INFLUENCE OF CHEMISTRY AND PROCESSING PARAMETERS ON YIELD RATIO OF LINEPIPE STEEL GRADE PIPES April , 2016 Middle East Steel Conference, Dubai

No. 1 INTRODUCTION Increasing demand of oil and gas Geographic location Stringent requirements on properties Safety of pipe line American Petroleum Institute (API) specification allows YS/TS ratio up to 0.93 Pipe manufacturers requirement is based on type of pipe production. The yield ratio (yield to tensile strength ratio) is governing factor for the safety, as low yield ratio indicates higher resistance to deformation from yielding to plastic instability in material.

ERW PIPE MANUFACTURING PROCESS

No. 3 ERW PIPE MANUFACTURING PROCESS Electric welded pipes are defined as pipes having one longitudinal seam formed by electric resistance or electric induction welding, without the addition of filler metal the edges to be welded are mechanically pressed together and the heat for welding is generated by the resistance to flow of electric current. low frequency process, welding is performed with alternating current frequencies from 50 to 400 Hz, longitudinally welded pipes from 10 mm to 114 mm in diameter. high frequency process involve application of the high frequency alternating current in range of Hz for welding, High frequency current has effect of building up magnetic field at the center core of contactor

No. 4 ERW PIPE MANUFACTURING PROCESS The contactor’s Ohmic resistance is highest in the field so that electron takes the path of least resistance at outer region of contactor giving skin effect. Thus current flow along the strip edges of open seam tube to reach to welding point. Curie temperature, it becomes non- magnetic and the depth of penetration rises to several tenths of millimeter. This helps in heating edges quickly to welding point and with applying optimum pressure, edges are welded.

No. 5 ERW PIPE MANUFACTURING PROCESS Figure Shows High frequency induction welding Welding Generator Squeeze Rolls Welding Gap Entry Angle Open Seam Tube Induction Coil Welding Point Weld

FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING

No. 7 FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING 1. Chemical Composition Carbon: Carbon promotes ferrite –pearlite structure and has strong influence on yield strength of material. Carbon in chemical composition can be increased, if the toughness requirements are not stringent like in case of grade J55. It is observed that higher the % carbon, lower is the ratio.

No. 8 FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING 1.Chemical Composition Molybdenum: with addition of Molybdenum the CCT curves shift to right indicating formation of acicular ferrite even at lower cooling rate. Figure shows the shift in CCT curve with addition of Molybdenum to steel. Mo: 0.12 Mo: 0.001

No. 9 FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING 1. Chemical Composition Niobium: Niobium strengthens the steel by precipitation hardening. Due to pan- caking phenomenon in Finishing Mill grain size is very fine and thus increases Yield Strength adversely affects Yield Ratio. Evolution of Grain structure during Rolling Original grains before rolling Deformed Elongated grains after rolling New Grains nucleation New Grains growing Recrystallized Grains

No. 10 FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING 2. Prior deformation of austenite Deformation of austenite below T nr has very strong influence on the YS/TS ration (YR) of the coil. Due to prior deformation, presence of higher density of dislocations in deformed austenite, appears to offer higher driving force for formation of acicular ferrite Deformation prior to transformation is far more effective in increasing fraction of AF than addition of Mo and increasing cooling rates. Prior deformation below T nr do not necessarily survive phase transformation to acicular ferrite, however induces finer and dislocated pancaked austenite grain size, which will provide higher nucleation rate of acicular ferrite and/or lead to finer ferrite grain size. The dislocation, presence of AF and fine grain acts as obstacles during plastic deformation and hence results in higher yield strength of steel and thereby raising the YR

No. 11 FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING 3. Cooling Rate of hot rolled strip Both YS and TS increases with increasing cooling rates. At lower cooling rates microstructure mainly consist of polygonal ferrite and pearlite At higher cooling rates ferrite and acicular ferrite are dominant, hence material strength increases. The volume fraction of Polygonal Ferrite (PF) decreases with increasing cooling rates. Volume fraction of PF/AF ratio decreases with increasing cooling rates and thus YS/TS ratio increases

No. 12 FACTORS AFFECTING YS/TS RATIO (YR) DURING HOT ROLLING 4. Hot rolled Strip Coiling Temperature Coiling temperature of Ti-Nb micro-alloyed grade does not have much effect on the YS/TS ratio, if coiled in °C In presence of Molybdenum and prior deformation of austenite before transformation, increases volume fraction of acicular ferrite and hence YR increases with lowering coiling temperatures.

FACTORS AFFECTING YS/TS RATIO (YR) DURING PIPE MANUFACTURING

No. 14 FACTORS AFFECTING YS/TS RATIO (YR) DURING PIPE FORMING 5. Influence of thickness to diameter (t/D) ratio 1.The wall thickness to diameter ratio t/D and strain hardening rate during cold forming have influence on the change in yield strength and its sign 2.Generally, in ERW pipes, with lower the t/D ratio, drop in yield strength than the original strength is observed due to Bauschinger effect 3.Whereas, for higher t/D ratio, gain in the yield strength is observed due to work hardening Graphical representation of effect of t/D on the yield ratio.

No Deformation in pre-forming and forming of pipe To reduces the tooling cost as well as production loss due to change of breakdown rolls set up for each size, most of the pipe manufacturers use common portion of tooling set to produce multiple sizes of pipe within a given rage with same set up. Sharing of common breakdown set up is possible as fin passes are designed to work on strip edges and on correct shape of the pipe prior to welding. The range of pipe size, which can share common breakdown passes should not exceed 20%. Using correct width is critical to produce good tube with respect to dimensional tolerance and also for the mechanical properties FACTORS AFFECTING YS/TS RATIO (YR) DURING PIPE FORMING

No Deformation in pre-forming and forming of pipe While calculating strip width for certain diameter, factors like initial trimming allowance in strip, thickness, material grade, welding allowance (diameter of pipe at the welding unit) and number of fin passes should be considered It is critical to calculate the amount of stretch during forming in tooling design to reach to correct diameter of pipe. The amount of stretching during forming also alters the mechanical properties of material due to strain hardening. Low strength material with better elongation can be spread more during forming but is also prone to increase yield strength due to work hardening FACTORS AFFECTING YS/TS RATIO (YR) DURING PIPE FORMING

No. 17 CONCLUSION 1. The major factors influencing YS/TS ratio (YR) in steel production are chemical composition, prior deformation of austenite during rolling, cooling rate and coiling temperature 2. The important factors influencing YS/TS ratio (YR) in pipe manufacturing are thickness to diameter ratio (t/D), helix angle in case of spiral pipes, forming forces during pre-forming and forming of pipes contributes to control YS/TS ratio. 3. The YS/TS ratio can be Kept on lower side by controlling processing parameters at steel manufacturing stage by selecting appropriate chemical composition and process parameters 4. To avoid strain hardening in ERW Pipes, manufacture should limit amount of spread in width during forming.

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