T
he cost of managing a warranty claim can be many times higher than a simple return of the cast metal part. T e value-added cost is much higher: An installed part usually requires disassembly, replacement and shipping, and it potentially incurs costs for lost time, labor or other peripheral damage or loss. T e expectations for a casting supplier normally are
to investigate the root cause of a problem and take prompt corrective action to eliminate it. Depending on the volume of parts supplied and the severity of the occurrence, the end user might require information about the probability of other parts in the fi eld having the same problem. T ey also will request, at a minimum, short-term corrective action to eliminate the defect from the stream of supply. T ese requests can overwhelm the casting supplier. Metal castings often are returned in a warranty environment where it is impos-
sible, in a subsequent forensic evaluation, to determine the exact cause of the defect. T e complexity of multiple potential common causes, along with equip- ment and human factors, requires more than simple corrective action. Fault tree analysis can help casting buyers and their suppliers address and resolve diffi cult quality problems.
Warranty Investigation Method A proper warranty investigation in such a situation must investigate six aspects,
as shown in Figure 1. In the diagram, design refers both to the casting supplied and the design of the equipment or system in which it is assembled. Manufacture refers to the process by which it was built. Communications refers to the informa- tion provided to the end user regarding use of the equipment, maintenance and any warning associated with it. T e casting supplier cannot investigate the assembly of the casting into the unit
or its fi eld use and potential abuse leading to the warranty claim. An evaluation of the communication and warnings delivered to the user also is out of the casting supplier’s reach, yet necessary to consider as part of a thorough investigation. If the part design came from the equipment provider, it is their responsibility to
evaluate. T e material that composes the casting and its specifi ed properties likely were incorporated into the design delivered to the metalcaster. Again, this falls on the casting purchaser to evaluate. If a claim comes to the attention of a metalcaster, at least some preliminary
evaluation of these aspects of warranty is accomplished. T e assumption is that the component’s manufacture was at fault, either in failing to deliver the speci- fi ed mechanical properties, dimensional conformity or part soundness required by the contract.
Typically, the metalcaster is pro- vided narrative information about the failure and a sample of the part that failed with which to perform forensic analysis. Good practice would indicate the part has some traceability to the process inspections and records of the casting or subsequent operations, such as heat treatment, that may have been performed at the metalcasting facility. T e casting provider will need access, in-house or purchased, to the means to understand the nature of the failure and identify the type and extent of any defect.
Common and Assignable Causes
In the nomenclature of quality, pro-
cess variability generally is attributed to two types of causes: assignable and common. Assignable causes can be named and singly create the objection- able variation in the process. If, for example, a misrun defect was observed in a casting and it was known that the metal was poured 45 degrees below the lower specifi cation limit established for the part, then the cause would be assignable. Corrective action would be applied such that greater reliability for pour temperature would be obtained. Unfortunately, a great many industrial processes exist where such obvious assignable causes are not present yet process variability is such that at some small rate, defects are created in the process output. For example, consider an alumi-
Fig. 1. The diagram illustrates the six possible categories of causes for a warranty (fi eld) failure.
num part cast in a permanent mold. T e initial pour temperature will have some variability; the cycle time (and temperature of the mold) also will be variable within a small range. T e thickness, thus the insulating value, of the die coat will be variable over a certain range, dependent on opera- tor skill, the materials involved and their concentration, and so on. T e forced cooling system will vary in its ability to remove heat, its timing within the solidifi cation cycle, etc. All of these relatively small variations, and others, can combine in unknown
Jul/Aug 2014 | METAL CASTING DESIGN & PURCHASING | 41
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60