The Kitchen Sink Too
Thermal Processing & Metals in Everyday Life
While considering the next Now You Know topic, it seemed like we had already covered “everything but the kitchen sink.” So, naturally, the kitchen sink was the obvious choice for this month.
While stainless steel kitchen sinks seem to be ubiquitous, they have only existed for the past 75 years or so. Kitchen sinks made from other materials (e.g., stone) trace back about 150 years. Even enameled cast iron sinks were not used before the 1890s. Prior to that and the availability of running water and sewage, a bowl or bucket was used inside a dry sink, which was a metal trough built into a wooden cabinet.
Although stainless steels – alloys of iron, chrome and nickel – had been studied as early as 1821, the process to make the steel corrosion resistant was not identified until 1909. The key was finding a way to eliminate the carbon in the steel, which would combine with the chromium and reduce the corrosion resistance.
Kitchen sinks formed from stainless steel are currently the most popular choice because they are lightweight, attractive, easy to clean and won’t stain or rust. Old kitchen sinks and other stainless material are recycled into stainless by melting in an electric furnace. The molten metal is transferred to an AOD (Argon-Oxygen-Decarburization) vessel for refining and blowing out the carbon. Without using argon, chromium would be oxidized into the slag during this process. The AOD allows the carbon to be preferentially oxidized while retaining most of the chromium. The balance is reduced back from the AOD slag at the deoxidation stage of the melt process.
The refined steel can be cast in a variety of ways, but for the manufacture of sink-bowl material, it is typically cast into slabs and rolled into hot-band coil. This is followed by a hot acid wash, known as the annealing and pickling process. It will be cold rolled to the final gauge in specialized mills called “Z” mills for the inventor, Tadeusz Sendzimer. The Z mill is able to roll stainless to very light gauges while maintaining uniform thickness across a wide strip. Sometimes the cold-rolled coil will need to be re-annealed to be able to roll it to the thinnest gauges.
The process anneals for stainless are required because it work-hardens when rolled. The annealing time and temperature are critical to forming a good kitchen sink from the material. If the annealing line stops beyond the time allowed by the strip accumulator, the furnace can overheat the steel, causing grain growth. When drawn into a sink, this will result in an “orange-peeled” condition, which is aesthetically unpleasant and may also affect drawability. A full anneal also helps the sink manufacturer because the bowl will not work-harden as quickly during drawing.
Of the three basic types of stainless – austenitic, ferritic and martensitic – austenitic stainless is used for sink bowls. Typically, a special alloy variation, usually indicating “SBQ” for sink-bowl quality, is used to enhance the drawability. Drawability was one of the limitations in the early use of stainless for sink bowls. Deeper bowls of the time were probably welded versus drawn.
The process for manufacturing a sink begins by cutting the steel coil into blanks. Under a 1,000-ton press, a punch stretches the blank into a bowl form, which also work-hardens the steel. Lubricant is brushed into the bowl, and it is stretched a second time, further hardening it. Next, a drain hole and lip is formed in the bottom of the bowl. A belt is then used inside the bowl to give it a brushed appearance.
For a double-bowl sink, two of these bowls are butted together and electrode welded. A roller flattens the weld followed by a sander to smooth it. The rim recesses are then formed, and it is trimmed. Next, holes are punched for the faucets and a sprayer or soap dispenser. The bowl is then completely brushed to give it a fine finish. This process takes about 25 minutes, and it requires a skilled operator. The manufacturer’s name or logo is then embossed into the bowl followed by coating the bottom with heavy latex, which helps to deaden the sound. The entire sink manufacturing process takes about 2.5 hours.
Now you know everything you need to know about the kitchen sink and how thermal processing influences the entire process. IH