Specifying Copper Castings
Copper castings are purchased to meet the particular requirements of their end use, and casting quality tests should therefore be specified to assure the parts do conform to such requirements. Requirements for copper castings go beyond setting limitations for materials and manufacturing method. Consider the requirements listed in ASTM B584—Standard Specifications for Copper-Alloy Sand Castings for General Applications, which lists five other criteria beyond “materials and manufacture” to stipulate:
- Materials and Manufacture – states the limitations on raw materials and manufacturing methods to be used.
- Chemical Requirements – lists the specified range and limits of elements comprising the alloy.
- Mechanical Requirements – lists the minimum mechanical property requirements as determined from separately cast test-bars and requirements for test-bar design.
- Other Requirements – comments on various special tests that may be specified, such as pressure-tightness, fracture testing, casting soundness compared to radiographic standards, etc.
- Workmanship and Finish – describes general requirements regarding appearance, dimensions, and visible defects.
- Casting Repair – where permissible, describes size limitations of repairable defects and methods and conditions of repair.
- All these factors, which are of importance to both purchaser and producer alike, should comprise part of the purchase agreement and should be included in the foundry’s checklist for quality control.
Specifications for the chemical composition of purchased castings include not only the acceptable range for each element desired in the alloy, but also the limits for the content of various metallic impurities. Conformance to chemical composition specifications is an indication that the material will exhibit satisfactory chemical and physical properties. It also implies that the castings, if otherwise sound, are likely to have adequate mechanical properties.
The customer’s purchase agreement should specify the frequency of the testing and reporting of chemical composition. This may vary considerably, depending on customer requirements and the severity of the application. For ordinary commercial castings, one analysis per order may suffice (or possibly one per day, in the case of long runs). Some casting buyers, having first made a “quality audit” to satisfy themselves with the supplier’s quality control program and capability may actually waive a report on chemical analysis while reserving the right to make random examinations of the laboratory records. For critical applications, it is not uncommon for the customer to require a certified chemical analysis for each heat of metal cast, with the castings themselves identified accordingly.
The most commonly specified mechanical properties are the static tensile properties (ultimate tensile strength, yield strength, and elongation). In general, these properties are measured by testing separately cast tensile bars. The choice of test-bar mold geometry for copper-base alloys, so far as gating and risering are concerned, depends somewhat on the solidification characteristics of the individual alloy. The specification most commonly used in the U.S. is ASTM B208 – Standard Practice for Preparing Tension Test Specimens for Copper Alloy Sand, Permanent Mold, Centrifugal and Continuous Casting. Included in this specification are three test-bar mold designs for sand castings. The first (double keel block test-bar) is intended for narrow-freezing-range alloys such as aluminum bronzes, which are sensitive to turbulence and oxidation during pouring. A second (double, vertical web-type) bar is recommended specifically for wide-freezing-range alloys, while a third is a double, horizontal full-web type.
It is important for casting buyers and producers alike to carefully consider the significance of separately cast test-bar properties. The casting designers would like to believe that test-bar properties reflect those found in the castings themselves. Unfortunately, this is not necessarily the case. Casting properties may actually be higher than those indicated by the test bar because of faster cooling rates in the mold. On the other hand, the casting is prone to many defects related to pouring, gating and risering, and molding, which the test bar is not, and may therefore contain defects of structure not indicated by a test bar. These potential defects of casting structure, visible or not, necessitate other methods of quality assurance testing. With these ideas in mind, it will be appreciated that the properties of separately cast test bars indicate only the quality of the metal as it is poured into molds. In a sense, the mechanical properties of separately cast test bars reinforce the results of chemical analysis, while providing added information as to whether the metal has been damaged by dissolved gases, foreign materials, or impurities not detected by chemical tests.
The foregoing idea is expressed in ASTM B208 as follows:
“3.1 The mechanical properties determined from test bars for sand, permanent mold, and centrifugal castings poured in accordance with this practice represent the properties of the metal going into castings poured from the same heat. These mechanical properties may not be the same as the properties of the corresponding castings because of the solidification effects of varying size, section and design.”
Although less prevalent than was once the case, some casting purchasers insist that test bars be cast as appendages to the castings themselves. This practice perpetuates the myth that the properties of such test bars reflect those of the casting. Actually, the attachment of the test bar to the casting could easily detract from the soundness of the casting at that location by disrupting the pattern of solidification. For large parts, the pouring temperature may be unnecessarily high in order to permit the smaller test bar to fill without misruns. This could easily damage casting properties in the case of wide-freezing-range alloys. As regards the test bar itself, its location and method of attachment are usually dictated by casting and mold geometry, making it prone to all manner of defects associated with poor gating practice. For these reasons, the stipulation that test bars be poured as attachments to the castings should be avoided whenever possible.
As a quality control tool, fracture tests made on the castings themselves can provide information to supplement that derived from facture testing of melt specimens. Fracture locations for test casting should be chosen selectively so as to study not only the more representative areas of the part but also the critical locations, particularly those which are last to solidify and are therefore prone to show shrinkage or internal gas porosity. The latter include sections adjacent to gates and risers, or isolated heavy walls and bosses. The soundness of areas which are to be machined is particularly important.
Fractures are most easily controlled by making saw cuts, from one or both ends, in the desired plane of fracture and then bending the casting while it is firmly held in a vise.
Many foundries use the fracture test routinely for evaluating the effectiveness of gating and risering systems on new or revised patterns, or in the establishment of proper pouring-temperature ranges. The same test can be used to evaluate or diagnose the cause of defects in castings rejected by visual inspection or other quality tests. For example, parts that show leakage during pressure-testing should be fractured as closely as possible through the exact point (or area) of leakage. If sand or slag inclusions are found, the remedial action to be taken is rather obvious. The same is true when a smooth, unfused surface is exposed, indicating the presence of a cold shut. If, however, the fracture is porous or discolored at the location of leakage, it is best to examine the fractures of adjacent areas before reaching a decision. If adjacent areas are homogeneous and sound in structure, and of uniform color, it is likely that the porosity is a localized condition which results from solidification shrinkage—perhaps the most common cause of leakers in copper-tin-lead-zinc alloys. Paradoxically, localized shrinkage porosity in unmachined areas of the casting are often associated with hot spots such as re-entrant angles and fillets. Most often, leakage occurs at machined surfaces. This is because the relatively dense, sound surface layers of the casting wall have been machined away, leaving a rather porous internal section which leaks under pressure.
If examination of fractured areas adjacent to the point of leakage likewise shows discoloration and evidence of porosity, then there is reason to suspect damaged metal conditions. This might mean that pouring temperature was excessive or that severe metal/mold reactions occurred. Gas porosity in itself does not usually cause leakage during pressure testing, unless it is extremely severe. This is because gas voids tend to occur as small, isolated bubbles which do not interconnect with each other. The surface of gas bubbles, when observed by fracture testing, appears relatively smooth and somewhat brassy in color. Shrinkage porosity, on the other hand, shows voids which are rough (dendritic). They may or may not be discolored. A further complication in viewing porous areas is the fact that both shrinkage porosity and gas porosity may be present simultaneously. Thus, it takes experience and a practiced eye to interpret fracture surfaces when diagnosing the causes of leakers.
Often times, during the review of the fractures of rejected castings, sub-surface defects may become exposed. These newfound problems may have nothing to do with the rejection and may even distract the determination of the deleterious defects. For this reason, it is also recommended that you fracture castings which have passed all test parameters or, better yet, castings which have successfully completed their life cycle. Evaluating these casting will help prevent wasting time on insignificant casting discontinuity.
Another important use of the fracture test is with polished ware such as that used for plumbing fixtures. It is not uncommon, after a polished casting has been chrome-plated, to find indications of surface porosity. Fracturing of parts will usually reveal whether the condition is restricted to the surface (and can probably be eliminated by a little extra polishing), or whether the porosity is widespread.
Purchasers of castings for use as pressure vessels (valves, fittings, etc.) almost invariably insist upon pressure-tightness as a casting requirement, reserving the right to subject the castings to tests for leakage, usually after machining, and returning all leakers for replacement. Foundries that produce such castings are faced with the problem of making sound parts without a reliable, economic method for testing them before shipment. In some cases, jobbing foundries have equipped themselves with apparatus for pressure testing on a sample basis; however, this is often neither effective nor practical. The most effective means of avoiding excessive customer returns due to leakage is to isolate the most common causes, based on past experience and trouble-shooting, and then establish reliable process control methods to avoid such defects.
Except for relatively critical applications and for certain alloys, test of soundness by radiography are seldom required for copper-base alloys as a part of a purchase specification. Such testing may be used, however, not only to verify the internal soundness of finished castings but as a means of evaluating gating and risering systems for new and experimental castings.
Workmanship and Finish
For purchase specifications related to workmanship and finish, refer to visual quality standards. Generally stated, the requirements may indicate only that parts be of “good” workmanship and “free from injurious defects.” Such general terms are obviously unsatisfactory to either producer or end-user, since they are subject to individual interpretation. The subjects of surface cleanliness and finish, presence of any or all kinds of visible defects, and dimensional tolerances of the parts should be dealt with explicitly in the purchase agreement and clearly understood by both buyer and seller. Quality can never be absolute since it depends on a variety of design, material, and process factors. The minimum standards of quality should reflect those required for the application and should be rigorously enforced. The costs of maintaining such quality should reflect the number and kind of tests required and the process parameters involved.
Foundries and casting users, in striving for quality and reliability, should consider supplementing or replacing the time-honored concept of 100% inspection with modern statistical quality control and acceptance sampling techniques.
Defect repair by such techniques as welding, plugging, impregnation or peening, although frequently effective and practical, should never be done without the full knowledge and consent of the purchaser. Although salvage by these techniques is permissible, the conditions and limitations must be defined and understood by both the supplier and purchaser. Cosmetic repair should not be considered in the same way as structural unsoundness repair. Weld repair is applicable to only a relatively few families of copper-base alloys, and purchase agreements should stipulate in those cases that acceptable techniques are used, such as a weldability test (see ASTM – Standard Specification for Copper Nickel Alloy Castings). CS