Material E
A
23
A MPTIAC
S
E
Wade G. Babcock, AMPTIAC Quarterly Editor-in-Chief The Advanced Materials and Processes Technology Information Analysis Center Rome, NY Ernest J. Czyryca Survivability, Structures, and Materials Directorate Carderock Division, Naval Surface Warfare Center West Bethesda, MD
THE ROLE OF MATERIALS IN SHIP DESIGN AND OPERATION INTRODUCTION
A sport fisherman fi sherman on a quiet lake or river chooses choo ses a shallow draft, flat bottom boat. A deep sea fishing charter chooses a larger, vee-bottom vessel with lots of power. A cargo company chooses a long, square cross section, boxy vessel. Some vessels are welded steel while others are wood and still others are made of glass or carbon fiber reinforced polymers. All of these boats share the same basic principles of keeping their operators or cargo dry, enabling them to cross a body of water, and accomplishing a certain task. But the differences in structure, speed, agility, sea-keeping, and overall abilities are enormous. It is obvious that the mission of an aircraft carrier is different from that of a cruiser. But why would a fisherman choose a shallow-draft, flat bottom boat to go bass fishing in? These questions are factors the designer must consider when building a new ship, and there are many more. In this issue’s MaterialEASE, we seek to provide a look at some of the most pressing considerations ship designers face when choosing materials for current and future Navy ships. While there is no way we can tackle all the issues unique to shipbuilding, what we seek to do here is present at least the “first tier” of considerations pertaining to materials that any ship designer will have to address. The information is compiled here such that the lay person can gain a basic appreciation for some of the most important materials issues in shipbuilding. Not only are there multiple grades of steel used throughout modern ships, but one can see from the articles in this special issue that there are efforts underway to change the fundamental materials and structures of combatants. The next 30 years may see the most dramatic shift in shipbuilding technology since steel replaced wood. First, take a look at modern ships. Since World War II, welded steel (so-called monolithic) hulls have almost completely replaced riveted steel hulls for all ships greater than 100 feet long. All of these structures are fabricated with a network of longitudinal and lateral “T”-shaped structural members covered with plate to form the hull shape. All joints are welded together and the grades of steel used typically fall into one of three categories: moderate yield strengths of 35 to 50 ksi, higher yield strengths of around 80 ksi, and the highest yield strengths of 100 ksi or more. In the United States, high yield strength Navy steels of 80 and 100 ksi were specified in the 1950s with compositions separate from those of general industrial steels. These “HY” series steels had to meet significant toughness and weldability requirements, especially at cold temperatures,
and were used in nuclear submarine pressure hulls and critical areas of surface ships for ballistic protection and as “crack arrestors.” These requirements were driven by the catastrophic failures incurred i n the first welded ships built from steels common in riveted construction. HY-80 and HY-100 have recently been augmented with even newer industrial HSLA (high strength, low alloy) grades which will meet the strength, toughness and weldability requirements, but should be significantly less expensive. (For more information on HSLA steels, please see the article by Czyryca, et. al. in this issue.) Any of the steels used in Navy ships ship s must adhere to t o certain criteria demanded by the Navy and shipbuilders. There are fundamental weldability, toughness, and forming characteristics for metals which must be met in the construction phase. Then there are additional parameters which dictate the ship’s eventual performance, such as weight, shock loads, vibration, fracture toughness in environmental extremes, and fire performance. Since these two phases of a ship’s lifetime are governed by different needs in terms of material performance, we have chosen to divide this report along these lines. Neither operation of the ship nor construction can be examined independently however, so care should be taken to consider each in the context of the other. The first section will cover the main materials issues of concern during the construction phase of a modern ship. The second section will discuss the most important materials considerations during the operation of the vessel. Most Navy ship hulls are constructed of steel, except for a few special purpose vessels which have either fiberglass or wood hulls. (These are used specifically for tasks where reduced magnetic signature is critical, such as mine clearing.) Many ships have structures above the waterline (called “topside”) fabricated of aluminum and more recently some use glass reinforced polymer (GRP) composites. Most of the discussion in this report will be in relation to steel, except where noted on topics where other materials have been applied. SECTION 1: MATERIAL ISSUES RELATING TO CONSTRUCTION OF NAVY SHIPS
It should be noted again that any material parameters considered during the construction phase will impact operational characteristics downstream. For instance, the time and procedures required to weld a structure together are major cost points during a ship’s construction.
The AMPTIAC Quarterly, Volume Volume 7, Number Number 3
31
Material E
A
S
There is significant pressure to reduce these costs, therefore the weldability of a metal is critical. However, if welds are not done carefully they are literally a breeding ground for flaws and will thus critically impact the service performance of the vessel in operation. Strength
Overall size of any structure is often a function of the strength of its com ponent materials. In the case of ships, high strength steels with yields stresses ranging from 30 to 130ksi are typically used. In general, most of a ship’s structure is made from 50 or 80ksi yield stress steel, with critical areas making use of stronger grades as needed. Less dense alloys of aluminum may be used for topside structures, and recently composites are regularly finding their way onto ships also (mostly for topside structures – less commonly for hulls.) The current ultimate load carrying abilities (tensile and compressive) of reasonably cost effective GRP composites limit all-composite hulls to about 200 feet in length. A more complicated facet of the overall strength issue is the ability of a structure to do its intended job. For instance, if a designer calls for 0.5 inch thick plate with a yield stress of 50 ksi, it might seem appro priate that a thinner, 80 ksi plate could be substituted. Ship structures (and most other large structures for that matter) are never that simple. Plate steel is welded into monolithic structures with interconnecting I- and T-cross-section beams. Hull or deck plating is then welded into place creating “grillage” structure. Thinner plate (even with higher yield stress) will behave differently, often buckling much sooner than thicker plate. For this reason, careful attention is paid to buckling modes of overall structures and thicker plate is often required even when its specific strength is well overmatched to the task. Similarly, the overall length of a vessel determines its loading characteristics. In a shorter vessel (up to ~150 feet), its structural com ponents must perform well in bending and be stiff. For a hull greater than 200 feet in length, the ultimate tensile and compressive behavior of its components takes over. While composites can be made with extremely high strength capabilities, the cost of their component materials and fabrication grows rapidly (especially as more exotic components are chosen). Steel structure can be fabricated to handle both loading scenarios (stiffness and ultimate strength, based on length) at a much more affordable cost. Weldability
An immense amount of welding is required to build a steel ship. Thousands of piece parts are cut and assembled, requiring miles of welds at the joints. Many of these joints require multiple passes to complete. By far, welding is the most labor intensive portion of constructing a ship. Welds in a ship structure are also very critical to its overall strength, durability, and toughness. Even small defects in weldments can create the initiation point for considerably larger cracks and eventual failures.
32
The AMPTIAC Quarterly, Volume 7, Number 3
E Because of the massive amount of welding required, and its importance to the structural integrity of the vessel, careful welding processes are stringently adhered to. While HSLA steels require diligent attention to detail and procedure during welding, the HY-series steels require even more weld preparation and post treatments. One of the reasons for the push to replace HY-series steels with HSLA grades in the 1990’s, was the reduction in labor required for welding. In general, any steel grade meeting the strength, toughness, and other requirements, but which is sim pler to weld with a lower predisposition to weld flaws, is desirable. The cost of alloying steels to increase weldability must be balanced against the added cost of welding labor associated with a less weldable grade. Toughness
In addition to strength and weldability, toughness is one of the most salient attributes of metallic structures. In shipbuilding, toughness is a critical feature of the structure and its component materials (both plate and weld metals), as they must be able to deform plastically to some extent, and tolerate cracks and flaws while maintaining overall structural integrity. This is obviously complicated by the fact that Navy ships must be capable of operating in every ocean environment, from the frozen arctic to the steamy tropics. Steels, however, have a ductile-to-brittle transition in toughness as temperature decreases. It is a function of temperature, loading rate, and microstructure of the steel. Below a temperature specific to each steel grade, the material will have little resistance to catastrophic crack growth. In the transition regime, the combination of dynamic loading and cracks or defects in areas of stress concentration, may result in unimpeded, rapid crack propagation through the material. For shipbuilding steels, it is imperative to select grades with a low toughness transition temperature (below the expected operational tem perature range). For the higher strength steels used by the Navy, alloying and processing methods are used to produce grades with very low toughness transition temperatures, but again, this can increase cost and reduce availability of the grade. Marine Corrosion
Ocean-going ships are intended for use in one of the most corrosive environments on the planet, and as such, corrosion is considered carefully in the design phase of any vessel. There are various materials options, design strategies, coating, and cathodic protection technologies available to the designer and shipbuilder. Painting a ship is probably the Navy tradition with the strongest love/hate relationship. The process is necessary to protect the vessel from the sea’s corrosive effects, yet it is a never ending task that eats up significant labor resources and time. There is rarely a moment in a ship’s service life when something aboard is not being painted. And these paints are not the kind of materials you find at a hardware store: there are many
A D V A N C E D M A T E RI A L S
different formulations, each specifically engineered to perform critical tasks around the ship. Many also require very rigorous surface preparation and curing processes which place additional drains on manpower. From the designer’s perspective, the ship must be built in such a way as to reduce corrosion (limiting entrapped areas where water can collect and stand for instance) and also allow for the significant corrosion pre ventative processes that will go on for the duration of the ship’s life. Structural elements must allow for periodic and sometimes often recoating, and some elements must allow for replacement if they are particularly susceptible. There have been efforts to replace corrosion-prone steels with other metals like aluminum or titanium. These alternative metals offer a lower density (which can reduce weight) and some corrosion resistance, but they present their own unique problems which are beyond the scope of this article. In short, marine grade aluminum (5000-series) is the only non-ferrous, corrosion resistant metal that has seen widespread use in Navy ships as topside structure over the past 30 years. This is mainly because of the availability and relatively low cost of marine aluminum, a consequence of commercial crew boat and high-speed ferry construction using well-understood fabrication and welding processes. Formability
Ship hull forms and grillage structures require component steel or composites to be formed into a multitude of complex shapes. The considerable amount of welding required to fabricate steel structures also imparts a significant amount of residual stress and often unwanted deformation in the finished structure. This combination of initial forming requirements coupled with post-assembly straightening of deformed plate, requires that the steels chosen for ship construction be amenable to a wide range of forming procedures. The labor associated with these forming procedures must be balanced with the cost of alloying or preprocessing of steel plate to increase formability. In addition, formability of the grade impacts toughness, weldability, and strength. Composites offer the ability to be formed into much more complex monolithic shapes, but are not as amenable to assembly of multiple subsections. Transferring structural loads and accommodating thermal expansion mismatch is not a trivial endeavor in large structures. In fact, joining technology (including composite-to-composite and compositeto-steel) is currently one of the most limiting (and potentially the most promising) area of development. All of these factors must be balanced carefully in the selection process. Availability
About 40-50% of the steel used by DOD is consumed in Navy ship fabrication. The HY-series steels were specific to Navy applications, forcing manufacturers to
AND
PROCESSES
TECHNOLOGY
A MPTIAC
divert production from common grades and increasing the lead time required for production. When combined with their unique alloying requirements, these factors make HY steels considerably more expensive than more common industrial grades. Some recent efforts to utilize industrial-standard HSLA steel grades have helped to increase the effective amount of steel available, as well as lower the cost of procurement. Affordability
Currently and for the foreseeable future, one of the most pressing factors in materials selection for Navy ships is affordability. With labor costs continually rising and often outpacing the increased cost of raw materials, the balance between these must carefully accommodate and antici pate how materials decisions impact construction labor expense. The long-term costs of maintenance and readiness must also be factored in. Choosing the cheapest material in the construction phase cannot be a considered a success if the decision requires a significant increase in construction labor and/or creates a maintenance nightmare during the 30 to 50 year expected lifetime of the platform. Traditional steels and newer composite materials each offer specific advantages. As compared to composite materials, steels are less expensive to purchase, relatively less expensive to fabricate, and potentially more expensive to maintain. When considering the cost of building a ship from steel, one must always factor in the long term cost of repeated painting and corrosion mitigation. Composites cost more to fabricate, but offer the promise of lowered maintenance cost through their corrosion resistance. There are however, questions as to the long-term maintenance of composite joining technologies, environmental attack causing delamination (as simple as water infiltration), and perhaps more as-yet-undiscovered issues. SECTION 2: MATERIAL ISSUES RELATING TO OPERATION OF NAVY SHIPS
Once a vessel enters the fleet, many of the materials selection decisions made years before during design continue to impact its day-to-day operation. Weight
In design of air and space craft, weight is the driving force for almost all materials and structural decisions. Wherever weight can be reduced, it typically is, no matter what the cost. Lower weight translates to greater speed, range, and payload capacity, as well as a generally reduced cost of operation in the form of fuel requirements. While not quite as critical as air and space craft, weight reduction in ships is very important. The same economies of increased payload, speed, and range translate, as well as a reduction in fuel requirements. There are additional benefits to reducing topside weight of ship
The AMPTIAC Quarterly, Volume 7, Number 3
33
Material E
A
S
structure, such as increasing the sea-keeping ability of the ship via improved stability of the platform in a seaway. (Reduced mass higher up on the ship keeps the center of gravity low and reduces roll moments.) While the overall weight of a ship is determined predominantly by decisions made in the design phase, they have the greatest impact on service performance, including the growth margin critical for added or improved capabilities over the life of the ship. Therefore we have chosen to include the discussion in our Operation section. Many ships currently in use have aluminum topside structures. While these structures employ modern high-strength alloys, the fabrication methods and design are still very traditional, utilizing conventional welded grillage construction. Composites have also been used very recently for some limited topside structures, such as mast enclosures and compartments. (For more information on composites aboard ships, see the article by Potter in this issue.) Most recent research efforts to reduce weight on ships have focused on using higher yield stress steels to replace those with lower yield stresses. This enables the use of thinner plate, and thus reduces weight. One cannot replace all structural plate with thinner material however, as buckling instability becomes an ever increasing issue. Much research and experimentation has gone into studying how ship structure performs with thinner, higher yield stress steel and this work will no-doubt continue. Other innovations such as the advanced double hull (ADH) seek to replace conventional construction with new techniques which reduce the overall number of metal piece parts needed. (For more information on the ADH concept, please see the article by Beach, et. al. in this issue.) Composites have been used on hulls shorter than 200 feet long with great success. The raw material and labor cost to build composite hulls is still too high for applications without a specific requirement (such as reduced signature) for composite materials. Fracture Toughness
As discussed earlier, fracture toughness is a critical property required in the steels and weld metals of warship hull structure. Structural components’ ability to withstand the day-to-day rigors associated with thermal extremes, impacts from piers and other boats, maintenance procedures, sea conditions, and other “normal” events is important. These, however, are not the most important conditions that impact readiness. Naval vessels are also subject to shock loading from hostile weapon effects, such as air and underwater explosive devices a nd projectiles require protection measures against shock, blast, and penetration. Steels used in ships are formulated and processed to have a greater ability to withstand fracture and a greater flaw tolerance under shock conditions than more common structural grades. This allows them to withstand high intensity loading and remain ductile, sustaining damage without rupture or fracture. Similarly, the welding processes are carefully engineered to produce very clean and uniform welds that
34
The AMPTIAC Quarterly, Volume 7, Number 3
E are also very tough and flaw tolerant. The qualification of Navy steels involves a progression of fracture toughness testing from small impact tests to large scale, full thickness explosion tests. These tests demonstrate the capability of the welded system to sustain gross deformation, in the presence of cracks and low temperature, without fracture. The specifications for both base metals (plate, forgings, castings, etc.) and welds contain toughness test requirements to assure their ability to resist fracture and tolerate flaws. Marine Corrosion
Someone once said that from the moment steel comes from the mill, it is constantly trying to get back to the earth in the form of rust. This is especially true of Navy steels, which are expected to do their job in one of the most corrosive naturally occurring environments. Salt water and sea spray cover every surface of an ocean-going vessel and are constantly attacking its structure. Not only the structural components, but every system aboard the vessel is being chemically corroded. Galvanic corrosion (produced by the differences in electronic potential between component metals aboard a ship electrically connected via the ship’s structure and ionically conductive seawater) is also a critical concern. In addition, the ship itself generates combustion products (in turbine and diesel powered vessels, as well as ancillary combustion engine-powered systems on other vessels) which are high in corrosive compounds of sulfur and some acids. Upper structures of the vessel are subject to these airborne pollutants. There are multiple ways of combating corrosive elements. As mentioned in the first section, there are various materials options, design strategies, cathodic protection, and coating technologies available to the designer and shipbuilder. Once the vessel is in service, combating corrosion becomes a major maintenance requirement. Coatings are used to protect the steel hull (or other substrates) from attack by salt water and other water- and airborne corrosive com pounds. They require a significant amount of care during application and frequent reapplication to maintain their protection. From simply scraping, sanding, or grinding a small area, to significant removal, surface preparation, and reapplication of a coating to a large area, coating maintenance is a never-ending process. Once there is a discontinuity in the coating, corrosive attack is mitigated by cathodic protection, either by sacrificial anodes or by impressed current systems. The sacrificial anodes are typically blocks of zinc alloy, electrically coupled with the steel hull and which preferentially corrode, protecting the exposed steel. The anodes are positioned about the hull of the ship and replaced periodically as they deteriorate. Impressed current systems use permanent anodes on the hull and generate a potential field to counteract the corrosion potential of exposed steel. Galvanic corrosion (caused when, steel is be connected to materials of higher corrosion resistance), is mitigated by utilizing coatings or insulators to help reduce the electrolytic connection enabled by the
A D V A N C E D M A T ER I A L S
seawater, and by sacrificial anodes. (There typically is a concentration of anodes in the stern, where the bronze propeller, galvanically connected to the hull via the shaft, would otherwise cause accelerated corrosion of any unprotected steel.) Biological Attack
In addition to corrosion protection, the hull coating below the waterline must posess anti-fouling properties to maintain hydrodynamic performance and fuel efficiency. The most commonly used coatings prevent or slow the attachment of marine organisms by containing compounds toxic to the animals, thus preventing them from attaching in the first place. Another method is to apply a coating whose chemistry or surface morphology hinders the creatures’ attachment, thus allowing them to be sloughed off by water flow once the vessel gets underway. Fire
Fire is probably one of the most dangerous events that can threaten the safety of a ship and its crew. Shipboard fires must be fought with fire fighting systems available on the ship. The crew must fight and defeat the fire if the vessel is to survive. Crew training in fire-fighting and damage control are critical to ship survival. Structural steels are vulnerable to annealing and softening with exposure to fire-generated heat, thus allowing structures to collapse. Shipboard composites can be a fuel source for fires, thus exacerbating an already serious situation. In both cases, active and passive fire insulation systems are used to keep structural members protected from heat and flame for a certain period of time – presumably long enough to get a fire under control. For composites, great care is taken to select component materials with higher levels of fire resistance. An additional fire concern is the production of smoke and toxic gases from burning materials. Ships contain many flammable and non-flammable compounds (liquids like fuel, oil, greases, paint, etc.; solids like furnishings, electronics, composite structures, metals, etc.) which when exposed to either heat or flame can burn, volatilize, or smolder. The smoke and gases generated are of just as much concern as the fire itself. First and foremost, they hinder the ability of crew to get near a fire’s source to extinguish it. Secondly, they can pose a significant threat to large portions of the vessel even if the fire itself is small and easily controllable. Naval vessels, given their intention to go into harm’s way, are designed with materials and structures that meet higher standards of fire resistance and control. Additionally, active fire suppression systems are often incorporated into combatants. All of these factors are balanced against affordability, but given that fire is one of the most dangerous threats to a surface combatant, more resources are usually allocated to this area.
AND
PROCESSES
TECHNOLOGY
A MPTIAC
Signature
Ships are very difficult to hide on the open water. A ship’s main defense against detection is to reduce the amount of electromagnetic, acoustic, or thermal radiation it emits or reflects. In order to do this, ship designers are employing any number of technologies to absorb and deflect enemy radar, reduce thermal and acoustic emission, and in general increase the stealth characteristics of surface combatants. As mentioned elsewhere, composite structures are being used on the topside of ships to reduce weight. They also offer the capability of integrating absorbing and reflecting materials into topside structures. By building composite shrouding around the very reflective metal masts on current ships, the electromagnetic signature of a ship can be dramatically reduced. Future vessels will use predominantly composite topside structures to further reduce signatures. Composite hull forms and new steel double-hull technologies offer the promise of reduced thermal and acoustic signatures. Composites can insulate the internal components from the water, while double hull designs allow for flooded compartments which can act as thermal and acoustic barriers. Composite and double hull technologies are also allowing more design freedom to create lower-profile, critically-shaped hull forms which further reduce all types of signatures, including reduced wake. Wave Loading
The structural demands of a Navy ship, and indeed any ship, are characterized by one particular load condition that most other structures will rarely see. Ships are subjected to a constant low-frequency, high-cycle fatigue stress induced by wave motion. This is due to ships not being rigid beams; they are in fact a structural beam uniformly supported by hydrostatic pressure along its bottom surface. As the ship moves through water whose surface is not flat, often the ends are supported more than the middle (or vice-versa), creating a repetitive bending moment in the structure. There are also repetitive lateral forces exerted on the structure which create other bending moments. While these moments are accounted for in design, the component materials still acquire damage over the service life of the vessel. The fatigue strength of a welded steel structure does not increase relative to the strength of the steel. Therefore, the use of higher strength steels requires detailed design against fatigue cracking over the life of the ship. Fatigue testing of structural joints and computational techniques are employed in modern ship design to characterize the fatigue life of the structure. Vibration
Vibration is a constant companion to any ship. There are hundreds of pieces of equipment aboard a vessel chugging away at their individual tasks, each one imparting its own characteristic vibration mecha-
The AMPTIAC Quarterly, Volume 7, Number 3
35
Material E
A
S
nism into the vessel’s structure. While most large equipment is built on vibration-isolating spring mounts, none of these systems is perfect and always some vibratory load is transferred to the substructure. Even when large equipment is well isolated, its acoustic noise induces vibrational loading into surrounding structure. Vibration can induce fatigue, fretting, and other forms of degradation into structural materials. It also produces detectable noise transmitted through the water and interferes with a ships’ own sonar system effectiveness. Constant small movements cause surfaces to wear, and tiny amounts of elastic deformation may induce fatigue-like failures over time. Since vibration cannot be eliminated, material and structure decisions must take these degradation mechanisms into account. Hull Damage
Ships are always susceptible to hull rupture from collision with fixed objects (rocks, piers, sea walls, etc) and other vessels. Navy ships also face the possibility of having explosives detonated close by (hull whip ping and blast loading) or being impacted by projectiles and fragments. Material choices are critical in reducing these threats, or at least reducing the amount of damage. Unlike merchant vessels, Navy ships are expected to maintain a high level of performance even when damaged. Therefore component materials’ ability to not only limit damage but also be quickly repairable is paramount. Hull survivability is part of the structural design of warships. Grillage is analyzed and built such as to limit the amount of damage caused by perceived threats. New designs such as double hulls (and some composite structures) have additional, built-in resistance to hull rupture. Materials with high fracture toughness are obviously prime choices to limit damage propagation. Specific materials are often used in critical areas of the hull. The hull structure must be fracture resistant under high intensity loading at temperatures as low as -40 °F. CONCLUSION
Materials play a key role in many aspects of the construction and operation of modern ships. For construction of Navy vessels, there are many standard and traditional procedures that determine most of these aspects. While steel is by far the most common, economical construction material, there is significant interest in aluminum,
36
The AMPTIAC Quarterly, Volume 7, Number 3
E titanium, stainless steels, and composites for future, high-speed ships. The Navy is aggressively pursuing improved structures and novel construction methods that could serve to dramatically revolutionize future vessels. They will have significantly different hull forms, be fabricated increasingly by automated methods, and will utilize even more light-weight metal, hybrid, and composite structures than we currently envision. The Navy is evolving its combatant fleet with the introduction of new materials and new construction methodologies. We have provided in these pages a brief look at the most pressing issues ship designers consider when choosing materials. Future vessels will utilize new materials to enable capabilities that far outpace their current sisters, just as the armored steel, steam turbine powered battleships of the early 20th century were revolutionary technological leaps ahead of their wooden, wind-powered predecessors. REFERENCES
American Society of Naval Engineers, www.navalengineers.org. C.R. Crowe, and D.F. Hasson, Materials Trends in Marine Construction. United States Naval Academy, Division of Engineering and Weapons, Annapolis, MD (1990) E.J. Czyryca, Advances in High-Strength Steel Plate Technology for Naval Hull Construction. Naval Surface Warfare Center, Carderock Division, Metals and Welding Department, Annapolis, MD Federation of American Scientists, www.fas.org T.C. Gillmer, Modern Ship Design, 2nd Edition. Naval Institute Press, Annapolis, MD (1977) National Shipbuilding Research Program, Advanced Shipbuilding Enterprise, www.nsrp.org Ship and Submarine Materials Block, Navy Exploratory Development Program FY1991 Block Plan. Naval Surface Warfare Center, Carderock Division (Formerly David Taylor Research Center), Bethesda, MD (1990) Carbon and Alloy Steels, ASM Specialty Handbook, Steels for Ships and Offshore Structures. J.R. Davis editor, ASM International, Materials Park, OH (1996) pp. 669
