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

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.

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

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

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.

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”.

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.

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

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

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

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

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

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

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

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.

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

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.

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.

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 ??? Test temperature: 273K 213K 233K 253K

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.

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.)

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

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. Re-melted All oxides Oxide diam.: 0.5 – 1 mm 1 - 3 mm > 3 mm < 0.5 mm As-welded

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

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!

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???

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

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.

Effect of Size Distribution of Oxides on Toughness. # 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.

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.)