The safety helmet has been hit by a heavy object, but there are no cracks on the surface. Does it need to be replaced?

Must be replaced! A safety helmet that has been hit by a heavy object, even if the surface looks intact, is equivalent to "internal injury", just like a mobile phone that has a broken screen but the motherboard is broken. The protective principle of a safety helmet is like the airbag of a car. The outer hard shell withstands the impact, and the internal buffer layer absorbs the force like a sponge. After it has been hit once, the internal plastic skeleton and buffer pad may have been deformed or broken, and the next time it encounters danger, it may directly "strike". To give a real example: someone did an experiment, using a 5-kilogram iron block to hit a safety helmet from a height of 1 meter, leaving only pits on the surface, but continued to use this hat for a second impact test, and the hat was directly smashed through, and the impact force on the head mold exceeded the standard by 3 times. This shows that the first impact has caused the internal structure to "fall apart", but it is not visible to the naked eye. This is like a flattened can, although it is not broken, it can no longer withstand the same pressure. 
In addition, the material of the safety helmet (such as ABS plastic) will change its molecular structure after being hit, just like repeatedly bending a wire will make it brittle. Even if there are no cracks, the toughness of the material is greatly reduced, and it may be directly shattered when it encounters the next impact. What is more dangerous is that some low-quality safety helmets are made of recycled plastic. They look fine on the surface, but the inside has been aged for a long time and will break at the touch. 
So don’t feel sorry for the hat, your life is much more valuable than it. Someone on the construction site did not change the safety helmet that was hit, and ended up with a serious head injury in the second accident. Remember: the safety helmet is a "disposable bodyguard". After withstanding a heavy blow, it has completed its mission and it is time to retire.