler sections in Hydrockrack unit Slovnaft, Improvement suggestion No. 55/90,Ferox Den.

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IX H 625 Repair Welding Dissimilar Joints