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3.3.6 Long-Term Creep of Lead-Free Soldered Copper Tube Joints

Charles Hagwood

Statistical Engineering Division, ITL

Roger Clough and Donald Harne

Metallurgy Division, MSEL

Copper Development Association (CDA)

In 1993, the Copper Development Assocation (CDA) approached NIST for answers to a problem. Lead-free solders will be required in plumbing used in new building construction. Would these solders work as well as the traditional lead-containing alloys? What is the maximum safe instantaneous pressure, and the maximum allowable pressure to give a creep life of 100 years? Jointly with CDA, a set of tests was designed to provide the answers to such questions, necessary for the establishment of maximum safety plumbing codes for these new solders.

One objective of this study is to establish more precise design stresses for water system connections made with three commonly used lead-free solders. Under the auspices of the Copper Development Association, another objective is to obtain data on the long-term warm creep failure of copper tubes joined with the new lead-free solders. These data are needed in formulating new strength codes, since these solders are already being used in water systems. Our final objective is to obtain stress-lifetime data at elevated temperatures for copper tubes joined with lead-free solders and develop, with the aim of design code implementation, lifetime probability models based on these.

Anticipated outcomes are: 1) Reduced health risks and environmental degradation. 2) Improved strength and creep resistance compared to Pb/Sn Solders. 3) New tube joint codes giving cost savings to the building industry.

Millions of plumbing joints are made each year in the construction of houses, office buildings and high-rise apartments. In terms of sheer volume of solder, the building industry's use of solder is substantial compared to that of the electronics industry. Cost savings can be appreciable if the required introduction of the new lead-free solders does not require the use of more expensive soldering techniques or the use of more expensive plumbing materials by the multi-billion dollar building construction industry. Therefore these test results, which will allow the safest allowable pressures to be used, will permit optimum savings to this industry.

Failure data have been collected on solder welds at various temperatures and pressures. Most of these data sets are censored. A failure time model of the form $hours=c_0\exp(\sigma_0 stress)$ was shown to hold and the parameters were estimated using the maximum likelihood technique under a Weibull distributional assumption. Prediction intervals have been derived.

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Date created: 7/20/2001
Last updated: 7/20/2001
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