What could be more common than the household refrigerator? It is the most-used appliance in America with more than 99.5% of homes having one. How did this come about, and what does heat treatment have to do with cooling things?

The process of keeping food cold by using evaporative cooling goes back to ancient times. The Romans used terra-cotta pots in water fanned by slaves to cool their food. Likewise, around 500 B.C. the Egyptians and Indians made ice on cold nights by setting water out in earthenware pots and keeping the pots wet. In relatively modern times, the icebox was used for about 150 years until the invention of the mechanical refrigerator in the late 1800s.

The first documentation of artificial refrigeration occurred at the University of Glasgow in 1748. William Cullen’s work relied on the vapor-compression refrigeration process explained by Michael Faraday. While different processes were investigated until the late 19th century, Carl von Linde is widely considered the father of the modern refrigerator. His invention of a continuous process of liquefying gases in large quantities formed a basis for today’s refrigeration technology.

Can you believe that about half of the households in the U.S. relied on melting ice – in an icebox – to keep food cold at the start of the 20th century? The other half had no cooled storage at all – ice was too expensive. Based on early patented work from France, General Electric manufactured the first refrigerating machine marketed by the Johns Manville Company. The first unit was sold in 1911 for about $1,000 – about twice the cost of an automobile that year.

GE developed and marketed the “Monitor-Top” refrigerator in 1927. It was the first to see widespread usage with over 1 million units being produced. Many of these units are still functioning today. Technological developments continued throughout the 20th century with the incorporation of a frozen-food compartment, automatic defrosting and automatic icemakers to name just a few.

Most common household refrigerators make use of the thermodynamic vapor-compression cycle. A circulating refrigerant (such as Freon) enters the compressor as a vapor at its boiling point. This vapor is then condensed into a liquid at its boiling point. The liquid refrigerant then passes through an expansion valve, where the pressure quickly decreases, resulting in the flash evaporation and auto-refrigeration of a portion of the liquid. This refrigerant requires kinetic energy, which is extracted in the form of heat from the food compartment via the evaporator.

No doubt several components of a modern refrigerator rely on thermal treatment for proper functioning. Items like springs come to mind. One that you may not think about is the lower door hinge. Why the lower door hinge and not the upper? The lower hinge carries all of the weight. Today’s refrigerator doors contain add-ons such as water dispensers and are designed to hold large items like gallons of milk. And that says nothing about the 5-year-old boy who enjoys hanging from the door while it swings open.

A supplier to a refrigerator maker like GE typically manufactures the lower hinge. These hinges are made from 1050 carbon steel and are through-hardened, quenched and tempered to the ideal hardness. A commercial heat treater is usually the one to perform this operation in continuous atmosphere furnaces. An endothermic atmosphere is generated that is neutral to the carbon in the hinge material. The post-hardening oil quench is followed by a wash, and the part is also coated on high-end refrigerators.

It is often remarkable to look at quantities of items such as this. At a single commercial heat treater in Ohio, approximately 275,000 ½-pound hinges are heat treated every month. That’s 3.3 million every year for the number crunchers out there.

This is just one more way that heat treatment makes the things we use in our day-to-day lives better.