mehanizam segregacije fosfora u zut cr...

NAUKA∗ISTRAŽIVANJE∗RAZVOJ SCIENCE∗RESEARCH∗DEVELOPMENT

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2003), str. 127-143 127

Koreaki Tamaki, Jippei Suzuki, Hiroshi Kawakami

MEHANIZAM SEGREGACIJE FOSFORA U ZUT Cr-Mo ČELIKA I NJEN UTICAJ NA POJAVU PRSLINA USLED PONOVNOG ZAGREVANJA

MECHANISM OF PHOSPHORUS SEGREGATION IN HAY OF Cr/Mo STEEL AND ITS EFFECT ON REHEAT CRACKING

Originalni naučni rad / Original scientific paper UDK/UDC: 621.791.051.4:620.192.46:669.15′26′28-194 Rad primljen / Paper received: 10.08.2003.

Adresa autora / Author's addres: Koreaki Tamaki*, Jippei Suzuki** and Hiroshi Kawakami** * Prof.Emeritus of Mie University ** Department of Mechanical Engineering, Mie University

Kamihama-cho, TSU, Mie 514-8507, Japan

Ključne reči: Prsline usled ponovnog zagrevanja; Cr-Mo čelici; zona pod uticajem toplote (ZUT); segregacija fosfora; granica zrna; termički ciklus zavarivanja; α/γ transformacija; EDX mikroanaliza; koeficijenat raspodele fosfora; α/γ granična površina; prsline po granicama zrna

Key words: reheat cracking, Cr/Mo steels, heat affected zone, phosphorus segregation, grain boundary, thermal cycle of welding, α/γ transformation, EDX micro-analysis, distribution coefficient of phosphorus, α/γ interface, intergranular fracture

Izvod

Prsline usled ponovnog zagrevanja se javljaju po granicama polaznih austenitnih zrna (PAGB) unutar zone pod uticajem toplote - ZUT u Cr-Mo čelicima. Jedan od osnovnih razloga nastanka ovih prslina je segregacija fosfora (P) na PAGB. Ispitivanje sklonosti na pojavu prslina usled ponovnog zagrevanja implant testovima na nekoliko različitih Cr-Mo čelika je pokazalo da je segregacija P na PAGB dominantni razlog nastanka prslina kod čelika sa nižim sadržajem Mo i višim sadržajem Cr. Ovoj grupi pripadaju najčešće korišćeni komercijalni čelici, kao što su 1.25Cr-0.5Mo i 2.25Cr-1Mo. U ostalim Cr-Mo čelicima glavni razlog nastanka prslina usled ponovnog zagrevanja je Mo2C.

″EDX mikroanaliza″ je rađena na PAGB. Rezultati, dobijeni u ZUT 1.25Cr-0.5Mo čelika, ukazuju da je do segregacije fosfora došlo već u zavarenom stanju. Ovo ukazuje da je segregacija fosfora izazvana α/γ transformacijom, do koje dolazi u toku termičkih ciklusa pri zavarivanju. Ovaj tip segregacija je nazvan ″transformacijom indukovana segregacija (TIS)″.

Proces nastanka TIS u 1.25Cr-0.5Mo čelicima je praćen korišćenjem metode ″nagrizanja granica zrna″. Rezultati ispitivanja ukazuju da je fosfor već bio raspoređen po granicama zrna kada se ferit transformiše u austenit. Sa sledećim zagrevanjem u austenitno područje udeo TIS se smanjuje. Takođe, ako je brzina zagrevanja reda 1000 K/s, kao u slučaju zavarivanja, određeni udeo TIS se može pojaviti i na sobnoj temperaturi.

Abstract

Reheat cracking occurs along the prior-austenite grain boundary (PAGB) of HAZ of Cr-Mo steels. One of the major causes of reheat cracking is the segregation of phosphorus in PAGB. Reheat cracking tests of the implant type on several Cr-Mo steels revealed that the segregatin of phosphorus is the principal cause of reheat cracking in the field of lower molybdenum and higher chromium contens. Major commercijal steels, such as 1.25Cr-0.5Mo and 2.25Cr-1Mo steels locate in this field. On the contrary, Mo2C is the principal cause of reheat cracking in the remaining Cr-Mo field.

″The EDX micro-analysis″ was made on the PAGB. The result on HAZ of 2.25Cr-1Mo steel informed that phosphorus was segregated already in the as-welded state. This fact suggests that the segregatin of phosphorus will be produced by the α/γ transformaton brought about by a thermal cycle of welding. This type of segregation is named ″the transformation-induced segregation (TIS) ″.

The process of the formation of TIS in 1.25Cr-0.5Mo steel was traced by an analyzing procedure of ″grain boundary etching″. Experimental results revealed that phosphorus was concentrated in the grain boundary at the time when ferrite transforms into austenite. The quantity of TIS was reduced by the succeeding heating in the austenite region. However, a certain quantity of TIS could be brought to the room temperature, if the heating rate was as large as 1000 K/s, as the case of welding.

1. UVOD Od ranije je poznato da su metalurški razlozi nastanka prslina usled ponovnog zagrevanja u ZUT Cr-Mo

1. INTRODUCTION It has been informed that the metallurgical causes of reheat cracking in HAZ of Cr-Mo steels were the

SCIENCE∗RESEARCH∗DEVELOPMENT NAUKA∗ISTRAŽIVANJE∗RAZVOJ

128 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE (3/2003), str. 127-143

čelika, segregacije elemenata nečistoća i taloženje karbida [1-5]. Neki autori naglašavaju odlučujuću ulogu fosfora u odnosu na sve ostale elemente [4]. U prvom delu ovog rada ispitan je uticaj fosfora na sklonost za nastanak prslina usled ponovnog zagrevanja za svaki od ispitanih Cr-Mo čelika, i potvrđen je opseg sastava u kome fosfor izaziva najveću štetu. U drugom delu je analizirana segregacija fosfora na PAGB, korišćenjem novorazvijene metode. U poslednjem delu, mehanizam segregacije fosfora razmatran je na osnovu eksperimentalnih podataka i proračuna.

Uticaj M2C karbida na nastanak prslina usled ponovnog zagrevanja je razmatran preko poređenja uticaja fosfora, koji je utvrđen u prvom delu [3]. Nastanak prslina usled ponovnog zagrevanja je diskutovan i u vezi otpusne krtosti i niskoduktilnog loma puzanjem, jer se svi javljaju u istom temperaturnom području [6-8].

2. UTICAJI SEGREGACIJE FOSFORA I IZDVAJANJA M2C NA POJAVU PRSLINA USLED PONOVNOG ZAGREVANJA

Glavni metalurški uzroci prslina usled ponovnog zagrevanja su segregacija fosfora i taloženje karbida M2C [3]. Prvonavedeni slabi PAGB, a drugonavedeni ojačava feritnu osnovu. Takođe, nije sasvim jasno da li u datim Cr-Mo čelicima ova dva faktora deluju simultano.

2.1 Osetljivost na prsline usled ponovnog zagrevanja Cr-Mo čelika

Osetljivost na prsline usled ponovnog zagrevanja u Cr-Mo čelicima je ispitivana korišćenjem implant testova ( sl.1) [3]. Implant (sl.1a,b) je ubačen u šupljinu unutar ploče od osnovnog materijala (sl.1c) kao i navar dužine 100mm, sa parametrima zavarivanja datim u tabeli 1. Na ovaj način, ZUT je dobijen u zoni sa zarezom (sl.1d).

Sklop za naprezanje-rasterećenje (″restraining″) mašine za ispitivanje prikazan je na slici 1e. Uzorak je opterećen početnim opterećenjem σaw, i zagrejan na 873K, sa brzinom zagrevanja od 200K/s. Uzorak je zadržan 20 časova na temperaturi 873K.

Relaksiranje napona se javlja u toku zagrevanja (sl.2). Ako je početni napon veličine σaw1 i σaw2, prsline se javljaju pod smanjenim naponom σpw1 i σpw2. Ako je veličina ″restraint″ napona jednaka σaw3 ili σaw4, prsline se ne javljaju.

Granična vrednost između σaw2 i σaw3 je nazvana "kritični restraint napon u početnom stanju (σaw-krit)" ili kraće "kritični restraint napon", dok su σpw2 i σpw3 nazvani "kritični restraint napon na visokim temperaturama (σpw-krit)".

Napon σaw-krit je uzet za meru osetljivosti prema prslinama u različitim Cr-Mo čelicima; što je niža vrednost σaw-krit, čelik je osetljiviji na pojavu prslina.

segregation of impurity elements and the precipitation of carbide [1-5]. The authors have pointed out that phosphorus is the most detrimental element among the impurities [4]. In the first part of this document, the effect of phosphorus on the cracking sensitivity was examined on each of the Cr-Mo steels, and the Cr- Mo contents field in which phosphorus exhibits most harmful effect was confirmed. In the second part, the segregation of phosphorus in the prior-austenite grain boundary (PAGB) was examined by the analyzing technique newly developed. In the last part, the mechanism of segregation of phosphorus was discussed by the experiments and the calculations.

The effect of M2C carbide on reheat cracking was also examined by referring to the effect of phosphorus in the first part [3]. Reheat cracking was discussed also in connection with the temper embitterment and the low ductility creep fracture, which arise at the same temperature range as the former [6-8].

2. EFFECTS OF SEGREGATION OF PHOSPHORUS AND PRECIPITATION OF M2C ON REHEAT CRACKING The major metallurgical causes of reheat cracking are the segregation of phosphorus and the precipitation of M2C [3]. The former weakens the PAGB, and the latter strengthens the ferrite matrix. It is not certain, however, whether those two factors work together in given Cr-Mo steel.

2.1 Sensitivity of reheats cracking of Cr-Mo steels The reheat cracking sensitivity of Cr-Mo steels was assessed by the implant test shown in Fig. 1 [3]. The implant (Fig. 1 (a), (b)) is inserted in the hole of the base metal plate (Fig. 1(c)) and a bead of 100 mm in length is deposited by the welding condition of Table 1. HAZ is produced at the notched portion (Fig. 1 (d)).

The restraining flame of test machine is shown in Fig. 1 (e). The specimen was loaded by the initial restraining stress of σaw, and heated up to 873K with the heating rate of 200 K/h and then kept at 873K for 20h.

The stress relaxation occurs in the course of heating (Fig. 2). If the initial stress is large enough as σaw1 or σaw2, cracking occurs under a reduced restraint stress of σpw1 or σpw2. If it is small as σaw3 or σaw4, cracking does not occur.

The boundary of σaw2 and σaw3 was named "the critical restraint stress at the initial state (σaw-crit)" or briefly "the critical restraint stress", and that of σpw2 and σpw3 was named "the critical restraint stress at high temperature σpw-crit". The σaw-crit was used for assessing the cracking sensitivity of several Cr-Mo steels; the smaller the σaw-crit, the more sensitive to the cracking.



Slika 1. Shema implant testa i uređaja: (a) implant; (b) deo sa zarezom; (c) lim osnovnog metala; (d) ZUT dobijen na delu sa

zarezom; (e) implant test uređaj Fig. 1. Implant test specimen and test machine (a) implant, (b) notched portion, (c) base metal plate, (d) HAZ produced in the

notched portion, (e) implant test machine (i: test specimen, s: supports, t: thermal insulator, r: restraint plates, c: restraining columns, j: screw jack, l: loading lod with load cell, v: loading lever).

Tabela 1. Parametri zavarivanja za implant test Elektroda Struja zavarivanja Napon luka Brzina zavarivanja predgrevanje

JIS DT2416 sušena na 625K 1 čas 180 A 24 V 15 cm/min 425 K

Slika 2. Odnos između kritičnih ″restraint″ napona, σaw-krit i σpw-krit, prikazan na krivoj relaksacije napona

Fig. 2. Relation between the critical restraint stresses, σaw-crit and σpw-crit shown on the stress relaxation curve.

Table 1. Welding conditions for implant test. Electrode Welding current arc voltage Welding speed Preheat

JIS DT2416 diried at 625K for 1h 180 A 25 V 15 cm /min 425 K

2.2 Change in σaw-crit with increasing phosphorus content Following two series of Cr-Mo steels were used.

− 0.5Mo series: 0 to 2.6%Cr-0.5%Mo steels − 1.0Mo series: 0 to 2.9%Cr-1.0%Mo steels Phosphorus content is varied from 0.005 to 0.05mass% keeping other elements in the following content ranges.

0.16-0.20C, 0.20-0.40Si, 0.60-1.10Mn, 0.014-0.020S

The effect of phosphorus content on the σaw-crit is summarized in Fig.3 and 4. A logalythmic scale is taken in the abscissa. The σaw-crit is changed with an increasing phosphorus content in the following manners:

Type A: The σaw-crit shifts from the upper level to the lower one in the range of Pcrit to Pc' as illustrated in Fig5.



Slika 3. Promena kritičnog ″restraint″ napona sa povećanjem sadržaja fosfora; Cr-05%Mo čelik

Fig. 3. Change in critical restraint stress with an increasing phosphorus content; Cr-0.5%Mo steels.

2.2 Promena σaw-krit sa povećanjem sadržaja fosfora Korišćene su sledeće dve serije Cr-Mo čelika:

– 0.5Mo serija: 0 do 2.6%Cr - 0.5%Mo čelici – 1.0Mo serija: 0 do 2.9%Cr - 1.0%Mo čelici

Sadržaj fosfora je variran između 0.005 do 0.05 mas%, dok je sadržaj ostalih elemenata održavan u sledećim granicama (mas.%):

0.16-0.20C; 0.20-0.40Si; 0.6-1.10Mn; 0.014-0.20S

Efekat fosfora na σaw-krit je prikazan na slici 3 i 4. Na apscisi je logaritamska podela. Veličina σaw-krit se menja sa povećanjem sadržaja fosfora na sledeći način:

TIP A: U oblasti od Pkrit do Pc', σaw-krit se pomera sa višeg nivoa na niži nivo, kao što je prikazano na slici 5. Ovom tipu ponašanja pripadaju čelici koji sadrže 0.5-2.6%Cr - 0.5%Mo, kao i 2.2-2.9%Cr - 1.0%Mo (sl.3b - f i sl.4e,f).

TIP B: σaw-krit zadržava niži nivo. Ovom tipu ponašanja pripadaju čelici koji sadrže 0 -2.0%Cr - 1.0%Mo (sl.4a-d). σaw-krit. za 1.0%Cr-1.0%Mo čelik je veoma nizak, tek 300 MPa (sl.4c).

TIP C: Visok nivo σaw-krit je zadržan pri bilo kojem sadržaju fosfora. Ovoj grupi pripada čelik sa 0%Cr - 0.5%Mo (sl.3a).

Na slikama 6a i 6b su prikazane zavisnosti višeg i nižeg nivoa σaw-krit od sadržaja hroma. Viši nivo, za seriju Cr-0.5%Mo, sa povećanjem sadržaja hroma se menja neznatno. Sa povećanjem sadržaja hroma do 1.0% niži nivo se dalje snižava, a zatim blago povećava (sl.6a). Za seriju Cr-1.0%Mo, promena σaw-krit je mala, i dostiže minimum na 1.0%Cr (sl.6b). Ako se pretpostavi

0.5 to 2.6%Cr-0.5%Mo steels and 2.2 to 2.9%Cr- 1%Mo steels belong to this type (Fig.3(b to f) and Fig.4(e, f)).

Type B: The σaw-crit still remains in the lower level. 0 to 2.0%Cr- 1.0%Mo steels belong to this type (Fig. 4 (a to d)). The σaw-crit of 1.0%Cr-1.0%Mo steel is as small as 300MPa (Fig. 4(c)).

Type C: The upper level of σaw-crit is kept through all the phosphorus content. 0%Cr-0.5%Mo steel belongs to this type (Fig. 3(a)).

The upper and the lower level values of σaw-crit are plotted against chromium content as Fig. 6 (a) and (b). The upper level of Cr-O.5%Mo series changes little with an increasing chromium content. The lower level, however, decreases with an increasing chromium content up to 1.0% and then increases a little (Fig.6(a)).

For Cr-1%Mo series, however, the σaw-crit of 0 to 2%Cr is very small, it becomes minimum at 1.0%Cr (Fig.6(b )). If it is assumed that an ideal σaw-crit in this case will be 650MPa (the dotted line), the decrement of σaw-crit (shaded area) should be brought about by a certain cause other than phosphorus.

2.3 Respective effects of phosphorus and M2C The Cr-Mo contents field is divided into three fields according to Fig.6 as shown in Fig.7.

Field A: The major cause of reheat cracking is phosphorus, and therefore, the cracking sensitivity can be reduced significantly by reducing its content below the Pcrit. The commercial Cr-Mo steels, 2 1/4 Cr-1Mo, 1 1/4Cr-1/2Mo and HT80 steels locate in this field.

Field B: Phosphorus is not the major cause of reheat cracking. Its cause will be M2C as explained later.



da idealna veličina σaw-krit iznosi 650MPa (tačkasta linija), onda je smanjenje σaw-krit posledica nekog drugog procesa, tj. nije povezana sa fosforom.

2.3 Uticaji fosfora i M2C Prema podacima prikazanim na slikama 6 i 7, čelici iz sistema Cr-Mo se mogu podeliti na tri oblasti.

Oblast A: Glavni razlog pojave prslina usled ponovnog zagrevanja je fosfor, te se sklonost ka pojavi prslina usled ponovnog zagrevanja može smanjiti ako se smanji sadržaj ispod Pkrit. U ovoj oblasti se nalaze komercijalni Cr-Mo čelici: 2.25Cr-1Mo; 1.25Cr-0.5Mo i HT80.

Oblast B: Fosfor nije glavni razlog pojave prslina usled ponovnog zagrevanja. Taj razlog je izdvajanje M2C, što će biti kasnije objašnjeno. Do danas ne postoje komercijalni Cr-Mo čelici koji se nalaze u ovoj oblasti. Ipak, čelici 2.25Cr-1Mo; 1.25Cr-0.5Mo se nalaze veoma blizu granica oblasti A i B; u strukturi ovih čelika M2C će se pojaviti ako se smanji sadržaj hroma u prvom, odnosno poveća sadržaj Mo u drugom.

Oblast C: Oblast u kojoj ne dolazi do prslina. Ovoj oblasti pripadaju 0.5Mo čelik, HT60 čelik i veći broj C-Mn čelika.

2.4. Cr-Mo čelici u oblasti taloženja M2C Sekundarno otvrdnjavanje izaziva taloženje finih čestica M2C karbida koji zadržavaju koherentnost sa feritnom osnovom [9]. Kako su talozi veoma sitni i fini, korišćenjem uobičajenih tehnika nije moguće odrediti njihovu kvantitativnu raspodelu. Cr-Mo čelici su kaljeni i otpuštani 24 časa na 873K. Karbidne čestice su izdvojene elektrolitički. Udeo za svaki karbid je određen korišćenjem rentgenostrukturne analize [3].

Dobijeni karbidi u Cr-Mo čelicima su prikazani na slici 8. M3C, M7C3, M23C6 i M2C su najčešće zastupljeni karbidi u obliku Fe3C, Cr7C3, Cr23C6 i Mo2C, gde su Fe, Cr i Mo ponekad zamenjeni drugim elementima. Sadržaj Mo2C je prikazan pored svake linije.

Osenčena površina u kojoj se izdvaja talog M2C se dobro slaže sa oblašću B na slici 7.

Ova činjenica pokazuje da je sniženje σaw-krit, u odnosu na idealnu vrednost sa slike 6b, prouzrokovano izdvajanjem (taloženjem) M2C karbida. Najveći deo ovog sniženja je verovatno izazvan zahvaljujući smanjenom efektu relaksacije napona pošto dolazi do sekundarnog ojačavanja [3,9]. Doprinos ojačavanju karbidnim česticama biće razmotren u nastavku ovog rada.

2.5 Uticaj sadržaja Cr i Mo na relaksaciju napona i napon loma σpw-krit Relaksacija napona R (sl.2) se izračunava prema sledećoj jednačini:

Formula... (1)

Commercial Cr-Mo steel does not exist generally in this field. However, 2 1/4Cr-1Mo and 1 1/4Cr-1/2Mo steels locates near to the boundary of fields A and B; if chromium content of the former is decreased a little, and molybdenum content of the latter is increased, the effect of M2C appears in those steels.

Field C: the field free from reheat cracking. 1/2Mo steel, HT60 steel and several C-Mn steels locate in this field.

2.4 Cr-Mo contnents field in which M2C precipitates The secondary hardening is caused by a fine precipitate of M2C being kept the coherency with ferrite matrix[9]. As the precipitate in this stage is too fine to measure its quantity, following quantifying procedure was conveniently used. Cr-Mo steels were quenched and tempered at 873K for 24h. Carbide particles were extracted electrolytically and the quantity of each type of carbide was determined by X-ray analysis[3].

The carbide types are shown in Cr-Mo content diagram as in Fig.8. M3C, M7C3, M23C6 and M2C represent the carbide forms, Fe3C, Cr7C3, Cr23C6 and Mo2C of which a part of Fe, Cr and Mo are replaced by other elements. The amount of M2C is shown beside each plot.

The shaded area in which M2C precipitates meets very well the field B in Fig.7. This fact informs that the decrease in σaw-crit from an ideal value in Fig.6(b) is induced mainly by the precipitation of M2C. The major part of this decrease may be caused by the decrease in stress relaxation due to the secondary hardening[3,9]. The contribution of secondary hardening will be discussed below.

2.5 Effect of Cr-Mo content on stress relaxation and fracture stress σpw-crit. The stress relaxation R is given by the follwing equation (Fig.2).

,%100xRaw

pwaw

σσσ −

= (1)

The stress relaxation for two series of specimens are shown in Fig.9. For both the series, the R decreases steeply with an increasing chromium content up to 1.0% and then changes little. With an increasing molybdenum from 0.5% to 1.0%, the R is decreased equally for each chromium content. These results suggest that:

1. Chromium will reduce the R both by the solution hardening to the matrix and the secondary hardening induced by carbide. It has been believed that the pure chromium carbide, Cr7C3 does not induce the secondary hardening [9]. However, Cr7C3 containing molybdenum as the fonn of M7C3 seems to induce the secondary hardening[3].



Slika 4. Promena kritičnog ″restraint″ napona sa povećanjem sadržaja fosfora; C r-1%Mo čelik

Fig. 4. Change in critical restraint stress with an increasing phosphorus content; Cr-1%Mo steels.

Na slici 9 prikazana je relaksacija napona za dve serije uzoraka. U obe serije, sa povećanjem sadržaja hroma do 1% vrednost R se strmo snižava, dok se neznatno menja pri daljem povećanju sadržaja hroma. Povećanjem sadržaja molibdena sa 0.5 na 1.0%, za svaki sadržaj hroma, vrednost R se identično smanjuje.

Ovi rezultati ukazuju da:

1. Hrom snižava vrednost R, kako ojačavanjem čvrstog rastvora matrice, tako i sekundarnim otvrdnjavanjem izazvanim karbidima. Veruje se da čisti karbid hroma Cr7C3 ne izaziva ovo otvrdnjavanje [9]. Indicije su da Cr7C3 koji sadrži molibden u obliku M7C3 doprinosi sekundarnom otvrdnjavanju [3].

2. Molibden snižava vrednost R uglavnom zbog sekundarnog otvrdnjavanja usled prisustva M2C karbida.

Kritični ″restraint″ napon na visokim temperaturama, σpw-krit, se može izračunati na osnovu poznatih vrednosti σaw-krit i R:

kritawkritpwR

−−

−= σσ

1001 (2)

Vrednost σpw-krit predstavlja lomnu čvrstoću uzorka.

Za čelike sa 0.5 % Mo, razlika između gornjeg i donjeg nivoa σpw-krit je relativno velika, u opsegu između 1.0 do 2.0 % Cr, dok se sa daljim povećanjem sadržaja Cr ova razlika smanjuje (sl. 10a).

Za čelike sa 1.0%Mo, u području u kome dolazi do taloženja M2C (0-2 % Cr), σpw-krit ima malu vrednost (sl. 10b).

Slika 5. Prelazak kritičnog ″restraint″ napona sa gornjeg na

donji nivo usled povećanja fosfora Fig. 5. Transition of critical restraint stress from the upper level to the lower one with an increasing phosphorus content.

2. Molybdenum will reduce the R mainly by the secondary hardening induced by M2C.

The critical restraint stress at high temperature, σpw-crit is calculated from σaw-crit and the R as; Formula...(2)

The σpw-crit corresponds to the fracture strength of the test specimen.

For 0.5%Mo series, the difference beween the upper and the lower levels of σpw-crit is large in the range of 1 to 2 % Cr, it seems to decrease in the range of higher chromium content (Fig. 10(a)). For 1.0 % Mo series, the σpw-crit in the range in which M2C precipitates (0 to 2 % Cr) is very small (Fig. 10(b)).



Slika 6. Smanjenje kritičnog ″restraint″ napona uzrokovanog fosforom i M2C

Fig. 6. Decrease in critical restraint stress brought about by phosphorus and M2C.

Slika 7. Fe-Cr-Mo dijagram stanja sa oblastima u kojima je

nastanak prslina usled ponovnog zagrevanja izazvan segregacijama fosfora (oblast A), i izdvajanjem M2C (oblast

B), i u kojoj nema prslina (oblast C) Fig. 7. Cr-Mo contents fields in which the reheat cracking is induced by phosphorus segregation (Fields A) and by M2C

precipitation (Fields B), and the field in which both the factors do not act (Fields C). Note: a: 1/2Mo, b: HT80, c: 1 1/4Cr-

1/2Mo, d: 2 1/4Cr1Mo steels.

2.6 Uticaj M2C i FeMoP na σpw-krit U dijagramu stanja Fe-Cr-Mo-P ugao bogat železom je određen prema slici 11 [10,11]. Krive rastvorljivosti u uglu bogatom železom pokazuju rastvorljivost fosfora u feritu; ucrtane su linije rastvorljivosti sve do sadržaja od 0.4%P, za koje se pretpostavlja da odgovaraju sadržaju na PAGB. Kada koncentracija fosfora na PAGB premaši liniju rastvorljivosti, u delovima bogatim na Mo i Cr, izdvajaju se FeMoP ili Cr2P, respektivno.

Razlog niske osetljivosti na prsline u uglu bogatom železom (oblast C na slici 7) se može objasniti da se u ovim čelicima fosfidi ne javljaju na PAGB (sl. 11). Među svim fosfidima FeMoP je termodinamički najstabilniji [10].

Slika 8. Količina M2C u Cr-Mo čelicima posle otpuštanja 24h

na 873K. Fig. 8. Quantity of M2C present in Cr-Mo steels tempered at

873K for 24h.

Slika 9. Zavisnost relaksacije napona R od sadržaja hroma

u Cr-Mo čelicima Fig. 9. Stress relaxation, R of Cr-Mo steels



Slika 10. Zavisnost lomne čvrstoće σpw-krit od sadržaja hroma u Cr-Mo čelicima. Oznake: kao na slici 6

Fig. 10. Fracture strength, (σpw-crit of Cr-Mo steels, Marks: see Fig. 6.

Molibden će uporedo sa fosforom segregirati na PAGB i formirati FeMoP i sprečiti poguban uticaj fosfora.

Ipak, prsline po granicama zrna javljaju se u Cr-1.0%Mo čelicima zbog niske vrednosti σpw-krit (sl.10b). Do pojave prslina u ovom slučaju ne dolazi zbog prisustva fosfora, već zbog izdvajanja M2C po PAGB. Zaključeno je da je u Cr-1.0%Mo čelicima M2C štetan, ne samo zbog sniženja R, već i slabljenja samih PAGB.

Cr2P je manje stabilni fosfid [10]. Njegovo ponašanje na PAGB još uvek nije sasvim jasno, ali se pretpostavlja da slabi PAGB.

Slika 11. Deo faznog dijagrama sistema Fe-Cr-Mo-0.2%P, aproksimiranog na bazi sistema Fe-Cr-P i Fe-Mo-P na 975K.

(Dijagram simulira stanje u oblasti granice zrna kod Cr/Mo čelika)

Fig. 11. Phase diagram of Fe-Cr-Mo-0.2%P system approximated on the basis of Fe-Cr-P and Fe-Mo-P diagrams at 975K. (It simulates the state of grain boundary region of Cr-

Mo steels.)

2.7 Rezime (1) Različiti sastavi Cr-Mo čelika se mogu podeliti u oblasti u kojima prsline usled ponovnog zagrevanja nastaju prevashodno usled fosfora ili M2C. U nekom čeliku ova dva mehanizma nikada ne deluju istovremeno, osim u slučaju kada se sastav čelika nalazi veoma blizu granice dva područja. (2) M2C je štetan, ne samo zbog sniženja R, već i zbog slabljenja PAGB.

2.6 Effects of M2C and FeMoP on σpw-crit The iron comer of Fe-Cr-Mo-P system diagram is estimated as shown in Fig. 11 [10,11]. The curves in the iron comer show the solubility of phosphorus in ferrite; the solubility lines as much as O.4%P are drawn for imaging the state of PAGB. When the phosphorus concentration in P AGB exceeds the solubility line, FeMoP is precipitated in the Mo-rich field and Cr2P in the Cr-rich field.

The reason why the cracking sensitivity is very small in the iron comer (Field C in Fig.7) may be explained by the fact that phosphide does not exist in PAGB of the steel of this field (Fig.11).

FeMoP is the phosphide thermodynamically most stable among several types of phosphide[10]. Molybdenum will cosegregate with phosphorus in PAGB forming FeMoP and prevent the harmful effect of phosphorus.

Nevertheless, an intergranular cracking occurs in Cr-1.0%Mo steels with a very small σpw-crit (Fig.10(b)). The cracking in this case will be induced not by phosphorus but by M2C precipitated in PAGB. It is concluded that M2C is harmful for Cr-1.0%Mo steels not only by reducing the R but also weakening PAGB itself.

Cr2P is less stable phosphide[10]. Its behavior in PAGB is not yet clear at present, but it possibly weakens the PAGB with a certain manner. 2.7. Summary 1. The Cr-Mo contents field can be devided into the field in which reheat cracking is caused predominantly by phosphorus or M2C. For a given steel, those two causes never act together, except for the steels locating very near to the boundary of both fields. 2. M2C is harmful not only by reducing the R but also by weakening the PAGB.

3. MECHANISM OF SEGREGATION OF PHOSPHORUS IN REHEAT CRACKING

3.1. Special feature of segregation of phosphorus in reheat cracking Reheat cracking exhibits special features in occurrence as:



3.MEHANIZAM SEGREGACIJE FOSFORA KOD PRSLINA USLED PONOVNOG ZAGREVANJA

3.1 Osobenosti segregacije fosfora pri pojavi prslina usled ponovnog zagrevanja Prsline usled ponovnog zagrevanja imaju sledeće osobenosti:

1. Javljaju se samo kada se početni ZUT ponovo zagreva;

2. Javljaju se samo kada je vreme ponovnog zagrevanja manje od 50 časova;

3. Ako je početni ZUT jednom ponovno zagrevan na 900K, pa potom zagrevan na 800K, do prslina ne dolazi.

Ove osobenosti ukazuju da do segregacije fosfora, koji je glavni uzročnik prslina, dolazi zapravo tokom zavarivanja. U ovom radu ispitan je mehanizam segregacije fosfora u toku zavarivanja.

Očekuje se da će rezultati istraživanja naći primenu u rešavanju problema koji nastaju u ZUT pri istim opsezima temperature i vremena, kao što su otpusna krtost kratkotrajnih tipova, i prsline usled smanjenja plastičnosti pri puzanju [6-8,12].

Postoje dva predložena koncepta za objašnjenje mehanizma segregacije fosfora izazvanih zavarivanjem. Prema prvom konceptu, do segregacija dolazi u toku hlađenja nakon zavarivanja. Kada se čelik ubrzano hladi kroz austenitno područje, nečistoće, kao npr. fosfor, se mogu nakupiti na granicama zrna usled ravnotežne segregacije [13].

Prema drugom konceptu, kada je čelik u austenitnom području, dolazi do likvacije po granicama zrna. Sama likvacija i očvršćavanje po granicama zrna izazivaju segregacije [14].

Prvi koncept je ipak samo plod proračuna zasnovanog na pretpostavkama i još nije potvrđen eksperimentima. Drugi koncept se može primeniti samo na veoma usko područje unutar ZUT, i to blizu linije stapanja [12].

U radu je velika pažnja posvećena merenju koncentracije fosfora na PAGB, [P]gb.

3.2 Merenje [P]gb (1) Problemi merenja [P]gb kod pojave prslina usled ponovnog zagrevanja Kao prva faza eksperimenta [P]gb je merena u ZUT zavarenog stanja. U toku merenja su se pojavili i određeni problemi. Za merenje [P]gb se uobičajeno koristi spektroskopija. Uzorak se prelomi na temperaturi tečnog azota, te se posmatra sveže prelomljena površina. Ova metoda je veoma korisna za ispitivanje otpusne krtosti dugotrajnog tipa (tradicionalna otpusna krtost), koja se javlja pri otpuštanju od preko 100 časova na 800K [7,12], pošto udarac u uređaju izaziva površinu preloma sličnu onoj dobijenoj Šarpi testom.

U slučaju prslina usled ponovnog zagrevanja ova metoda nije primenjiva, pošto u uzorcima ponovo

1. It occurs only when the original HAZ is reheated.

2. It appears only in the time range less than 50h of reheating.

3. If the original HAZ is once reheated at 900K and then heated at 800K, reheat cracking does not occur.

Those items suggest that the segregation of phosphorus, which is the major cause of reheat cracking, is prepared as early as in the welding process. The mechanism of segregation of phosphorus during welding was examined in this research.

The result of the research will be applicable as well to the phenomena arising in HAZ in the time-temperature range same as that of reheat cracking, such as the temper embrittlement and low ductility creep fracture of the short-term types [6-8,12].

Two concepts have been proposed for the mechanism of segregation of phosphorus induced by welding. In the fIrst concept, the segregation occurs during cooling process of welding. When the steel is cooled rapidly in the austenite region, the impurities, such as phosphorus can be concentrated in the grain boundary by the equilibrium segregation [13].

In the second concept, when the steel is in the austenite state, liquation will occur at the grain boundary. The liquation itself and the solidifIcation in grain boundary will induce the segregation [14].

The first concept, however, is assumed only by the calculation, it is not yet confIrmed by experiments. The second concept can be applied only for the very narrow area in HAZ and very near to the bond (the fusion line) [12].

Much attention has been paid in the present research on the measurement of the concentration of phosphorus in PAGB, [P]gb.

3.2. Measurement of [P]gb (1) Problem in the measurement of [P]gb in reheat cracking As the fIrst step of experiment, the [P]gb of HAZ in the as-welded state was measured: Following problem, however, arose necessarily in the measurement. Auger spectroscopy is applied usually for analyzing the [P]gb. The specimen is fractured at the temperature of liquid nitrogen, and a surface fresh - prepared is analyzed. This method is very usuful for the temper embrittlement of long-term type (the traditional temper embrittlement), which appears by tempering for more than 100h at 800K [7,12], because a blow in the apparatus produces the fracture surface same as obtained by the Charpy impact test.

This method, however, is never applicable for reheat cracking, because a blow at the low temperature produces a cleavage fracture alone in the specimen which is reheated for less than 50h at 800K.

In the present research, the grain boundary lying on the cross-section of specimen was analyzed, instead of the



zagrevanim kraće od 50h na 800K udarac na niskim temperaturama izaziva lom cepanjem.

U ovom istraživanju ispitivane su granice na poprečnom preseku uzorka, a ne na površini preloma. Upotrebljeno je nagrizanje granica zrna i specijalna EDX mikroanaliza.

fracture surface. A special EDX micro-analysis and the grain boundary etching were adopted.

(2) Procedure of EDX micro-analysis

A micro-region of PAGB was analyzed by the following procedure[12] (Fig.12).

Slika 12. Postupak EDX analize u mikro-oblasti granice zrna

Fig. 12. Procedure of EDX analysis on a micro-region across a grain boundary; (a) Surface in etched state (b) Surface in re-polished state

(2) Procedura EDX mikroanalize Mikropodručje PAGB je analizirano preko sledeće procedure[12], (sl.12):

1. PAGB su otkrivene koršćenjem sledećeg dvokomponentnog reagensa za nagrizanje(sl.12a): - Komponenta A: zasićen vodeni rastvor pikrinske kiseline; - Komponenta B: 3% vodeni rastvor natrijumove soli «laurylbenzenesulfonic» kiseline.

Komponente se pomešaju neposredno pre upotrebe u odnosu 4:6.

2. Oznake su načinjene tvrdomerom na obe strane PAGB, (sl.12a). Uzorak se zatim nanovo prepolira, sve do trenutka kada nestane trag PAGB (sl.12b).

3.EDX analiza se sprovodi na mestu koje je na sredini između dve oznake, na rastojanju 70nm.

4.Intenziteti refleksija X zraka u opsegu 6.40±0.23keV i 2.01±0.17keV, sabirani su za Fe i P respektivno (E1 do Eu na slici 13). Realni intenziteti X zraka, Ife i Ip određeni su umanjenjem ukupnog intenziteta, Imes za intenzitet pozadine, Ibg (sl.13). [P]gb je određen odnosom Ip / Ife.

(3)[P]gb u ZUT u zavarenom stanju Simulacija ZUT je urađena na uzorcima čelika sa 2.25%Cr-1%Mo i 1.25%Cr-0.5%Mo. Hemijski sastav ispitivanih čelika je dat u tabeli 2.

Tabela 2. Hemijski sastav ispitivanih Cr-Mo čelika (mas. %) Table 2. Chemical compositions of Cr-Mo steels (mass %).

Tip čelika C Si Mn P S Cr Mo

2.25Cr-1Mo 0.15 0.12 0.52 0.004 0.001 2.40 1.05

1.25Cr-0.5Mo 0.15 0.28 0.57 0.011 0.009 1.09 0.55

Primer rezultata je prikazan na slici 14. Koncentracija [P]gb iznosi 0.3%, što je čak 80 puta veća od nominalnog sadržaja fosfora u čeliku (0.004 %).

1. PAGB was revealed by the etching reagent; (Fig. I2(a)). - A: saturated aqueous solution of picric acid. - B: 3% aqueous solution of laurylbenzenesulfonic

acid sodium salt.

A and B are mixed together just before the use with the ratio 4 : 6.

2. Marks are indented in both sides of PAGB by a hardness tester (Fig. I2(a)). The surface is repolished until the image of PAGB disappears (Fig. I2(b)).

3. The center portion of the two indents is spot-analyzed by a EDX with the spacing of 70nm.

4. X-ray intensities in the ranges of 6.40 ± 0.23keV and 2.01 ± 0.17keV were summarized for Fe and P, respectively (E1 to Eu in Fig. 13). A real X-ray intensity, Ife or Ip was obtained by subtracting the back ground, Ibg from the summarized X-ray intensity, Imes (Fig. 13). The [P]gb was given by the ratio Ip/Ife.

Slika 13. Određivanje intenziteta rentgenskog zračenja za jedan element, Ie (=Imes-Ibg)

Fig.13. Determination of X-ray intensity of an element, le(=Imes-Ibg).



Takođe je zapaženo da:

1. Segregacije fosfora na PAGB nisu zapažene u stanju isporuke;

2. Položaj PAGB u ZUT se razlikuje u odnosu na čelik u stanju isporuke;

3. [P]gb u ZUT (0.3% na sl.14) nije u saglasnosti sa isporučenim stanjem, već nastaje pri zavarivanju.

Slika 14. Segregacija fosfora na granicama zrna izazvana termičkim ciklusom tokom zavarivanja, rezultati EDX analize Fig.14 Phosphorus segregation in grain boundary induced by

weld-thermal-cycle; result of EDX micro-analysis.

Segregacije izazvane zavarivanjem su obavezno povezane sa α/γ transformacijom koja se javlja u toku zagrevanja. Ovaj novi tip segregacija naziva se "transformacijom izazvana segregacija".

Ova tvrdnja je dokumentovana u nastavku rada.

3.3. Uticaj brzine zagrevanja na α/γ transformaciju Fazne transformacije, koje se odigravaju u toku zagrevanja velikom brzinom kao kod zavarivanja, razlikuju se od onih koje se dobijaju uobičajenim brzinama zagrevanja, npr. zagrevanjem zajedno sa peći. Da bi se ovaj efekat registrovao, urađena je serija eksperimenata.

(1) Eksperimentalna procedura Ispitivanje je izvršeno na 1.25Cr-0.5Mo čeliku (tab.2). Mikrostrukturu ovog čelika u stanju isporuke čine ferit i perlit. Uzorci su zagrevani na 975-1325K, sa brzinama zagrevanja od 0.05K/s do 1800k/s, a zatim kaljeni u vodi.

Brzina zagrevanja od 1800k/s je dobijena propuštanjem struje kroz veoma tanak uzorak (0.5x5x70mm). Brzina hlađenja je bila 500K/s.,.Za ostale brzine zagrevanja oblice (Φ6x50mm) su zagrevane indukciono ili u peći. Brzina hlađenja je bila 300K/s [8,12].

Temperature početka i kraja α/γ transformacije (Ac1 i Ac3, respektivno) su određene merenjem tvrdoće i mikrostrukturnim ispitivanjem serije zakaljenih uzoraka.

(2) Uticaj brzine zagrevanja na temperature transformacije Poprečni presek za Fe-1.25%Cr-0.5%Mo u faznom dijagramu sistema Fe-Cr-Mo-C je prikazan na slici 15

(3) [P]gb in HAZ of as-welded state A simulated HAZ specimen made of 2 1/4Cr-1Mo steel (Table 2) was used.

An example of the results is shown in Fig. 14. The [P]gb is as large as 0.3%, this value is as much as 80 times of the bulk phosphorus content (0.004%). It is noted here that:

1. Segregation of phosphorus in PAGB was not observed in as-received steel.

2. The location of PAGB in HAZ is quite different from that in as-received steel.

3. The [P]gb in HAZ (0.3% in Fig. 14) is not inherent of as-received steel but is produced newly by welding.

Slika 15. .Ravnotežni dijagram sistema, Fe-Cr-Mo-C; presek za 1.25%Cr-0.5%Mo Fig. 15. Equilibrium diagram of Fe-Cr-Mo-C system; a cross-section at 1.25%Cr-0.5%Mo

The segregation caused by welding is necessarily connected to the α/γ transformation which occurs in the heating process of welding. This new type of segregation is named "the transformation-induced segregation".

Other evidences, which hold this type of segregation are shown below.

3.3 Effect of heating rate on the process of α/γ transformation The transforming process in a great heating rate, such as by welding may differ from that in an usual heating rate, such as by furnace. The effect of heating rate was confmned by a series of experiments.

(1) Experimental procedure Specimens were taken from 1 1/4Cr-1/2Mo steel (Table 2). Its original microstructure in as-received state is ferrite and pearlite.

The specimens were heated up to 975 to 1325K with the heating rate of 0.05 to 1800K/s, and quenched in water.



[15, 16]. U ravnotežnom stanju, čelik sa 0.15%C započinje i završava transformaciju na Ae1 i Ae3 temperaturama, respektivno.

Temperature Ac1 i Ac3 su određivane za svaku brzinu zagrevanja, i na slici 16 prikazane su strelicama orijentisanim naviše i naniže, respektivno.

The heatig rate of 1800 K/s was obtained by flowing the current through a thin specimen (0.5x5x70 mm). The cooling rate was 500 K/s. For other heating rates, round bar specimens (Φ6x50mm) were heated by an induction current or in a furnace. The cooling rate was 300 K/s [8, 12].

Slika 16. Temperature početka i kraja α/γ transformacije za svaku brzinu zagrevanja određene merenjem tvrdoće; 1.25%Cr-

0.5%Mo čelik Fig. 16. Beginning and finishing temperatures of �/� transformation at each heating rate determined by the hardness

measurement; 1 1/4Cr-1/2Mo steel.

Slika 17. Uticaj brzine zagrevanja na temperature početka i

i završetka α/γ transformacije; 1.25%Cr-0.5%Mo Fig. 17. Influence of heating rate on the beginning and

finishing temperatures of α/γ transformation; 1 1/4Cr-1/2Mo steel.

Na slici 17 prikazana je zavisnost temperatura Ac1 i Ac3 od brzine zagrevanja. Treba napomenuti da je temperatura data u recipročnoj funkciji apsolutne temperature, a brzina zagrevanja kao logaritamska vrednost za K/s. Prema podacima sa slike 17, može se zaključiti:

1. Ac1 i Ac3 temperature se povećavaju sa povećanjem brzine zagrevanja. Ova zavisnost je linearna, sve do brzine od 1800K/s.

The beginning and the finishing temperatures of α/γ transformation (Ac1 and Ac3, respectively) were determined by hardness measurements and microscopic observations on a series of quenched specimens.

(2) Effect of heating rate on the temperature of transformation The cross-section of Fe-1.25%Cr-0.5%Mo in the phase diagram of Fe-Cr-Mo-C system is shown in Fig. 15 [15, 16]. In the equilibrium state, 0.15%C steel begins and finishes the transformation at Ae1 and Ae3 temperatures, respectively.

The temperatures, Ac1 and Ac3 at each heating rate were determined as shown by upward and downward arrows, respectively in Fig.16.

Ac1 and Ac3 were plotted against the heating rate as shown in Fig.17. 1/T is taken in the ordinate with a normal spacing. Heating rate is shown in the abscissa of the logarithmic scale. This figure informs following facts.

1. The Ac1 and Ac3 rise simply with an increasing heating rate. 1/T and logarithm of heating rate are in a linear relationship up to 1800K/s.

2. This fact suggests that even in a great heating rate the transformation is performed still by the diffusional process (not by the lattice transformation).

3.Even in welding of the heating rate as large as 1000K/s, the transformation occurs by the diffusion process, although Ac1 and Ac3 are raised significantly.



2. Ova činjenica takođe ukazuje da je i pored veoma velike brzine zagrevanja transformacija i dalje difuziono kontrolisana (a ne promena vrste kristalne rešetke).

3. Čak i u slučaju zavarivanja, gde je brzina zagrevanja veća od 1000K/s, transformacija se odigrava difuzionim procesom, iako su Ac1 i Ac3 značajno povećane.

(3) Nukleacija i rast austenitne faze Na slici 18 je prikazana mikrofotografija početne faze tranformacije. Austenitni tanki film (bela polja) se javlja po granicama feritnih zrna (tamna polja), kao i na graničnoj površini feritnih i perlitnih (sferoidizirani karbidi) zrna. Faza nastanka austenitnih zrna je praćena posmatranjem serije mikrostruktura. Reprezentativni set etapa je dat na slici 19.

1. Neposredno iznad Ac1 temperature, austenit nastaje na granicama između feritnih zrna i ferita i perlita, i raste duž tih površina (sl.19a).

2. Između Ac1 i Ac3 austenit raste sa granica prema unutrašnjosti feritnih i perlitnih zrna (sl.19b).

3. Na Ac3 temperaturi preostala feritna zrna nestaju te austenit prekriva celokupnu zapreminu, pošto se zrna austenita sudaraju. Kada se zagrevanje dalje nastavi, dolazi do porasta veličine austenitnog zrna (sl. 19c).

(3) Initiation and growth process of austenite Fig.18 shows the microphotograph at the beginning stage of transformation. Austenite strings (white region) is initiated at the grain boundary of ferrite (dark, flat region) and the boundary of ferrite and pearlite (region with spherodized carbide). The initiation and growth processes of austenite were traced by observing a series of microstructures; a representative sequence is illustrated in Fig 19.

Slika 18. SEM mikrofotografija na početku α/γ transformacije, elektrolitički nagriženo, A-Austenit; F-Ferit; P-

Perlit; 1.25Cr-0.5Mo čelik Fig.18. SEM microphotograph at the beginning stage of �/� transformation, etched electrolytically, A: austenite, F: ferrite,

P: pearlite, 1 1/4Cr-1/2Mo steel.

(a) Austenitna zrna se stvaraju na 20K iznad Ac1 i počinju da rastu

(b) Austenitna zrna rastu na temperaturi 50K iznad Ac1

(c) Transformacija se završava na Ac3

Slika 19. Nastanak austenitnih zrna iz ferita i perlita unutar temperaturnog područja Ac1 – Ac3; toplovaljano, 1.25%Cr-0.5%Mo čelik; brzina zagrevanja 0.05K/s

Fig. 19. Process of producing austenite grains from ferrite and pearlite in the temperature range of Ac1 to Ac3; hot-rolled 11/4Cr-1/2Mo steel, heating rate 0.05K/s.

Takođe, sa slike 19c je jasno da se položaji austenitnih i feritnih zrna potpuno razlikuju.

Na slici 19 su dati rezultati za zagrevanje malom brzinom (0.05K/s); slični rezultati su dobijeni i za slučaj većih brzina zagrevanja.

Kvantitativni podaci rasta austenitnog zrna su prikazani na slici 20. Na ordinati je data prosečna površina pokrivena jednim austenitnim zrnom na poprečnom preseku uzorka. Nula i jedinica na apcisi se odnose na trenutak početka i završetka transformacije, respektivno.

U slučaju male brzine zagrevanja austenitna zrna nastaju u malom broju (svega nekoliko) mesta. Ova zrna rastu veoma brzo, te strukturu čini mali broj velikih zrna i preko 80µm2.

1.Just above the Ac1, austenite nucleates at the ferrite grain boundary and the ferrite/pearlite boundary, and grows along them (Fig. 19(a)).

2. Between Ac1 and Ac3, austenite grows toward the inside of ferrite and pearlite region (Fig. 19(b)).

3. At the Ac3, remaining ferrite disappears and blocks of austenite grains meet at the portion where the final ferrite located. By succeeding heating, austenite grains grow (Fig. 19(c)).

It is noted here that the location of austenite grain boundary differs completely from that of ferrite (Fig. 19(c)).

Fig. 19 is the results of a smaller heating rate (0.05K/s); similar results were also obtained on the larger heating rates.



U slučaju povećane brzine zagrevanja nukleacija se odigrava na velikom broju mesta, što rezultira veoma finim austenitnim zrnom. Za brzinu zagrevanja od 200K/s, veličina zrna se smanjuje na 20µm2.

3.4 Segregacije izazvane transformacijom Ponašanje fosfora u toku α/γ transformacije praćeno je korišćenjem metode nagrizanja granica zrna (GBE) [17].

Slika 20. Povećanje prosečne veličine austenitnog zrna u toku transformacije; toplovaljani 1.25%Cr-1.0%Mo čelik

Fig. 20. Increase of average area of austenite grains with progressing transformation; hot-rolled 1 1/4Cr-1/2Mo steel.

Quantitative data of austenite grain growth is shown in Fig.20. The ordinate shows the average area occupied by one austenite grain in a cross-section of specimen. Zero and the unit in the abscissa show the times at which the transformation begins and fInishes, respectively.

With a small heating rate, austenite nucleates at a few sites and grows to a large sized grain as large as 80µm2.

With the increased heating rate, the nucleation occurs at many sites resulting in the fine austenite grains. For the heating rate of 200K/s, the grain size becomes as small as 20µm2.

3.4. Process of segregation induced by transformation The behavior of phosphorus during the air transformation was traced by the grain boundary etching method (GBE)[17].

The GBE is useful for semi-quantifying the [P]gb of a group of specimens of the similar microstructure. Ogura informed that the depth of etched grain boundary, "d" is proportional to the [P]gb[ 17].

(1)Procedure of GBE The depth d was measured by the following procedure.

1.A cross-section of specimen is polished and etched by the etching reagent shown in the section 3.2(2) for 60min at 283K. 80ml of reagent for the area of 30mm2 is recommended. PAGB is grooved in a certain depth.

Slika 21. Merenje dubine nagrižene granice zrna d:

Fig. 21. Measurement of depth of etched grain boundary d; d = 0.14 (D1-D2); (a) Surface in polished and etched state; (b) Surface in re-polished state

GBE metoda je pogodna za polu-kvantifikaciju [P]gb za grupu uzoraka slične mikrostrukture. Ogura je saopštio da je dubina nagrizanja granice zrna proporcionalna [P]gb [17].

(1)Postupak GBE metode Dubina d je merena na sledeći način:

1. Poprečni presek uzorka je poliran i nagrižen sredstvom za nagrizanje koje je opisano u 3.2 (2) i to 60 minuta na 283 K. Preporučuje se da se za svakih 30mm2 površine za nagrizanje koristi 80 ml reagensa. PAGB su tako nagrižene do određene dubine.

2.Oznake se utiskuju u okolini PAGB, korišćenjem uređaja za merenje tvrdoće (sl. 21a). Površina se zatim ponovo polira sve dok oznake ne nestanu (sl. 21b).

2. A marks is indented near the PAGB by a hardness tester (Fig.21(a)). The surface is polished again until the groove begins to disappear (Fig.21(b)).

3. The depth of groove, d is given by the following equation. d =0.14(D1 - D2)

A thin specimen(0.5x5x70mm) was taken from 1 1/4Cr-1/2Mo steel (Table 2) and it was heated by the current flowing through it with the heating rates of 10, 100 and 1000K/s up to 1225, 1425 and 1625K, and water-quenched immediately. The microstructure obtained is martensite + bainite. The depth d was measured on those specimens.

3.Dubina nagrizanja se određuje prema sledećem izrazu:



d = 0.14(D1−D2)

Tanki uzorak (0.5x5x70mm) od čelika 1.25Cr-0.5Mo je zagrevan propuštanjem električne struje sa brzinama zagrevanja od 10, 100 i 1000K/s, do 1225, 1425, i 1625K, a zatim trenutno kaljen u vodi. Dobijena je struktura kombinacija martenzita i beinita. Na ovako kaljenim uzorcima merena je dubina d.

(2)Segregacija fosfora indukovana transformacijom Na slici 22 je prikazana promena [P]gb u toku procesa zagrevanja od sobne temperature do 1625K. Na ovoj slici promena [P]gb je prikazana upotrebom dubine nagrizanja d. Osenčena oblast označava temperaturno područje između Ac1 i Ac3 (sl.17), tj. područje u kome se transformacija odigrava. Sa slike se vidi da:

1. [P]gb dostiže najveću vrednost upravo unutar temperaturnog područja (osenčena površina) u kome dolazi do transformacije. Nakon završene transformacije, sa povećanjem temperature [P]gb se snižava. Ovakvo ponašanje je primećeno pri svim brzinama zagrevanja.

2. U slučaju brzine zagrevanja 1000K/s, [P]gb ima najveću vrednost, kako u blasti transformacije, tako i nakon završetka transformacije.

Ovi podaci ukazuju da:

1. Segregacije fosfora je nezaobilazna pojava pri faznim transformacijama.

2. Rastvaranje segregacija započinje neposredno nakon završetka transformacije.

3. Ako je brzina zagrevanja velika, a vreme za rastvaranje kratko, [P]gb se zadržava na visokom nivou.

4. U ZUT najveća vrednost [P]gb će biti dostignuta u grubozrnom području, usled najveće brzine zagrevanja u toj zoni.

Slika 22. Uticaj prvobitne temperature i brzine zagrevanja i

na koncentraciju fosfora na granici zrna prikazan preko parametra d

Fig. 22. Influence of ultimate heating temperature and heating rate on the phosphorus concentration in grain

boundary shown by the parameter d.

(2)Segregation of phosphorus induced by transformation Fig. 22 shows the change in [P]gb in the heating process from the room temperature up to 1625K. In this figure, the change of [P]gb is shown by using the parameter d. The shaded area between the lines, Ac1 and Ac3(Fig.17) is the temperature range in which the transformation occurs.

This figure informs the important facts that:

1. [P]gb reaches maximum in the range of transformation (shaded area). After the transformation, it decreases with a rasing temperature. This change is observed for all the heating rates.

2. With the heating rate of 1000K/s, the [P]gb is largest even for the temperature range in which the transformation has been completed.

Those results suggest that;

1. Segregation of phosphorus arises necessarily whenever the transformation occurs.

2. However, the dissolution of segregation begins to occur immediately after the transformation.

3. If the heating rate is large enough and hence, the time for the dissolution of segregation is short, the [P]gb remains still in a large quantity.

4. In HAZ, the maximum [P]gb will be attained in the coarse-grained zone due to the largest heating rate in this zone.

Slika 23. Ugao železa u Fe-P ravnotežnom dijagramu stanja ukazujući na ravnotežni koeficijent raspodele

ko;ko=(Cγ/Cα)=0.20. Fig. 23. Iron corner of Fe-P equilibrium diagram showing

equilibrium distribution coefficient ko; ko=Cγ/C α =0.20

4. DISCUSSION ON THE MECHANISM OF TRANSFORMATION-INDUCED SEGREGATION The segregation of phosphorus during transformation is discussed in this section with the concept of "equilibrium distribution".

In case of "solidification-induced segregation", its mechanism has been explained very well by the



4. DISKUSIJA O MEHANIZMU SEGREGACIJA IZAZVANIH TRANSFORMACIJOM Segregacije fosfora su ovde razmatrane preko koncepta “ravnotežne raspodele”.

U slučaju “segregacija indukovanih očvršćavanjem”, mehanizam je objašnjen veoma jasno i detaljno, korišćenjem koncepta koeficijenta ravnotežne raspodele razmatranog elementa (nečistoće) u tečnoj (polazna faza) i u čvrstoj fazi (produkt); koeficijent ko=(CS/CL) mora biti manji od jedinice [18].

U slučaju transformacijom indukovanih segregacija, koeficijent fosfora (rastvoreni) u feritu (α-izvor) i austenitu (γ-produkt), ko=(Cγ/Cα) je manji od jedinice (0.20), (sl.23) [19,20]. Prema ovom stanovištu, pored očvršćavanja, segregacije fosfora su moguće i u toku transformacije [12,21]. Transformacija ferita u austenit se odigrava u svakoj elementarnoj zapremini kretanjem α/γ granične površine, kao što je prikazano na slikama 24a i 24b. Na graničnoj površini fosfor je prisutan u obe faze, tj. i u austenitu i u feritu, u odnosu od 1 do 5 [19,20]. To znači da se fosfor može rastvoriti tek delimično u odnosu na ukupnu količinu rastvorenu u feritu. Zato višak fosfora sa austenitne strane biva odbačen na feritnu stranu, i on se tu akumulira, kao što je prikazano na slici 24c. Kada se transformacija u ovoj oblasti završi, granična površina postaje granica austenitnog zrna. U tom trenutku je na granicama zrna velika koncentracija fosfora. Naknadna zagrevanja u austenitnom području podstiču prirodnu težnju granice zrna da difuzijom fosfora unutar zrna smanji koncentraciju. Ipak, u slučaju velikih brzina zagrevanja i hlađenja kao pri zavarivanju, određena količina fosfora i dalje zaostaje na granicama zrna.

Taj fosfor se hlađenjem na sobnu temperaturu registruje kao ″fosfor koji je segregirao po PAGB″.

5. ZAKLJUČAK

1. Segregacija fosfora na granice polaznih austenitnih zrna je glavni razlog pojave prslina usled ponovnog zagrevanja u najvećem broju komercijalnih čelika sa niskim sadržajem Mo i visokim sadržajem Cr. U ostalim CrMo čelicima Mo2C je najodgovorniji za prsline. 2. EDX mikroanaliza je otkrila prisustvo segregacija fosfora u zavarenom stanju odmah posle zavarivanja. 3. Segregacije fosfora su izazvane feritno-austenitnom transformacijom u toku zagrevanja tokom zavarivanja. 4. Koeficijenat raspodele fosfora je manji od jedinice, tako da neizbežno dolazi do akumulacije fosfora na α/γ graničnoj površini u toku α/γ transformacije, te je pojava transformacije neophodan uslov da bi došlo do segregacija fosfora. 5. Segregacije fosfora su još utvrđene u slučaju velike brzine zagrevanja i nakon nekog vremena pošto se transformacija završila.

concept that the coefficient of equilibrium distribution of solute element (impurity) in liquid (source) and solid (product), ko (=Cs/CL) is smaller than the unit[18].

In case of the transformation-induced segregation, the coefficient of phosphrus (solute) in ferrite (α; source) and austenite (γ; product), ko (=C γ /C α) is smaller than the unit (0.20) as shown in Fig.23[19,20]. From this point of view, it will be possible that phosnhorus can be segregated by the transfonnation as well as by the solidification. [12, 21].

The transformation from ferrite (α) to austenite (γ) is progressed in each small area in steel by movihg the α/γ interface as illlustrated in Fig.24 (a) and (b). At the interface, phosphorus' is distributed in austenite and ferrite with a given ratio of 1 to 5 [19,20]. That is, austenite can dissolve only a small part of phosphorus which was formerly dissolved in ferrite in total amount. The excess phosphorus in the austenite side should be rejected back to the ferrite side. The rejected phosphorus is accumulated in the ferrite side as shown in Fig.24 (c). When the transformation is completed in this area, the interface becomes the grain boundary of austenite. At this time period, the grain boundary contains phosphorus with a high concentration.

The grain boundary of austenite, however, tends to loss phosphorus by diffusion by the succeeding heating in the austenite range. Nevertheless, a certain amount of phosphorus is retained in grain boundary, if the heating and the cooling rates are very large as the case of weling.

Phosphorus in grain boundary is then brought to the room temperature, and results in "the segregated phosphorus in PAGB".

5. CONCLUSION

1. The segregation of phosphorus in PAGB is the major cause of reheat cracking in the field of lower molybdenum and higher chromium contents. Major commercial steels locate in this field. On the contrary, Mo2C is the principal cause in the remaining Cr-Mo contents field. 2. EDX micro-analysis revealed that phosphorus was already segregated in the as-welded state. 3. The segregation of phosphorus is initiated by the α/γ( transformation during the heating process of welding. 4. The distribution coefficient of phosphorus (Cγ/Cα) is smaller than the unit, and hence it is necessarily accumulated at the α/γ interface during α/γ transformation. That is, the occurrence of the transformation is the necessary condition of segregation of phosphorus. 5. The segregation of phosphorus thus established is kept in certain amount even after the transformation if the heating rate is very large. That is, the transformation occurring in a short time is the enough condition of the segregation of phosphorus.



Slika 24. Koncentracija fosfora na α/γ graničnoj površini u toku transformacije (šematski prikaz) Fig. 24. Concentration of phosphorus to the α/γ interface during transformation (Schematic representation).

LITERATURA

[1] A. Dhooge, A. Vinckier: Reheat cracking-review of recent studies (1984-1990), Welding of the World, Vol.40, No.3/4 (1992), 44-71.

[2] C.D. Lundin, K.K. Khan: Fundamental studies of the metallurgical causes and mitigation of reheat cracking in 1 1/4Cr-1/2Mo and 2 1/4Cr-1Mo steels, WRC Bulletin 409-Feb.1996, Welding Research Council.

[3] K.Tamaki, J.Suzuki, et al: Effect of molybdenum carbide on reheat cracing sensitivity of Cr/Mo steels, IIW Doc. IX-1159-80 (1980).

[4] K. Tamaki, J. Suzuki: Combined influence of phosphorus, chromium and molybdenum on reheat cracking of steels, IIW Doc. IX-1408-86 (1986).

[5] K. Tamaki, J. Suzuki, et al: Effect of restraint stress on crack-initiating temperature and fracture mode of reheat cracking of Cr-Mo steels, IIW Doc. IX-1409-86 (1986).

[6] K. Tamaki, H. Kawakami, J. Suzuki: Temper embrittlement in HAZ of Cr-Mo steels, Welding in the World, Vol.43, No.2 (1999), 36-48.

[7] K. Tamaki, H. Kawakami, J. Suzuki: Metallurgical causes of low ductility creep fracture in HAZ of 14Cr-1Mo steel, IIW Doc. IX-1996-01 (2001).

[8] K. Tamaki, H. Kawakami, et al: Effect of SR treatment on creep-ductility creep of HAZ of 21/4Cr-1Mo and 1 1/4Cr-1/2Mo steels, IIW Doc. XI-703/99(1999).

[9] N. Kohda: Precipitation in Alloys, 1976, Maruzen, pp. 359-401 (in Japanese).

[10] H. Kaneko, T. Nishizawa, K. Tamaki: Phosphide phases in ternary alloys of iron, phosphorus and other elements, J. Japan Inst. Metals, 27-7 (1963), 159-165 (in Japanese).

[11] H. Kaneko, T. Nishizawa, K. Tamaki et al: Solybility of phosphorus in α and γ-iron, J. Japan Inst. Metals, 27-7 (1963), 166-170 (in Japanese).

[12] K. Tamaki, J. Suzuki, H. Kawakami, et al: Phosphorus segregation induced by α/γ tansformation during Welding, Quatery J. Japan, Welding Soc., 20-4 (2002), 60-69.

[13] T. Ogura: A method for evaluation of the amount of grain boundary segregation during quenching, Trans. Japan Inst. Metals, 22-2 (1981), 109-117.

[14] F. Matsuda, H. Nakagawa et al: Calculation of phosphorus segregation at grain boundary during weld thermal cycle, Quatery J. Japan Welding Soc. , 7-14 (1989), 490-495 (in Japanese).

[15] K. Bungardt et al: Untersuchungen über den Aufbau des System Eisen-Chrom-Kohlenstoff, Arch. Eisenhüttenves. 29-3 (1958), 192-203.

[16] Amer. Soc. Met.:Metals Handbook 8th Ed., Vol.8, “Metallography, Structures and Phase Dijagrams”, (1973), pp.409-411.

[17] T. Ogura et al: Study on the grain boundary etching method as a technique analyzing the P segregation an the grain boundary, J. Japan Inst. Metals, 45-10 (1981), 1093-1101. (in Japanese).

[18] F. Matsuda: Welding Metallurgy, 1976, Nikkan-Kogyo, pp.137-142 (in Japanese).

[19] M. Hansen: Constitution of Binary Alloys, McGraw Hill (1952), pp.962-965.

[20] E. Hornbogen: Precipitation of phosphorus from alfa iron and its effect on plastic deformation, Trans. ASM, 53 (1961), 569-589.

[21] K. Tamaki, J. Suzuki, H. Kawakami: Segregation of phosphorus induced by a thermal cycle of welding and its effect on reheat cracking, IIW Doc. IX-2061-03, (2003).

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