FLUX-CORED ARC WELDING OF HIGH ' STRENGTH LOW ALLOY STEEL
by
NASROLLAH BANI MOSTAFA
A THESIS SUBMITTED
IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
`oe
f o y4
°1 (%
2 ,
Department of Mechanical Engineering
INDIAN INSTITUTE OF TECHNOLOGY, DELHI INDIA
August, 1992
CERTIFICATE
This is to certify that the thesis entitled, "FLUX- CORED 4.RC WELDING OF HIGH STRENGTH LOW ALLOY STEEL " being submitted by Mr.N.B.MOSTAFA to the Indian Institute of Technology, Delhi, for the award of the degree of "Doctor of Philosophy" in Mechanical Engineering is a record of bonafide research work carried out by him. He has worked under my guidance
& supervision and has fulfilled the requirement for the submission of this thesis which has reached the requisite standard.
The results contained in this thesis have not been submitted, in part or in full, to any other university or Institute for the award of any degree or diploma.
a G_
Prof.(Dr.) R.S.Parmar, Department of Mechanical Engineering, Indian Institute of Technology, New Delhi- 110 016, INDIA.
ii ACKNOWLEDGEMENTS
I would like to express my gratitude and thanks to Dr. R.S.
Parmar, Professor of Mechanical Engineering Department, Indian Institute of Technology,Delhi, for his guidance, encouragement and understanding without which it would have not been possible to accomplish this work.
Professors N.K. Tiwari, Head and U.R.K. Rao of the Mechanical Engineering Department are thanked for their valuable help extended in providing the necessary materials and facilities at various stages of this work.
Thanks are also due to Mr. B. Majumdar of the Indian Statistical Institute and my colleagues Messrs. A. Mandal, T.
Srihari, V.K. Gupta and N. Murgan for rendering all possible help and fruitful discussions.
I also thank Messrs. Shiv Kumar and Mahender Singh of the Welding Lab. and all other supporting staff of Production and Metrology Labs. for their cooperation and help in conducting the experiments.
I am also thankful to Messrs. P. Thakkar of ITMMEC and Raina of Applied Mechanics Department for extending their help in carrying out the experiments on microhardness and residual stresses respectively.
Messrs. D.C. Sharma and R.K. Arora, Department of Textile Technology are thanked for their help in taking SEM
iii photomicrographs and typing of the thesis respectively.
Finally, I sincerely thank my parents and wife who stood by me during difficult times and provided a lot of moral support and encouragement in accomplishing this work.
August 1992 N.B. Mostafa
iv
ABSTRACT
A grade of steels known as Titsen-55 high strength low alloy (HSLA) or microalloyed steel manufactured indigenously in India has found an increasing engineering applications because of its better mechanical properties and greater resistance to atmospheric corrosion than carbon steels This steel is frequently incorporated in structural and low-temperature services such as bridges, storage tanks & spheres, pipelines, etc. Although welding is the process most often employed for fabrication of these components, but in spite of this fact not much information exists on the welding aspects of this HSLA steel. This lack of information regarding the different aspects of welding of this grade of steel in particular in conjunction with flux-cored arc welding (FCAW) process was the driving force behind the present research work .
To investigate into the different aspects of welding of Tisten-55 microalloyed steel, experiments were conducted using the two versions of gas-shielded FCAW viz., constant potential and pulsed-current FCAW under different combinations of wire diameter and shielding gas composition
The quality of a joint depends on its mechanical properties which in turn are determined by weld joint dimensions as well as joint microstructure . Therefore to ensure an adequate welded joint, these dimensions which are influenced by various welding parameters must be predicted and their optimum values be
V
selected . To accomplish the aim of predicting the effects of welding variables such as welding current, arc voltage, peak current, electrode-to-work angle, etc. on the weld bead dimensions like width, depth of penetration, etc.; mathematical models were developed through the use of the statistical technique of central composite rotatable design . These models which have been developed under different wire diameter - shielding gas - FCAW process combinations are able to predict the main or linear, quadratic , and two-way interaction effects of various welding parameters on the resulting weld bead dimensions.
Another important factor influencing the mechanical properties of a weld is its type of microstructure . Different microstructures exhibit different hardness values If the hardness values of welded joints are very high, they indicate the susceptibility to cracking and subsequent failure of the structure . Hence, the microhardness survey of weld beads deposited under different conditions was conducted using Vickers method of hardness testing . The :weld beads deposited with pulsed-current FCAW process recorded a maximum microhardness value of about 500 VHN in the coarse-grain heat-affected zones . The maximum hardness in the coarse grain HAZ decreased with increase in the heat input.
The metallographic observation of the welds using different etching reagents was carried out to obtain black & white, and colour photomicrographs of the various zones of the weldments Optical and scanning electron microscopes were used to examine
Vi
the presence of different phases at different magnifications. The presence of hard microstructures such as bainite and martensite was observed in the coarse-grain region of heat-affected zones which were responsible for the high hardness values of these zones .
The Charpy-impact testing of some of the welded joints was also conducted. The fractured surfaces of the samples showed mixed mode of fracture under scanning electron microscope . Welds deposited under 75% Ar + 25% CO2 exhibited lower transition temperature and higher upper shelf energy than those deposited under 100% CO2.
Finally the residual stresses developed due to welding which may cause failure of a welded structure were studied . Magnitude and distribution of longitudinal residual stresses in some of the welded plates were determined using the centre-hole drilling technique . The results obtained indicated the presence of residual tensile stresses in the plates upto a distance of about 55 mm from the weld centreline beyond which the residual tensile stresses changed over to residual compressive stresses . Increase in heat input and nozzle to plate distance increased the residual stresses and width of the tension zones slightly.
CONTENTS
Page
CERTIFICATE --- i
ACKNOWLEDGEMENTS
ABSTRACT --- iv-vi
CONTENTS --- vii-x
CHAPTER-1 INTRODUCTION --- 1-24
1.1 Introduction --- 1
1.2 High Strength Low Alloy (HSLA) --- 1-4 steels
1.3 Welding of HSLA Steels --- 4-5 1.4 Welding Process Selection --- 6-7 1.5 Historical Development --- 7-9
of Flux-Cored Wires
1.6 Flux-Cored Arc Welding (FCAW) --- 9 Process
1.7 Flux-Cored Electrode Wires --- 10-11
1.8 Shielding Gases --- 11-12
1.9 Flux-Cored Arc Welding Power --- 12 Sources
1.10 Specific Aspects of Investigation --- 13-20 1.11 Plan of Research Work --- 20-21
1.12 References --- 22
Table --- 23
Figure ---
24vii
viii
CONTENTS(Cont.)
CHAPTER-2 LITERATURE SURVEY --- 25-60
2.1 Introduction --- 25
2.2 High Strength Low Alloy (HSLA) --- 25-35 Steels
2.3 Weld Bead Geometry and Shape --- 35-47 Relationships Prediction
2.4 Metallurgical and Fracture --- 47-53 Toughness Studies
2.5 Study of Residual Stresses --- 53-58
Figures --- 59-60
CHAPTER-3 EXPERIMENTAL DETAILS AND ESTABLISHMENT --- 61-77 OF OPERATING RANGES OF PROCESS VARIABLES
3.1 Introduction --- 61
3.2 Materials --- 61-63
3.3 Power Source --- 63-65
3.4 Electrode wire-Shielding Gas- --- 66 Welding Process
Combinations3.5 Experimental Set-up --- 66 3.6 Identification and Selection --- 66-67
of Welding Variables
3.7 Establishment of Operating --- 67-70 Ranges of Factors
3.8 Pulse Parameters Relationships --- 70
Tables --- 71-73
Figures --- 74-77
CONTENTS(Cont.)
CHAPTER-4 DEVELOPMENT OF MATHEMATICAL MODELS --- 78-170 FOR WELD BEAD GEOMETRY PREDICTION
4.1 Introduction --- 78-79
4.2 Design of Experiments --- 79-80
4.3 Definitions --- 80-81
4.4 Factorial Design of Experiments --- 81-83 4.5 Central Composite Rotatable Designs --- 83-85 4.6 Development of Design Matrix --- 85-87 4.7 Plan of Investigation --- 87 4.8 Experimental Procedure --- 88 4.9 Selection of Mathematical Model --- 88-89 4.10 Estimation of the Coefficients --- 90
of the Models
4.11 Checking the Adequacies --- 91-92 of the Models
4.12 Testing the Significance of the --- 92-94 Coefficients of the Models
4.13 Analysis of Results --- 94-117
Tables --- 118-140
Figures --- 141-170
CHAPTER-5 METALLURGICAL AND FRACTURE --- 171-219 TOUGHNESS STUDIES
5.1 Introduction --- 171-173
5.2 Classification of a Weldment --- 173-176
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