T-702-G Shipyard Productivity Enhancement Opportunities. Darren Begg Manager, Welding Engineering Technology BMT Fleet Technology

Transcription

1 T-702-G Shipyard Productivity Enhancement Opportunities Darren Begg Manager, Welding Engineering Technology BMT Fleet Technology Biography Darren Begg is an IIW certified Welding Engineering Technologist and the manager for Welding Engineering Technology at BMT Fleet Technology Limited. For the last 15 years he has been responsible for investigating and developing welding and related manufacturing processes to improve productivity, reduce costs, and streamline production primarily in the energy and shipbuilding industries. Description The National Shipbuilding Research Program (NSRP) is a US Government initiative that provides funding to researchers and shipyards to improve the techniques and technologies used in shipbuilding. The tools developed under this program should help to reduce overall shipbuilding costs and to improve the structural integrity of ships built to both commercial and naval standards with the objective of making them more affordable and prolonging their life cycle, respectively. A very large percentage of shipbuilding costs is related to welding and so the NSRP has a sub panel that is dedicated to continuously advancing welding technologies and related joining techniques. As ship designers turn to the use of higher strength steels the shipbuilders must also evolve and adopt processes that can achieve the desired joint properties and to as well minimize distortion and rework. BMT Fleet Technology (BMT) has conducted extensive research under the NSRP to help shipyards achieve their goals when welding these materials. An overview of the results achieved in these programs is discussed herein. Mari-Tech 2012 Exhibition and Conference Re-birth of the Marine Technical Community

2 311 Legget Drive Kanata, Ontario Canada K2K 1Z8 Tel: Fax: Shipyard Productivity Enhancement Opportunities Darren Begg, BMT Fleet Technology Limited MARITECH 2012 ABSTRACT The National Shipbuilding Research Program (NSRP) is a US Government initiative that provides funding to researchers and shipyards to improve the techniques and technologies used in shipbuilding. The tools developed under this program should help to reduce overall shipbuilding costs and to improve the structural integrity of ships built to both commercial and naval standards with the objective of making them more affordable and prolonging their life cycle, respectively. A very large percentage of shipbuilding costs is related to welding and so the NSRP has a sub panel that is dedicated to continuously advancing welding technologies and related joining techniques. As ship designers turn to the use of higher strength steels the shipbuilders must also evolve and adopt processes that can achieve the desired joint properties and to as well minimize distortion and rework. BMT Fleet Technology (BMT) has conducted extensive research under the NSRP to help shipyards achieve their goals when welding these materials. An overview of the results achieved in these programs is discussed herein. BACKGROUND High strength low alloy steels are being used in ship construction to optimize the life-cycle costs of both naval and commercial vessels however their use can bring upon some major challenges in production. The very nature of these steels makes it difficult for shipbuilders to use techniques that were once considered reliable joining methods. As designers aim to achieve lighter structures, through the use of thinner high strength steels, shipbuilders are also now faced with challenges related to controlling distortion and reducing costly rework. multi-mission Destroyers, advanced Cruisers, and a new breed of focused mission ships such as the Littoral Combat Ship (LCS). These programs often have requirements that steel structures not have their primers removed prior to welding. In order to produce sound welds when welding over primers, welding processes such as flux cored arc welding (FCAW) with special electrode formulations have been traditionally used. Slow welding speeds are employed to allow the gases generated from the decomposed primers to sufficiently escape the weld prior to solidification, but often this practice results in large levels of distortion as well as low production rates. In commercial shipbuilding it is common practice to remove the primers prior to welding by grit blasting and then reapply after the welding operations are complete. This method of construction adds time and money to the build process and shipyards are always looking for productive methods to effectively weld over primers to reduce their costs. To increase the productivity of welding, processes with high deposition rates need to be employed; however, to effectively weld over primer while minimizing distortion processes that offer high travel speeds and low heat inputs and suitable degasification are required. Welding processes that provide a suitable balance between productivity and distortion control need to be considered as well as provide a significant return on the equipment investment. A number of research projects have been conducted under the NSRP to help shipbuilders achieve their goals, and these include: The US Navy has several shipbuilding programs that utilize high strength steels including aircraft Carriers,

3 The evaluation of advanced tandem submerged arc welding (T-SAW) processes with waveform control and metal cored electrodes to join high strength steels; Tandem gas metal arc welding (T-GMAW) with metal cored electrodes for welding thin primed HSLA-80 steel; The evaluation of mechanical tensioning to reduce distortion in thin high strength steels. Previous research results for the above projects are discussed below. PROBLEM DEFINITION AND APPROACH The results from previous research have been divided into the following sections. Evaluation of Advanced T-SAW with Waveform Control and Metal Cored Electrodes to Join High Strength Steels Some shipyards have improved their productivity in thick steels by employing one-sided SAW for long panel seam welds, in which two or three electrodes are used to fill the joint in a single run. With this technique, a flux filled contoured copper backing bar is used to support the weld pool on the back side of the joint. The flux protects the weld from atmospheric contamination and also serves to shape the weld back bead profile. However, the high heat input of these welding procedures can lead to large regions of coarse grain HAZ (CG-HAZ) with poor impact toughness. For HSLA-100 steels, specific heat input restrictions are specified by NAVSEA to ensure that minimum specified weld metal yield strengths are achieved, and, to also maintain adequate HAZ toughness. It is for this reason in practice that often several small low heat input weld passes are deposited to fill a joint which is not very cost effective. One potential resolution, without sacrificing quality, is to make use of SAW power sources with wave form control technology along with specially formulated metal cored electrodes to provide high weld metal deposition rates, enhanced joint filling, and lower effective heat inputs. Power sources, such as digital square wave alternating current (AC) with variable balance control (i.e. independent control of the positive and negative polarity dwell times in the AC welding cycle), have been developed for SAW. In conventional SAW, a greater portion of heat is generated at the cathode (negative pole), and therefore when the polarity is direct current electrode positive (DCEP), the work piece becomes the cathode and a greater portion of the heat is generated at the work piece below the arc column. This is why when welding on DCEP polarity that welds with deep penetration characteristics are achieved, compared to DC electrode negative (DCEN). When welding on DCEN, the electrode then becomes the cathode and therefore a greater portion of the heat is generated at the electrode tip, and provides enhanced electrode melt-off and deposition rates compared to straight DCEP polarity. A conventional AC welding sine wave provides a 50/50 balance of negative and positive polarity, and therefore a balance between the characteristics of conventional DCEP and DCEN polarity welding. In Figure 1, conventional sine wave AC (in red) as well as the modified squarewave AC waveform (in black) is shown. The square waveform was developed to increase the time at peak current, as well as eliminate the slope as the output passes through the neutral axis. Both of these modifications greatly enhanced arc stability compared to conventional sine wave output. Figure 1: Conventional and Square AC Waveforms Variable balance AC (VBAC) waveform technology was developed to allow welding procedures to be fine tuned for specific applications. VBAC can be used to control the duration the cycle spends in each of the EP and EN polarities, as shown in Figure 2. Figure 2: VBAC Waveform The example shown in Figure 2 illustrates a 70% EP and 30% EN polarity setting, which will provide greater penetration compared to conventional AC welding however improved deposition rates compared to DCEP for the same current level. Increasing negative polarity dwell time beyond 50% EN also reduces the amount of heat into the work piece compared to a conventional balanced sine

4 wave output, thus promoting lower peak temperatures and higher weld zone cooling rates. This therefore has the potential to improve the weld properties and reduce distortion, and as well reduce dilution between the weld and base metal. In addition, advances in consumable design utilize metal cored electrodes for SAW that are manufactured using a mild steel jacket with a core of specifically selected iron and other metal powders and alloys. Stabilizers and arc enhancers are added to its core, typically providing a wider operating window compared to welding with solid wires. Versatility is possible with these electrodes because of the infinite alloy compositions that can be easily made by electrode manufacturers. Special alloy combinations can be achieved that would be difficult or impractical with solid electrodes, including special types for welding high strength steels. Metal cored electrodes are a more economical alternative to solid wire electrodes because the manufacturing process involves blending metal powders instead of creating a special melt of steel, and therefore small quantities are easier to produce, and minimum order quantities are typically much lower. With metal cored electrodes approximately 90% of the current is carried by the thin sheath surrounding the electrode, thereby increasing resistive heating of the electrode which can demonstrate increases in weld metal deposition rate compared to a solid wire electrode of the same electrode diameter for the same current level. T-GMAW with Metal Cored Electrodes for Welding Thin Primed High Strength Steels When welding thin high strength steels shipyards are faced with challenges of controlling distortion within acceptable limits. Thinner materials don t often offer the same buckling resistance as thicker steels and therefore practices must be put into place to counteract shrinkage stresses and resulting distortion. Some of these methods include lowering the weld heat input, using strategic weld deposition sequences, and/or heavily restraining the materials during welding. To add complication, practices that have been demonstrated suitable for welding over primer on thicker materials, produce unacceptable levels of distortion when welding thin high strength steel. T-GMAW offers the potential to produce high quality welds when welding over primer at high speeds and as well reduce distortion. T-GMAW is a process that uses a special torch with two closely spaced electrical isolated contact tips, with each electrode separately controlled by its own independent power supply, as shown in Figure 3. The filler metal can be spaced in various configurations however when welding they are fed into a common puddle producing a long weld pool. The longer weld pool of the T-GMAW process allows for longer degasification times when welding over primers, and therefore, in combination with the increased deposition rates, allow for speeds to be significantly increased over the traditional single electrode FCAW and GMAW processes. The faster travel speeds also reduce heat input and therefore resulting in less panel distortion. Figure 3: Tandem GMAW Electrode Configurations Mechanical Tensioning Techniques for Distortion Control in High Strength Steels The very nature of the arc welding process (local and non-uniform heating and cooling), is such that it is usually accompanied by distortion of the structure being fabricated. The magnitude of the distortion is controlled in practice within specified tolerances, not only for aesthetic purposes but also to maintain the structural integrity in service. It is preferable to implement techniques and procedures that minimize distortion in the first place since its correction at a later stage entails substantial hidden costs, including an adverse effect on the quality of the subsequent welds and of the overall fabrication (e.g., poor fit-up, greater amount of weld volume, possibly higher residual stresses, etc). Mechanically applied tension loading parallel to the weld axis during welding has been used by fabricators to minimize the buckling of thin plate during butt welding. Kawasaki Heavy Industries developed several methods to reduce out-of-plane distortion involving mechanical tensioning, and the methods were so successful that they were called the Kawasaki Perfect Panel Production method. The tension load forces the weld and thermally upset zone to stretch longitudinally and transverse to the weld to conform to the geometry of the balance of the sheet. This method of distortion control was applied and the results are discussed herein. RESULTS Evaluation of Advanced T-SAW with Waveform Control and Metal Cored Electrodes to Join High Strength Steels Highly productive welding procedures and metal cored electrode formulations were developed for T- SAW of HSLA-65 and HSLA-100 steels. The chemical analysis of the electrodes (formulation) and resulting deposited weld metal (with flux type) is shown in Table 1.

5 HSLA-65 Procedures Table 1: Chemical Analysis High heat input tandem welding procedures were developed for HSLA-65 using variable balance AC technology and metal cored electrodes and a modified flux copper backing bar. The procedures developed for the 1 thickness allow for the joints to be welded from one side onto a flux filled copper backing in a single run. The parameters used, including the comparing benchmark shipyard procedures, are shown in Table 2. The mechanical property requirements were comfortably met, and are shown in Table 3. The weld cross section is shown in Figure 4. Table 2: Welding Procedure, 1 Pass, HSLA-65 HSLA-100 Procedures One sided T-SAW procedures were also developed for HSLA-100 using variable balance AC technology and metal cored electrodes, where the first pass was welded from one side onto a flux copper backing bar, and the fill passes were made at high speed, each pass using T-SAW. This process allowed for all welding to be completed from one-side in as little as three runs, without having to flip the plate over, back gouge, and complete the weld from the second side. The parameters for each pass, as well as the shipyard benchmark travel speeds, are shown in Table 4. The weld cross-section is shown in Figure 5. Table 4: One Side Welding Procedure, 4 Passes, HSLA-100 Table 3: Figure 5: Weld Marco; 4 Pass, HSLA-100 Figure 4: Weld Macro, 1 Pass, HSLA-65 In addition, two sided two pass T-SAW procedures were developed, where the first pass was made at high speed, the panel was flipped, and the second weld was deposited without having to back gouge between passes. The weld parameters, as well as the benchmark shipyard welding speeds, are shown in Table 5. The weld cross section is shown in

6 Figure 6. Table 5: Welding Procedure, 2 Pass, HSLA-100 T-GMAW with Metal Cored Electrodes for Welding Thin Primed High Strength Steels T-GMAW uses two wires which are fed through a single torch, into the weld pool simultaneously. The T-GMAW process results in a significant increase in deposition rate, requiring higher rates of travel speeds to complete welds of various types and sizes. The higher travel speeds generate lower heat inputs and resulting distortion. The use of metal cored electrodes with the T-GMAW process resulted in a 221% increase in travel speed compared to benchmark FCAW procedures that were used in production by the project sponsoring shipyard. The welding procedures are shown in Table 7, and a sample weld cross section from the T-GMAW weld is shown in Figure 7. Table 7: T-GMAW Welding Parameters Figure 6: Weld Macro, 2 Passes, HSLA-100 The minimum mechanical property requirements for all the HSLA-100 procedures were met satisfactorily, as shown in Table 6. Radiography results for each procedure met the requirements of MIL-STD 2035A. (a): Weld Macro Table 6: Mechanical Test Results, HSLA-100 The combination of variable balance AC and metal cored electrodes offer significant productivity enhancement compared to current SAW practice that use conventional DC / AC technology and solid wire electrodes. Cost reductions (electrode and labor) demonstrated savings as much as 65% compared to current practice. Figure 7(b): Weld Surface With no clean-up operations to remove slag after welding, the actual productivity improvements are estimated at 250%. Procedures for T-GMAW were developed for welding over primers to utilize the elongated puddle of the tandem arc to extend the degasification period allowing higher travel speeds to

7 be employed without generating weld defects. These procedures resulted in an increase of 140% in travel speed compared to the benchmark FCAW procedure. Porosity counts on fractured weld samples revealed that the T-GMAW process was capable of achieving maximum porosity requirements; however, variations in the primer coating thickness resulted in sections of weld where resulting porosity was not acceptable. It appears that the T-GMAW process is less tolerant to the amount of primer on the material compared to the benchmark FCAW process. It is believed that a mechanical process to mill the edge of the stiffener square, removing primer from only the bottom surface of the stiffener and eliminating ridges where primer settles would make the T-GMAW more feasible. Joint fit-up would be improved with the machine edge reducing the amount of rework and productivity would be improved due to the increased travel speeds achieved with the T-GMAW process. Panel distortion (vertical displacement) measurements were recorded at points A and B from locations T1 through T23 along the entire length, using a laser level. For all plates welded under mechanical tension, the distortion data was recorded at all three stages: Before welding, no tension applied Before welding with 10,000lb tension applied After welding with tension released The results show that a tension load of 10,000, when applied to the base plate of the T-stiffener assembly, that significant distortion reductions can be achieved. Figures 9 and 10 show the results of the before and after welding displacement measurements and the level of distortion reduction achieved. Mechanical Tensioning Techniques for Distortion Control in High Strength Steels Welding of HSLA-80 stiffeners was performed using the FCAW process and 10 inch wide by 10 foot long base plates. All the stiffened panels (5mm thick) welded were marked up as illustrated in Figure 8. Figure 9, Distortion Reduction, Along Location A Figure 8: Distortion Measurement Locations

8 Figure 10: Distortion Reduction, Along Location B CONCLUSIONS A number of research projects were conducted by BMT under the NSRP, to investigate how advances in welding power sources and consumable design can aid to improve productivity rates, increase integrity of welds, and reduce distortion and rework in high strength steels. It was found that: SAW with advanced waveform control and metal cored electrodes can produce high heat input highly productive welds in high strength steels that consistently meet the strict weld and HAZ mechanical property requirements of naval standards; T-GMAW with metal cored electrodes has the ability to produce sound welds over primed high strength steel surfaces at very high rates of travel speeds; and Mechanical tensioning offers the potential to reduce distortion in thin high strength steel welded structures.

9 TW-702 Shipyard Productivity Enhancement Opportunities

10 Shipyard Productivity Enhancement Opportunities Darren Begg BMT Fleet Technology Limited

11 Welding Costs Welding and associated activities is one of the largest components of shipbuilding costs Labour (material preparation, welding, QA and NDT) Material (electrodes, gases, flux, contact tips, etc.) Overhead (hydro for welding and cutting equipment)

12 Welding Costs How do we reduce welding costs? The biggest savings can be had by >> Welding faster Having the ability to weld thick plate in fewer passes Controlling distortion and reducing rework

13 NSRP National Shipbuilding Research Program (NSRP) Welding Technology Panel (SP7) Focus on developing and implementing innovative technologies that enhance productivity, reduce distortion and rework, lower shipbuilding costs

14 Sponsored NSRP Projects BMT has conducted research under the NSRP with-in the following focus areas: High Heat Input Submerged Arc Welding of High Strength Steels Tandem Gas Metal Arc Welding of Primed High Strength Steels Mechanical Tensioning to Reduce Distortion of Thin High Strength Steels

15 Submerged Arc Welding High Heat Input Submerged Arc Welding of High Strength Steels Newport News and NSWC-Carderock Sponsors HSLA-65 and 100 Steels Inconsistent Weld and Heat Affected Zone (HAZ) Properties Need More Robust Procedures / Process

16 Submerged Arc Welding Advantages: High Deposition Rate and High Current Process (ability to weld very thick sections in fewer passes), Low Hydrogen Process, Low Fume Generation Disadvantages: High Input Process (can result in poor mechanical properties), Distortion (need heavy restraint)

17 Submerged Arc Welding Variable Balance AC SAW with Metal Cored Electrodes Develop Metal Cored Electrodes for High Heat Input Welding of HSLA-65 and HSLA-100 Steels. Comfortably Meet Mechanical Property Requirements 20ft-lbs at -20F for HSLA-65 60ft-lbs at 0F and 35ft-lbs at -60F for HSLA-100 Develop Optimized Procedures for Single Pass and Multi-Pass One-sided Welding, and, High Speed Two Sided Welding without Back Gouging Compare Results Against Benchmarks

18 Submerged Arc Welding Metal Cored Electrodes consist of a mild steel sheath filled with specific alloying ingredients. The current is only carried by the metal sheath, and therefore results in higher resistive heating and higher deposition rates compared to solid electrodes. Metal cored electrodes have the advantage of controlling very accurately the alloying ingredients and thus the final weld chemistry. Metal Cored Solid

19 Submerged Arc Welding Metal Cored Electrode Design Comfortably Achieve Minimum Mechanical Property Requirements for High Heat Input Welding of HSLA-65 and HSLA-100

20 Submerged Arc Welding 1 HSLA-65 One Pass - Tandem

21 Submerged Arc Welding HSLA-65 Weld Mechanical Test Results

22 Submerged Arc Welding HSLA-100 Welding NAVSEA restricts heat input to 85kJ/in to achieve weld and HAZ properties Requires small unproductive weld passes One sided welding prohibited Two sided welding requires backgouging

23 Submerged Arc Welding 1 HSLA-100, One Pass per Side, No Backgouging Travel Direction

24 Submerged Arc Welding 1 HSLA-100, One Pass per Side, No Backgouging

25 Submerged Arc Welding 1 HSLA-100, High Speed One Sided Welding, 4 Passes

26 Submerged Arc Welding HSLA-100 Mechanical Test Results

27 Submerged Arc Welding Productivity Improvements Single Pass Tandem Arc OSW Tandem Procedure Resulted in 2.5X Improvement in Welding Speed Over Benchmark HSLA-65 OSW Two Pass Procedure Four Pass Tandem Arc OSW Procedure Resulted in 5.5X Improvement in Welding Speed Over Benchmark Heat Input Limited 8 Pass HSLA-100 Procedure Two Pass (One Pass per Side) High Speed Tandem Arc Procedure, Without Backgouging, Resulted in 11X Improvement in Welding Speed Over Benchmark Heat Input Limited 8 Pass HSLA-100 Procedure Can exceed the 85kJ/in Heat Input Requirement

28 Tandem Gas Metal Arc Welding Littoral Combat Ship (LCS) Marinette Marine was using FCAW and slow welding speeds (approx 25 inches per minute) to weld over primer and achieve porosity free welds Primer thickness of 0.5 to 1.0mil Distortion! Tandem Gas Metal Arc Welding (TGMAW) Two wires into same weld pool 2X s the deposition rate Higher Welding Speeds Longer weld pool, ability to weld over primers?

29 Tandem Gas Metal Arc Welding Tandem Gas Metal Arc Welding (TGMAW) with Metal Cored Electrodes

30 Tandem Gas Metal Arc Welding Two 1.2mm diameter metal cored electrodes 5mm thick HSLA-80 steel plate and T-stiffeners Fillet Weld Size 5mm Primer thickness 0.5 to 1.0 mil 3.5X s faster than FCAW procedure

31 Tandem Gas Metal Arc Welding Set-up and Welds

32 Tandem Gas Metal Arc Welding Weld Fracture Specimen

33 Distortion Control Thinner steels (under 12mm) become more susceptible to welding induced distortion due to their lower buckling resistance. Most effective ways of controlling distortion are by reducing the heat input and/or by adding restraint (shrinkage resistance). Research was conducted to determine if mechanically tensioning the plates would offer sufficient restraint to reduce distortion.

34 Distortion Control

35 Test Set-up Distortion Control 5mm thick HSLA-80 T-stiffeners attached to 5mm thick plate with button tack welds. Measurements made at predetermined points along plate and stiffener. Test set-up welded with benchmark FCAW procedure and no tension. Distortion measurements made. Test set-up repeated with bottom plate mechanically tensioned to 10,000 lbs. Distortion measurements made. Results between no tension and tension cases compared.

36 Distortion Control

37 Distortion Control Benchmark Welding Procedure FCAW Weld Size: 3/16 Electrode: Trimark MIL-101TM Shielding Gas: 100% CO2 Amperage: 210A Voltage: 30V Travel Speed: 25ipm Heat Input : 15 kj/in

38 Distortion Control

39 Distortion Control SIDE A SIDE B TOP OF STIFFENER

40 Distortion Control The Net Distortion Displacement Reduction of the 10,000lb Tension Loaded FCAW Procedure Results in a 144% Improvement Compared to the Same Welding Procedure Without Any Tension Load Applied.

41 QUESTIONS?