High Strength Steel Welding Research

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High Strength Steel Welding Research

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ABSTRACT

The application of high strength low alloy (HSLA) steels has been limited by the availability of suitable filler metals. Specifically, as the weld metal strength increases, the susceptibility to hydrogen- assisted cracking increases. Therefore, to take full advantage of the developments in HSLA steel base metals, weld filler metals which minimize the effects of diffusible hydrogen develop tough microstructures must be designed. The benefit of yttriumn-containing inclusions to provide effective hydrogen traps and reduce diffusible hydrogen levels, as well as, to act as intragranular nucleation sites to produce tough microstructures has been demonstrated. Furthermore, fluoride- containing consumables have been demonstrated to reduce weld metal diffusible hydrogen levels through reactions within the arc atmosphere. The current research was undertaken to study the effects on welding characteristics and weld metal properties when these two concepts are integrated into a single welding consumable.

To study the effects of integrating fluoride additions to yttrium- containing consumables, flux-cored arc welding (FCAW) consumables were manufactured. A total of twenty-one wires of varying compositions were fabricated and used to produce beadon plate welds and bead-on-bead chem-pads. A target weld metal composition of 2.5 0hweight percent nickel, 1.1 weight percent manganese, 0.2 weight percent molybdenum, 280 ppm titanium, and 300 to 900 ppm yttrium. was selected because it has been observed to produce a fine-grained microstructure with low diffusible hydrogen content.

Characterization of these welds included arc stability, weld bead morphology, microstructural development, chemical composition, and inclusion development. Consumables that produced welds with good characteristics were then selected for multiple-pass welds so evaluations of impact toughness, tensile strength, and microhardness could be performed. In addition, diffusible hydrogen analysis was
performed with these consumables.

Potassium fluoride additions were found to be detrimental to weld metal characteristics compared to welds produced with metal-cored, yttrium-containing consumables. Powder fills containing an excess of ten percent potassium fluoride produced welds with significant amounts of porosity due to an increase in arc stability. Up to ten percent, good quality welds were produced but had poor wetting characteristics and an insufficient volume of fine-grained, acicular ferrite.

To improve wetting characteristics, ferrosilicon and other flux ingredients such as calcium fluoride, calcium oxide, silica, and alumina were integrated into the welding process. These additions not only improved wetting characteristics, but also improved alloy recovery and microstructure.

Evaluation of mechanical properties was performed on welds produced with the metal-cored, yttrium-containing consumables, consumables with the powder fill containing five and ten percent potassium fluoride, and consumables integrating the additional ingredients. Consumables that produced welds with microstructures containing a fraction of acicular ferrite of 65 percent or more met the requirements for impact toughness and tensile strength, but only had about 18 percent elongation. Welds with microstructures containing between 45 and 60 percent acicular ferrite were within fifteen percent of the minimum toughness standards. The U.S. Navy set minimum requirements of 35 and 60 foot-pounds (48 and 81 J) at -60 and 0°F, respectively, 88 to 115 ksi (607 to 793 MPa) yield strength with 20 percent elongation for high strength steel weld metals.

TOC

ABSTRACT iii
LIST OF FIGURES . ix
LIST OF TABLES . xvii
ACKNOWLEDGEMENTS xx 0
CHAPTER 1: INTRODUCTION 1
CHAPTER 2: LITERATURE REVIEW 5
2.1 Welding of High Strength Steels 6
2.2 Effect of Alloying Weldments . 10
2.3 Weld Metal Microstructural Development . 13
2.3.1 Weld Metal Solidification 13
2.3.2 Inclusion Characteristics and Development 21
2.3.3 Austenite Decomposition and Microstructural Constituents . 40
2.3.3.1 Grain Boundary Ferrite 42
2.3.3.2 Widmanstitten Ferrite 43
2.3.3.3 Bainite . 44
2.3.3.4 Acicular Ferrite . 45
2.3.3.5 M A Constituent . 47
2.3.4 Influence of Weld Metal Inclusions on Microstructural Development 47
2.3.5 Effects of Microstructure on Mechanical Properties 54
2.4 Hydrogen Assisted Cracking . 56
2.4.1 General Features of HAC 57
2.4.1.1 Macroscopic Observations 57
2.4.1.2 Microscopic Observations 58
2.4.2 Factors Responsible for HAC and their Control . 60
2.4.2.1 Hydrogen Level . 61
2.4.2.2 Stress Level . 62
2.4.2.3 Types of M icrostructure . 63
2.4.2.4 Tem perature . 64
2.5 Hydrogen Pickup in Weldments . 65
2.5.1 Hydrogen Sources . 65
2.5.2 Forms of Hydrogen in the Flux Covered and Cored Electrodes 65
2.5.3 Influence of Hydrogen in the Flux of Weld Metal Diffusible Hydrogen . 67
2.5.4 Influence of the Atmosphere and Surrounding Environment on Steel Weld Metal Diffusible Hydrogen 69
2.6 Hydrogen Management in Low Carbon and Low Alloy Steel Weldments . 71
2.6.1 Modification of Flux Ingredients . 71 0
2.6.2 Control of Oxygen Content in the Steel Weld Metal . 72
2.6.3 Hydrogen Trapping in the Steel Weld Metal 72
2.6.4 Dilution of Hydrogen in the Arc with Inert Gas . 73
2.6.5 A ddition of Fluorides . 74
2 .7 Y ttrium . 77 m
2.7.1 Effects on Inclusion and Microstructural Development 85
2.7.2 Effects on Hydrogen Management . 88
2.8 Effective Fluorides and their Effects on Hydrogen Management 89
2.8.1 Consideration of Mechanism of Reduction of Hydrogen Pickup 89
2.8.2 Thermodynamic Behavior of Species Involved in HF Formation . 92
2.8.2.1 Form s of Hydrogen 92 0
2.8.2.2 Vaporization of Fluorides 94
2.8.3 Model of HF Formation Effect 97
2.8.3.1 Reaction Occurring in the Surroundings of the Molten Pool and M olten D roplet 97
2.8.3.2 Concerns for Hydrogen Absorption from the Arc Atmosphere to the W eld Pool . 99
CHAPTER 3: OBJECTIVES OF RESARCH . 107 0
CHAPTER 4: EXPERIMENTAL METHODS . 109
4.1 Electrode Form ulation . 109
4.2 Materials Used . 113
4.2.0 Fabrication of Ferroyttrium 115
4.3 Fabrication of Experim ental Electrodes . 115
4.3.1 Tubular Wire Making Facility at Colorado School of Mines . 116
4.3.2 Forming and Closing the Strip and Adding Flux in the Wire . 117
4.3.3 Concerns for Flux-Cored Wire Making 119
4.4 Welding Procedures . 121 0
4.5 Macroscopic Analysis 124
4.6 Weld Bead Chemical Analysis 124
4.6.1 Inductively Coupled Plasm a . 126
4.6.2 Diffusible Hydrogen Analysis 126
4.6.2.1 Weld Test A ssem bly 127
4.6.2.2 Welding Fixture 127
4.6.2.3 Welding A nalysis . 128
4.6.2.4 Diffusible Hydrogen Measurements 129
4.7 Light Optical Microscopy 130
S4.8 Mechanical Properties Evaluation . 131
4.8.1 Charpy V-Notch Impact Testing . 131
4.8.2 Tensile Testing 134
4.8.3 Hardness Measurements . 136
CHAPTER 5: RESULTS AND DISCUSSION 1379
5.1 Integrating Fluoride Additions to Yttrium- Containing Consumnables. 1370
5.1.1 Welding Characteristics. 137
5.1.2 Weld Bead Morphology Analysis 140
5.1.3 Microstructural Analysis 143
5.1.4 Chemical Analysis. 151
5.1.5 Inclusion Analysis 156
5.1.6 Summary of the Integration of Fluorides to Yttrium Containing Consumnables 164
5.2 Integrating Additional Flux Ingredients to Yttrium Containing Consumnables . 164
5.2.1 Improving Molten Weld Metal Fluidity with Ferrosilicon 165
5.2.2 Addition of Other Flux Ingredients to Yttrium Containing Consumnables. 1680
5.3 Integration of Flux Ingredients Through the Use of a Paste . 1710
5.3.1 Welding Characteristics. 1710
5.3.2 Weld Bead Morphology 1720
5.3.3 Microstructural Analysis 1730
5.3.4 Chemical Analysis. 175
5.3.5 Inclusion Analysis . 177
5.4 Summary of Consumnables Selected for Further Observations. 178
5.5 Diffusible Hydrogen Measurements . I 186
5.6 Comparisons of Mechanical Properties 188
CHAPTER 6: CONCLUSIONS 1979
CHAPTER 7: RECOMMENDATIONS FOR FUTURE WORK 1990
CHAPTER 8: REFERENCES CITED . 201