Specifying and Ordering Carbon and Low-Alloy Steel Castings
Steel casting costs may reflect variations resulting from differences in material specifications, tolerance limits, inspection requirements and certification requirements. The cost may escalate to several times the basic manufacturing cost depending on the complexity of these requirements. In addition, a wide range in estimated casting costs from several casting supplier bidders may reflect that a purchaser was not specific to the properties and requirements desired.
When specifying castings, it’s important to know how each of these requirements can affect the total cost of an order.
When making inquiries or ordering cast steel components, end-users should include information on:
- Casting shape—either by drawing or pattern. Drawings should include dimensional tolerances, indications of surfaces to be machined and datum ports for locating. If only one pattern is provided, the dimensions of the casting are predicted by the pattern;
- Material specification and grade, such as American Socciety for Testing and Materials (ASTM) A 27/A 27M-95 Grade 60-30 Class 1);
- Number of parts;
- Supplementary requirements, such as ASTM A 781/A 781M-95 S2 Radiographic Examination, test methods sucs as ASTM E 94, and acceptance criteria such as ASTM e 186 severity level 2;
- Any other information that might contribute to the production and use of the part.
When specifying castings, the three most important production criteria are the material to be used, design of the part and the testing required.
Carbon steels contain only carbon as the principal alloying element, but other elements such as silicon and manganese may be added for deoxidation. In addition to carbon, low-alloy steels contain other alloying elements up to a total 8% alloy content.
Many grades of carbon and low-alloy steel exist that meet specific end-use requirements, such as structural strength and resistnace to wear, heat and corrosion. Material for steel castings is generally ordered to ASTM requirements, although other specifications may be used. The most common specifications are:
- ASTM 781/A 781M-97—This specification covers mandatory requirments for steel and alloys castings in general industrial use;
- ASTM A 703/A 703M-97—This specification covers common requirments that apply to steel castings for pressure-containing parts;
- ASTM A 957-96—This specification covers requirements for steel and alloy castings produced by the investment casting process;
- ASTM A 985-98—This specification covers common requirements for steel investment castings used in pressure-containing applications.
The processes and procedures most commonly specified by the customer are heat treatment (which may enhance the properties of specific alloys) and welding. These processes have the greatest effect on metallurgical quality and serviceability of the casting, and both are routine functions of the steel casting facility. The temperature-controlling and recording equipment on heat treatment furnaces should be calibrated at specified intervals. The casting facility can be required to submit procedures and qualifications for review before starting an order.
Considerations of Steel Design
To obtain the highest quality product, the part should be designed to take advantage of the casting process. Reasonable dimensional tolerances must be indicated where a drawing is provided. Tolerances are normally decided by agreement between the end-user and the casting facility. Close cooperation between the customer’s design engineers and the casting engineers is essential to optimize casting design in terms of cost and performance.
Minimum Section Thickness
The stiffness requirements often govern the minimum thickness to which a section can be designed. There are cases, however, in which a thin section will suffice, depending on 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 unit length be adopted in order for the molten steel to completely fill the mold cavity.
Molten steel cools rapidly as it enters the mold. In a thin section close to the gate, which delivers the hot metal into the casting cavity, 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 section thickness of 0.25 in. (6 mm) is suggested for conventional steel castings. Wall thicknesses of 0.06 in. (1.5 mm) and sections tapering down to 0.03 in. (0.76 mm) are common for steel investment castings.
Draft is the amount of taper or angle that 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 metal thicknesses. The amount of draft recommended under normal conditions is 0.1875 in./ft. or approximately 1.5 degrees.
Draft can be eliminated by using cores, but this adds significant cost. In cases where the amount of draft may affect the subsequent use of the casting, the drawing should specify whether the draft is to be added or subtracted from the casting dimensions given.
Parting in one place facilitates the production of a pattern as well as the production of the mold. Patterns with straight parting lines parallel to a single plane can be produced more easily and at lower cost than patterns with irregular parting lines.
Casting shapes that are symmetrical about one center line or plane simplify molding and coring and should be used whenever possible.
A core is used to create openings and cavities that cannot be made by the pattern alone. Every attempt should be made by the designer to reduce or eliminate the cores needed for a particular design to reduce the final casting cost.
The minimum core diameter for a steel casting is dependent on three factors: the thickness of the metal section surrounding the core, the length of the core, and the special precautions and procedures used by the casting facility.
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, and as the severity of the thermal condition increases, the cleaning of the castings and core removal becomes more difficult and expensive.
In addition, the thickness of the metal section surrounding the core and the length of the core affect the bending stresses induced by buoyancy forces, and therefore, the ability of the casting facility to obtain the tolerances required. If the size of the cores is large enough, rods often can be used to strengthen it. Naturally, as the metal thickness and the core length increase, the reinforcement required to resist bending stresses also increases. Therefore, the minimum core diameter also must increase to accommodate the extra reinforcement required.
The casting design should provide for openings large enough to permit ready access for the removal of the core.
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 cross-section of a solidifying casting must be fed by liquid metal from an adjoining heavier section or from a riser, which serves as an adjoining feed metal reservoir. Shrinkage cavities are found in sections that must be fed through thinner sections by design. The thinner sections solidify too quickly to permit liquid metal to pas from the riser to the thicker sections and compensate for the volumetric contraction.
The casting facility’s engineers are responsible for providing a component that can be machined to meet the specific functional requirements. To accomplish this goal, a close relationship must be maintained between the end-user’s engineering and purchasing staff and the casting producer. The following points must be considered:
- Molding process, including its advantages and limitations;
- Machining stock allowance to assure cleanup 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 be checked at the outset to determine if all machining requirements are achievable. This may be accomplished with a complete layout of the sample casting to make sure that adequate stock allowance exists on all surfaces requiring machining. For many simple designs that can be measured with a simple rule, a complete layout of the casting may not be necessary. In other cases, in which the machining dimensions are more complicated, the casting should be checked more completely for target points and scribing to indicate all machining surfaces.
Mandatory testing ensures that the material meets the minimum requirements of the specification. In addition to specifying test methods, acceptance criteria must be agreed upon. The more testing and the tighter the acceptance criteria, the more expensive the steel casting produced—without necessarily increasing the quality or serviceability of the component.
The final cost of castings is affected by overspecification as well as maintaining specifications, codes, standards and other documents at the casting facility. Overspecifying a part may include requiring a more highly alloyed steel than necessary, specifying unnecessary or redundant tests, or specifying exceedingly stringent acceptance standards.
Steel castings are specially designed and manufactured parts, therefore their cost will depend on the complexity of design as well as the purchaser’s requirements. The cost of one casting cannot necessarily be compared to the cost of another similar casting because differences in quality and service requirements may exist.
--David Poweleit and Malcom Blair, Steel Founders’ Society of America.