A soldered joint is defined as a gas-tight joint obtained by joining metallic mixtures that melt below what temperature?

A soldered joint is specifically characterized by the use of materials that melt at relatively low temperatures to create a strong, gas-tight seal between metals. In the context of soldering, this process involves using solder, which is an alloy, to bond two metal surfaces together.

The correct answer identifies the temperature range for soldered joints as being above 400° F, which aligns with the common understanding in mechanical and electrical contexts. The melting point of most solders, traditionally made from lead-tin or other alloys, usually falls below 1,000° F, ensuring that the joint can be easily formed and not damage the base metals involved.

This temperature criterion is critical because it ensures that the joint not only achieves a gas-tight seal but also maintains structural integrity during operation. The temperature limit ensures that the solder solidifies adequately to create the necessary bond without compromising the integrity of the materials being joined.

In contrast, the other options suggest higher temperature thresholds that do not align with the typical definitions of soldering processes. Such temperatures would likely refer to welding or brazing processes instead, which involve joining materials at much higher temperatures, therefore making those options unsuitable for the definition of a soldered joint.