Branch Out to Prevent Failures in the Field
Resolve warranty risk through fault tree analysis. THEODORE J. SCHORN, ENKEI AMERICA INC., COLUMBUS, INDIANA
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 disas- sembly, 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 elimi- nate 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 impossible, in a subsequent forensic evaluation, to determine the exact cause of the defect. T e complexity of mul- tiple potential common causes, along with equipment and human factors, requires more than simple corrective action. Fault tree analysis can help cast- ing 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
34 | MODERN CASTING September 2014
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. Com- munications refers to the information provided to the end user regarding use of the equipment, maintenance and any warning associated with it. T e casting supplier cannot inves- tigate 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 respon- sibility to evaluate. T e material that composes the casting and its specifi ed properties likely were incorporated into the design delivered to the met- alcaster. Again, this falls on the casting purchaser to evaluate. If a claim comes to the atten-
tion of a metalcaster, at least some preliminary evaluation of these aspects of warranty is accomplished. T e assumption is that the com- ponent’s manufacture was at fault, either in failing to deliver the speci- fi ed mechanical properties, dimen- sional 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-
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
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