ENGINEERING WITH STEEL CASTINGS 1.0 Casting versus fabrication. Fabrication generally relies on the use of available rolled steel sections, worked into the finished product by bending, machining, and welding. Castings can be moulded to a much greater extent, and hence lend themselves to complex shapes. 2.0 Materials which can be cast. At Flowserve Davies & Baird, we are generally making castings in mild (low carbon) steel, stainless steels (austenitic, martensitic, duplex and super duplex), and manganese steel. We also have extensive experience making castings in structural, alloy, and heat resistant steels. Figure 1. Fully machined steel castings 3.0 Types of castings methods. Some of the commonly used methods for making steel castings include; Sand Casting and Investment Casting. 3.1 Sand Casting Sand Casting is the least costly, most widely applicable method. It is suitable for small or large production runs. This is the method in use at Flowserve Davies & Baird. 3.2 Investment Casting Investment Casting is a precision casting method, typically more expensive and generally limited to smaller castings of mass <45 kg. Investment Casting is suitable for small castings where dimensional precision or premium surface finish is required, and can be used for large production runs. 4.0 The sand casting process. From engineering drawings either of the casting or of finished product, pattern equipment is manufactured. Put simply, the pattern is representative of the finished casting, but with a dimensional allowance built in for contraction (as the molten steel cools), and with taper to allow the pattern to be withdrawn from the mould. Frequently, the pattern equipment will comprise of multiple components, for example to make a core/s to form the internal shape/s of a casting. Figure 2. A Pattern in mould box filled with moulding sand. A sand mould is created by positioning the pattern in a rigid mould box, and ramming moulding sand around the pattern to create the outer shape of the casting (see Figure 2). Frequently, this is done in two parts – the cope and drag half such that when the two halves are put together, the external shape of the casting is encapsulated inside the mould (essentially as a solid negative). More complex castings or larger shapes may require that the mould be constructed in more than two sections. Figure 3. Core box with a ready-to-be-removed core Frequently, one or more cores are fitted into the mould prior to closure, to create the internal shape of the casting. At Flowserve Davies & Baird, moulding sand is a mixture of new and recycled sand, plus epoxy resin. Once moulded, this mixture hardens such that it can support the flow and mass of the molten steel without collapse or distortion. Figure 4 Tapping from furnace into ladle. Molten steel of the chosen grade is tapped from the melting furnace into a preheated and insulated pouring ladle, and then into the mould through a system of runners and risers. Excess molten steel is provided in the runner system with the intent that a supply of molten metal continues to be supplied to the solidifying casting after the pour from the ladle has in fact been completed. Calculating and positioning the methods is a key foundry skill, where success relies as much on experience as it does on calculation. Generally, castings are knocked out of the sand mould once they have solidified and cooled. The methods (runners and risers) are removed and the casting proceeds to dressing and machining, where a finished shape is achieved. Weld repairs are done as needed, heat treatment for desired properties, shot blast to clean the surface of the casting and NDT to determine the quality of the casting. 5.0 The advantages and disadvantages of engineering with steel castings. The particular advantage of engineering with steel castings is the ability to custom manufacture i.e. the possibilities for complex shapes, and demanding material and test specifications, for quantities from 1-off to multiple units. The disadvantages derive from the fact that the process is highly variable, skill sensitive, and has a low degree of automation and hence control. The degree of repeatability is low, especially for castings manufactured in jobbing quantities. Castings are generally not as “clean” as wrought (rolled) product and are considerably more subject to discontinuities. 5.1 Taper The withdrawal of pattern equipment from a sand mould dictates the use of taper. In many cases what could be an exactly vertical or horizontal surface on a casting may not be able to be so, depending on the orientation of the casting during moulding and because of the essential requirement for taper. 6.0 Casting quality: Figure 5. Pouring molten metal into sand moulds is a high-risk undertaking! 6.1 Pouring molten metal into sand moulds is a high risk undertaking! Manufacturing wrought metal is usually a semi continuous process - for example rolling or extruding - which gives rise to higher degrees of process control. Pouring castings into sand moulds is a start-stop process, and involves numerous conditions that are not entirely predictable including: linearity of metal flow, rate of fill, rate of solidification, solidification front, gas pick up and other gas effects. Many operations are performed by hand including slag removal prior to pour. Preparing the moulds (moulding) for sand castings in jobbing quantities is typically labour intensive, with high levels of reliance on the skill of individual moulders, and this adds to the degree of variability in the casting process. 6.2 When is a discontinuity a defect? Defects in castings are discontinuities that are defined as being a defect, by a test method and acceptance specification eg. AS4738.1 or as specified by the purchaser of the casting. Test methods and acceptance specifications have been developed over decades of best practice engineering in casting manufacture and use, and are reflected in applicable Standards. A key point in engineering with castings is that there is no test method; not dye penetrant, not magnetic particle, not ultrasonic, not radiographic testing, that can detect every type and size of discontinuity. Thus all castings should be expected to contain discontinuities, and the specifying engineer must provide for that fact in the design and specification. Specifying simply that “castings shall be free from defects”, without defining test methods and acceptance specifications is futile, unprofessional, and exposes the unwary supplier and purchaser to unnecessary conflict. Whereas by specifying appropriate test methods and acceptance criteria, the engineer can control the size, concentration, and frequency of discontinuities in castings to an acceptable level. 6.3 Dimensional control and tolerances in sand castings. Sand casting is a relatively imprecise method of metal forming, when it comes to dimensional control, and care is needed when specifying castings with respect to dimensions and tolerances. For example, the total combined tolerance on a raw casting dimension between 100 and 160mm could be expected to be in the range 3.6-7mm from a typical hand moulded sand casting process. Generally, the smaller the raw casting dimension, the larger the total tolerance as a percentage of the raw dimension. ISO 8062:1994 Castings – System of dimensional tolerances and machining allowances is a useful reference. 6.4 Pattern equipment, “methods” and trial castings. The design of pattern equipment and the “methods” is not an exact science. Exactly how a casting will solidify in the mould, and the way it will move as it contracts is estimated rather than calculated with absolute accuracy. When new pattern equipment is manufactured, the first casting/s off is a trial. In some percentage of instances, pattern equipment modifications, or fine tuning of the “methods” are necessary to achieve dimensional or other quality conformance. Occasionally, the first casting/s off – the trial – is not merchantable and must be scrapped. Pattern makers and methods technicians occasionally find that the pattern equipment has to be “made wrong” in order to “make the casting right”. The essential point is that the manufacture of a new casting design must make allowance for the trial process to take place, before the foundry can commit to making and supplying production castings. 6.5 Modelling of solidification and mould filling. The advent of three dimensional (3D) modelling technology has created the possibility of modelling how a mould will fill, as well as the pattern of solidification of molten metal in the mould. This helps to remove some of the uncertainty described in the preceding paragraph, without eliminating it completely. Completion of a trial for a new design of casting remains necessary, however the probability of a successful trial is improved. This becomes of great importance for one off castings of high complexity and cost. Flowserve Davies & Baird encourage clients to provide new casting designs in 3D format. 6.6 Quality as a function of quantity. It follows that the more castings are manufactured of a particular design, the greater the opportunity to tune pattern equipment and “methods”. The rate of “first time sound” castings produced would generally rise as production numbers rise, quickly reaching an optimum level. It follows that the highest risk castings, as far as the foundry is concerned and therefore also the purchaser, are one offs of a new design. Figure 6. Cast Pelton wheel bucket 7.0 Making steel. It’s not over until it’s over! In steel castings manufacture, the process of creating the steel specification is not complete until the final heat treatment of the casting – and there could be as many as five heat treatments depending on the material specification. That is, the pouring of the molten metal into the sand mould is only one step in the steel making process: A manganese steel casting for example is extremely brittle when first knocked out of the mould after solidification, whereas the finished product is relatively soft and ductile. Between the pour and the final heat treatment, the foundry has some degrees of freedom in creating a casting of merchantable quality and that meets the client’s specification. This includes weld repair when necessary. 7.1 Welding as part of the foundry process. Repairing castings by welding to make them of merchantable quality is central to making steel castings. For the reasons described above, this is especially so in the manufacture of castings in jobbing quantities. Weld repair during manufacture is carried out under controlled conditions using weld procedures - established by the foundry metallurgist. Importantly, the metallurgist’s expertise dictates at what stage of the creation of the steel, any the weld repairs required will be carried out. For example in many cases this will be before final heat treatment of the steel. In other cases, weld repairs may be conducted after a first heat treatment of the casting but before the final heat treatment. Weld repair of steel castings during manufacture is generally a benign process, and should not be confused with field welding of finished cast steel components. The key point is that field weld repair is attempted after the process of creating the steel has been completed. A key point in engineering with steel castings is that it is not practical for a foundry to accept a specification that excludes welding. Recognising this, experienced engineers may define a major weld, and specify that such welds be documented on a weld map to be included in the quality documentation for a highly stressed and/or safety critical casting. Specifying zones in which welding is excluded, or requiring that a weld must be approved by the engineer (which in itself presupposes the possibility that the engineer may not give approval), represent high risk for the foundry and may be rejected. 7.2 Field weld repairs By comparison with weld repair during casting manufacture, field weld repair may or may not be benign, depending on the material and the size of the weld. Generally, most steels are weldable, however in some cases the requirement for pre or post heat treatment is difficult if not impossible to effect and control in the field, and may render field welding inadvisable. Flowserve Davies & Baird are generally able to advise our clients, as regards suitable field welding methods for castings of our manufacture. 8.0 Specifying castings. In specifying castings, the purchaser would consult the foundry to ascertain the particulars necessary to allow manufacturing and testing to proceed. The purchaser shall, as a minimum, provide the following information at the time of inquiry and order (As stated on AS 4738.1 section 4). This includes a description of the casting by pattern number and/or drawing number and the number of castings to be made from each pattern or drawing. Variations in the ordered quantities should be agreed to at the time of enquiry and/or order. Delivery information shall be agreed upon (required date or schedules, delivery location and terms of delivery). The material Standards and grade designation for the casting should also be stated. Mechanical and chemical tests required and acceptance criteria for these tests. Patterns, drawings or samples of the casting with finished casting dimensions, tolerances and machining locations clearly indicated. When no drawing is supplied, the casting is purchased to pattern supplied and the manufacturers shall not be responsible for the shape and dimensions of the castings. When the purchaser supplies both a pattern and drawing, the pattern shall be capable of producing a casting in the specified material complying with the drawing. Note also that a pattern is designed for use with one material. When used with different materials, a variation in the design weight and dimensions may result. Available information regarding significant difficulties previously encountered in connection with castings of the same or similar pattern and any supplementary requirement including surface defects should be made known to the manufacturer. The purchaser who visits and/or inspects to exercise these options should not cause interference to the normal operations of the manufacturer, and all visitors need to comply with the manufacturer’s plant rules for visitors. It is recommended that before the patterns are made, and preferably while the design is still on the drawing board, that the purchaser confer with the manufacturer regarding casting requirements. The exact service conditions under which the ferrous castings will have to operate are rarely known to the manufacturer and the quality of the ferrous castings required to match the service conditions may only be determined by the purchaser, who should give an indication of the quality Standards required at the time of enquiry. In this regard, the manufacturer’s experience can often assist the purchaser in obtaining castings best suited for his overall needs. 9.0 Useful references AS 4738.1 Metal castings - Ferrous sand moulded ISO 8062:1994 Castings – System of dimensional tolerances and machining 10.0 Definitions Mild (low carbon) Steel Steels in which carbon is the chief alloying element and the specified minimum of other elements does not exceed 1.65% manganese, 0.6% silicon and 0.4% for all other elements. Stainless Steel A family of alloy steels containing chromium approx. 8-25%. They are characterised by their resistance to corrosion. • Austenitic stainless steels have the iron atoms arranged in a face center cubic pattern. (Found at high temperature in carbon steels but at room temperaute in some alloy steels, eg. Austenitic stainless steel) • Martensitic stainless steels have the atoms arranged in a body center tetragonal pattern and supersaturated with carbon. (Produced by rapid quenching of austenite) • Duplex & Super duplex stainless steels have a microstructure of ferrite and austenite. It provides better corrosion resistance and about twice the yield strength of the austenitic stainless steels. Manganese Steel Austenitic manganese steel contain at least 11% Manganese as the main alloying addition and is used in applications where high impact will cause work hardening of the wear surface whilst the inherent toughness of the sub surface steel is retained. Structural Steel Steel as a construction material, a profile, formed with a specific shape or cross section and certain standards of chemical composition and strength. Structural steel shape, size, composition, strength, storage, etc, is regulated in most industrialised countries. Alloy steel Alloy steels provides many benefits over regular steel alloys. In general, alloy steels are much stronger and tougher than ordinary mild steels. They are used in cars, trucks, cranes, bridges, and other structures that are designed to handle large amounts of stress, often at very low temperatures. Heat resistant Steel Heat resistant steels are designed for use at elevated temperatures, often having superior oxidation resistance and strength at high temperatures. Runner A channel through which molten metal or slag is passed from one receptacle to another; in a mould, the portion of the gate assembly that connects the downgate or sprue with the casting ingate or riser. The term also applies to similar portions of master patterns, pattern dies, patterns, investment moulds and finished castings. Riser Reservoir of molten metal from which casting feeds as it shrinks during solidification. Dressing The process of removing all runners and risers and cleaning off adhering sand from the casting. NDT Non-Destructive Testing is testing or inspection that does not destroy the object being tested or inspected. This includes, but are not limited to, Visual, Dimensional, Liquid & Magnetic Penetrant and Radiography inspections.
Pages to are hidden for
"ENGINEERINGWITHSTEELCASTINGSr2"Please download to view full document