Get Ready for the New Low Lead Requirements
In January 2014, federal legislation was enacted, aimed at reducing the allowable lead content of components used in potable water systems.
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In January, federal legislation was enacted, aimed at reducing the allowable lead content of components used in potable water systems. For machined components, the new rules essentially prohibit the use of traditional free machining leaded brass in components that “touch” the water flowing through the systems. This has already promoted design and materials choice changes and will encourage the more creative use of non-traditional materials such as plastics as well as potentially increasing the volume of “lead free or low lead” brasses used in machined parts.
One of the issues for the precision parts industry is the difference in machining characteristics that the low lead brasses exhibit, compared with those of traditional, leaded, free machining brass. This situation is quite similar to the differences between free machining steel grades (1215 and 12L14) and is obviously dependent on machine type, layout and tooling selection and part complexity. The brass industry has, however, made the selection of “no lead/low lead” brass material a more complex choice.
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Traditional leaded free machining brasses have as much as 3 percent lead (Pb) to enhance machining characteristics.
The practical definition of no-lead/no-lead brass in the U.S. is a maximum of 0.25 percent lead content. The brass rod industry, globally, has developed three primary alternatives:
Reduce lead content to below 0.25 percent and essentially maintain the other ingredients of the “standard” free machining brass (that is, approximately 62 percent Cu, balance Zn + minor residuals).
Replace the lead (Pb) content with Bismuth (Bi), which has similar chemical characteristics and is right next to lead in the periodic table (Pb 82/207; Bi 83/209).
Modify the recipe significantly (including increasing Copper (Cu) content to approximately 75 percent) and add a different element (Silicon (Si)) to enhance machinability.
Each option has its pros and cons. The first option is very straightforward from a metallurgy and engineering viewpoint since the basic functional properties of the material for the final application are unchanged, except the lead content is lower, and there is a concomitant reduction in machinability, which may result in changes in tooling such as insert selection, layout and cycle times.
The second option, mostly adopted by offshore suppliers, replaces lead with bismuth, which maintains more or less the original machinability characteristics. However, the presence of bismuth does have some metallurgical implications and might, under certain conditions of heat or stress or both, result in increased risk of cracking, either via hot shortness during the bar extrusion process, or after manufacturing in the final application. Also, the effect of bismuth being introduced into the nominal (mostly leaded) brass scrap material stream has not yet been closely studied and may have long-term implications, perhaps even ultimately requiring segregation of scrap sources.
The third option, which is really a patented recipe, provides generally acceptable machinability. However, this option has two significant issues that need to be managed by the machinist and ultimately by the end user. First, the significantly higher copper content (75 percent versus 62 percent) has a direct impact on cost since the price of copper is much higher than the price of zinc (the other main ingredient). Second, the presence of silicon in deliberately added quantities makes it necessary to segregate the scrap to avoid what can be a significant cross contamination penalty on the sale of turnings. Complete scrap segregation can be difficult, particularly in job shops where only one chip spinner is used.
In the end, each individual parts maker will make their own selection, based on their particular situation and economics. However, the new legislation has a significant and wide reaching impact on our industry, which will provide additional business opportunities for parts manufacturers who have available capacity and have developed the process know-how necessary to capably supply components from this new family of alloys.
