The short answer is: metal legs can definitely get hot under direct summer sun, but they are usually not the part you actually sit on. What really decides whether a bench is too hot is the seat surface material, its color, and how much direct sunlight the bench receives.
Metal is an excellent conductor of heat. When sunshine hits steel, aluminum, or iron legs for hours, the surface temperature can climb well above 120°F, and sometimes much higher. If you touch the legs, they can feel burning. However, because the legs are narrow and often partially shaded by the seat itself, they do not always become as dangerously hot as a flat metal seat panel.
If the bench has a metal seat pan or metal slats, that is a different story. A dark metal seat in full sun can easily reach 140°F to 170°F, which is hot enough to cause pain or even a mild burn after a few seconds of contact. In that case, yes, the bench is too hot to sit on comfortably and may be unsafe.
If the bench has metal legs but a wooden, composite, or plastic seat, the seat usually stays much cooler than metal. Wood absorbs and releases heat more slowly and does not conduct heat the way metal does. A light-colored wooden seat in full sun might feel very warm but rarely becomes painful. A dark composite or plastic seat can still get hot, though usually less than metal.
Color matters more than many people expect. Black or dark brown surfaces absorb more solar energy, while white, light gray, or natural wood tones reflect more and stay cooler. So a bench with dark metal legs and a dark seat will be the worst combination in summer.
Location and airflow also make a difference. A bench in open sun with no breeze will heat up more than a bench under a tree, pergola, or shade sail. Even partial shade during the hottest part of the day can significantly reduce surface temperatures.
So how do you know if it is safe? Use the back-of-the-hand test. Place the back of your hand on the seat for about five seconds. If it feels uncomfortable or you have to pull away, it is too hot to sit on. If you must use a sun-baked bench, wear long pants or put down a towel first.
If you are shopping for an outdoor bench for a sunny spot, choose a metal frame with a wood or composite seat, or select lighter colors. Avoid all-metal benches in full sun unless you plan to add cushions or keep them in shade. If you already own a metal bench, consider moving it under shade, adding light-colored seat cushions, or painting metal parts with a lighter high-temperature outdoor paint.
So yes, the metal legs can get hot, but the real question is what you are sitting on. A well-designed bench with metal legs and a non-metal seat can still be comfortable in summer, while an all-metal bench in direct sun may be too hot to use during midday.