Honestly, the short answer is no—not under normal outdoor conditions. I hear this a lot from people installing benches in places like Phoenix, Las Vegas, or anywhere that gets brutally hot, so let me give you a real, no-nonsense answer.
During a heat wave, air temperatures can hit 100°F to 115°F. In direct sunlight, the metal surface of a park bench can get much hotter—sometimes 140°F to 170°F. That is definitely too hot to touch comfortably, but it is still far below the temperature where steel or aluminum starts to lose structural strength or permanently deform.
Metal does expand when it heats up. A long steel slat or aluminum frame can grow slightly in length on a blazing afternoon. Quality benches are designed for this: slats have small expansion gaps, fasteners allow a tiny bit of movement, and welded frames distribute thermal stress instead of concentrating it in one weak spot. You might notice a faint creak or a tiny shift in alignment. That is just thermal expansion doing its job, and it returns to normal when the metal cools.
Where people run into problems is usually with cheap, very thin sheet metal or flimsy tubing. Under extreme sun exposure, those can flex, ripple, or get that wavy “oil canning” look because the material is simply too thin to hold its shape under thermal stress. A heavy-gauge steel or cast aluminum park bench will not have that issue in ordinary weather.
The finish matters too. A quality powder-coated surface protects the metal underneath and reflects enough heat to keep temperatures stable. Dark colors absorb more heat than light colors, but even a black bench will not reach temperatures that warp thick steel or cast aluminum on a normal summer day.
If you are shopping for a bench for a truly extreme environment—like a desert installation with constant direct exposure—look for heavy-gauge steel or cast aluminum, heat-resistant powder coating, slats with expansion gaps, and bolted or welded joints designed for outdoor thermal movement.
So unless the bench is exposed to a fire, a malfunctioning heater, or some industrial heat source, you do not need to worry about the sun alone warping the metal. It will get hot, so warn people before they sit, but it will hold its shape.