Osjetljivost čelika X70 na hladne pukotine pri mokrom podvodnom zavarivanju

doctoral thesis

• Title (english): Susceptibility of X70 steel on cold cracking during underwater wet welding
• Author: Ivica Garašić
• Mentor: Slobodan Kralj (mentor)
• Committee member: Vinko Ivušić (predsjednik povjerenstva)
• Committee member: Slobodan Kralj (član povjerenstva)
• Committee member: Zoran Kožuh (član povjerenstva)
• Committee member: Ivan Samardžić (član povjerenstva)
• Granter: University of Zagreb; Faculty of Mechanical Engineering and Naval Architecture; Zagreb
• Defense date and country: 2008-04-30, Croatia
• Scientific / art field, discipline and subdiscipline: TECHNICAL SCIENCES; Mechanical Engineering
• Universal decimal classification (UDC): 621 - Mechanical engineering. Nuclear technology. Machinery ; 669 - Metallurgy

Abstract

U ovom radu obrađena je problematika podvodnog mokrog zavarivanja čelika za cjevovode
API 5L X70 s aspekta osjetljivosti na nastanak hladnih pukotina.
U uvodnom dijelu obrazložena je motivacija istraživanja te je dan presjek dosadašnjih
spoznaja i provedenih istraživanja na području mokrog podvodnog zavarivanja, dodatnih
materijala, mehanizama nastanka hladnih pukotina, utjecaja katodne zaštite i razvoja čelika za
cjevovode. Definirane su hipoteze te je osmišljen plan eksperimenta za dobivanje relevantnih
informacija nakon čije statističke obrade će biti moguće donijeti određene zaključke te
odbaciti ili prihvatiti hipoteze.
Eksperimentalni rad proveden je u Laboratoriju za zavarivanje, Fakulteta strojarstva i
brodogradnje. Iz osnovnog materijala API 5L X70 izrađene su epruvete za određivanje
difundiranog vodika i Implant test. Primijenjena su tri postupka zavarivanja-REL, zavarivanje
praškom punjenom žicom u plinskoj zaštiti i zavarivanje samozaštićujućom praškom
punjenom žicom , svaki na tri razine unosa topline. Epruvete su prije zavarivanja držane u
katodnoj zaštiti na tri razine potencijala. Nakon zavarivanja izmjereni su difundirani vodik,
određeno je kritično Implant naprezanje te su izmjerene tvrdoće HV10.
Analiza rezultata je pokazala da postupak zavarivanja, ovisno o unosu topline, kao i potencijal
katodne zaštite imaju utjecaja na difundirani vodik i na kritično Implant naprezanje. Također
je pokazano da se unosom topline može utjecati na mikrostrukturu u ZUT-u što se direktno
očitava na vrijednostima tvrdoća i smanjenju rizika od pojave hladnih pukotina.
Mikrostrukturna analiza ukazala je da su kod većine uzoraka u grubozrnatom ZUT-u
detektirane hladne pukotine te da postoji značajan utjecaj nehomogenosti na pojavu hladnih
pukotina. Statističkom analizom potvrđeno je da su unos topline i potencijal katodne zaštite
značajni faktori koji utječu na difundirani vodik i kritično Implant naprezanje. Metodom
odzivnih površina dobiveni su matematički modeli koji su provjereni i uspoređeni sa
postojećim podacima i modelima te je ustanovljena dobra korelacija.
Na osnovi provedenog ekperimentalnog rada i analize podataka dobivenih mjerenjem
difundiranog vodika i kritičnog Implant naprezanja zaključeno je da je moguće podvodno
mokro zavarivanje na čeliku X70 uz postizanje zavara određene kvalitete i bez pojave hladnih
pukotina. Međutim, pri tome za svaki postupak zavarivanja postoji određeno područje
parametara čija primjena daje optimalne rezultate. Uz to, pokazano je da potencijal katodne
zaštite ima utjecaja na cjelovitost podvodnih cjevovoda s gledišta pojave hladnih pukotina.

Abstract (english)

This doctoral thesis deals with susceptibility of API 5L X70 steels on cold cracking during underwater wet welding. In introduction, the motivation of this research is given as well as brief overview of present research and gained knowledge on the field of underwater welding, filler materials, hydrogen induced cracking mechanism, influence of cathodic protection and development of pipeline steels. Scientific hypothesis is defined together with design of experiment for gaining data necessary for confirmation or rejection of proposed hypothesis. Experimental work is conducted in Laboratory for welding at Faculty of Mechanical Engineering and Naval Architecture. API 5L X70 pipe was used for production of samples needed for determination of diffused hydrogen and critical Implant stress. Three welding processes are used- MMAW, FCAW with additional gas protection and FCAW with selfshielded wire, each on three different heat input levels. Samples were held in cathodic protection before welding on three different potential levels. After welding, diffusible hydrogen and critical Implant stress were determined and HV10 hardness was measured. Analysis of results showed that heat input and cathodic protection level influenced the amount of diffusible hydrogen and intensity of .critical Implant stress. Moreover, it was showed that heat input could decrease occurrence of cold cracking by means of decreasing maximal hardness in HAZ. Beside that, inhomogenities also increase possibility of cold cracking. Statistical analysis showed that heat input and cathodic protection level presents significant factors which have influence on diffusible hydrogen and critical Implant stress. Response surface method was applied to deliver mathematical models that were checked on existing research data and it was proofed that significant correlation exists in prediction of diffusible hydrogen and critical Implant stress. On the basis of conducted experimental work and achieved data analysis from measurement of diffusible hydrogen and critical Implant stress it was concluded that it is possible to achieve acceptable underwater wet weld quality on pipeline steel API 5L X70 without appearance of cold cracking. However, it is necessary to use the optimal welding parameter range for each welding process. Regarding cathodic protection it was showed that its intensity could affect underwater pipeline integrity by means of cold cracking appearance.

• Keywords: podvodno mokro zavarivanje; cjevovodi; hladne pukotine; difundirani vodik
• Keywords (english): underwater wet welding; pipelines; cold cracking; diffusible hydrogen
• Language: croatian
• URN:NBN: urn:nbn:hr:235:446833
• Study programme: Title: Mechanical Engineering and Naval Architecture; Study programme type: university; Study level: postgraduate; Academic / professional title: doktor/doktorica znanosti, područje tehničkih znanosti (doktor/doktorica znanosti, područje tehničkih znanosti)
• Type of resource: Text
• File origin: Born digital
• Access conditions: Open access
• Created on: 2020-12-03 07:37:43