Impact of Oxygen Concentration on Toughness in Steel Weld Metals

1. Changes in Toughness at Low Oxygen Concentrations in Steel Weld Metals. S. Terashima and H. K. D. H. Bhadeshia Phase Transformations and Complex Properties Research Group Department of Materials Science and Metallurgy University of Cambridge

2. Index of Today’s Talk. Today, we are going to…….. [Section 1] Clarify an impact-toughness behaviour of steel weld metal. (780 MPa class in UTS) Find out that oxygen (or oxides) has important roles. [Section 2] Improve the toughness by changing size-distribution of oxides.

3. Effect of Oxygen on Toughness (UTS: 490 MPa class). Acicular Ferrite Coarse Ferrite In 490 MPa class welds, a peak appeared by acicular ferrite formation. Coarse Bainite

4. Effect of Oxygen on Toughness (UTS: 980 MPa class). In 980 MPa class welds, toughness decreased with increasing oxygen because oxides do not cause microstructural change in weld metals. Martensite

5. Effect of Oxygen on Toughness (UTS: 580-780 MPa). Some mild contradictions: (1) Simply decreases because oxides initiate cracks or voids. (2) Makes a peak because coarsened bainite is formed at lower oxygen level.

6. Requirement of 780 MPa Class Weld Metal. Requirement of 780 MPa class steel (in UTS) is increasing. Structural thick steel plates (Ship, Building, etc.) Thin steel plates (Vehicle, Electronics, etc.) In order to create weld metals with both high strength and toughness, systematic understanding is needed concerning “Relationship between toughness and oxygen content in weld metal”.

7. Objective. Relationship between toughness and oxygen content in weld metal is studied over a wide range of oxygen content especially from the viewpoint of microstructures, using weld metals with 580, 680 and 780 MPa class ultimate tensile strength.

8. How to Change Oxygen Concentration in Welds? # MAG (Metal Active Gas shielded arc welding) was used to produce weld metals. # It is well known that a composition of a shielding gas during MAG welding affects an oxygen concentration of a weld metal. # Therefore, a composition of a shielding gas was changed: Pure Ar 0.99Ar-0.01CO2 0.98Ar-0.02CO2 0.90Ar-0.10CO2 0.80Ar-0.20CO2 Pure CO2

9. Relationship between CO2 in Gas and Oxygen in Welds. Specimens are prepared with a wide range of oxygen concentration. High: 780 MPa Medium: 680 MPa Low: 580 MPa Method: Combustion analysis

10. 20ppmw O ac Test temperature: 293K 10mm 10mm 273K 253K 110ppmw O 233K aa Toughness and Microstructure (580 MPa). Coarsened Structure Acicular Ferrite

11. 90ppmw O ac 10mm 10mm 10mm Test temperature: 273K 210ppmw O 253K 233K 280ppmw O 213K aa a Toughness and Microstructure (680 MPa). Acicular Ferrite Coarsened Structure Contains Coarsened Structure

12. Test temperature: 273K 253K 233K 213K Toughness and Oxygen Concentration (780 MPa). Complex behaviour. Is there any microstructural change due to oxygen level ?

13. Optical & SEM Micrographs (780 MPa). 20 ppmw O 110 ppmw O 140 ppmw O alb aub alb alb aub aub 5mm 5mm 5mm 10mm 10mm 10mm 270 ppmw O 350 ppmw O 560 ppmw O aa aa a

14. Test temperature: 273K 253K 233K 213K Toughness and Microstructure (780 MPa). Allotriomorphic Ferrite Acicular Ferrite Martensite with Bainite Still not clear between 20 - 140 ppm O. i.e. Why toughness changed even though microstructures were the similar ?

15. Fractured Surface Tested at 273 K (780 MPa). (a) 20 ppm O (b) 110 ppm O (c) 140 ppm O 10mm 5mm Inclusions (oxides) at dimples initiated cracks or voids.

16. Test temperature: 273K 253K 233K 213K Toughness and Microstructure (780 MPa). Martensite with Bainite Cracks or Voids Martensite with Bainite Roles of oxygen in welds: Shifting microstructures, and Initiating cracks or voids.

17. Effect of Oxygen on Impact Toughness of Steel Welds. • In the 780MPa weld, • high impact toughness was observed • not only at the intermediate oxygen level (270 and 350ppmO) • but also at the very low level (20 ppm O). • (2) In the 580 and 680MPa welds, • impact toughness was high only at the intermediate oxygen level (190ppmO and 110-210ppmO, respectively). • (3) Oxygen shows two roles: • one is to change microstructure of the weld metals, • and the other is to degrade impact toughness of welds • by forming oxides which act as crack or void initiation site.

18. Index of Today’s Talk. Today, we are going to…….. [Section 1] Clarify an impact-toughness behaviour of steel weld metal. (780 MPa class in UTS) Find out that oxygen (or oxides) has important roles. [Section 2] Improve the toughness by changing size-distribution of oxides.

19. Test temperature: 273K 253K 233K 213K How to Improve Toughness of Steel Welds? In the literature, # Smaller oxides (about 0.5 mm in diam.): initiate acicular ferrite. # Larger oxides (over 1 mm in diameter): initiate cracks or voids. ---> If larger oxides are eliminated from welds, will their toughness be improved ???

20. Objective. To minimise the oxide size so as to delay oxide-related damage to the later stages of stress and strain and to improve impact toughness of steel weld microstructure.

21. How to Change Size Distribution of Oxide? # MAG (Metal Active Gas shielded arc welding) was used to produce weld metals. # It is known that larger oxides (say, > 1mm) tend to float to the surface of a specimen after TIG (Tungsten Inert Gas shielded arc welding). # Therefore, re-melting by TIG was carried out after MAG to eliminate larger oxides. (Re-melting was carried out only for the as-welded specimen with 350 ppmw O.)

22. Relationship between CO2 in Gas and Oxygen in Welds. Re-melting decreased oxygen concentration. Method: Combustion analysis

23. All oxides Re-melted Oxide diam.: 0.5 – 1 mm As-welded < 0.5 mm 1 - 3 mm > 3 mm Size Distribution of Oxides in Steel Welds. [Re-melted & 350 ppm O] # Smaller ones (< 1 mm) were decreased by re-melting. # Larger ones (> 1 mm) were decreased drastically (almost all) by re-melting. [Re-melted & 110-140 ppm O] # Re-melted had more smaller oxides. # Re-melted had smaller number of larger oxides, instead.

24. Micrographs. 110ppmw O 140ppmw O alb alb aub aub 5mm 5mm 10mm 10mm 270ppmw O 350ppmw O 90 ppmw O aa aa aa 10mm As-welded Re-melted Re-melted consisted of a’+ aa Smaller oxides initiated acicular ferrite even in martensitic microstructure.

25. Acicular ferrite content as a function of oxygen concentration. As-welded: 20 – 140: a’ + ab 270, 350: aa 560：allotriomorphic ferrite Re-melted: a’ + aa # Higher toughness can be expected!

26. Impact Toughness. Re-melted showed good toughness. [Re-melted > 20-140 ppm O] Because acicular ferrite appeared in re-melted. [Re-melted > 270 ppm O] Fractographs???

27. 5mm 5mm 5mm 5mm Fractured Surface tested at 273 K. As-welded As-welded As-welded 20 ppmw O 110 ppmw O 350 ppmw O Re-melted Visible particles were hardly observed at dimples. 90 ppmw O Particles at dimples.

28. Impact Toughness. Re-melted showed good toughness. [Re-melted > 20-140 ppm O] Because acicular ferrite appeared in re-melted. [Re-melted > 270 ppm O] Because larger oxides are eliminated from re-melted.

29. Effect of Size Distribution of Oxides onToughness. # The refinement of oxide particles and a reduction in the total oxygen concentration from the usual hundreds of parts per million, is beneficial to the toughness of strong weld metals. # It is again confirmed that oxides in steel weld metals can change microstructures of welds and initiate cracks or voids in welds.

30. Summary and Some Future Works. Two conventional methods to improve impact toughness of steel welds are # Producing acicular ferrite (Popular, but not so easy for stronger welds.) # Decreasing oxygen concentration down to 20 ppm O (Cost would be high.) According to the present research, the following method can be proposed: # Refining oxide particles (Useful even for stronger welds, however, difficult to achieve in practice.)