Friction Welding for Cladding Applications: Processing, Microstructure
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FRICTION WELDING FOR CLADDING APPLICATIONS: PROCESSING, MICROSTRUCTURE AND MECHANICAL PROPERTIES OF INERTIA FRICTION WELDS OF STAINLESS STEEL TO LOW CARBON STEEL AND EVALUATION OF WROUGHT AND WELDED AUSTENITIC STAINLESS STEELS FOR CLADDING APPLICATIONS IN ACID- CHLORIDE SERVICE by Nathan Switzner Copyright by Nathan Switzner 2017 All Rights Reserved A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Metallurgical and Materials Engineering). Golden, Colorado Date _______________________________ Signed: _______________________________ Nathan Switzner Signed: _______________________________ Dr. Zhenzhen Yu Thesis Advisor Golden, Colorado Date _______________________________ Signed: _______________________________ Dr. Angus Rockett Professor and Head George S. Ansell Department of Metallurgical and Materials Engineering ii ABSTRACT Friction welding, a solid-state joining method, is presented as a novel alternative process step for lining mild steel pipe and forged components internally with a corrosion resistant (CR) metal alloy for petrochemical applications. Currently, fusion welding is commonly used for stainless steel overlay cladding, but this method is costly, time-consuming, and can lead to disbonding in service due to a hard martensite layer that forms at the interface due to partial mixing at the interface between the stainless steel CR metal and the mild steel base. Firstly, the process parameter space was explored for inertia friction butt welding using AISI type 304L stainless steel and AISI 1018 steel to determine the microstructure and mechanical properties effects. A conceptual model for heat flux density versus radial location at the faying surface was developed with consideration for non-uniform pressure distribution due to frictional forces. An existing 1-D analytical model for longitudinal transient temperature distribution was modified for the dissimilar metals case and to account for material lost to the flash. Microstructural results from the experimental dissimilar friction welds of 304L stainless steel to 1018 steel were used to discuss model validity. Secondly, the microstructure and mechanical property implications were considered for replacing the current fusion weld cladding processes with friction welding. The nominal friction weld exhibited a smaller heat softened zone in the 1018 steel than the fusion cladding. As determined by longitudinal tensile tests across the bond line, the nominal friction weld had higher strength, but lower apparent ductility, than the fusion welds due to the geometric requirements for neck formation adjacent to a rigid interface. Martensite was identified at the dissimilar friction weld interface, but the thickness was smaller than that of the fusion welds, and the morphology was discontinuous due to formation by a mechanism of solid-state mixing. Thirdly, the corrosion resistance of multiple austenitic stainless steels (types 304, 316, and 309) processed in varying ways was compared for acid chloride environments using advanced electrochemical techniques. Physical simulation of fusion claddings and friction weld claddings (wrought stainless steels) was used for sample preparation to determine compositional and microstructural effects. Pitting resistance correlated firstly with Cr content, with N and Mo additions providing additional benefits. The high ferrite fraction of as-welded samples reduced their corrosion resistance. Wrought type 309L outperformed as- welded type 309L in dissolved mass loss and reverse corrosion rate from the potentiodynamic scan in 1.0 N HCl/3.5% NaCl solution. Electrochemical impedance results indicated that wrought 309L and 316L developed a corrosion resistant passive film more rapidly than other alloys in 0.1 N HCl/3.5% NaCl, and also performed well in long term (160-day) corrosion testing in the same environment. Fourthly, to prove the concept of internal CR lining by friction welding, a conical workpiece of 304L stainless steel was friction welded internally to 1018 steel. iii TABLE OF CONTENTS ABSTRACT ................................................................................................................................................. iii LIST OF FIGURES ..................................................................................................................................... xi LIST OF TABLES ..................................................................................................................................... xvi LIST OF SYMBOLS ............................................................................................................................... xviii ACKNOWLEDGEMENTS ..................................................................................................................... xxiii CHAPTER 1 : INTRODUCTION ................................................................................................................ 1 1.1 Industrial Relevance .......................................................................................................... 1 1.2 Motivation for the Research .............................................................................................. 1 1.3 Current Processes for Producing CR Lined Pipe .............................................................. 1 1.4 A Proposed Process for Producing CR Lined Pipe ........................................................... 4 1.5 Objectives ......................................................................................................................... 4 1.6 Approach ........................................................................................................................... 5 CHAPTER 2 : LITERATURE REVIEW ..................................................................................................... 6 2.1 Cladding of Steel Pipe for Corrosion Resistance .............................................................. 6 2.2 Dissimilar Welding and Fusion Cladding Issues .............................................................. 6 2.2.1 Power Plant Applications ............................................................................................. 6 2.2.2 Nuclear Applications ................................................................................................... 7 2.2.3 Petrochemical Processing Applications: ...................................................................... 7 2.2.4 Partial Mixing and Martensite Formation .................................................................... 8 2.3 Friction Welding ............................................................................................................... 9 2.3.1 Thermal Models of Friction Welding ........................................................................ 11 2.3.2 Thermo-Mechanical Models ...................................................................................... 12 2.3.3 Friction Welding of Stainless Steels .......................................................................... 13 2.3.4 Friction Welding of Low Alloy Steels ....................................................................... 14 2.3.5 Friction Welding of Stainless Steel to Low Alloy Steel ............................................ 14 2.4 Corrosion of Stainless Steel ............................................................................................ 16 iv 2.4.1 Thermodynamics of Stainless Steel Corrosion .......................................................... 16 2.4.2 Kinetics of Stainless Steel Corrosion ......................................................................... 19 2.4.3 Passivation of Stainless Steels ................................................................................... 24 2.4.4 Localized Corrosion and Pitting of Stainless Steel .................................................... 25 2.4.5 Pitting Resistance Equivalence Number .................................................................... 26 2.4.6 Acid Chloride Environments in Petrochemical Processing ....................................... 28 2.5 Corrosion Testing of Stainless Steels .............................................................................. 29 2.5.1 Electrochemical Corrosion Testing ............................................................................ 29 2.5.2 Mass Loss Studies ...................................................................................................... 34 CHAPTER 3 : EXPERIMENTAL PROCEDURES ................................................................................... 35 3.1 Comparison of IF Butt Welding and Fusion Weld Cladding .......................................... 35 3.1.1 IF Butt Welding ......................................................................................................... 35 3.1.2 FCA Cladding ............................................................................................................ 41 3.1.3 IF and FCA Weld Interface Macrographs .................................................................. 42 3.1.4 Micrographs ............................................................................................................... 42 3.1.5 Hardness Mapping ..................................................................................................... 43 3.1.6 Hardness Testing and Tracing ...................................................................................