Zavarivanje i Navarivanje Visokoegiranih Celika

SCIENCE∗RESEARCH∗DEVELOPMENTNAUKA∗ISTRAŽIVANJE∗RAZVOJ

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (2/2008), str. 51-60 51

J. Pecha, J. Hakl, T. Vlasák Prevod: Sonja Papak

ZAVARIVANJE OPREME ZA ENERGETIKU – SADAŠNJOST I BUDUĆNOST

WELDING IN POWER EQUIPMENT - PRESENT AND FUTURE

Originalni naučni rad / Original scientific paper

UDK / UDC: 621.791:621.31

Rad primljen / Paper received: Mart 2008.

Adresa autora / Author's address: Jozef Pecha, Ján Hakl Slovenské energetické strojárne, Továrenska 210, Tlmače, Slovačka Tomáš Vlasák Státny výskumný ústav materiálů, Praha-Běchovice, Republika Češka

Ključne reči: Membranski zid, zavarivanje pod praškom, termička obrada, otpornost na puzanje, sekundarno ojačavanje.

Keywords: Membrane wall, submerged arc welding, postweld heat treatment, creep resistance, secondary hardening.

Izvod U radu je prikazan kratak pregled automatskog zavarivanja u prizvodnji membranskih zidova za delove pod pritiskom kod energetske opreme. Analizirani su problemi trenutnog stanja i moguće perspektive u ovoj ključnoj tehnologiji zavarivanja. Analizirani su aspekti zavarivanja savremenog čelika tipa 23. Ispitivani su efekti termičke obrade zavarenog spoja na otpornost na puzanje. Prikazani su problemi zavarljivosti cevi od čelika T23 sa primerima iz prakse.

Abstract A brief summary of automatic welding in manufacture of membrane walls for pressure parts of power equipment is presented. Problems of current state and possible expectation in this key technology of welding were analyzed. Analyses of material aspects of welding the progressive steel type 23 are presented. Weld heat treatment effect on creep resistance were examined. Problems of T23 steel tube weldability and conclusions from practice are shown.

INTRODUCTION

Even welding in power equipment faces various technological problems. However, solving any problems helps to increase the level of every human activity. This presentation will show the level reached by welding technologies of pressure parts.

So as the power unit parameters were increased, it was also developed basis of alloying additions to applied creep resisting steels. Development of creep resistant steels has been accompanied with problems how to manage their weldability. How were the problems succeeded to be managed? This paper will try to answer it briefly, too.

Steels in power equipment are applied for increased temperatures, mostly in creep zone.

The weld seams forming the pressure part shall face the same exposition as base material. Their properties try to be as close to the base material properties as possible. This is the reason why the welding technology development has followed the target mentioned above.

In the very beginning, tube systems were connected almost exclusively manually by coated electrode, ofcourse except for flame technology. An attempt to achieve higher productivity resulted in dynamical start of resistance butt welding in pressure systems. The technology was widely used in ll0 and 200 MW boilers

UVOD

Iako se zavarivanje opreme za energetiku suočava sa različitim tehnološkim problemima, rešavanje svakog problema povećava nivo svih ljudskih delatnosti. U radu će biti prikazan nivo koji su tehnologije zavarivanja dostigle u oblasti opreme pod pritiskom.

Kako su povećavani energetski parametri uređaja, tako se razvijala i osnova za povećanje legirajućih dodataka primenjivanih čelika otpornih na puzanje. Usavršavanje čelika otpornih na puzanje je praćeno problemima njihove zavarljivosti. Kako se došlo do rešenja tih problema? Ovaj rad će to ukratko pokušati da objasni.

Čelici za energetsku opremu se primenjuju na povišenim temperaturama i to uglavnom u zonama gde se javlja puzanje.

Zavareni spojevi kojima se spajaju delovi pod pritiskom su izloženi istim uticajima kao i osnovni materijal, stoga se pokušavaju dobiti njihova svojstva što približnija osnovnom materijalu. To predstavlja razlog zbog kojeg je razvoj u tehnologiji zavarivanja sledio gore pomenuti cilj.

Na početku su cevni sistemi skoro isključivo bili ručno spajani korišćenjem obložene elektrode, naravno, sem tehnologije zavarivanja sa gasnim plamenom. Pokušaj da se postigne veća produktivnost rezultirala je razvojem sučeonog otpornog zavarivanja u


52 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (2/2008), str. 51-60

sistemima pod pritiskom. Tehnologija je široko primenjivana u kotlovima od 110 i 200 MW izastupljena je kod oko 90 % zavarenih cevi.

Usled nesavršenosti koje su se uglavnom ogledale u pojavi visoke tvrdoće, stvaranja suvišnog metalaspoja i problema vezanih za protok fluida, ta tehnologija je postepeno izbegavana.

Tokom sedamdesetih godina prošlog veka, lansirani su metodi zavarivanja sa zaštitnim inertnim gasom, koji su postali dominantni. Čak i kod cevovoda sa debelim zidovima, oni su bili nezamenljivi za zavarivanje korenog prolaza. Podesna geometrija zavarenog spoja i dobra mogućnost formiranja područja korenog prolaza garantuju minimum defekata koji predstavljaju presudni faktor za životni vek zavarenog spoja. Za zavarivanje relativno manjih debljina, razvijeno je mehanizovano zavarivanje u zaštitnom gasu argonu, čime su ograničene ljudske greške, iako to ne znači da su smanjene potrebe za stručnošću operatera.

Sa stanovišta automatizacije zavarivanja, zanimljiva je proizvodnja sabirnih i distributivnih cevi sa debelim zidovima. Dok se spoljašnji šavovi debelih delova zidova zavaruju spomenutom kombinacijom 141 (GTAW) + 111 (REL), cevi brizgaljke se takođe zavaruju kombinacijom 141 (GTAW), ali se prolazipopune rade pomoću automatskog zavarivanja pod praškom 121 (EPP), ako prostorni uslovi to omogućavaju. Ako to nije moguće, ovi zavari setakođe prave pomoću kombinacije 141 (GTAW) + 111 (REL).

Početkom šezdesetih godina, delovi isparivača energetskih uređaja su konstruisani isključivo od „solo“ zasunskih cevi. Ovo su bile cevi sa navarenim ravnim delovima, međusobno spojeni zavarivanjem. Zidovi isparivača koji su bili zavarivani su na taj način formirali kompaktne komore za sagorevanje, ali one nisu bile toliko nepropustljive na gas. Idealizovani Karnotov ciklus je primorao traženje načina povećanja efikasnosti još u prvoj fazi, to jest kod pretvaranja vode u paru. Umesto do tada korišćenih takvih komora, zavarivanjem pod praškom su izrađivani zidovi koji su bili nepropusni za gas.

Komoru za sagorevanje koja se sastoji od membranskih zidova karakteriše mala težina (isključujući teške delove od opeke), ali uglavnom sa mogućnošću sagorevanja pod izvesnim preopterećenjem i predstavlja bitan doprinos povećanju efikasnosti energetskog uređaja kao celine. Današnja tehnologija zavarivarivanja spojenih cevi sa ravnim limovima od gvožđa između predstavlja ekskluzivnu osnovu za stvaranje zidova bilo koje veličine, slika 1.

Jasno je da automatsko zavarivanje pod praškom (121, EPP) dominira u proizvodnji zidova. Geometrija zavara, posebno veličina dela cevi gde nema provara se redovno proverava makro pregledom. Kao dodatni materijal se dugo vremena koristila puna žica ø 2 mm.

and applied in about 90 % of workshop tube welds.

Due to imperfections manifested mainly in high peaks of hardness, creation of excess welding metal and related problems in medium flowing, that technology was gradually avoided.

In 70-ties of the last century there were launched methods of shielded inert gas metal arc welding that became dominant. Even in thick wall piping they were indispensable for welding the root areas. Suitable weld geometry and good capability to form the root area guarantee minimum of defects being a crucial factor for service life of weld. For welding relatively smaller thicknesses, argon shielded welding has succeeded to be mechanized and consequently, human errors are limited, although it does not mean reduced requirements for operators qualification.

From the point of welding automation it is interesting manufacture of thick wall collecting and distribution headers. While circumferential seams of thick wall parts are welded by the mentioned combination 141 (GTAW) + 111 (SMAW), nozzle tubes are also welded by the combination 141 (GTAW), but with filling passes done by automatic submerged-arc welding 121(SAW), if possible due to spatial conditions. If it is not possible, then these welds are welded by the combination 141 (GTAW) + 111 (SMAW), too.

At the beginning of 60-ties the evaporator parts of power units were constructed exclusively of “solo” register tubes. There were tubes with welded-on flat parts, mutually interconnected by welding. The evaporator walls welded in that way formed combustion chambers being compact, but less gas-tight. The idealized Carnot cycle forced to look for ways of increasing the efficiency still in the first phase i.e. at changing water into vapour. Instead of “register” combustion chambers used till then, there were manufactured gas-tight walls by submerged-arc welding.

The combustion chamber consisting of membrane walls is characterized by small weight (excluded heavy brickwork), but mostly by possible combustion under a certain overpressure and it is an important contribution to increased efficiency of power unit as a whole. Today welding technology of tube couple with flat iron in-between is an exclusive basis for creation of walls of any size, fig.1.

Automatic submerged-arc welding (121, SAW) dominates clearly in manufacture of walls. The weldgeometry, particularly the size of tube part lacking fusion is checked regularly on macrosections. For a long time it was used a solid wire ø 2 mm as filler material.

Recently, it has been an attempt to increase welding rates. A possible way to increase the welding rate is to use a tubular wire. To achieve the required effect it is important to optimalize the combination of wire and flux. Having done a lot of tests of wire and flux



Creep resistance of weld seams P23 has been

U zadnje vreme pokušava se sa povećanjem brzine zavarivanja. Jedan od mogući načina povećanja jeste korišćenje punjene žice. Da bi se postigao traženi efekat, bitno je optimizovati kombinaciju žice i punjenog jezgra. Posle mnogobrojnih obavljenih ispitivanja uticaja žice i jezgra na poroznost i geometriju sva četiri šava popune, brzina zavarivanja punjenom žicom se ustalila u opsegu od 1,1-1,4 m/s.

U svetu, posebno u Japanu, za zavarivanje membranskih zidova koristi se MAG postupak. Firma Kusakabe nudi liniju sa 12 glava za zavarivanje, dok 6 glava zavaruje u horizontalnoj poziciji od gore, a 6 glava zavaruje u suprotnoj poziciji. Za ovaj raspored glava za zavarivanje, brzine zavarivanja su 1,5 m/s.

U Evropi za proizvodnju membranskih zidova prevladava postupak zavarivanja pod praškom. Vodeća finska firma PEMA za proizvodnju linija za zavarivanje membranskih zidova nudi opremu sa 6 glava za zavarivanje i dodatnom opremom za okretanje limova, kao i potpunu automatizaciju tokom operacija zavarivanja. Garantovana brzina zavarivanja iznosi 1,2 m/s.

UTICAJ TOPZ NA OTPORNOST NA PUZANJE DEBELOZIDNIH ZAVARENIH SPOJEVA OD ČELIKA P23

Neki od rezultata ispitivanja su prikazani u literaturi [21]. Prezentovani su rezultati ispitivanja zavarenih spojeva na spoljnoj temperaturi. Pošto je u pitanju novi čelik, bitna je otpornost na puzanje zavarenih spojeva. Ovaj zahtev je od velike važnosti

influence on porosity and geometry of all 4 “fillet” welds, the welding rate with tubular wire settles within the range of 1.1-1.4 m/s.

In the world, particularly in Japan, for membrane walls welding it is use MAG method. Kusakabe offers a line with 12 welding heads, while 6 heads weld in the horizontal position from above and 6 heads weld in opposite position overhead. In the company presentation there are given welding rates of 1.5 m/s for this welding heads arrangement.

Within Europe, the submerged-arc welding method prevails for membrane wall manufacture. The Finnish PEMA that is a leading company producing welding lines for manufacture of membrane walls supplies equipment with 6 welding heads and accessories for turning the panels and complete mechanization during welding operations. The guaranteed welding rate is 1.2 m/s.

PWHT INFLUENCE ON CREEP RESISTANCE OF THICK WALL WELDS P23

Some results about P23 steel welding were previously presented in the literature [21]. There were presented test results of weld seams at ambient temperature.

As it is concerning new steel, the creep resistance of weld seams is important. This requirement is important because there are just a few records of long-term properties of weld seams. I would also like to mention experience with welding the tubes of this steel.

Slika 1: Zavarivanje membranskog zida isparivača Figure 1: Welding of evaporator membrane wall



jer postoji mali broj podataka o osobinama zavarenih spojeva nakon dugotrajnog rada. Takođe će biti pomenuta iskustva sa zavarivanjem cevi od ovog čelika.

re tested three alternatives of post-weld heat

nt

• ent

• ent

C ut with bars at

he time

tive III with PWHT at the parameters

s ones, the alternative III

specimens were mostly

studied in cooperation with SVÚM Praha (National Research Institute for Materials) and VUZ PI Bratislava (Welding Research Institute – Industrial Institute). These test results are included in the paper of Vlasák, Hakl and Pecha [1] referring to the first knowledge of this steel welding in Slovakia [2]. The paper of the mentioned authors examines heat treatment influence on creep resistance of weld seams.

There wetreatment parameters and the first alternative was taken as a basis. The results of the other tests of this alternative are given in the paper at the conference in Zlatibor. The parameters of weld heat treatment for creep resistance tests were as follows:

• 1st alternative - scope of heat treatmetemperatures 750-760 °C/2h, (HT1) 2nd alternative - scope of heat treatmtemperatures 740-750 °C/2h, (HT2) 3rd alternative - scope of heat treatmtemperatures 730-740 °C/1h, (HT3)

reep resistance tests were carried otemperatures of 500, 550 and 600 °C and pressure. To compare the weld heat treatment temperatureseffects on creep resistance it was used the creep resistance diagram of the base material P23.

In creep testing of the first alternative welds tto rupture represented just about one tenth of base material values. The creep resistance results achieved in the 2nd alternative were slightly higher, but they cannot be considered to be satisfactory. The lower post-weld heat treatment temperature confirmed the trend of creep resistance improving. Therefore it was decided to confirm the assumption by reducing the heat treatment temperature and the holding time at the temperature. Sensitive modification of heat treatment parameters (fig. 2) would not be possiblewithout exact control and recording instrumentation provided in z common electric box furnace HN 27/12. The issues of precise measurement of temperatures within the furnace were solved by Macko et al., Research Institute of Power Engineering Equipment Levice [3].

The alternaaccording to the previous figure confirmed in creep resistance testing the assumption of possible reaching of satisfactory time to rupture with the specified testing parameters, fig. 3.

Comparing to previoureached the best results. The test results are within the permissible range of variations -20 % of results of creep resistance tests of base material used for weld seams. The achieved values of weld creep resistance are comparable to the results of Metrode, Vallourec (Thyssen), Air Liquide [4, 5].

During creep tests the testingruptured in case of low-heated heat affected zone

Otpornost zavarenih spojeva od čelika P23 na puzanje je proučavana u saradnji sa SVÚM iz Praga (Nacionalni institut za istraživanje materijala) i VUZ PI iz Bratislave (Insititut za istraživanje zavarivanja -Industrijski institut). Ovi rezultati ispitivanja prikazani u radu Vlasák, Hakl i Pecha [1] u kojem se spominju neka od prvih saznanja zavarivanju ovih čelika u Slovačkoj [2]. Rad spomenutih autora ispituje uticaj termičke obrade na otpornost na puzanje zavarenih spojeva.

Ispitivane su tri alternative parametara termičke obrade, uzevši prvu opciju kao osnovnu. Rezultati ostalih alternativa testova su predstavljeni u radu [21].Parametri termičke obrade zavarenih spojeva za ispitivanje otpornosti na puzanje su sledeći:

• Prva alternativa - termička obrada na temperaturi 750-760 °C/2h, (TO1)

• Druga alternativa - termička obrada na temperaturi 740-750 °C/2h, (TO2)

• Treća alternativa - termička obrada na temperaturi 730-740 °C/1h, (TO3)

Ispitivanje otpornosti na puzanje je vršeno na šipkama, na temperaturama od 500, 550 i 600 °C i pritisku. Radi poređenja uticaja temperatura termičke obrade na otpornost na puzanje, korišćen je dijagram otpornosti na puzanje osnovnog materiala P23.

U testovima na puzanje kod prve alternative (TO1)vreme do loma predstavlja nešto više od jedne desetine vrednosti za osnovni materijal. Rezultati testova otpornosti na puzanje kod druge alternative (TO2) su bili neznatno viši, ali ne mogu se smatrati zadovoljavajućim. Niža temperatura termičke obrade nakon zavarivanja je potvrdila trend poboljšanja otpornosti na puzanje. Stoga je odlučeno da se pretpostavka potvrdi smanjivanjem temperature termičke obrade i vremena držanja na zadatoj temperaturi. Fine modifikacije parametara termičke obrade (slika 2) ne bi bile moguće bez tačne kontrole i instrumenata za beleženje na elektro peći HN 27/12. Probleme tačnog merenja temperatura unutar peći rešio je Macko sa saradnicima, Institut za istraživanje opreme u elektranama iz Levica [3].

Alternativa (TO3) sa TOPZ sa parametrima termičke obrade prema slici 2 potvrdila je sa testovima otpornosti na puzanje, pretpostavku o mogućempostizanja zadovoljavajućeg vremena do loma sa navedenim parameterima, slika 3.

U poređenju sa prethodnim, alternativa termičke obrade (TO3) je postigla najbolje rezultate. Rezultati ispitivanja se nalaze unutar dozvoljene granice od -20 % od rezultata otpornosti na puzanje osnovnog materijala koji se koristio za zavarene spojeve.Postignute vrednosti otpornosti zavara na puzanje se



Program: COST 536WELD D219*30mm

materiál P23

725727729731733735

737739741743745

04:44:00 04:54:00 05:04:00 05:14:00 05:24:00 05:34:00 05:44:00 05:54:00

Čas [hh:mm:ss]

Tepl

ota

[°C

]

TIR1.1-teplota materiálu v bode1TIR2.1-teplota materiálu v bode 2Žiadaná hodnotaPriemer hodnôt výdrže (TIR1.1+TIR2.1)/2

Žíhané: 21.04.2006

hodnota výdrže 735°C -

TIR1.1 – temperature materijala u tački 1 / temperature of material in point 1; TIR2.1 – temperatura materijala u

tački 2 / temperature of material in point 2; žiadaná hodnota – zahtevana vrednost / required value; Priemer hodnôt výdrže – prosečno vreme držanja / holding time average

Slika 2: Temperature tokom TOPZ zavarenih spojeva P23, TO3 , (TIR1.1, TIR2.1 temperature na uzorku zavara)

Figure 2: Progress of temperature during PWHT of welds P23, HT3, (TIR1.1, TIR2.1 temperatures at the sample weld)

10

100

1000

10000

100000

50 100 150 200 250 300 350Napětí [MPa]

Dob

a do

lom

u [h

]

Alternativa III-550°CAlternativa III-600°CAlternativa III-500°CAlternativa III-550°CAlternativa III-600°C

500°C550°C600°C

Prázdné symboly znázorňují pokračující zkoušky

Napätie [MPa]

Doba do lomu – vreme do loma / time to rupture; Napätie – napon / stress; Prázdné symboly znázorňujúce pokračujúce skúšky – prazni simboli predstavljaju testove koji traju / empty symbols represent tests in progress

Slika 3: Rezultati testiranja karakteristika otpornosti na puzanje zavarenih šavova od P23, TOPZ, TO3

Figure 3: Results of testing of creep-resistance properties of welds of P23, PWHT, HT3

of (HAZ). During the thermal-deformation cycle welding the zone was several times heated up to the temperatures between Ac1 and Ac3. The zones heated up to the temperatures above Ac1 were subject to multiple martensitic transformations. On the other side, the zones heated under Ac1 were subject to multiple high-temperature tempering resulting in a drop of hardness accompanied by carbidic phase increase.

mogu porediti sa rezultatima iz literature [4, 5].

Tokom ispitivanja puzanja, uzorci koji su testirani su se uglavnom lomili u slučaju da je zona pod uticajem toplote (ZUT) bila izložena nižim temperaturama termičke obrade. Tokom termičko-deformacionog ciklusa zavarivanja, zona je nekoliko puta zagrevana do temperatura između Ac1 i Ac3. Zone koje su zagrevane na temperature iznad Ac1 su bile predmet višestrukih martenzitnih transformacija. Sa druge



at treatment resulted in further tempering

treatment representing the 3rd

HIN-WALL TUBES T23

G

s D 42.2 x 3.6 mm of

Post-weld heof these zones. The result of the above mentioned processes is a softer microstructure. Consequently, creep rupture is initiated preferentially in a softer HAZ as shown in fig. 4.

Samples of heat alternative were subject to further tests with regard to favourable creep resistance results. Impact strength testing showed the impact work values 94, 111, 120 J from the centre of weld seam. The weld seam hardness (before creep tests) reached the maximal values of 250 HV.

WELD SEAMS OF T

Testing welds were made by an automatic TIwelding machine. At welding the welding torch is in a stable position and tube is rotating. Tube end preparation is simple alignment of end faces, but there are high requirements for perpendicularity of aligned ends to the tube centre line. When the ends are moved to contact each other (zero gap), the automatic welding machine will make one weld layer up to the tube wall thickness of 5.6 mm.

For testing there were used tubeabove mentioned quality. As filler it was used a wire D 1.0 mm on a spool marked WZ CrWV 2 1.5 from Böhler Thyssen Welding (Union I P23). What made us

strane, zone koje su zagrevane do ispod Ac1 su bile podvrgnute višestrukim visoko-temperaturnim otpuštanjima, što je dovodilo do pada tvrdoće koju je pratilo povećanje udela karbidne faze.

Termička obrada posle zavarivanja je rezultirala daljim otpuštanjem ovih zona. Rezultat gore pomenutih procesa jeste omekšavanja u mikrostrukturi. Stoga, lom usled puzanja se prvenstveno inicira u mekšoj zoni pod uticajem toplotekao što je prikazano na slici 4.

Termički obrađeni uzorci spojeva prema alternativi (TO3) su bili predmet daljih ispitivanja obzirom na povoljne rezultate otpornosti na puzanje. Ispitivanja energije udara su pokazala vrednosti od 94, 111 i120 J iz centra zavarenog spoja. Tvrdoća zavarenog spoja (pre testova puzanja) je dostigla maksimalne vrednosti od 250 HV.

ZAVARIVANJE TANKOZIDNIH CEVI OD ČELIKA T23

Zavareni spojevi za ispitivanje su napravljeni uređajem za automatsko TIG zavarivanje. Prilikom zavarivanja, brener za zavarivanje se nalazi u fiksnompoložaju, a cev se rotira. Priprema kraja cevi predstavlja jednostavno poravnanje zadnjih strana, ali postoje visoki zahtevi za upravnost poravnatih krajeva sa centralnom linijom cevi. Kada se krajevi pokreću

Slika 4: SEM analiza izabranog uzorka posle testa puzanja [6]

gore – mikrostruktura uzorka posle testa puzanja; a)-mikrostruktura osnovnog materijala (1000x); b)-mikrostrukture zone uticaja toplote (1000x); c)-mikrostruktura metala šava (1000x)

Figure 4: SEM analysis of selected sample after creep test [6]

above – microstructure of the sample after creep test; a)-microstructure of parent material (1000x); b)-microstructure of heat-affected zone (1000x); c)-microstructure of weld metal (1000x)

SCIENCE∗RESEARCH∗DEVELOPMENTNAUKA∗ISTRAŽIVANJE∗RAZVOJ da bi dotakli jedni druge (nema međuprostora), aparat za automatsko zavarivanje će napraviti jedan sloj zavara do debljine zida cevi od 5,6 mm.

Za ispitivanje su korišćene cevi dimenzija42,2 x 3,6 mm kvaliteta T23. Kao dodatni materijal je korišćena žica Ø 1,0 mm oznake WZ CrWV 2 1.5 od proizvođača Böhler Thyssen Welding (Union I P23). Razlog zbog kojeg je iz tehnološke procedure zavarivanja izostavljeno predgrevanje, jesu navodi iz literature [7], sopstveno iskustvo sa zavarivanjem čeličnih cevi T24 [8], kao i neophodnost da zavarivanje cevi za supergrejače bude efikasnije.

Heuser sa saradnicima [9] opisuje rezultate ispitivanja T23 zavarenih cevi sprovedenih orbitalnom glavom na zidu debljine 7,5 mm bez predgrevanja. Uticaj predgrevanja se manifestuje na tvrdoći zavarenih spojeva. Rezultati testova tvrdoće zavarenih spojeva T23 bez predgrevanja i termičke obrade posle zavarivanja koja se spominje u radu nisu dokazali povećanje vrednosti iznad 310 HV. Slični rezultati su postignuti ispitivanjima pri zavarivanju cevi od T24 (debljina zida 6,3 mm) kod proizvođača opreme za energetiku [8].

Pitanje da li zavareni spojevi trebaju biti podvrgnuti termičkoj obradi ili ne i dalje predstavlja predmet širih proučavanja mnogih autora i programa istraživanja[10-12]. Zavareni spojevi sličnih, prethodno razvijenih ½ Cr ½ Mo ¼ V čelika trebaju biti podvrgnuti termičkoj obradi [13] usled većeg sadržaja ugljenika i rezultujuće tvrdoće. Vrednost dopuštene tvrdoće (maks. 350 HV10) je bio jedan od razloga zbog kojeg se počelo sa razmatranjima o izostavljanju mekogžarenja zavarenih spojeva modernih CrMo modifikovanih tipova čelika T23 i T24 [14, 15].

Izostavljanje mekog žarenja zavarenih spojeva ovih čelika predstavlja siguran debalans u mikrostrukturi.Čestice sekundarne faze ojačavaju čvrsti rastvor (Cr23C6 i Cr7C3) i poseduju visoku dimenzionalnu stabilnost koja je postignuta otpuštanjem pri proizvodnji poluproizvoda. Zavarivanje u zoni pod uticajem toplote osnovnog materijala stvara privremene uslove da se ne postigne potpuno taloženje ojačavajućih čestica. Ako se prelazni uslovi drže tako da zavareni spoj nije bio izložen mekom žarenju, onda je tokom rada pri visokim temperaturama došlo do pojave dodatnog taloženja ovih čestica [10, 12]. Pojava sekundarnog ojačavanja zavara sa CrMoV legirajućim dodacima u osnovi praćeno je smanjenjem njihovih plastičnih osobina što znači povećanje granice popuštanja i smanjenje udarne žilavosti. Iako je primarni zahtev za karakteristike zavara kod ovih čelika dovoljna otpornost na puzanje, pojava sekundarnog ojačavanja ne može biti potcenjivana pošto može da se pojavi tokom početnih faza rada energetskog postrojenja. Pucanje zavarenih spojeva u CrMo, CrMoV čelicima na početku dugotrajnog rada može izazvati obimne štete čak i kada su zavari zadovoljavajući sa tačke gledišta otpornosti na puzanje. Prelazni režimi rada

test results of T 23 tube

e subject to heat

e steels

sient conditions are kept so that a weld

to leave out preheating from the technological procedure of welding, it was literary knowledge [7], our own experience with welding of tube steel T24 [8] and a necessity to make tube welding for superheaters more efficient.

Heuser et al. [9] describes the welds carried out by an orbital head on a wall thickness of 7.5 mm without preheating. Preheating influence is manifested in weld seam hardness. Hardness test results of weld seams T23 without preheating and post-weld heat treatment mentioned in the paper did not prove increase of values above 310 HV. Similar results were achieved at tests of T24 tube welding (wall thickness 6,3 mm) at a manufacturer of power engineering equipment [8].

The issue if weld seams are to btreatment or not, is still a subject of large studies of a lot of authors, research programmes [10-12]. Weld seams of similar, previously developed ½ Cr ½ Mo ¼ V steels shall be subject to heat treatment [13] due to a higher content of carbon and resulting hardness. The value of permissible hardness (max. 350 HV10) was one of reasons why it started to consider about leaving out stress annealing of weld seams of modern CrMo modified steels type T23, T24 [14, 15].

Leaving the weld seam annealing for thesmeans a certain type of unbalance of microstructure. Secondary phase particles hardening the solid solution (Cr23C6 and Cr7C3) are of high dimensional stability achieved by tempering at manufacture of semi-finished products. Welding in heat-affected zone of parent material creates transient conditions at there is not occurred complete precipitation of hardening particles.

If the transeam was not subject to annealing, then during operation at high temperatures there is occurred additional precipitation of these particles [10, 12]. The event of secondary hardening of weld seams CrMoV of alloying additions basis is accompanied by reduction of their plastic features and it means increase of ultimate strength and decrease of impact strength. Although the primary requirement for weld properties in these steels is sufficient creep strength, the occurrence of secondary hardening cannot be underestimated as we can meet with it during the initial stages of power plant operation. Weld cracking in CrMo, CrMoV steels at the beginning of long-term operation can cause extensive damages even when the welds from the creep resistance point of view are satisfactory. Transient modes of boilers are the most critical from the point of secondary hardening i.e. shut-down of plant when welded nodes made of these steels reach ambient temperature and there is not enough plasticity reserve in a hard weld to transfer stress of thermal expansion at shutdown as well as at subsequent starting-up.

Assessment of hazard


level of this phenomenon should consider several factors e.g. tightness of



ding was carried out by Pecha et

eration of thermal

kotlova su najkritičniji sa tačke sekundarnog ojačavanja, to jest isključivanje postrojenja kada zavari napravljeni od ovih čelika dostignu spoljnu temperaturu, a nema dovoljno rezerve plastičnosti u zavaru da bi preneo naprezanje termalnog širenja pri isključivanju kao i pri naknadnom pokretanju.

Za procenu nivoa opasnosti od ovog fenomena bi trebalo da uzeti u razmatranje nekoliko faktora, na primer zaptivenost zavara, debljinu zavara, period i trajanje isključenja itd.

Mohyla i Koukal [12] su objavili rezultate merenja tvrdoće zavarenog spoja i pokazali da je u krupnozrnoj zoni uticaja toplote zavarenih spojeva od T23 i T24 materijala koji nisu bili podvrgnuti termičkoj obradi došlo do jačeg sekundarnog ojačavanja tokom dugotrajnog izlaganja (500°C) u relativno kraćim vremenskim periodima (T23-100 h, T24-2000 h), vrednosti tvrdoće zavarenih spojeva bez termičke obrade nakon zavarivanja iznose od 410-430 HV10, a to nije dopustivo sa stanovišta standarda. S druge strane, rezultati testova zavarenih spojevaproučavanih čeličnih cevi tokom rada, ali koji nisu bili izloženi termičkoj obradi nakon zavarivanja nisu dokazali pojavu sekundarnog ojačavanja niti njegove posledice.

Publikacije Lunda i Halda [16] i japanskih autora [15]su to dokazale. Opsežna ispitivanja zavarivanja sličnog čelika T24 su sprovedena od strane Pecha i saradnika [8, 17]. U saradnji sa Institutom za istraživanje materijala slovačke Akademije nauka, takođe je provereno sekundarno ojačavanje ovih zavarenih spojeva od čelika T24 [18]. Istraživane su karakteristika zavarenih spojeva cevi koji su bili izloženi dugotrajnom mekom žarenju na 550 °C, tj. za 50 °C više nego u ovom radu [12]. Ova temperatura se smatra maksimalnom temperaturom za primenu čelika T24 u sistemima super-grejača. Trajanje mekog žarenja pri temperaturi od 550 °C je iznosilo 1000 i 3000 časova, jer se tada mogu očekivati maksimalne vrednosti sekundarnog ojačavanja.

Uz metalografsku analizu, proveravana je i tvrdoća i udarna žilavost zavarenih spojeva nakon različitih perioda izlaganja visokoj temperaturi pod uslovima sa i bez žarenja. Rezultati pokazuju da je u ovom slučaju nakon 1000 časova izlaganja toploti tvrdoća u zoni pod uticajem toplote pala za 60 jedinica HV, ali nakon 3000 časova tvrdoća je dostigla vrednosti uslova nakon zavarivanja bez žarenja. U ovom slučaju nije došlo do pojave ekstremnog ojačavanja. Vrednosti tvrdoće od 280-290 HV 10 su prihvatljive, a za CrMo čelike čak i očekivane. Žareno stanje pokazuje logično niže vrednosti nakon oba perioda izlaganja visokim temperaturama. Pojava sekundarnog ojačavanja može delovati opasno, ako je kombinovano sa uslovima i veličinom zaptivenosti određenog elementa. Na primer, ako debljina zavarenog spoja koji nije bio izložen termičkoj obradi prelazi 10 mm, onda može da se očekuju komplikovani uslovi zaptivenosti i ekstremi tvrdoće

welded node, weld thickness, period and duration of shutdown, etc.

Mohyla and Koukal [12] published results of weld seam hardness measurement and showed that in coarse-grained heat affected zone of weld seams of T23 and T24 materials not subject to post-weld heat treatment there was occurred stronger secondary hardening during long-term exposition (500°C) in relatively shorter time periods (T23 – 100h, T24 –2000h), the hardness values of welds without post-weld heat treatment range within 410-430 HV10, and that is not permissible from the point of standards.

On the other hand the test results of weld seams of studied steel tubes in operation, but not subject to post-weld heat treatment proved neither the occurrence of secondary hardening nor its consequences.

It is proved by publications of Lunda and Halda [16] and Japanese authors [15]. Extensive testing of similar T24 steel welal. [8, 17]. In cooperation with the Research Institute of Material of the Slovak Academy of Science there was also checked secondary hardening of these steels welds [18].

There were examined properties of weld seams of T24 tubes subject to long-term annealing at 550 °C, i.e. by 50 °C higher than in paper [12]. This temperature is considered to be the maximal temperature at application of T24 steel in superheater systems. Annealing duration at 550 °C was 1000 and 3000 hours, because then the maximal values of secondary hardening can be expected.

In addition to metallographic analysis it was also checked hardness and impact strength of weld seams after various periods of high-temperature exposition under the conditions with / without annealing. The results show that in this case after 1000 hours of thermal exposition the hardness in HAZ dropped by 60 units HV, but after 3000 hours the hardness achieved the values of the conditions after welding without annealing. That is to say, no extreme hardening occurred in this case.

The hardness values of 280-290 HV 10 are acceptable and for CrMo steels they are even expected. The annealing state shows logically lower values after both periods of high-temperature exposition. Occurrence of secondary hardening can seem to be hazardous, if it is joined with the conditions and size of tightness of a particular node. For example if a thickness of weld seam not subject to heat treatment will exceed 10 mm, then there can be expected complicated conditions of tightness and extremes of hardness can be hazardous. Therefore weld seams of bigger thicknesses of studied steels must be subject to annealing.

Transient modes of thermal equipment (shutdown, starting-up) can seem to be risky due to secondary hardening. During initial op



regular

s HV 10 was measured in the weld seam centre. The maximal values were reached in heat

eld metal. In these zones the

f weld seams of modern steel T/P23 of thick-wall and thin-wall parts for

ponents of power engineering

u

e and

mogu biti opasni. Stoga zavareni spojevi većih debljina proučavanih čelika moraju biti podvrgnuti žarenju. Prelazni režimi rada termoenergetske opreme (isključenje, pokretanje) mogu delovati rizično usled sekundarnog ojačavanja. Pri početku rada opreme ovi režimi su kratkotrajni (optimizacija parametara), nisu česti, dok temperatura cevi, usled izolacije ne pada ispod 20 °C. Duža isključivanja iz rada postaju redovna tokom kasnijih perioda rada postrojenja, nakon maksimuma sekundarnog ojačavanja. Takođe, zbog toga nisu zabeleženi kvarovi postrojenja sa zavarenim spojevima proučavanih čelika koji nisu bili podvrgnuti termičkoj obradi. Ipak je neophodno dalje obraćati pažnju na istraživanja sekundarnog ojačavanja pošto je naophodno posebno za membranske zidove termo-energetskih postrojenja.

REZULTATI ISPITIVANJA

Tvrdoća HV 10 je merena u centru zavarenog spoja. Maksimalne vrednosti su postignute u zoni pod uticajem toplote i u metalu šava. U ovim zonama su izmerene vrednosti HV10 od 297 do 309. U poređenju sa osnovnim materijalom, tvrdoća je povećana za oko 100 jedinica. Tvrdoća zavarenog spoja čelika koji nisu bili izloženi termičkoj obradi je ograničena poznatom vrednošću od 350 HV [19]. Standard proizvoda [20] prihvata čak i tvrdoću od 380 HV za zavarene spojeve čelika koji nisu bili izloženi termičkoj obradi.

Čvrstoća zavarenog spoja proverena zateznim ispitivanjem je pokazala vrednosti od 620-652 MPa, dok su zahtevi za osnovni materijal min. 510 MPa. Karakteristike plastičnosti zavarenih spojeva su ispitivane savijanjem. Uzorci savijani oko korenog zavara kao i oko pokrivnih slojeva su izdržali savijanje od 180° bez ikakvih znakova oštećenja.

Obzirom na gore pomenute rezultate, zavareni spojevi će biti podvrgnuti dugotrajnom izlaganju temperaturi od 550°C, nezavisno od njihove primene u radu i ispitivaće se njihova čvrstoća i karakteristike plastičnosti. Uz to, zavareni spoj će biti podvrgnut ispitivanju otpornosti na toplotu.

ZAKLJUČAK

Obavljena su ispitivanja na zavarenim spojevima od savremenog čelika T/P23 na delovima sa debelim i tankim zidovima za komponente pod visokim pritiskom u energetskim postrojenjima. Rezultati ispitivanja potvrđuju da su primenjeni tehnološki parametri zavarivanja bili ispravni. Međutim, potrebno je znati ponašanje zavarenih spojeva tokom eksploatacije. Poznato je da postoji vrlo malo rezultata ispitivanja na puzanje zavarenih spojevamodifikovanog CrMo čelika. Rezultati ovog rada će doprineti povećanju baze podataka o ispitivanju puzanja. Optimizacijom termičke obrade uspeo se pronaći kompromis između žilavosti i dobre otpornosti na puzanje zavarenog spoja cevovoda sa debelim

equipment these modes are short (optimalization of parameters), not very frequent, while the tube temperature due to insulation does not drop under 20°C. Long operation shutdowns becomeduring later periods of plant operation, after the maximum of secondary hardening. Also due to that reason there were recorded no failures of plants with weld seams of studied steels not subject to heat treatment. However, it is necessary further to pay attention to research of secondary hardening as it is required particularly by membrane walls of thermal plants.

TEST RESULTS

Hardnes

affected zone and wvalues of 297 up to 309 HV 10 were measured. Comparing to the parent material, the hardness was increased by about 100 units. Hardness for weld seams of these steels not subject to heat treatment is limited by the known value of 350 HV [19]. The product standard [20] accepts even the hardness of 380 HV for welds of these steels not subject to heat treatment.

Weld seam strength checked by tensile test showed the values of 620-652 MPa, while the requirement for the parent material is min. 510 MPa. Plastic properties of weld seams were tested by bending test. The samples stressed from the side of root as well as from the side of covering layers withstood the bend of 180° without any marks of damage.

With regard to the above mentioned issues the weld seams will be subject to long-term application of temperature 550°C, independently from their application in operation and their strength and plastic features will be examined. Additionally, the weld will be subject to heat resistance testing.

CONCLUSION

There were carried out a lot of tests o

high-pressure complants. The test results confirm that the applied technological parameters of weld execution were correct. However, it is necessary to know behaviour of weld seams during exploitation. It is known that there are very few results of creep test of weld seams of modified CrMo. Results of this paper will contrib te to extension of database of creep tests. By optimalization of heat treatment it succeeded to find a compromise between toughness and good creep resistance of weld seam of thick-wall piping.

It was confirmed that steel type 23 requires special technology for large thickness welding. To manage weldability, it is necessary to observe strictly the technological principles, particularly heat modtechnique of welding. Recommendations for practice



e in the range of 730-

ent shall be provided control technique.

of thick-wall piping welding:

• Welding shall be in heat mode under the line Ms.

• Strictly limited heat input.

• Stress annealing to be don

zidovima.

Potvrđeno je da čelik tipa 23 zahteva posebnu tehnologiju za zavarivanje velikih debljina. Da bi se kontrolisala zavarljivost, neophodno je da se tehnološki principi striktno poštuju, posebno toplotni režim i tehnika zavarivanja. Preporuke za zavarivanje cevi sa debelim zidovima su:

740 °C.

• Equipment for heat treatmwith precise recording and

• Zavarivanje mora biti u toplotnom režimu ispod

Ms temperature.

• Strogo ograničen unos toplote.

• Žarenje za uklanjanje naprezanja raditi u opsegu od 730-740 °C.

• Oprema za termičku obradu mora da poseduje tehničke mogućnosti preciznog zapisa i kontrole.

LITERATURE [1] Vlasák, T., Hakl, J., Pecha, J.: Žárupevnost základního materiálu a svarových spoju oceli P23, 15th Int. Conf. METAL 2007, Ostrava, 2007, s. 37. [2] Pecha, J., Meško, J.: Weldability of new creep resistant Cr-Mo steel with tungsten content. Materials Engineering, Vol. 13, 2006, No. 2, s. 58. [3] Macko, M. et al: Modernizácia merania teplôt v žíhacích peciach, Technická správa 03/2006, Výskumný ústav energetických zariadení, Levice,

2006. [4] Heuser,H., Jochum, C.: Development of new bainitic and martensitic filler material for the use in advanced coal-fired power plants. Prezentation

in Skoda Power. 2006. [5] Bauné, E. et al: Weldability and properties of new creep resistant materials for use in ultra supercritical coal fired power plants. Materials for

Advanced Power Engineering 2006, Forschjungszentrum Julich, "8th Liege Conference" september 2006 p. 871. [6] Pašák, M.: Štúdium degradačných mechanizmov porušovania zvarových spojov žiarupevných ocelí. Bakalárská praca. Trnava 2007. [7] Gabrel, J.: Development of T and P 23 steel grade. Final Report COST 522, Vallourec Research Centre CEV, 2003. [8] Pecha, J., Peleš, O.: Zváranie rúrok z ocele T24. Priebežná správa k európskemu Projektu SmartWeld. Limerick 2003. [9] Heuser, H. – Jochum, C. – Meyer, F.W. : Entwicklung von Schweißzusatzwerkstoffen für moderne Kraftwerkstähle. Tagungsband der 6.

Werkstoffseminar TU Graz, 2003, s.1-14. [10] Foldyna, V., Koukal, J.: Vývoj nových ocelí pro energetiku a chemické strojírenství na bázi 2 až 3% Cr. Zborník prednášok zo seminára Nové

materiály a technologie svařování v prvním desetiletí nového milénia. Ostravice 2002, s.35-49. [11] Marshall, A.W., Zhang, Z.: Development weld metals for advanced 2,5 and 9-11Cr steels. COST 536, UK 10, Louhborough, 2005 [12] Mohyla, P., Koukal, J. Svařitelnost perspektivních nízkolegovaných žáropevných ocelí. In Sborník konference TechMat 04 : 9.11.2004, Česká

Třebová. Pardubice: TU Pardubice, 2004, s. 96-101. [13] Technické informace o vlastnostech a technologickém zpracováni výrobků z oceli 15128. Ostrava 1968. [14] The T23/T24 Book. Vallourec & Mannesmann tubes. 2002. [15] Sawaragi, Y., Iseda, A., Yamamoto, S., Masuyama, F.: Devolopment of High Strenght 2%Cr Steel Tubes (HCM2S) for Boilers. The Sumitomo

Search No 59 Sep. 1977. [16] Lund, E., Hald, J.: In Plant Testing of HCM 12, HCM2S and 7CrMoTiB10-10 for membrane Wals. Separát. [17] Pecha, J. et al.: Nové materiály z V. Rámcového programu a ich zváranie. Zborník z XXXVI. Konferencie Rozvoj zvárania na Slovensku z pohľadu

členstva SR v EÚ. Vysoké Tatry, 2003, s.52. [18] Jedinaková, M., Výrostková, A.: Precipitácia fáz vo zvarových spojoch membránových stien feritických ocelí po dlhodobom žíhaní. Camin

Conference, Smolenice, apríl 2005. [19] STN EN ISO 15614-1. Stanovenie a schválenie postupov zvárania kovových materiálov. Skúška postupu zvárania. Časť 3. Oblúkové a plameňové

zváranie ocelí a oblúkové zváranie niklu a zliatin niklu. 2006. [20] STN EN 12952-5 Vodorúrkové kotly a pomocné zariadenia. Časť 6: Kontrola pri výrobe, dokumentácia a značenie časti kotla namáhaného

tlakom, 2005. [21] J. Pecha, Welding of progressive steel P23 in power engineering, Zavarivanje i zavarene konstrukcije, Vol.51, No. 3, 91-100, Beograd, 2006.

Documents

Zavarivanje i Navarivanje Visokoegiranih Celika