In the course of the welding cycle three events occur:
- 1. Filler metal and contiguous base metal are melted and resolidify to form a fused connection.
- 2. The heat effect of welding subjects the adjacent base metal to a thermal gradient ranging from above the base metal melting point to ambient temperature. The heat affected zone, HAZ, and deposited weld metal then cool to ambient temperature. As indicated in the picture, the variety of metallurgical structures produced in this heat affected zone includes those exposed to the various temperatures which produce resolidification of melted base metal, grain growth, grain refinement, or modification of metallurgical microstructure.
- 3. The solidification of molten metal, as well as the metallurgical phase changes, induce plastic flow and develop residual stresses which may exceed the yield point in magnitude; thus resulting in structural distortion
When cracking occurs at elevated temperatures, the crack is usually intergranular, meaning between grain boundaries. Such cracking is associated with excessive solidification and cooling stresses acting on constituents present at the grain boundaries which are relatively weak at elevated temperatures. The weakened grain boundary may consist of specific low melting constituents such as sulfides in steel. In other cases the deposition of a weld bead of unfavorable geometry may impose excessive cooling stresses on the hot weld deposit which has relatively low strength at elevated temperature.
For example, in submerged arc welding, weld beads such as those shown in the picture, would tend to form a center section which solidifies last and remains at an elevated temperature after the surrounding metal has solidified and cooled. The low strength at the grain boundaries of the material at elevated temperature is inadequate to resist the thermal stresses, and hot cracking occurs. Such cracking, can usually be readily prevented by changing weld parameters to produce a bead of more favorable contour.
Cold Cracking - Hydrogen Cracks or Delayed Cracks
The role of hydrogen is an important consideration in the welding of ship steels. Hydrogen-bearing compounds such as water or organic compounds present on the filler metal surface, in electrode coverings, or on base metal surfaces may dissociate in the welding arc to form atomic hydrogen.
The atomic hydrogen penetrates and is highly soluble in molten steel weld metal and the zone of adjacent heat affected steel which has been transformed to a phase known as austenite; the austenite forms when the heat affected zone of a steel is heated above a critical temperature, (approximately 900°C (1,640°F), for structural steels.
As the solidified weld metal and austenitized heat affected zone cool to ambient temperatures, they are transformed into non-austenitic phases which release most of the dissolved hydrogen from solution, since hydrogen is practically insoluble in these phases.
When hydrogen is released from solution in the presence of a hard zone in the microstructure and a high residual stress field, a condition known as hydrogen cracking may occur. Since the time of such cracking varies from immediate to several days or weeks after the completion of welding, the phenomenon is also known as delayed cracking.
The tendency for such cracking varies directly with the magnitude of:
- 1. hydrogen concentration,
- 2. local metal hardness,
- 3. residual stress.
Hydrogen delayed cracking is the most important and troublesome form of cracking encountered in welding of the higher strength ship steels.
The vessel is ordinarily launched from the building position. The vessel's weight is transferred from the keel block, bilge crib and shoring building supports to the launching cradle. The cradle is supported on one or more ground ways which extend longitudinally under water. Attachments between sliding and ground ways prevent movement of the former. Restraining attachments may be burn off sole plates or triggers, release of which allows the cradle-supported vessel to slide down the ground ways under the influence of gravity and enter the water. As the stern acquires buoyancy, it lifts, and when this happens the ship pivots about its fore poppet. Checking of the sternward motion may be required to prevent grounding of the vessel on an opposite bank or shoal.
If the vessel is supported by a translation system, on arrival in the launching position the vessel's weight is transferred to the sliding ways or sleds resting on ground ways perpendicular to the vessel's centerline. If the vessel is built in the launching position, its weight is transferred as described above for end launching.
Floating Dry Dock
Varying deformations due to changes in solar radiation, air and water temperatures and varying loads, as well as the absence of a horizontal reference plane, makes a floating drydock unsuitable for the new building of vessels of any great size or weight.
Ground Supported and Floating Platform
The platform, which is essentially a floating drydock, is ballasted to rest on a horizontal underwater pile-supported grid, or in another version, has one side or one end of the platform temporarily connected to, and aligned with, the shore. The vessel is moved onto the platform, the inshore wing walls of the platform being temporarily removed if the vessel is moved transversely. Alignment between shore side and platform rails, tracks or ways is maintained by the relative fixity of platform deck and shore. If only side or end sup¬port is provided, the platform must be progressively dewatered as the vessel is moved on to it to keep the platform deck horizontal and aligned vertically. Even if the platform has the overall support afforded by a grid, progressive dewatering may be required to reduce piling loads if a heavy vessel is being moved onto the platform. With the vessel centered on the platform, any removed wing walls are reinstalled and ballast tanks are dewatered to float the platform clear of any submerged or above water supports. The platform is moved to a dredged area of enough depth for it to operate in a conventional floating drydock mode and submerge sufficiently to float the vessel being launched.
The launching of a newly constructed vessel is deservedly considered one of the most critical events in the whole building process and one that is potentially hazardous if the movement of the large, yet fragile, mass that is supported on a comparatively frail structure is not properly planned and executed. Configuration of design and selection of materials for suitability and strength and the control of, and making use of, the natural forces of gravity, buoyancy, water resistance and friction is, in essence, the art of ship launching. A properly engineered launching can be accomplished safely and efficiently. The development of this engineering plan is one of the most important tasks facing the shipyard naval architect.
At the time of receipt of a request for a proposal, major preliminary calculations are commonly made in order to verify that the vessel can be constructed and launched, in one or mоге pieces, using facilities existing or suitably modified, and this would typically allow to make some basic planning. Following contract award, further calculations are prepared to exactly locate the vessel on the building slip, determine forces acting on the ground ways, cradle and ship, and to provide a basis for the design and delineation of the launching arrangement.
Ordering of Steel
Mill orders are normally prepared by the drawing room, with the plate sizes being lifted from ship plans or plating models or, alternatively, obtained directly from computer printouts. Each piece of steel is assigned an identifying mark on the plan and on the bill of material. Plates to be severely hot-formed are ordered somewhat thicker than the required size in order to allow for the thinning down that occurs during the forming process.
There are price extras for very narrow, very wide and odd gage plates. Plates in the 1830 to 2286 mm (72 to 90-in.) width range are the least expensive per unit weight. The most economical sizes for the shipyard are determined after considering all related factors including the number of welded seams. Selected even-gage plates cost less per unit weight. In English units, the even gages are normally every 1/32 in. for plates up to 1/2 in. thick and every 1/16 in. for plates over 1/2 in. thick. In metric units, even gages are normally every one mm for thin plates and every two or more mm for the thicker plates.
The exact called-for grade of steel may not be obtainable for some ship repairs or when a small amount of steel or a special grade of steel is required. It is then necessary to obtain a proper substitute. Specific classification society approval of the substitute might be required, especially when ASTM and other non-ship grades are involved.
There has been little effort to increase the amount of standardization of plate or shape sizes beyond that normally adopted because such increases have been considered to be of little, if any, economic value.
Design work on structures incorporating large castings or forgings, such as a stern frame, is started early due to the long lead time required for these large items./p>
Let us talk a bit more about the application of non-ferrous materials in ship construction. The previous article covered the use of the GRP while in this one we will deal with concrete. Concrete consists of a mixture of stone aggregate bonded by a hardened cement, and Portland cement is normally used for marine applications. The aggregate consists of sand, gravel, and crushed stone. Specific gravity of concrete normally varies between 2.2 and 2.5, primarily depending upon the sizes and density in the stone mixture. Lighter weight concrete with specific gravities in the 1.6 to 2.0 range are made by using clay and shale aggregate. The ratio of water to cement is one of the most significant factors in determining concrete quality and properties.
Ordinary structural concrete with a water/cement/ratio approximating 0.40 by weight is usual for marine work. The long term durability of concrete in sea water has been well established on the basis of service experience and testing of samples from structures that have been submerged. However, in certain situations when concrete is ex¬posed to sulfate in soils or fresh water, it may react with the sulfate, and degrade. Sea water, however, minimizes or prevents such deterioration. Where sulfate deterioration is of concern, special sulfate resistant concretes are used.
Ferrocement is a form of reinforced concrete wherein layers of steel mesh are used as the reinforcing medium. The material has been used for making small boats up to 50m with skin thickness of 10 mm to 40 mm. The low cost and availability of the mesh and concrete ingredients make ferrocement particularly attractive where sophisticated industrial facilities are not available. Its most extensive commercial use has been for fishing vessels up to 50 m in length.
In the present article we will have a short talk about one of the non-metallic materials used for construction ship hulls. Though not used in ship building as widely as the steel, for instance, the these plastics are becoming more are more popular and therefore deserve due attention of the ship designers and shipbuilders.
Glass reinforced plastics, GRP, are a form of fiber reinforced plastics, FRP, which were introduced for marine structural applications in the 1940's in the form of Navy personnel boats. Since that time GRP have found widespread acceptance for yachts and small boats such as fishing trawlers up to 34 m in length. Although the future of glass reinforced plastics for larger ship structures is very promising, economic factors, and to some degree, questions of durability, limit their applicability.
Reinforced plastics used for ship structures are composed of glass fibers embedded in unsaturated polyester resins. Properties of GRP that are particularly useful for marine service, and have led to their extensive use for small boats, are high strength-to-weight ratio combined with good resistance to deterioration upon prolonged exposure to sea water. Lower maintenance costs for GRP hulls compensate for their relatively high initial cost as compared with steel or wood.
In this article we will talk about the engineering and design in the shipbuilding industry of today. First of all, let's see what exactly the engineering department of the shipyard normally deals with. The engineering department is called upon to aid in preliminary planning due principally to the numerous applicable rules and regulations which must be satisfied and to the complexity of both yard and ship-board equipment, it is important to note that new regulations.
These rules and regulations would normally include internationally recognized regulatory documents published by such entities as, for example, the Intergovernmental Maritime Consultative Organization (IMCO) and the U. S. Coast Guard (USCG) and many others, have greatly increased the amount of engineering work necessary for both the preliminary and final designs of the ship. Engineering works are normally started at the earliest possible time, even before schedules are prepared. That is very important for the smooth engineering design and subsequent construction of the vessel.
Scheduling methods used in shipbuilding are unique to each yard and generally reflect practices developed from experience. An overall schedule which is most useful to both management and production departments is one which highlights major tasks and events, and which shows the sequence of work and the relation of the various tasks to each other and to the whole project.
The basic principle in network flow is the task-to-task relationship. That is, task С cannot start until its two prerequisite tasks A and В are completed. There is, of course, the usual task-to-time relationship for each task. These principles have always been employed in one form or another throughout industry, but the computer has now made it possible to utilize to the fullest these principles in network form.
Network Flow Scheduling technique is often used for controlling large, complex, and possibly non-repetitive projects. Examples of the technique are PERT (Program Evaluation and Review Technique) and CPM (Critical Path Method), both of which provide a means of representing graphically the different operations that make up a project. These networks can be revised to show the effects of adjustments to a schedule necessitated by changes in design, delays, etc. It is also possible to treat the network statistically in order to obtain an idea of the probable longest and shortest times for completion of a project.