Suction Roll Material Comparison 11th International Symposium on Corrosion in the Pulp & Paper Industries Paul E Glogowski Metso Paper Aiken, SC

Nine Principal Material Features 1 Microstructure 2 Chemical composition 3 Mechanical properties 4 Corrosion resistance 5 Corrosion fatigue 6 Residual stress 7 Threshold fatigue crack growth 8 Thermal Fatigue Resistance 9 Experience This morning I will be talking about the nine material features that are important in selecting a suction roll material. These features will be demonstrated using the contemporary premium alloys available today. Metso designed MetShell Pr is designated A, Sandusky Alloy 86 is B and Kibota ACX-100 is C. These duplex stainless steel alloys are the available as suction rolls today. Other stainless steel alloys and bronze are also available. I will give a brief explanation of the 9 features and some recent results. A more indepth presentation on the material science will be given at11th International Symposium on Corrosion in the Pulp and Paper Industries in Charleston. All of these factors are important in designing and or selecting a suction roll material First a quick look at the two types of shell blank manufacturing.

Hot Rolled Plate Slab ready Refining in CLU-converter Melting in electric arc furnace Refining in CLU-converter Slab ready Hot Rolling to plate Slab heating Continuous casting to slab Converter treated low carbon duplex stainless steel is cast to 300 mm (12”) thick slab. After actual shell order the slab will be hot rolled into thickness which equals to wall thickness of the shell added with small (5-7 mm (2-2.5”) / surface) working allowance. The cutting length of one plate equals to circumferential length of the shell. Due to dimensions of hot rolling machine the width of the plate is app. 2800 mm (110”). After cooling down to room temperature the quality of the plate will be inspected by 100% UT-inspection. If defects are found the plate will be rejected or defects will be repaired by specified welding procedure. Inspection 39831-90

Shell Blank Hot preforming Cutting inc. test piece Hot forming Joint preparation and welding of circumferential welds (SAW) Joint preparation and ES-welding of longitudinal weld Hot Forming Depending on the final length of the shell, the shell blank will be fabricated from 2-5 segments. Axial weld seams are welded with Electro Slag (ES) welding. In ES-welding process the I-shaped joint will be filled with one pass. The filler metal is having a matching chemistry so the final microstructure after heat treatment will be equal to base metal. Circumferential weld seams are welded with Submerged Arc Welding (SAW) method. The first pass is done on the inner surface followed by multipass outside welding. During welding the shell blank is rotating on a roller so the welding position will remain same all the time. After welding all seams are inspected by ultrasonic inspection. Heat treatment 100% NDT of welds (x-ray or isotope)

Centrifugal Casting Melting in electric arc furnace Refining in AOD-converter Centrifugal casting into rotating mold Solidification and cooling in rotating mold Taking the shell out of mold Heat treatment Inside and outside machining The pouring of molten steel is done on the inner surface of a rotating mould. Due to high rotational speed the centrifugal force is keeping the steel on the surface of the mold. The solidification starts on the surface of the mold and it moves towards inner surface of the shell. The idea is that moving solidification front pushes all impurities to the inner surface where they can be removed by machining. Centrifugally cast shells are heat treated in horizontal position. The final machining is performed after heat treatment and if any new casting defects are found they can be removed by grinding. Repair by welding is not allowed after heat treatment.

Microstructure Side by side comparison of hold rolled A and cast microstructures. Note the uniform grain structure throughout the thickness of the hot rolled material. Hot rolling result in re-crystallization of the microstructure, in other words the grains are broken and reformed, producing a fine homogeneous structure. Subsequent, heating and cooling to form the shell blank(thermal history) modifies the structure through grain growth Centrifugal casting usually results in columnar grains at the OD, equiaxed grains towards the ID. This is a direct result of the casting procedure and does not change duing the manufacturing process

Chemical Composition Carbon (C): Ferrite former. Provides added strength and mechanical properties. Can cause precipitation of chromium carbides during heat treatment and welding, resulting in intergranular corrosion. Manganese (Mn) Austenite former. Added for strength and as a flux to tie up sulfur and other contaminants. Silicon (Si):Provides some additional strength and fatigue resistance. Chromium (Cr):Ferrite former. Through oxidation creates the clear thin passive film on stainless steels (13% chromium required). The oxide film is susceptible to attack by chloride ions. Nickel (Ni):Austenite former. Lends to ductility of an alloy. Adds to corrosion resistance because it is more noble. Copper (Cu):Can enhance strength. Typically precipitates out as a separate phase. Provides A86 with added chloride ion and thiosulfate ion corrosion resistance. Molybdenum (Mo):Austenite former. Provides chloride ion corrosion resistance. Additions reduce both pitting and crevice attack. Alloys containing molybdenum require rapid cooling during heat treatment to prevent the formation of brittle and deleterious phases.

Chemical Composition The pitting resistance equivalence number (PREn) is based on alloy chemistry. This number can compare pitting resistance of several alloys based on results in a standard ferric chloride solution. PREn = Cr% + 3.3 X Mo% + 16 X N%). As can be seen from the equation, the additions of molybdenum and nitrogen greatly increase the pitting resistance of an alloy.

Mechanical Properties Duplex stainless steels give high strength and corrosion resistance. Austenitics like 316, give good corrosion resistance but low strength, Ferritics and martensitics , give good strength but low corrosion resistance.

Pitting Electrochemistry Log Current, amps (corrosion rate) Potential, volts Ecorr Electrochemical Corrosion Scan in typical newsprint whitewater Environment pH = 4.5 Cl- = 100 ppm Thiosulfate ion (S2O3=) = 40 ppm Temperature = 140°F (60°C) Conductivity = 4000 microsiemens Ecorr is the corrosion potential about -450 millivolts (MV) In the example on the left, 3RE60 is placed in the above environment and the potential is measured compared to a standard electrode (H)

Pitting Electrochemistry Ecorr Log Current, amps (corrosion rate) Potential, volts Epit Erepassivate Electrochemical Corrosion Scan in typical newsprint whitewater Electrochemical Corrosion Scan in typical newsprint whitewater For suction roll alloys, many manufacturers report the margin of safety (MS) for the stainless steel. For our example, we have a pitting MS of 1250 mV or 1.25 volts. A more conservative MS would be the pit repassivation which is 1150 mV or 1.15 volts for this example. Margin of Safety Ecorr Electrochemical pitting test, explanation of terms

Electrochemical Corrosion Test Results

Corrosion Fatigue Strength Alloy tensile strength Microstructure Environment Frequency Higher tensile strength relates to higher fatigue strength (to crack initiation) Smaller grains give higher strength and higher corrosion fatigue strength. The more corrosive the environment especially combined with slower test frequencies will cause lower CF strength for the same alloy.

Corrosion Fatigue Strength Test Methods Tatnal-krause reverse plate bending A drilled hole to simulate the stress concentration from suction roll drilled holes (KT = 2) R.R. Moore rotating bending A better simulation of a suction roll. Stress concentrations can be increased by notching the specimen Previous work by Bowers and Butterfield for TAPPI showed no correlation in the different test methods So, you have to be careful when comparing corrosion fatigue strength data.

Corrosion Fatigue -LOW pH -CHLORIDES -SULPHUR COMPOUNDS -HIGH TEMPERATURE -POOR ROLL CLEANING -CHEMICALS AND DETERGENTS -FEQUENCY LAB FASTER THAN MACHINE -DIFFERENCE BETWEEN LAB TESTS AND ACTUAL CONDITION S T R E S S -RESIDUAL STRESS -THERMAL STRESSES -ABNORMAL LOADS -DEFECTS IN SHELL Factors affecting roll performance The corrosion fatigue strength of a suction roll shell material is defined in laboratory under standardized test environment. The frequency of the test is usually between 1750 and 4000 cpm, Shells run at about 250 up to 500 rpm in a tissue machine. However the corrosion fatigue strength is always dependent on current chemical environment. Dimensions of suction roll shell are designed according to external mechanical load (Nips, vacuum level, felt tension) of the roll. Theoretically the maximum stress level is located on the edges of suction holes on the mid area of the shell. In reality there are factors which may locally increase the calculated operating stress level of the shell. In order the reach acceptable shell life time (109 cycles), the shell is designed so that there is always a certain factor of safety against corrosion fatigue crack initiation and propagation. Due to specific nature of corrosion fatigue there is no sharp fatigue limit which could be used as a design criteria. Lifetimes of the shells: - For stress limited positions (Press & some couch positions). 10 exp. 9 cycles = Calculated lifetime is about 7-10 years in operation (= calendar time is about. Double). - For bending limited positions (pick-up, transfer, forming- and some couch positions) lifetime of the shell is much longer because the stress level is low. CALCULATED OPERATING STRESS LEVEL 9 10 C Y C L E S

Published Corrosion Fatigue Strengths Estimated at 1 Billion cycles That said, the published data on the top three alloys shows good corrosion fatigue strength in the same environment

Residual Stress Sach’s test Destructive expensive For ID measurement gages are mounted on OD and sample is incrementally bored For OD measurement strain gages are mounted on ID and sample is incrementally turned The best way to determine residual stress is through the Sach’s test. Strain gages are mounted as shown, and the shell is incrementally machined away,with strain gage readings after every step The changes in strain are converted to stress by the following equations

Sach’s Residual Stress Calculations Equations for longitudinal and circumferential stresses: turning nL = - E/1-2 [(n-N/2)(n+1 - n-1+n] nC = - E/1-2 [(n-N/2)( n+1 - n-1+ (n+N/2n) n] boring nL = - E/1-2 [(n-N/2)( n+1 - n-1+(n+N/2n) n] nC = - E/1-2 [(n-N/2)( n+1 - n-1+ n} I won’t spend any time in this. Suffice it to say, that the cross wall residual stress profile is accurately determined by this method

Sach’s Residual Stress Calculations nL & nC = longitudinal and circumferential stress values for the removed layer n  = the change in longitudinal strain plus the quantity of Poisson’s ratio times the circumferential change in strain  = the circumferential change in strain plus the quantity of Poisson’s ratio times the longitudinal change in strain.

Residual Stress As a Function of Heat Treatment Typical residual stress profiles based on heat treatments. All data except fog quench were measured by Sach’s residual stress test method. The fog quench curve was interpolated from the air and water quench data. Short term failures of water quenched A63 were attributed to the high residual stresses from water quenching. Beloit and Sandusky went to very low residual stress alloys, intermediate stress levels, were never used, the success of SRG and MetShell Pro how that it is possible to design and run a suction roll with intermediate levels of residual stress

Turning Sample Boring Sample Strain gage Strain gage Strain gage

Comparison of Residual Stress to Ultimate Tensile Strength The 3 top alloys have modest residual stress when compared to UTS, On the otherhand, A63 was at 50%

Typical Stages of Fatigue Metals fail by this general rule. Corrosion fatigue testing generally determines the time to crack initiation. While Fatigue crack growth rate measurements determine the stress field necessary for crack growth in Phase 1. Phase 2 or the Paris Law regimen show a linear crack growth with a linear increase in the log of the stress cycles. This regimen is commonly used by material scientist to determine the remaining useful life. However, in suction rolls, this linear crack growth regimen cannot be used. Crack growth rates are too high and failure would occur in a matter of weeks.

Threshold Fatigue Crack Growth DEFINITIONS DK = The driving force or stress intensity factor DK is a scale factor which defines the magnitude of the stress field at the crack tip Crack growth rate, da/dn = The amount of crack growth per load cycle Threshold fatigue crack growth has been used to try and predict the life of new suction roll alloys. It is a materials science term that defines the stress field at a crack tip.

Threshold Fatigue Crack Growth Driving force for crack (or flaw) growth Instantaneous length of crack at any time, t Geometry of cracked material The magnitude of cyclic loading Dkthreshold = Ds(acritical *)1/2 Where: Ds = range of cyclic stress acritical = 1/2 the critical flaw size required for fatigue crack growth Factors that affect FCGR

Threshold Fatigue Crack Growth Test Environments Test environments typically reported in NA

Threshold Fatigue Crack Growth K = [P/B(W1/2)] [(2+)/(1-)3/2 ] (0.866+4.64-13.322 + 14.723 - 5.64) B W a P where: P is the range of load  is the crack length B is the specimen thickness W is the specimen width Slide showing the compact tension specimen configuration used for testing the Threshold fatigue crack growth rate of MetShell Pro. Also shown is the specimen orientation as removed from the test shell.

Threshold Fatigue Crack Growth Specimen Orientation Slide showing the compact tension specimen configuration used for testing the Threshold fatigue crack growth rate of MetShell Pro. Also shown is the specimen orientation as removed from the test shell.

Threshold Fatigue Crack Growth

Threshold Fatigue Crack Growth

Critical Flaw Size Example comparing the critical flaw size required for crack growth when the cyclic stress range is 55 MPa (8000 psi) this does not take into consideration the residual stress. Note that for the higher the threshold fatigue crack growth value, the larger flaw size is required for crack growth. To consider residual stress, the ratio of 0.5 could be used. Consistent with this is the fact that the failure rate of MetShell Pro shells is low. Failure rates of suction roll shell materials with low threshold stress intensity factors (VK-A378) are considerably higher than either A86 or MetShell Pro.

Critical Flaw Size (cyclic stress range 55 MPa, 8000 psi) The increase in threshold stress intensity factor for the commercially prepared suction roll shell is related to the additional thermal processing steps involved in making a shell from plate. Recall that the plate is heated to 1100°C and the edges are formed, then heated again to roll the cylindrical segment. Electroslag longitudinal welding is used to close the cylinder. The segment is then reheated and rolled to final shape. Submerged arc welding forms the length of the shell. The entire fabrication is then placed in a vertical furnace and heat-treated at 1010°C. The temperature is maintained for 1 hour per 50 mm of wall thickness. The shell is then immediately air-cooled. The entire process causes further refinement and an increase in the grain size. Since grain size is important in fatigue crack propagation, the increased grain size results in threshold stress intensity factors greater than the cast shells. In comparing the critical flaw size required for crack growth when the cyclic stress range is 55 MPa, the higher threshold stress intensity factor results in a larger critical flaw size. You will also notice that the critical flaw size is similar independent of the cracking direction. Plate rolling has not led to anisotropy in the material properties. This is another indication of the microstructure refinement as a result of the thermal history. It should also be noted that the manufacturing process and quality control employed during fabrication of MetShell Pro rules out defects of this size.

Critical Stress Required for Crack Growth Comparison of some alloys and the previous test results. Linear regression analysis in the low crack growth regimen was conducted to determine the threshold values. Data for MetShell Pro is different from the 3RE60 plate because of the increased grain size.

Threshold Fatigue Crack Growth Critical Stress for a 2.54 mm edge flaw The increase in threshold stress intensity factor for the commercially prepared suction roll shell is related to the additional thermal processing steps involved in making a shell from plate. Recall that the plate is heated to 1100°C and the edges are formed, then heated again to roll the cylindrical segment. Electroslag longitudinal welding is used to close the cylinder. The segment is then reheated and rolled to final shape. Submerged arc welding forms the length of the shell. The entire fabrication is then placed in a vertical furnace and heat-treated at 1010°C. The temperature is maintained for 1 hour per 50 mm of wall thickness. The shell is then immediately air-cooled. The entire process causes further refinement and an increase in the grain size. Since grain size is important in fatigue crack propagation, the increased grain size results in threshold stress intensity factors greater than the cast shells. In comparing the critical flaw size required for crack growth when the cyclic stress range is 55 MPa, the higher threshold stress intensity factor results in a larger critical flaw size. You will also notice that the critical flaw size is similar independent of the cracking direction. Plate rolling has not led to anisotropy in the material properties. This is another indication of the microstructure refinement as a result of the thermal history. It should also be noted that the manufacturing process and quality control employed during fabrication of MetShell Pro rules out defects of this size.

Threshold Fatigue Crack Growth NOTE: The stress intensity factor, K, allows one to Correlate between the behavior of a cracking lab Specimen and a cracked component because it Characterizes the stress field at the crack tip

Conclusions Microstructure Chemical composition A fine and homogeneous microstructure, give improved strength and corrosion fatigue strength. Chemical composition Duplex stainless steels have similar alloying elements Adding molybdenum provides improved corrosion resistance, including crevice and chloride pitting attack. Corrosion fatigue strength Higher strength and higher corrosion resistance leads to higher corrosion fatigue strength. All of these factors are important in designing and or selecting a suction roll material

Conclusions Threshold fatigue crack growth Thermal fatigue strength High threshold fatigue crack growth rate values contribute to long lived suction roll shells. Thermal fatigue strength Smaller grain size gives better resistance to thermal fatigue. Experience Design and application of suction rolls requires experience and machine operating tim eto prove an alloy All of these factors are important in designing and or selecting a suction roll material

Conclusions Material A has the suction roll shell properties and the experience to handle all paper making suction roll applications. All of these factors are important in designing and or selecting a suction roll material

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