The ordering of steel castings takes a certain amount of time and energy to qualify a potential supplier. To get the best value from the steel casting also requires a cooperative effort on the part of the buyer and the seller from the early stages of the design through the manufacturing process. Good planning ahead of time will pay dividends for both you and your supplier.
How to Order Steel Castings
As with any manufacturing process, in order to produce a part, it is necessary to know:
- Design – What is the part?
- Material – What should the part be made of?
- Testing – How should the part be tested before delivery?
Inquiry and Ordering
All pertinent information must be stated on both the inquiry and order including:
- Casting shape–either by drawing or pattern. Drawing should include dimensional tolerances, indications of surfaces to be machined, datum points for locating.
- Material specification and grade (e.g. ASTM A27-87 Grade 60-30 Class I).
- Number of Parts
- Supplementary Requirements (e.g. ASTM A781-87a S2 Radiographic Examination).
- Test Methods (e.g. ASTM E94)
- Acceptance Criteria (e.g. ASTM E186 severity level 2 or MSS-SP-54)
- Any other information that might contribute to the production and use of the part.
To achieve the most efficient production and the highest quality product, the part should be designed to take advantage of the flexibility of the casting process.
The foundry must have either the designer’s drawings or pattern equipment and know the length of the run (number of parts to be made).
To take advantage of the casting process, the foundry should also know which surfaces are to be machined and where datum points are located. The acceptable dimensional tolerances must be indicated when a drawing is provided.
Tolerances are normally decided by agreement between the foundry and customer. SFSA Handbook Supplement 3 represents a common starting point for such agreements. It is not a specification and care should be taken to reach agreement on what tolerances is required.
Minimum Section Thickness
The rigidity of a section often governs the minimum thickness to which a section can be designed. There are cases, however, when a very thin section will suffice, depending upon strength and rigidity calculations, and when castability becomes the governing factor. In these cases it is necessary that a limit of minimum section thickness per length be adopted in order that liquid metal will completely fill the mold cavity in these thinner sections.
Molten steel cools rapidly as it enters a mold. In a thin section, close to the gate, which delivers the hot metal, the mold will fill readily. At a distance from the gate, the metal may be too cold to fill the same thin section. A minimum thickness of 0.25 in (6 mm) is suggested for design use when conventional steel casting techniques are employed. Wall thickness of 0.060 in (1.5 mm) are common for investment castings and sections tapering down to 0.030 in (0.76 mm) can readily be achieved.
Draft is the amount of taper or the angle, which must be allowed, on all vertical faces of a pattern to permit its removal from the sand mold without tearing the mold walls. Draft should be added to the design dimensions while maintaining minimum metal thickness.
Regardless of the type of pattern equipment used, draft must be considered in all casting designs. (Draft can be eliminated by the use of cores; however, this adds significant costs.) In cases where the amount of draft may affect the subsequent use of the casting, the drawing should specify whether this draft is to be added to or subtracted from the casting dimensions as given.
The necessary amount of draft depends upon the size of the casting, the method of production, and whether molding is by hand or machine. Machine molding will require a minimum amount of draft. Interior surfaces in green sand molding usually requires more draft than exterior surfaces. The amount of draft recommended under normal conditions is about 3/16 in. per ft. (approximately 1.5 degrees), and this allowance would normally be added to design dimensions.
Parting in one plane facilitates the production of the pattern as well as the production of the mold.
Patterns with straight parting lines, that is, with parting lines in one plane, can be produced more easily and at lower cost than those with irregular parting lines.
Casting shapes which are symmetrical about one center line or plane readily suggest the parting line. Such casting design simplifies molding and coring, and should be used wherever possible. They should always be made as ” split patterns ” which require a minimum of handwork in the mold, improve casting finish, and reduce costs.
Number of Cores
A core is a separate piece (often made from molding sand) placed inside the mold to create openings and cavities which cannot be made by the pattern alone. Every attempt should be made by the designed to eliminate or reduce the number of cores needed for a particular design to reduce the final cost of the casting.
The minimum diameter of a core which can be successfully used in steel castings is dependent upon three factors:
- The thickness of the metal section surrounding the core
- The length of the core
- The special precautions and procedures used by the foundry.
The adverse thermal conditions to which the core is subjected increase in severity as the metal thickness surrounding the core increases and the core diameter decreases. These increasing amounts of heat from the heavy section must be dissipated through the core. As the severity of the thermal conditions increases, the cleaning of the castings and core removal becomes much more difficult and expensive.
The thickness of the metal section surrounding the core, and the length of the core, both affect the bending stresses induced in the core by buoyancy forces and, therefore, the ability of the foundry to obtain the tolerances required. If the size of the core is large enough, rods can often be used to strengthen the core. Naturally, as the metal thickness and the core length increase, the amount of reinforcement required to resist the bending stresses also increases. Therefore, the minimum diameter core must also increase to accommodate the extra reinforcing.
The cost of removing cores from casting cavities may become prohibitive when the areas to be cleaned are inaccessible. The casting design should provide for openings sufficiently large to permit ready access for the removal of the core.
Internal Soundness-Directional Solidification
Steel castings begin to solidify at the mold wall, forming a continuously thickening envelope as heat is dissipated through the mold-metal interface. The volumetric contraction that occurs within a solidifying cast member must be compensated by liquid feed metal from an adjoining heavier section, or from a riser which serves as a feed metal reservoir and which is placed adjacent to, or on top of, the heavier section.
The lack of sufficient feed metal to compensate for volumetric contraction at the time of solidification is the cause of shrinkage cavities. They are found in sections which, owing to design, must be fed through thinner sections. The thinner sections solidify too quickly to permit liquid feed metal to pass from the riser to the thicker sections.
Design for Machining
In the final analysis the foundry casting engineer is responsible for giving the designer a cast product that is capable of being transformed by machining to meet the specific requirements intended for the function of the part. To accomplish this goal a close relationship must be maintained between the customer’s engineering and purchasing staff and the casting producer. Jointly, and with a cooperative approach, the following points must be considered:
- The casting process, its advantages and its limitations.
- Machining stock allowance to assure clean up on all machined surfaces.
- Design in relation to clamping and fixturing devices to be used during machining.
- Selection of material specification and heat treatment.
- Quantity of parts to be produced.
It is imperative that every casting design, when first produced, be checked to determine whether all machining requirements called for on the drawings may be attained. This may be best accomplished by having a complete layout of the sample casting to make sure that adequate stock allowance for machining exists on all surfaces requiring machining. For many designs of simple configuration that can be measured with a simple rule, a complete layout of the casting may not be necessary. In other cases, where the machining dimensions are more complicated, it may be advisable that the casting be checked more completely, calling for target points and the scribing of lines to indicate all machined surfaces. Additional guidelines for casting design are given in the Steel Castings Handbook and Supplements 1, 3 and 4 of the Handbook.
The material to be used to produce the part must be identified in the order. Material for steel castings is generally ordered to ASTM requirements, although other specifications may be used. This section contains a summary of the scope, chemical composition requirements and mechanical property requirements of these materials or product specifications. Many requirements are common to several specifications and are given in: ASTM A781/A781M-89 and/or ASTM A703/703M-89
A781/ CASTINGS, STEEL AND ALLOY, COMMON REQUIREMENTS,
FOR A781M-89 GENERAL INDUSTRIAL USE
This specification covers a group of requirements that are mandatory requirements of the following steel casting specifications issued by the American Society of Testing and Materials: A27, A128, A148, A297, A447, A486, A494, A560, A743, A744 and A747.
If the product specification specifies different requirements the product specification shall prevail.
This specification also covers a group of supplementary requirements, some of which may be applied to the above specifications as required. These are provided for use when additional testing or inspection is desired and applies only when specified individually in the order by the purchaser.
A703/ STEEL CASTINGS, GENERAL REQUIREMENTS,
FOR PRESSURE-A703M-89 CONTAINING PARTS
This specification covers a group of common requirements which, unless otherwise specified in an individual specification, shall apply to steel castings for pressure-containing parts under each of the following ASTM specifications: A216, A217, A351, A352, A389, and A487.
This specification also covers a group of supplementary requirements which may be applied to the above specifications as indicated therein. These are provided for use when additional testing or inspection is desired and applies only when specified individually by the purchaser in the order.
Testing ensures that the material meets the requirements of the specification; consequently, testing is mandatory. More frequent testing or other tests may be imposed, by use of supplementary requirements of product specifications or general requirement specifications.
In addition to specifying test methods, acceptance criteria must be agreed upon. The more testing and tighter the acceptance criteria–the more expensive the product will be–without necessarily increasing quality or serviceability. Hence, the extent of testing and acceptance criteria should be based on the design and service requirements.
The mechanical properties are verified by the use of test bars cast either separately or attached to the castings.
The mechanical properties obtained represent the quality of the steel, but do not necessarily represent the properties of the castings themselves, which are affected by solidification conditions and rate of cooling during heat treatment, which in turn are influenced by casting thickness, size and shape. In particular, the hardenability of some grades may restrict the maximum size at which the required mechanical properties are obtainable.